Dissertation for the Degree of Doctor of Philosophy

A Chemical Study

of Arsenic, Boron and Gases

in High-Temperature Geothermal

Fluids in Iceland

Niels Giroud

FACULTY OF SCIENCE - UNIVERSITY OF ICELAND

August 2008

Accademic DissertationA dissertation Presented to the University of Iceland Faculty of Science

in Candidacy for the Degree of Doctor of Philosophy

SupervisorProf. Stefán Arnórsson

Institute of Earth Sciences, University of Iceland

Doctoral CommitteeProf. Stefán Arnórsson

Institute of Earth Sciences, University of Iceland

Dr. Sigurdur R. GíslasonResearch Professor

Institute of Earth Sciences, University of Iceland

Dr. Andri StefánssonResearch Scientist

Institute of Earth Sciences, University of Iceland

OpponentsDr. Bruce Christenson

GNS ScienceLower Hutt, New Zealand

Dr. Giovanni ChiodiniDirector of Research at Vesuvius Observatory

National Institute of Geophysics and VolcanologyNaples, Italy

A Chemical Study of Arsenic, Boron and Gases

in High-Temperature Geothermal Fluids

in Iceland

c© Niels Giroud, 2008Printed in Iceland by Oddi hf.

ISBN 978-9979-70-479-9

Contents

List of Figures vii

List of Tables ix

List of Symbols xi

Abstract xiii

Útdráttur (in Icelandic) xv

Acknowledgments xvii

1 Introduction 11.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Selected geothermal �elds . . . . . . . . . . . . . . . . . . . . . . . 4

2 Gas chemistry in �ve Icelandic high-temperature geothermalsystems 72.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2 Geological background . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.1 Kra�a and Námafjall . . . . . . . . . . . . . . . . . . . . . . 92.2.2 Nesjavellir . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.3 Svartsengi and Reykjanes . . . . . . . . . . . . . . . . . . . 11

2.3 Alteration mineralogy . . . . . . . . . . . . . . . . . . . . . . . . . 122.4 Fluid compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.5 Well discharge enthalpy . . . . . . . . . . . . . . . . . . . . . . . . 152.6 Aquifer temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . 162.7 Sampling and analysis . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.7.1 Water phase . . . . . . . . . . . . . . . . . . . . . . . . . . 182.7.2 Steam phase . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.8 Thermodynamic data . . . . . . . . . . . . . . . . . . . . . . . . . 19

iii

2.8.1 Mineral solubility constants and gas solubilities . . . . . . . 192.9 Data handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.9.1 Note on pH measurement . . . . . . . . . . . . . . . . . . . 212.9.2 Calculation of aquifer water compositions and aqueous spe-

ciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.10 Mineral-gas equilibria . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.10.1 CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.10.2 H2S and H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.11 Individual mineral-solution equilibria . . . . . . . . . . . . . . . . . 302.11.1 Calcite and wollastonite . . . . . . . . . . . . . . . . . . . . 302.11.2 Andradite-grossular, epidote-clinozoisite, prehnite . . . . . . 322.11.3 Magnetite, pyrite and pyrrhotite . . . . . . . . . . . . . . . 332.11.4 N2 and Ar . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.12 Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . 35

3 Processes in�uencing As, B and Cl concentrations in �uids ofvolcanic geothermal systems in Iceland 393.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.2 Geological features . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2.2 Kra�a and Námafjall . . . . . . . . . . . . . . . . . . . . . . 423.2.3 Landmannalaugar . . . . . . . . . . . . . . . . . . . . . . . 433.2.4 Geysir area and Hveravellir . . . . . . . . . . . . . . . . . . 443.2.5 Hveragerdi . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.2.6 Nesjavellir . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.2.7 Reykjanes and Svartsengi . . . . . . . . . . . . . . . . . . . 45

3.3 Fluid compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.4 Sampling and analytical methods . . . . . . . . . . . . . . . . . . . 463.5 Data handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.5.1 Calculation of aquifer �uid composition . . . . . . . . . . . 463.5.2 Arsenic, B and Cl concentration in the aquifer �uid . . . . 513.5.3 Sources of As, B and Cl to geothermal �uids . . . . . . . . 54

3.6 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.6.1 Boron and Cl distribution in sampled waters . . . . . . . . 553.6.2 As and Cl relationships . . . . . . . . . . . . . . . . . . . . 583.6.3 Rock derived concentrations . . . . . . . . . . . . . . . . . . 59

3.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.8 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . 70

4 Formation of a vapor cap subsequent to production in the Svart-sengi geothermal �eld, Iceland 734.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.2 Geology and hydrology . . . . . . . . . . . . . . . . . . . . . . . . . 74

iv

4.3 Production history . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.4 Sampling, analyses and calculations . . . . . . . . . . . . . . . . . 784.5 Fluid composition . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.6 Evolution of gas concentrations . . . . . . . . . . . . . . . . . . . . 81

4.6.1 Wet-steam wells . . . . . . . . . . . . . . . . . . . . . . . . 814.6.2 Dry-steam wells . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.7 Changes in dissolved salts and pH . . . . . . . . . . . . . . . . . . 854.8 Formation of the vapor cap . . . . . . . . . . . . . . . . . . . . . . 87

4.8.1 Vapor fraction calculations . . . . . . . . . . . . . . . . . . 874.8.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

4.9 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 894.10 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . 90

A Trace element analyses 93

Bibliography 101

v

List of Figures

1.1 Hot-spring and fumarole at Hveravellir . . . . . . . . . . . . . . . . 31.2 Map of Iceland showing the location of the studied high-temperature

areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.3 Well testing at Kra�a . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 Location map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2 Na/K and quartz geothermometers compared for both the �phase

segregation� model and the �heat transfer model�. . . . . . . . . . . 172.3 Potential mineral bu�ers controlling CO2 gas concentrations in the

initial aquifer �uid . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.4 Mineral bu�ers controlling H2S in the initial aquifer �uid . . . . . 282.5 Mineral bu�ers controlling H2 in the initial aquifer �uid . . . . . . 292.6 Saturation indices of selected alteration minerals . . . . . . . . . . 312.7 Ca2+/(H+)2 ratio versus temperature . . . . . . . . . . . . . . . . 322.8 Argon versus N2 in initial aquifer �uids for all �elds . . . . . . . . 342.9 In�uence of air addition and boiling on the N2/Ar ratio versus Ar

concentration in aquifer �uid . . . . . . . . . . . . . . . . . . . . . 34

3.1 Location map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.2 Phase segregation excess enthalpy model . . . . . . . . . . . . . . . 473.3 Boron distribution between vapor and liquid phases discharging

from wet-steam wells versus sampling temperature . . . . . . . . . 533.4 Example of the in�uence of the selected vapor pressure at which

phase segregation occurs on the calculated aquifer B concentrationfor three high-enthalpy wells. . . . . . . . . . . . . . . . . . . . . . 54

3.5 Measured Cl and B concentrations in liquid water samples fromall the study areas . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.6 Measured Cl and As concentrations in liquid water samples fromall the study areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.7 Rock derived As, B and Cl concentrations in aquifer liquid in Gey-sir and Hveravellir �elds . . . . . . . . . . . . . . . . . . . . . . . . 60

vii

3.8 Rock derived As, B and Cl concentrations in aquifer liquid in Land-mannalaugar and Hveragerdi �elds . . . . . . . . . . . . . . . . . . 61

3.9 Rock derived As, B and Cl in aquifer liquid. . . . . . . . . . . . . . 643.10 Time evolution of non-marine B in the aquifer water of three wells

of the Námafjall area. . . . . . . . . . . . . . . . . . . . . . . . . . 643.11 Cl and B of non-marine origin . . . . . . . . . . . . . . . . . . . . . 66

4.1 Location map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.2 Annual discharge and injection . . . . . . . . . . . . . . . . . . . . 764.3 Cumulated discharge and injection . . . . . . . . . . . . . . . . . . 774.4 Annual CO2 emissions from wet-steam and dry-steam wells at

Svartsengi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.5 Chloride concentration in the aquifer �uid of wet-steam wells . . . 804.6 CO2, H2S and H2 concentrations versus time in the aquifer �uid

of wet-steam wells . . . . . . . . . . . . . . . . . . . . . . . . . . . 824.7 CO2, H2S and H2 concentrations versus time in the discharging

vapor from the dry-steam wells . . . . . . . . . . . . . . . . . . . . 844.8 Magnesium concentration in the aquifer �uid of wet-steam wells . . 854.9 Fluoride concentration in the aquifer �uid of wet-steam wells . . . 864.10 Mg+2 versus −2F− activities . . . . . . . . . . . . . . . . . . . . . 864.11 Fraction of the total liquid that forms the vapor cap . . . . . . . . 89

viii

List of Tables

2.1 Major elements analyses of the sampled waters. Concentrations inmg/kg. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2 Gas analyses of the sampled vapor phase . . . . . . . . . . . . . . . 162.3 Individual minerals dissolution reactions . . . . . . . . . . . . . . . 202.4 Aquifer water composition . . . . . . . . . . . . . . . . . . . . . . . 242.5 Mineral assemblages potentially controlling CO2, H2S and H2 con-

centrations and respective K-temperature equations . . . . . . . . 26

3.1 Arsenic, B and Cl concentrations in liquid and vapor phases ofwet-steam wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.2 Arsenic, boron and chloride concentrations in spring waters fromhigh-temperature areas . . . . . . . . . . . . . . . . . . . . . . . . . 57

4.1 Major and trace elements and major gas concentrations in aquiferliquid of wet-steam wells . . . . . . . . . . . . . . . . . . . . . . . . 79

4.2 Gas concentrations in the vapor discharge from dry-steam wells . . 804.3 Gas concentrations and estimates of the fraction of vapor �owing

into wells 7 and 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

A.1 Trace element analyses of liquid water samples from wet-steam wells 94A.1 Trace element analyses of liquid water samples from wet-steam

wells (cont.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95A.1 Trace element analyses of liquid water samples from wet-steam

wells (cont.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96A.2 Trace element analyses of vapor samples from wet-steam and dry-

steam wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97A.2 Trace element analyses of vapor samples from wet-steam and dry-

steam wells (cont.) . . . . . . . . . . . . . . . . . . . . . . . . . . . 98A.2 Trace element analyses of vapor samples from wet-steam and dry-

steam wells (cont.) . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

ix

List of Symbols

Ai subscript i: any speciesAr subscript r: any non-volatile speciesAs subscript s: any volatile speciesAd superscript d: well discharge conditionsAe superscript e: conditions where phase segregation occurs between the

aquifer and discharge of the �uid, before it occursAf superscript f : conditions of the deep aquifer (feed zone)Ag superscript g: conditions where phase segregation occurs between the

aquifer and discharge of the �uid, after it has occuredAl superscript l: liquid phaseAm superscript m: marine componentAs superscript s: seawaterAt superscript t: total �uidAv superscript v: vapor phaseal activity in aqueous solutionDs distribution coe�cient of species s between liquid and vaporfs fugacity of gaseous species shl speci�c enthalpy of vapor-saturated water (kJ/kg)ht speci�c enthalpy of total �uid (kJ/kg)hv speci�c enthalpy of vapor (kJ/kg)Ks solubility constant of gaseous species s (moles/kg/bar, Henry's Law

Coe�cient)Md,t total mass �ow rate from wet-steam well (kg/s)Me,l mass �ow rate of boiled water retained in aquifer (kg/s)Mf,t mass �ow rate of initial aquifer �uid (kg/s)md,l

i concentration of component i in discharging liquid water (moles/kg)md,t

i concentration of component i in total well discharge (moles/kg)md,v

i concentration of component i in discharging vapor (moles/kg)me,l

i concentration of component i in liquid water retained in aquifer(moles/kg)

me,vi concentration of component i in vapor phase at conditions P e

xi

mf,li concentration of component i in initial aquifer liquid (moles/kg)

mf,mi marine component concentration of component i in initial aquifer �uid

(moles/kg)mf,t

i concentration of component i in initial aquifer �uid (moles/kg)ms

i concentration of component i in seawaterNs number of moles of volatile species sne,l

s mole fraction of volatile species s in the liquid phase at contition ene,v

s mole fraction of volatile species s in the vapor phase at contition eP e vapor pressure at which phase segregation occurs (bar)Ps partial pressure of species s (bar)T e Temperature at which phase segregation occurs (◦C)T f Average aquifer temperature (◦C)V e,l (Me,l/Md,t) relative mass (to well discharge) of boiled (and degassed)

water retained in aquiferV f,t (Mf,t/Md,t) relative mass (to well discharge) of initial aquifer �uid

that has boiledXd,v vapor fraction at well discharge conditionsXe,v vapor fraction at condition P e

xii

Abstract

Volatile species provide valuable information on physical and chemical processes oc-curring in high-temperature (volcanic) geothermal systems. This study focuses on anassessment of the processes that control selected volatile species and arsenic concen-trations in the deep aquifers of �ve volcanic geothermal �elds in Iceland. It is basedon analytical data on water and vapor samples collected from wet- and dry-steam welldischarges. The �elds considered are Kra�a, Námafjall, Nesjavellir, Svartsengi andReykjanes. The �rst three systems are of the dilute �uid type; the source �uid is con-sidered to be meteoric water. The systems of Svartsengi and Reykjanes, on the otherhand, contain saline �uid. The source water at Svartsengi is 70% seawater and 30%meteoric water but seawater only at Reykjanes. Some of the wells have �excess� en-thalpy, that is the enthalpy of the discharged �uid is higher than that of vapor saturatedwater at the aquifer temperature. Excess well discharge enthalpy is not considered tore�ect such an enthalpy of the initial aquifer �uid. It is considered to be produced inthe depressurization zone around wells, where extensive boiling occurs. The results ofthe quartz and Na-K geothermometers are taken to indicate that phase segregation inproducing aquifer is the cause of the �excess� well discharge enthalpy; a fraction of theliquid is retained in the aquifer due to its adsorption onto mineral grain surfaces bycapillary forces, whereas all the vapor �ows into the wells.

The �rst paper presented in this thesis focuses on the reactive gases carbon dioxide(CO2), hydrogen sul�de (H2S), and hydrogen (H2), and their state of equilibrium withvarious mineral assemblages, as well as the unreactive argon (Ar) and the relativelyunreactive nitrogen (N2). The concentrations of CO2 in the aquifer �uid of many of thewells corresponds closely to equilibrium with the mineral assemblage epidote-prehnite-calcite-quartz in the case of the dilute waters but to equilibrium with epidote-garnet-calcite-quartz in the case of the saline waters. Some of the wells in Kra�a, however, areenriched in CO2 relative to equilibrium, likely because of high �ux from the magmaticheat source, whereas most of the wells at Nesjavellir show low CO2 concentrations,probably as a consequence of insu�cient supply of this gas from the magma. H2Sand H2 are likely to be controlled by the same mineral assemblage. For most wells,aquifer �uid H2S concentrations are close to equilibrium with the assemblage epidote-prehnite-pyrite-pyrrhotite, whereas H2 concentrations are often higher. The high H2

concentrations are considered to re�ect the presence of a small fraction of equilibriumvapor in the aquifer �uid. Such a vapor fraction strongly a�ects the H2 concentrationin the aquifer �uid but not so much the much more soluble H2S, as well as the moresoluble CO2.

xiii

The main sources of N2 and Ar to the aquifer �uid is air-saturated groundwater andseawater. The �uid is mostly depleted in both gases relative to the source water. N2/Arratios show that N2 has another source, potentially entrapped air bubbles, magmaticgas or decaying organic matter.

The second paper aims at describing the sources and behavior of arsenic (As), boron(B) and chloride (Cl) using data from the �ve above mentioned geothermal areas, aswell as samples from boiling and warm hot springs and hot water wells in four otherhigh-temperature geothermal �elds in Iceland, and from four wells in the Southern Low-lands low-temperature area. Arsenic, B, and Cl concentrations in geothermal �uids inIceland are low compared to such �uids in many parts of the worlds. The concentrationsin the dilute �uids are typically in the range 50�150 ppb, 0.5�5 ppm, and 50�150 ppmfor As, B, and Cl, respectively. Arsenic concentrations in the saline �uids are similar,but B and Cl concentrations are much higher. Boron and Cl are highly mobile on thewhole range of water temperatures (10�320 ◦C) considered in this study. Arsenic isalso very mobile in the deep primary geothermal �uids but boiling of this �uid as wellas its mixing with cold groundwater removes As e�ectively from solution, especiallythe mixing. Data on Cl/B and Cl/As ratios from boiling hot springs in mature high-temperature geothermal systems show that all these elements are dissolved from thebasalt in stoichiometric proportions. In the young geothermal system at Kra�a, theB and Cl of the deep aquifer �uid is largely derived from magmatic volatiles but onlyto a limited extent from the basalt host rock. At Nesjavellir, most of the Cl in thedeep �uid has been dissolved from the host rock, but a large fraction of the dissolvedB has a magmatic source. In the saline �uids, As is less mobile than B and Cl and islikely to be removed from solution into sul�de minerals. If virtually all the Cl in theSvartsengi and Reykjanes saline �uids is of seawater origin, which is considered likely, itis evident that a signi�cant fraction of the B in these �uids is magmatic. Arsenic doesnot partition signi�cantly into the vapor phase, at least at vapor pressures as high as40 bar, but B does. Boron distribution between liquid and vapor is closely approachedat the wellhead where samples were collected.

The third paper deals with the vapor cap in the Svartsengi geothermal system thatformed over the 240 ◦C liquid-dominated reservoir subsequent to pressure drawdownby exploitation of the �eld. The mass of �uid withdrawn from the reservoir in theperiod 1976�2006 amounts to 243 megatonnes (Mt), of which 217 came from wet-steamwells (the liquid reservoir) and 25 Mt from dry-steam wells (the vapor cap). In 2006the cumulated drawdown was 30 bar. Boiling has reduced the gas content of the liquidreservoir and in turn increased and decreased aqueous �uoride (F) and magnesium (Mg)concentrations, respectively, due to rapid re-equilibration of the degassed water with Mgand OH-bearing minerals. The gas content of the vapor cap has decreased with time.Initially CO2 concentrations were ∼80,000 ppm but at present they are ∼30,000ppm.It is estimated that to date as much as 3% w/w of the initial aquifer liquid water hasboiled to supply vapor to the vapor cap. Wells have been drilled into the vapor cap forelectricity production with the consequence of higher CO2 and H2S emissions into theatmosphere per produced energy units.

xiv

Útdráttur (in Icelandic)

Ýmis rokgjörn efnasambönd veita mikilvægar upplýsingar um eðlis- og efnaræn ferli íháhitasvæðum. Sú rannsókn sem hér er fjallað um lýsir athugun á nokkrum rokgjörnumefnum í vatni af íslenskum háhitasvæðum með áherslu á hvaða ferli ráða styrk þeirra ívatninu. Efnagreining á uppleystum steinefnum í vatnssýnum og gösum í gufusýnumsem safnað úr renni blaut- og þurrgufuhola var notuð til að reikna efnainnihald djúp-vökva í �mm háhitasvæðum, Krö�u, Námafjalli, Nesjavöllum, Svartsengi og Reykjanesi.Djúpvökvinn í þremur fyrst töldu svæðunum er snauður af uppleystum steinefnum.Upprunavökvinn er talinn vera úrkomuvatn. Vökvinn í jarðhitakerfunum í Svartsengiog á Reykjanesi er saltur. Í Svartsengi er upprunavatnið 70% sjór og 30% regnvatn enhreinn sjór á Reykjanesi. Sumar blautgufuholurnr sem safnað var úr eru hávermishol-ur, þ.e. vermi holurennis er hærra en gufumettaðs vatns við þann hita sem ríkir í æð.Hávermi í renni borhola er ekki talið endurspegla hávermi vökva í djúpvökva. Taliðer að hávermisvökvi myndist við ákafa suðu í aðfærsluæð borhola. Niðurstöður kvars-og alkalí-hitamælanna fyrir hávermisholur benda til þess að vatn og gufa í aðfærsluæðaðskiljist, a.m.k. að hluta, og þannig myndist hávermisvökvi sem �æðir inn í borholur.Hluti vatnsins situr eftir í æðinni með því að hann loðir við y�rborð steinda fyrir áhrifhápípukra�a. Gufan �æðir hins vegar inn í borholurnar.

Fyrsta greinin í þessari doktorsritgerð fjallar um hvarfgjarnar lofttegundir, koltvíoxíð(CO2), brennisteinsvetni (H2S) og vetni (H2) og ástand efnajafnvægis þeirra við nokkurfylki ummyndunarsteinda, en einnig um argon (Ar) og köfnunarefni (N2) en síðarnefndalofttegundin er tiltölulega óhvarfgjörn. Í djúpvökva margra borhola með lágseltu svar-ar styrkur CO2 vel til efnajafnvægis við steindafylkið epídót-prenít-kalsít-kvars en tiljafnvægis við epídót-granat-kalsít-kvars í söltu svæðunum í Svartsengi og á Reykjanesi.Í sumum borholum í Krö�u er styrkur CO2 í vökva í aðfærsluæð hærri en svarar tiljafnvægis við nefnt steindafylki líklega vegna mikils streymis þessarar lofttegundar frákviku undir jarðhitaker�nu. Hins vegar er styrkur CO2 í djúpvökva á Nesjavöllum lægrien svarar til jafnvægis, sennilega vegna takmarkaðs �æðis þessa gass frá kviku. Líklegter talið að styrkur H2S og H2 í djúpvatni stýrist af efnajafnvægi við sama steindafylkið.Fyrir �estar holur er svarar styrkur í æð vel til jafnvægis við fylkið epídót-prenít-pýrít-pyrrótít. Hins vegar er styrkur H2 stundum of hár. Hinn hái styrkur H2 er ekki talinnráðast af fráviki frá efnajafnvægi helfur af tilvist efnajafnvægisgufu í djúpvökva. Tilvistslíkrar djúpgufu hefur mikinn áhrif á styrk H2 í djúpvökva. Hún hefur hins vegar lítiláhrif á H2S sem er miklu auðleystara í vatni en H2 og einnig lítil áhrif á CO2 sem er

xv

tiltölulega auðleyst í vatni. Köfnunarefni í djúpvökva er aðallega komið út uppruna-vatninu og Ar eingöngu, loftmettuðu grunnvatni og loftmettuðum sjó. Djúpvökvinnhefur y�rleitt tapað báðum þessum gösum að hluta miðað við upprunavatnið. N2/Arhlutföll í einstökum sýnum benda til þess N2 geti átt uppruna víðar en í loftmettuðuvatni, hugsanlega loftbólum í þessu vatni, í kvikugösum eða jafnvel rotnandi lífrænu efni.

Önnur greinin lýsir uppruna og hegðun arsens (As), bórs (B) og klóríðs (Cl) í jarð-hitavökva fyrrnefndra háhitasvæða en einnig vatni í laugum, hverum og heitavatnsholumaf fjórum öðrum háhitasvæðum á Íslandi og í vatni úr fjórum borholum á lághitasvæð-um á Suðurlandsundirlendi. Styrkur As, B og Cl í jarðhitavökva á Íslandi er lágur ísamanburði við slíkan vökva í jarðhitasvæðum erlendis. Einkennandi styrkur As, B ogCl í lágseltuvökva á háhitasvæðum er 50�150 ppb, 0,5�5 ppm og 50�150 ppm. StyrkurAs í ósöltu jarðhitavatni er sambærilegur við lágseltuvökva en miklu hærri fyrir B ogCl. Bór og Cl eru mjög hreyfanleg efni (eftir að þau hafa borist í vatn haldast þauí lausn) við öll hitastig (10�320 ◦C) í íslenskum jarðhitakerfum. Arsen er einnig mjöghreyfanlegt í uppha�egum djúpvökva jarðhitasvæða en suða á þessum vökva og blöndunhans við kalt vatn leiðir til útfellinga á As og þá sérstaklega blöndun. Hlutföllin Cl/B ogCl/As í vatni á tiltölulega gömlum háhitasvæðum (liggja í útjaðri gosbeltanna) eru þausömu og í basalti sem bendir til þess að þessi efni skolast úr berginu í sömu hlutföllumog þau eru í þessu bergi. Í hinu tiltölulega unga háhitasvæði (liggur á �ekamótum) íKrö�u er B og Cl í djúpvökva aðallega ættað frá kviku en aðeins tiltölulega lítill hlutier komin úr berginu sem vatnið hefur hvarfast við. Á Nesjavöllum er mestur hluti Cl ídjúpvökva hins vegar ættaður úr bergi en stór hluti B er kvikuættaður. Í söltu djúp-vatni er hreyfanleiki As lægri en B og Cl og tapast líklega úr lausn við upptöku þessí súlfíðsteindir. Ef allt Cl í djúpvatni í Svartsengi og á Reykjanesi er sjávarættað, enlíklegt er talið að svo sé, er ljóst að verulegur hluti B í þessu vatni hefur borist í þaðfrá kviku. Arsen leitar ekki að neinu ráði y�r í jarðgufu, jafnvel þótt gufuþrýstingur sé40 bör. Sömu sögu er ekki að segja um B. Jafnvægisdrei�ng næst fyrir B milli vatns oggufu á holutoppi þar sem sýnum var safnað.

Þriðja greininfjallar um gufupúðann í háhitasvæðinu í Svartsengi sem myndaðist y�r240 ◦C heitu vatnsker� vegna niðurdráttar í svæðinu í kjölfar vinnslu. Massataka vatnsog gufu úr svæðinu á tímabilinu 1976�2006 nam 243 megatonnum (Mt). Af honumkomu 217 Mt frá blautgufuholum (úr vatnsker�nu) en 25 Mt úr þurrgufuholum (úrgufupúðanum). Árið 2006 samsvaraði uppsöfnuð þrýstilækkun í svæðinu 30 börum.Suða í vatnsker�nu hefur leitt til lækkunar á gasstyrk í djúpvatninu sem aftur hefurorsakað hækkun á styrk �úors (F) en lækkun á styrk magnesíums (Mg) vegna hraðrarnálgunar á efnajafnvægi hins afggasaða vatns við Mg- og OH-steindir. Styrkur gassí gufupúðanum hefur farið lækkandi. Í uppha� var hann um 80,000 ppm fyrir CO2

en er nú um 30,000 ppm. Áætlað er að allt að 3% af djúpvatni efst í vatnsker�nuha� soðið til þessa til að mynda gufu fyrir gufupúðann. Holur hafa verið boraðar ígufupúðann til raforkuframleiðslu með þeim a�eiðingum að hækka útstreymi CO2 ogH2S út í andrúmsloftið á hverja framleidda raforkueiningu.

xvi

Acknowledgments

Many, many people deserve my immense gratitude, for helping me in some way bringingthis thesis to completion.

First of all, I will never be grateful enough to my supervisor Stefán Arnórsson foro�ering me this PhD position, and for his in�nite patience in trying to turn a geologistinto a geochemist. With him, thermodynamics always seems simple, and the natureof geothermal systems self-explanatory. His passion is very communicative, and hissupport invaluable, even after months of struggle on rebellious noble gas data or whenthe motivation was running low.

I would like to thank Ingvi Gunnarsson and Andri Stefánsson for all the valuablediscussions and for their help in the �eld and the laboratory, as well as for their ex-cellent company. Special thanks go to Jón Örn Bjarnason for his priceless advices andextremely accurate reviews, and for making the great pieces of software WATCH andSteamandwater available.

Combining a part-time job and the redaction of a PhD dissertation may be partic-ularly di�cult. However, my experience was extremely positive, and for this I wouldlike to express my deep gratitude to my colleagues at Iceland GeoSurvey, and in partic-ular Magnús Ólafsson and Ingibjörg Kaldal for giving me total freedom for managingmy work hours. Thráinn Fridriksson and Halldór Ármannsson, together with Magnús,spent a lot of their precious time helping me through the world of geochemical explo-ration and monitoring of geothermal systems in Iceland. All my colleagues have beenvery supportive, and I am very thankful for that.

I want to thank warmly all my fellow colleagues in Askja and friends in Iceland fortheir great company, Jesper, Alex, Jamie, Fredrik, Lillemor and all the others, thankyou so much for sharing so many adventures and good times.

Special thanks to my friend Caroline; without her friendship and support, I wouldprobably never have come to Iceland to start a new adventure.

I would never have achieved such project without permanent support from my familyduring so many years, no matter how far I could be. Pap, mam et toute la famille, merci.

No words can express my gratitude to my better half Tanja, for being the light inmy life, for enduring my stress in procrastination, and supporting me all the way tocompletion of this thesis.

I would like to thank the National Power Company Landsvirkjun, Reykjavik Energy,and the Sudurnes Regional Heating Company for their �nancial support.

xvii

Chapter 1

Introduction

The main focus of this study is the behaviour of various volatile species in high-temperature volcanic geothermal �uids in Iceland. This doctoral thesis constitutesof the three following papers, which will be submitted for publication in internationaljournals upon completion of the PhD thesis. They are included here as Chapters:

1. Giroud, N. and Arnórsson, S., 2008. Gas chemistry in �ve Icelandic high-temperature geothermal systems. Journal of Volcanology and Geothermal Re-search. Chapter 2.

2. Giroud, N. and Arnórsson, S., 2008. Processes in�uencing As, B and Cl con-centrations in �uids of volcanic geothermal systems in Iceland. Geochimica etCosmochimica Acta. Chapter 3.

3. Giroud, N., Arnórsson, S. and Bjarnason, J. Ö., 2008. Formation of a vapor capsubsequent to production in the Svartsengi geothermal �eld, Iceland. Geother-mics. Chapter 4.

In addition to these publications, the author participated in related research projectsduring his studies at the University of Iceland that are not part of this thesis:

• Arnórsson, S., Bjarnason, J. Ö., Giroud, N., Gunnarsson, I., Stefánsson, A.,2006. Sampling and analysis of geothermal �uids. Geo�uids 6 (3), 203�216

• Stefánsson, A., Gunnarsson, I., Giroud, N., 2007. New methods for the directdetermination of dissolved inorganic, organic and total carbon in natural watersby Reagent-FreeTMIon Chromatography and inductively coupled plasma atomicemission spectrometry. Analytica Chimica Acta 582 (1), 69�74

1

1.1. OBJECTIVES

Parts of the work were also presented in various meetings, including:

• Giroud, N., Arnórsson, S., 2005. Estimation of long-term CO2 and H2S releaseduring operation of geothermal power plants. World Geothermal Congress 2005paper 0233

• Giroud, N. and Arnórsson, S., 2004. Formation of a steam-cap in the geothermalreservoir of Svartsengi, Reykjanes peninsula, Iceland. Poster at the GeoscienceSociety of Iceland Meeting

During the last two years, while writing the present thesis, the author worked asa geochemist at Iceland GeoSurvey, on di�erent projects related to geochemistry ofhigh-temperature (> 200 ◦C) and low-temperature geothermal systems.

1.1 Objectives

Study of the geochemical characteristics of geothermal systems is of great importance,both to improve the knowledge of hydrothermal and volcanic systems, and to providetools for an optimal use of geothermal resources. The unique geological setting of Icelandon a mid-oceanic ridge provides an outstanding opportunity to study geothermal ac-tivity in relation with volcanic systems. Electrical power generation from geothermalenergy started in Iceland almost 40 years ago, in 1969. Many of the high-temperature�elds have been drilled for this purpose and six are being exploited today, whereas sev-eral others are being developed.

The main objectives and the principal results of this study are the following:

• Quantify the processes which a�ect the concentrations of the most abundantgases, i.e. CO2, H2S, H2, N2 and Ar. The concentrations of reactive gases inthe deep aquifer �uid in geothermal systems is a�ected both by geochemical andphysical processes, i.e. chemical reactions within the �uid, interactions of the�uid with the host rock minerals, the presence or absence of equilibrium vapor,�uxes from a magmatic source, boiling/condensing and phase separation. Theconcentrations of three of the reactive gases (CO2, H2S, and H2) in the �uid arecompared with predicted concentrations calculated by equilibrium with selectedmineral assemblages, using thermodynamic data from the literature.The source of Ar to the �uid in the high-temperature geothermal �elds is air-saturated groundwater and seawater, and trapped air bubbles. These are alsothe principal source for N2 although magmatic gases and decaying organic mattermay contribute, at least in some �elds. As a consequence, their concentrationsare largely or solely a�ected by physical processes and the respective solubilitiesof the gases. Results on this subject are presented in Chapter 2.

• Assess the processes that govern As, B and Cl concentrations in �uids of high-temperature geothermal systems. The results indicate that B, as Cl, acts asessentially conservative component in the high-temperature geothermal systemsin Iceland, i.e. once in solution it stays there. In volcanically young �elds, such asKra�a, NE-Iceland, a large part of the aqueous B and Cl is derived from volatilesescaping from the underlying magma heat source. In mature, high-temperature

2

CHAPTER 1.

Figure 1.1: Hot-spring and fumarole at Hveravellir

�elds, on the other hand, the source of these elements to the water is the rock withwhich the geothermal �uid has interacted. Like B, As is highly mobile in the high-temperature geothermal systems. However, it may be removed from solution intosul�de minerals, in particular when extensive boiling occurs and in particularwhen geothermal �uid mixes with cold groundwater and ferric hydroxides areprecipitated. The details of this study are included in Chapter 3.

• Evaluate from geochemical data how extensively the 240 ◦C liquid water reservoirat Svartsengi, southwest Iceland, has boiled to produce the above lying vapor cap.Data on samples collected on a regular basis by Iceland GeoSurvey, together withsamples collected speci�cally for this study indicate progressive boiling withinthe liquid reservoir. At present as much as 3% w/w of the liquid has boiled toyield vapor to the above lying vapor cap. Results of this study can be found inChapter 4.

1.2 Methods

A very important part of research on geothermal �uids, which is based on data onthe composition of well discharges, is calculation of deep �uid compositions, individualspecies activities in this �uid, assessment of speci�c mineral-solution equilibria as wellas consideration of the origin of the various component present in the geothermal �uid.

For the present study, samples were collected from wet-steam and dry-steam wells in�ve producing high-temperature geothermal �elds in Iceland and from four wet-steamwells in the Southern Lowlands area. Both liquid and vapor phases were sampled fromwet-steam wells, i.e. wells producing a mixture of liquid water and vapor. Additionally,vapor was collected from four dry-steam wells at Svartsengi (see Chapter 4). Samplesof both phases were subsequently analyzed for major components at the University

3

1.3. SELECTED GEOTHERMAL FIELDS

of Iceland as described by Arnórsson et al. (2006). Trace elements were analyzed byAnalytica (ALS Laboratory Group), Luleå, Sweden. A more detailed description ofsampling and analytical methods can be found in each chapter, as well as in Arnórssonet al. (2006). Chemical data on samples from boiling and warm hot springs and hot-water wells in four other high-temperature �elds collected in 2001 and 2002 by theInstitute of Earth Sciences, University of Iceland, were also used.

Analyzed liquid and vapor compositions were used to calculate the temperature,composition, and aqueous species distribution of the deep aquifer �uid for each well.This calculation is straightforward for liquid enthalpy wells, i.e. wells whose dischargeenthalpy can be taken to be the same as that of the aquifer �uid. When this is thecase, the total well discharge composition equals that of the initial aquifer �uid. Themajority of the wells included in the present study, however, have �excess� dischargeenthalpy, due either to segregation in the aquifer of the vapor and liquid phases �owinginto wells, or to enhanced boiling of the liquid by heat-transfer from the aquifer rock.In the latter case, the total discharging �uid is representative of the deep aquifer �uid,but this is not the case if the cause of the �excess� enthalpy is phase segregation. Athorough description of the calculation method is given in each chapter.

The data interpretation involves calculation of solubility constants for minerals, andactivity products from calculated species activities. Stability of various mineral phasesin contact with the solution can thus be estimated.

1.3 Selected geothermal �elds

This study focuses on �ve high-temperature areas that have been drilled for electricalpower generation and are being exploited today. These are Kra�a and Námafjall, north-east Iceland, and Nesjavellir, Svartsengi and Reykjanes, southwest Iceland (Fig. 1.2).As mentioned before, four wells in the Southern Lowlands were also sampled. Ad-ditionally, data on samples from warm and boiling hot springs, and wells, from fourhigh-temperature areas (Hveravellir, Geysir, Hveragerdi, and Landmannalaugar) werestudied with respect of As, B and Cl.

All the areas except the Southern Lowlands are located within the active volcanicbelts, the axes of which represent the plate boundary between the Eurasian and North-American lithospheric plates. The Southern Lowlands are located on the Hreppar mi-croplate, between the eastern and the western volcanic belts. The geology of the areasshows an interesting variety of settings; some �elds are located within the boundaryof central volcanoes, directly above their magma heat source, whereas others are lo-cated just outside such volcanoes, astride the �ssure swarms that intersect the centralvolcanoes. Additionally, some �elds, like those on the Reykjanes peninsula, southwestIceland, are not linked to any central volcano but have a sheeted dyke complex as heatsource. The �elds of Hveravellir and Geysir are located by the margins of the activevolcanic belts and have likely drifted away from their heat source. These systems areconsidered to be mature, and may eventually evolve into low-temperature systems. Therock type hosting the geothermal systems is mostly basaltic volcanics. However, silicicrocks occur in some areas. A more detailed description of the geology is presented ineach chapter.

4

CHAPTER 1.

Figure 1.2: Map of Iceland showing the location of the studied high-temperatureareas.

5

1.3. SELECTED GEOTHERMAL FIELDS

Figure 1.3: Well testing at Kra�a

6

Chapter 2

Gas chemistry in �ve Icelandichigh-temperature geothermal systems

Giroud, N. and Arnórsson S.

Abstract

Many processes control reactive and unreactive gas concentrations in aquifer �uids ofhigh-temperature geothermal systems. This study focuses on CO2, H2S, H2, N2 andAr concentrations in aquifer �uids of �ve liquid dominated volcanic geothermal �elds.These are Kra�a, Námafjall and Nesjavellir, which are of dilute �uid type, and Svart-sengi and Reykjanes where �uids are saline. Dissolved solids and gas analyses of waterand vapor samples were used to calculate aquifer �uid compositions. The calculationsaccount for the excess enthalpy of some wells, where the discharge enthalpy is higherthan liquid enthalpy at the aquifer temperature due to phase segregation of the �owingliquid and vapor. This model yields consistent results for quartz and Na/K geother-mometers.In the dilute �uids, H2S concentrations correspond to equilibrium with a mineral assem-blage containing epidote, prehnite, pyrite and pyrrhotite. In the saline �uids, equilib-rium generally seems to be closely approached with a grossular, epidote and wollastonitemineral assemblage. In some of the dilute �uids, CO2 concentrations are close to equi-librium with the clinozoisite, prehnite, calcite and quartz assemblage. However, the�uids at Nesjavellir are generally depleted in CO2 relative to equilibrium, most likelybecause of insu�cient supply from the magma heat source. Some wells at Kra�a showexcess CO2 due to recent magmatic activity. In the saline �uids, CO2 appears to becontrolled by equilibrium with the clinozoisite, grossular, calcite and quartz assemblage.The same mineral bu�er is expected to control aquifer liquid H2 and H2S concentra-tions. Indeed many samples of the dilute �uids correspond well with equilibrium withthe epidote, prehnite, pyrite and pyrrhotite assemblage. Other samples show higher H2

7

2.1. INTRODUCTION

concentrations. This is considered to be due to the presence of equilibrium steam in theaquifer �uid, which raises much the sparingly water soluble H2 concentrations in theaquifer �uid but not so much the concentrations of the more soluble CO2 and H2S. Inthe saline �uids, H2 concentrations are somewhat lower for Reykjanes than predicted byequilibrium with the epidote, wollastonite, grossular and pyrrhotite assemblage. Two ofthe Svartsengi saline waters are close to equilibrium with this bu�er, whereas four havelower H2, most likely due to degassing of the aquifer liquid by boiling and consequentformation of the vapor cap above the liquid reservoir. Fluids are mostly depleted in N2

and Ar relative to air-saturated water. N2/Ar ratios show that N2 has another sourcethan air-saturated water, potentially entrapped air bubbles, magmatic gas or decayingorganic matter.

2.1 Introduction

Many geochemical processes a�ect the concentrations and ratios of reactive and un-reactive gases in �uids of volcanic geothermal systems. They include supply of gasesfrom the magma heat source, from the atmosphere and from the rock with which thegeothermal �uid interacts. Within a particular geothermal system, variations in gasconcentrations of the geothermal �uid are also a�ected by gas-gas and mineral-gas re-actions and boiling and phase separation.

Dissolution experiments involving basalt and pure water at temperatures > 250 ◦Cdo not produce actual geothermal water compositions. The experimental solution pHis too high. The geothermal �uid parameters can, however, be duplicated by additionof CO2 (Kacandes and Grandsta�, 1989). Data from volcanic geothermal systems inIceland hosted by basalt, show that sulfur and carbon contents of the altered aquiferrock is much higher in the up�ow zone where extensive depressurization boiling occursthan those of fresh basalt. These elements are hosted by sul�de minerals and calcite(Gunnlaugsson, 1977; Arnórsson, 1995). These minerals precipitated as a consequenceof changes in the liquid water composition by its boiling. Modal calcite as high as20% have been reported (Björnsson et al., 1972) which correspond to C concentrationsin the rock in excess of 2%. Sulfur concentrations as high as 5% have been reported(Gunnlaugsson, 1977). In fresh undegassed basalt S and C concentrations are around800 ppm (Moore and Fabbi, 1971) and 1400 ppm as CO2 (Flower et al., 1982), respec-tively. The value of 1400 ppm is the average of 78 analyses of dyke samples from easternIceland. The median value for a total of 250 samples of lavas and dykes is 500 ppmCO2 (see Sveinbjörnsdóttir et al., 1995). This lower value probably re�ects variabledegassing of the lavas with respect to CO2.

Several studies have demonstrated that the aquifer water concentrations of the mostabundant reactive gases (CO2, H2S and H2) in volcanic geothermal systems are of-ten controlled by a close approach to equilibrium with hydrothermal mineral bu�ers(Giggenbach, 1981; Arnórsson, 1985; Gudmundsson and Arnórsson, 2002; Stefánssonand Arnórsson, 2002; Fridriksson et al., 2006; Karingithi et al., 2008). However, inother systems, such as at Kra�a, Iceland and in the western part of Olkaria, Kenya,the �ux of CO2 from the magma heat source is considered to govern its concentrationin aquifer �uids (Ármannsson et al., 1982; Ármannsson et al., 1989; Gudmundsson and

8

CHAPTER 2.

Arnórsson, 2002; Karingithi et al., 2008).Carbon dioxide emissions from active volcanic geothermal systems are largely dif-

fuse and cover large areas. The more visually con�ned emissions from fumarole or hotsprings discharges are only a portion of the total emission (Fridriksson et al., 2006;Chiodini et al., 2005; Aiuppa et al., 2004). This is apparently not the case for the otherabundant reactive gases (H2S and H2) indicating that they are removed from the risingdispersed �uid, either by reactions with other �uid species or by reactions with the hostrock minerals.

This study focuses on quantifying the geochemical and physical processes whichgovern the concentrations of the most abundant gases (CO2, H2S, H2, CH4, N2 andAr) in the aquifer �uid of wet-steam wells drilled into �ve volcanic geothermal �eldsin Iceland. All these systems are hosted by basaltic rocks. In two of the systems theparent �uid is essentially seawater but meteoric water in the other three systems.

The methodology involves calculation of mineral equilibrium constants with ther-modynamic data from the literature, and comparison with the reaction quotient of theminerals, calculated from the chemical analyzes of water and steam samples.

Essentially data interpretation involves two steps. One is modelling of initial aquifer�uid compositions from data on liquid and vapor samples collected at the wellhead. Theother step, which includes the reactive gases, involves calculation of activity productsfor speci�c mineral-gas and gas-gas reactions and derivation of the equilibrium constantfor these reactions with thermodynamic data from literature.

2.2 Geological background

Volcanic geothermal systems in Iceland are located within the active volcanic belts,most of them in the center of these belts but some close the their margins (Fig. 2.1).The volcanic belts represent the diverging boundary between the North-American andEurasian plates. The marginal volcanic geothermal systems are considered to be ma-ture and have drifted away from their magma heat source within the active volcanicbelts. Relatively young geothermal systems are characteristically located on the plateboundary where it is intersected by a �ssure swarm that runs perpendicular to theplate movements. Except for geothermal systems on the Reykjanes Peninsula, thesesystems typically coincide with central volcanic complexes, which represent points ofmaximum volcanic activity within the volcanic belts. Silicic volcanics are typically as-sociated with these complexes considered to have formed by partial melting of basalt atthe base of the crust. The �ve volcanic geothermal systems in Iceland (Kra�a, Náma-fjall, Hengill, Svartsengi and Reykjanes), which are the subject of the present study,have been extensively drilled. Nesjavellir represents one of the well�elds within theHengill system. In these systems, the drilled reservoir rocks either contain no silicicrocks or they are very minor (Franzson, 1998).

2.2.1 Kra�a and Námafjall

The Kra�a �eld lies within the caldera of the Kra�a central volcano which has eruptedboth basaltic and silicic magma (Jónasson, 1994). The aquifer rock is, however, largely

9

2.2. GEOLOGICAL BACKGROUND

Figure 2.1: Map showing the active volcanic belts and high-temperature geother-mal areas.

basaltic, sub-aerially erupted lavas, sub-glacially erupted hyaloclastites as well as smallintrusive bodies of basalt, dolerite and gabbro. Intrusions of granophyre also occur.

The volcanics dominate the succession above about 1200 m depth but at deeperlevels intrusions become more abundant and they dominate below about 2000 m. Avolcanic episode occurred in the area between 1975 and 1984 and fresh magma wasintruded into the roots of the Kra�a geothermal system at 4�7 km depth (Einarsson,1978). On nine occasions partial emptying of the magma chamber did not reach thesurface but �owed laterally north or south along the tensional fractures that intersect theKra�a central volcano. This subsurface discharge was witnessed by earthquake activityof de�ation of the Kra�a caldera (Björnsson et al., 1979). Degassing of the fresh magmawas accompanied by an increase in gas �uxes from the geothermal system, particularlywith respect to CO2 (Ármannsson et al., 1982; Ármannsson et al., 1989). About twentyyears after cessation of the 1975�84 volcanic episode, many well discharges were stilla�ected by this gas pulse (Gudmundsson and Arnórsson, 2002).

The drilled geothermal reservoir at Kra�a has been divided into two main depthzones, an upper sub-boiling zone at 190�200 ◦C and a lower two-phase zone with tem-peratures of up to 350 ◦C (Ármannsson et al., 1989; Arnórsson, 1995). Geothermometer(quartz and Na-K) temperatures indicate that most aquifers producing into wells arebelow 300 ◦C (Gudmundsson and Arnórsson, 2002).

The Námafjall �eld, which lies 8 km south of Kra�a, may be regarded as a parasitesystem to the Kra�a volcano. This �eld was also a�ected by the 1975-84 volcanicepisode. During periods of fracturing, magma sometimes �owed to the south along thefractures from the magma chambers under Kra�a. This magma, however, did not reachthe surface except on one occasion when a geothermal well erupted 3 m3 of magma(Larsen et al., 1978). The 1975�84 volcanic episode suggests that the heat source to theNámafjall geothermal system is represented by the dykes formed by magma intrusion

10

CHAPTER 2.

into tensional �ssures from the magma body in the roots of the Kra�a system. Theaquifer rock at Námafjall is the same as at Kra�a except that silicic rocks are absent.Intrusive formations dominate below about 1500 m depth.

In wells drilled at Námafjall prior to the 1975�84 volcanic episode (well nos. 1�9)temperatures follow the boiling point curve with depth. In wells drilled in another partof the �eld and after the 1975�84 volcanic episode, the temperature at the top of thereservoir is sub-boiling. These low temperatures are considered to be the consequenceof cold shallow groundwater incursion along fractures that were activated during the1975�84 volcanic episode. Maximum recorded temperatures in drillholes at Námafjall is320 ◦C (Gudmundsson and Arnórsson, 2002). As at Kra�a, geothermometry indicatesthat aquifers producing into wells at Námafjall have considerably lower temperaturesthan this maximum (Gudmundsson and Arnórsson, 2002).

Fumarolic activity at both Kra�a and Námafjall increased during the early part ofthe 1975-84 volcanic episode. However, at Námafjall no changes in the gas content ofwell discharge and fumarole steam was observed (Arnórsson and Gunnlaugsson, 1985).The increase in the fumarolic activity is likely caused by increased boiling by heat �ux toalready two-phase aquifer �uid. At Kra�a, the gas �ux from the fresh magma may alsohave contributed to increased boiling. The apparent lack of increase in the gas contentof the �uid in the Námafjall system during the 1975-84 volcanic episode indicates thatthe magma intruded from the chambers below Kra�a had already been largely degassed.

2.2.2 Nesjavellir

The Nesjavellir �eld lies in a tectonically active graben to the northeast of the Hengillcentral volcano. The drilled formations at depth are largely basalt lavas and basaltichyaloclastites as wells as small intrusions of basalt and dolerite. Granophyre is sporadic(Franzson, 1998). Intrusions are dominant below some 1500 m depth. A volcanic�ssure, which lies immediately to the north of the well�eld erupted about 2000 yearsago (Saemundsson, 1963, 1992; Bull et al., 2005). The permeability is mostly fracturecontrolled (Bödvarsson et al., 1990).

The drilled geothermal system at Nesjavellir is two-phase, at least to the depthpenetrated by the deepest wells (∼2200 m). The maximum recorded temperature is> 380 ◦C. The well in which this temperature was recorded (the maximum for theinstrument used) was drilled close to the �ssure which erupted ∼2000 years ago.

2.2.3 Svartsengi and Reykjanes

The Svartsengi and the Reykjanes geothermal systems are both located on the Reykja-nes peninsula in southwestern Iceland which represents the landward continuation ofthe Reykjanes Ridge. The peninsula consists mostly of Holocene lavas protruded byhyaloclastite hills formed by sub-glacial eruptions during the last glaciation (Jakobssonet al., 1978; Clifton and Kattenhorn, 2006). They lie astride two of the �ve active �ssureswarms on the Reykjanes peninsula. In view of their geological setting, the heat sourceto these systems are considered to be sheeted dyke complexes and the permeabilityis fracture controlled. The two exploited geothermal systems are hosted by basaltic

11

2.3. ALTERATION MINERALOGY

rocks, subglacial hyaloclastites, breccias, and pillow lavas as well as tu�aceous sedi-ments. Abundance of intrusions increases with depth. They dominate the successionat the greatest depth reached by the wells.

The temperature in Svartsengi is very homogeneous. Between 500 and 2000 m depthit is ∼240 ◦C, re�ecting good vertical permeability in the system. At Reykjanes, thehighest temperature recorded is ∼320 ◦C at 2000 m depth. The aquifer is two-phaseabove ∼1000 m but subboiling at greater depths.

2.3 Alteration mineralogy

All systems considered in this study are located in basaltic environments and have sim-ilar mineralogy. The di�erent salinity of the �uid is not su�cient to change drasticallythe alteration mineralogy from one system to another.

Alteration mineralogy in the high temperature geothermal systems in Iceland hasbeen summarized by Arnórsson (1995). All �elds show a zonation with depth re�ectingthe temperature stability range of the minerals within the system.

Calcite and pyrite are stable over the whole range of temperature. Chalcedony isstable up to ∼180 ◦C and above that temperature quartz is formed. Prehnite appearsabove 200 ◦C and epidote at slightly higher temperature (Hreggvidsdóttir, 1987; Lonkeret al., 1993). Albite is common, especially at temperatures above 150 ◦C, whereas K-feldspar (adularia) is scarce, due to the low K content of the host rocks. Hydrothermalsheet silicates are represented by smectite, that is interstrati�ed with chlorite above200 ◦C and above 230�240 ◦C, discrete chlorite is formed. Garnet, represented by thegrossular-andradite solid solution is widely present in Reykjanes and Svartsengi (Lonkeret al., 1993). Magnetite is mostly present as primary mineral but is generally unstablein high-temperature geothermal systems. The present study focuses on ten alterationminerals that were considered as potential gas bu�ers. In alphabetical order these are,calcite, pyrite, pyrrhotite, magnetite and wollastonite, and the solid-solutions epidote-clinozoisite and grossular-andradite. Quartz is assumed to be at equilibrium in all thegeothermal systems considered in this study.

2.4 Fluid compositions

The geothermal �uid at Reykjanes represents seawater which has reacted with basalt(Björnsson et al., 1972; Mottl and Holland, 1978; Arnórsson, 1995). The Cl concentra-tions of the unboiled aquifer water is thus close to that of seawater. The deuteriumcontent of the aquifer water is, however, signi�cantly negative and thus lower than thatof seawater (Sveinbjörnsdóttir, 1983; Sveinbjörnsdóttir et al., 1986). This has beenexplained by mixing of seawater with fresh groundwater and dissolution of Cl fromthe basalt to account for the observed Cl concentration in the geothermal water of theaquifer (Árnason, 1976; Ólafsson and Riley, 1978). In view of the low Cl content ofbasalt (Sigvaldason and Óskarsson, 1976; Floyd and Fuge, 1982) this explanation ap-pears unlikely. However, recent studies by Pope et al. (2008) conclude that the lowδ2H values of the geothermal seawater at Reykjanes is a consequence of equilibration of

12

CHAPTER 2.

the geothermal �uids with hydrous alteration minerals. Alteration mineralogy seems tohave formed from a glacial dominated �uid source with a signi�cantly lower δ2H signa-ture than that of seawater. As a result, the present geothermal �uid source at Reykjanesis seawater that has reacted and experienced isotope exchange with alteration minerals.

The Svartsengi geothermal system has a mixed meteoric-seawater �uid origin withCl content corresponding to about 2/3 that of seawater (Arnórsson, 1978; Ragnarsdóttiret al., 1984; Lonker et al., 1993) and the deuterium content of this water is consistentwith such an origin. Reaction of the seawater at Reykjanes and the seawater/meteoricwater mixture at Svartsengi has lead to almost quantitative removal of Mg and SO4 fromsolution, a slight decrease in Na but substantial increases in the concentrations of Caand K. Silica is increased strongly but F decreased much (Table 2.1). Experiments in-volving reaction of seawater with basalt have reproduced the �uids at Reykjanes (Mottland Holland, 1978).

13

Table2.1:Majorelem

ents

analysesofthesampledwaters.Concentrationsin

mg/kg.

Sample#Sampling

Well

Discharge

Sampling

meas.

pHpHcorr.

CO

2H

2S

FCl

SO

4SiO

2Na

KCa

Mg

Fe

Al

Sr

Bdate

enthalpy

pressure

/at◦C

forsilicac

kJ/kg

bar-g

04-3007

19.10.2004N-12

1759

20.0

7.86/21.7

21.7

90.9

0.5

22

107.8

3.7

621

133

24.4

0.3

98

0.0

042

0.0

044

0.9

66

0.0

03

3.5

41

04-3008

19.10.2004N-9

1060a

14.0

8.90/18.6

8.6

738.6

62.1

0.5

85

35.6

43.9

457

151

19.4

2.5

81

0.0

016

0.0

012

0.9

23

0.0

25

0.7

24

04-3010

20.10.2004K-13

1941

15.5

8.43/18.1

8.3

467.9

72.6

0.9

33

31.8

303.9

432

244

26.6

4.4

73

0.0

017

0.0

040

1.1

66

0.0

21

0.9

00

04-3015

20.10.2004K-15

1499

14.5

9.26/18.0

8.6

816.0

93.2

1.2

71

36.4

247.9

730

222

39.1

2.0

59

0.0

005

0.0

012

1.8

78

0.0

09

1.1

28

04-3022

21.10.2004K-16

2451

11.0

7.36/19.0

159.3

61.9

2.1

14

133.2

12.4

662

194

32.8

0.8

83

0.0

020

0.0

038

1.0

26

0.0

05

1.7

35

04-3021

21.10.2004K-17

2547

28.5

7.88/18.9

69.4

108.5

1.8

47

14.9

4.7

653

118

19.8

0.2

02

0.0

003

0.0

017

1.9

29

0.0

01

1.3

59

04-3019

21.10.2004K-20

2543

11.0

7.80/18.1

158.0

75.2

1.2

72

227.0

5.8

859

259

52.4

1.9

66

0.0

016

0.0

022

1.0

72

0.0

08

2.1

75

04-3009

20.10.2004K-21

1167a

15.0

8.80/17.8

8.5

849.7

62.1

1.0

17

72.2

67.0

502

150

19.8

1.1

88

0.0

008

0.0

035

1.5

11

0.0

09

0.6

03

04-3023

21.10.2004K-24

887a

2.9

9.59/16.9

9.2

920.3

31.7

0.8

25

45.6

241.3

381

221

17.5

3.5

85

0.0

007

0.0

027

0.8

55

0.0

27

0.5

50

04-3011

20.10.2004K-27

1199a

12.0

9.51/19.7

9.0

338.3

46.1

1.0

47

49.5

262.4

558

237

34.5

2.6

54

0.0

041

0.0

137

1.5

50

0.0

13

0.6

73

04-3014

20.10.2004K-32

1910

14.5

8.89/19.1

8.4

017.9

89.5

1.3

80

147.8

167.5

867

235

53.9

1.5

58

0.0

009

0.0

022

1.6

74

0.0

08

1.6

19

04-3012

20.10.2004K-34

2636

17.5

7.24/19.4

90.6

88.9

1.3

41

99.6

44.5

554

162

30.7

1.1

81

0.0

037

0.0

137

1.2

36

0.0

06

0.6

93

04-3040

10.12.2004NJ-5

1800

15.5

8.70/22.6

8.4

158.5

59.6

0.9

36

117.0

8.1

713

164

31.4

0.3

34

0.0

011

0.0

014

1.7

49

0.0

02

1.5

88

04-3042

10.12.2004NJ-6

2136

15.8

8.45/22.8

8.2

712.9

46.3

0.7

23

164.8

6.0

758

156

32.2

0.2

46

0.0

004

0.0

010

1.6

05

0.0

02

2.4

31

04-3038

10.12.2004NJ-7

1182a

15.5

9.11/22.6

8.6

656.0

69.6

1.2

32

95.4

14.1

671

171

30.0

0.3

97

0.0

018

0.0

007

1.7

45

0.0

03

1.4

00

04-3043

10.12.2004NJ-9

1700

15.3

8.25/22.6

8.1

836.5

47.1

0.9

28

172.3

8.8

753

167

33.5

0.3

67

0.0

005

0.0

018

1.4

25

0.0

04

2.2

09

04-3039

10.12.2004NJ-10

1133a

15.0

9.46/22.7

8.8

025.7

50.2

1.5

82

122.3

12.7

739

171

32.3

0.3

22

0.0

009

0.0

020

2.0

52

0.0

02

1.5

23

04-3048

15.12.2004NJ-11

1948

7.8

9.19/21.8

8.5

915.1

111.7

1.1

60

78.1

20.8

796

150

28.6

0.3

08

0.0

023

0.0

010

2.3

23

0.0

02

2.0

42

04-3001

09.09.2004NJ-13

1937

10.5

8.95/21.7

8.4

22.8

72.1

0.9

54

151.4

3.2

881

150

33.7

0.1

36

0.0

017

0.0

164

1.9

06

0.0

01

2.2

41

04-3049

15.12.2004NJ-14

1195a

16.1

9.22/21.9

8.6

718.1

28.8

1.1

36

182.7

10.7

721

171

31.2

0.2

90

0.0

009

0.0

009

1.8

18

0.0

02

1.8

19

04-3037

10.12.2004NJ-16

1252a

15.5

8.92/22.3

8.5

237.5

86.1

1.3

43

65.0

25.8

713

157

28.4

0.4

24

0.0

071

0.0

049

2.0

67

0.0

02

1.8

90

04-3002

09.09.2004NJ-19

1711

6.0

7.13/21.7

25.7

85.3

1.1

05

147.9

1.9

861

147

32.9

0.0

87

0.0

007

0.0

093

2.0

93

0.0

01

3.3

24

04-3050

15.12.2004NJ-20

1400

16.2

8.93/22.0

8.5

432.9

49.9

0.9

44

169.1

12.8

693

172

32.4

0.4

45

0.0

007

0.0

015

1.7

90

0.0

04

1.8

28

04-3046

15.12.2004NJ-21

2400

16.0

8.10/21.8

8.1

151.2

83.8

1.1

57

110.5

1.9

786

142

31.4

0.2

98

0.0

053

0.0

011

1.6

42

0.0

02

1.8

91

04-3045

15.12.2004NJ-22

1612

15.6

8.55/21.7

8.2

814.3

56.8

1.0

47

164.1

4.3

859

153

33.4

0.2

10

0.0

080

0.0

031

1.9

91

0.0

01

2.0

08

04-3041

10.12.2004NJ-23

2662

15.5

8.54/22.6

8.2

89.2

59.6

1.3

61

124.8

5.5

829

144

30.5

0.4

16

0.0

019

0.0

026

1.8

86

0.0

03

7.6

78

04-3005

10.09.2004SV-7

1028a,b

15.5

6.24/20.2

24.8

2.2

0.1

40

13831

26.0

468

7263

1050

1167

0.4

279

0.0

397

0.0

66

8.0

03

7.0

72

04-3003

10.09.2004SV-8

1028a,b

14.0

6.77/21.7

16.9

1.2

0.1

62

13493

28.2

488

7043

1011

1141

0.6

780

0.0

110

0.0

70

7.8

43

7.1

34

04-3032

01.12.2004SV-9

1028a,b

14.8

7.19/23.3

6.9

1.2

0.1

61

13230

28.2

492

6995

1004

1125

0.3

068

0.1

568

0.1

19

7.6

32

6.6

33

04-3029

01.12.2004SV-11

1028a,b

19.0

5.72/23.5

46.6

3.8

0.1

47

12916

29.1

460

6825

982

1102

0.5

668

0.1

331

0.1

63

7.5

30

6.7

67

04-3004

10.09.2004SV-18

1028a,b

14.0

7.24/20.1

12.3

1.3

0.1

58

14255

27.6

471

7522

1114

1129

0.4

712

0.0

090

0.0

63

7.7

35

8.1

28

04-3006

10.09.2004SV-19

1028a,b

14.5

7.58/20.3

8.6

0.8

0.1

47

14235

28.5

463

7490

1093

1170

0.4

063

0.0

037

0.0

65

7.9

99

7.6

50

05-3004

14.01.2005R-12

1316a

42.0

4.88/21.9

103.9

11.5

0.1

29

19288

15.8

679

10166

1434

1680

0.7

960

0.4

315

0.0

23

9.1

04

6.7

86

05-3003

14.01.2005R-15

1256a

25.0

5.16/21.7

47.4

3.4

0.1

38

19251

24.3

672

10089

1447

1637

1.0

748

0.2

930

0.0

45

9.1

89

6.7

18

aLiquid

enthalpywells

bSelected,correspondsto

enthalpyofsteam-saturatedliquid

at

238◦C

cpHvaluesabove8.0

were

correctedforsilicapolymerization.Ifpresent,thecorrectedvaluewasusedforthecalculations.

CHAPTER 2.

At Kra�a, Námafjall and Nesjavellir the source water to the geothermal �uid islocal or near local meteoric water according to δ2H data (Darling and Ármannsson,1989; Arnórsson, 1995). The total dissolved solids content of the aquifer water in these�elds is very low (1000�2000 ppm) compared to most other geothermal systems inthe world (e.g. Ellis and Mahon, 1977). The low salt content is considered to be theconsequence of the low Cl content of the host basalt with which the water has reacted(Arnórsson, 1995). The most important anions are Cl− and SO−2

4 , the most abundantcation is Na+ and silica as SiO2 is typically the most abundant dissolved solid (Table2.1).

The gas content of vapor samples is quite variable. N2/Ar ratios are most oftenbetween that of air-saturated water and that of air. Carbon dioxide is generally the mostabundant gas (18�600 mmol/kg of condensate), followed by H2 (<0.5�110 mmol/kg) orH2S (<2�51 mmol/kg) (Table 2.2). Methane concentrations are quite low. Systematicdi�erences are observed in the aquifer �uid composition of the study areas. Thus, thesaline waters are low in both H2S and H2 compared to the dilute waters. The magmaticactivity at Kra�a in the late 1970's and the early 1980's had an important impact onthe CO2 content of well discharges at Kra�a (Ármannsson et al., 1989). This CO2

anomaly is still quite strong in some of the Kra�a wells (Gudmundsson and Arnórsson,2002, 2005). In the parasitic systems of Nesjavellir and Námafjall, aquifer �uid CO2

concentrations are relatively low, whereas those of H2S and H2 are relatively high. On amolar basis, the gas content of dilute aquifer �uids is typically much higher than thoseof the dissolved solids. The pH of water samples, as measured at room temperature istypically below 7 for the saline water but above 8 for samples of dilute water. The pHof the unboiled and undegassed water in the aquifer is lower.

2.5 Well discharge enthalpy

The enthalpy of the well �uids sampled for the present study ranges between that ofsaturated vapor and liquid enthalpy, i.e. the enthalpy of the discharged �uid equals theenthalpy of vapor saturated liquid at the aquifer temperature. Many of the studiedwells have �excess� enthalpy, i.e. the enthalpy of the discharged �uid is higher thanthat of liquid enthalpy (Table 2.1). All the geothermal �elds considered for the presentstudy are liquid-dominated, as de�ned by White et al. (1971), meaning that the aquifer�uid is liquid only or liquid with a small vapor fraction, even in terms of volume. Theenthalpy of the �uid �owing into wells has increased from the initial liquid enthalpyin the depressurization zone around the wells by one or two processes. Cooling of the�uid due to depressurization boiling creates a temperature gradient between aquiferrock and the �owing �uid which favour conductive transfer of heat from rock to �uidthus enhancing vapor formation. Capillary forces between mineral grain surfaces andliquid reduce partly or even completely the mobility of the liquid.

At Svartsengi, exploitation of the reservoir has led to the formation of a vapor capon top of the hot water reservoir. Reservoir pressure drawdown has enhanced boilingin the uppermost part of the liquid reservoir and gravity segregation of the two �uidsled to the formation of the vapor cap.

15

2.6. AQUIFER TEMPERATURES

Table 2.2: Gas analyses of the sampled vapor phase in mmol/kgof condensate.

Sample # Well N2 Ar O2 CH4 H2 CO2 H2S N2/Ar

04-3007 N-12 4.35 0.236 0.000 0.757 103.95 92.6 25.7 18.404-3008 N-9 8.07 0.201 0.000 3.788 105.82 64.8 24.1 40.204-3010 K-13 13.12 0.222 0.000 0.117 46.99 157.7 11.9 59.104-3015 K-15 3.22 0.072 0.000 0.055 11.69 58.0 12.6 44.804-3022 K-16 3.59 0.097 0.000 1.025 78.27 600.0 32.2 37.004-3021 K-17 3.62 0.081 0.000 0.628 32.18 165.9 18.9 44.904-3019 K-20 1.97 0.052 0.000 0.333 41.47 437.0 25.8 37.804-3009 K-21 1.08 0.024 0.000 0.240 5.26 96.6 7.2 44.204-3023 K-24 3.73 0.091 0.000 0.353 0.35 23.8 3.4 41.104-3011 K-27 9.47 0.174 0.000 0.511 6.86 50.0 3.6 54.404-3014 K-32 1.95 0.042 0.000 0.045 24.35 79.8 24.2 46.004-3012 K-34 7.99 0.175 0.000 0.344 110.42 413.0 27.6 45.604-3040 NJ-5 9.73 0.153 0.000 1.357 34.00 142.8 40.8 63.504-3042 NJ-6 2.90 0.040 0.000 0.060 30.14 37.0 44.8 72.404-3038 NJ-7 9.40 0.144 0.000 2.006 61.49 95.9 31.6 65.104-3043 NJ-9 5.00 0.079 0.000 0.741 31.51 122.4 40.8 63.204-3039 NJ-10 8.63 0.150 0.000 1.611 1.60 41.7 18.5 57.604-3048 NJ-11 5.82 0.110 0.000 0.684 51.05 32.7 49.8 52.904-3001 NJ-13 5.41 0.075 0.000 0.110 32.50 25.2 37.3 72.304-3049 NJ-14 1.26 0.026 0.000 0.154 1.54 39.8 8.9 48.504-3037 NJ-16 5.46 0.081 0.000 0.992 51.60 49.4 44.5 67.104-3002 NJ-19 1.98 0.040 0.000 0.089 30.06 30.4 37.9 48.904-3050 NJ-20 1.15 0.012 0.000 0.325 12.30 72.4 21.2 92.104-3046 NJ-21 4.52 0.083 0.000 0.414 41.45 139.5 51.1 54.704-3045 NJ-22 9.85 0.147 0.000 0.043 19.89 43.0 33.6 67.104-3041 NJ-23 2.10 0.039 0.000 0.145 40.38 18.3 43.5 54.204-3005 SV-7 2.21 0.044 0.000 0.050 2.22 167.8 4.8 49.804-3003 SV-8 13.12 0.160 0.000 0.038 0.19 93.5 0.0 82.204-3032 SV-9 3.33 0.053 0.000 0.010 0.22 56.4 1.4 62.604-3029 SV-11 30.95 0.410 0.000 0.088 3.64 314.8 3.4 75.504-3004 SV-18 1.70 0.035 0.000 0.011 0.09 74.0 1.9 48.204-3006 SV-19 2.17 0.036 0.000 0.012 0.12 53.4 1.2 60.205-3004 R-12 7.02 0.109 0.000 0.062 0.87 446.9 14.7 64.505-3003 R-15 7.88 0.122 0.000 0.067 0.87 389.7 10.3 64.4

2.6 Aquifer temperatures

Aquifer temperatures have been estimated by the quartz and Na/K geothermometers forall wells. The Na/K geothermometer results are independent of the process producing�excess� well discharge enthalpy because it is based on a component ratio. This is,however, not the case for the quartz geothermometer. If �excess� well discharge enthalpyis produced by conductive heat transfer from the aquifer rock to the �uid �owing intowells, component concentrations, including silica, in the total well discharge are thesame as in the aquifer �uid. If, on the other hand, phase segregation is responsible forthe �excess� enthalpy, silica concentrations are lower in the total well discharge than inthe aquifer �uid because some of the liquid with its dissolved silica is retained in theformation.

Quartz equilibrium temperatures have been calculated according to both models for�excess� enthalpy wells together with the Na/K temperature (Fig. 2.2). It is seen thatthe two geothermometers yield similar results by the phase segregation model, althoughquartz equilibrium temperatures are systematically higher (13 ◦C on average). Theconductive heat transfer model yields quartz equilibrium temperatures that are lowerthan the Na/K temperatures for �excess� enthalpy wells and the di�erence increaseswith increasing discharge enthalpy. These results indicate that phase segregation islargely the cause of �excess� enthalpy. It is therefore reasonable to calculate componentconcentrations in the initial aquifer �uid according to this model.

For the Svartsengi wells, which receive �uids from sub-boiling aquifers, quartz equi-

16

CHAPTER 2.

Figure 2.2: Na/K and quartz geothermometers compared for both the �phasesegregation� model and the �heat transfer model�.

17

2.7. SAMPLING AND ANALYSIS

librium and Na/K temperatures compare well (Fig. 2.2), the average di�erence forSvartsengi being 3.3 ◦C suggesting that the calibration of the two geothermometers isinternally consistent likely, at least, in the temperature range of 200�300 ◦C. For liquidenthalpy wells discharging dilute waters (see Table 2.1), average Na/K temperatures are18 ◦C lower than average quartz equilibrium temperatures. This di�erence is consid-ered to be due to cooler water recharge into producing aquifers and insu�cient time forequilibration with alkali-feldspars (the Na/K geothermometer) as the recharging watergains heat by �ow through the aquifer rock (Arnórsson et al., 2000; Gudmundsson andArnórsson, 2002). Such recharge has not been observed in Svartsengi. For �excess�enthalpy wells, Na/K temperatures are on average 13 ◦C lower than quartz equilibriumtemperatures.

At Reykjanes, Na/K temperatures are signi�cantly lower than quartz equilibriumtemperatures (Fig. 2.2). The cause is believed to be insu�cient supply of K from thebasalt to the �uid to saturate it with K-feldspar. The seawater source �uid contains∼400 ppm K but the geothermal seawater 1400�1500 ppm. The di�erence, which isdue to leaching from the basalt host rock, is comparable, or even higher, than the Kcontent of fresh basalt in the Reykjanes area.

The calibration (quartz solubility constant) used here for the quartz geothermometeris that proposed by Gunnarsson and Arnórsson (2000). The calibration of Fournier andPotter (1982) is more widely used. The two calibrations, however, yield similar resultsin the temperature range of the wells under study, the di�erence being only 6 ◦, 1 ◦ and17 ◦C at 200 ◦, 250 ◦ and 300 ◦C, respectively. The Na/K geothermometer equation usedis based on thermodynamic data on low-albite and microcline solubility (Arnórsson andStefánsson, 1999; Arnórsson et al., 2000).

2.7 Sampling and analysis

Only a short description of the sampling and analytical techniques used in this studyis given here. For further details, see Arnórsson et al. (2006).

Water and steam samples were collected from the two-phase pipeline (or steampipeline in case of dry steam wells) close to the wellhead by mean of a stainless (N316)steel Webre separator.

2.7.1 Water phase

Sample treatment

Water samples were �ltered through 0.2 µm cellulose acetate membrane using a Te�on�lter holder and collected into low-density polyethylene bottles. An untreated 200 mlsample was analyzed on a Reagent-FreeTMion-chromatograph (RFICTM, Dionex 2000)for F, Cl and SO4. A 200 ml sample was acidi�ed with 2 ml Suprapur R©HNO3 (E. Merck1.00441) for major element analyses on an Inductively Coupled Plasma - Atomic Emis-sion Spectrometer (ICP-AES) (Al, B, Ca, Cl, Fe, Mg, Na, Si, Sr). Another 100 mlacidi�ed sample was used for trace elements determination on an ICP-MS.

Glass bottles with air-tight cap were used for pH and total carbonate carbon deter-mination in the laboratory shortly after sampling. Hydrogen sul�de was measured on

18

CHAPTER 2.

site by titration with standard mercuric acetate solution, using dithizone as endpointindicator (Arnórsson et al., 2006).

Note on sulfate analysis

Special samples were collected for sulfate determination. One 1 ml of 1% Zn-acetate so-lution was added to 100 ml sample to remove H2S. Subsequently, sulfate concentrationswere determined by RFICTM. Sulfur was also analyzed by ICP-AES. In the absence ofsulfur bearing species other than sulfate, this measurement yields a value for sulfate.Upon sample storage the ZnS precipitate tends to dissolve. Subsequent oxidation ofH2S will cause an increase in the SO4 concentrations in the sample. Accordingly, thereported SO4 numbers in Table 2.1 are maximum values.

2.7.2 Steam phase

Steam samples for analysis of major gases were collected into evacuated 300 ml glassbulbs containing 10 or 20 ml of KOH 50% w/v solution. Carbon dioxide and H2Sdissolve quantitatively in the hydroxide solution, concentrating the non-condensablegases in the gas phase. This allows a better precision of the non-condensable gasanalysis. Hydrogen, N2, O2, Ar and CH4 were analyzed on a PerkinElmer-ARNEL4019 gas-chromatograph. Condensate was analyzed for CO2 and H2S by titration.

2.8 Thermodynamic data

2.8.1 Mineral solubility constants and gas solubilities

Equilibrium constants for the reactions given in Tables 2.3 and 2.5 were taken fromKaringithi et al. (2008). They are based on thermodynamic data (∆G◦f , S

◦, V◦, C◦p) onthe minerals from Holland and Powell (1998) except for pyrite, pyrrhotite and calcite.Data on the sul�de minerals were taken from Robie and Hemingway (1995). The calcitesolubility constants is from Arnórsson et al. (1982). The standard Gibbs energy of thedissolved gases are based on the gas solubility constants given by Fernández-Prini et al.(2003) assuming ideal behavior of the gases. The thermodynamic properties of theaqueous species and liquid water entering the reactions in Table 2.3 were taken fromvarious sources; data on H4SiO0

4 are from Gunnarsson and Arnórsson (2000), those onH2Ol, Ca+2, Fe+2, and OH− are from SUPCRT92 (Johnson et al., 1992) using theslop98.dat data set and those on Fe(OH)−4 and Al(OH)−4 from Diakonov et al. (1999)and Pokrovskii and Helgeson (1995), respectively.

Three of the hydrothermal minerals considered for the present study form extensivesolid solutions (epidote, garnet and prehnite). The other minerals considered (calcite,magnetite, pyrite, pyrrhotite, quartz and prehnite) are taken to be pure. Activitiesof endmembers in solid solution have been retrieved from their analyzed compositionassuming ideal behavior.

At Kra�a and Nesjavellir, the composition of epidote, expressed as Xps, the molefraction of pistacite (Ca2Fe3Si3O12OH), is in the range 0.18�0.32 (Bird and Spieler,

19

2.8. THERMODYNAMIC DATA

Table 2.3: Individual minerals dissolution reactions

1. and + 4H+ + 8H2O = 3Ca2+ + 2Fe(OH)−4 + 3H4SiO04

2. cal + 2H+ = Ca2+ + H2Ol + CO2,aq

3. czo + 12H2Ol = 2Ca2+ + 3Al(OH)−4 + 3H4SiO04 + OH−

4. epi + 12H2Ol = 2Ca2+ + Fe(OH)−4 + 2Al(OH)−4 + 3H4SiO04 + OH−

5. gro + 4H+ + 8H2Ol = 3Ca2+ + 2Al(OH)−4 + 3H4SiO04

6. mag + 4H2Ol = 2Fe(OH)−4 + Fe2+

7. pre + 10H2Ol = 2Ca2+ + 2Al(OH)−4 + 3H4SiO04 + 2OH−

8. pyr + 2H+ + H2,aq = 2H2Saq + Fe2+

9. pyrr + 2H+ = H2Saq + Fe2+

10. qtz + 2H2Ol = H4SiO04

11. wol + 2H+ + H2Ol = Ca2+ + H4SiO04

2004) and 0.14�0.38 (Árnason and Bird, 1992), respectively. This gives an average epi-dote endmember (Ca2FeAl2Si3O12OH) activity of 0.75. The corresponding clinozoisiteactivity is therefore 0.25. In Svartsengi and Reykjanes, Xps ranges between 0.19 and0.44, which corresponds to an endmember epidote activity of 0.9 (Bird and Spieler,2004). The substitution of Al by Fe in epidote occurs mostly in the M3 site, and tocalculate the epidote and clinozoisite activities, we considered a full substitution of Alby Fe on this site. Nevertheless, the M1 site can also accommodate some Fe, and thiscan lead to a modest overestimation of the calculated activities (Deer et al., 1992).

In prehnite (Ca2Al(Al,Fe)Si3O10(OH)2), Al-Fe substitution occurs on one site andthe activity, assuming ideal behavior, is proportional to the ratio of Fe to one Al. Theselected activities for the dilute aquifers is equal equal to 0.9, corresponding to an XFe

of 0.1 (Hreggvidsdóttir, 1987) and 0.8 (XFe = 0.2) (Lonker et al., 1993) for the saline�elds.

Garnet (grossular-andradite, Ca3(Fe,Al)Si3O12) has only been reported from thesaline �uid �elds in Svartsengi and Reykjanes. Substitution occurs on two sites, soai,ideal = X2

i where ai,ideal is the activity of the endmember i, and X its mole fraction.The average Al mole fraction in the garnet at Svartsengi and Reykjanes is very variable(Lonker et al., 1993). From the average XAl in grossular and XFe in andradite, activitiesof 0.2 and 0.3 is obtained for the grossular and andradite endmembers, respectively,assuming ideal solid-solution.

Mineral compositions used in this study are averages of chemical analyses from thecited authors. The question raises whether the numbers selected are representative ofthe mineral composition with which the present �uid is reacting. Some minerals showa zonation and the core of those grains is not expected to be reacting with the present�uid, neither will be the rim if the grain has ceased to grow. In addition, analyses aremore likely to be performed on coarse-grained rather than �ne-grained crystals thatrecently formed from solution. The average mineral compositions were selected becauseit is the simplest one. Also, the �uids sampled must be a mixture of many componentsand individual components likely have reacted with garnets of di�erent compositions.

20

CHAPTER 2.

2.9 Data handling

2.9.1 Note on pH measurement

The waters sampled for the present study contain 380�880 ppm of dissolved silica (asSiO2). Some of these waters are slightly amorphous silica supersaturated at samplingconditions, whereas other waters become supersaturated upon cooling to room tem-perature subsequent to sampling. Silica polymerization a�ects the solution pH as itinvolves removal of silica monomers, which act as weak acid, from solution and theformation of silica oligomers, which are stronger acids than silica monomers (Tosselland Sahai, 2000). Therefore, removal of monomers from solution tends to raise its pH,whereas the formation of oligomers has the opposite e�ect. At high pH (above ∼9),when the dissolved silica is signi�cantly ionized, decrease in silica monomer concen-tration by its polymerization dominates the pH change over oligomer formation andcauses it to increase. In contrast, when ionized silica concentrations are insigni�cant(pH below ∼8), oligomer formation dominates the pH change over decrease in silicamonomer concentrations. As the acid dissociation constants for silica oligomers areonly poorly known from ab initio calculations their e�ects on pH cannot be quanti�ed.Besides their concentration cannot be measured and their concentrations change withtime, sometimes rapidly. The pH of the samples collected for the present study wasmeasured when practically all silica in excess of amorphous silica polymerization hadpolymerized. When the measured pH was above 8, the pH was corrected for the e�ect ofsilica polymerization on the assumption that the solution pH was essentially governedby the silicic acid bu�er and following the results of Gunnarsson and Arnórsson (2005).The results are shown in Table 2.1. The only way to obtain reliable values for pH ofamorphous silica supersaturated solutions is to measure it on site before onset of silicapolymerization.

2.9.2 Calculation of aquifer water compositions and aque-

ous speciation

As demonstrated in section 2.6 above, the quartz and Na/K geothermometer resultsindicate that excess enthalpy of well discharges is essentially produced by phase seg-regation. Therefore, the phase segregation model was used to calculate initial aquifer�uid compositions for �excess� enthalpy wells, i.e. model 3 in Arnórsson et al. (2007).For liquid enthalpy wells, it is reasonable to take total well discharge compositions torepresent the initial aquifer �uid.

The pressure at which phase segregation occurs a�ects the composition of the ini-tial aquifer �uid in relation to the total well discharge. It is considered likely thatsegregation occurs over a range of pressures between the aquifer vapor pressure andthe wellhead vapor pressure. Since the segregation pressure range is not known, weselected this pressure to be halfway between aquifer and wellhead vapor pressures. Theinitial temperature selected for calculating aquifer �uid component compositions wasthe average of the quartz and Na/K geothermometers (Table 2.1).

Aquifer �uid compositions and speciation distribution in this �uid were calculatedwith the aid of theWATCH speciation calculation software, version 2.1 (Arnórsson et al.,

21

2.9. DATA HANDLING

1982; Bjarnason, 1994). For liquid enthalpy wells, the initial aquifer �uid compositionfor component i is obtained from

mf,ti = md,t

i = md,li (1−Xd,v) +md,v

i Xd,v (2.1)

mi designates concentration on component i. The superscript d stands for discharge,i.e. sampling pressures, t for total �uid and f , v and l for initial aquifer �uid, vaporand liquid, respectively. For components that partition almost exclusively in the liquidphase, at sampling conditions, md,v

i is taken to be zero by the WATCH program and forgaseous components that are present in insigni�cant concentrations in liquid samples,and therefore not analyzed, md,l

i is taken to be zero. In fact the only componentsanalyzed in both phases are carbonate carbon, sul�de sulfur and boron.

For excess enthalpy wells, the WATCH calculations involve two steps. The �rst stepconsists of calculating the vapor fraction and the liquid and vapor compositions at thepressure (P g) at which phase segregation is assumed to occur. The second step involvescalculating aquifer �uid composition from the liquid and vapor phase compositions atP g, assuming that the �owing �uid enthalpy before phase segregation occurred is thesame as that of vapor saturated liquid at the aquifer temperature. For the �rst step wehave

md,ti = mg,l

i (1−Xg,v) +mg,vi Xg,v (2.2)

where Xg,v denotes the steam fraction in the �uid immediately after phase segregationoccurs, at vapor pressure P g. Other symbols have the same notation as in equation(2.1).

The �owing �uid enthalpy after phase segregation is taken to be the same as thatof the well discharge. Therefore

hd,t = hg,l(1−Xg,v) + hg,vXg,v (2.3)

so

Xg,v =hd,t − hg,l

hg,v − hg,l(2.4)

h stands for speci�c enthalpy. Superscripts have the same notation as in equation(2.1) through (3.5). As before, mg,v

i is taken to be zero for dissolved solids that donot partition signi�cantly into the vapor phase at conditions P g. Gaseous speciesare assumed to be present in both phases at P g. The distribution coe�cient (Ds forspecies s) describes its relative concentrations in the two phases at equilibrium. Atvapor pressure P g we have

Dgs =

ng,vs

ng,ls

and for dilute �uids: (2.5)

Dgs =

mg,vs

mg,ls

(2.6)

n denotes mole fraction and subscript s gaseous species s (see Arnórsson et al., 2007).From Ps = xv

sPtot and mls = KsPs, it follows that

Ds =55.508

KsPtot(2.7)

22

CHAPTER 2.

Ks is the Henry's Law constant for gas s, Ps and Ptot are partial pressure of gas s andtotal pressure, respectively, and xv

s the mole fraction of gas s in the vapor phase. Thenumber 55.508 is the number of moles of water in one kg. Now equation (2.2) can berewritten as

md,ti = mg,l

i (1−Xg,v) +Dgsm

g,li Xg,v (2.8)

allowing calculations of the concentrations of all components in both liquid and vaporphases at pressure P g. The second calculation step by the WATCH program for �excess�enthalpy wells involves calculating the steam fraction at pressure P g on the basis thatthe �owing �uid enthalpy before phase segregation occurs is that of the initial aquifer�uid. This steam fraction is symbolized as Xe,v

Xe,v =hf,t − hg,l

hg,v − hg,land, (2.9)

mf,ti = mg,l

i (1−Xe,v) +mg,vi Xe,v (2.10)

As before for non-volatile species, mg,vi is taken to be zero. The results for component

concentrations in the initial aquifer �uid of �excess� enthalpy wells, as calculated by themethod just described, is given in Table 2.4, together with the aquifer composition of�uids feeding liquid enthalpy wells.

2.10 Mineral-gas equilibria

In the discussion below (section 2.12) it is assumed that the initial aquifer �uid isrepresented by liquid only, i.e. the calculated gas concentrations in the initial aquifer�uid represent aqueous gas concentrations. If some vapor is present in the aquifer �uid,the true gas concentration in the aquifer liquid would be lower than assumed here,particularly for the less water soluble gases, such as H2.

2.10.1 CO2

Figure 2.3 shows that the calculated aquifer liquid concentrations of carbon dioxide(CO2,aq) as a function of temperature. Also shown are equilibrium constants for twopotential mineral assemblages that would �x CO2,aq if equilibrium was attained. Therespective reactions for dilute and saline �uids are shown in reactions 1 and 2 in Table2.5.

23

2.10. MINERAL-GAS EQUILIBRIA

Table2.4:Aquifer

watercomposition.Tem

perature

isin

◦ C,dissolved

solidconcentrationsare

inmg/kgbutgases

inmmol/kg.

Sample#Well

Aquifer

SiO

2Na

KCa

Mg

Cl

FB

SO

4Fe

Al

CO

2H

2S

N2

Ar

O2

CH

4H

2N

2/Ar

temp.b

04-3007

N-12

268

536

115

21.1

0.3

50.0

04

93.1

0.4

53.1

63.2

0.0

04

0.8

37.6

13.4

60.3

09

0.0

169

0.0

00.0

536

7.4

11

18.3

04-3008

N-9a

232

421

139

17.9

2.3

80.0

01

32.8

0.5

40.9

140.4

0.0

01

0.8

55.9

73.6

00.6

42

0.0

160

0.0

00.0

605

8.4

38

40.2

04-3010

K-13

222

411

232

25.3

4.2

60.0

02

30.3

0.8

91.1

6289.3

0.0

04

1.1

15.6

41.1

20.2

95

0.0

050

0.0

00.0

025

1.0

67

58.6

04-3015

K-15

273

598

182

32.0

1.6

80.0

00

29.8

1.0

40.9

7203.0

0.0

01

1.5

47.0

32.9

30.3

44

0.0

077

0.0

00.0

056

1.2

55

44.6

04-3022

K-16

263

521

153

25.8

0.6

90.0

02

104.8

1.6

61.2

49.8

0.0

03

0.8

152.2

03.6

90.2

85

0.0

077

0.0

00.0

866

6.2

45

36.8

04-3021

K-17

268

596

108

18.1

0.1

80.0

00

13.6

1.6

91.2

74.3

0.0

02

1.7

610.3

22.3

70.1

70

0.0

038

0.0

00.0

293

1.5

28

44.5

04-3019

K-20

293

614

185

37.5

1.4

00.0

01

162.3

0.9

11.3

84.1

0.0

02

0.7

752.8

24.1

60.2

27

0.0

060

0.0

00.0

368

4.5

94

37.7

04-3009

K-21a

238

458

137

18.1

1.0

80.0

01

65.9

0.9

30.7

761.1

0.0

03

1.3

89.4

92.2

90.0

94

0.0

021

0.0

00.0

212

0.4

61

44.2

04-3023

K-24a

199

337

195

15.5

3.1

70.0

01

40.3

0.7

30.5

4213.3

0.0

02

0.7

61.6

90.7

70.4

31

0.0

105

0.0

00.0

150

0.0

15

41.1

04-3011

K-27a

247

485

206

30.0

2.3

10.0

04

43.1

0.9

10.7

8228.3

0.0

12

1.3

57.2

61.6

51.2

31

0.0

226

0.0

00.0

667

0.8

93

54.4

04-3014

K-32

299

648

176

40.3

1.1

60.0

01

110.5

1.0

31.6

4125.2

0.0

02

1.2

511.8

05.0

30.2

63

0.0

057

0.0

00.0

062

3.2

99

45.8

04-3012

K-34

263

461

135

25.6

0.9

90.0

03

82.9

1.1

24.6

337.1

0.0

11

1.0

329.5

62.8

10.5

06

0.0

112

0.0

00.0

218

7.0

34

45.3

04-3040

NJ-5

276

582

134

25.6

0.2

80.0

01

95.4

0.7

61.1

56.6

0.0

01

1.4

315.0

55.8

80.9

22

0.0

146

0.0

00.1

284

3.2

39

63.2

04-3042

NJ-6

285

600

123

25.5

0.1

90.0

00

130.4

0.5

71.7

84.8

0.0

01

1.2

74.0

56.3

10.2

91

0.0

040

0.0

00.0

062

3.0

41

72.1

04-3038

NJ-7a

268

563

144

25.2

0.3

30.0

02

80.1

1.0

31.1

211.8

0.0

01

1.4

616.4

86.7

91.5

09

0.0

232

0.0

00.3

223

9.9

01

65.1

04-3043

NJ-9

283

602

133

26.8

0.2

90.0

00

137.7

0.7

41.4

87.0

0.0

02

1.1

414.5

76.1

80.5

59

0.0

089

0.0

00.0

810

3.4

77

62.9

04-3039

NJ-10a

278

598

138

26.2

0.2

60.0

01

99.0

1.2

81.1

310.3

0.0

02

1.6

68.4

04.7

11.6

41

0.0

285

0.0

00.3

067

0.3

03

57.6

04-3048

NJ-11

282

587

111

21.1

0.2

20.0

02

57.6

0.8

61.4

015.3

0.0

01

1.7

14.2

58.1

60.7

11

0.0

135

0.0

00.0

823

6.1

76

52.7

04-3001

NJ-13

297

638

109

24.4

0.1

00.0

01

109.7

0.6

91.8

12.3

0.0

12

1.3

83.8

07.2

30.7

60

0.0

106

0.0

00.0

150

4.5

84

71.9

04-3049

NJ-14a

275

596

141

25.8

0.2

40.0

01

151.1

0.9

41.3

18.9

0.0

01

1.5

07.2

32.2

40.2

22

0.0

046

0.0

00.0

268

0.2

68

48.5

04-3037

NJ-16a

273

589

130

23.5

0.3

50.0

06

53.7

1.1

11.5

021.3

0.0

04

1.7

19.3

09.8

30.9

48

0.0

141

0.0

00.1

727

8.9

94

67.1

04-3002

NJ-19

293

601

103

23.0

0.0

60.0

01

103.3

0.7

72.6

41.3

0.0

06

1.4

65.0

27.1

20.3

26

0.0

067

0.0

00.0

137

4.7

67

48.8

04-3050

NJ-20

274

573

142

26.8

0.3

70.0

01

139.9

0.7

81.2

510.6

0.0

01

1.4

89.0

33.7

30.1

30

0.0

014

0.0

00.0

368

1.3

89

91.6

04-3046

NJ-21

291

609

110

24.3

0.2

30.0

05

85.6

0.9

01.2

21.5

0.0

01

1.2

716.1

17.9

30.4

73

0.0

087

0.0

00.0

430

4.3

60

54.5

04-3045

NJ-22

296

655

117

25.5

0.1

60.0

06

125.2

0.8

01.3

43.3

0.0

02

1.5

26.8

16.7

21.4

88

0.0

223

0.0

00.0

062

2.9

47

66.8

04-3041

NJ-23

292

624

108

23.0

0.3

10.0

02

93.9

1.0

33.8

94.1

0.0

02

1.4

22.0

96.5

30.2

19

0.0

041

0.0

00.0

150

4.2

17

53.9

04-3005

SV-7a

239

429

6650

961

1069

0.3

92

12664

0.1

36.4

123.8

0.0

36

0.0

614.6

70.4

60.1

92

0.0

038

0.0

00.0

044

0.1

89

49.8

04-3003

SV-8a

240

442

6380

916

1033

0.6

14

12222

0.1

56.5

625.5

0.0

10

0.0

69.1

60.0

31.2

99

0.0

158

0.0

00.0

037

0.0

20

82.2

04-3032

SV-9a

241

448

6373

915

1025

0.2

80

12054

0.1

56.6

325.7

0.1

43

0.1

15.1

60.1

60.3

20

0.0

051

0.0

00.0

006

0.0

20

62.6

04-3029

SV-11a

239

431

6394

920

1032

0.5

31

12100

0.1

47.0

627.3

0.1

25

0.1

520.8

80.3

22.0

33

0.0

269

0.0

00.0

056

0.2

28

75.5

04-3004

SV-18a

240

427

6813

1009

1023

0.4

27

12912

0.1

46.9

225.0

0.0

08

0.0

67.2

20.2

10.1

68

0.0

035

0.0

00.0

012

0.0

10

48.2

04-3006

SV-19a

239

421

6809

994

1064

0.3

69

12942

0.1

37.1

225.9

0.0

03

0.0

65.0

30.1

30.2

03

0.0

034

0.0

00.0

012

0.0

10

60.2

05-3004

RN-12a

287

613

9172

1294

1516

0.7

18

17402

0.1

27.7

314.3

0.3

89

0.0

245.8

31.7

40.6

85

0.0

106

0.0

00.0

062

0.0

84

64.5

05-3003

RN-15a

284

567

8511

1220

1381

0.9

07

16241

0.1

27.1

620.5

0.2

47

0.0

461.8

51.6

91.2

31

0.0

191

0.0

00.0

106

0.1

34

64.4

aLiquid

enthalpywells

bSelected,correspondsto

theaverageofNa/K

andquartzgeotherm

ometertemperatures,exceptforReykjanes,where

quartzgeotherm

ometertemperature

wasused

(seetext).

24

CHAPTER 2.

Figure 2.3: Potential mineral bu�ers controlling CO2 gas concentrations in theinitial aquifer �uid. For the dilute �uid activities of clinozoisite of 0.25 and 0.1 areshown. Prehnite activity was selected as 0.9. For the saline �uids, the activitiesof clinozoisite, grossular and prehnite are 0.1, 0.2 and 0.82, respectively.

25

Table2.5:Mineralassem

blages

potentiallycontrollingCO

2,H

2SandH

2concentrationsandrespectiveK-tem

perature

equations(K

aringithiet

al.,2008).

Tem

perature

equationsare

validin

therange0�350◦ C

atP

sat.Unitactivitywas

selected

forallmineralsandliquid

water.

#SpeciesReaction

Log

K(T)

1.CO

2cz

o+

cal+

3 2qtz

=3 2pre

+C

O2,a

q−

1.8

31−

22843/T

2−

1344.4

/T

+0.0

0829T−

0.4

55

log

T

2.CO

22 5cz

o+

cal+

3 5qtz

=3 5gro

+1 5H

2O

l+

CO

2,a

q−

2.3

27−

58215/T

2−

1829.2

/T

+0.0

1059T−

0.5

61

log

T

3.H

2S

1 3pyr+

1 3pyrr

+2 3pre

+2 3H

2O

l=

2 3ep

i+

H2S

aq

−2.0

52−

215218/T

2−

1658.7

/T

+0.0

0856T−

0.5

09

log

T

4.H

2S

1 4pyr+

1 2pyrr

+H

2O

l=

1 4m

t+

H2S

aq

−2.0

20−

188233/T

2−

1504.4

/T

+0.0

0760T−

0.5

28

log

T

5.H

2S

2 3gro

+1 3pyr+

1 3pyrr

+2 3qtz

+4 3H

2O

l=

2 3ep

i+

2 3w

ol+

H2S

aq−

1.8

62−

253145/T

2−

1595.2

/T

+0.0

0780T−

0.4

52

log

T

6.H

2S

2gro

+1 4pyr+

1 2m

t+

2qtz

+2H

2O

l=

2ep

i+

2w

ol+

H2S

aq

−1.5

49−

383405/T

2−

1774.0

/T

+0.0

0820T−

0.3

03

log

T

7.H

24 3pyrr

+2 3pre

+2 3H

2O

l=

2 3ep

i+

2 3pyr+

H2,a

q−

1.3

58−

1420.4

/T

+6.7

77·1

0−

4T

+5.6

11·1

0−

6T

2−

0.3

91

log

T

8.H

23 2pyrr

+H

2O

l=

3 4pyr+

1 4m

t+

H2,a

q−

1.4

36−

1131.3

/T−

1.8

66·1

0−

4T

+5.3

77·1

0−

6T

2−

0.4

54

log

T

9.H

22 3gro

+4 3pyrr

+2 3qtz

+4 3H

2O

l=

2 3ep

i+

2 3w

ol+

2 3pyr+

H2,a

q−

1.2

41−

1519.9

/T

+8.8

18·1

0−

4T

+4.6

93·1

0−

6T

2−

0.3

36

log

T10.H

26gro

+2m

t+

6qtz

+4H

2O

l=

6ep

i+

6w

ol+

H2,a

q1.5

70−

5346.3

/T

+0.0

0880T−

6.4

79·1

0−

6T

2+

1.1

13

log

T

CHAPTER 2.

The equilibrium constant curves incorporate the average activities of the clinozoisite(0.25 and 0.1), Al-prehnite (0.9 and 0.82) and grossular (0.8) components in epidote,prehnite and garnet, respectively (see Fig. 2.3). Many of the data points for the dilute�uids fall close to the equilibrium curve. Yet there are some systematic di�erencesbetween �elds. At Kra�a, three wells (16, 20 and 24) have high CO2 relative to equi-librium. The CO2 content of fumaroles and wells rose drastically in Kra�a followingan episode of magma intrusion into the roots of the geothermal system in the period1975�1984 (Ármannsson et al., 1989; Gudmundsson and Arnórsson, 2002). Since themagmatic event, the CO2 concentrations in the well discharges have declined in somewells (Gudmundsson and Arnórsson, 2002), but increased in others, depending on theirlocation in relation to the newly intruded magma body. It is concluded that the highCO2 in wells 16, 20 and 24 is the consequence of high CO2 �ux from the magma in-trusion, too high for equilibrium with mineral assemblages to be closely approachedin the respective aquifers. In the other wells at Kra�a and at Námafjall, the CO2

concentration is close to equilibrium for a clinozoisite activity of 0.1. At Nesjavellir,the CO2 concentrations for most of the wells are lower than the equilibrium concen-trations. Likely, this is a consequence of inadequate supply of CO2 from the magmaheat source to the geothermal system, making calcite unstable. Equilibrium betweenepidote, prehnite, quartz and solution �xes aqueous Ca+2/(H+)2 activity ratios. Fromthe following reaction CaCO3,calcite + 2H+ = Ca+2 + CO2,aq + H2Ol it is seen thata speci�c CO2 partial pressure is required to stabilize calcite, if Ca+2/(H+)2 activityratios are �xed.

For the saline �uids at Svartsengi and Reykjanes comparison is made with anothermineral assemblage, one including garnet rather than prehnite (see reaction 2 in Table2.5). Garnet is abundant as hydrothermal mineral in these areas (Lonker et al., 1993)whereas prehnite is rare. Carbon dioxide concentrations at Reykjanes are very close toequilibrium with this bu�er. Two of the wells at Svartsengi show CO2 concentrationssigni�cantly above the equilibrium curve but the four remaining are close to the curve.

2.10.2 H2S and H2

These two gases are likely to equilibrate with the same mineral assemblage. The as-semblages likely to be involved, as deduced from the hydrothermal minerals in thegeothermal systems considered are given in Table 2.5 (reactions 3 to 6 for H2S and 7to 10 for H2).

The mineral assemblages involving pyrite and pyrrhotite and either prehnite +epidote or magnetite yield very similar equilibrium constants in the range 180�320 ◦C forthe selected epidote and prehnite compositions, both for dilute and saline �uid systems(Figs. 2.4 and 2.5). The di�erence at 200 ◦C is close to zero, increasing to about 0.3logK units at 300 ◦C. The dilute �uids at Kra�a, Námafjall and Nesjavellir are closeto equilibrium with these two assemblages. Overall, the match is, however, better forthe prehnite-epidote bearing assemblage although it is not possible to determine forindividual samples which assemblage may be involved. The average departure fromequilibrium for the prehnite-epidote containing assemblage for Kra�a and Nesjavellir is0.05 and -0.04 log mol/kg with a standard deviation of 0.2. These deviations are withinthe error in the thermodynamic data of the minerals. A systematic di�erence is observed

27

2.10. MINERAL-GAS EQUILIBRIA

Figure 2.4: Mineral bu�ers controlling H2S in the initial aquifer �uid. Selectedactivities are 0.75 and 0.9 in the dilute �uids for epidote and prehnite, respec-tively, and 0.9, 0.82 and 0.2 in the saline �uids for epidote, prehnite and grossular,respectively.

between areas, the Nesjavellir samples tending to be the highest in H2S in relationto the aquifer temperature. At Kra�a and Nesjavellir, and in particular Námafjall, astrong magnetic low is associated with the geothermal �elds (Pálmason, 1975; Björnssonet al., 1986). This has been taken to indicate that the primary magnetite of the basaltis unstable, dissolved by the geothermal �uids. By this interpretation it seems likelythat H2Saq concentrations in the initial aquifer liquids are controlled by close approachto equilibrium with the pyrite, pyrrhotite, epidote and prehnite mineral assemblage(reaction 3 in Table 2.5).

The H2 concentrations are similar to or higher than those predicted by equilibriumwith the epidote-prehnite assemblage and by as much as 0.4 log mol/kg (Fig. 2.5).Three samples are close to equilibrium, two from Kra�a (wells 21 and 27) and onefrom Nesjavellir (well 20). Another three aquifer �uids are signi�cantly lower than theequilibrium values, two from Nesjavellir (wells 10 and 14) and one from Kra�a (well 24).All these wells, except Nesjavellir 20 have liquid enthalpy. The apparent inconsistency

28

CHAPTER 2.

Figure 2.5: Mineral bu�ers controlling H2 in the initial aquifer �uid. The activityof epidote is 0.75 and 0.9 in the dilute and in the saline �elds, respectively andthe activity of prehnite is 0.9 and 0.82, respectively. For grossular, the selectedactivity is 0.2.

between H2S and H2 is considered to result from the presence of vapor in the initialaquifer �uid. The vapor fraction is apparently su�ciently large to signi�cantly raise theH2 concentrations but not those of the more soluble H2S. This vapor fraction has beenestimated on the assumption that both aqueous H2S and H2 concentrations are �xed byequilibrium with the mineral assemblage pyrite, pyrrhotite, epidote and prehnite. Thehighest vapor fraction value for �excess� enthalpy wells is 0.04 by mass and the averageis 0.018. A value of 0.04 by mass at 300 ◦C corresponds to 40% by volume and 0.018corresponds to 22%.

The H2S concentrations in the saline �uids are close to equilibrium with a mineralassemblage containing garnet, epidote and wollastonite (equation 5 in Table 2.5). Par-ticular feature is the large variation in the H2S temperatures in the Svartsengi aquifer�uid, where both the Na/K and quartz geothermometers and the measured down-holetemperatures are very constant, around 240 ◦C. One data point for Svartsengi (well 8),which yields the lowest aquifer �uid H2S concentration, involves an air contaminated

29

2.11. INDIVIDUAL MINERAL-SOLUTION EQUILIBRIA

vapor sample. Is is known that sul�de reacts fast with O2 in alkaline medium, andtherefore, it is concluded that the low H2S temperature is faulty and due to air con-tamination of the sample. The remaining �ve samples scatter around the equilibriumcurve, the maximum departure being around 0.3 log mol/kg. Four of the samples inSvartsengi and both samples in Reykjanes show too low H2 equilibrium temperaturesrelative to that mineral assemblage.

2.11 Individual mineral-solution equilibria

If the aqueous concentrations of CO2, H2S and H2 are controlled by equilibria withparticular mineral assemblages, the individual minerals in these assemblages should bein equilibrium with the solution. Below, we present results obtained for the state ofmineral-solution equilibria for individual minerals. Mineral saturation is expressed interms of saturation index (SI) de�ned as

SI = log

„Q

K

«(2.11)

where Q represents activity product and K the equilibrium constant.

2.11.1 Calcite and wollastonite

The majority of the aquifer �uids are rather close to calcite saturation (Fig. 2.6). ForNámafjall and Kra�a, the results indicate some over-saturation, the average SI valuesbeing 0.28 and 0.46, respectively. On average aquifer waters at Nesjavellir are veryclose to calcite saturation, average SI value is -0.11. At Svartsengi and Reykjanes, theaquifer water is systematically calcite under-saturated, or by -0.57 SI units in the case ofSvartsengi and it falls down to -1.03 at Reykjanes. Monitoring studies at Svartsengi haveindicated some depletion in the aquifer liquid concentration of CO2 and slight cooling(Steingrímsson, 1989). The cause is boiling and simultaneous formation of a steam capover the liquid reservoir (Gudmundsson and Thórhallsson, 1986). The cooling has ledto slight decrease in the pH of the aquifer water and this, together with the decreasein dissolved CO2 concentration has turned the aquifer liquid calcite under-saturated.The calculated pH of the aquifer water for the two samples from Reykjanes is 4.47 and4.39. These values seem low. The Ca+2/(H+)2 activity ratios for the Reykjanes andSvartsengi aquifer waters are correspondingly low relative to those at equilibrium withmineral bu�ers that potentially could control this activity ratio (Fig. 2.7). The tworeactions considered are:

3

2pre + 2H+ = czo +

3

2qtz + 2H2Ol + Ca+2 and, (2.12)

3

5gro + 2H+ =

4

5H2Ol +

2

5czo +

3

5qtz + Ca+2 (2.13)

The low Ca+2/(H+)2 activity ratios correlate well with negative SI values for calcite.The cause of the low calculated aquifer �uid pH-value for Reykjanes and some of the

30

CHAPTER 2.

Figure 2.6: Saturation indices of selected alteration minerals. Endmember activ-ities are the same as described in Fig. 2.3 to 2.5.

31

2.11. INDIVIDUAL MINERAL-SOLUTION EQUILIBRIA

Figure 2.7: Ca2+/(H+)2 ratio versus temperature. Symbols for various areas arethe same as in Fig. 2.3 to 2.5. Mineral assemblages clinozoisite+prehnite+quartzand grossular+clinozoisite+quartz are also shown. Equations are modi�ed fromKaringithi et al. (2008), with activities of clinozoisite, prehnite and grossular of0.1, 0.9 and 0.2, respectively.

Svartsengi samples is not known, nor is the calcite under-saturation observed at Reykja-nes but equilibrium with calcite is rapidly attained at high temperatures (Zhang andDawe, 1998). A process that could produce calcite under-saturated reservoir �uid isdegassing with respect to CO2 by depressurization boiling and rapid re-equilibration ofpH, possibly by uptake of OH− into layer silicates.

The dilute aquifer waters are close to wollastonite saturation whereas the saline wa-ters are signi�cantly under-saturated. As for calcite, the wollastonite under-saturationin the saline aquifer �uids is attributed to low aqueous Ca+2/(H+)2 activity ratios.

2.11.2 Andradite-grossular, epidote-clinozoisite, prehnite

A large scatter in SI values is observed for the endmembers of all these solid solutionminerals. Part of the large scatter is the consequence of the stoichiometry of the respec-tive mineral-solution reactions. As an example, in the garnets, H+ is raised to the power4, which ampli�es in the same proportion the uncertainties on pH values. The resultsfor the Fe endmembers of the garnet and epidote indicate systematic over-saturation.This is not considered real, but due to over-estimation of the Fe(OH)−4 species activitydue to faulty thermodynamic data on iron hydrolysis constants and possibly also in theselected thermodynamic properties of Fe(OH)−4 as given by Diakonov et al. (1999).

The saturation indices for all the Al-endmembers, grossular, clinozoisite and Al-prehnite show an apparent over-saturation of the aquifer �uids with respect to thesephases. As for Fe-bearing minerals, one possible cause may be faulty thermodynamicdata on Al-species and in particular on the Al-Si dimer. Ca+2 removal by calciteprecipitation from the �owing �uid may also be involved if insu�cient time is allowedfor re-equilibration.

32

CHAPTER 2.

2.11.3 Magnetite, pyrite and pyrrhotite

The over-estimation of the Fe(OH)−4 due to faulty thermodynamic data leads to anunder-estimation of the Fe2+ species activity. This has an in�uence on the equilibriumcalculations of all Fe2+ bearing minerals, i.e. pyrite and pyrrhotite and magnetite,which all show a similar pattern of under-saturation, with more negative SI values athigher temperature (Fig. 2.6).

2.11.4 N2 and Ar

In vapor samples, N2/Ar molal ratios range between 18.4 and 92.1 (Table 2.2). In air-saturated water (ASW) and seawater (ASS) at 5 ◦C the N2/Ar ratios are 37.2 and 39.4,respectively. If two samples are exempted (sample 04-3007 from well 12 at Námafjalland sample 04-3050 from well 20 at Nesjavellir), N2/Ar ratios range between 37.0 and82.2 or between that of air-saturated water and air (Fig. 2.8). A possible explanationof these ratios is that the initial aquifer �uid is represented by ASW containing minoramount of air bubbles. However, to produce a N2/Ar ratio of 50, requires additionof 1.3 mmol of N2 from air to 1 kg of water and correspondingly 0.026 mmol for Ar.Fig. 2.9 shows the e�ect of addition of air bubbles to air-saturated water. As the Arconcentration increases, the N2/Ar ratio approaches that of air. On the other hand,boiling of ASW and of a selected mixture of ASW and air with a N2/Ar ratio of 70produces higher N2/Ar ratios, for a given Ar concentration, than allowed by air additiononly. Two di�erent types of boiling are shown. One that is a closed system boiling, wherethe �uid is allowed to equilibrate with the total amount of steam at once, and an open

system boiling, which is in fact a multi-step boiling where the steam formed at eachstep of temperature drop (0.01 ◦C in this case) is removed before the next fraction ofsteam is formed. The latter model produces faster degassing of the aquifer �uid for agiven total mass fraction of steam. On the other hand, Fig. 2.9 shows that this boilingmodel yields higher N2/Ar ratios for a given Ar concentration.

The initial aquifer �uid concentrations for N2 and Ar have been calculated for allthe wells considered for the present study. For wells with discharge enthalpies close toliquid enthalpy (Table 2.1) the total well discharge composition was taken to representthe initial aquifer liquid composition. For excess enthalpy wells, two models were se-lected to calculate N2 and Ar concentrations in the initial aquifer �uid. One of themodels is the same as that used to calculate reactive gases in initial aquifer �uid, thesegregation model. The other model allows for addition of gas-free steam to the �uid�owing into wells (see Arnórsson et al., 2007, model 4). It is envisaged that evaporationof capillary water in the formation may add steam to the �owing �uid. Since all thewells considered for the present study have been producing for many years, it is con-sidered reasonable that this capillary water has already been largely degassed. Model4 yields higher N2 and Ar concentrations in the initial aquifer �uid because it assumesdilution by addition of gas-free steam to the �owing �uid. The magnitude of this dilu-tion is directly related to the amount of steam formed by vaporization of capillary water.

The average aquifer Ar concentrations in the dilute �uids producing into liquidenthalpy wells is 0.015 mmol/kg, the range being 0.002 to 0.028 mmol/kg. Two wells

33

2.11. INDIVIDUAL MINERAL-SOLUTION EQUILIBRIA

Figure 2.8: Argon versus N2 in initial aquifer �uids for all �elds. Also shownare air-saturated water (ASW, Fernández-Prini et al., 2003) and air-saturatedseawater (ASS, Weiss, 1970) at 5 ◦C, as well as ASW ratio and air ratio.

Figure 2.9: In�uence of air addition and boiling on the N2/Ar ratio versus Arconcentration in aquifer �uid. Continuous line shows addition of air bubblesto air-saturated water (ASW), broken and dotted lines show e�ect of boiling ina closed system and an open system, respectively, on the N2/Ar ratio and Arconcentration in the boiled liquid (see text). Samples calculated in this studyare also shown. Symbols for various areas have the same meaning as on Fig. 2.8,open and full symbols showing liquid and excess enthalpy wells, respectively.

34

CHAPTER 2.

are very low in Ar, well 21 at Kra�a and well 14 at Nesjavellir (Table 2.4). These twowells are marginal to their respective well �elds and may represent the degassed �uid�owing away from the major up�ow. If these two samples are exempted, the averageAr concentration is 0.019 mmol/kg, which is that of air-saturated water at 5 ◦C, whichis about the average annual air temperature at Nesjavellir. The saline aquifer water atSvartsengi is signi�cantly degassed with respect to Ar, due to its boiling and formationof a steam cap on the top of the liquid reservoir, the average being 0.010 mmol/kg.The geothermal seawater in producing aquifers at Reykjanes contains 0.015 mmol/kgof Ar, which corresponds very well with air-saturated seawater at 5 ◦C. The segregationmodel yields Ar aquifer �uid concentrations which are, with one exception, below thatof air-saturated water, the average being 0.009 mmol/kg. By contrast, the model thatallows addition of gas-free steam to the �owing �uid gives on average 0.021 mmol/kg,the range being 0.003 to 0.063 mmol/kg.

In view of the modelling of aquifer �uid Ar concentrations, it seems very likely thatthe combination of several processes explains the observed N2/Ar ratios (Fig. 2.9). Pos-sible processes causing the elevated ratios, above air-saturated water, are air enrichedsource water, boiling, contribution of N2 from the magma heat source or possibly de-caying organic matter in paleosoils. Air contamination during sampling may also con-tribute, however in this case Ar concentrations would likely be higher than the observedvalues.

2.12 Discussion and conclusions

Wells drilled into liquid dominated geothermal reservoirs sometimes have liquid en-thalpy but commonly excess enthalpy. The selection of a model to calculate the initialaquifer �uid composition is likely one of the major sources of error in calculating theaquifer �uid compositions from wellhead data on excess enthalpy wells. For wells withliquid enthalpy it is considered to be a reasonable assumption to take total well dis-charge composition to represent the initial aquifer �uid composition. Excess enthalpymay be caused by phase segregation in producing aquifers or conductive heat transferfrom the aquifer rock to the �uid �owing into wells. Aquifer �uid compositions cal-culated for excess enthalpy well discharges applying the phase segregation model yieldresults consistent with compositions calculated from liquid enthalpy wells when assum-ing the total discharge composition of the latter to be equal to the total aquifer �uidcomposition. The segregation model also yields consistent results for the quartz andNa/K geothermometers. It is therefore concluded that the excess enthalpy of the wellsconsidered for the present study is for the most part produced by phase segregationalthough conductive heat �ow from aquifer rock may also contribute.

The composition of a well discharge may di�er from that of the initial aquifer �uidnot only as a consequence of phase segregation but also by precipitation or dissolutionreactions that can occur when extensive boiling causes changes in the chemical prop-erties of the �uid but also by reaction with casing and wellhead material. Elementspresent in low concentrations in the �uid, which also are chemically reactive, are sub-ject to the largest concentration changes between undisturbed aquifer and wellhead.They include Ca in dilute waters, Fe and possibly Al, along with many trace elements.Even if equilibria are upset between solution and individual mineral in assemblages that

35

2.12. DISCUSSION AND CONCLUSIONS

control the concentrations of components present in high concentrations in the �uid,such as the major gases (CO2, H2S and H2), the well discharge composition will re�ectthe correct concentrations of such components in the initial aquifer �uid.

Calculation of initial aquifer �uid composition is only the �rst step in assessing thestate of equilibrium between particular species in the aqueous phase and minerals. Thenext step involves calculation of speciation distribution in the aquifer �uid to obtainactivity products (Q) for speci�c mineral-solution reactions and the �nal step is tocalculate equilibrium constants for these reactions from thermodynamic data. It is notpossible to quantitatively assess the overall error. Yet, is is evident that the cumulativeerror on the minerals alone is greater that the departure from equilibrium of aqueousH2S with the mineral assemblage pyrite, pyrrhotite, epidote and prehnite in the caseof the dilute waters. Calculation of the equilibrium constant for this reaction assumedpure FeS and FeS2. If the FeS is de�cient in iron, the equilibrium curve is shifted tolower values. The presence of S−2 in FeS2 has the same e�ect. Sul�de mineral analysis ismissing for quantitative estimation of the e�ect of the iron-sul�de mineral compositionon the equilibrium constant. Secondly, the uncertainties on the thermodynamic dataon these mineral, as given by Robie and Hemingway (1995) may produce an error ashigh as 0.3 log mol/kg on the equilibrium H2S concentration, in addition to the errorproduced by the silicate minerals, epidote and prehnite.

In the saline waters at Svartsengi and Reykjanes, H2S concentrations in the aquifer�uids match well equilibrium with the mineral assemblage containing pyrite, pyrrhotite,quartz, garnet, epidote and wollastonite.

Aquifer �uid CO2 concentrations in the dilute �uids show larger variation than thoseof H2S. The CO2 content of the Nesjavellir aquifer �uids is low relative to equilibriumwith the clinozoisite+prehnite+calcite+quartz assemblage. The selected activity of cli-nozoisite has a strong in�uence on the value taken by the equilibrium constant because ofthe sensitivity of the clinozoisite activity to the composition of the epidote. The selectedclinozoisite activity of 0.1 is within the measured clinozoisite composition in this area,or even 0.05 which brings the equilibrium curve close to the calculated aquifer �uid CO2

concentration. On the other hand, low CO2 concentrations in the aquifer �uid couldbe the consequence of insu�cient supply of CO2 to the geothermal system from themagma heat source. At Námafjall and Kra�a, CO2 aquifer �uid concentrations matchgenerally well equilibrium with the clinozoisite+prehnite+calcite+quartz assemblage.In a few instances it is higher due to rapid degassing of magma intruded into the rootsof the geothermal system in the period 1975�84. At Reykjanes and Svartsengi, equilib-rium with �uid CO2 and clinozoisite+calcite+quartz+grossular rather than prehnite isclosely approached. Grossular garnet is abundant in the reservoir rock in these �eldsbut prehnite is rare. Production from the Svartsengi reservoir has led to decreasingaquifer �uid CO2 concentrations in some of the wells, whereas two wells have excessiveCO2.

The same mineral assemblage appears to control H2 and H2S aquifer �uid concen-trations in the saline �uid. Most of the wells at Kra�a, Námafjall and Nesjavellir havehigher H2 aquifer �uid concentrations than expected at equilibrium with the mineralassemblage involving H2S. The �uid concentrations of the gases were taken to rep-resent liquid concentrations, when calculating the respective mineral-gas equilibrium

36

CHAPTER 2.

constants. The apparent high aquifer liquid H2 concentrations are considered to re�ectthe presence of equilibrium vapor in the initial aquifer �uid. This vapor has a cleare�ect on H2 because it is sparingly soluble in water but not on the more soluble CO2

and H2S. The excess H2 indicates an equilibrium vapor fraction that varies between0.04 and 4% by weight.

Although the initial aquifer liquid concentrations of the reactive gases CO2, H2Sand H2 seem to be generally controlled by close approach to equilibrium with speci�cmineral bu�ers, the same does not appear to be the case for many of the minerals whichform parts of these assemblages. This apparent discrepancy is considered to be partlydue to changes in minor element concentrations in the �uid between aquifer and well-head due to precipitation reactions. In the case of Fe-bearing minerals it is also due tofaulty data on iron hydrolysis constants leading to under-estimation of the Fe+2 speciesbut over-estimation of Fe(OH)−4 . The thermodynamic data selected for Fe(OH)−4 toretrieve equilibrium constants may also contribute.

Nitrogen and Ar concentrations in the initial aquifer �uid were calculated accordingto a segregation model and model 4 in Arnórsson et al. (2007) which assumes thatthe excess enthalpy is due both phase segregation and addition of gas-free steam byevaporation of capillary water. The two models yield almost the same N2/Ar ratios,but they di�er in Ar and N2 concentrations, the latter giving higher gas concentrationsin the initial aquifer �uid. Our results show large variation of N2 and Ar concentrationsand N2/Ar ratios within �elds. Most of the well discharges are depleted in Ar, indicatingthat the initial aquifer �uid is represented by degassed �uid. The observed N2/Ar ratioscannot be produced only by boiling of air-saturated water. Boiling of such water causesdecrease in the N2/Ar ratio of the boiled liquid. Early formed vapor that may mixwith �uid at higher levels in the reservoir can only produce N2/Ar ratios up to around50. Liquid N2/Ar ratios of up to 50�60 can be produced by boiling of air-saturatedwater containing entrapped air. However, higher ratios can only be explained by anadditional source of N2 to the �uid, either magmatic or organic.

37

Chapter 3

Processes in�uencing As, B and Clconcentrations in �uids of volcanicgeothermal systems in Iceland

Giroud, N. and Arnórsson, S.

Abstract

The concentrations of arsenic, boron and chloride are relatively low in �uids of high-temperature geothermal systems in Iceland. In systems, where the convecting �uidis meteoric by origin, As, B and Cl contents are typically 50�150 ppb, 0.5�5 ppmand 50�150 ppm, respectively. The reason is low concentrations of these elementsin the basaltic volcanics of the country. In the seawater systems of Reykjanes andSvartsengi, As concentration levels are about the same as in the dilute �uid systems.By contrast, B concentrations are substantially higher and those of Cl much higher duetheir presence in the source seawater �uid. Boron and Cl act as incompatible in thesegeothermal systems. Arsenic in the deep �uid is also highly mobile. During boilingand mixing of the geothermal water with cold groundwater, As is e�ectively removedfrom solution, probably mostly by its uptake into ferri-hydroxides and clay minerals butalso into sulphide minerals, particularly when the liquid boils extensively. In hot springwaters of mature high-temperature systems, representing boiled deep water, Cl/B andCl/As ratios are about the same as those of basalt indicating near stoichiometric basaltdissolution with respect to these elements. Arsenic does not partition signi�cantly intothe vapor phase, at least at vapor pressures as high as 40 bar, but B does. From availableexperimental data on the distribution coe�cient for B between liquid and vapor, it isconcluded that equilibrium distribution is closely approached at wellheads of wet-steamwells. In the high-temperature systems of Námafjall and Kra�a, B and Cl contents of the�uid indicates that it is determined by the mixing of components from two sources. One

39

3.1. INTRODUCTION

is the basaltic rock with which the �uid has interacted and the other magma volatiles.The rock interaction component represents �uid that has reacted to a relatively limitedextent. It is similar to the sub-boiling liquid in the upper Kra�a reservoir. The Cl and Bcontent of the magmatic component is some 200 and 30 ppm, respectively. Fresh magmawas intruded into the roots of the Kra�a geothermal systems during a volcanic episodein 1975�84 and magma from Kra�a �owed along �ssures into the Námafjall geothermalsystem. A substantial fraction of the B and Cl in the geothermal �uid of the Kra�aand Námafjall systems is derived from the magmatic source. At Nesjavellir similar kindof mixing explains the B and Cl content of the �uid. However, the rock interactioncomponent is represented by quite extensively reacted water. Here little Cl appears tocome from the magma but a substantial fraction of the B. The well�eld at Nesjavellir isadjacent to a volcanic �ssure that erupted ∼2000 years ago. By comparison of data fromthe seawater systems at Reykjanes and Svartsengi with the meteoric-water systems, itseems likely that a substantial fraction of the B in the �uid of these seawater systemsis of magmatic origin. The mobility of As is signi�cantly lower than those of Cl andB in aquifer �uids producing into wet-steam wells. This is considered to be due to itsremoval from solution into sulphide minerals. Their formation is most intense in zonesof extensive boiling. Magmatic volatiles contribute little if any As to the geothermal�uid.

3.1 Introduction

Arsenic, boron and chloride are important components in geothermal �uids. Both Asand B are environmentally important (Smedley and Kinniburgh, 2002; Axtmann, 1975;Badruk and Kabay, 2003; Arnórsson, 2004). Arsenic is one of the most poisonous andcarcinogenic element found in natural waters and elevated B concentrations in suchwaters may have adverse e�ects on the growth of many types of plants (Cengiz, 2007).Boron and Cl provide important information on the source �uid to geothermal sys-tems and various processes occurring within such systems including mixing and boiling(Giggenbach, 1991; Arnórsson, 1985; Ellis and Mahon, 1977). All these elements arehighly mobile in geothermal systems. Chloride is generally taken to be conservative,i.e. once in solution it stays there (Giggenbach, 1991). Boron may also be conservative,depending on the type of minerals that form from solution and take up B. Arnórs-son and Andrésdóttir (1995) concluded that B essentially behaves as a conservativeelement in the basaltic environment of Iceland. Due to its correlation with Cl, As ingeothermal systems has also been taken to be conservative although closer examinationshows that it is not. Arsenic can be removed from solution by its uptake into sulphideminerals and iron hydroxides (Ballantyne and Moore, 1988; Smedley and Kinniburgh,2002). In common types of volcanic rocks all these elements are present to some extentin easily soluble compounds. Thus hydrothermal experiments have demonstrated thatthey are largely transferred into solution without appreciable alteration of the rock-forming minerals (Ellis and Mahon, 1964, 1967). Both As and B form volatile speciesat elevated temperatures (Kuritani and Nakamura, 2006; Glover, 1988; Ballantyne andMoore, 1988).

The only types of minerals that e�ectively extract B from solution are clays, in par-ticular illite (Harder, 1969). Such minerals may also incorporate As (Onishi, 1978) al-

40

CHAPTER 3.

though sulphides and ferri-hydroxides are more important (Bamford et al., 1980; Chris-tensen et al., 1983). The ratios of Cl/B and Cl/As in seawater are much higher thanin common types of igneous and metamorphic rocks due to uptake of As and B intomarine sediments.

The concentrations of As, B and Cl in oceanic basalts formed at diverging plateboundaries, such as Iceland, are much lower than in volcanic rocks formed on convergingplate boundaries (Arnórsson, 2002; Arnórsson, 2003; Arnórsson and Andrésdóttir, 1995;Sigvaldason and Óskarsson, 1976; Ellis and Mahon, 1964, 1967). The same applies to�uids in volcanic geothermal systems. Thus B concentrations of less than a few ppm aretypical for �uids of volcanic geothermal systems in Iceland whereas they may be as highas 100 ppm in such systems by converging plate boundaries (Ellis and Mahon, 1977).Corresponding numbers for As are <0.1 ppm and as much as 50 ppm, respectively(Arnórsson, 2003; Ballantyne and Moore, 1988). It is likely that these elements areadded to andesitic and related magmas that form by partial melting of hydrated mantlerock, by transfer of volatile compounds rising from the heated underlying slab of marinesediments. Consequently rocks formed from this magma are rich in these elements aswell as �uids of geothermal systems forming over magmatic intrusions in this geologicalenvironment.

Present interest to enhance development of geothermal resources to reduce emis-sion of greenhouse gases into the atmosphere by fossil fuel combustion requires carefulassessment of the long-term environmental impact of such development. One aspectof this is atmospheric and surface and groundwater contamination with respect to Clbut in particular B and As as well as an understanding of the geochemistry of theseelements in volcanic and other types of geothermal systems.

The present study focuses on the distribution of As, B and Cl in �uids of ninevolcanic geothermal systems in Iceland. Almost all known volcanic geothermal systemsat diverging plate boundaries are sub-marine except those of Iceland. The �uids of theIcelandic systems provide information of value for the understanding of the processesdetermining the content of these elements not only in basaltic environment at divergingplate boundaries but they also throw some light on the global cycle of these elementsand therefore also their distribution in volcanic rocks and associated �uids by convergingplate boundaries.

3.2 Geological features

3.2.1 Overview

High-temperature volcanic geothermal systems in Iceland are located within the activebelts of volcanism and rifting (Fig. 3.1). These volcanic belts represent the boundaryof the American and European lithospheric plates. Yet, between them in South Icelandthere is a triangular micro-plate, the Hreppar plate. The high-temperature systems thatare marginal to the active volcanic belts (see Fig. 3.1) are most likely mature and in theprocess of cooling down as they drift out of these belts and are cut from their magmaticheat source. The high-temperature geothermal systems may be divided into groupsdepending on their geological setting and the nature of their heat source. This heat

41

3.2. GEOLOGICAL FEATURES

Figure 3.1: Map of Iceland showing the active volcanic belt systems and high-temperature areas.

source may be a sheeted dyke complex, high-level sills, cone-sheets, irregularly shapedsmall intrusive bodies and more deep-seated stocks in the roots of a major volcaniccomplex, typically associated with calderas. Parasitic systems to volcanic complexesare also known, where magma has been intruded laterally along �ssures from a majormagma body below the central volcano, such as Kra�a (Björnsson et al., 1977). Dueto crustal accretion at the plate boundary, geothermal systems initially on the plateboundary will drift with time out of the volcanic belts and become displaced from themagmatic heat source. In this way high-temperature geothermal systems can developinto low-temperature systems.

For the present study, geothermal �uids from springs and wells were collected fromnine active high-temperature areas. They are Kra�a, Námafjall, Landmannalaugar,Nesjavellir, Hveragerdi, Hveravellir, Geysir area, Svartsengi and Reykjanes. Further,several samples were collected from wells in the Southern Lowlands low-temperaturearea. The location of these areas is shown on Fig 3.1. Námafjall is considered to bea parasitic system to the Kra�a area. Nesjavellir and Hveragerdi are separate parts ofthe larger Hengill area. Below a brief description will be given of the geology of eacharea.

3.2.2 Kra�a and Námafjall

The Kra�a geothermal �eld in northern Iceland lies astride the plate boundary and islocated within the caldera of the Kra�a central volcanic complex (Stefánsson, 1981).The Námafjall geothermal area lies outside this caldera, about 10 km to the south of thehill of Leirhnjúkur which is located in the center of the Kra�a caldera. An active �ssureswarm runs through the Kra�a caldera. The Námafjall system lies across the eastern

42

CHAPTER 3.

part of this �ssure swarm (Saemundsson, 1991) which is about 100 km long and 5�8 km wide (Saemundsson, 1974, 1978, 1983). The volcanic rocks in the area are mostlybasaltic in composition although silicic magma has also been erupted (Jónasson, 1994).Basaltic and silicic eruptions have occurred in sub-glacial and Recent times in the area,both within and outside the Kra�a central volcano (Saemundsson, 1991). Temperaturesin deep drillholes (∼2000 m) at Kra�a and Námafjall are as high as 350 ◦C and 320 ◦C,respectively (Ármannsson et al., 1987).

A major volcanic-rifting episode started on the Kra�a �ssure swarm at the end of1975. A total of nine volcanic eruptions took place during this episode that lasted untilSeptember 1984. Fresh magma was intruded into the roots of the Kra�a volcano form-ing chambers at 3�7 km depth (Einarsson, 1978). Periodic discharge of magma fromthis chamber occurred into the �ssure swarm to the north or south. On one occasion,a geothermal well at Námafjall erupted 3 m3 of magma (Larsen et al., 1978). The con-tent of some common geothermal gases (CO2 and H2) rose in well �uids and fumarolesat Kra�a subsequent to the intrusion of the fresh magma but not at Námafjall (Ár-mannsson et al., 1982). These observations indicate that other components, which formfugitive compounds at magmatic temperatures, may have been added to the convectinggeothermal �uid at Kra�a, e.g. As, B and Cl.

3.2.3 Landmannalaugar

The Landmannalaugar �eld is part of the greater Torfajökull area in central southIceland which is located at the plate boundary within the eastern active volcanic beltin South Iceland. Torfajökull hosts the largest geothermal area in the country, coveringsome 200�300 km2. No drillings have been carried out in this area. Alkaline springwaters, representing the boiled fraction of the deep �uid in the geothermal system, areonly found in the Landmannalaugar part of the Torfajökull area. In most of the area,surface manifestations are characterized by fumaroles, steam-heated surface waters andhot altered ground. Geochemical geothermometers indicate temperatures as high as300 ◦C in the Landmannalaugar �eld. At Landmannalaugar mixed water occurs inwarm and hot springs (Arnórsson, 1985).

The Torfajökull region constitutes the largest complex of silicic volcanics in Iceland.Within the Landmannalaugar �eld only silicic volcanics are exposed. They include rhy-olites and comendites as well as subglacially formed pitchstone and obsidian (Ívarsson,1992). It is considered that almost all the exposed volcanics formed by sub-glacialeruptions during late Quaternary times, probably during the last glaciation (Saemu-ndsson, 1972). A large caldera structure characterizes the Torfajökull region, whichenvelopes practically all the thermal manifestations. The silicic formations produce aconspicuous gravity low within the area (Pálmason, 1973). A distinct positive gravityanomaly occurs within gravity low taken to re�ect the existence of basaltic intrusives(Walker, 1974). Likely, the basaltic magma has been trapped under the less dense sili-cic volcanics forming the heat source to the geothermal system. By this model, it isexpected that the chemical composition of the boiled hot spring water at Landmanna-laugar is characterized by interaction with silicic rocks although volatile componentsfrom the basaltic magma heat source may in�uence the gas chemistry of the �uid inthe geothermal system.

43

3.2. GEOLOGICAL FEATURES

3.2.4 Geysir area and Hveravellir

These areas are described together because their surface manifestations have manythings in common. Both areas are quite small and marginal to the active volcanic belts(Arnórsson, 1985). Most of the geothermal activity in both �elds occurs within an areathat is only a few hundred meters across. It consists of hot springs with boiled deepwater. Some of these springs show geyser activity. Warm springs occur in both areasthat represent a mixture of the geothermal water and cold groundwater. At Geysir,the mixing occurs at depth, i.e. before the rising hot water becomes degassed as aconsequence of boiling. At Hveravellir, on the other hand, the mixing occurs close tothe surface between boiled and degassed water and cold groundwater (Arnórsson, 1985).

The heat output in both areas is apparently low as deduced from the integrated�ow from springs. The rocks in the vicinity of the Hveravellir �eld are all basaltic.At Geysir, on the other hand, rhyolitic rocks outcrop although most of the bedrockconsists of basalts. The main �uid up�ow feeding the hot springs at Geysir is consideredto be along the contact of a rhyolite plug. The spring water at Geysir indeed re�ectsreaction with silicic rocks, such as high �uoride and relatively high concentrations of Cland B compared to thermal waters that have interacted with basalt only. Undergroundtemperatures in these area have been estimated as ∼250 ◦C or a little higher (Arnórsson,1985).

Deposits of silica sinter are extensive around the hot springs in both areas. It isconsidered likely that such deposits cover fracture surfaces in up�ow channels below thehot springs limiting interaction between water and rock and thus impeding changes inthe chemical composition of the rising �uid. By contrast, this is not the case for themixed waters, particularly when the mixing involves a gaseous hot water component.Such mixing leads to the formation of acidic water that is highly reactive (Arnórsson,1985).

3.2.5 Hveragerdi

The Hveragerdi �eld is located at the southwestern boundary of the Hengill area. Drill-hole data reveal a temperature of 180�230 ◦C (Arnórsson and Gunnlaugsson, 1985). Hotsprings are abundant in the area, as well as fumaroles and hot altered ground. Silica sin-ter deposits are limited. The area is tectonically active although Hveragerdi lies outsidethe main zone of rifting. In valleys intersecting the hills north of Hveragerdi hydrother-mally altered rocks are exposed containing minerals indicative of temperatures in excessof 200 ◦C. Alteration in rocks at depth penetrated by wells in Hveragerdi is intensewith minerals indicating temperatures higher than those measured today (Sigvaldason,1963). Either the system has cooled down or erosion has brought high-temperaturehydrothermal minerals closer to the surface. Both explanations are consistent with thegeological location of the �eld that indicates that it is mature and is in the process ofdrifting out of the active volcanic belt. Possibly the Hveragerdi system has drifted asidefrom its original magmatic heat source.

44

CHAPTER 3.

3.2.6 Nesjavellir

The Nesjavellir �eld forms the northeasternmost part of the Hengill area in SouthwestIceland. It lies astride the plate boundary and is located within a tectonically activegraben. A volcanic �ssure that erupted ∼2000 years ago forms the northwest side ofthe graben (Saemundsson, 1963). Permeability is fracture controlled (Bödvarsson et al.,1990). Faults running through the area can be traced through the Hengill mountainousarea and towards northeast across the bottom of Lake Thingvallavatn and farther north.The reservoir �uid is two-phase (liquid and vapor), at least down to the depths of thedeepest wells in the area (∼2200 m). The highest temperature recorded is > 380 ◦C(the maximum measurable by the instrument) in a well drilled close to the volcanic�ssure. This high temperature is perhaps related to proximity to a magmatic intrusionbelow the 2000 year old eruptive �ssure.

The geological formations at Nesjavellir above 600 m depth consist primarily ofbasaltic hyaloclastites and basaltic lavas with the latter becoming more abundant below600 m (Franzson et al., 1986). Intrusive bodies also increase with depth and exceed 50%of the volume of the rock below 1500 m (Franzson, 1988). Rare granophyric bodies havebeen identi�ed within the intrusive complex.

3.2.7 Reykjanes and Svartsengi

The Svartsengi and Reykjanes �elds lie astride the divergent plate boundary where itis intersected by active �ssure swarms that form an angle of about 50◦ to the plateboundary (Lonker et al., 1993). Sheeted dyke complexes are considered to serve asthe magmatic heat source for the Reykjanes and Svartsengi geothermal systems, al-though sills may also be involved. The rock exposed at the surface and penetrated bywells is solely basaltic, subglacially erupted hyaloclastites, breccias and pillow lavas andtu�aceous sediments as well as lava �ows formed during inter-glacial periods (Fridleifs-son and Albertsson, 2000; Tómasson and Kristmannsdóttir, 1972). Intrusions becomeincreasingly abundant with depth, being more than 60 and 80% of the rock below1500 m depth at Svartsengi and Reykjanes, respectively. Both �elds have been exten-sively drilled. Permeability is fracture controlled. In the deep geothermal reservoir atSvartsengi temperatures vary very little from 500 to 2000 m, being about 240 ◦C. AtReykjanes, the reservoir is two-phase above about 1000 m but sub-boiling at greaterdepths. The highest temperature recorded is about 320 ◦C at 2000 m depth.

3.3 Fluid compositions

In seven of the nine high-temperature �elds sampled the waters are meteoric by originand low in dissolved solids compared to most high-temperature geothermal waters inthe world. This is the consequence of the low Cl content of the basalts with which thewaters have interacted (Arnórsson and Andrésdóttir, 1995). The higher Cl content ofwaters from the Landmannalaugar and Geysir �elds is due to interaction of the waterwith silicic rocks. Data on δ2H for these water are similar to that of local precipitationor more negative. When more negative, the source water may be distantly derived

45

3.4. SAMPLING AND ANALYTICAL METHODS

precipitation from higher elevation and further inland or they contain a component orPre-Holocene water (Arnórsson and Andrésdóttir, 1995; Arnórsson, 1995).

In two areas, Reykjanes and Svartsengi, the geothermal �uid is relatively salineattributable to seawater recharge to the systems (Björnsson et al., 1972; Arnórsson,1978; Ragnarsdóttir et al., 1984) The concentrations of the unreactive Cl of the deepunboiled Reykjanes geothermal �uid is ∼100% seawater (Lonker et al., 1993), measuredbetween ∼19,500 and 23,500 ppm (Arnórsson, 1995; Stefánsson and Arnórsson, 2002;Arnórsson, 1978). At Svartsengi Cl concentrations correspond to about 2/3 seawaterrecharges and 1/3 meteoric water (Arnórsson, 1978; Ragnarsdóttir et al., 1984).

3.4 Sampling and analytical methods

Only a brief description of sampling and analytical techniques is given here. For a moredetailed description, the reader is referred to Arnórsson et al. (2006).

In the case of wet-steam wells, water and steam samples were taken from two phasepipeline at the wellhead using a stainless steel (N316) Webre separator to separate thephases. Water samples at wells, as well as boiling hot springs, were �ltered through a0.2 µm cellulose acetate membrane using a polypropylene or Te�on �lter holder and col-lected into low-density polyethylene bottles. An untreated 200 ml sample was analyzedon a Reagent-FreeTMion-chromatograph (RFICTM, Dionex 2000) for F, Cl and SO4. A200 ml sample was acidi�ed with 1 ml Suprapur R© HNO3 (E. Merck 100441) for majorelement analyses on an Inductively Coupled Plasma - Atomic Emission Spectrometer(ICP-AES) (Al, Ca, Cl, Fe, Mg, Na, Si, Sr). Another 100 ml sample acidi�ed with 1 mlof the same acid was used for trace elements determination on an Inductively CoupledPlasma - Mass spectrometer (ICP-MS), including As and B.

Glass bottles with air-tight cap were used for collecting samples for pH and totalcarbonate carbon determination in the laboratory shortly after sampling. Hydrogensul�de was measured on site by titration with standard mercuric acetate solution, usingdithizone as endpoint indicator (Arnórsson et al., 2006). Total carbonate carbon wasdetermined by titration with standard (0.1 M) HCl solution as described by Arnórssonet al. (2006).

Steam phase was collected into evacuated Giggenbach bottles (see for exampleArnórsson et al., 2006) containing 10 ml of 50% w/v KOH solution and condensedsteam was collected into 100 ml polyethylene bottles and acidi�ed with Suprapur R©HNO3 for ICP-MS analysis in a manner similar to that for trace element samples of thewater phase. Carbon dioxide and H2S, which dissolved quantitatively in the alkalinesolution, were determined as in water samples. Non-condensable gases (H2, N2, CH4,Ar) were analyzed by gas chromatography.

3.5 Data handling

3.5.1 Calculation of aquifer �uid composition

Deep aquifer �uid composition for wells and boiling hot springs was calculated usingthe WATCH speciation program (Arnórsson et al., 1982), version 2.3 (Bjarnason, 1994).

46

CHAPTER 3.

Figure 3.2: Phase segregation excess enthalpy model (modi�ed from Arnórssonet al., 2007)

Some of the wet-steam wells included in the present study have liquid enthalpy, i.e. thedischarge enthalpy equals that of steam saturated water at the aquifer temperature.Other wells have �excess� enthalpy, i.e. the enthalpy of the discharged �uid is higher thanthe enthalpy of steam-saturated water at the aquifer temperature. All the geothermalsystems considered here are hot-water (liquid dominated) systems (see White et al.,1971) implying that the aquifer �uid is liquid water only or liquid water with a smallvapor fraction, even in terms of volume, so the enthalpy of the initial aquifer �uidis equal to or only slightly higher than that of steam saturated water at the aquifertemperature.

Excess well discharge enthalpy is the consequence of increase in the �owing �uidenthalpy in the depressurization zone around discharging wells. Depressurization boilingin this zone lowers the temperature of the �uid, thus creating a temperature gradientbetween �uid and aquifer rock and favoring conductive transfer of heat from rock to�uid. Addition of heat to the two-phase �uid will not a�ect its temperature but enhanceboiling. Flowing liquid and vapor may separate, at least partly (segregate), in two-phaseaquifers, leading to an increase in the steam to water ratio (enthalpy) of the discharge.Such separation results from the di�erent �ow properties of vapor and liquid, the e�ectsof capillary pressure (adhesive forces between mineral grain surfaces and liquid) andrelative permeability (e.g. Horne et al., 2000; Pruess, 2002; Li and Horne, 2004). Bythese processes the mobility of liquid is reduced relative to that of vapor (Fig. 3.2).

In this contribution it is assumed that �excess� well discharge enthalpy is producedby phase segregation. When this model is selected, conformity between the quartzand the Na/K geothermometers is good. If, on the other hand, it was assumed thatthe excess enthalpy was caused by conductive heat transfer from aquifer rock to the�owing �uid, low quartz equilibrium temperatures are obtained, particularly when the

47

3.5. DATA HANDLING

discharge enthalpy is approaching that of dry steam.The steam fraction in the initial aquifer �uid has been taken to be zero. This

steam fraction can be obtained from data on the concentrations of two gases in thetotal discharge (preferably H2 and H2S) and by assuming that their concentrations inthe initial aquifer liquid (mf,l

i ) is controlled by a speci�c mineral bu�er (see Arnórssonet al., 1990, 2007). Studies indicate that the steam fraction in the initial aquifer �uidin hot-water volcanic geothermal systems is small, even in terms of volume, such as atKra�a, Iceland (Gudmundsson and Arnórsson, 2002) and Olkaria, Kenya (Karingithi,2002).

When phase segregation occurs in producing aquifers, the mass �ow rate of the welldischarge is given by

Md,t = Mf,t −Me,l (3.1)

whereM denotes mass �ow rate. The superscripts d and f designate well discharge andinitial aquifer �uid of feed zone, respectively. Superscript e stands for the intermediatezone between f and d where phase segregation occurs. The second superscripts indicatethe �uid phase. Thus, l means liquid and t refers to total �ow. From conservation ofenthalpy and mass we further have

hd,t ·Md,t = hf,t ·Mf,t − he,l ·Me,l (3.2)

andmd,t

i ·Md,t = mf,ti ·Mf,t −me,l

i ·Me,l (3.3)

where h denotes speci�c enthalpy and mi concentration of the i-th component. Su-perscripts have the same notation as in equation (3.1). Dividing with Md,t throughequations (3.1) to (3.3) leads to

1 = V f,t − V e,l (3.4)

hd,t = hf,t · V f,t − he,l · V e,l (3.5)

md,ti = mf,t

i · V f,t −me,li · V e,l (3.6)

Common solution of equations (3.4) and (3.5) and isolation of V f,t yields

V f,t =hd,t − he,l

hf,t − he,l(3.7)

By selecting a value for he,l (the enthalpy of liquid water at which phase segregationoccurs), the value of V f,t can be obtained from equation (3.7) since hd,t is a measuredvalue, hf,t is known when a value for the aquifer temperature has been selected, becauseit is assumed that the steam fraction in the initial aquifer �uid is zero, so hf,t = hf,l.Steam Tables give hf,l for steam saturated water.

Combination of equation (3.4) and (3.6) by elimination of V e,l leads to

md,ti = mf,t

i · V f,t −me,li (V f,t − 1) (3.8)

The concentrations of individual analyzed components in the deep unboiled aquifer�uid can be obtained from analytical data on liquid and vapor samples collected at thesurface with the aid of equation (3.8) and the distribution coe�cient Ds for species

48

CHAPTER 3.

between the liquid and vapor phases at the temperature at which phase segregation istaken to occur:

Ds =ne,v

s

ne,ls

(3.9)

where Ds represents the distribution coe�cient for species s, superscript v stands forvapor and n is the mole fraction given by

ns =Ns

NH2O +P

i Ni(3.10)

where Ns stands for the number of moles of volatile species s, and the sum is for allsuch species except H2O. If we consider a mass of �uid containing 1 kg of H2O, thenNH2O = 55.51 and

ns =ms

55.51 +P

i mi(3.11)

where ms denotes molal concentration of species s. For dilute �uids,P

i mi � 55.51so

ns∼=

ms

55.51(3.12)

and

Ds∼=mv

s

mls

(3.13)

The partial pressure Ps of species s is given by

Ps = nvs · Ptot (3.14)

where Ptot is the total pressure of all gases including H2O. The solubility constant(Henry's Law Coe�cient) of species s in aqueous solution is given by

als = Ks · fs (3.15)

where Ks is the solubility constant (in moles kg−1 bar−1), als stands for the activity

of species s in liquid water and fs its fugacity over the solution. For dilute �uids andmoderate pressure, equation (3.15) may be replaced by

mls = Ks · Ps (3.16)

Combining equations (4.4), (3.12), (4.9) and (3.16) yields

Ds∼=

55.51

Ks · Ptot(3.17)

The steam fraction (Xe,v) of the �owing �uid that has formed by depressurizationboiling at temperature T e but before phase segregation occurs is given by

Xe,v =hf,t − he,l

he,v − he,l(3.18)

As it is assumed that the steam fraction of the initial aquifer �uid is zero, it followsthat hf,t = hf,l. It also follows from conservation of mass that the concentration of

49

3.5. DATA HANDLING

species s in the liquid and vapor phases at temperature T e is related to its concentrationin the aquifer �uid by

mf,ti = me,v

i ·Xe,v +me,li (1−Xe,v) (3.19)

As for the enthalpy in equation (3.18),mf,ti = mf,l

i . Combination of equations (3.17)to (3.19) by inserting equation (3.17) into (3.18) and isolating me,l

i , the concentrationof species s in the liquid phase yields

me,ls =

md,ts

V f,t(Xe,v(Des − 1)) + 1

(3.20)

Having obtained me,ls from equation (3.20), mf,t

s (equal to mf,ls ) can be obtained

from equation (3.8). For non-volatile species, r, that does not partition signi�cantlyinto the vapor phase, equation (3.19) reduces to

mf,tr = me,l

r (1−Xe,v) (3.21)

asme,vi is zero. In this case, mf,t

r can be obtained by inserting equation (3.21) into (3.8).

mf,tr =

md,tr

V f,t − V f,t−11−Xe,v

(3.22)

V f,t and Xe,v can be obtained from equations (3.7) and (3.18), respectively.For wells with liquid enthalpy, the concentrations of all components in the total

discharge were taken to be equal to that of the initial aquifer �uid. Thus equation (3.1)reduces to

Md,t = Mf,t (3.23)

From this it follows that: hf,t = hd,t and md,ti = mf,t

i . Thus the steam fraction atsampling conditions becomes

Xd,v =hd,t − hd,l

hd,v − hd,l(3.24)

and the concentration of component i in the initial aquifer �uid is given by an equationanalogous to (3.19), or

mf,ti = md,v

i ·Xd,v +md,li (1−Xd,v) (3.25)

For boiling hot springs, the quartz equilibrium temperature was taken to representthe temperature (T f ) of the source aquifer to the hot spring water. This gives theenthalpy of the initial aquifer �uid, hf,l. Equations (3.24) and (3.25) were thus used tocalculate initial aquifer �uid compositions. Degassing of the deep aquifer �uid duringits boiling in the up�ow zones was adjusted to such a degree that it matched calcitesaturation in the unboiled aquifer water. In the WATCH program this is done bydividing into Ks an arbitrary factor that can take a value between 0.01 and 1 (seeequation (3.16)). This e�ectively means that the value of the solubility constant isincreased, i.e. the gases are made more soluble than they really are. The WATCH

50

CHAPTER 3.

program was run until the arbitrary factor gave a CO2 concentration that matchedcalcite saturation. It is reasonable to assume calcite saturation in the unboiled waterbelow hot springs because studies of drillhole data reveal that calcite saturation isclosely approached in the aquifer of hot water (> 50 ◦C) and wet-steam wells in Iceland(Arnórsson, 1978; Arnórsson et al., 1983).

3.5.2 Arsenic, B and Cl concentration in the aquifer �uid

Boron and As were determined in both liquid and vapor samples of wet-steam wells(Table 3.1). The concentrations of As in vapor samples is with two exceptions low, lessthan 0.8% of the concentration in liquid samples and no more than would be expectedfor carry-over of liquid to vapor. The high As concentration in the vapor of two samples(04-3014 and 04-3032) is considered to be due to carry over as these samples also containrelatively high concentrations of trace elements such as Br and Ti, which do not partitionsigni�cantly into the vapor phase. On the other hand, measured B concentrations aresigni�cant in vapor samples and they increase with sampling pressure relative to B liquidconcentrations, indicating that B partitions signi�cantly in the vapor phase (Table 3.1).For these reasons equation (3.21) has been used to obtain As concentrations, as wellas those of Cl, in the initial aquifer �uid. The of B concentrations in this �uid, on theother hand, were derived from equation (3.20).

51

Table3.1:Arsenic,BandClconcentrationsin

liquid

andvaporphasesof

wet-steam

wells.CalculatedconcentrationsofAs,

BandClleached

from

therock

inthedeepaquifer

liquid

are

alsoshow

n.

Sample#

Sampling

Locationa

Discharge

Sampling

Calc.

Aquifer

Cl

BB

As

As

Cl

BAs

Cl

BAs

date

enthalpy

pressure

aquifer

temp.b

liquid

liquid

vapor

liquid

vapor

aquifer

aquifer

aquifer

non-

non-

non-

liquid

sample

sample

sample

sample

sample

�uid

�uid

�uid

marinec,fmarinefmarinef

kJ/kg

bar-g

pH

◦C

mg/kg

µg/kg

µg/kg

µg/kg

µg/kg

mg/kg

µg/kg

µg/kg

mg/kg

µg/kg

µg/kg

04-3010

20.10.2004

K-13

1941

15.5

6.84

222

31.8

1230

18.8

38.8

<0.05

30.5

1161

37.3

27.5

1160

37.3

04-3015

20.10.2004

K-15

1499

14.5

7.03

273

36.4

1200

22.8

102.0

0.061

29.7

971

83.3

26.7

970

83.3

04-3022

21.10.2004

K-16

2451

11.0

6.22

263

133.2

1670

23.1

105.0

0.056

110.1

1240

87.0

107.1

1239

87.0

04-3021

21.10.2004

K-17

2547

28.5

7.01

268

14.9

1400

55.4

42.3

<0.05

14.2

1272

40.3

11.2

1271

40.3

04-3019

21.10.2004

K-20

2543

11.0

6.55

289

227.0

2200

27.4

88.9

<0.05

176.3

1382

69.0

173.3

1381

69.0

04-3009

20.10.2004

K-21

1167

15.0

6.91

238

72.2

845

12.8

16.8

<0.05

65.6

771

15.3

62.6

770

15.3

04-3023

21.10.2004

K-24

887

2.9

7.05

199

45.6

615

5.5

0.8

0.329

39.4

544

0.7

36.4

544

0.7

04-3011

20.10.2004

K-27

1199

12.0

7.04

247

49.5

896

11.5

23.8

0.125

42.6

779

20.5

39.6

779

20.5

04-3014

20.10.2004

K-32

1910

14.5

6.99

299

147.8

2000

231.9

213.0

12.3e

114.0

1642

164.3

111.0

1642

164.3

04-3012

20.10.2004

K-34

2636

17.5

6.37

263

99.6

6220

114.0

129.0

<0.05

87.7

4633

113.5

84.7

4632

113.5

04-3007

19.10.2004

N-12

1759

20.0

6.86

268

107.8

3680

84.5

148.0

0.10

94.6

3162

129.9

91.6

3162

129.9

04-3008

19.10.2004

N-09

1060

14.0

6.87

232

35.6

984

17.8

24.3

<0.05

32.6

907

22.2

29.6

906

22.2

04-3040

10.12.2004

NJ-05

1800

15.5

6.72

276

177.0

1429

22.1

24.4

<0.05

146.9

1153

20.3

141.9

1152

20.3

04-3042

10.12.2004

NJ-06

2136

15.8

6.74

285

164.8

2310

43.7

90.3

<0.05

136.4

1780

74.8

131.4

1779

74.8

04-3038

10.12.2004

NJ-07

1182

15.5

6.89

268

95.4

1330

23.1

14.1

<0.05

78.6

1120

11.6

73.6

1119

11.6

04-3043

10.12.2004

NJ-09

1700

15.3

6.62

282

172.3

1860

30.9

57.1

<0.05

139.1

1477

46.1

134.1

1476

46.1

04-3039

10.12.2004

NJ-10

1133

15.0

7.00

278

122.3

1390

24.1

8.1

<0.05

96.7

1130

6.4

91.7

1129

6.4

04-3048

15.12.2004

NJ-11

1948

7.8

6.80

282

78.1

1950

11.4

50.6

<0.05

58.6

1401

38.0

53.6

1399

38.0

04-3001

09.09.2004

NJ-13

1937

10.5

6.92

297

151.1

2550

28.5

80.5

0.186

112.4

1806

60.1

107.4

1805

60.1

04-3049

15.12.2004

NJ-14

1195

16.1

6.90

274

182.7

1580

27.7

53.1

<0.05

148.1

1311

43.1

143.1

1310

43.1

04-3037

10.12.2004

NJ-16

1252

15.5

6.94

273

65.0

1810

51.4

28.3

<0.05

52.8

1504

23.0

47.8

1503

23.0

04-3002

09.09.2004

NJ-19

1711

6.0

6.38

293

147.9

3830

27.7

84.0

<0.05

101.7

2645

57.8

96.7

2644

57.8

04-3050

15.12.2004

NJ-20

1400

16.2

6.86

274

169.1

1510

32.1

88.6

<0.05

138.8

1251

72.7

133.8

1250

72.7

04-3046

15.12.2004

NJ-21

2400

16.0

6.79

291

110.5

1710

25.9

40.4

<0.05

92.0

1224

33.6

87.0

1223

33.6

04-3045

15.12.2004

NJ-22

1612

15.6

6.86

296

164.1

1760

35.1

48.1

<0.05

125.5

1342

36.8

120.5

1341

36.8

04-3041

10.12.2004

NJ-23

2662

15.5

6.88

292

124.8

6790

83.0

93.4

<0.05

103.8

3894

77.7

98.8

3893

77.7

05-3004

14.01.2005

RN-12

1316

42.0

4.73

261

19288

7890

197.0

139.0

0.348

18951

7728

136.6

3192

134.9

05-3003

14.01.2005

RN-15

1256

25.0

4.37

260

19250

7860

119.0

156.0

1.90

17517

7157

142.3

2964

140.7

04-3005

01.12.2004

SV-07

1028d

15.5

4.86

239

13831

7000

172.0

78.3

0.193

12542

6407

71.0

3405

69.9

04-3003

10.09.2004

SV-08

1028d

14.0

5.10

240

13493

7260

157.0

98.6

0.317

12041

6556

88.0

3674

86.9

04-3032

01.12.2004

SV-09

1028d

14.8

5.18

241

13230

7340

0.0

89.4

14.2e

11853

6633

80.1

3796

79.0

04-3029

01.12.2004

SV-11

1028d

19.0

4.69

239

12916

7540

183.0

57.5

0.051

12024

7057

53.5

4179

52.4

04-3004

09.10.2004

SV-18

1028d

14.0

5.21

240

14255

7660

210.0

104.0

0.922

12723

6922

92.9

3877

91.8

04-3006

09.10.2004

SV-19

1028d

14.5

5.36

239

14235

7840

173.0

105.0

0.283

12800

7125

94.4

4061

93.3

04-3027

30.11.2004

Bödmódstadir

525

0.0

7.77

125

44.3

132

1.5

41.9

126

1.4

37.9

155

1.4

04-3026

30.11.2004

EfriReykirS-24

637

2.7

7.75

151

53.3

163

7.1

2.1

<0.05

52.1

160

2.1

47.1

205

2.1

04-3025

30.11.2004

ReykholtS-1

576

1.1

7.68

137

79.1

350

16.9

76.6

340

16.4

71.6

350

16.4

04-3024

30.11.2004

ReykjabólS-1

654

3.6

7.61

155

29.4

541

9.7

15.2

<0.05

29.0

534

15.0

23.0

697

15.0

aK=Kra�a,N=Námafjall,NJ=Nesjavellir,RN=Reykjanes,SV=Svartsengi,S=Southern

Lowlands.

Thenumberindicate

wellnumber.

bAverageofquartzandNa/Kgeotherm

ometertemperatures,exceptforReykjanesandSouthern

Lowlands.

ForReykjanes,quartzequilibrium

temperature

wasusedandchalcedony

equilibrium

temperature

inthecase

oftheSouthern

Lowlands.

cIt

isnotpossibleto

calculate

thenon-m

arinederivedClin

theReykjanesandSvartsengisystems.

Itisexpectedto

beless

thantheanalyticalerroroftheCldeterm

ination.

dSelected,correspondsto

enthalpyofsteam-saturatedliquid

at

238◦C

eSuspect.

Causedbycarry-overofliquid

into

vaporsample.

fAmountofCladdedto

�uid

byrockdissolutionand/orfrom

magma.

CHAPTER 3.

The value of P e, the vapor pressure at which phase segregation occurs, was taken tobe half way between the aquifer vapor pressure and the vapor pressure at the wellheadat which samples were collected.

Glover (1988) summarized experimental results on the distribution coe�cient (DB)for B between liquid water and vapor. He proposed the following equation for DB

log(DB) =T − 729

149.5(3.26)

where T is in K. Here it is assumed that the DB value refers to the H3BO3 species andthat practically all the B in both the liquid and vapor phases at sampling conditionsand higher temperatures occurs as this species. Fig. 3.3 shows the analyzed B ratiobetween liquid water and vapor samples. The equilibrium distribution (equation (3.26)above) is also shown.

Figure 3.3: Boron distribution between vapor and liquid phases discharging fromwet-steam wells versus sampling temperature. Equilibrium distribution (Glover,1988) is also shown.

As already pointed out, phase segregation is assumed to occur at a vapor pressurewhich is halfway between the sampling vapor pressure and the vapor pressure of theinitial aquifer �uid. It is, however, likely that phase segregation occurs over a rangeof pressures. This pressure range is not known. The choice made here is thereforereasonable because it is the simplest one. The choice of segregation pressure a�ects thecalculated concentration of B in the initial aquifer �uid, particularly when the dischargeenthalpy of wells is close to that of saturated steam.

Fig. 3.4 shows how calculated B concentrations in the initial aquifer �uid vary withthe selected vapor pressure value for phase segregation for three selected wells. For

53

3.5. DATA HANDLING

wells with discharge enthalpy below about 1800 kJ/kg, the calculated concentration ofB in the initial aquifer �uid is not sensitive to the choice of P e. By contrast, this is notthe case for well NJ-23 that has an enthalpy of 2662 kJ/kg (the enthalpy of dry steamat atmospheric pressure is 2676 kJ/kg). If the segregation occurred at 20 bar abs., thecalculated aquifer B concentration would be 4540 ppb, but 3360 ppb if it occurred at60 bar abs. Selection of phase segregation vapor pressure also a�ects the calculatedconcentrations of non-volatile components in the initial aquifer �uid, such as As andCl. As for B, the choice of P e is only important for wells with discharge enthalpy closeto that of dry steam.

Figure 3.4: Example of the in�uence of the selected vapor pressure at whichphase segregation occurs on the calculated aquifer B concentration for three high-enthalpy wells.

3.5.3 Sources of As, B and Cl to geothermal �uids

The principal sources of As, B and Cl to the �uids of the high-temperature geothermalsystems in Iceland include precipitation, in�ltration of seawater into the bedrock, therock with which the �uid has interacted and magma volatiles. In order to estimatethe contribution of Cl from precipitation, we have used the information provided bySigurdsson and Einarsson (1988) on Cl in precipitation in Iceland. It ranges from over10 ppm for high-temperature systems in coastal areas to about 3 ppm in those systemslocated farthest inland. Corresponding numbers for B and As are 0.7�2.4 ppb and0.0003�0.0009 ppb, respectively.

From this it is considered that Cl contribution from precipitation to the high-temperature �uids is insigni�cant and trivial in the case of B and As. Seawater in�ltra-tion into geothermal systems may occur under present-day hydrological conditions, suchas at Svartsengi and Reykjanes. According to Arnórsson and Andrésdóttir (1995), suchin�ltration also occurred around the end of the last glaciation in many other lowlandareas, when these areas were transgressed by the ocean. The only high-temperaturesystem considered here that may contain an old seawater component is Hveragerdi. For

54

CHAPTER 3.

Figure 3.5: Measured Cl and B concentrations in liquid water samples from allthe study areas. Included are samples from Arnórsson and Andrésdóttir (1995)

a Cl/B rock dissolution ratio of 38, the Cl marine groundwater component accounts for42�58% of the total Cl in the area. In the case of the seawater systems at Reykjanesand Svartsengi, a large proportion of the B in the waters is of marine origin, 59% atReykjanes and 44% at Svartsengi. Very likely almost all the Cl is marine (>99%) buttrivial part of As. In the dilute water systems, the sources of As, B and Cl to the deep,undiluted geothermal �uid are dominantly non-marine, or ∼100% for As, >99% for Band 90�98% in the case of Cl. The sources include the rock with which the �uid hasinteracted and, at least for some areas, a gaseous phase from the magma heat source.In a later section an attempt is made to evaluate the contribution of As, B and Cl frommagma volatiles to high-temperature �uids.

3.6 Results

3.6.1 Boron and Cl distribution in sampled waters

The relation between analyzed Cl and B concentrations is shown in Fig. 3.5. The solidline corresponds to seawater Cl/B ratio, as given by Krauskopf and Bird (1994), whereasthe dotted and broken curves show relationship between Cl and B for Cl/B molal rockdissolution ratio of 38 (molal) and an initial concentration of 5 ppm Cl and 1.2 ppbB, and 2 ppm Cl and 0.5 ppb B, respectively. The selected Cl/B ratio is based ondata on Cl and B in Icelandic tholeiites as reported by Arnórsson and Andrésdóttir(1995). The average Cl and B concentrations of these tholeiites are 177 and 1.43 ppm,respectively. The data given by Arnórsson and Andrésdóttir (1995) show considerablescatter. Accordingly the selected Cl/B ratio carries considerable error.

55

3.6. RESULTS

According to Sigurdsson and Einarsson (1988), local precipitation in the Geysir,Landmannalaugar, Nesjavellir and Námafjall areas contains ∼5 ppm Cl, whereas itis somewhat lower in Kra�a and at Hveravellir (∼3 ppm) and higher (∼10 ppm) inHveragerdi and the Southern Lowlands. Therefore a value of 5 ppm Cl and 1.2 ppb Bserves as a good proxy for the concentrations of these elements in the local precipitationin the study areas.

Data on Cl and B from Arnórsson and Andrésdóttir (1995) have been plotted inFig. 3.5 together with the data presented in Tables 3.1 and 3.2. All samples, except thosefrom the seawater systems of Reykjanes and Svartsengi, have much lower Cl/B ratiosthan seawater. In the seawater systems, Cl/B ratios are half that of seawater. Samplesfrom hot springs in the Geysir, Hveravellir and Landmannalaugar areas plot a littlebelow the Cl-B dissolution line. The average Cl/B ratio for all these areas is almost thesame, 36�40, if one heavily diluted sample from the Geysir area is excluded. However,some samples from Landmannalaugar have lower Cl/B ratios, i.e. those samples thathave reacted the most with the rocks as well as mixed waters from warm springs thatcontain a large cold water component. In view of the uncertainty in the Cl/B leachingratio, both for basalts and silicic rocks, it is considered that the slightly low Cl/B ratiosof the most reacted Landmannalaugar waters are not signi�cant.

Compared to the waters at Geysir, Hveravellir and Landmannalaugar, spring watersfrom Hveragerdi and discharges from three of the four wells from the Southern Low-lands have higher Cl/B ratios. This is attributed to a small component of seawater-groundwater component in these waters. Hveragerdi and most of the Southern Lowlandswere transgressed by the ocean in early Holocene times. At this time, seawater musthave in�ltrated the bedrock and this old seawater is still seeping into permeable tec-tonically active fractures in which more recent water of meteoric origin is convecting(Arnórsson and Andrésdóttir, 1995). The Cl/B ratio of the Hveragerdi and SouthernLowlands waters can be explained by assuming that Cl of marine origin in these watersis 20�40% of the total Cl except for one sample (04-3024 in Table 3.1) that does notcontain a marine groundwater component. This water is from a well in an elevated partin this area that was not submerged in early Recent times.

Many of the dilute waters from wet-steam wells in high-temperature areas havelower Cl/B ratios than basalt, in particular those from Námafjall and Kra�a. Their Bconcentrations lie in the range from ∼500 ppb to almost 6800 ppb. The most commonconcentration is, however 1000�2000 ppb. The corresponding number for Cl is 15�230 ppm. When the marine derived B content of the geothermal seawaters at Reykjanesand Svartsengi are subtracted from the total B, the amount of B added to the water fromother sources is 3.5�4 ppm in the case of Svartsengi and ∼3 ppm for Reykjanes. This ishigher than the average for the dilute waters, yet within the same range. The average Bcontent of tholeiites in Iceland is close to 1.4 ppm (Arnórsson and Andrésdóttir, 1995).It is thus evident that many high-temperature waters contain higher B concentrationsthan the rock with which they have reacted.

The observation that the data points in Fig. 3.5 for the �elds of Geysir, Hveravellir,Hveragerdi and Landmannalaugar fall quite close to the basalt Cl-B dissolution curveis taken to indicate that these elements are dissolved from the rock in stoichiometricproportions. Since it is generally accepted that Cl displays conservative behavior, i.e.once in solution it stays there, it is concluded that B also acts as conservative. It is

56

CHAPTER 3.

Table 3.2: Arsenic, boron and chloride concentrations in spring waters from high-temperatureareas. Also shown are calculated aquifer water pH and As, B and Cl concentration originatingfrom rock dissolution (leached).

Sample Spring name Coordinates Spring Aquif. Aquif. Calc. Cl B As Cl B As Cl B Astemp. temp.a liq. steam sample sample sample aquif. aquif. aquif. leach. leach. leach.

N/E Deg. ◦C ◦C pH fract. mg/kg µg/kg µg/kg mg/kg µg/kg µg/kg mg/kg µg/kg µg/kgGeysir01-3201 Geysir 64.31387/-20.29934 87.0 233 6.91 0.26 124.3 1047 84.5 92.0 775 62.5 87.0 773 62.501-3202 Blesi 64.31352/-20.30140 87.5 231 6.92 0.25 124.8 1061 83.4 92.9 790 62.1 87.9 789 62.101-3203 Ótherrishola 64.31208/-20.30237 98.0 231 7.12 0.25 105.5 1040 71.0 78.7 776 53.0 73.7 774 53.001-3204 Litli-Geysir 64.31180/-20.30126 93.0 214 7.25 0.22 122.7 1068 83.2 95.6 832 64.8 90.6 831 64.801-3205 Litli-Strokkur 64.31115/-20.30230 88.0 207 6.59 0.21 111.5 926 77.9 88.5 735 61.8 83.5 734 61.801-3206 Smidur 64.31057/-20.30184 99.0 204 7.34 0.20 117.0 1001 55.8 93.7 801 44.7 88.7 800 44.701-3207 Konungshver 64.31386/-20.30186 95.0 226 7.11 0.25 123.4 1025 81.5 93.1 774 61.5 88.1 772 61.501-3208 Helludalur 1 64.32171/-20.30826 30.1 81 7.77 0.00 20.5 114 2.0 20.5 114 2.0 15.5 113 2.001-3209 Laugarfell west 64.31280/-20.31019 38.8 101 7.39 0.00 42.5 257 2.3 42.6 257 2.3 37.6 256 2.301-3210 �82-015� 64.31833/-20.30478 26.0 72 7.60 0.00 21.2 103 1.2 21.1 103 1.2 16.1 102 1.201-3211 Spring by Beiná 64.31793/-20.29345 19.6 39 7.73 0.00 7.9 22 0.3 7.9 22 0.3 2.9 21 0.301-3212 Laugarfell east 64.31390/-20.30607 97.5 208 7.01 0.21 114.0 953 66.8 90.1 754 52.8 85.1 752 52.801-3213 Strokkur 64.31269/-20.30095 86.0 214 7.31 0.22 120.5 1019 89.6 93.9 795 69.8 88.9 793 69.801-3214 Nedridalur 1 64.30280/-20.33033 65.2 142 7.68 0.00 20.7 204 0.4 20.7 204 0.4 15.7 203 0.401-3215 Múli 1 64.27408/-20.32108 48.0 123 7.35 0.00 33.2 228 6.4 33.2 228 6.4 28.2 227 6.401-3216 Nyihver 64.31039/-20.30155 98.0 210 7.31 0.21 124.8 1012 76.2 98.3 797 60.0 93.3 796 60.0Hveravellir01-3217 Hveravellir I 64.85924/-19.55620 96.0 246 6.91 0.29 68.6 543 51.6 48.9 388 36.8 46.9 387 36.801-3218 Graenihver 64.85908/-19.55500 93.3 251 6.72 0.30 70.7 551 50.4 49.7 387 35.4 47.7 387 35.401-3219 Braedrahver 64.85906/-19.55479 96.2 250 6.63 0.30 64.2 549 52.6 45.2 387 37.0 43.2 386 37.001-3220 Bláhver 64.85900/-19.55502 90.2 229 6.65 0.25 68.3 540 59.2 51.2 405 44.4 49.2 404 44.401-3221 Eyvindarhola 64.85895/-19.55567 95.5 219 6.8 0.23 59.5 493 46.6 45.8 380 35.9 43.8 379 35.901-3222 Hveravellir II 64.85930/-19.55578 81.6 240 6.57 0.27 70.1 572 50.5 50.9 415 36.7 48.9 415 36.701-3223 Hveravellir III 64.85904/-19.55622 62.2 183 6.39 0.16 54.3 421 17.0 45.6 354 14.3 43.7 353 14.301-3224 Hveravellir IV 64.85904/-19.55545 80.6 193 4.29 0.00 32.6 277 0.7 32.5 276 0.7 30.5 276 0.701-3225 Fagrihver 64.85917/-19.55489 84.0 245 6.71 0.28 56.4 540 50.8 40.3 386 36.3 38.3 386 36.301-3226 Hveravellir V 64.85943/-19.55430 92.9 243 6.77 0.28 61.0 513 65.2 43.9 369 46.9 41.9 369 46.901-3227 Hveravellir VI 64.85966/-19.55370 82.5 195 6.62 0.18 46.5 415 8.9 38.0 339 7.2 36.0 339 7.201-3228 Hveravellir VII 64.85986/-19.55328 38.6 78 7.85 0.00 37.3 266 5.8 37.3 266 5.8 35.3 265 5.8Landmannalaugar01-3229 Landmannal. I 63.98779/-19.05774 77.2 184 6.61 0.00 277.0 1706 17.1 276.9 1705 17.1 272.9 1704 17.101-3230 Landmannal. II 63.98776/-19.05783 14.5 84 6.48 0.00 64.0 345 1.5 64.1 345 1.5 60.1 344 1.501-3231 Landmannal. III 63.98767/-19.05774 5.8 53 6.26 0.00 17.5 122 0.4 17.4 121 0.4 13.4 120 0.401-3232 Landmannal. IV 63.98814/-19.05776 58.3 152 6.51 0.00 49.7 600 6.5 49.6 600 6.5 45.7 599 6.501-3233 Landmannal. V 63.98848/-19.05761 60.0 154 6.51 0.00 58.0 610 6.1 58.0 610 6.1 54.0 609 6.101-3234 Landmannal. VI 63.98873/-19.05752 55.6 151 6.55 0.00 51.9 577 4.9 51.9 578 4.9 47.9 577 4.901-3235 Skriduhver 63.98862/-19.10836 96.2 132 7.75 0.06 173.0 1356 48.7 162.6 1276 45.8 158.7 1275 45.801-3236 Bóluhver 63.98840/-19.11728 96.7 94 8.85 0.00 182.0 1208 17.7 182.1 1209 17.7 178.1 1208 17.701-3237 Litli-Sullur 63.98899/-19.10857 96.9 180 7.02 0.15 254.4 1746 42.9 215.7 1481 36.4 211.7 1480 36.401-3238 Svartaauga 63.98901/-19.10858 96.9 123 7.34 0.04 164.8 1174 18.3 157.6 1123 17.5 153.6 1122 17.501-3239 Svuntuhver 63.98686/-19.10689 97.1 195 7.22 0.18 271.5 2483 154.0 222.6 2036 126.3 218.6 2035 126.301-3240 Eyrarauga 63.98734/-19.10622 96.5 182 7.26 0.16 287.7 2631 127.0 242.8 2221 107.2 238.8 2220 107.201-3241 Eyrarhver 63.98877/-19.10031 96.0 182 7.38 0.16 498.3 5082 252.0 420.2 4286 212.5 416.2 4285 212.501-3243 Raudanefskelda 63.96988/-19.01900 11.7 72 5.59 0.00 4.2 25 1.5 4.2 25 1.5 0.2 24 1.501-3244 Graenalaug 63.97107/-19.02336 55.6 145 7.06 0.00 4.5 89 0.4 4.5 89 0.4 0.5 88 0.501-3245 Stefánsauga 63.97322/-19.02608 75.9 194 6.18 0.18 6.9 104 0.2 5.7 85 0.2 1.7 84 0.201-3246 Landmannal. VII 63.97236/-19.07565 18.0 88 6.82 0.00 22.2 200 2.8 22.2 200 2.8 18.1 199 2.8Hveragerdi02-3201 Hverag. waterfall I 64.00064/-21.10875 90.2 158 6.63 0.11 76.5 342 0.4 68.1 305 0.3 58.1 302 0.302-3202 Hverag. waterfall II 64.00149/-21.10923 99.5 163 7.41 0.12 84.0 354 0.5 73.9 312 0.4 64.0 309 0.402-3203 By swimming pool 64.00250/-21.10896 99.8 182 7.24 0.16 148.7 539 2.4 125.4 454 2.1 115.4 452 2.102-3204 By well nr. 3 64.00898/-21.11228 65.1 104 7.25 0.00 36.2 173 0.4 36.3 173 0.4 26.3 171 0.402-3205 By Frosti&Funi 64.00349/-21.11051 99.2 191 7.19 0.17 160.7 577 4.7 132.8 476 3.9 122.8 474 3.902-3206 Bóluhver 64.00397/-21.11089 98.1 185 7.30 0.16 135.8 502 3.1 113.7 420 2.6 103.7 418 2.602-3207 Above Bóluhver 64.00402/-21.11083 99.4 169 7.21 0.13 86.2 331 2.2 74.9 288 1.9 64.9 285 1.902-3208 By river, Bóluhver 64.00397/-21.11053 99.9 154 6.48 0.10 38.3 169 0.7 34.4 152 0.6 24.4 150 0.602-3209 By well nr. 8 64.01514/-21.12764 75.3 126 6.78 0.00 14.9 64 0.1 15.0 64 0.1 5.0 61 0.102-3210 Spring Hverag. I 64.00067/-21.11305 92.2 196 7.27 0.18 166.0 582 5.5 135.7 476 4.5 125.7 473 4.502-3211 Spring Hverag. II 64.00091/-21.11291 66.6 187 7.70 0.00 148.1 571 5.5 148.1 571 5.5 138.2 569 5.5

aThe quartz and chalcedony geothermometer were used for boiling and sub-boiling springs, respectively.

57

3.6. RESULTS

further concluded that the source of Cl and B to these waters is essentially the rockwith which these waters have reacted.

Theoretically, the low Cl/B ratios of the Nesjavellir, Námafjall and Kra�a waterscould be due to lower ratios of these elements in the rock, loss of Cl from solution relativeto B or preferential addition of B relative to Cl from a magmatic heat source. All theseareas are closely associated with recent volcanism. Fresh magma was intruded into theroots of the Kra�a caldera immediately below the present well�elds during a volcanicepisode in 1975�84 (Björnsson et al., 1977) and this magma �owed along �ssures intothe Námafjall area (Larsen et al., 1978). The well�eld at Nesjavellir is adjacent to avolcanic �ssure that erupted 1880±65 years ago (Saemundsson, 1963). The Reykjanesand Svartsengi areas, as well as Landmannalaugar, are also in areas of young volcanismbut the other study areas are not. The Reykjanes and Svartsengi �uids have receivedconsiderable B in addition to that from the parent seawater. Landmannalaugar ismarginal to the Torfajökull geothermal area and may not be overlying a magmatic heatsource. Thus geological setting of areas and their link with volcanic activity favor thatthe low �uid Cl/B ratios are due to supply of this element from magma in excess of Cl.Boron is known to form volatile compounds at magmatic and lower temperatures andmost of the B in basalt is readily extractable, as indicated by the leaching experimentsof Ellis and Mahon (1964, 1967), presumably because it exists in a soluble compound onmineral grain surfaces. Such compound is believed to form by sublimation of volatileB compounds after the magma has consolidated. Boron-containing minerals and B-minerals have been found as sublimates around volcanic fumaroles (Garavelli and Vurro,1994; Garavelli et al., 1997) and on cooling new lava (Kuritani and Nakamura, 2006).Like B, Cl forms volatile compounds at magmatic temperatures. It occurs mostly asHCl in magmatic gases. Likely, the release of Cl from magma is limited relative to B dueto its relatively high solubility in melt. Sigvaldason and Óskarsson (1976) concludedthat only a small fraction of the Cl in basaltic magma is lost as a volatile duringits consolidation.

3.6.2 As and Cl relationships

Fig. 3.6 shows the As-Cl relationship in water samples. The As-Cl values show morescatter than those of B-Cl (Fig. 3.5). The dotted and broken curves in Fig. 3.6 showhow aqueous Cl/As ratios change by basalt dissolution for precipitation containing 5and 2 ppm Cl, and a Cl/As leaching ratio of 3129. This ratio is based on the averageCl content of Icelandic basalts of 177.5 ppm (Sigvaldason and Óskarsson, 1976) andan As content of 0.12 ppm. The latter number represents the average As content oftholeiites in the Skagafjördur area of northern Iceland. It is thus assumed that basaltdissolves congruently with respect to As and Cl. This assumption is supported by thedissolution experiments of Ellis and Mahon (1964, 1967), who demonstrated that As,like Cl, is easily extractable and almost quantitatively leached from basalt and othertypes of volcanic rocks by hydrothermal solutions.

The Cl/As ratio of the geothermal seawater at Reykjanes and Svartsengi are almosttwo orders of magnitude higher than that of seawater. For other �elds, the Cl-Asrelationship di�ers in some respect from that of Cl-B. Like Cl-B, undiluted waters fromthe Geysir area and Hveravellir fall more or less on the basalt dissolution curve. On

58

CHAPTER 3.

Figure 3.6: Measured Cl and As concentrations in liquid water samples from allthe study areas.

the other hand mixed waters from these areas containing a large cold water component,have As concentrations that plot above the Cl/As dissolution curve. In the case ofLandmannalaugar, this is considered to be due to precipitation of As from solution.For Hveragerdi, which has a marine Cl content of 50�100 ppm, except for the mostdiluted sample, the low Cl/As ratios can be accounted for by the presence of the highmarine Cl content. (Fig. 3.6).

The waters at Kra�a and Námafjall possess Cl/As ratios which typically are lowerthan that of basalt, whereas this ratio is higher in the case of the Nesjavellir waters.

3.6.3 Rock derived concentrations

Fig. 3.7 and 3.8 show the relationships between the rock-derived concentrations of As, Band Cl for the �elds of Geysir, Landmannalaugar, Hveragerdi and Hveravellir. The termrock-derived (or leached) represents the di�erence between analyzed concentration insamples and the estimated concentration of these elements in the parent precipitation.The two �rst plots from the top (parts A and B) for each area show the calculated rock-derived (or leached) B and As concentrations in the aquifer liquid water, respectively,versus the calculated rock-derived Cl concentration. The third plot (part C) shows therock-derived As concentration versus aquifer temperature. The broken lines representthe respective elemental ratios in Icelandic tholeiites. For Cl/B, they are based onArnórsson and Andrésdóttir (1995) but for Cl/As and B/As they are based on a valueof 0.12 ppm for As, which is the average As content of Tertiary tholeiites in northernIceland (Arnórsson, 2002) and on Arnórsson and Andrésdóttir (1995) for Cl and B asbefore.

59

3.6. RESULTS

Figure 3.7: Rock derived As, B and Cl concentrations in aquifer liquid in Geysirand Hveravellir �elds. The broken line represents the respective elemental ratiosin Icelandic tholeiites according to Arnórsson and Andrésdóttir (1995).

60

CHAPTER 3.

Figure 3.8: Rock derived As, B and Cl concentrations in aquifer liquid in Land-mannalaugar and Hveragerdi �elds. The broken line represents the respectiveelemental ratios in tholeiites from various parts of Iceland (Arnórsson and An-drésdóttir, 1995).

61

3.6. RESULTS

Geysir and Hveravellir

The relationship between Cl and B is linear for the �elds of Geysir and Hveravellir.A line passing through the data points goes through the point of origin. This line isthe consequence of mixing between the geothermal water and cold water. The Cl/Bratios of the waters are very similar to the average Cl/B ratio of Icelandic tholeiites.From this it is concluded that B behaves conservatively as Cl in these areas and thatdissolution of Cl and B from the basalt is stoichiometric.

The As versus Cl graphs for the Geysir and Hveravellir �elds (Fig. 3.7B) show twodi�erent groups. The group with the high Cl and As contents probably representsundiluted geothermal water. The other group plots above the mixing line. The watersof this group which contain a large cold water component are highly depleted in Asrelative to Cl, with an average of 2.1 ppb As for six samples from the Geysir areaand 7.0 ppb for four samples from Hveravellir. The same kind of pattern appears onthe As versus calculated aquifer temperature graph. This temperature correspondsto the last temperature of equilibrium with quartz for high As samples but to thetemperature of last equilibrium with chalcedony in the case of the mixed waters. Theresults for the mixed waters indicate that As is removed to substantial degree fromsolution subsequent to mixing. As demonstrated by Arnórsson (1985), mixing leadsto a decrease in pH, particularly if mixing involves unboiled and therefore undegassedgeothermal water. The decrease in pH enhances dissolution of the primary ma�c basaltminerals leading to increased formation of secondary iron-hydroxides and clays. Likelythe As coprecipitates with these minerals.

Landmannalaugar

Arsenic, B and Cl concentrations variations are larger in this �eld than in other areasconsidered for the present study, the range being 10�400 ppm, 5�4500 ppb and 1�220 ppb for Cl, B and As, respectively. The highest observed Cl and As concentrationsare higher than in other areas and the highest B concentrations are similar to thehighest values observed in wet-steam well discharges. The bedrock exposed in theLandmannalaugar area is all composed of silicic volcanics. This leads to the conclusionthat the high As, B and Cl content of the geothermal waters is the consequence ofreaction of these waters with silicic volcanics. The relationship between Cl and Bis linear and the ratio is almost the same as in Icelandic tholeiites (Fig. 3.8). Allexcept one water from boiling hot springs show considerable mixing with cooler or coldgroundwater. Warm springs, which also have high �ow rates, represent extensivelydiluted geothermal waters, i.e. they contain a large cold water component. The watersample containing the highest As, B and Cl concentration may or may not representundiluted reservoir �uid.

Relative to Cl, and therefore also B, As is highly depleted in all the extensivelydiluted warm spring waters and in all but two samples of mixed boiling spring waters(Fig. 3.8) indicating its removal from solution subsequent to mixing. Unlike the resultsfor the areas of Geysir and Hveravellir, described above, we observe As precipitation notonly for mixed sub-boiling waters but also for mixed boiling waters. Geothermal watersat Landmannalaugar di�er from those at Geysir and Hveravellir by their higher H2Scontent. It is possible that removal of As from boiling waters involves its incorporation

62

CHAPTER 3.

into sul�des that precipitate from solution upon extensive boiling due to cooling anddegassing. Degassing with respect to the acid gases, CO2 and H2S raises the pH of theboiling water. Both decrease of temperature and increase in pH reduce sul�de mineralsolubilities.

Hveragerdi

At Hveragerdi, the aqueous Cl/B ratio is higher, the average being 68 (molal), whereasthe average for the Icelandic tholeiites is 38. As can be seen from Fig. 3.8, the Cl-Bdata points form a curve, rather than a line, and a linear regression through these datapoints does not pass through the point representing local precipitation (10 ppm Cl,2 ppb B) as would be expected if this line represented mixing. The Cl/B ratios increasewith increasing concentrations, being around 80 (molal) for the waters containing thehighest Cl (100�130 ppm). In the most dilute waters, by contrast, Cl/B ratios are ∼45(molal) or a little higher than such ratios in geothermal waters at Geysir, Hveravellirand Landmannalaugar. The curvature of the Cl/B ratios of the Hveragerdi waters canbe explained by continuous mixing of rising geothermal water with progressively lessreacted water in which Cl/B ratios equal that of Icelandic tholeiites. The high Cl/Bratios of the deep geothermal water at Hveragerdi are probably due to the presence of aseawater-groundwater component that in�ltrated the bedrock around the end of the lastglaciation when the area was transgressed by the ocean (Arnórsson and Andrésdóttir,1995).

Arsenic is very low and the Cl/As ratio is the highest compared to �uids from other�elds considered for the present study. Mixing of the deep water with shallower water,whether limited or extensive leads to precipitation of As from solution. Studies of hy-drothermal alteration at depth in Hveragerdi show that it is very pervasive (Sigvaldason,1963). The primary basalt minerals have been totally replaced by hydrothermal miner-als and extensive removal of As from the initial basalt accompanied the hydrothermalalteration. Data are lacking to verify this. However, Arnórsson (1969) demonstratedthat Mo and Ge are excessively depleted from the altered rock at Hveragerdi, relativeto fresh basalt.

Kra�a, Námafjall and Nesjavellir

Unlike the results just presented for the hot spring data from Geysir, Hveravellir, Land-mannalaugar and Hveragerdi, the relative concentrations of As, B and Cl in dilutewet-steam well �uids from the �elds of Kra�a, Námafjall and Nesjavellir show muchgreater scatter, both within and between �elds (Fig. 3.9). In these �elds Cl/B ratiosare similar to those of Icelandic tholeiites and the hot spring waters or lower. At Nesja-vellir Cl/B ratios vary between 8 and 39 (molal) whereas those from Kra�a have arange of 6�39. When considering older data on Cl and B from Námafjall and Kra�a(Gudmundsson and Arnórsson, 2002; Arnórsson and Gunnlaugsson, 1985), the range iseven larger, 3�39 and 1�24 for Kra�a and Námafjall, respectively. At Námafjall, B con-centrations have clearly decreased with time (Fig. 3.10), but in two of the wells, Cl hasstayed constant but in the third well it is erratic, possibly due to variable productionfrom more than one aquifer.

63

3.6. RESULTS

Figure 3.9: Rock derived As, B and Cl in aquifer liquid.

Figure 3.10: Time evolution of non-marine B in the aquifer water of three wellsof the Námafjall area.

64

CHAPTER 3.

Some of the wells with the highest B concentrations discharge almost dry steam. Asdiscussed in section 3.5.2 above, the calculated value for aquifer �uid B concentrationfrom analyzed concentrations in liquid and vapor samples collected at the wellheaddepends on the value selected for the segregation pressure (P e). Selection of too a lowvalue for P e leads to over-estimation of the aquifer �uid B concentration. Some wellswith discharge enthalpy close to that of dry steam (e.g. 16, 17 and 20 at Kra�a, seeTable 3.1) do not contain anomalously high B whereas other wells have high B but notvery high discharge enthalpy (e.g. well 12 at Námafjall and well 19 at Nesjavellir). It istherefore concluded that calculated aquifer �uid B concentrations cannot be explainedby use of an erroneous selection of P e.

A plot of Cl/B against 1/B of data from Nesjavellir, Kra�a and Námafjall showsthat most of the data points scatter around two lines (Fig. 3.11) indicating that theCl and B contents of the aquifer �uid in these geothermal systems are determined, foreach line, by mixing of two components. For the line with the steeper slope in Fig. 3.11,one of these components is taken to be that at the intersection point of the two lines.This component contains 26.2 ppm B and 188 ppm Cl. Assuming a Cl/B ratio (molal)of 38 for the other component, yields a B concentration of 0.75 ppm and 94 ppm forCl. The selected Cl/B ratio is the average Cl/B ratio of Icelandic tholeiites. The lowB end-member for the mixing line (not shown in Fig. 3.11) with the lower slope gives0.15 ppm B and 19.2 ppm Cl for a Cl/B molal ratio of 38. The value of 0.15 ppm Bcorresponds to 1/B = 70 (molal).

The low B end-member for the low-slope mixing line represents geothermal �uid thathas reacted to a relatively limited extent with basalt. It contains Cl in concentrationsquite similar to that of water in aquifers in the upper Kra�a reservoir. By contrast, thelow B end-member for the high-slope line represents quite extensively reacted water,like that discharged from boiling hot springs in the Geysir area. The component whichcorresponds to the point of intersection of the two mixing lines, is considered to contain arelatively high proportion of magmatic volatiles. Essentially, the two component mixingalways involves substantial addition of B to the geothermal �uid from a magmaticsource. Addition of Cl is limited to �uids falling on the steep slope mixing line butlarge in the case of �uids de�ning the low-slope line.

The data depicted in Fig. 3.11 indicate that the Nesjavellir well �uids are representedby liquid that has reacted extensively with the basalt either before or subsequent tomixing with the component rich in magmatic volatiles. At Kra�a and Námafjall, onthe other hand, the magmatic component has mixed with relatively little reacted water.There are, however, exceptions. They include wells 16, 20, 21 and 32 at Kra�a, as well asone sample from well 13. Wells 16 and 20 are located south of the Kra�a mountain butwells 13 and 32 in the easternmost part of the Leirbotnar well�eld by the lower slopeson the west side of Kra�a mountain. Well 21 is located some 2 km to the south of otherproductive wells. (see Fig. 2 in Gudmundsson and Arnórsson, 2002, for well locations).A large groundwater current �ows from the north to the west of the mountain of Kra�a,considered to be represented by the upper sub-boiling reservoir of the Kra�a system.Wells drilled through the upper reservoir and into the lower two-phase zone at Kra�adischarge �uid represented by a mixture of the magmatic volatile component and littlereacted water of the upper reservoir. The wells to the east, particularly those south ofKra�a mountain, are outside the main groundwater current, in a �shadow� of the Kra�a

65

3.6. RESULTS

Figure 3.11: A plot of Cl and B of non-marine origin. Filled symbols representsamples from this study, open symbols represent data from Gudmundsson andArnórsson (2002),Arnórsson and Gunnlaugsson (1985). The solid and the brokenlines represent the linear regression of data from Nesjavellir and the lower Kra�areservoir and data from the upper Kra�a reservoir and Námafjall, respectively.These two lines represent mixing of two end-members, a magma-volatile compo-nent and liquid meteoric that has reacted to a di�erent extend with the rock (seetext).

66

CHAPTER 3.

mountain. The mixed �uid in these wells contain a component of water that has reactedextensively with the basalt, just as is the case at Nesjavellir. The �uid discharged fromwell 21 represents a mixture of moderately reacted water containing a fair contributionof the magmatic volatile component.

Four samples have been plotted for well 13 in Fig. 3.11. Initially this well had twofeed zones, one from the deeper two-phase reservoir and one from the upper sub-boilingreservoir. The sample with the high Cl/B ratio from well 13 was collected in 1981during the early years of production when the deeper feed zone was active. The threesamples collected at a later date (1988 to 2004), are all derived from the shallower sub-boiling zone because at that time the deeper zone had become inactive due to its lowpermeability.

As already mentioned, B concentrations have decreased in aquifers producing intowells at Námafjall. Such decrease has also been observed for couple of wells at Kra�abut most show no change or erratic variation with time. Decrease with time in theconcentration of B in the aquifer �uid of a particular well indicates a decrease in themagmatic gas component of the well discharge.

A correlation is not observed between As and Cl concentrations in �uids dischargedfrom the wet-steam wells at Kra�a, Námafjall and Nesjavellir (Fig. 3.9). This is tobe contrasted with the results described previously for the hot spring waters. Theundiluted hot springs waters have relatively constant concentrations of Cl and As, andtherefore also of Cl/As ratios. In the wet-steam wells, concentration variation for Clis roughly 15-fold but 25-fold for As. Cl/As ratios in the Nesjavellir �uids are withfew exceptions higher than those at Kra�a and Námafjall. On average they are 4600(molal) and 2800 for these �elds, respectively, if a total of four samples are omittedthat have very high Cl/As due to low As concentrations. These four samples also havehigh B/As ratios suggesting that the cause of these high ratios is removal of As fromsolution through its incorporation into precipitating sul�des.

If four samples are omitted, there is a signi�cant positive correlation between theconcentrations of As and B in the aquifer �uids at Kra�a plus Námafjall and Nesja-vellir, respectively (Fig. 3.9). The four exempted samples all have anomalously highB relative to Cl (Fig. 3.9), attributable to large supply of B to the �uid by magmaticvolatiles. As for Cl/As ratios, those of B/As are higher at Nesjavellir than at Kra�a.The di�erence in the Cl/As and B/As ratios of the �uids at Nesjavellir on one handand Kra�a and Námafjall on the other, indicate lower mobility of As at Nesjavellir thanat Kra�a and Námafjall. This is, however, not conclusive due to limited data on theAs content of basaltic to silicic igneous rocks in Iceland that would permit B/As andCl/As ratios of these rocks to be well established and correlated with these elementalratios in geothermal �uids. There may be several reasons for the lower mobility of Asrelative to those of Cl and B. They include loss of As from solution by its incorporationinto precipitating sul�de minerals at depth, which precipitate, in particular in zones ofextensive boiling, and variable addition of As to the geothermal �uids from magmaticvolatiles.

67

3.7. DISCUSSION

Reykjanes and Svartsengi

The concentrations of As, B and Cl in the aquifer �uid at Svartsengi lie in the range of52�93 ppb, 6.4�7.1 ppm and 11,850�12,800 ppm, respectively (Table 3.1 and Fig. 3.9).For the two samples from Reykjanes, average As and B concentrations are somewhathigher, 139 ppb and 7.4 ppm, respectively. The concentrations of As and B from thenon-marine source have been estimated from the reported As, B and Cl in seawaterusing

mf,mi =

mf,mCl

msCl

·msi (3.27)

where m represents concentration. Subscript i denotes either As or B. Superscriptsf,m and s stand for the marine component in the initial aquifer �uid and seawater,respectively. The di�erence between mf,l

i and mf,mi is the concentration of component

i in the initial aquifer �uid of non-marine origin.Almost all the As in the Reykjanes and Svartsengi �uids is of non-marine origin

(Table 3.1) and 41% (3.1 ppm) and 56% (3.8 ppm) of the B. Practically all the Cl istaken to be of marine origin. Both As and B concentrations are generally higher in thesaline Reykjanes and Svartsengi �uids than observed for the drilled dilute-water high-temperature areas. At Svartsengi, B/As relationship is similar to that of the high Bsamples from Nesjavellir and Kra�a, whereas the same relationships for the Reykjanes�uids compares with those at Nesjavellir. Fig. 3.9 also shows that the As concentrationsrelative to the aquifer temperature are higher than for the other areas. This could eitherbe due to more extensive rock leaching or a higher fraction of magmatic volatiles in thesaline �uids.

3.7 Discussion

The concentrations of As and B in �uids of volcanic geothermal systems in Iceland arelow compared to �uids in geothermal systems located on converging plate boundaries.An important fraction of the B and As present in the oceans is trapped within theoceanic sediments, mostly adsorbed on clay minerals (Spivack et al., 1987) and ironoxyhydroxides (Sullivan and Aller, 1996; Pierce and Moore, 1982). Some of the trappedAs and B is remobilized on converging plate boundaries at the high temperature in thesubducted plate below the mantle wedge (James and Palmer, 2000; You and Gieskes,2001), which explains the high concentrations of these elements in volcanic rocks andassociated geothermal systems in this environment.

The amount of Cl in the meteoric geothermal �uids derived from the parent pre-cipitation has been estimated from data on Cl in precipitations in Iceland given bySigurdsson and Einarsson (1988). Subsequently, precipitation derived B and As wereestimated by assuming that the Cl/B and Cl/As ratios of the precipitation were thesame as that of seawater. For individual samples, they represent rounded values inspeci�c areas rather than being evaluated for individual samples, as e.g. described byArnórsson and Andrésdóttir (1995). This may produce a large error for heavily dilutedgeothermal spring waters but not for little or undiluted waters as almost all the As, Band Cl are of non-marine origin.

68

CHAPTER 3.

Calculation of the deep aquifer �uid composition from wellhead data involves someassumptions and approximations. The �uid source of a geothermal well or spring istaken to be chemically homogeneous and with a �xed temperature. In formations ofheterogeneous permeability, the discharge of hot springs and wells will be composedof many components that have �owed over di�erent distances at di�erent rates fromtheir point of origin to the well or hot spring. These components may di�er, both incomposition and temperature. Strictly speaking, therefore, there is no large body ofaquifer �uid with a speci�c temperature and composition.

In the studied wet-steam wells, all the As and Cl occupies the liquid phase atwellhead conditions but a fraction of the B is in the vapor phase and this fractionincreases with increasing vapor pressure, indicating that the amount of B in the vaporphase is higher at aquifer pressure conditions than at wellhead conditions. Analysis ofB in both liquid and vapor phases sampled at wellheads shows that the B partitioningbetween the two phases corresponds well with equilibrium distribution according toexperimental data compiled by Glover (1988). This implies that B concentration insteam discharged from wet-steam wells can be calculated with reasonable con�dencefrom its concentration in the water when data on B in steam are lacking.

Segregation of vapor and liquid in producing aquifers of wet-steam wells was as-sumed to be the cause of excess well discharge enthalpy. The segregation was assumedto occur at a particular vapor pressure, taken to be half way between the wellheadand initial aquifer vapor pressures. Likely, phase segregation occurs over a range ofpressures. Therefore the selection of a single value for phase segregation involves anapproximation. This approximation, however, does not produce a signi�cant error incalculating elemental concentrations in the initial aquifer �uid except when the dis-charge enthalpy is approaching that of dry steam as can be inferred from Fig. 3.3.

Mixing of geothermal �uid with cold groundwater, particularly if it has not beendegassed by boiling prior to mixing, produces an acid solution leading to enhancementof primary basalt mineral dissolution, particularly olivine, pyroxene and glass and sub-sequent precipitation of ferric hydroxides. Chloride and B act as incompatible duringthe mixing process as indicated by their linear concentration relationship in variablymixed waters from speci�c geothermal areas. On the other hand, As is largely removedfrom solution. The cause is considered to be its co-precipitation with ferric hydroxides,or even clays, or adsorption onto such minerals.

Limited data are available on the contents of As, B and Cl in Icelandic basalts,and especially silicic volcanics. Values selected for Cl/B ratios of Icelandic tholeiitesare based on data on Cl from Sigvaldason and Óskarsson (1976) and B data on thesame samples presented by Arnórsson and Andrésdóttir (1995). From the average Clcontent of 177 ppm in these basalts and 1.43 ppm B, an average Cl/B molal ratio of 38is obtained. The range of the Cl and B concentrations in individual samples is, however,considerable, leading to an uncertainty in the Cl/B basalt ratios. Cl/B ratios of hotsprings waters from the studied high-temperature areas are almost the same on average(∼40), as that obtained for the basalts.

Estimated Cl/As and B/As ratios in basalt carry a large uncertainty. They are basedon the average As content of tholeiites (0.12 ppm) from the province of Skagafjördur in

69

3.8. SUMMARY AND CONCLUSIONS

northern Iceland and Cl and B data on di�erent samples (Sigvaldason and Óskarsson,1976; Arnórsson and Andrésdóttir, 1995). More extensive data on all these elementsin Icelandic rocks, in particular in the studied high-temperature �elds would permita more accurate evaluation of whether these elements are dissolved from basalts andsilicic volcanics in stoichiometric proportions, or whether rock dissolution is incongruentwith respect to them.

3.8 Summary and conclusions

The concentrations of As, B and Cl in meteoric �uids of volcanic geothermal systems(high-temperature > 200 ◦C) in Iceland are low compared to such �uids in volcanicgeothermal systems by converging plate boundaries. They typically lie in the range of20�140 ppb, 500�5000 ppb and 30�150 ppm, respectively, when the host rock is basalt.The reason for the low concentrations of these elements in the Icelandic �uids is theirlow content in the host basalt and the parent basaltic magma.When the meteoric �uidshave reacted with silicic volcanics, which formed by remelting of basaltic crust, theircontents of As, B and Cl are higher. In the seawater geothermal systems of Reykjanesand Svartsengi, As and B concentrations are similar to those of the meteoric watersystems when correction has been made for the marine contribution which is signi�cantfor B but trivial for As.

Many of the wells included in this study have higher discharge enthalpy than thatof the initial aquifer �uid. In calculating the aquifer �uid composition from analysis ofliquid and vapor samples collected at the wellhead, it was assumed that the enthalpyincrease was produced by phase segregation, i.e. partial or complete retention of theliquid in the aquifer due to its absorption onto mineral grain surfaces by capillaryforces. The phase segregation model was selected because it yields consistent resultsfor the quartz and Na-K geothermometers.

Boron partitions signi�cantly into the vapor phase of wet-steam well discharges butAs does not, at least at vapor pressures as high as 40 bar. Equilibrium distribution isclosely approached for B between liquid and vapor phases. The calculated concentra-tions of As, B and Cl in the initial aquifer �uid are a�ected by the value selected forthe initial aquifer temperature values as well as the selected vapor pressure at whichphase segregation is assumed to occur. The e�ect of the latter selection is, however,small (a few percent) except for wells with enthalpy approaching that of dry steam. Inview of this, the aquifer �uid concentrations of As, B and Cl, reported in Table 3.1 areconsidered to be accurate within several percent with possibly two exceptions (samplesnos. 04-3012 and 04-3041).

Chloride and B are dissolved from the rock in stoichiometric proportions and bothelements display conservative behavior; once dissolved they stay in solution. In hotspring areas that are mature, as indicated by their location outside the zones of activerifting within the volcanic belts, the Cl/B �uid ratios are very similar to those ofbasalt except in one �eld (Hveragerdi) where the �uid contains a small fraction ofseawater groundwater thought to have in�ltrated the bedrock around the end of thelast glaciation. In young systems with meteoric �uid, located at the divergent plateboundary, Cl/B �uid ratios are lower than those of basalt due to excessive supply of B

70

CHAPTER 3.

to the geothermal �uid from the magmatic heat source in relation to Cl. Such magmatic�ux of B also occurs in the seawater systems of Svartsengi and Hveragerdi. When freshmagma is being degassed Cl/B ratios may be as low as 1 (molal).

Arsenic possesses relatively high mobility in aquifer �uids like Cl and B. Frequentlyits concentration in deep aquifers is comparable to that of tholeiitic basalt. However,some As is considered to be removed from solution at depth due to its incorporationinto secondary sul�de minerals. When hot �uid rises from depth and mixes with coldgroundwater at shallow levels, As is e�ectively removed from solution by its coprecipi-tation with or adsorption onto ferric hydroxide and possibly also clays.

71

Chapter 4

Formation of a vapor cap subsequent toproduction in the Svartsengi geothermal

�eld, Iceland

Giroud, N., Arnórsson, S. and Bjarnason, J. Ö.

Abstract

Production from the Svartsengi geothermal �eld, SW-Iceland, started in 1976. Atpresent, the installed electric capacity of the geothermal plant is 76 MW and the in-stalled thermal capacity for direct use is 150 MW. The total �uid mass withdrawn fromthe reservoir at the end of 2006 was 243 megatonnes (Mt) of which 217 came fromwet-steam wells and 25 Mt from dry-steam wells. Production has led to a pressuredrawdown of approximately 30 bar in the 240 ◦C liquid reservoir. A vapor cap hasformed above the liquid reservoir as a consequence of enhanced boiling by the pressuredrawdown that presently extends down to over 400 m depth. Boiling of the liquid hasreduced its gas content. This in turn has caused an increase and decrease of liquid Fand Mg concentrations, respectively, due to re-equilibration of the boiled water withMg and OH-bearing minerals. Initially, the vapor cap was very rich in gas, but it hasdecreased with time, i.e. continued boiling of the underlying liquid water. Shallow wellshave been drilled to tap the vapor cap for electricity production. This is economicallyattractive but not environmentally favorable due to the high content of CO2 and H2Sin the vapor. It is estimated that to date as much as 3% w/w of the initial liquid waterhas boiled to supply vapor to the vapor cap.

73

4.1. INTRODUCTION

4.1 Introduction

Vapor-dominated geothermal systems are considered to have formed by boiling down ofliquid-dominated geothermal systems (Go� and Janik, 2000). In the East ProductionField of Olkaria geothermal �eld in Kenya, a vapor zone existed under natural conditionson top of a two-phase liquid-dominated reservoir (Arnórsson et al., 1990, and referencestherein). At Wairakei in New Zealand and Svartsengi and Reykjanes in Iceland a vaporcap has formed over liquid-dominated reservoirs as a consequence of a production-induced pressure drawdown (Clotworthy, 2000; Gudmundsson and Thórhallsson, 1986,and unpubl. data). At Wairakei, as well as Svartsengi, production from the steam capby the drilling of shallow holes into it has proved to be economic. Environmentally, onthe other hand, it is not so favourable due to the high CO2 content of the vapor. Plansare under way at Reykjanes to drill into the steam cap for its exploitation.

At Svartsengi the best permeability is associated with near-vertical active tectonicfractures and also with scoriaceous tops of lava �ows and minor contraction fractures.The formation of the steam cap at Svartsengi has been associated with much increasedfumarole activity, which was minor prior to exploitation. In the light of data on hy-drothermal alteration from the nearby �eld at Reykjanes (Pope et al., 2008), it seemsunlikely that such alteration can lead to the formation of a self-sealing cap on top of theSvartsengi reservoir that would impede escape of vapor to the surface. Tectonic move-ments are frequent, as witnessed by earthquake activity, and formation of hydrothermalminerals is unlikely to cope with opening of fractures by the these movements. It ap-pears more likely that ascent of vapor is restricted by volume expansion of the risingvapor.

The chemistry of the �uids discharged from all the wells at Svartsengi has beenmonitored by Iceland GeoSurvey, formerly part of the National Energy Authority, sincethe beginning of production. This has resulted in a database of nearly 30 years ofchemical data that allows assessment of the various processes that have caused �uidcompositions to vary as a consequence of pressure drawdown by production. The presentarticle is based on these monitoring data as well as on 10 samples speci�cally collectedfor the present study. These data provide important information about the mode offormation of the vapor cap at Svartsengi and allow predictions to be made regardingchanges in the gas content of the vapor cap.

4.2 Geology and hydrology

The Svartsengi high-temperature geothermal �eld is one of �ve high-temperature geother-mal �elds on the Reykjanes peninsula, in South-West Iceland (Fig. 4.1). It is located inthe Grindavík �ssure swarm, one of �ve �ssure swarms that lie across the plate boundaryon the Reykjanes peninsula (Jakobsson et al., 1978). The rocks consist predominantlyof sub-glacially erupted hyaloclastites, breccias and pillow-lavas, intercalated with Pleis-tocene and Holocene lava �ows and dyke intrusions. At depths greater than 1500 m,dykes, and to a lesser extent sills, predominate. The heat source of the geothermalsystem is considered to consist of a shallow (1�3 km) sheeted dyke complex (Arnórssonet al., 1978; Arnórsson, 1995).

Abundant vertical fractures provide very high permeability across the reservoir with

74

CHAPTER 4.

Figure 4.1: Map of the Reykjanes peninsula with the Svartsengi, Eldvörp andReykjanes high-temperature geothermal �elds (modi�ed from Fridriksson et al.,2006).

values up to 100�150 · 10−15 m2 (Arnórsson, 1995). The reservoir in its natural statewas liquid-dominated and the temperature across the �eld is very uniform at about240 ◦C.

The deep groundwater consists of seawater-groundwater. Drillings in the westernpart of the Reykjanes peninsula outside the geothermal �elds reveal the existence of afresh water lens �oating on top of that seawater-groundwater (Sigurdsson, 1986). Inthe middle of the peninsula, north of Svartsengi, the lens is about 50 m thick and thegroundwater level at ∼2 m above sea level. The fresh water lens thins towards the coastin south, west and north.

The producing �eld is hydrologically connected to the high-temperature area ofEldvörp, located about 6 km to the southwest (Fig. 4.1), as shown by resistivity surveys(Stefánsson et al., 1976), geochemical studies (Bjarnason, 1984), as well as by pressuremonitoring, subsidence measurements, and gravity surveys. The total area of the �eldis considered to be close to 7 km2 and shaped like a rectangle extending in an east-west direction (Gudmundsson and Thórhallsson, 1986). The depth of the producingliquid-dominated aquifer extends from 600 to at least ∼2000 m depth (Björnsson andSteingrímsson, 1992). The volume of the total geothermal system has been estimatedas 15�25 km3 (Bjarnason, 1996).

4.3 Production history

Production from the Svartsengi high-temperature �eld started in 1976. Delivery ofhot-water for district heating was followed shortly by electric power generation. Theinstalled generating capacity was 2 MW in 1977, increasing to 8 MW in 1980, 11.6 MW

75

4.3. PRODUCTION HISTORY

Figure 4.2: Annual discharge and injection in million tonnes of �uid.

in 1989, 16.4 MW in 1993, 46.4 MW in 1999, and �nally to 76.4 MW in April 2008(Arnórsson et al., 2008). The annual and cumulated �uid withdrawal from the reservoirfrom 1976 to 2006 is shown in Fig. 4.2 and Fig. 4.3, respectively. The total �uid�liquidand vapor�withdrawn from wet-steam wells increased sharply in the �rst years ofproduction up to a maximum of 9.84 Mt (megatonnes, 109 kg) in 1986. Since then, �uidwithdrawal from wet-steam wells has ranged between 6.0 and 9.7 Mt/year. Reservoirpressure drawdown caused by production from the �eld has led to the formation ofa vapor cap at the top of the reservoir. Well 10, which is only 425 m deep, stoppeddischarging liquid water in 1984 and became the �rst well to produce dry steam fromthe vapor cap. Three other wells were drilled into the vapor cap and brought on streamin 1996, 1999 and 2001, respectively. The spent �uid has been mostly disposed o� at thesurface, but a fraction of it was injected irregularly between 1984 and 1999. Injectionwas resumed in 2001 and has been continued since, reaching 4.40 Mt in 2006 (Fig. 4.2).The total mass of �uid withdrawn from the aquifer between 1976 and 2006 is 243 Mt,and the cumulated mass of injected liquid is 27 Mt. The net mass extraction from the�eld is thus 216 Mt, corresponding to a volume of 0.26 km3 of liquid water at 238 ◦C.

Regular downhole pressure measurements carried out by Iceland GeoSurvey showthat the pressure drawdown in monitored wells ranges between 25 and 32 bar. If weassume a �uid density of 816 kg/m3, the corresponding water level drawdown is 312 to400 m.

Between 96 and 98% of the gas discharged from the wells at Svartsengi is CO2. Theemissions from the wet-steam wells have been relatively constant over time, rangingbetween 2,700 and 4,700 tonnes per year (Fig. 4.4). Emissions from the dry-steamwells, on the other hand, are much higher, and they have increased with increasingproduction from these wells. Carbon dioxide emissions from the four wells tappingsteam from the vapor cap reached a maximum in 2002 (∼60,000 tonnes) but havedeclined since, to ∼43,000 tonnes in 2006 (Fig. 4.4).

76

CHAPTER 4.

Figure 4.3: Cumulated discharge and injection in million tonnes of �uid.

Figure 4.4: Annual CO2 emissions (in tonnes) from wet-steam and dry-steamwells at Svartsengi (Fridriksson et al., 2007).

77

4.4. SAMPLING, ANALYSES AND CALCULATIONS

4.4 Sampling and analytical techniques and cal-

culation of aquifer �uid compositions

Ten of the Svartsengi wells were sampled for the present study, six wet-steam wells andfour dry-steam wells. The sampling and analytical methods used were those describedby Arnórsson et al. (2006). Monitoring samples, which were extracted from IcelandGeoSurvey reports to Sudurnes Regional Heating Company, were collected as describedby Arnórsson et al. (2006) except for the gases. Initially gases and condensate werecollected separately as described by Arnórsson et al. (2000) but later by the methodgiven by Arnórsson et al. (2006). However, gas analytical methods have been the samethroughout the monitoring period. Dissolved solids in monitoring water samples havebeen analyzed by a variety of methods.

For wet-steam wells, the total discharge composition is taken to represent the aquiferliquid composition. The vapor fraction of the discharge was calculated from the mea-sured sampling pressure, taking the discharge enthalpy to be that of vapor-saturatedliquid at the aquifer temperature. We have

Xd,v =hf,l − hd,l

hd,v − hd,l(4.1)

where Xd,v is the vapor fraction of the discharge at sampling conditions, hf,l is thespeci�c enthalpy of the aquifer liquid, and hd,l and hd,v are the speci�c enthalpies ofthe liquid and the vapor at sampling conditions, respectively. For any component i, wehave

mf,li = md,l

i (1−Xd,v) +md,vi Xd,v (4.2)

where m denotes concentration. The aquifer liquid compositions including pH werecalculated with the aid of the WATCH speciation program (Arnórsson et al., 1982),version 2.1 (Bjarnason, 1994). The results are shown in Table 4.1.

Vapor samples from dry-steam wells are taken to represent the composition of thevapor cap.

4.5 Fluid composition

The deep reservoir �uid at Svartsengi is sub-boiling and consists of a mixture of two-thirds seawater and one-third meteoric water (Arnórsson, 1978; Ragnarsdóttir et al.,1984; Bjarnason, 1988, 1996; Lonker et al., 1993). The composition of the reservoir liquidis remarkably uniform, with a Cl content of 12�13,000 mg/kg. Slight but systematicdi�erences are observed in the aquifer �uid Cl content of individual production wells(Fig. 4.5). The deuterium content of the aquifer �uid is consistent with its origin, asdeduced from its Cl content (Lonker et al., 1993; Pope et al., 2008). The fresh watercomponent in the Svartsengi reservoir �uid is considered to originate from the freshwater lens mentioned in section 4.2.

Relative to seawater and its Cl content, the Svartsengi brine is enriched in Si, K,Ca and B, practically devoid of Mg and SO4, much depleted in F and slightly in Na(Table 4.1). The in situ aquifer water pH is around 5, as calculated by the WATCHspeciation program. The trace element composition of the Svartsengi brine generally

78

CHAPTER 4.

Table 4.1: Major and trace elements and major gas concentrations in aquiferliquid of wet-steam wells. Sampling pressure and vapor fraction are also shown.Concentrations of major components and trace elements in mg/kg and µg/kg,respectively.

Well SV-7 SV-8 SV-9 SV-11 SV-18 SV-19Sample # 04-3005 04-3003 04-3032 04-3029 04-3004 04-3006

Sampling date 10.09.2004 10.09.2004 01.12.2004 01.12.2004 10.09.2004 10.09.2004

Pd (bar-g) 15.5 14.0 14.8 19.0 14.0 14.5

Xd,v (% w/w) 8.93 9.91 9.38 7.40 9.91 9.57pH (at 238 ◦C) 4.88 5.11 5.20 4.72 5.22 5.37SiO2 429 442 448 431 427 421Na 6650 6380 6373 6394 6813 6809K 961 916 915 920 1009 994Ca 1069 1033 1025 1032 1023 1064Mg 0.392 0.614 0.280 0.531 0.427 0.369Cl 12664 12222 12054 12100 12912 12942F 0.13 0.15 0.15 0.14 0.14 0.13B 6.41 6.56 6.63 7.06 6.92 7.12SO4 23.8 25.5 25.7 27.3 25.0 25.9Fe 0.036 0.010 0.143 0.125 0.008 0.003Al 0.06 0.06 0.11 0.15 0.06 0.06Sr 8.00 7.84 7.63 7.53 7.74 8.00CO2 646 403 227 919 318 221H2S 15.8 1.1 5.3 10.9 7.3 4.5H2 0.38 0.04 0.04 0.46 0.02 0.02N2 5.4 36.4 9.0 56.9 4.7 5.7O2 0.00 0.00 0.00 0.00 0.00 0.00Ar 0.15 0.63 0.20 1.08 0.14 0.13CH4 0.070 0.060 0.010 0.090 0.020 0.020Trace elements

Ag <0.05 <0.05 <0.05 <0.05 <0.05 <0.05Al 102.5 95.1 169.5 123.7 110.5 119.1As 71.7 89.3 81.5 53.9 94.2 95.5Au 0.011 0.010 <0.001 0.012 0.013 0.023Ba 1474 1721 1203 1415 1920 1573Br 44,865 39,856 40,999 41,220 45,290 47,275Cd 0.124 0.043 0.091 0.117 0.134 0.072Co 0.127 0.065 0.156 0.155 0.100 <0.05Cr 1.227 0.495 1.367 3.279 1.839 0.559Cs 54.8 54.5 53.7 55.5 57.6 56.0Cu <1 <1 <1 1.2 <1 <1Ga 0.615 0.218 0.436 0.609 0.063 <0.05Ge 16.1 14.1 19.2 20.9 12.5 15.1Hg 0.0066 0.0111 0.0312 <0.0020 0.0092 0.0088I 70.6 110.5 119.4 72.4 120.5 99.1Li 2,371 2,790 2,342 2,557 2,699 2,518Mn 139 210 118 191 185 156Mo 7.03 7.89 5.62 5.56 9.15 5.85Ni 4.93 2.23 3.50 3.58 2.27 1.96P 9.71 <10 13.85 10.59 <10 12.00Pb 0.110 <0.1 <0.1 0.154 <0.1 <0.1Rb 2,765 2,736 2,806 2,857 2,980 2,927Sb 0.890 1.522 1.403 0.541 1.667 1.682Sn <0.5 <0.5 <0.5 <0.5 <0.5 <0.5Th <0.02 <0.02 <0.02 <0.02 <0.02 <0.02Ti 2.02 1.97 1.38 3.61 3.65 1.42Tl 7.73 7.81 6.17 6.66 8.43 7.56U <0.005 <0.005 <0.005 <0.005 <0.005 <0.005V 2.77 2.09 7.34 3.75 3.4 4.01W 0.539 0.651 <0.5 <0.5 0.572 <0.5Zn 1.95 2.53 <2 3.63 <2 2.25

79

4.5. FLUID COMPOSITION

Figure 4.5: Chloride concentration in the aquifer �uid of wet-steam wells. Openand full symbols represent data from Iceland GeoSurvey and samples collectedfor this study, respectively.

di�ers much from that of seawater, in particular the concentrations of As, Ba, Fe, Ge,Mo, Rb and Sr. The major dissolved gas is CO2. It accounts for 96�98% of the total gas,the remainder being mostly H2S, H2 and N2. At present, CO2 concentrations are 200�800 mg/kg in the liquid reservoir, but some 20,000 mg/kg in the vapor cap (Table 4.2).The di�erence in the compositions of the geothermal brine and the source �uid is theconsequence of interaction of the source �uid with the basaltic rock. Additionally, someof the CO2 and the As may be derived from a magmatic source (Giroud and Arnórsson,2008a,b).

Table 4.2: Gas concentrations in the vapor discharge from dry-steam wells. Con-centrations in mg/kg.

Well SV-10 SV-14 SV-16 SV-20Sample # 05-3001 04-3030 05-3002 04-3031

Sampling date 01.12.2004 01.12.2004 14.01.2005 01.12.2004CO2 29,292 21,698 26,467 19,625H2S 411 179 418 349H2 9.94 6.47 9.66 11.33N2 391.1 461.1 93.8 91.2O2 0.00 0.00 0.00 0.00Ar 8.00 8.41 2.60 2.52CH4 2.12 1.18 2.82 1.77

80

CHAPTER 4.

4.6 Evolution of gas concentrations

4.6.1 Wet-steam wells

Until 1990, gas concentrations (CO2, H2S and H2) in the aquifer �uid feeding wet-steam wells (nos. 7, 8, 9 and 11) are on the whole about the same (Fig. 4.6). After1990 large di�erences are on the other hand seen between wells. Fig. 4.6 shows thatthe CO2 concentration decreases between 1982 and 1990 in all of the wet-steam wells.Since 1990�1991, an increase is observed in the CO2 concentration in wells 7 and 11.This increase becomes very sharp between 1999 and 2000, but in the last few years,concentrations have stabilized and vary between 500 and 800 mg/kg, well 11 showingthe highest concentrations. In well 8, CO2 concentrations remain mostly stable in theperiod 1990�1999. The CO2 in well 9 continues to decrease. As in wells 7 and 11, anincrease is also seen between 1999 and 2000 in wells 8 and 9, but it is not as sharp.This increase coincides with increased production from the reservoir when wells 18 and19 were put on line. The CO2 content of the discharge of these two wells, as well asthat of wells 8 and 9, changed little until about 2005, but has decreased since.

The H2S concentrations in the aquifer �uid of wet-steam wells range mostly between5 and 10 mg/kg until 1990 (Fig. 4.6). There is complete overlap between wells, andapparently, concentrations do not vary with time. After 1990, H2S concentrationsbecome di�erent between wells in a manner comparable to that for CO2. Some samplesshow almost no H2S, specially after 2003. Analyzed H2S concentrations are sensitive toair contamination upon sampling, because H2S reacts rapidly with O2, particularly inan alkaline medium like that used by Iceland GeoSurvey for vapor sampling in recentyears (see section 4.4). As a result, air contamination likely leads to an underestimateof the H2S content of the vapor, particularly when its concentrations are low. Smallair contamination of the vapor upon sampling leads to high N2 and Ar concentrations.Indeed, samples low in H2S tend to be high in N2.

Analyzed H2 concentrations are close to the detection limit of the analytical methodused. No clear trend is observed before 1990 for any of the wells. Then, wells 7 and 11show an increase, particularly between 1991 and 2000, as for CO2 and H2S. A smallerincrease is observed for wells 8 and 9, again just as for CO2 and H2S (Fig. 4.6).

The increase in the gas concentrations in wells 7 and 11 after 1990�1991 is believedto be a consequence of the drawdown of the water level and a corresponding downwardmigration of the interface between the vapor cap and the liquid reservoir. The pressuredrawdown will cause the depth level of �rst boiling in the liquid-dominated reservoir tomigrate downwards. The slight excess vapor in the discharge of wells 7 and 11 may comefrom the vapor cap, or from rising vapor bubbles in the liquid reservoir, depending onthe depth level of producing horizons intersected by these wells. Enhanced drawdownin the reservoir in 1999 to 2000, when wells 18 and 19 came on line, led to a furtherincrease in the gaseous vapor of wells 7 and 11, and such vapor also appeared in wells8 and 9.

The fraction of vapor forming the vapor cap in the total �uid discharge from wells 7and 11 has been estimated using average concentrations of CO2, H2S and H2 in the otherfour wet-steam wells and in the dry-steam wells. Table 4.3 shows a three-fold increasein the vapor mass fraction between 1996�1997 and 2006�2007, from 0.7% to 2.1% andfrom 0.9% to 3.3% in wells 7 and 11, respectively. The vapor fractions calculated with

81

4.6. EVOLUTION OF GAS CONCENTRATIONS

Figure 4.6: CO2, H2S and H2 concentrations versus time in the aquifer �uid ofwet-steam wells. Symbols have the same meaning as in Fig. 4.5.

82

CHAPTER 4.

Table 4.3: Gas concentrations and estimates of the fraction of vapor (in % w/wand % v/v) �owing into wells 7 and 11. Concentrations are averages (in mg/kg)of all samples taken in the periods 1996�1997 and 2006�2007, respectively.

1996�1997 2006�2007CO2Well 7 453 546Well 11 473 778Other wet-steam wells 215 236Dry-steam wells 37,422 18,445H2S

Well 7 7.97 12.68Well 11 9.94 18.60Other wet-steam wells 4.26 6.06Dry-steam wells 433.67 285.18H2Well 7 0.060 0.225Well 11 0.070 0.253Other wet-steam wells 0.013 0.030Dry-steam wells 8.843 8.828

Vapor fraction % w/w % v/v % w/w % v/vWell 7

CO2 0.60 11.7 1.70 26.3H2S 0.86 15.2 2.37 33.3H2 0.54 10.0 2.22 31.8Average 0.68 12.4 2.10 30.6Well 11

CO2 0.69 12.6 2.98 38.7H2S 1.32 21.6 4.49 49.2H2 0.65 11.9 2.54 34.9Average 0.89 15.6 3.34 41.5

H2S (Table 4.3) are higher than those calculated from CO2 and H2. This is likely to bedue to an underestimate of the H2S concentration in the wet-steam well �uids due toair contamination of the sample, as mentioned above.

The decrease in gas concentrations of well 9 is most likely due to in�ow of progres-sively more degassed liquid due to its boiling and subsequent segregation of the boiledliquid and the vapor. The increase in the gas content in wells 8 and 9 around the year2000, is likely to be due to boiling of previously undegassed reservoir liquid.

4.6.2 Dry-steam wells

At the outset, well 10 discharged wet steam, but since 1984 it has produced dry steamonly.Initially, CO2 concentrations in the vapor cap were close to 50,000 mg/kg on av-erage (Fig. 4.7). They have been decreasing since at a rate of about 1,700 mg/kg peryear. There is a small di�erence between wells, well 16 showing usually the highest CO2

concentrations, and well 20 the lowest. The average CO2 concentration in the four wellsin the period 1999�2000 was 20,385 mg/kg, decreasing to 15,020 mg/kg in the period2006�2007. A very similar trend is observed for H2S and H2: Between 1999�2000 and2006�2007, the average H2S and H2 concentrations decreased from 365 to 308 mg/kg,and from 6.33 to 5.37 mg/kg, respectively.

83

4.6. EVOLUTION OF GAS CONCENTRATIONS

Figure 4.7: CO2, H2S and H2 concentrations versus time in the discharging vaporfrom the dry-steam wells. Open and full symbols represent data from IcelandGeoSurvey and samples collected for this study, respectively.

84

CHAPTER 4.

Figure 4.8: Magnesium concentration in the aquifer �uid of wet-steam wells.Symbols have the same meaning as in Fig. 4.5.

4.7 Changes in dissolved salts and pH

The concentration of Cl (Fig. 4.5), Na, K, Ca and SiO2 (Fridriksson et al., 2007) in wellsproducing from the liquid reservoir have not changed with time over the 25 years coveredby this study. The concentrations of Mg have decreased, however, and those of F haveincreased (Figs. 4.8 and 4.9). It is thought that the cause of these changes is loss of CO2

from the reservoir liquid due to boiling, and rapid re-equilibration between solution andOH-bearing minerals (chlorite and smectite, see Lonker et al., 1993). Arnórsson et al.(1983) suggested that F concentrations in geothermal �uids reacting with basaltic rockare controlled by exchange equilibria with OH-bearing minerals, such as chlorite andsmectite. They observed that F−/OH− activity ratios were constant at any particulartemperature. In seawater, F concentrations are 1.0 mg/kg and therefore 0.7 mg/kgin the source �uid to the Svartsengi geothermal system. In the Svartsengi geothermalbrine, F concentrations were about 0.1 mg/kg during the early years of production,but they have increased to about 0.2 mg/kg. If it is assumed that the increase in Fconcentration is accompanied in an equal change in OH−, then H+ activity would havedecreased by a factor of 2, that is pH increased by 0.3 units. The decrease in Mgconcentration by a factor of 2 to 4 could result from an equal decrease in (H+)2 activityif hydrolysis reactions controlled the Mg+2/(H+)2 ratio, as can be seen from

�MgO� + 2H+ = H2O + Mg+2 (4.3)

where �MgO� represents the magnesium oxide component in a Mg-silicate mineral. Adecrease of Mg concentrations by a factor of 2 to 4 corresponds to an increase in pHof 0.15 to 0.3. Fig. 4.10 shows a plot of log(Mg+2) activity versus -2log(F−) activity.If a change in solution pH was the cause of changes in both Mg+2 and F− activities,one would expect the data points in Fig. 4.10 to fall on a line with unit slope. This is

85

4.7. CHANGES IN DISSOLVED SALTS AND PH

Figure 4.9: Fluoride concentration in the aquifer �uid of wet-steam wells. Sym-bols have the same meaning as in Fig. 4.5.

Figure 4.10: Mg+2 versus −2F− activities in the aquifer �uid of wet steam wells.Symbols have the same meaning as in Fig. 4.5. The line represents the unit slope.

86

CHAPTER 4.

indeed the case with most of the data points. Notable exceptions are data from wells 7and 11, the discharges of these wells contain gaseous steam. This gaseous steam wouldnot a�ect signi�cantly the true pH of the boiled liquid, but it a�ects the calculated insitu pH and therefore underestimates F− activity and correspondingly overestimatesthe activity of the HF species. It is thus concluded that the calculated F− activity forwells 7 and 11 are in error (too low F−) when the discharge of these wells containssigni�cant gaseous steam. As a consequence, the calculated pH does not re�ect theactual increase in the OH− concentration, but shows instead a slow decrease with time(Fridriksson et al., 2006).

4.8 Formation of the vapor cap

As mentioned in section 4.3, the pressure drawdown in the liquid reservoir at Svartsengiof more than 25 bar has led to the formation of a vapor cap above the liquid reservoir.The pressure drawdown has enhanced boiling in the upper part of the liquid reservoir.The vapor formed subsequently separates from the liquid by gravity to accumulateat higher levels. Some of the steam escapes to the surface but apparently most of itcumulates in the bedrock. Evidently, gravity separation becomes operative at low vaporfractions, even in terms of volume (considerably less than 10%). The fraction of theliquid body which has vaporized to form the vapor cap has been estimated from the gascontent of the discharges wet-steam and dry-steam wells. The calculation procedure isgiven below.

4.8.1 Vapor fraction calculations

Progressive boiling of liquid water in the Svartsengi reservoir leads to a decrease in itsgas content. It also leads to a decrease in the gas content of the vapor cap; during theearly stages of boiling, the vapor of the vapor cap is rich in gas, but it decreases withcontinued vapor formation. If equilibrium distribution is closely approached betweenliquid and vapor, the distribution coe�cient describes the relative concentration of gass in the two phases. The distribution coe�cient Ds is de�ned as

Ds =nv

s

nls

(4.4)

where superscript v and l designate vapor and liquid, and n mole fraction. When gaspressures are low, relative to vapor pressure, we have

ns∼=

ms

55.508so : (4.5)

Ds∼=mv

s

mls

(4.6)

where m designates concentration of gas s in a kg of vapor (Arnórsson et al., 2007).The solubility constant (Henry's Law constant) for gas s, Ks, de�ned as

Ks =al

s

fs(4.7)

87

4.8. FORMATION OF THE VAPOR CAP

where als and fs are the activity of species s in the liquid and its fugacity over the solu-

tion, respectively, can be expressed in the following way for dilute �uids and moderatepressures:

mls = Ks · Ps (4.8)

where Ps is the partial pressure of species s, de�ned as

Ps = ns · Ptot (4.9)

where Ptot is total pressure. Combining equations (4.5), (4.4), (4.8) and (4.9), leads to

Ds∼=

55.508

Ks · Ptot(4.10)

Vapor fractions were calculated from an initial temperature of 240 ◦C, with coolingsteps due to adiabatic boiling of 0.01 ◦C in an open system where the vapor produced ateach temperature step is immediately removed. The steam fraction Xg at temperatureT g < 240 ◦C, is expressed as,

Xg,v =hf,t − hg,l

hg,v − hg,l(4.11)

where hf,t, hg,l and hg,v are the enthalpy of the total �uid at 240 ◦C, and the enthalpyof the liquid and vapor at T g, the temperature of phase separation, respectively.

A mass balance for gas s in the reservoir liquid and vapor cap is given by

mf,ls = mg,l

s (1−Xg,v) +mg,vs Xg,v (4.12)

where superscript f denotes initial reservoir liquid, g temperature T g at which liquidand vapor separate. Superscripts l and v have the same notation as before. Combinationof equation (4.10) and (4.12) gives

mf,ls = mg,l

s

»1 +Xg,v 55.508

Ks · P tot

–(4.13)

We have selected the average gas content (CO2, H2S and H2) of the initial reservoirliquid to be represented by the average composition of the samples collected from wet-steam wells in the period 1983�1985. From the selected mf,l

s , mg,ls has been calculated

for selected X values with the aid of equation (4.13) and subsequently mg,vs from equa-

tion (4.6). Values for the gas solubility constants were taken from Fernández-Prini et al.(2003).

4.8.2 Results

The decline of the CO2 concentration in the �uid of well 9 before 1999, before the sharpincrease of the mass of �uid withdrawn from the reservoir, amounts to 310 mg/kg (from432 down to 122 mg/kg). If this decline in gas concentration is taken to be solely aconsequence of adiabatic boiling, the fraction of steam produced from the boiling of theliquid during this period would be close to 0.85% w/w. As mentioned in section 4.6.1,the changes in the H2S and H2 content of the liquid aquifer do not show trends thatare as clear as for CO2, mostly because of their overall lower concentrations.

88

CHAPTER 4.

Figure 4.11: Fraction of the total liquid that forms the vapor cap. Each point isan average of all data available that year for all four dry-steam wells.

Fig. 4.11, shows how the vapor fraction forming the vapor cap has increased withtime. As based on the analyzed gas concentrations in the dry-steam wells. The resultsfor CO2 and H2S compare reasonably, but those for H2 are low. As already mentioned,the H2 data are not very reliable, particularly those in the initial aquifer liquid, becauseanalyzed concentrations are close to the detection limit of the analytical method used.According to the results in Fig. 4.11, about 1% w/w (or 17% v/v) of the initial liquidhad boiled in 1990, to yield steam for the vapor cap, this number had increased to3% w/w (or 49% v/v) in 2006. Vapor formation amounting to 1% w/w assumingadiabatic boiling would cool the reservoir liquid by 4 ◦C, that is from 240 to 236 ◦C.3% w/w vapor formation produces cooling by 11 ◦C. This latter �gure is signi�cantlyhigher than measured cooling in producing wells below the depth level of �rst boiling.The reason for this is considered to be escape of early formed vapor to the surfaceand withdrawal of vapor from the vapor cap through the dry-steam wells. Both escaperoutes will cause the actual gas in the vapor cap to be lower than that calculated foradiabatic boiling.

4.9 Discussion

The best permeability at Svartsengi is provided by near-vertical �ssures and faults asindicated by the geological structure and the homogeneity of reservoir temperatureover large depth ranges. Distribution of hydrothermal minerals does not indicate a self-sealing zone on top of the reservoir. Also renewal of permeability by tectonic movementsis frequent relative to the growth rate of hydrothermal minerals. At the nearby high-temperature �eld at Reykjanes, epidote, which formed in Pre-Holocene times fromfresh water geothermal �uid has not yet re-equilibrated with seawater that has beencirculating through the system for the last ∼10,000 years (Pope et al., 2008). Isotopic

89

4.10. SUMMARY AND CONCLUSIONS

exchange equilibria are relatively rapid so the results from Reykjanes suggest that therate of hydrothermal mineral formation by alteration of the basaltic rock is very slow,certainly in relation to the frequency of tectonic movements.

Recharge into the Svartsengi reservoir, both natural and by injection, may cause thegas content of the �uid discharged from wells to decrease. The e�ect of injected wastewater should also increase the Cl content of the well �uid. Such an increase has notbeen observed, indicating that the e�ect of used brine on decrease in the gas contentof the well �uid is not signi�cant. According to Eysteinsson (2000), the geothermal�eld at Svartsengi has 70% natural recharge. These results are based on monitoring ofgravity in the area. These result indicate that cold recharge into the reservoir couldcause a decrease in the gas content of the reservoir liquid but hot recharge would not,or to a lesser extent.

Boiling of the 240 ◦C reservoir liquid at Svartsengi causes much expansion of �uidvolume as the speci�c volume of 240 ◦C vapor is about 20 times that of liquid at thesame temperature. This volume increase will impede �uid �ow. It is likely that thise�ect, together with enhanced boiling by pressure drawdown, are the causes of theformation of the vapor cap. A self-sealing zone by hydrothermal mineral formation orimpermeable strata, are not needed for the formation of such a cap whether it formsunder natural conditions or by exploitation.

Unfortunately, many of the vapor samples collected for the monitoring studies areair-contaminated, so it is not possible to use the unreactive gases like N2 and Ar toassess boiling processes and the fraction of liquid that has boiled to generate vapor forthe vapor cap. It would be particularly interesting to have data on the noble gases forsuch an assessment, because the only source of these gases to the geothermal �uid isair-saturated meteoric water and seawater, and the di�erent noble gases have di�erentsolubilities in liquid so limited vapor formation by boiling will fractionate these gasesto a di�erent extent between boiled liquid and vapor.

4.10 Summary and conclusions

The pressure drawdown and subsequent enhanced boiling in the geothermal reservoirat Svartsengi due to important �uid withdrawal for electricity production led to theformation of a vapor cap on top of the liquid dominated reservoir. The study of thechanges in gas concentrations with time in both the liquid aquifer and the vapor capallows us to estimate to up to 3% w/w the fraction of the liquid that has vaporized toform the vapor cap.

Extensive boiling in the reservoir also leads to degassing of the liquid water. Removalof CO2 and H2S decreases the pH of the remaining liquid, i.e. increases the activity ofthe OH− ion. As a consequence, rapid equilibration with Mg and OH-bearing mineralsleads to a decrease, respectively an increase, of Mg and F concentrations with time inthe liquid reservoir.

The gas concentrations in the vapor cap are several orders of magnitude higher thanthose in the liquid reservoir. The amount of CO2 released into the atmosphere per unitof electricity produced is therefore less favourable when the �uid is withdrawn from thevapor cap rather than from the liquid dominated reservoir.

90

CHAPTER 4.

The presence of a cap rock or of a self-sealing process at the top of the aquifer maynot be required for the formation of a vapor cap. The much enhanced vapor formationand volume increase in relation to permeability may be su�cient for the vapor cap toform.

91

Appendix A

Trace element analyses

Trace elements were analyzed on liquid water and vapor samples collected from most ofthe wells considered in this study. Results of the analyses are presented in the followingtables.

93

TableA.1:Trace

elem

entanalysesofliquid

watersamplesfrom

wet-steam

wells.

Concentrationsinµg/kg.

Sample

Well

Ag

Al

As

Au

BBa

Br

Cd

Co

Cr

Cs

04-3007

N-12

0.019

989

148.0

0.0311

3680

0.904

350

0.004

0.190

0.13

2.520

04-3008

N-09

0.002

1040

24.3

0.0051

984

2.200

115

0.035

<0.005

0.052

1.150

04-3010

K-13

0.001

1200

38.8

0.0051

1230

1.640

82

0.024

<0.005

0.081

7.790

04-3015

K-15

<0.001

1660

102.0

0.0092

1200

1.920

72

<0.002

0.015

0.089

8.460

04-3022

K-16

0.001

969

105.0

0.0094

1670

0.623

260

<0.002

<0.005

0.117

9.660

04-3021

K-17

<0.001

1690

42.3

0.0005

1400

0.265

39

<0.002

<0.005

0.105

5.290

04-3019

K-20

0.002

1060

88.9

0.0242

2200

1.340

440

<0.002

<0.005

0.04

15.900

04-3009

K-21

<0.05

1540

16.8

0.0008

845

0.895

260

<0.002

<0.005

0.036

3.920

04-3023

K-24

<0.001

877

0.8

0.0008

615

0.579

114

<0.002

<0.005

0.018

4.010

04-3011

K-27

0.002

1390

23.8

0.0014

896

3.940

97

<0.002

<0.005

0.045

8.460

04-3014

K-32

0.001

1850

213.0

0.0103

2000

4.010

240

<0.002

<0.005

0.047

6.830

04-3012

K-34

0.002

1340

129.0

0.0026

6220

1.900

120

<0.002

<0.005

0.102

7.210

04-3040

NJ-5

0.003

1560

24.4

0.0054

1420

0.479

430

<0.002

<0.005

0.08

3.120

04-3042

NJ-6

<0.001

1360

90.3

0.0038

2310

0.423

600

<0.002

<0.005

0.045

3.910

04-3038

NJ-7

0.007

1590

14.1

0.0118

1330

0.608

370

<0.002

<0.005

0.118

4.280

04-3043

NJ-9

<0.001

1150

57.1

0.0054

1860

0.484

620

<0.002

<0.005

0.193

3.210

04-3039

NJ-10

0.005

1860

8.1

0.0098

1390

0.508

490

<0.002

<0.005

0.095

6.140

04-3048

NJ-11

0.002

2160

50.6

0.0059

1950

0.361

260

<0.002

0.006

0.088

3.050

04-3001

NJ-13

0.013

1540

80.5

0.0642

2550

1.300

510

<0.002

<0.005

0.086

7.390

04-3049

NJ-14

<0.001

1680

53.1

0.0010

1580

0.370

680

<0.002

<0.005

0.064

6.880

04-3037

NJ-16

0.027

1930

28.3

0.0333

1810

0.488

230

<0.002

<0.005

0.229

2.660

04-3002

NJ-19

0.014

2280

84.0

0.0258

3830

0.814

550

<0.002

<0.005

0.114

4.660

04-3050

NJ-20

0.010

1500

88.6

0.0102

1510

0.374

590

<0.002

0.007

0.057

6.610

04-3046

NJ-21

0.001

1420

40.4

0.0111

1710

0.386

320

<0.002

<0.005

0.069

3.820

04-3045

NJ-22

0.008

1770

48.1

0.0645

1760

0.278

670

<0.002

0.006

0.101

6.140

04-3041

NJ-23

0.001

1720

93.4

0.0021

6790

0.569

420

<0.002

0.010

0.111

2.020

04-3005

SV-7

<0.05

112

78.3

0.0115

7000

1610.000

49000

0.135

0.139

1.34

59.900

04-3003

SV-8

<0.05

105

98.6

0.0114

7260

1900.000

44000

0.047

0.072

0.546

60.200

04-3032

SV-9

<0.05

186

89.4

<0.001

7340

1320.000

45000

0.100

0.171

1.5

58.900

04-3029

SV-11

<0.05

132

57.5

0.0127

7540

1510.000

44000

0.125

0.165

3.5

59.200

04-3004

SV-18

<0.05

122

104.0

0.0139

7660

2120.000

50000

0.148

0.110

2.03

63.600

04-3006

SV-19

<0.05

131

105.0

0.0255

7840

1730.000

52000

0.080

<0.05

0.615

61.600

05-3004

RN-12

0.113

61

139.0

0.0012

7890

6480.000

74000

0.417

0.119

0.291

62.600

05-3003

RN-15

0.073

92

156.0

0.0325

7860

6650.000

78000

0.923

0.320

0.268

61.600

04-3027

Bödmódstadir

<0.001

54

1.5

<0.0001

132

0.223

140

<0.002

0.005

0.026

0.770

04-3026

EfriReykirS-24

<0.001

476

2.1

0.0005

163

0.171

150

<0.002

0.005

0.086

0.660

04-3025

ReykholtS-1

<0.001

191

16.9

<0.0001

350

0.241

250

<0.002

<0.005

<0.01

0.758

04-3024

ReykjabólS-1

<0.001

544

15.2

0.0016

541

0.066

83

<0.002

0.006

0.038

0.342

94

CHAPTER A.

TableA.1:(cont.)

Sample

Well

Cu

Ga

Ge

Hg

ILi

Mn

Mo

Ni

PPb

04-3007

N-12

0.631

3.21

23.7

<0.0020

10.20

213.0

2.240

0.91

0.718

2.21

0.042

04-3008

N-09

3.910

4.28

23.7

0.006

4.25

112.0

1.070

0.34

0.090

<1

0.105

04-3010

K-13

0.446

3.85

23.7

<0.0020

2.61

156.0

1.210

2.88

0.053

1.02

0.038

04-3015

K-15

1.730

6.15

27.2

<0.0020

2.35

283.0

1.480

2.56

0.269

1.38

0.018

04-3022

K-16

0.175

4.92

24.7

<0.0020

5.68

264.0

1.590

3.87

<0.05

1.25

<0.01

04-3021

K-17

0.542

6.92

16.2

<0.0020

1.10

247.0

0.967

4.58

<0.05

1.14

0.011

04-3019

K-20

0.116

5.68

28.6

<0.0020

9.68

370.0

1.760

2.57

0.078

2.32

0.011

04-3009

K-21

0.526

5.12

22.1

<0.0020

11.20

182.0

0.755

8.13

<0.05

1.08

0.050

04-3023

K-24

0.107

3.38

29.9

<0.0020

3.17

91.6

0.337

1.34

<0.05

<1

0.023

04-3011

K-27

0.257

4.74

28.7

0.003

3.00

285.0

0.653

5.59

<0.05

1.21

0.027

04-3014

K-32

0.395

6.61

33.2

<0.0020

6.87

474.0

1.660

2.19

<0.05

1.96

0.017

04-3012

K-34

0.605

4.30

31.7

<0.0020

3.11

215.0

2.540

4.21

<0.05

1.31

0.019

04-3040

NJ-5

0.271

5.72

46.9

0.003

8.50

186.0

0.372

1.47

<0.05

1.03

0.011

04-3042

NJ-6

0.271

4.08

40.1

<0.0020

10.40

194.0

0.698

5.98

<0.05

<1

<0.01

04-3038

NJ-7

0.966

5.09

42.0

0.004

6.61

225.0

0.155

1.12

<0.05

1.23

0.021

04-3043

NJ-9

0.241

4.26

33.7

<0.0020

11.10

188.0

0.773

6.17

0.133

1.23

<0.01

04-3039

NJ-10

0.627

6.18

51.0

0.005

8.55

278.0

0.158

1.48

<0.05

1.16

0.018

04-3048

NJ-11

0.617

7.80

49.9

<0.0020

5.46

187.0

0.328

1.37

<0.05

1.29

<0.01

04-3001

NJ-13

0.787

4.95

43.0

0.008

11.70

381.0

0.405

3.72

0.295

1.91

0.126

04-3049

NJ-14

0.417

5.05

32.2

<0.0020

12.10

274.0

0.209

7.48

<0.05

1.02

0.015

04-3037

NJ-16

0.663

7.09

47.7

0.013

5.28

146.0

0.158

1.56

0.139

1.47

0.140

04-3002

NJ-19

1.260

6.96

39.6

0.007

11.90

312.0

0.864

3.47

0.089

30.10

0.142

04-3050

NJ-20

0.270

5.06

31.9

<0.0020

34.80

223.0

0.393

8.91

<0.05

1.18

<0.01

04-3046

NJ-21

0.257

5.31

29.7

<0.0020

6.85

234.0

0.531

1.90

<0.05

1.13

0.026

04-3045

NJ-22

0.318

5.90

36.6

0.003

11.00

277.0

0.223

2.66

<0.05

1.52

0.055

04-3041

NJ-23

0.461

5.77

40.5

<0.0020

7.81

197.0

0.754

3.14

<0.05

1.58

<0.01

04-3005

SV-7

<1

0.67

17.6

0.007

77.10

2590.0

152.000

7.68

5.380

10.60

0.120

04-3003

SV-8

<1

0.24

15.6

0.012

122.00

3080.0

232.000

8.71

2.460

<10

<0.1

04-3032

SV-9

<1

0.48

21.1

0.034

131.00

2570.0

130.000

6.17

3.840

15.20

<0.1

04-3029

SV-11

1.280

0.65

22.3

<0.0020

77.30

2730.0

204.000

5.94

3.820

11.30

0.164

04-3004

SV-18

<1

0.07

13.8

0.010

133.00

2980.0

204.000

10.10

2.510

<10

<0.1

04-3006

SV-19

<1

<0.05

16.6

0.010

109.00

2770.0

172.000

6.44

2.160

13.20

<0.1

05-3004

RN-12

<1

5.70

22.3

0.101

292.00

3840.0

2210.000

64.40

0.535

21.50

<0.1

05-3003

RN-15

1.740

2.29

23.0

0.068

300.00

3930.0

1500.000

92.50

1.330

15.40

<0.1

04-3027

Bödmódstadir

0.802

4.52

7.4

<0.0020

2.43

25.2

0.308

8.57

0.245

6.70

0.226

04-3026

EfriReykirS-24

<0.1

6.16

9.3

0.004

2.60

43.6

0.299

7.36

0.322

<1

0.021

04-3025

ReykholtS-1

0.115

5.40

8.3

<0.0020

5.87

44.1

0.540

25.80

0.375

<1

0.023

04-3024

ReykjabólS-1

0.156

4.49

10.7

0.019

1.21

40.5

0.633

24.90

1.300

2.62

0.059

95

TableA.1:(cont.)

Sample

Well

Rb

Sb

Sn

Sr

Th

Ti

Tl

UV

WZn

04-3007

N-12

93.6

1.170

<0.05

2.55

<0.002

0.283

0.017

<0.0005

0.781

2.070

0.432

04-3008

N-09

104.0

0.854

<0.05

32.90

<0.002

0.312

0.020

<0.0005

0.158

2.220

0.992

04-3010

K-13

210.0

1.020

<0.05

22.50

<0.002

0.324

0.052

<0.0005

1.060

4.650

0.844

04-3015

K-15

196.0

1.880

<0.05

10.10

<0.002

0.272

0.055

<0.0005

2.120

6.870

<0.2

04-3022

K-16

206.0

1.680

<0.05

5.16

<0.002

0.178

0.011

<0.0005

1.730

6.410

0.505

04-3021

K-17

124.0

1.210

<0.05

0.97

<0.002

0.245

0.011

<0.0005

1.570

3.870

0.509

04-3019

K-20

339.0

2.310

<0.05

6.93

<0.002

0.198

0.021

<0.0005

1.890

6.990

0.272

04-3009

K-21

156.0

0.811

<0.05

12.20

<0.002

0.246

0.046

<0.0005

1.150

4.210

0.214

04-3023

K-24

167.0

0.236

<0.05

28.70

<0.002

0.064

0.020

<0.0005

0.492

3.640

1.600

04-3011

K-27

176.0

1.160

<0.05

16.80

<0.002

0.153

0.095

<0.0005

1.880

4.470

<0.2

04-3014

K-32

245.0

3.030

<0.05

9.42

<0.002

0.254

0.097

<0.0005

7.940

6.590

0.544

04-3012

K-34

184.0

3.730

<0.05

5.41

<0.002

0.301

0.019

<0.0005

4.590

3.500

0.350

04-3040

NJ-5

121.0

0.499

<0.05

2.20

<0.002

0.077

<0.005

<0.0005

2.270

5.730

0.720

04-3042

NJ-6

153.0

1.130

<0.05

2.23

<0.002

0.137

0.006

<0.0005

2.700

5.350

0.464

04-3038

NJ-7

150.0

0.506

<0.05

2.74

<0.002

0.140

<0.005

<0.0005

0.602

4.480

0.923

04-3043

NJ-9

127.0

0.840

<0.05

3.18

<0.002

0.221

0.007

<0.0005

1.760

5.050

0.485

04-3039

NJ-10

142.0

0.563

<0.05

2.28

<0.002

0.162

<0.005

<0.0005

2.190

6.020

0.780

04-3048

NJ-11

97.8

0.751

<0.05

1.87

<0.002

0.141

<0.005

<0.0005

1.960

5.200

0.610

04-3001

NJ-13

161.0

1.750

<0.05

1.16

<0.002

0.392

0.031

<0.0005

1.760

4.930

0.444

04-3049

NJ-14

173.0

0.676

<0.05

2.19

<0.002

0.150

0.009

<0.0005

3.060

5.440

0.692

04-3037

NJ-16

93.6

0.640

<0.05

2.19

<0.002

0.154

0.012

<0.0005

0.639

4.140

11.200

04-3002

NJ-19

137.0

1.060

<0.05

0.85

<0.002

0.300

0.015

<0.0005

1.770

4.870

0.897

04-3050

NJ-20

152.0

1.970

<0.05

3.12

<0.002

0.174

<0.005

<0.0005

2.910

5.320

0.707

04-3046

NJ-21

113.0

0.746

<0.05

1.54

<0.002

0.147

0.006

<0.0005

1.500

4.720

2.240

04-3045

NJ-22

143.0

0.880

<0.05

0.87

<0.002

0.155

0.019

0.001

4.100

5.570

6.510

04-3041

NJ-23

71.2

0.664

<0.05

2.33

<0.002

0.599

<0.005

<0.0005

2.100

5.210

0.633

04-3005

SV-7

3020.0

0.972

<0.5

8050.00

<0.02

2.210

8.440

<0.005

3.020

0.589

2.130

04-3003

SV-8

3020.0

1.680

<0.5

8410.00

<0.02

2.170

8.620

<0.005

2.310

0.719

2.790

04-3032

SV-9

3080.0

1.540

<0.5

7660.00

<0.02

1.510

6.770

<0.005

8.060

<0.5

<2

04-3029

SV-11

3050.0

0.577

<0.5

8140.00

<0.02

3.850

7.110

<0.005

4.000

<0.5

3.880

04-3004

SV-18

3290.0

1.840

<0.5

7820.00

<0.02

4.030

9.310

<0.005

3.750

0.631

<2

04-3006

SV-19

3220.0

1.850

<0.5

8160.00

<0.02

1.560

8.320

<0.005

4.410

<0.5

2.470

05-3004

RN-12

4150.0

1.090

<0.5

9010.00

<0.02

6.150

9.940

<0.005

9.970

0.884

16.300

05-3003

RN-15

4200.0

1.370

<0.5

9120.00

<0.02

<0.1

17.600

<0.005

9.080

1.000

50.900

04-3027

Bödmódstadir

18.6

0.065

<0.05

11.00

<0.002

0.283

0.015

0.003

4.400

4.480

0.859

04-3026

EfriReykirS-24

30.7

0.163

<0.05

7.72

<0.002

0.068

0.008

<0.0005

1.890

5.010

0.828

04-3025

ReykholtS-1

24.2

0.334

<0.05

16.90

<0.002

0.075

0.015

<0.0005

0.921

7.190

0.524

04-3024

ReykjabólS-1

19.4

0.653

<0.05

6.34

<0.002

0.147

0.011

<0.0005

1.410

6.400

0.848

96

CHAPTER A.

Table

A.2:Trace

elem

entanalysesofvaporsamplesfrom

wet-steam

anddry-

steam

wells.Concentrationsinµg/kg

Sample

Well

Ag

Al

As

Au

BBa

Cd

Co

Cr

Cs

04-3007

N-12

<0.001

3.21

0.100

<0.0001

84.5

1.180

0.030

0.009

11.500

0.089

04-3008

N-9

<0.001

2.95

<0.050

<0.0001

17.8

0.533

0.003

0.007

2.790

0.127

04-3010

K-13

<0.001

9.54

<0.050

<0.0001

18.8

0.252

<0.002

0.007

0.765

0.201

04-3015

K-15

<0.001

4.41

0.061

<0.0001

22.8

0.203

<0.002

<0.005

0.175

0.258

04-3022

K-16

<0.001

3.10

0.056

<0.0001

23.1

0.198

<0.002

<0.005

0.177

1.130

04-3021

K-17

<0.001

1.61

<0.050

<0.0001

55.4

0.250

<0.002

<0.005

0.915

0.267

04-3018

K-19

<0.001

2.64

<0.050

<0.0001

169.0

0.294

<0.002

<0.005

0.359

0.272

04-3019

K-20

<0.001

12.00

<0.050

<0.0001

27.4

0.445

<0.002

<0.005

1.140

0.689

04-3009

K-21

<0.001

4.90

<0.050

<0.0001

0.395

<0.002

0.008

1.060

0.062

04-3023

K-24

<0.001

9.13

0.359

<0.0001

5.5

0.068

<0.002

<0.005

0.202

0.756

04-3011

K-27

0.003

9.43

0.125

<0.0001

11.5

0.142

0.005

0.014

2.970

0.187

04-3017

K-30

<0.001

3.89

<0.050

<0.0001

188.0

0.185

<0.002

0.008

0.150

0.259

04-3020

K-31

<0.001

4.80

0.969

<0.0001

342.0

0.241

0.008

0.008

0.613

0.447

04-3012

K-33

<0.001

5.65

<0.050

<0.0001

114.0

0.339

<0.002

<0.005

0.693

0.171

04-3013

K-35

<0.001

5.65

<0.050

<0.0001

98.8

0.471

<0.002

<0.005

1.670

0.405

04-3040

NJ-5

<0.001

3.68

<0.050

<0.0001

22.1

0.168

0.002

<0.005

1.240

0.363

04-3042

NJ-6

<0.001

4.34

<0.050

<0.0001

43.7

0.167

<0.002

0.007

2.420

0.321

04-3038

NJ-7

<0.001

2.99

<0.050

<0.0001

23.1

0.245

<0.002

<0.005

3.230

0.232

04-3043

NJ-9

0.001

3.12

<0.050

<0.0001

30.9

0.203

0.005

<0.005

0.649

0.384

04-3039

NJ-10

<0.001

7.79

<0.050

<0.0001

24.1

0.142

<0.002

0.010

1.360

0.361

04-3048

NJ-11

<0.001

3.82

<0.050

<0.0001

11.4

0.309

<0.002

0.006

0.788

0.229

04-3001

NJ-13

<0.001

5.45

0.186

<0.0001

28.5

1.840

<0.002

0.068

16.300

0.150

04-3049

NJ-14

<0.001

5.76

<0.050

<0.0010

27.7

0.346

<0.002

<0.005

0.501

0.325

04-3037

NJ-16

0.003

147.00

<0.050

0.0011

51.4

1.350

0.015

0.178

35.200

0.065

04-3002

NJ-19

<0.001

8.65

<0.050

<0.0001

27.7

1.620

0.002

0.005

6.090

0.769

04-3050

NJ-20

<0.001

2.55

<0.050

<0.0001

32.1

0.236

<0.002

<0.005

0.553

0.433

04-3046

NJ-21

<0.001

2.56

<0.050

<0.0001

25.9

0.318

<0.002

<0.005

0.681

0.485

04-3045

NJ-22

<0.001

6.19

<0.050

<0.0001

35.1

0.446

<0.002

0.006

5.330

0.043

04-3041

NJ-23

<0.001

4.57

<0.050

<0.0001

83.0

0.053

<0.002

0.045

0.424

0.129

04-3005

SV-7

<0.001

3.44

0.193

<0.0001

172.0

0.735

0.028

0.022

2.710

0.489

04-3003

SV-8

<0.001

4.39

0.317

<0.0001

157.0

0.537

0.007

0.023

6.740

0.141

05-3001

SV-10

0.010

22.10

0.055

<0.0001

46.7

0.261

0.050

0.169

113.000

0.033

04-3029

SV-11

<0.001

7.17

0.051

<0.0001

183.0

1.110

<0.002

0.066

9.920

0.292

05-3002

SV-16

<0.001

1.72

<0.050

<0.0001

74.3

0.175

<0.002

0.023

12.800

<0.030

04-3004

SV-18

0.001

8.40

0.922

<0.0001

210.0

0.461

0.017

0.104

31.500

0.472

04-3006

SV-19

<0.001

11.90

0.283

<0.0001

173.0

0.910

0.004

0.017

1.360

0.773

04-3031

SV-20

<0.001

6.04

0.083

0.0001

66.3

2.730

<0.002

0.010

1.830

0.511

05-3004

RN-12

<0.001

2.55

0.348

<0.0001

197.0

0.394

0.015

0.022

1.520

<0.030

05-3003

RN-15

<0.001

2.00

1.900

<0.0001

119.0

1.660

0.005

0.016

8.450

0.041

04-3026

EfriReykir

<0.001

10.10

<0.050

0.0001

7.1

0.244

<0.002

0.021

0.753

0.060

04-3024

Reykjaból

0.003

6.83

<0.050

<0.0001

9.7

6.840

0.003

4.510

10.600

1.740

97

TableA.2:(cont.)

Sample

Well

Cu

Ga

Ge

Hg

Li

Mn

Mo

Ni

Pb

04-3007

N-12

0.104

0.031

0.020

0.006

0.157

17.50

<0.050

1.730

0.4930

04-3008

N-9

0.114

0.017

0.016

0.008

0.427

13.00

<0.050

0.806

0.6840

04-3010

K-13

0.153

0.051

0.012

0.005

0.585

5.57

<0.050

0.533

0.2810

04-3015

K-15

<0.100

0.063

<0.001

0.014

0.887

1.55

<0.050

0.108

0.0915

04-3022

K-16

<0.100

0.055

0.008

0.015

1.350

1.28

<0.050

0.136

0.1680

04-3021

K-17

<0.100

0.004

<0.001

0.012

0.181

3.41

<0.050

0.343

0.0147

04-3018

K-19

<0.100

0.009

<0.001

0.010

0.714

4.95

<0.050

0.278

0.0233

04-3019

K-20

<0.100

0.026

0.013

0.006

0.882

7.80

0.069

0.491

0.0278

04-3009

K-21

<0.100

0.049

0.018

0.008

0.808

14.10

<0.050

3.250

0.2940

04-3023

K-24

0.140

0.036

<0.001

0.010

0.647

0.66

0.177

0.517

0.1950

04-3011

K-27

0.188

0.039

0.021

0.008

0.432

2.99

0.340

1.610

1.2400

04-3017

K-30

<0.100

0.013

<0.001

0.013

0.745

3.88

<0.050

0.184

0.1260

04-3020

K-31

1.510

0.015

<0.001

0.003

0.328

4.03

<0.050

0.354

0.1570

04-3012

K-33

<0.100

0.032

0.018

0.011

0.300

9.66

<0.050

0.528

0.1060

04-3013

K-35

<0.100

0.038

<0.001

0.008

0.219

10.40

<0.050

0.506

0.0773

04-3040

NJ-5

0.145

0.090

0.036

0.014

1.130

2.80

<0.050

0.436

0.1430

04-3042

NJ-6

0.102

0.037

0.026

0.034

0.723

2.78

<0.050

0.560

0.1040

04-3038

NJ-7

<0.100

0.043

0.043

0.019

1.030

3.90

<0.050

0.661

0.1270

04-3043

NJ-9

0.679

0.060

0.025

0.036

0.578

3.89

0.119

0.406

0.2880

04-3039

NJ-10

<0.100

0.056

0.063

0.018

1.160

2.32

<0.050

0.391

0.1000

04-3048

NJ-11

<0.100

0.027

0.048

0.014

0.290

5.56

<0.050

0.624

0.0240

04-3001

NJ-13

0.104

0.080

0.038

0.007

0.354

42.50

0.105

12.000

0.0176

04-3049

NJ-14

<0.100

0.041

0.029

0.033

0.488

5.47

<0.050

0.099

0.0114

04-3037

NJ-16

0.145

0.146

0.096

0.022

0.475

14.80

0.068

15.300

3.7500

04-3002

NJ-19

0.348

0.060

0.109

0.004

2.100

58.80

<0.050

0.317

1.3700

04-3050

NJ-20

<0.100

0.040

0.049

0.016

0.336

4.02

<0.050

0.178

0.0180

04-3046

NJ-21

<0.100

0.043

0.015

0.017

0.477

2.93

<0.050

0.190

0.0506

04-3045

NJ-22

<0.100

0.061

0.030

0.011

0.012

8.32

<0.050

0.609

0.0827

04-3041

NJ-23

<0.100

0.151

0.045

0.018

0.894

0.84

<0.050

0.588

0.0731

04-3005

SV-7

0.173

0.013

0.033

0.041

2.100

3.99

0.076

1.760

11.9000

04-3003

SV-8

0.362

0.010

0.021

0.010

0.868

6.26

0.154

2.910

9.8000

05-3001

SV-10

0.816

0.016

0.035

0.007

0.188

6.10

1.060

7.460

1.8700

04-3029

SV-11

0.117

0.024

0.023

0.102

0.437

12.30

0.091

8.760

0.2520

05-3002

SV-16

<0.100

0.001

0.009

0.055

0.059

2.16

0.253

1.650

0.6280

04-3004

SV-18

0.646

0.011

0.045

0.037

2.970

11.50

1.060

7.700

128.0000

04-3006

SV-19

0.173

0.040

0.025

0.088

1.630

5.47

0.164

1.490

1.2700

04-3031

SV-20

<0.100

0.022

0.007

0.035

0.478

15.40

0.065

2.470

0.1870

05-3004

RN-12

0.473

0.011

0.026

0.025

0.233

1.71

0.217

1.140

1.6600

05-3003

RN-15

<0.100

0.010

0.021

0.061

1.460

25.40

0.217

0.955

1.4900

04-3026

EfriReykir

<0.100

0.034

0.018

0.014

0.896

7.95

0.196

3.310

0.1710

04-3024

Reykjaból

1.420

0.037

0.057

0.201

0.408

41.80

0.293

383.000

2.1800

98

CHAPTER A.

TableA.2:(cont.)

Sample

Well

Rb

Sb

Sr

Te

Ti

Tl

UV

Zn

04-3007

N-12

1.560

<0.010

2.640

<0.005

0.117

0.0123

0.0005

0.1250

2.070

04-3008

N-9

5.150

<0.010

1.280

<0.005

0.065

0.0054

<0.0005

0.0569

0.457

04-3010

K-13

5.860

<0.010

0.911

<0.005

0.414

0.0295

<0.0005

0.1300

3.300

04-3015

K-15

5.430

<0.010

0.241

<0.005

0.134

0.0470

<0.0005

0.0732

<0.200

04-3022

K-16

15.300

<0.010

0.181

<0.005

0.046

0.0571

<0.0005

0.0303

0.315

04-3021

K-17

2.460

<0.010

0.320

<0.005

0.024

0.0102

<0.0005

0.0395

<0.200

04-3018

K-19

3.950

<0.010

0.475

<0.005

0.086

0.0170

<0.0005

0.0175

0.209

04-3019

K-20

7.330

<0.010

0.650

<0.005

0.021

0.0179

<0.0005

0.0621

0.245

04-3009

K-21

4.640

<0.010

1.330

<0.005

0.231

0.0188

<0.0005

0.0727

0.355

04-3023

K-24

9.710

<0.010

0.091

<0.005

0.999

0.0355

<0.0005

0.0633

21.000

04-3011

K-27

5.210

<0.010

0.520

<0.005

0.224

0.0236

<0.0005

0.0849

0.640

04-3017

K-30

5.000

<0.010

0.353

<0.005

0.106

0.0186

<0.0005

0.0508

0.424

04-3020

K-31

4.910

<0.010

0.380

<0.005

0.169

0.0149

<0.0005

0.0544

1.380

04-3012

K-33

3.910

<0.010

1.120

<0.005

0.040

0.0106

<0.0005

0.0742

<0.200

04-3013

K-35

4.010

<0.010

1.210

<0.005

0.130

0.0149

<0.0005

0.0482

0.345

04-3040

NJ-5

10.300

<0.010

0.441

<0.005

0.089

0.0577

<0.0005

0.0548

0.656

04-3042

NJ-6

7.300

<0.010

0.437

<0.005

0.320

0.0223

<0.0005

0.1060

0.663

04-3038

NJ-7

8.730

<0.010

0.362

<0.005

0.097

0.0334

<0.0005

0.0371

0.836

04-3043

NJ-9

10.500

<0.010

0.436

0.010

0.051

0.0648

<0.0005

0.0620

0.913

04-3039

NJ-10

9.790

<0.010

0.244

<0.005

0.703

0.0249

<0.0005

0.0829

0.702

04-3048

NJ-11

4.590

<0.010

0.494

<0.005

0.130

0.0144

<0.0005

0.0601

0.544

04-3001

NJ-13

3.530

0.010

6.120

<0.005

0.120

0.0102

<0.0005

0.0612

0.892

04-3049

NJ-14

6.350

<0.010

0.674

<0.005

0.111

0.0064

<0.0005

0.1090

0.499

04-3037

NJ-16

1.540

<0.010

2.470

<0.005

11.000

0.0312

0.0007

0.5150

7.360

04-3002

NJ-19

9.160

<0.010

6.110

<0.005

0.093

<0.0050

<0.0005

0.0219

0.862

04-3050

NJ-20

7.510

<0.010

0.539

<0.005

0.026

0.0089

<0.0005

0.0284

0.530

04-3046

NJ-21

7.500

<0.010

0.385

<0.005

0.121

0.0579

<0.0005

0.1130

1.090

04-3045

NJ-22

2.430

<0.010

0.721

<0.005

0.385

0.0191

<0.0005

0.6750

2.190

04-3041

NJ-23

4.880

<0.010

0.155

<0.005

0.059

0.0395

<0.0005

0.0555

0.521

04-3005

SV-7

18.800

<0.010

2.720

<0.005

0.047

0.0866

0.0008

0.0487

1.260

04-3003

SV-8

2.930

<0.010

4.420

<0.005

0.040

0.0368

<0.0005

0.0448

3.000

05-3001

SV-10

1.110

0.018

0.281

<0.005

3.100

0.0737

<0.0005

0.3210

10.500

04-3029

SV-11

5.000

<0.010

2.890

<0.005

0.187

0.0195

<0.0005

0.0758

4.670

05-3002

SV-16

0.584

<0.010

0.185

<0.005

0.101

0.0777

<0.0005

0.0640

2.050

04-3004

SV-18

13.200

0.012

2.540

<0.005

0.054

0.1630

<0.0005

0.4780

3.790

04-3006

SV-19

17.500

<0.010

4.280

<0.005

0.131

0.3030

<0.0005

0.0583

0.735

04-3031

SV-20

9.090

<0.010

5.530

<0.005

0.106

0.0596

<0.0005

0.0825

1.360

05-3004

RN-12

0.951

0.020

0.312

<0.005

0.119

0.1010

<0.0005

0.0291

8.350

05-3003

RN-15

1.570

<0.010

2.400

<0.005

0.069

0.0277

<0.0005

0.0539

3.700

04-3026

EfriReykir

1.350

<0.010

1.090

<0.005

0.177

0.0046

<0.0005

0.0336

0.542

04-3024

Reykjaból

3.950

<0.010

2.540

<0.005

0.281

1.3600

<0.0005

0.0591

3.970

99

Bibliography

Aiuppa, A., Caleca, A., Federico, C., Gurrieri, S., Valenza, M., 2004. Di�use degassing ofcarbon dioxide at Somma-Vesuvius volcanic complex (Southern Italy) and its relationwith regional tectonics. Journal of Volcanology and Geothermal Research 133, 55�79.

Ármannsson, H., Benjamínsson, J., Je�rey, A. W. A., 1989. Gas changes in the Kra�ageothermal system, Iceland. Chemical Geology 76, 175�196.

Ármannsson, H., Gíslason, G., Hauksson, T., 1982. Magmatic gases in well �uids aid themapping of the �ow pattern in a geothermal system. Geochimica et CosmochimicaActa 46, 167�177.

Ármannsson, H., Gudmundsson, Á., Steingrímsson, B. S., 1987. Exploration and devel-opment of the Kra�a geothermal area. Jökull 37, 13�30.

Árnason, B., 1976. Groundwater systems in Iceland traced by deuterium. Societas sci-entiarum Islandica 42, 236 pp.

Árnason, J. G., Bird, D. K., 1992. Formation of zoned epidote in hydrothermal systems.In: Water-Rock Interaction-7. Vol. 2. WRI-7, pp. 1473�1476.

Arnórsson, S., 1969. A geochemical study of selected elements in thermal waters inIceland. Ph.D. thesis, Imperial College, Univ. London.

Arnórsson, S., 1978. Major element chemistry of geothermal sea-water at Reykjanesand Svartsengi, Iceland. Mineralogical Magazine 42 (322), 209�220.

Arnórsson, S., 1985. The use of mixing models and chemical geothermometers for es-timating underground temperatures in geothermal systems. Journal of Volcanologyand Geothermal Research 23, 299�335.

Arnórsson, S., 1995. Geothermal systems in Iceland: Structure and conceptual models.1. High-temperature areas. Geothermics 24, 561�602.

Arnórsson, S., 2002. Chemical environmental impact of geothermal resource utilization.Geochimica et Cosmochimica Acta 66, A31�A31.

Arnórsson, S., 2003. Arsenic in surface- and up to 90°C ground waters in a basaltarea, N-Iceland. Applied Geochemistry 18, 1297�1312.

101

BIBLIOGRAPHY

Arnórsson, S., 2004. Environmental impact of geothermal energy utilization. In: Gieré,R., Stille, P. (Eds.), Energy, Waste and the Environment: a Geochemical Perspective.Vol. 236. Geological Society, London, Special Publication, pp. 297�336.

Arnórsson, S., Andrésdóttir, A., 1995. Processes controlling the distribution of B and Clin natural-waters in Iceland. Geochimica et Cosmochimica Acta 59 (20), 4125�4146.

Arnórsson, S., Axelsson, G., Saemundsson, K., 2008. Geothermal systems in Iceland.Jökull 58, in press.

Arnórsson, S., Bjarnason, J. Ö., Giroud, N., Gunnarsson, I., Stefánsson, A., 2006.Sampling and analysis of geothermal �uids. Geo�uids 6 (3), 203�216.

Arnórsson, S., Björnsson, S., Muna, Z. W., Bwire-Ojiambo, S., 1990. The use of gaschemistry to evaluate boiling processes and initial steam fractions in geothermalreservoirs with an example from the Olkaria �eld, Kenya. Geothermics 19 (6), 497�514.

Arnórsson, S., D'Amore, F., Gerardo-Abaya, J., 2000. Isotopic and Chemical Tech-niques in Geothermal Exploration, Development and Use. International Atomic En-ergy Agency, Vienna.

Arnórsson, S., Grönvold, K., Sigurdsson, S., 1978. Aquifer chemistry of four high-temperature geothermal systems in Iceland. Geochimica et Cosmochimica Acta 42,523�536.

Arnórsson, S., Gunnlaugsson, E., 1985. New Gas Geothermometers for Geothermal-Exploration - Calibration and Application. Geochimica et Cosmochimica Acta 49 (6),1307�1325.

Arnórsson, S., Gunnlaugsson, E., Svavarsson, H., 1983. The chemistry of geothermalwaters in Iceland. II. Mineral equilibria and independent variables controlling watercompositions. Geochimica et Cosmochimica Acta 47, 547�566.

Arnórsson, S., Sigurdsson, S., Svavarsson, H., 1982. The chemistry of geothermal watersin Iceland. 1. Calculation of aqueous speciation from 0-degree-C to 370-degree-C.Geochimica et Cosmochimica Acta 46 (9), 1513�1532.

Arnórsson, S., Stefánsson, A., 1999. Assessment of feldspar solubility constants in waterin the range 0 degrees to 350 degrees C at vapor saturation pressures. AmericanJournal of Science 299 (3), 173�209.

Arnórsson, S., Stefánsson, A., Bjarnason, J. Ö., 2007. Fluid-�uid interaction in geother-mal systems. Reviews in Mineralogy & Geochemistry 65, 259�312.

Axtmann, R. C., 1975. Chemical aspects of the environmental impact of geothermalpower. In: Second United Nations Symposium on the Development and Use ofGeothermal Resources, San Francisco, California. Vol. 20-29. pp. 1323�1327.

102

BIBLIOGRAPHY

Badruk, M., Kabay, N., 2003. Removal of boron from Kizildere-Denizli geothermalbrines using ion-exchange method. In: Second United Nations Symposium on theDevelopment and Use of Geothermal Resources, San Francisco, California. Vol. S14.pp. 8�13.

Ballantyne, J. M., Moore, J. N., 1988. Arsenic geochemistry in geothermal systems.Geochimica et Cosmochimica Acta 52, 475�483.

Bamford, R., Christensen, O., Capuano, R., 1980. Multielement geochemistry of solidmaterials in geothermal systems and its application, Part 1: The hot-water systemat the Roosevelt Hot Springs KGRA, Utah. Earth Science Laboratory/University ofUtah Reasearch Institute Report DOE/ET/27002-7, 168 p.

Bird, D. K., Spieler, A. R., 2004. Epidote in geothermal systems. Reviews in mineralogyand geochemistry 56, 235�300.

Bjarnason, J. Ö., 1984. Eldvörp�Chemical composition of the seawater-groundwaterand vapor from well EG-2. National Energy Authority Report OS-84071/JHD-11,20, (in Icelandic).

Bjarnason, J. Ö., 1988. Svartsengi�Geochemical monitoring 1980�1987. National En-ergy Authority Report OS-88001, 98 pp., (in Icelandic, with an English abstract).

Bjarnason, J. Ö., 1994. The speciation program WATCH, version 2.1. National EnergyAuthority .

Bjarnason, J. Ö., 1996. Svartsengi�Geochemical monitoring 1988�1995. National En-ergy Authority Report OS-96082, 125 pp., (in Icelandic, with an English abstract).

Björnsson, A., Hersir, G. P., Björnsson, G., 1986. The Hengill High-Temperature Areain SW-Iceland. Regional Geophysical Survey. Geothermal Resources Council Trans-actions 10, 205�210.

Björnsson, A., Johnsen, G., Sigurdsson, S., Thorbergsson, G., Tryggvason, E., 1979.Rifting of the plate boundary in North Iceland 1975�1978. Journal of GeophysicalResearch 84, 3029�3038.

Björnsson, G., Steingrímsson, B., 1992. Fifteen years of temperature and pressure mon-itoring in the Svartsengi high-temperature geothermal �eld in SW-Iceland. Geother-mal Resources Council Transactions 16, 627�634.

Björnsson, S., Arnórsson, S., Tómasson, J., 1972. Economic evaluation of the Reykjanesthermal brine area, Iceland. American Association of Petroleum Geologists Bulletin56 (12), 2380�2391.

Björnsson, S., Saemundsson, K., Einarsson, P., Tryggvason, E., Grönvold, K., 1977.Current rifting episode in north Iceland. Nature 266, 318�323.

103

BIBLIOGRAPHY

Bödvarsson, G., Björnsson, S., Gunnarsson, A., Gunnlaugsson, E., Sigurdsson, O.,Stefánsson, V., Steingrímsson, B., 1990. The Nesjavellir Geothermal Field, Iceland;1. Field Characteristics and Development of a Three-dimensional Numerical Model.Journal of Geothermal Science and Technology 2 (3), 189�228.

Bull, J., Minshull, T., Mitchell, N., Dix, J., Hardardottir, J., 2005. Magmatic andtectonic history of Iceland's western rift zone at Lake Thingvallavatn. GeologicalSociety of America Bulletin 117 (11�12), 1451�1465.

Cengiz, K., 2007. E�ects on environment and agriculture of geothermal wastewater andboron pollution in Great Menderes Basin. Environ. Monit. Assess. 125, 377�388.

Chiodini, G., Granieri, D., Avino, R., Caliro, S., Costa, A., Werner, C., 2005. Carbondioxide di�use degassing and estimation of heat release from volcanic and hydrother-mal systems. Journal of Geophysical Research (Solid Earth) 110, 8204�+.

Christensen, O., Capuano, R., Moore, J., 1983. Trace-element distribution in an ac-tive hydrothermal system, Roosevelt Hot Springs thermal area, Utah. Journal ofVolcanology and Geothermal Research 16, 99�129.

Clifton, A., Kattenhorn, S., 2006. Structural architecture of a highly oblique divergentplate boundary segment. Tectonophysics 419, 27�40.

Clotworthy, A., 2000. Response of Wairakei geothermal reservoir to 40 years of produc-tion. In: World Geothermal Congress 2000, Kyushu - Tohoku, Japan. pp. 2057�2062.

Darling, W. G., Ármannsson, H., 1989. Stable isotopic aspects of �uid-�ow in the Kra�a,Námafjall and Theistareykir geothermal systems of northeast Iceland. Chemical Ge-ology 76 (3�4), 197�213.

Deer, W. A., Howie, R. A., Zussman, J., 1992. An introduction to the rock-formingminerals - 2nd ed. Pearson.

Diakonov, I. I., Schott, J., Martin, F., Harrichourry, J. C., Escalier, J., 1999. Iron(III)solubility and speciation in aqueous solutions. Experimental study and modelling:Part 1. Hematite solubility from 60 to 300 degrees C in NaOH-NaCl solutionsand thermodynamic properties of Fe(OH)4(aq). Geochimica et Cosmochimica Acta63 (15), 2247�2261.

Einarsson, P., 1978. S-wave shadows in the kra�a caldera in NE-Iceland, evidence for amagma chamber in the crust. Bulletin of Volcanology 73, 1�9.

Ellis, A. J., Mahon, W. A. J., 1964. Natural hydrothermal systems and experimentalhot water/rock interactions. Geochimica et Cosmochimica Acta 28, 1323�1357.

Ellis, A. J., Mahon, W. A. J., 1967. Natural hydrothermal systems and experimentalhot water/rock interactions. Part II. Geochimica et Cosmochimica Acta 31, 519�538.

Ellis, A. J., Mahon, W. A. J., 1977. Chemistry and geothermal systems. AcademicPress, New York.

104

BIBLIOGRAPHY

Eysteinsson, H., 2000. Elevation and gravity changes at geothermal �elds on the Reykja-nes peninsula, SW-Iceland. In: World Geothermal Congress 2000, Kyushu - Tohoku,Japan. pp. 559�564.

Fernández-Prini, R., Alvarez, J. L., Harvey, A. H., 2003. Henry's constants and vapor-liquid distribution constants for gaseous solutes in H2O and D2O at high tempera-tures. Journal of Physical and Chemical Reference Data 32 (2), 903�916.

Flower, M. F. J., Pritchard, R. G., Brem, G., Cann, J. R., Delany, J., Emmerman,R., Gibson, I. L., Oakley, P. J., Robinson, P. T., Schmincke, H.-U., 1982. Chemi-cal stratigraphy, Iceland Research Drilling Project Reydarfjordur, eastern Iceland.Journal of Geophysical Research 87, 6489�6510.

Floyd, P. A., Fuge, R., 1982. Primary and secondary alkali and halogen element distri-bution in Iceland Researd Drilling Project basalts from eastern Iceland. Journal ofGeophysical Research 87, 6477�6488.

Fournier, R., Potter, R., 1982. An equation correlating the solubility of quartz in waterfrom 25-degrees-C to 900-degrees-C at pressures up to 10,000 bars. Geochimica etCosmochimica Acta 46 (10), 1969�1973.

Franzson, H., 1988. Nesjavellir, borehole geology and permeability in the reservoir.National Energy Authority Report OS-88046/JHD-09, (in Icelandic with an Englishsummary).

Franzson, H., 1998. Reservoir geology of the Nesjavellir high-temperature �eld, SW-Iceland. In: 19th Annual PNOC-EDC Geothermal Conference, Manila, Philippines.pp. 13�20.

Franzson, H., Gudmundsson, A., Fridleifsson, G., 1986. Nesjavellir High-TemperatureField, SW-Iceland - Reservoir Geology. In: 5th International Symposium on Water-Rock Interaction, Reykjavik, Iceland. pp. 23�27.

Fridleifsson, G., Albertsson, A., 2000. Deep Geothermal Drilling on the Reykjanes RidgeOpportunity for International Collaboration. In: World Geothermal Congress 2000.pp. 3701�3706.

Fridriksson, T., Kristjánsson, B. R., Ármannsson, H., Margrétardóttir, E., Ólafsdóttir,S., Chiodini, G., 2006. CO2 emissions and heat �ow through soil, fumaroles, andsteam heated mud pools at the Reykjanes geothermal area, SW Iceland. Appliedgeochemistry 21, 1551�1569.

Fridriksson, T., Ólafsson, M., Giroud, N., 2007. Production geochemical survey of Svart-sengi and Hafnir. Iceland GeoSurvey report 2007/040, (in Icelandic).

Garavelli, A., Laviano, R., Vurro, F., 1997. Sublimate deposition from hydrothermal�uids at the Fossa crater - Vulcano, Italy. European Journal of Mineralogy 9 (2),423�432.

105

BIBLIOGRAPHY

Garavelli, A., Vurro, F., 1994. Barberiite, NH4BF4, a new mineral from Vulcano, Aeo-lian Islands, Italy. American Mineralogist 79, 381�34.

Giggenbach, W. F., 1981. Geothermal mineral equilibria. Geochimica et CosmochimicaActa 45, 393�410.

Giggenbach, W. F., 1991. Chemical techniques in geothermal exploration. In: D'Amore,F. (Ed.), Application of Geochemistry in Geothermal Reservoir Development. UNI-TAR, pp. 119�144.

Giroud, N., Arnórsson, S., 2008a. Gas chemistry in �ve Icelandic high-temperaturegeothermal systems. In prep.

Giroud, N., Arnórsson, S., 2008b. Processes in�uencing As, B and Cl concentrations in�uids of volcanic geothermal systems in Iceland. In prep.

Giroud, N., Arnórsson, S., 2005. Estimation of long-term CO2 and H2S release duringoperation of geothermal power plants. World Geothermal Congress 2005 paper 0233.

Glover, R., 1988. Boron distribution between liquid and vapour in geothermal �uids.In: New Zealand Geothermal Workshop, Proceedings. pp. 223�227.

Go�, F., Janik, C. J., 2000. Encyclopedia of Volcanoes. Pergamon Press, Ch. Geother-mal systems, pp. 817�834.

Gudmundsson, B. T., Arnórsson, S., 2002. Geochemical monitoring of the Kra�a andNámafjall geothermal areas, N-Iceland. Geothermics 31 (2), 195�243.

Gudmundsson, B. T., Arnórsson, S., 2005. Secondary mineral-�uid equilibria in theKra�a and Námafjall geothermal systems, Iceland. Applied Geochemistry 20 (9),1607�1625.

Gudmundsson, J. S., Thórhallsson, S., 1986. The Svartsengi reservoir in Iceland.Geothermics 15 (1), 3�15.

Gunnarsson, I., Arnórsson, S., 2000. Amorphous silica solubility and the thermodynamicproperties of H4SiO4 degrees in the range of 0 degrees to 350 degrees C at P-sat.Geochimica et Cosmochimica Acta 64 (13), 2295�2307.

Gunnarsson, I., Arnórsson, S., 2005. Impact of silica scaling on the e�ciency of heatextraction from high-temperature geothermal �uids. Geothermics 34, 320�329.

Gunnlaugsson, E., 1977. The origin and distribution of sulphur in fresh and geothermallyaltered rocks in Iceland. Ph.D. thesis, Univ. Leeds, U.K.

Harder, H., 1969. Tungsten. In: Wedepohl, H. K. (Ed.), Handbook of Geochemistry.Vol. 1. Springer, New York.

Holland, T. J. B., Powell, R., 1998. An internally consistent thermodynamic data setfor phases of petrological interest. Journal of Metamorphic Geology 16 (3), 309�343.

106

BIBLIOGRAPHY

Horne, R., Satik, C., Mahiya, G., Li, K., Ambusso, W., Tovar, R., Wang, C., Nassori,H., 2000. Steam-water relative permeability. In: World Geothermal Congress 2000,Kyushu - Tohoku, Japan. pp. 2609�2615.

Hreggvidsdóttir, H., 1987. The greenschist to amphibolite facies transition in the Nesja-vellir hydrothermal system, Southwest Iceland. Master's thesis, University of Stan-ford.

Ívarsson, G., 1992. Geology and petrochemistry of the Torfajökull central volcano incentral south Iceland, in association with the Icelandic hot spot and rift zones. Ph.D.thesis, University of Hawaii.

Jakobsson, S. P., Jónsson, J., Shido, F., 1978. Petrology of the Western ReykjanesPeninsula, Iceland. Journal of Petrology 19 (4), 669�705.

James, R. H., Palmer, M. R., 2000. Marine geochemical cycles of the alkali elements andboron: The role of sediments. Geochimica et Cosmochimica Acta 64 (18), 3111�3122.

Johnson, J. W., Oelkers, E. H., Helgeson, H. C., 1992. SUPCRT92 - A software packagefor calculating the standard molal thermodynamic properties of minerals, gases, aque-ous species, and reactions from 1-bar to 5000-bar and 0-degrees-C to 1000-degrees-C.Computers and Geosciences 18, 899�947.

Jónasson, K., 1994. Rhyolite volcanism in the Kra�a central volcano, north-east Iceland.Bulletin of Volcanology 56, 516�528.

Kacandes, G., Grandsta�, D., 1989. Di�erences between geothermal and experimentallyderived �uids: How well do hydrothermal experiments model the composition ofgeothermal reservoir �uids? Geochimica et Cosmochimica Acta 53, 343�358.

Karingithi, C., 2002. Hydrothermal Mineral Bu�ers Controlling Reactive Gases Con-centrations in the Greater Olkaria Geothermal System, Kenya. Master's thesis, Uni-versity of Iceland, Universtiy of Iceland.

Karingithi, C. W., Arnórsson, S., Grönvold, K., 2008. Hydrothermal mineral bu�erscontrolling reactive gas concentrations in the Olkaria geothermal system, Kenya Inprep.

Krauskopf, K., Bird, D., 1994. Introduction to Geochemistry, 3rd edition. McGraw-Hil.

Kuritani, T., Nakamura, E., 2006. Elemental fractionation in lavas during post-eruptivedegassing: Evidence from trachytic lavas, Rishiri Volcano, Japan. Journal of Volcanol-ogy and Geothermal Research 149, 124�138.

Larsen, G., Grönvold, K., Thorarinsson, S., 1978. Volcanic eruption through a geother-mal borehole at Námafjall, Iceland. Nature 278, 707�710.

Li, K., Horne, R., 2004. Universal capillary pressure and relative permeability modelfrom fractal characterization of rock. In: Twenty-Ninth Workshop on GeothermalReservoir Engineering, Stanford University, California. Vol. SGP-TR-75.

107

BIBLIOGRAPHY

Lonker, S. W., Franzson, H., Kristmannsdóttir, H., 1993. Mineral-�uid interactionsin the Reykjanes and Svartsengi geothermal systems, Iceland. American Journal ofScience 293, 605�670.

Moore, J. G., Fabbi, B. P., 1971. An estimate of the juvenile sulfur content of basalt.Contrib. Mineral. Petrol. 33 (2), 118�127.

Mottl, M., Holland, H., 1978. Chemical exchange during hydrothermal alteration ofbasalt by seawater�i. Experimental results for major and minor components of sea-water. Geochimica et Cosmochimica Acta 42, 1103�1115.

Ólafsson, J., Riley, J., 1978. Geochemical studies on thermal brine from Reykjanes(Iceland). Chemical Geology 21, 219�237.

Onishi, H., 1978. Arsenic. In: Wedepohl, H. K. (Ed.), Handbook of Geochemistry.Vol. 3. Springer, New York.

Pálmason, G., 1973. Kinematics and heat �ow in a volcanic rift zone with applicationto Iceland. Geophys. J. Royal Astronom. Soc. 26, 515�535.

Pálmason, G., 1975. Geophysical methods in geothermal exploration. In: Proceedings2nd UN Symposium on the Development and Use of Geothermal Resources. Vol. 2.pp. 1175�1184.

Pierce, M., Moore, C., 1982. Adsorption of arsenite and arsenate on amorphous ironhydroxide. Water Research 16, 1247�1253.

Pokrovskii, V. A., Helgeson, H. C., 1995. Thermodynamic properties of aqueous speciesand the solubilities of minerals at high pressures and temperatures: The systemAl2O3-H2O-NaCl. American Journal of Science 295 (10), 1255�1342.

Pope, E., Bird, D., Arnórsson, S., Fridriksson, T., Elders, W., Fridleifsson, G., 2008.Fluid origin and evolution in the Reykjanes geothermal system, Iceland: A stableisotope study of hydrothermal epidote. American Journal of Science Submitted.

Pruess, K., 2002. Mathematical modeling of �uid �ow and heat transfer in geothermalsystems. United Nations University Geothermal Program, Reykjavik, Iceland. Tech.rep., Earth Science Division, Lawrence Berkeley National Laboratory, University ofCalifornia.

Ragnarsdóttir, K. V., Walther, J. V., Arnórsson, S., 1984. Description and interpreta-tion of the composition of �uid and alteration mineralogy in the geothermal system,at Svartsengi, Iceland. Geochimica et Cosmochimica Acta 48 (7), 1535�1553.

Robie, R. A., Hemingway, B. S., 1995. Thermodynamic properties of minerals andrelated substances at 298.15 K and 1 bar (105 Pascals) pressure and at higher tem-peratures. U.S. Geol. Survey Bull. 2131, 461 p.

Saemundsson, K., 1963. The age of Nesjahraun. Náttúrufraedingurinn 33 (41), (inIcelandic with an English summary).

108

BIBLIOGRAPHY

Saemundsson, K., 1972. Notes on the geology of the Torfajökull area (in Icelandic).Náttúrufraedingurinn 42, 81�144.

Saemundsson, K., 1974. Evolution of axial rifting zone in northern Iceland and Tjörnesfracture zone. Geological Society of America Bulletin 85 (4), 495�504.

Saemundsson, K., 1978. Fissure swarms and central volcanoes of the neovolcanic zonesof Iceland. Geological Journal Special Issue (10), 415�432.

Saemundsson, K., 1983. Fractures in the Kra�a area. on the status of the Kra�a geother-mal power station. In: Meeting held in Akureyri, Iceland 2�3 March 1983. pp. 4�8.

Saemundsson, K., 1991. Geology of the Kra�a system. In: The Natural History of LakeMývatn. The Icelandic Natural History Society, Reykjavik, pp. 24�95.

Saemundsson, K., 1992. Geology of the Thingvallavatn area. Oikos 64 (1�2), 40�68.

Sigurdsson, F., 1986. Hydrogeology and groundwater on the Reykjanes Peninsula. Jökull36, 11�29.

Sigurdsson, F., Einarsson, K., 1988. Groundwater resources of Iceland, Availability anddemand. Jökull 38, 35�53.

Sigvaldason, G., 1963. Epidote and related minerals in two deep geothermal drill holes,Reykjavik and Hveragerdi. Iceland. US Geol Surv Prof Paper 450E, E77�79.

Sigvaldason, G. E., Óskarsson, N., 1976. Chlorine in basalts from Iceland. Geochim.Cosmochim. Acta 40, 777�789.

Smedley, P. L., Kinniburgh, D. G., 2002. A review of the source, behavior and distri-bution of arsenic in natural waters. Applied geochemistry 17, 517�568.

Spivack, A., Palmer, M. R., Edmond, J., 1987. The sedimentary cycle of the boronisotopes. Geochimica et Cosmochimica Acta 51 (7), 1939�1949.

Stefánsson, A., Arnórsson, S., 2002. Gas pressures and redox reactions in geothermal�uids in iceland. Chemical Geology 190, 251�271.

Stefánsson, A., Gunnarsson, I., Giroud, N., 2007. New methods for the direct de-termination of dissolved inorganic, organic and total carbon in natural waters byReagent-FreeTMIon Chromatography and inductively coupled plasma atomic emis-sion spectrometry. Analytica Chimica Acta 582 (1), 69�74.

Stefánsson, V., 1981. The Kra�a geothermal �eld, Northeast Iceland. In: Rybach, L.,Mu�er, L. (Eds.), Geothermal Systems and Case Histories. John Wiley, New York,pp. 273�294.

Stefánsson, V., Georgsson, L. S., Sigfússon, R., 1976. Resistivity measurments at Eld-vörp and Svartsengi. National Energy Authority Report OS-JHD-7639.

109

BIBLIOGRAPHY

Steingrímsson, B., 1989. Geothermal exploitation experience, case histories for threegeothermal �elds in Iceland. In: GRC workshop on responses of a geothermal �eldduring exploitation, Berkeley, California, June 15�16, 1989. pp. 305�359.

Sullivan, K., Aller, R., 1996. Diagenetic cycling of arsenic in Amazon shelf sediments.Geochimica et Cosmochimica Acta 60 (9), 1465�1477.

Sveinbjörnsdóttir, A., 1983. Water-rock interaction in Kra�a and Reykjanes geothermalsystems, Iceland � An extended abstract. Journal of the Geological Society 140,549�550.

Sveinbjörnsdóttir, A., Coleman, M., Yardley, B., 1986. Origin and history of hydrother-mal �uids of the Reykjanes and Kra�a geothermal �elds, Iceland � A stable isotopestudy. Contributions to Mineralogy and Petrology 94, 99�109.

Sveinbjörnsdóttir, A., Heinemeier, J., Arnórsson, S., 1995. Origin of 14C in Icelandicgroundwater. Radiocarbon 37 (2), 551�565.

Tómasson, J., Kristmannsdóttir, H., 1972. High temperature alteration minerals andthermal brines, Reykjanes, Iceland. Contributions to Mineralogy and Petrology 36,123�134.

Tossell, J. A., Sahai, N., 2000. Calculating the acidity of silanols and related oxyacidsin aqueous solution. Geochimica et Cosmochimica Acta 64 (24), 4097�4113.

Walker, G., 1974. Eruptive mechanisms in Iceland. In: Kristjansson, L. (Ed.), Geody-namics of Iceland and North Atlantic Area. Reidel, Dordrecht, the Netherlands, pp.177�188.

Weiss, R., 1970. The solubility of nitrogen, oxygen and argon in water and seawater.Deep-Sea Research 17, 721�735.

White, D., Mu�er, L., Truesdell, A., 1971. Vapor-dominated hydrothermal systemscompared with hot-water systems. Economic Geology 66, 75�97.

You, C., Gieskes, J., 2001. Hydrothermal alteration of hemi-pelagic sediments: exper-imental evaluation of geochemical processes in shallow subduction zones. AppliedGeochemistry 16, 1055�1066.

Zhang, Y., Dawe, R., 1998. The kinetics of calcite precipitation from a high salinitywater. Applied Geochemistry 13 (2), 177�184.

110