Mineralogical speciation of sulfur in
acid sulfate soils from Luleå, Sweden
Niklas Gunnarsson
Natural Resources Engineering, master's
2018
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering
i
Preface This master thesis ends five years of intense studying and a wonderful time at Luleå University
of Technology. The thesis covers both geochemistry and mineralogy and was financially
supported by Interreg Nord in cooperation with the County Administrative Board of
Norrbotten. It was part of a larger project called “Coastal watercourses in Bottenviken: Method
development and ecological restoration- A cross-border Swedish-Finnish cooperation project”
that is in progress between 2015-2018.
I would like to start to express my gratitude to my external supervisor Fredrik Nordblad from
the County Administrative Board of Norrbotten for sharing his knowledge and experience
from working with these soils.
Next, I want to thank Bernhard Dold and Thomas Aiglsperger my advisors at the university
for their support and knowledge with sulfide-rich materials. A special thanks to Thomas for
sharing his mineralogical knowledge, for helping me with sample preparation and with
microscope and SEM-interpretation. Finally, I would like to thank my classmate Astrid
Theander for the company and helping hands with sampling during cold winter days.
Niklas Gunnarsson
Luleå, October 2018
ii
Abstract
Marine sulfide – bearing sediments that oxidize when in contact with oxygen and leach out
elements in high concentrations to small watercourses have been a problem for many years all
over the world especially around the Bothnian Bay. The purpose of this study was to further
investigate the sulfur mineralogy present in acid sulfate soils in the area of Luleå, Sweden. A
secondary aim was to see if elements leach out and accumulate in an acid sulfate soil closer to
the recipient. Samples were taken in two profiles (one oxidized and one waterlogged) from
four sites (sites A-D) and were analyzed for whole rock geochemistry. Two sites were further
investigated for mineralogy in polished samples with an optical microscope, Raman
spectroscopy and SEM-EDS. Each profile consisted of three layers: oxidation zone, transition
zone and reduced zone. The oxidation zone above the groundwater table was light grey with
brown iron hydroxide staining. Parts that lied under the water table were dark grey-black with
in general strong odor (“rotten eggs”) due to its sulfur content. It was usually straightforward
to distinguish and separate the layers from each other directly in the field, however in some
cases pH was needed for confirmation.
A general feature of investigated polished samples is the presence of abundant framboidal
pyrites that are common in reduced marine sediments. The transition zone was formed in sub-
oxic conditions and this feature is reflected by the mineralogy. Many morphologies of the
framboidal pyrite were observed in this layer and signs of both dissolution and formation
occur. In the sample from site C one could observe elemental sulfur in form of large (up to 50
µm) euhedral crystals. In the samples with pH<4, no sulfides occur as they have been replaced
by jarosite (site B). Site C lacks these sulfur-bearing hydroxides which is thought to be due to
a sulfur concentration of <0.2 %. Sulfur shows extensive leaching at most sites but at site B and
D1, it accumulates in the transition zone. Elements like cobalt (Co), nickel (Ni) and zinc (Zn)
are leached out or are accumulated further down in the profile. Elements that could have been
transported and have accumulated in the waterlogged profiles are Co, Ni, Zn and chromium
(Cr) and in some profiles also copper (Cu) and vanadium (V).
Keywords: acid sulfate soil, metal leakage, low pH, SEM, elemental sulfur, framboidal pyrite
iii
Preface ........................................................................................................................................ i
Abstract .................................................................................................................................... ii
Abbreviations and definitions ............................................................................................ iv
1 Introduction ..................................................................................................................... 1
1.1 Background ........................................................................................................................... 1
1.2 Aim ......................................................................................................................................... 1
2 Literature research........................................................................................................... 2
2.1 Forming acid sulfate soils .................................................................................................. 2
2.2 Oxidation of sulfides .......................................................................................................... 3
2.3 Improving ecological status ............................................................................................... 4
2.4 Trace elements in sulfidic sediments .............................................................................. 4 2.4.1 Association to organic matter ....................................................................................................... 5 2.4.2 Association to hydroxides ............................................................................................................. 5
2.5 Character of sediment ......................................................................................................... 7
2.6 Classification ........................................................................................................................ 7
2.7 Acid sulfate soils around the Baltic Sea .......................................................................... 7
2.8 Prediction .............................................................................................................................. 9 2.8.1 Comparison with ARD .................................................................................................................. 9
3 Materials and methods ................................................................................................. 10
3.1 Study site ............................................................................................................................. 10
3.2 Sample collection .............................................................................................................. 12
3.3 Sample preparation ........................................................................................................... 14
3.4 Polished samples ............................................................................................................... 15
3.5 Whole rock analyses ......................................................................................................... 15
3.6 Optical microscope, Raman spectroscopy and SEM-EDS ......................................... 16
4 Results and discussion ................................................................................................. 17
4.1 Gammelstad (Site B) ......................................................................................................... 19 4.1.1 Optical microscope and SEM ...................................................................................................... 19 4.1.2 Geochemistry of the profiles ....................................................................................................... 27
4.2 Björsbyn (Site C) ................................................................................................................ 30 4.2.1 Optical microscope and SEM ...................................................................................................... 30 4.2.2 Geochemistry of the profiles ....................................................................................................... 34
4.3 Sunderbyn (Site A) ............................................................................................................ 37 4.3.1 Geochemistry of profiles ............................................................................................................. 37
4.4 Björsbyn (Site D) ............................................................................................................... 39 4.4.1 Geochemistry of profiles ............................................................................................................. 39
4.5 Geochemical comparison ................................................................................................. 41
4.6 Classification ...................................................................................................................... 42
5 Conclusion ...................................................................................................................... 44
5.1 Future work ........................................................................................................................ 44
6 References ....................................................................................................................... 45
7 Appendix ........................................................................................................................ 51
7.1 Appendix 1: Profile descriptions .................................................................................... 51
7.2 Appendix 2: Mass balance ............................................................................................... 53
7.3 Appendix 3: Immobile element vertical distribution ................................................. 57
7.4 Appendix 4: Whole rock geochemical data ................................................................... 57
iv
Abbreviations and definitions
Abbreviation Explanation
AASS Actual acid sulfate soil (oxidation layer)
ABA Acid base accounting
AMD Acid mine drainage
AP Acid potential
ARD Acid rock drainage
ASS Acid sulfate soil
AVS Acid volatile sulfides
bgs Below ground surface
BP Before present
Eh Redox potential
GTK Geological Survey of Finland
ICP-SFMS Inductively coupled plasma mass spectrometry
LOI Loss on ignition
NAG Net acid generation
NNP Net neutralizing potential
NP Neutralization potential
OZ Oxidation zone
PASS Possible acid sulfate soil (reduced layer + transition zone)
RZ Reduced zone
SEM-EDS Scanning electron microscope-energy dispersive x-ray spectroscopy
SGU Swedish Geological Survey
TS Total solids
TZ Transition zone
XRD X-ray diffraction
100 ha 1 km2
A1,A2…D2 Profile names according to site
Chemical Elements Arsenic As Silicon Si
Aluminum Al Sodium Na
Cadmium Cd Sulfur S
Calcium Ca Titanium Ti
Cobalt Co Uranium U
Chromium Cr Vanadium V
Copper Cu Yttrium Y
Iron Fe Zinc Zn
v
Lead Pb Zirconium Zr
Magnesium Mg Heavy rare-earth elements (Tb (65)-Lu (71)) HREE
Manganese Mn Light rare-earth elements (La (57)-Gd (64)) LREE
Molybdenum Mo Rare-earth elements (La (57)-Lu (71)) REE
Nickel Ni
Minerals and chemical compounds
Anglesite PbSO4 Bicarbonate HCO3−
Barite BaSO4 Bisulfide HS−
Calcite CaCO3 Ferric iron Fe3+
Elemental sulfur S0 Ferrous iron Fe2+
Gibbsite Al(OH)3 Hydrochloric acid HCl
Greigite Fe3S4 Hydrofluoric acid HF
Goethite FeOOH Hydrogen gas H2
Hematite Fe2O3 Hydrogen ion (proton) H+
Ilmenite FeTiO3 Hydrogen peroxide H2O2
Iron(III) hydroxide Fe(OH)3 Hydrogen sulfide H2S
Jarosite KFe3(OH)6(SO4)2 Iron (II) hydrogen sulfide Fe(HS)2
Mackinawite Fe(1+x)S Lithium metaborate LiBO2
Magnetite Fe3O4 Nitric acid HNO3
Manganite MnOOH Organic matter CH2O
Marcasite FeS2 Oxygen gas O2
Pyrite FeS2 Polysulfide, anion Sn2−, S(n−1)
2−
Pyrolusite MnO2 Sulfate SO42−
Pyrrhotite Fe(1−x)S Sulfuric acid H2SO4
Schwertmannite Fe16O16(OH)12(SO4)2 Water H2O
Siderite FeCO3
Xenotime YPO4
Zircon ZrSiO4
1
1 Introduction
1.1 Background
Acid sulfate soils are “nasty” soils (Dent and Pons, 1995). H₂S is formed as a product from
sulfate reduction due to decomposing organic matter when oxygen is absent, that will later
favor the formation of iron sulfides (Berner, 1984). The term “acid sulfate soil” means that
sulfides come in contact with oxygen and start to oxidize, changing the character of the
sediment (Dent and Pons, 1995). This can be enhanced by anthropogenic activities that require
ditching such as farming or forestry that lower the groundwater table (Sohlenius et al., 2004)
or by shafting when building large structures (Pousette et al., 2007). The oxidation lowers the
pH to <4 leading to increased weathering and mobilization of aluminum (Al), to some extent
iron (Fe) and trace elements such as cobalt (Co), nickel (Ni) and zinc (Zn) (Åström, 1998;
Sohlenius and Öborn, 2004). If the acid leachate reaches smaller watercourses with minor
dilution effect, it has a significant effect on the water quality (Åström and Björklund, 1996;
Filppa, 2012). Especially Al is very toxic to the environment as aluminum hydroxide
precipitates with fish kills as a result (Erixon, 2009). Acid sulfate soils are spread over several
continents and cover more than 17 million hectares worldwide (Andriesse and van Mensvoort,
2006) where its environmental impact is a global problem. Lots of studies have been done in
other regions of the world, but it was not until recent years that the topic started to be
investigated further in Sweden.
1.2 Aim
Recent studies have concluded which and to what extent elements leach out from acid sulfate
soils but there is still a lack of knowledge about the soil mineralogy connected to these metals.
This master thesis was focused on investigating the sulfur mineralogy (both primary and
secondary) present in the acid sulfate soil stratigraphy from the Luleå area in Norrbotten. One
other aim was to compare sample geochemistry and connect trace element trends to sulfide
mineralogy. This was to increase the knowledge on both actual acid sulfate soil (AASS) and
potential acid sulfate soil (PASS) so that lowering of groundwater table and acid sulfate soil
management will have a minimum influence on nearby watercourses.
Questions to answer in this thesis were:
• What sulfide minerals can be observed in acid sulfate soil from the Luleå area?
• What secondary sulfur minerals are present in the acid sulfate soil?
• Are there any differences in sulfur mineralogy between the sites and what are they?
• Are elements leached out from a more oxidized profile, transported and then
accumulated in a waterlogged profile downstream?
To answer these questions, samples from four sites and in two profiles at each site were
collected and investigated on their whole rock geochemistry. Furthermore, polished sections
from two of the sites were prepared to study their mineralogy.
2
2 Literature research
2.1 Forming acid sulfate soils
Acid sulfate soils (ASS) cover an area worldwide as large as 1.5*105 km2 (Pons et al., 1982) and
can be found in southeast Asia, West Africa, South America, eastern Australia and Europe
(Andriesse and van Mensvoort, 2006). In Finland, ASS is distributed over an area of 3500 km2
(Palko, 1994) which is the largest area in the world where up to 130 000 ha acid sulfate soil is
cultivated land (Yli-Halla et al., 1999). Around the Baltic Sea, these sediments were formed
during the Littorina Sea which was a former stage of the Baltic Sea with higher salinity than
present levels (Sohlenius, 1996). Today they can be found 0-100 m above current sea level
(Erviö, 1975; Palko, 1994) due to an isostatic uplift of 9-10 mm/year around the Bothnian Bay
(Johansson et al., 2004; Kaniuth and Vetter, 2004). This is a result from being forced down by
a 2-2.7 km thick ice sheet during the last ice age (Siegert et al., 2001).
Sulfide formation is the first step in the formation of acid sulfate soil where the reduction of
sulfate (SO42−) by bacterial activity can be illustrated by the reaction (Berner, 1984):
2CH2O + SO42− → H2S + 2HCO3
− (1)
where sulfate is reduced to hydrogen sulfide (H₂S) by using organic matter as an energy
source. The metastable iron sulfides that are most common in sediments are mackinawite
(Fe(1+x)S, x=0-0.11 simplified to FeS) and greigite (Fe3S4) (Morse and Rickard, 2004; Rickard
and Morse, 2005). It is considered that mackinawite and greigite are responsible for the black
color in acid sulfate sediments but there is only limited evidence that actually supports the
theory (Rickard and Morse, 2005). Formation of mackinawite has been described with two
reactions between ferrous iron and hydrogen sulfide in reduced conditions (Rickard, 1995):
Fe2+ + H2S → FeS + 2H+ (2)
Fe2+ + 2HS− → Fe(HS)2 → FeS + H2S. (3)
Greigite is considered to form from mackinawite by a solid- state transformation when oxygen
is present but also elemental sulfur could be used (Rickard and Luther, 2007; Rickard and
Morse, 2005). Production of iron sulfides may be limited by organic carbon for sulfate
reduction, sulfate or ferrous iron to react with (Berner, 1984). Mackinawite is quite common in
marine sediments such as ASS, but it is thought to be replaced fast by pyrite (FeS2) that is
formed by reaction between FeS and dissolved H2S or with polysulfides (Morse et al., 1987;
Rickard and Luther, 1997; Rickard, 1975). The transformation of mackinawite to pyrite can be
illustrated in the reactions by (Butler et al., 2004):
FeS + H2S → FeS2 + H2 (4)
FeS + Sn2− → FeS2 + Sn−1
2− (5)
3
where reaction (4) is dominating in anoxic conditions (Rickard and Luther, 1997) and reaction
(5) in partly oxic/anoxic conditions (Rickard, 1975). Iron sulfides are often only a minor part of
marine sediments although some systems may be enriched in sulfides. Generally, iron sulfides
in sulfide-rich sediments can be classed into two categories namely acid volatile sulfide (AVS)
and pyrite (Howarth and Jørgensen, 1984; Morse et al., 1987). AVS includes minerals such as
amorphous- FeS, mackinawite, greigite and pyrrhotite (Fe(1−x)S, x=0-0.2) (Cornwell and
Morse, 1987). They are presumed to give the black sulfide coating in reducing sediments. In
marine sediments pyrite is the most abundant iron sulfide (Berner et al., 1979) and it can form
either aggregates that occur in spherical shapes known as “framboids” or form separate
crystals (Sweeney and Kaplan, 1973). Enrichment of acid volatile sulfides has been connected
to low pyrite formation by several factors such as depletion of H2S due to a more rapid
formation of FeS in systems that are rich in iron (Gagnon et al., 1995). Other reasons that may
affect both rate and extent of pyrite formation are that H2S is converted to HS− as a dominating
sulfide specie in alkaline systems (Rickard and Morse, 2005).
2.2 Oxidation of sulfides
Oxidation of sulfide-rich sediments occurs when they get in contact with oxygen that will
produce sulfuric acid and lowers the pH<4 which creates the acid sulfate soil (Cook et al.,
2000). This process is accelerated by artificial drainage and excavation (Toivonen and
Österholm, 2011). Studies have concluded that oxidation of metastable iron sulfide gives
elemental sulfur as the first product of oxidation (Bloomfield, 1972a) that will oxidize further
to sulfate (Van Breemen, 1973). Half-times for oxidation of acid volatile sulfides have been
measured between 29-60 min (Burton et al., 2009, 2006) and the sulfides are considered to be
completely oxidized within a month (Purokoski, 1958). Two oxidants have been considered
important for iron sulfide oxidation: oxygen and ferric iron (Van Breemen, 1973). Sometimes
oxidation could reach down close to 2 m (Österholm and Åström, 2002) with secondary iron
minerals as a common precipitate in this layer (Collins et al., 2010). Oxidation of metastable
iron sulfide that occurs rather fast is illustrated by the reaction (Berner, 1962; Boman et al.,
2008):
10FeS1.1 + 24O2 + 26H2O → 10Fe(OH)3 + 11H2SO4. (6)
Pyrite that has a slower oxidation rate may instead oxidize according to (Hart, 1962):
4FeS2 + 15O2 + 14H2O → 4Fe(OH)3 + 8H2SO4. (7)
When pH drops below 4.5, ferric iron is more important as oxidant than oxygen and increases
the oxidation rate with a lower pH as a result (Nordstrom, 1982). The bacteria formerly called
Thiobacilli ferrooxidans (Van Breemen, 1973) nowadays Acidithiobacillus ferrooxidans has a
significant role since it catalyzes the oxidation of ferrous iron to ferric iron in natural systems
(Johnson and Hallberg, 2003). This oxidation of sulfides lowers the pH down to values 2.5-4
(Dent and Pons, 1995; Van Breemen, 1973).
4
Oxidized iron hydrolyzes and precipitates as iron hydroxides when pH rises above 3.5 (Van
Breemen, 1973). During pH<4 jarosite (KFe3(OH)6(SO4)2) (Brown, 1971) and schwertmannite
(Fe16O16(OH)12(SO4)2) (Sullivan and Bush, 2004) can be found. Both jarosite and
schwertmannite will later hydrolyze to goethite (FeO(OH)) and may do so over several months
or years (Burton et al., 2007). Oxidation of iron sulfides may cause deoxygenation and acidic
drainage water with a high metal release as a result (Burton et al., 2009; Morgan et al., 2012).
Metals can be mobilized in acid sulfate soils by either sulfide oxidation or by the weathering
of phyllosilicates that are accelerated by the more acidic pH (Åström and Björklund, 1996;
Stumm and Furrer, 1987). When pH in the recipient is getting below 5.2 several organisms will
not survive (Eilers et al., 1984). Studies show that elements released include aluminum (Al),
calcium (Ca), cadmium (Cd), cobalt (Co), copper (Cu), magnesium (Mg), manganese (Mn),
sodium (Na), nickel (Ni), silicon (Si), and uranium (U) are occurring as free ions as long as pH
is low and are therefore bioavailable (Nystrand and Österholm, 2013).
2.3 Improving ecological status
Some studies have tried to improve the quality of drainage water and to minimize the leaching
of trace metals by raising the pH in the soil with e.g. liming (Åström et al., 2007). A problem
in that study was that iron hydroxide particles coated the lime particles and therefore
hampered the improvement (Åström et al., 2007). Powell and Martens (2005) tested controlled
tidal flooding in combination with lime to remediate ASS with improved pH and trace metal
redistribution toward reducing depths as a promising result. Tidal flooding is likely by either
anthropogenic reflooding or sea-level rise due to climate change and will transport reactive
trace elements down to the redox boundary of the soil where they will be enriched (Keene et
al., 2014). Waterlogging of ASS has shown to decrease the Al concentration in porewater in
boreal climate and also that formation of iron sulfides was low despite increased reducing
condition (Virtanen et al., 2014). Waterlogging of acidified acid sulfate soils has been tried in
northern Australia with an improved water quality as a result (Johnston et al., 2009). This is
possible due to dilution of acidity, buffering ocean water and reducing conditions consuming
H+. Remediation with waterlogging where schwertmannite is present increased the pH up to
6.5 and reduced the iron to ferrous iron with a release of sulfate and alkalinity that later gave
precipitation of siderite (FeCO3) (Burton et al., 2007). In Finland, they have included a
regulation plan for groundwater adjustment to minimize sulfide oxidation from ASS (Uusi-
Kämppä et al., 2013).
2.4 Trace elements in sulfidic sediments
Metals are generally much more mobile when pH is low and therefore pH is a crucial factor
that controls metal mobility (Gundersen and Steinnes, 2003). Formation and sedimentation of
iron sulfides into marine sediments is one of the main removal paths for iron and sulfur from
marine waters (Berner, 1984). Both AVS and pyrite are sinks for trace elements with adsorption
and coprecipitation processes to regulate their mobility and bioavailability (Morse et al., 1987).
5
Huerta-Diaz and Morse (1992) and Scholz and Neumann (2007) have concluded that some
trace elements of geochemical importance can be included in sedimentary pyrite in varying
proportions. These trace elements could include large amounts of arsenic (As), cobalt (Co) and
manganese (Mn), moderate amounts of copper (Cu), nickel (Ni) and chromium (Cr) and small
amounts of lead (Pb), zinc (Zn) and cadmium (Cd) that may form their own sulfide minerals
instead. Huerta-Diaz and Morse (1992) also conclude that trace metal content in pyrite is
limited by pyrite production or trace metal depletion. Typical sulfide metals such as Zn, Ni
and Co have been shown to commonly be incorporated in iron sulfides (Volkov and Fomina,
1974). Cadmium has a trend of forming insoluble sulfides in anoxic, sulfide-bearing sediments
when H2S is present (Jacobs et al., 1987).
2.4.1 Association to organic matter
In the oxidation layer the elements zinc, copper and cadmium can be associated with organic
matter and then be released to the water column when the organic matter is being consumed
with diffusion upward or downward in the profile as a result (Scholz and Neumann, 2007).
Aluminum (Nordmyr et al., 2008) and chromium are usually also associated with organic
matter (Shaw et al., 1990) where chromium together with vanadium and nickel forms
dissolved complexes (Olson et al., 2017).
2.4.2 Association to hydroxides
Leaching of elements from both iron and manganese hydroxides are common when pH is
lowered resulting in their dissolution. The elements vanadium (Morford et al., 2005), cobalt
(Shaw et al., 1990) and chromium (Algeo and Maynard, 2004) are associated with manganese
hydroxides and will be liberated when the hydroxides are unstable. When iron hydroxides are
reduced, elements such as arsenic (As) are released to the environment (Widerlund and Ingri,
1995).
The group of lanthanoids (also known as the rare earth elements) that are the elements La (57)-
Lu (71) in the periodic table can be found in moderate to strongly elevated concentrations (ca.
36-4700 µg/l) in acidic water e.g.(Åström, 2001). Of importance for the mobility of lanthanoids
is the phyllosilicate matrix (clay minerals) holding the elements in varying accessory minerals
(Harlavan et al., 2009) but also the grain size where (sub-) micrometric particles offer a large
surface area for reactions (Åström et al., 2010). Organic matter has also been found to be an
important carrier of these elements (Åström et al., 2010). Figure 1 shows how elements are
liberated from different phases and get more accessible for uptake and leaching from less
stable phases when pH drops from near neutral to more acidic.
6
Figure 1: Element cycling among different phases depending on pH, where the major mobilization paths
are numbered (Claff et al., 2011). CDB=crystalline iron oxide phase.
In a study from Finland they compared trace element concentrations between the three layers
of an acid sulfate soil (Åström, 1998). The conclusion was that manganese (Mn), zinc (Zn),
nickel (Ni) and cobalt (Co) were frequently released from the soil when it was oxidized but
also that small proportions of Zn, Ni and Co had reprecipitated further down in the transition
zone. They also concluded that only a small proportion of copper (Cu) was released and no
extensive leaching of iron (Fe), titanium (Ti), chromium (Cr) and vanadium (V) was observed.
This is thought to be due to that Cr, V and Ti are connected to weathering resistant fractions
or to organic matter and the small losses of Fe are thought to be due to precipitation of iron
hydroxides in the oxidation zone (Åström, 1998). In Sweden 15-45 % of Mn is lost from sulfide-
bearing sediments upon oxidation (Öborn, 1991). Factors affecting geochemistry of ASS are
grain size, mineral distribution and composition, concentration of organic matter and where
the groundwater table is located (Sohlenius et al., 2015).
7
2.5 Character of sediment
Acid sulfate soils are recognized by their odd color due to iron precipitates, bad odor due to
sulfur, sparse vegetation due to pH<4 and the speed of transformation from black to grey color
as a result of oxidation (Dent and Pons, 1995). The sulfur concentration in these soils could
vary between 0.6-2.1 % in the transition zone and the sulfidic layer while it is usually between
0.04-0.46 % in the oxidation zone (Åström, 1998). It is common that pyrite with 1-5 % is the
most abundant sulfide mineral in acid sulfate soils (Bloomfield, 1972a). Other minerals such
as marcasite (FeS2), mackinawite (FeS), greigite (Fe3S4), and amorphous iron sulfides could
also be present (Burton et al., 2008; Bush et al., 2004; Bush and Sullivan, 1997) with content
below 0.01 % (Bloomfield, 1972b) and up to 0.6 % (Berner, 1971). Åström (1998) found in their
study that the uppermost two layers had a pH between 3-5 while in the reduced layer a pH of
6-7. The low pH is primarily due to oxidation of sulfides that forms sulfuric acid but also by
hydrolysis of iron (Van Breemen, 1973).
Toivonen and Österholm (2011) concluded in their study that fine-grained phyllosilicates are
the main source for metals and that metal concentration variation is controlled by grain size.
2.6 Classification
Acid sulfate soil can be separated into “acid sulfate soil” and reduced sulfide-rich sediment
that may “potentially form acid sulfate soil” (PASS) (Brinkman and Pons, 1973). The
nomenclature used for acid sulfate soil layers is: actual acid sulfate soil (AASS) where pH is
low and iron hydroxide mottles occur and potential acid sulfate soil (PASS) where pH is near
neutral and has a sulfur content of at least 0.2 % (Edén et al., 2012). The Swedish Geological
Survey (SGU) classifies soil with in-field pH <4 as acid sulfate soil , samples with in-field pH
between 4.4< pH <6 and pH <4 after oxidation as semi-acid (TZ) and in-field pH > 6 and with
pH<4 after oxidation as reduced soil (Sohlenius et al., 2015). They also classify them into
different classes after field pH and sulfur content, where A+B are for pH<4, C for 4-4.5 and
pH> 4.5 is concluded not acid sulfate soil. For sulfur content, these classes were > 1 % class
one, 0.6-1 % class two and 0.2-0.6 % for class three, below that limit was not classified as sulfide
bearing (Edén et al., 2012; Sohlenius et al., 2015). The iron/sulfur ratio has been proposed as a
tool for classifying the acid potential where ratios < 3 is very acidifying and >60 as non-sulfidic
(Pousette et al., 2007).
2.7 Acid sulfate soils around the Baltic Sea
A rough areal estimate of the acid sulfate soils in Sweden was done to 140 000 ha (Öborn, 1994).
Mapping of acid sulfate soils in Sweden has had a low priority through the history and
therefore the area is not well defined (Öborn, 1989). Most acid sulfate soils occur along the
coast of the Baltic Sea and down around Lake Mälaren and Hjälmaren (Öborn, 1989). These
were deposited in the Litorina Sea (7000–4000 BP) (Sohlenius, 1996) but are now due to
isostatic land rebound lifted over the sea level (Figure 2).
8
Figure 2: Map showing the maximum sea level (light grey) along the Swedish coast where the majority of the
sulfide-rich sediments occur below this level, modified from (Sohlenius, 2011).
Acid sulfate soil form when the top meter is oxidized e.g. due to excavation or drainage for
agricultural use (Öborn, 1991). This was common in the early 19th century when farmlands
were prepared by draining wetlands accelerating the oxidation of underlying sulfides
(Österholm and Åström, 2004). They are characterized by low pH (3.9-4.5) and almost no
sulfur content due to the oxidation of sulfides (Sohlenius and Öborn, 2004). Potential acid
sulfate soil (PASS) both in Finland and Sweden is characterized by a black color with high AVS
content that indicates metastable iron sulfides (Backlund et al., 2005; Georgala, 1980). Around
Lake Mälaren the sulfide-bearing sediments are classified as muddy clay while along the coast
in northern Sweden as muddy silt (Sohlenius et al., 2004). The muddy clay is characterized by
a blue-grey color and is dominated by pyrite while the muddy silt along the northern coast
has black color due to dominating FeS minerals (Sohlenius et al., 2004). The soil is usually
characterized by a silicate matrix of granitic composition where soils in Sweden and Finland
show similarities (Åström, 1998; Öborn, 1989). A connected acid sulfate soil stratigraphy can
be seen along the Norrbotten coastline from Piteå up to Torneå where typical soil profiles
along this line have been described by (Fromm, 1965).
9
The most important mobilization factor is increased weathering when pH is lowered
(Sohlenius and Öborn, 2004). Mobile elements are mostly accumulated in the transition zone
where varying reducing and oxidizing conditions occur (Sohlenius and Öborn, 2003). When
the soil reaches above current sea level, iron monosulfides are oxidized in the top layer and
pyrite is preserved and therefore trapping elements such as Co, Ni and Zn (Boman et al., 2010).
Aluminum and copper were found to be strongly correlated with organic matter while Cd,
Co, Mn, Ni and Zn were likely associated with hydroxides (Nordmyr et al., 2008). In Finland
is iron not abundantly leached out from acid sulfate soils (Åström, 1998; Österholm and
Åström, 2002). This is due to iron hydroxide precipitation in the oxidation zone that may trap
elements such as arsenic that likes to adsorb to iron hydroxides when pH<7 (Wällstedt et al.,
2010). Areas with sulfide bearing sediments usually have elevated concentrations of sulfur,
nickel and copper in biota and have a lower resistivity than other fine-grained sediments
(Puranen et al., 1997). This makes it easier to find PASS by biogeochemistry and geophysics
(Sohlenius et al., 2004). Most sulfide bearing sediments found in Sweden lies close to the sea
and have been uplifted within the last 2000 years (Sohlenius, 2011). Porsöfjärden north of Luleå
has in periods been extremely affected by low pH and high metal concentrations (Erixon, 2009;
Filppa, 2012). Cadmium (Cd), nickel (Ni), manganese (Mn), copper (Cu) and cobalt (Co) have
been found to commonly leach out from ASS in general (Sohlenius and Öborn, 2004) while the
elements Co, Ni and Zn are typically found in drainage from acid sulfate soils in boreal climate
(Åström and Corin, 2000). In western Finland the concentrations of Zn, Ni and Co are 30-40
times higher in stream water draining sulfidic sediments compared to other sediments (Åström
and Björklund, 1996).
2.8 Prediction
In a study done at the University of Uppsala, they recommend that net neutralization potential
(NNP) should be investigated in sulfide-bearing sediments and if <5 also metal content should
be analyzed to prevent sudden acidification (Lax and Sohlenius, 2006). The company MRM
Konsult AB have developed a leaching test specifically for ASS where one reduced and then
several oxidized leaching tests are done with cycles of wetting and drying (MRM, 2007). In a
master thesis from the Luleå University of Technology, the MRM leaching test and Acid-base
accounting (ABA) test was compared for predicting ASS leaching where the MRM test was
concluded to give a more complete picture (Paulsson, 2012). Pousette (2010) describes a
leaching test developed by Luleå University of Technology for ASS that is similar to the MRM
leaching test where only the sample weight (10 g vs 25 g), water ratio and the number of
leaching steps were different for the test procedures.
2.8.1 Comparison with ARD
Acid-base accounting (ABA) test is the most common prediction method used for predicting
acid rock drainage (ARD) since it is fast and cheap. It is generally a result from calculating the
NNP as the difference between neutralization potential (NP) and acid potential (AP) where all
sulfur is assumed to be from pyrite and all neutralizing minerals as calcite (CaCO3) (Sobek et
al., 1978).
10
In most cases this is not accurate since the sulfur mineralogy is more complex (Dold, 2010).
This has led to a more high-resolution ABA-test where sulfate and iron hydroxide mineralogy
can be extracted from the sulfide mineralogy giving a more accurate AP (Dold, 2003). Skousen
et al. (1997) shows how to take siderite (FeCO3) into consideration to get a more accurate NP.
Another test that is frequently used is the NAG-test where hydrogen peroxide (H2O2) is added
to accelerate sulfide oxidation. The pH is measured in the leachate and then titrated stepwise
to neutral pH to get an acid forming capacity (Ian Wark Research Institute, 2002). Pyrite is the
most common sulfide mineral in mine tailings and is oxidized when oxygen is present (Dold,
2014). Mine tailings are very similar to ASS in many ways when looking at sulfide oxidation
and hydrolysis of minerals. One major difference is that ASS oxidizes much faster due to
dominating iron monosulfides compared to pyrite as dominating specie in mine tailings (see
section 2.2, (Dold, 2017)). See Dold, (2017) for more about prediction.
3 Materials and methods
3.1 Study site
This study has been carried out on actual acid sulfate soil (AASS) and potential acid sulfate
soil (PASS) from four locations around the city of Luleå in northern Sweden. The studied areas
are Site A close to Sunderbyn, Site B near Gammelstad, Site C and D both in Björsbyn (Figure
3).
Figure 3: Map showing sample points and studied areas.
11
Site A is located on a field ca. 40 m from a rainwater outlet, site B in a thicket ca. 30 m next to a
tributary to the Luleå river, site C profile 1 in a thicket and profile 2 close to the reeds behind
a garden business premises and site D profile 1 on a small meadow and profile 2 close to the
reeds ca 1 km from site C (Figure 4; Table 4).
Figure 4: Sample pictures from site B and C.
The geological units at site A and B are acidic intrusive rocks from the svecokarelian orogeny
ca. 2.85-1.87 Ga. At site C and D the units are a metasedimentary unit like greywacke or
quartzite, a basic intrusion like gabbro and an acidic intrusion all 2.85-1.87 Ga. Close-by there
is a younger acidic porphyritic intrusion, ca. 1.88-1.74 Ga (Figure 5). According to the soil map
from the Swedish Geological Survey, silt and clay dominate the sample areas.
12
Figure 5: Geological map of sample areas. The geological maps were generated via the SGU map generator
(http://apps.sgu.se/kartgenerator/maporder_sv.html).
3.2 Sample collection
Sample locations were chosen using a soil map from SGU that uses soil type and the shore-
level model as parameters. According to Sohlenius et al., (2015) this highlights areas with
silt/clay that have been uplifted the last 2000 years as the highest probability of finding AASS
and PASS.
13
Samples were taken in two profiles at each location, one on elevated levels showing extensive
oxidation and one closer to the shore that is still waterlogged with limited oxidation (Figure
6).
Figure 6: A simplified overview of the spatial relationship between profile 1, profile 2 and a nearby stream.
Brown: humus, light grey-orange: OZ, dark grey-black: TZ-RZ.
An Edelman auger was used for collecting samples at different depths. Soil samples were first
placed in a complete profile on plastic bags to visually distinguish and measure at which depth
each layer was located. In general the color was enough to separate layers (Figure 7) but to
ensure these visual assumptions the pH was measured with a WTW pH 330i pH-meter and a
Sentix 41 pH-electrode.
Figure 7: A: An example of a brownish acid sulfate soil (AASS). B: An example of a dark grey
potential acid sulfate soil (PASS).
14
To do the measurement a piece of material from a layer was mixed with a small part of milli-
Q water and stirred to a paste in which the pH was measured. This was done at least once per
layer but at some of the locations with more uncertainty, it was necessary with more frequent
measurements. After measuring the pH, one representative sample per layer (350-1000 g) was
collected in plastic bags. After collection, the samples were stored in a cold storage at the
university laboratory for 1-3 weeks until sample preparation.
3.3 Sample preparation
The most important minerals in ASS are the iron sulfides, so minimizing mineralogical
changes to samples during sample preparation is therefore important (Claff et al., 2010).
Drying of anoxic soils is thought to likely affect redox-sensitive elements that are present
(Bordas and Bourg, 1998; Rapln et al., 1986). Despite the risk, it is still common to use drying
as a pre-treatment method (Åström, 1998). When all samples had been collected they were
placed in aluminum forms and weighted before put into an oven where they were dried for 2-
3 days at 50 °C. When all samples were dry they were weighted again to reconstruct the initial
water content of each sample. During the drying, the color changed from a dark grey-black to
more light grey-brown on reduced samples (Figure 8).
Figure 8: A: Sample (site C profile 1, RZ) before drying. B: Sample (site C profile 1, RZ) after drying.
One piece from each layer, both profiles from site B and C (total 12) were chosen for preparing
polished samples. Homogenization was done with a glass bottle against a flat surface that
firmly crushed the sample into evenly sized grains. When the sample was homogenous
enough it was split with a riffle splitter at least three times until the remaining part was ground
with an agate mortar to reduce the grain size further. Ground samples of representative
sample aliquots were put into small plastic bags marked with sample-name that were
subsequently sent to ALS Scandinavia AB for whole-rock analysis. The remainder of the
samples were put back into plastic bags and stored in the cold storage.
15
3.4 Polished samples
The 12 pieces from site B and C were put into plastic molds for molding the pieces into epoxy.
One-part resin and one-part hardener were stirred together in a cup until entirely mixed. The
molds were filled up close to the top and then placed in a vacuum pump to take away all
bubbles from the epoxy. When all bubbles had been removed, they were hardened for 24 hours
before polishing. Polishing was done in a Struers LaboPol-6 polishing machine at 1200 rpm
with a lubricant as a cooling agent. The first three samples (Site B1 (RZ), Site B2 (RZ) and site
C2 (RZ)) were going through the entire polishing program with 4 minutes each on P500, P800
and P1200 abrasive paper as a start and then using 15, 9 and 3 microns diamond paste on
separate disks for polishing (Figure 9).
Figure 9: Polishing machine and polished samples after rough polishing.
A problem was that the samples were too soft, and a relief occurred. Therefore, on the
remaining samples, grinding with abrasion paper for 30 seconds at a time was done without
the additional use of diamond polishing. The samples C2 (OZ), B1 (OZ) and B2 (OZ) got deep
cavities from the polishing and were covered in a new layer of epoxy and then re-polished
with no improved result. Sample C1 (RZ) was also remolded and re-polished since it cracked
when trying to free it from the mold.
3.5 Whole rock analyses
ALS Scandinavia prepared the samples for analysis with two methods. The first one was total
digestion with a mixture of hydrochloric acid (HCl), nitric acid (HNO3) and hydrofluoric acid
(HF) that is mixed with a small part of a sample in a test tube that is heated until digestion is
reached. The other method is mixing lithium metaborate (LiBO2) with the sample and heating
it to 1000 °C melting it to a pearl that is digested in nitric acid so-called fusion analysis. The
samples from total digestion were then analyzed with 2nd gen ICP-SFMS, Element 2/XR and
from fusion analysis with first generation ICP-SFMS. Analysis with ICP-SFMS were been done
according to SS-EN ISO 17294-1, 2 (mod) and EPA-method 200.8 (mod). Samples were dried
in 50 °C but were total solid (TS) corrected to 105 °C according to SS 02 81 13-1.
16
After that loss of ignition (LOI) was determined for nearly all samples where samples were
heated in an oven to 1000 °C and their weight before and after were compared.
To highlight element enrichment or depletion it is possible to calculate a coefficient τ with the
equation (Anderson et al., 2002):
τi,j =Cj,w∗Ci,p
Ci,w∗Cj,p− 1. (8)
Where (j) is the mobile element and (i) is the immobile element in weathered (w) and parent
material (p). Positive values denote enrichment of the mobile element and a negative value a
depletion (Chadwick et al., 1990). When the coefficient reaches -1 it implies total depletion and
around zero neither enrichment or depletion (Ling et al., 2014).
3.6 Optical microscope, Raman spectroscopy and SEM-EDS
All samples were thoroughly investigated with reflecting light using a Nikon Eclipse LV
100POL optical microscope equipped with a Nikon DS-Fi1 camera. Interesting reflecting
grains were photographed and marked for further investigation with SEM and Raman at the
university. Raman spectroscopy was done on samples from site B profile 2 (OZ+TZ) and site
B profile 1 (OZ+RZ) using a Bruker Senterra equipped with an Olympus BXFM optical
microscope. Raman spectra were obtained using a 532 nm laser beam (d=1 micron) at 0.2 mW
with 45 s integration time and 3 repetitions. The monochromatic laser light (wavelength=530
nm) is sent on the sample, reacting with a mineral grain (inelastic scattering) that changes the
energy level of the laser photons that can be measured as Raman shifts (Colthup et al., 1975).
This shift is characteristic for each Raman-active mineral and gives information about the
mineral structure, hence minerals that have similar chemistry in scanning electron microscopy
e.g. iron oxides (hematite Fe2O3; magnetite Fe3O4) or iron sulfides (pyrite FeS2; marcasite FeS2)
can be distinguished. Scanning electron microscopy with energy-dispersive spectroscopy
(SEM-EDS) was performed on samples from site B profile 1 (OZ-RZ), from site B profile 2 (OZ)
and from site C profile 1 (TZ) to determine the chemical composition and get high-resolution
images of interesting grains. For this a ZEISS Merlin FEG-SEM was used at 20 keV accelerating
voltage and 1.1 nA beam current. The scanning electron microscope shoots a high-energy
beam of electrons at a selected part of a sample. The electrons of the beam interact with the
sample and produce secondary electrons, backscattered electrons and X-rays as a result of the
reaction (Reimer, 1998). These electrons are collected by detectors that compute an image of
the sample. The emitted X-rays are used in the energy dispersive spectroscope that gives a
distinct energy signature for every element that is collected in a spectrum making it possible
to interpret the mineralogy (Reimer, 1998).
17
4 Results and discussion Most profiles in this study and in earlier research consist of three layers with an oxidation zone
near the ground surface, a transition zone further down and a reduced zone at the bottom.
Figure 10 visualizes these layers in photographs and in idealized profile sketches with typical
colors and gradual transitions between layers. Typical colors in this soil are light grey with
orange mottles in the oxidation layer that turn darker grey further down when it gradually
turns into a more reduced environment. The bottom layer is often dark grey-black caused by
iron sulfides and/or by organic matter. The stratigraphy of profile 1 at each site starts with a
15-30 cm humus layer followed by a 43-110 cm thick oxidation layer.
Underlying is a transition zone (40-82 cm) that is depending on the groundwater table that lies
65-110 cm below ground surface (bgs). For profile 2 the humus layer is 0-30 cm in thickness
with a 15-45 cm underlying oxidation layer. The transition zone varies here between 0-74 cm
with a groundwater table between 0-40 cm below ground surface. One general trend that can
be seen at each site is that at site A the oxidation layer and transition zone are equal in
thickness, at site B the oxidation zone is thicker than the transition zone, at site C the oxidation
zone is thinner than the transition zone and at site D the oxidation zone is thicker than the
transition zone. Since the transition zone is formed through groundwater fluctuation one can
interpret that where the oxidation zone is thicker than the transition zone the groundwater
table is permanently lowered on a stable level whilst where the opposite result is seen the
groundwater level is varying more naturally with weather and flood.
For better visibility of the dominating geochemical processes, site B and C will be first
discussed with evidence of the mineralogy that will be linked to the chemical trends followed
by interpretation of processes at site A and D with a final comparison between all sites.
18
Fig
ure 10: F
ield p
rofiles w
ith id
ealized sk
etches fo
r clarification
. Bro
wn
: hu
mu
s,
oran
ge-lig
ht g
rey: O
Z, d
ark
grey
: TZ
and
black
: RZ
.
19
4.1 Gammelstad (Site B)
4.1.1 Optical microscope and SEM
In general, the samples from profile 1 have light grey color with a 1-5 mm brown oxidation
rim but the sample from the OZ is orange/red-grey in color. Samples from Profile 2 have a
light brown/beige-dark grey color with a 1-4 mm brown-grey rim. Samples from this site have
a trend where large silicate grains occur high up in the profile and decrease in numbers with
depth.
Oxidation zone (profile 1)
The entire sample was oxidized which gave no oxidation rim. In this position no sulfides were
found in the microscope. The sample contained lots of iron hydroxides and one of them was
jarosite (Figure 11).
Figure 11: A+B: Secondary electron image. C: Raman spectrum of grain with reference spectrum for
comparison. D: EDS spectrum with a picture from the optical microscope.
Jarosite is usually formed at pH < 3 but is stable at pH< 4 (Brown, 1971). One major problem
of forming jarosite is usually the source for potassium but it is quite common in heavily
weathered sulfide-rich mine tailings (Dold, 2014). As can be seen in Figure 11A and 11B the
large white spot is composed of many small jarosite grains. This area was first detected in
optical microscope due to its characteristic yellow color. This was later confirmed by the
Raman spectrum that corresponds well to the reference spectrum for jarosite, except for a small
shift to the right (Figure 11C). Later also the chemical composition was confirmed in SEM
where the EDS spectrum showed in Figure 11D matches the composition of jarosite
(KFe3(OH)6(SO4)2) except for a minor sodium content.
20
Transition zone (profile 1)
Further down in the profile one can find microcrystals of pyrite that together forms the
spherical shape “framboidal pyrite”. Many observations of framboidal pyrite in a variation of
sizes were done with an optical microscope. Some of them had a growth rim around the main
framboidal core (Figure 12A).
Figure 12: SEM image of framboidal pyrites with a sulfide rim. D: Chemical composition of the rim.
The circular grains are the framboidal pyrite that is composed of several small cubic-pyramidal
pyrite grains. The EDS spectrum shown in Figure 12D confirms that also the rim around is
composed of iron sulfides indicating the grain has been growing. It is uncertain which iron
sulfide this rim is formed from but since the iron peak is small this supports that it could be
pyrite rather than an iron monosulfide. Some observations of framboidal pyrite with other
morphologies have also been done (Figure 13).
21
Figure 13: A+B: Framboidal pyrite with internal structure. C: Backscatter image of unevolved framboidal
pyrite. D: Secondary electron image of unevolved framboidal pyrite.
Figure 13A and 13B show a framboidal pyrite without the growth rim, instead it has a well-
organized pattern like it has been growing one rim at the time until it reaches its spherical
shape. This can be compared to the unorganized core in Figure 12. The grain in Figure 13C
and 13D is chemically the same as pyrite and has a similar shape to a framboid but could be
either at the formation or in the dissolution phase of formation since the microcrystals are
welded together which makes it near impossible to distinguish individual grains. No
secondary iron minerals were documented with images but some barite (BaSO4) was observed
(Figure 14).
Figure 14: Barite grain with confirmation from the EDS spectrum.
22
Barite is thought to precipitate with bacterial help (Hanor, 2000), this due to its incorporation
with the redox-sensitive sulfur form sulfate. The observation of barite indicates that some
oxygen has reached down to this level which is in accordance with the sub-oxic transition
zone.
Reduced zone (profile 1)
In the reduced zone, black layers filled with framboidal pyrites can be seen with microscope
(Figure 15) but also with the naked eye. Especially in the central part of the sample a lot of
both small (4 µm) and large (16 µm) framboidal pyrites were observed.
Figure 15: Framboidal pyrite layer in an optical microscope with reflected light (10x ocular).
All the rounded black dots are framboidal pyrite grains that are arranged along a certain line
that appears black. In some cases, the framboidal pyrite had formed aggregates of different
sizes and shapes called polyframboids (Figure 16).
Figure 16: Framboidal pyrite forming aggregates of different shapes.
23
Figure 16A shows a rod-shaped aggregate with framboidal pyrites where each framboid has
been compressed together. They are showing a varying size distribution from 5-30 µm where
some grains show signs of a hexagonal cross-section (Figure 16B). It is also interesting to note
that, they have a 120° angle between each framboid. Figure 16C and 16D show another large
aggregate with framboidal pyrite where they are staked on each other in a larger heap. One
can see in Figure 16D that the pyrite microcrystals are growing on the outside of the framboid,
making it grow. Some grains have holes where microcrystals have been or going to be later.
Sometimes framboidal aggregates keep their spherical shape and grow larger instead (Figure
17).
Figure 17: Polyframboid formed by smaller framboids.
Figure 17A shows that this sample contains a lot of framboidal pyrites (all white dots). The
larger framboidal aggregate is formed from 10 µm framboidal pyrites (Figure 17C). As can be
seen in the well-shaped framboids, the microcrystals have been welded together making it
hard to distinguish each microcrystal. Around this, an unstructured pattern of pyrite
microcrystals lies on the surface (Figure 17D). This is most likely the case before the framboidal
texture is established. Like in the transition zone, some barite was found also down in the
reduced layer (Figure 18).
24
Figure 18: Diffuse barite crystal.
The barite grain has an anhedral shape and is diffuse which could indicate that it is becoming
unstable in more reducing environments. According to the stability diagram (Tokunaga et al.,
2016), barite is stable at all pH but is depending on the redox potential where it is stable
between Eh -0.65-0.25 V.
Oxidation zone (profile 2)
Like for profile 1, there are large weather-resistant silicate grains present. In this sample, only
a few unoxidized framboidal pyrites were observed. Barite that was found in profile 1 was
also observed in this layer (Figure 19).
Figure 19: Diffuse barite grain.
Like in the reduced layer in profile 1, barite shows a diffuse morphology which could indicate
metastability and that this grain is near dissolution. Other interesting grains that were found
are some that may be either churchite and/or xenotime (Figure 20).
25
Figure 20: Churchite or xenotime grains (𝑌𝑃𝑂4).
Although both churchite and xenotime both have the same chemistry it is more likely that the
grains in Figure 20 are xenotime. This since xenotime is a more common mineral, which could
contain small amounts of HREE and is associated with more common minerals such as zircon
(ZrSiO4), ilmenite (FeTiO3) and magnetite that also were observed. The yttrium peak (k alpha)
and the phosphorus peak are nearly at the same energy level which makes them hard to
distinguish from each other.
Transition zone (profile 2)
Large silicate grains that are visible with the naked eye are also present in this sample. More
framboidal pyrites than in the oxidation layer but less than in profile 1 were observed. In this
sample oxidation did not affect the pyrite grains like it did in the oxidation zone. Examples of
sparse packed framboidal pyrites taken with 50x ocular in the optical microscope can be seen
in Figure 21.
26
Figure 21: Poorly packed framboidal pyrites (50x ocular, optical microscope).
Very few framboidal pyrites were observed in the TZ sample and when observed they were
in cross-section with microcrystals that were not densely packed as in profile 1.
Reduced layer (profile 2)
In the reduced layer, the amount of framboidal pyrites increases only slightly. Black layers
filled with framboidal pyrites like the layers in profile 1 can be seen in one corner (Figure 22).
Figure 22: Framboidal pyrites (approx. 4 µm) in layers (5x ocular, optical microscope).
In this sample many of the framboidal pyrites in these lines were laying in an oxidation rim
and showed signs of oxidation. One could also see areas with iron hydroxides over the sample
that went into the central part of the sample from the rim.
27
4.1.2 Geochemistry of the profiles
The geochemical composition of the profiles was compared to study if mobile elements from
the oxidized profile accumulate downstream in the waterlogged profile. Soil samples at site B
were taken near a pedestrian bridge next to a small river between the neighborhoods
Gammelstad and Sunderbyn. The height above mean sea level were ca. 2.5 m and 1.5 m with
ca. 30 m apart. Element concentrations from profile 1 can be seen in Figure 23 below.
In this profile, pH is close to 3.8 in the oxidation zone due to intense sulfide oxidation. This
has led to the low sulfur concentration seen in Figure 23 which explains why no observations
of sulfides were done. For the elements sulfur, manganese, molybdenum and arsenic one can
see a trend with leaching through the entire profile where sulfur has leached 1.5 % and
manganese as much as 950 ppm. This could mean that at least Mn but possibly also Mo and
As have been incorporated in sulfides in the soil and released with sulfur when oxidation
starts. One can see that iron is retained in the oxidation zone which is also supported by the
observed iron hydroxides in the polished sample. Under the optical microscope jarosite was
found among the hydroxides in the oxidation zone which is supported by a pH<4 observed in
Figure 23, that keeps jarosite stable. One can also see that vanadium follows this trend since it
is known to adsorb to e.g. ferrihydrite (Larsson, 2014) which is also known to occur at low pH
(Naeem et al., 2007). Copper shows a similar trend which could indicate adsorption or
coprecipitation to iron hydroxides (Benjamin and Leckie, 1981) and/or association to organic
matter (Scholz and Neumann, 2007). Further down in the transition zone, pH increases >6
which gives a significant increase in sulfur concentration leading to the framboidal pyrites
observed here. Varying between oxidizing and reducing conditions with ground water level
is most likely the reason for the different morphologies (growth and dissolution) observed in
the transition zone. Some pyrites could be observed in the transition zone but in the reduced
layer, the amount increased rapidly. This correlates well with the increasing sulfur
concentration which increases with depth. One can see that iron follows sulfur in the reduced
parts which could indicate its association with iron sulfides.
Another trend observed is a slight increase of aluminum in the transition zone that could be
related to either precipitation of hydroxides or formation of clay minerals. It is though
important to note that Cr, the light rare earth elements (LREE) and the heavy rare earth
elements (HREE) follow this trend.
Figure 23: Element concentrations site B profile 1.
28
Chromium is a trivalent cation and will follow the REE that are commonly associated with
clay minerals (Åström et al., 2010; Harlavan et al., 2009). Also Zn and Co follow this trend and
some research suggest adsorption to illite (clay) particles (Chester, 1965). Zinc and Cobalt
follow each other and have a trend with leaching from the OZ (16 and 4 ppm) with small
accumulation in the transition zone (3 and 1 ppm). Nickel shows the same leaching rate
through the entire profile and do not follow any other trend. Lead shows increasing
concentration upward which could be due to either small amount of anglesite (PbSO4) that has
precipitated or association to organic matter.
Barite was observed in both the transition zone and reduced zone. A pH between 6.5-7 in these
layers gives a redox potential (Eh) of -0.25 V at lowest for barite to be stable (Tokunaga et al.,
2016). Framboidal pyrite is also stable in these two layers and according to the stability
diagram for pyrite (Figure 5.6) (Ingri, 2012), it is stable only in a small window. If connecting
the redox potential (Eh) of -0.25 V for barite to the stability for pyrite it gives a stable pH of 6-
8 which overlaps the existing pH in these layers. Rickard (1970) illuminates the ongoing debate
about the formation of framboidal texture for pyrite that has been in progress for several years.
Wilkin and Barnes (1997) debate about the possibility of formation through either reaction
forming greigite or replacement of greigite that commonly has a framboidal texture. Some of
the framboidal pyrites down in the reduced layer had oxidized during storage and then started
to get a coating of hematite that showed during Raman spectroscopy. Transition zone sample
had a 4-6 mm oxidation rim while reduced sample a 1-3 mm rim. This profile was stored for
12 days before preparation making the oxidation rate 0.33-0.5 mm/day and 0.083-0.25 mm/day
for the transition zone and reduced zone. The difference in oxidation rate could indicate that
the transition zone is dominated by more easily oxidized iron sulfides and the reduced zone
dominated by framboidal pyrites but this is just speculation. Element concentrations from
profile 2 can be seen in Figure 24 below.
In the waterlogged profile, pH is near 4.5 in the oxidation layer and increases further down.
Sulfur continues to leach out (0.8 %) even though the total concentration is lower than in profile
1. Very few framboidal pyrites were observed in both the oxidation zone and transition zone
which correlates well with the low sulfur content. Elements that follow sulfur exactly are Co
(3 ppm) and Cr (26 ppm) suggesting they could be included into sulfides. The sulfur
concentration increases with nearly 1% from the transition zone to the reduced layer. The
increase in number of framboidal pyrites is far too low to correspond to that sulfur increase.
Figure 24: Element concentrations site B profile 2.
29
Therefore, the source of sulfur must be some other mineral than framboidal pyrite like iron
monosulfide or nanoparticulate pyrite. In profile 1 was barite found in the reduced parts but
in profile 2 it was found in the oxidation zone. With a pH of 4.5 in the oxidation zone this gives
a redox potential (Eh) of -0.05 V at lowest in this sample for barite to still be stable (Tokunaga
et al., 2016). For pyrite to also be stable in this condition the redox potential could vary between
-0.05-0 V (Figure 5.6) (Ingri, 2012). In the transition and reduced zone when pH goes up to 7-
7.5 the redox potential could vary between -0.35-(-0.20) V for pyrite to be stable.
Like in profile 1 one can see that iron is retained in the OZ due to precipitation of iron
hydroxides although it was not that prominent in the polished sample like it was in profile 1.
Elements that follow the iron trend are Zn, Ni, Cu, Mo and V that could be due to adsorption
or coprecipitation to the iron particles. Manganese and the light rare earth elements (LREE)
show a leaching trend through the entire profile. Arsenic and the heavy rare earth elements
(HREE) show a trend with accumulation in the transition zone (5 and 1 ppm). Aluminum
shows a trend with the highest concentration in the oxidation layer but instead of decreasing
downward it is stable between the transition zone and reduced layer. Lead is stable through
the entire profile.
Oxidation zone has a 1-2 mm rim, transition zone a 2-4 mm rim and reduced zone a 0.5-1 mm
rim. After storage for 18 days this gives an oxidation rate of 0.056-0.11 mm/day, 0.11-0.22
mm/day and 0.028-0.056 mm/day. The oxidation zone sample from profile 2 was already
oxidized when taking the sample even though it was more reduced than the OZ in profile 1.
Like in profile 1, the transition zone shows a higher oxidation rate which could indicate
minerals that oxidize faster than pyrite such as iron monosulfides but this has to be further
investigated since also grain size affects the oxidation rate. It looks like profile 1 is dominated
by framboidal pyrite but not excluding iron monosulfides and profile 2 is dominated by more
easily oxidized minerals like iron monosulfides but has some framboidal pyrite as well.
If comparing the concentrations of Mn and S in the two profiles one can see that it is lower in
profile 2 (840 ppm and 0.95 %, respectively) compared to the first profile (1480 ppm and 1.9 %,
respectively). Their trends are quite similar in both profiles and overlap each other. Profile 2’s
reduced samples are taken closer to the surface than those of profile 1 which could lead to
similar concentrations if taken deeper down even though the profile lies closer to the mean sea
level. When comparing element concentrations, one can see that many of the elements show
higher concentrations in the waterlogged profile. There is a difference of 1-20 ppm between
the oxidation zones for Zn, Co, Ni, Cu, V, and LREE. For Cr, As and Pb higher concentrations
can be seen in the more reduced parts of the profile. Of these elements Zn and Co have leached
out more from the OZ than are being accumulated in the transition zone, Ni has leached out
through entire profile 1 and Cu and V have leached out from the transition zone. Zinc, nickel
and cobalt show the best possibilities for transportation to profile 2 but this has to be
monitored before any conclusions can be drawn. Arsenic has been liberated but leaching too
little to be able of giving such an increase downstream, there could either be a naturally higher
concentration or more transported from other acid sulfate soils upstream the river. All other
elements are having naturally higher concentrations compared to profile 1.
30
4.2 Björsbyn (Site C)
4.2.1 Optical microscope and SEM
Samples from profile 1 are mostly light grey-grey colored where the sample from OZ is brown
in color due to iron hydroxides. All samples have a 0.5-3 mm brown-light grey oxidation rim
surrounding the main matrix. Profile 2 has black-grey colored samples with a 4 mm diffuse
oxidation rim in the oxidation layer and a thin rim partially surrounding the transition zone
sample. Profile 1 has large silicate grains that can be seen with the naked eye which becomes
fewer with depth.
Oxidation zone (profile 1)
The sample from the oxidation zone is covered with iron hydroxides that give a dark brown
color but no jarosite was found on the surface. A beige oxidation rim surrounds the polished
sample and a breccia with large silicate grains lies outside the rim. One can observe many
euhedral reflecting grains that could be silicate grains.
Transition zone (profile 2)
This zone contains smaller framboidal pyrites compared to site B that are evenly distributed
over the sample. Out in the oxidation rim the pyrite has been altered but it is not possible to
say how far the oxidation extends. Some parts of the sample contained thin layers with a
different color than the original matrix (Figure 25A).
Figure 25: Layering due to mobilization and sulfur concentrating in framboidal pyrite.
Figure 25A shows one of the layers that appears brighter than the surrounding matrix. When
comparing the chemistry in point B inside the layer with point C in the matrix one can see that
the layer has lower sulfur concentration than the matrix (Figure 25B and Figure 25C). One can
also see a line with framboidal pyrites that lies within the layer, indicating that sulfur has been
concentrated in framboidal pyrites (Figure 25A).
31
In one area of the sample one can see a typical cubic pyrite grain together with a framboidal
pyrite sphere (Figure 25D). Another observation in this sample was a zoned iron sulfide
(Figure 26).
Figure 26: Zonation of metastable iron sulfide.
Zonation in Figure 26 has most likely been formed as a result from oxidation where the bright
spots (B) are the intact iron sulfide. Further out in zonation (spectrum C and D), sulfur and
iron concentrations decrease while other elements increase their concentrations (e.g. Figure
26C). This grain has a diffuse morphology that could indicate dissolution. This makes it
impossible with only the chemistry to interpret if it is a pyrite grain or an iron monosulfide
and should therefore be called metastable iron sulfide until more investigation has been done.
Iron peaks are very small which supports pyrite as the correct mineral rather than a
monosulfide. The most interesting observation in this sample was euhedral grains of elemental
sulfur (Figure 27).
32
Figure 27: Elemental sulfur next to a framboidal pyrite grain. ES= elemental sulfur, Py=pyrite.
Figure 27A shows three (50 µm) grains of native sulfur where the spectrum in Figure27D
shows the single sulfur peak from the grain in focus in Figure27C. In Figure27C one can also
see that the grain is quite soft since the electron beam from the instrument has burned three
holes in it. This is only a small area with native sulfur of many with more sulfur grains.
Reduced zone (profile 1)
The reduced layer has fewer framboidal pyrites than observed in the transition zone. Many of
the observed pyrites show signs of oxidation, both in the matrix and in the oxidation rim
(Figure 28). Some iron hydroxides have precipitated both in the rim and in the matrix of the
sample.
Figure 28: Framboidal pyrites with oxidation surface (20 x ocular, optical microscope).
The few framboidal pyrites found in the sample had an orange color and a rim around them
(Figure 28). This rim could either be an oxidation rim or a growth rim like those seen at site B.
33
Oxidation zone (profile 2)
The sample is black with a 4-5 mm thick dark grey oxidation rim. Larger silicate grains are still
intact, and no iron hydroxides have been observed. Some framboidal pyrites are preserved
due to a higher groundwater table that limits oxygen penetration and slows down the
oxidation in this sample. Instead the pyrite grains had grown larger (Figure 29).
Figure 29: Framboidal pyrites, not densely packed (50 x ocular, optical microscope).
Like site B profile 2 the framboidal pyrites are seen in a cross-section and are not as densely
packed like they usually are. The tiny sparkling grains in the background (in B) could either
be iron monosulfides or some silicate grains.
Transition zone (profile 2)
Grey colored sample with a 0.5 mm oxidation rim only at the downside of the sample. Many
small (4-8 µm) framboidal pyrites were observed evenly distributed over the sample (Figure
30). There was at least 15-25 framboidal grains per 5x ocular view.
Figure 30: Distribution of (up to 6 µm) framboidal pyrites (black dots) (10 x ocular, optical microscope).
Reduced zone (profile 2)
Some large areas were covered with iron hydroxides but are most likely only surface staining.
In this sample, only 1-3 framboidal grains per 5x ocular were observed compared to 15-25 in
the TZ (Figure 31).
34
Figure 31: Framboidal pyrites (20x ocular, optical microscope).
Some of the framboidal pyrites show signs of orange alteration on the surface and have a thin
rim around the grains that could be a result of oxidation or growth. Grains here are more
densely packed than in the oxidation zone.
4.2.2 Geochemistry of the profiles
Soil samples from site C were taken in a thicket close to the water, behind “Stadsträdgården”
in Björsbyn. Profiles were taken ca. 5.5 m and 2.5 m above mean sea level with ca. 30 m apart.
Element concentrations for profile 1 can be seen in Figure 32 below.
In this profile, pH reaches down to 3.8 near the ground surface. In the sample from the
oxidation zone one can see lots of iron hydroxides but no jarosite. The low pH supports the
presence of jarosite like at site B and the first thought is that potassium is the limiting factor
but when comparing potassium concentration between site B and C (Table 13 and Table 14)
they differ very little. That leaves the limiting factor to sulfur concentration. Site B has 0.45 %
in the oxidation zone and site C < 0.2 % which then most likely is the reason jarosite has not
formed at site C profile 1. In the OZ nearly all elements have been liberated, due to the low
pH. One major trend that can be seen is that sulfur is leached out (0.95 %) and reprecipitated
further down (1.2 %). This means that at least 0.25 % of the sulfur is already in the TZ from the
beginning before a possible reprecipitation. Manganese (213 ppm and 1170 ppm, respectively),
the light rare earth elements (2 ppm and 54 ppm, respectively), molybdenum (0 ppm and 13
ppm, respectively) and arsenic (2 ppm and 1.5 ppm, respectively) follow this trend and
support that it has to be a naturally higher occurrence of elements further down than are
leached out from the OZ.
Figure 32: Element concentrations site C profile 1.
35
In this profile, most processes occur in the TZ. Mobilization and concentration of sulfur into
pyrite, oxidation forming zonation in iron sulfides and large native sulfur grains have been
observed in this sample. The pH in this layer is near 7 which leads to a redox potential (Eh)
between -0.35-(-0.25) V for pyrite to be stable according to the iron stability diagram (Figure
5.6) in (Ingri, 2012). The most peculiar is the presence of native sulfur in the TZ. When looking
at the sulfur stability diagram (Figure 5.7) in (Ingri, 2012) at pH 7 with the required redox
potential for pyrite to coexist, one can see that the stable sulfur species in this system should
be H2S and/or HS−. Elemental sulfur is only stable in a narrow redox window at pH 0-5. Only
other documented finding of native sulfur with SEM was in the sulfate-methane transition
zone of marine sediments in China (Lin et al., 2015), which is a different environment than in
this study. Some possibilities are that native sulfur is an oxidation product from metastable
iron sulfides (Bloomfield, 1972a), from oxidation of hydrogen sulfide or from reduction of
sulfate (Berner, 1984). Boman et al. (2008) had a similar discovery near Vasa, Finland with
elemental sulfur together with pyrite in the TZ although it was only measured through a
separation sequence. Their conclusion was that no metastable iron sulfides are present since
they would have reacted with the elemental sulfur and formed pyrite like proposed by e.g.
(Morse et al., 1987). Therefore, in this sample it had to be more elemental sulfur present than
it was metastable iron sulfide and all the latter have been converted into pyrite. The sulfur
content reaches up to 2.4 % in the TZ with both framboidal pyrite and native sulfur as observed
species. The reduced layer has a sulfur concentration right below 1.4 % and it is reflected on
the fewer framboidal pyrites observed in this layer compared to the TZ. For the reduced layer
that has a pH of 8.5, the redox potential could vary between -0.4-(-0.3) V for pyrite to still be
stable (Figure 5.6) (Ingri, 2012). A major trend seen is for iron where leaching (OZ 1.6 %) occurs
through the entire profile but less in the more reduced part. This can be compared to site B
where iron was precipitating as hydroxides in the oxidation zone instead of leaching. The
elements following this trend are Zn (OZ 56 ppm), Co (OZ 10 ppm), Ni (OZ 28 ppm), Cu (OZ
22 ppm), Cr (OZ 30 ppm) and V (OZ 30 ppm). Since iron and sulfur are not following each
other this could indicate that the majority of the iron is incorporated in other minerals than
sulfides hence the trend. The low pH makes aluminum and lead to leach out through the entire
profile. At site B these two elements had increasing concentrations upward instead of leaching.
The only element that increases upward in this profile is the heavy rare earth elements (HREE)
which could be due to incorporation in weather-resistant minerals or association to clay
minerals. Oxidation rims are 0.5-1 mm in the oxidation zone, 0.7-3 mm in the transition zone
and 0.7 mm in the reduced layer. With storage for 20 days, this gave an oxidation rate of 0.025-
0.05 mm/day, 0.035-0.15 mm/day and 0.035 mm/day for each layer. Element concentrations for
profile 2 can be seen in Figure 33 below.
36
Like profile 2 at site B, pH in this waterlogged profile is higher than in profile 1. At this site
pH is near 7 near the ground surface and it increases above 8 with depth. This makes it possible
to observe intact framboidal pyrites in the oxidation zone although the sulfur concentration is
as low as in profile 1. This could also explain why only larger grains are still intact. With a pH
close to 7 this gives a redox potential between -0.35-(-0.25) V for pyrite to be stable (Figure 5.6)
(Ingri, 2012). In this profile one can see the same trend as in profile 1 for sulfur (0.4 % and 1.5
%, respectively) together with Mn (110 ppm and 1063 ppm, respectively), Mo (0 ppm and 7
ppm, respectively) and As (0.5 ppm and 5 ppm, respectively), with leaching from the OZ and
accumulation in the TZ. This supports the theory of a layer in the transition zone naturally
enriched in sulfides which connects these two profiles. The amount of framboidal pyrites
correlates well with the sulfur content that reaches up to nearly 2.4 % in this layer. In the
transition zone pH gets close to 7.8 which gives a redox potential between ca -0.37-(-0.27) V
for pyrite to be stable (Figure 5.6) (Ingri, 2012). With depth the pH increases above 8 and with
it the sulfur concentration decreases down close to the same level as in the OZ. This sample
has also framboidal pyrite but fewer than the transition zone which is reflected on the lower
sulfur concentration. At this pH the redox potential in the reduced zone lies between -0.4-(-
0.3) V for pyrite to be stable (Figure 5.6) (Ingri, 2012).
One can see that iron leach out (OZ 1.8 %) like in profile 1 but small amounts are accumulated
in the transition zone (0.2 %) indicating participation in sulfide formation in the transition
zone. This is different than for profile 1 that showed no accumulation further down. The
leaching of iron without retaining concentration due to hydroxide precipitation could indicate
that this site contains less Acidithiobacillus ferrooxidans bacteria that oxidizes Fe2+ to Fe3+
(Nordstrom, 1982). This means Fe2+ is kept in solution and is transported downward until it
reaches the sulfur front and precipitates as sulfides or is transported out of the system where
it can precipitate as iron hydroxides. In this profile Ni, Cu, Pb and V follow the same trend as
iron did in profile 1 with only intense leaching in the OZ. Zinc together with the light rare
earth elements show a continuous leaching pattern from top to bottom while chromium shows
intense leaching in the OZ and TZ. Aluminum shows the same trend as it has done before with
increasing concentration upward due to either higher silicate content or association with
organic matter. The heavy rare earth elements have a trend with liberation from the TZ (2
ppm) and retaining their concentrations in the OZ and cobalt is stable or shows a tiny increase
upward throughout the entire profile. This could indicate incorporation into more weather
Figure 33: Element concentrations site C profile 2.
37
resistant silicate minerals that are containing both Al and HREE and possibly small amounts
of Co. Oxidation rims are 4-5 mm around the oxidation sample and 0.5 mm around the
transition zone. With 20 days storage, this gives a 0.2-0.25 mm/day oxidation rate in the
oxidation zone and a 0.025 mm/day oxidation rate in the transition zone.
If comparing element concentrations between the two profiles one can see that Zn (7 ppm and
28 ppm, respectively), Ni (10 ppm and 5 ppm, respectively) and Co (9 ppm and 1 ppm,
respectively) are higher in the OZ and TZ in profile 2. Copper is 2 ppm and vanadium is 5
ppm higher in the TZ and Cr is (20 ppm and 45 ppm, respectively) higher in the OZ and RZ.
All these elements are being leached out from profile 1. Of these elements it is possible that all
of them could be transported from profile 1 to profile 2 even though most of them also show
leaching from profile 2. Cobalt has an increasing concentration but in profile 2 conclusion is
that it probably is included in some weather resistant mineral making it less presumable that
the increase is due to transportation from the upstream profile. The other elements either show
no leaching trend upstream or show no increase in element concentrations.
4.3 Sunderbyn (Site A)
4.3.1 Geochemistry of profiles
Soil samples for site A were taken on the field across from the “Kläppen school” in Sunderbyn.
Profiles from site A were taken 6 m and 1 m above mean sea level with ca. 100 m apart. The
element concentrations in profile 1 can be seen in Figure 34 below.
It can be observed that pH is close to 4.5 in the OZ leading to intense liberation of mostly S
(1.45 %) but also Mn (235 ppm), Zn (12 ppm), Co (4 ppm), Ni (8 ppm) and Cu (2 ppm) are
liberated from this layer. Like at site B iron is only liberated from the transition zone (0.2 %).
This is due to precipitation of iron hydroxides in the oxidation zone limiting the loss compared
to the RZ. A similar trend was also observed in studies from Finland (Åström, 1998; Nordmyr
et al., 2008). Iron follows the trend of sulfur in the TZ and RZ which could indicate
incorporation in sulfides further down. Arsenic follows this trend and is likely to be associated
with the iron particles (Widerlund and Ingri, 1995). Lead is also following this trend, but are
commonly precipitating as anglesite (PbSO4) particles at least in an acid mine drainage (AMD)
environment (Dold, 2010). Soil studies instead suggest the possibility of adsorption to
ferrihydrite or organic matter (Gustafsson et al., 2011). Downward in the profile, pH increases
Figure 34: Element concentrations with depth from site A profile 1.
38
to near neutral stopping the liberation of several elements. At this pH one can see that 36 ppm
of the liberated manganese accumulates in the TZ. This could be related to precipitation of
manganese hydroxides/oxides such as MnOOH and/or MnO2 (Stumm and Morgan, 1996).
Elements that follow this trend and possibly are associated with these hydroxides are Zn (15
ppm), Co (2 ppm), Ni (4 ppm) and Cu (1 ppm), which is in agreement with studies by (Algeo
and Maynard, 2004) and (Shaw et al., 1990). Some of these elements (Zn, Co and Ni) that
accumulate in the transition zone are following earlier research results from this environment
(Åström, 1998). Since more Zn has been accumulated in the transition zone than released by
oxidation this could be explained by a naturally higher Zn content in this layer.
Chromium and the light rare earth elements (LREE) have a similar trend with accumulation
in the transition zone without liberation from the OZ. Aluminum shows a decreasing
concentration with depth which could be due to either more silicate minerals, precipitation of
gibbsite (Al(OH)3) or association with organic matter higher up in the profile. The only
elements that show a similar trend as Al are the heavy rare earth elements (HREE) since these
elements are stable as trivalent cations. When looking at the mass balance in Figure 38 one can
see that aluminum show a slight accumulation in the transition zone probably due to clay
mineral formation which then most likely Cr and the REE are related to. Molybdenum and
vanadium show also decreasing concentrations with depth which could indicate that these
elements are associated with weather-resistant minerals. Element concentrations from profile
2 for comparison are shown in Figure 35 below.
In the waterlogged profile, pH is close to 6.2 due to the higher groundwater table that limits
oxygen penetration and slows down the oxidation. All elements show leaching from the
oxidation layer despite the high pH. If comparing immobile element concentrations (Ti and
Zr) with depth (Figure 46) one can see that Ti shows a similar trend like Fe and S while Zr
follows Mn. This could indicate a dilution effect due to organic matter in the top layer, which
gives a false leaching result. Iron (1.7 %), sulfur (1.5 %) and arsenic (4 ppm) are strongly
liberated from the oxidation layer while they are stable further down in the profile. Manganese
(494 ppm) and molybdenum (3 ppm) show a similar trend but are leaching more in the
transition zone than the other elements. In the transition zone, pH is slightly decreased.
Figure 35: Element concentrations with depth from site A profile 2.
39
Aluminum is accumulated (0.3 %) in this layer which could indicate precipitation of aluminum
particles like gibbsite (Al(OH)3) or formation of clay particles that gives a higher concentration.
One can also see that Zn (24 ppm), Co (4 ppm), Ni (7 ppm), Cu (1 ppm), Cr (7 ppm), V (3 ppm),
the LREE (18 ppm) and the HREE (1 ppm) are following this trend. Zn, Co, Ni and sometimes
Cu are commonly trapped in the transition zone (Åström, 1998; Nordmyr et al., 2008). The REE
are commonly associated with clay mineralogy and therefore would an enrichment support
such a hypothesis (Åström et al., 2010; Harlavan et al., 2009).
If looking at each element and compare the profiles one can see that only Co, Ni and Cr reach
higher concentrations in profile 2. Only Co and Ni of these show a leaching pattern with an
excess of mobile concentrations from the adsorption in the transition zone. This could mean
that these elements are transported downstream and accumulated in the waterlogged profile.
Due to the dilution effect, this is only kept as a possibility.
4.4 Björsbyn (Site D)
4.4.1 Geochemistry of profiles
Site D lies on a small meadow within 1 km from site C in Björsbyn. Profiles were taken 3 m
and 2 m above mean sea level with ca. 30 m apart. Element distribution through profile 1 can
be seen in Figure 36 below.
At this site, pH is higher (⁓ 4.8) near the ground surface than further down where it reaches 3
before increasing toward neutral pH. Like at site C sulfur leaches out (0.7 %) and is
accumulated in the transition zone (1.3 %). This backs up the theory of a sulfide-rich layer in
the transition zone. Site C and D are connected through this layer. Elements that follow sulfur
are Mo (0 ppm and 7 ppm, respectively), As (3 ppm and 9 ppm, respectively) and the LREE
(12 ppm and 40 ppm, respectively). This is very similar to site C where also Mn followed this
trend. Chromium shows minor accumulation in the transition zone (2 ppm) that could indicate
incorporation into sulfides. Iron has a trend where it leaches quite intense in the oxidation
layer (1.1 %) and further down stabilizes. which is similar to the trend seen at site C. Cobalt
and nickel follow the trend of iron (10 ppm and 24 ppm, respectively) which could be due to
incorporation in pyrite (Huerta-Diaz and Morse, 1992; Scholz and Neumann, 2007). Zinc,
copper and manganese show a continuous leaching trend (OZ 74 ppm, 26 ppm and 388 ppm,
respectively) through the entire profile. This could indicate that Zn and Cu are associated with
manganese particles and when pH is lowered they are dissolving making them go into
Figure 36: Element concentrations site D profile 1.
40
solution. Another trend is aluminum that is intensely leached out. This time lead and
vanadium follow this trend indicating that they could be associated with clay particles.
Element concentrations for profile 2 can be seen in Figure 37 below.
This waterlogged profile has like profile 2 at the sites A and C a pH above 6 in the OZ. The
transition zone is missing in this profile and only a very thin (10 cm) OZ is present which
makes it hard to interpret the trends. One can see one dominating trend in this profile, with
increasing concentrations upward which could be due to association with organic matter
(Nordmyr et al., 2008; Scholz and Neumann, 2007; Shaw et al., 1990) and some of them possibly
due to incorporation into weather-resistant minerals (Mo, Pb, V, HREE). The other trend to see
is one with minimal leaching and/or stable trend. In general, this profile has very low sulfur
content through the entire profile. This may indicate that the profile had oxidized and now are
reduced again although sulfides are not formed again as it did in Littorina time (Virtanen et
al., 2014). One possibility that would be great to investigate further is if the sulfur
concentration increases further down beyond the profile depth. This to determine if this profile
has been an old oxidized layer and the real reduced layer is located even further down. If
checking the trend for Zr and Ti (Figure 47), they follow the stable/minimum leaching trend
which means that this profile may have been affected by organic dilution like profile 2 at site
A.
Since profile 2 is without a transition zone, trends are not possible to completely compare
between the two profiles. Comparing the waterlogged profile concentrations to those of the
oxidation profile, one can see that Zn, Co and Ni concentrations are 2-2.5 times higher in the
oxidation layer and vanadium is 16 % higher. This could mean that these elements have been
transported from profile 1 and been accumulated in profile 2 like the case with accumulation
in the TZ presented by (Åström, 1998). The total concentrations of Cu, Al, Cr and Pb are higher
through entire profile 2 where the RZ in profile 2 has similar concentrations as the OZ and/or
the TZ in profile 1. Remaining elements either show no leaching trend upstream or are in
general lower downstream such as sulfur that is much lower than in profile 1.
Figure 37: Element concentrations site D profile 2.
41
4.5 Geochemical comparison
Site C has the highest Mn (1900 ppm and 1700 ppm, respectively), S (2.3 % and 2.15 %,
respectively), Ni (43 ppm and 41 ppm, respectively) and V (103 ppm and 103 ppm,
respectively) content of all sites and the lowest of As (5.3 ppm and 5.7 ppm, respectively).
Profile 1 has the highest Mo content (15 ppm) and the lowest Cr (65 ppm) and REE (LREE 119
and HREE 9 ppm) content.
Profile 2 has the highest Cr (139 ppm) and lowest Mo (1.2 ppm) content. Since a transportation
trend of Cr was seen from profile 1 (Figure 32) and an increase in concentration occurred
downstream (Figure 33), it strengthens the probability for Cr to has been transported, leaving
profile 1 with the lowest concentration of this element. Profile 1 at Site D shows the lowest
concentrations of Al (4.9 %), S (0.07 %), Zn (48 ppm), Co (6 ppm), Cu (8 ppm), Ni (12 ppm), Pb
(13 ppm) and V (66 ppm) all in the OZ. In the TZ it has the highest concentration of As (18
ppm) and in the RZ the highest concentrations of Zn (122 ppm) and Pb (21 ppm) where the
latter ones possibly have been transported downward in the profile and enrich the reduced
layer while depleting the oxidation layer of those elements. Profile 2 shows enrichment of Cu
(40 ppm) in the OZ making it possible to has been transported downstream, depleting it in the
oxidation profile. This profile also has the highest concentrations of rare earth elements
(253+21 ppm) compared to the other profiles, located in the OZ. The oxidation zone in profile
1 at site A has been enriched in aluminum (6.69 %). The transition zone at profile 2 is enriched
in cobalt (18 ppm) while the oxidation zone is depleted in iron (2.62 %) and manganese (439
ppm) compared to the other sites.
When comparing these chemical trends, note that the aluminum concentration mostly
increases upward despite many papers highlighting aluminum leaching out, leaving high
concentration in waterways e.g. (Filppa, 2012; Nordmyr et al., 2008). When comparing this
trend with the mass balance diagrams in 7.2 to denote if this is true, one can see that this is in
most cases not true. This could be due to complications during the analysis with precipitation
of aluminum minerals as a result or loss of volatile elements concentrating the solution in
aluminum. To investigate this further an x-ray diffraction (XRD) analysis has to be done to see
what matrix mineralogy the samples have. For example, profile A1 Figure 34) shows that Al
increase upward while in Figure 38 it is believed that aluminum accumulates in the transition
zone which may indicate the formation of clay particles that Cr and LREE adsorb to. In most
profiles, the mass balance shows a trend of leaching for Al other than in D1, B2 and A1 where
it is accumulated in the transition zone or is leached from the transition zone (section 7.2). A
possible reason for aluminum to plot with increasing concentration upward is the association
to organic matter (Nordmyr et al., 2008).
In profile 1 at site D (Figure 36) nearly all elements including Fe and Al are being liberated
from the OZ but according to the mass balance (Figure 44) they should instead be accumulated
in the transition zone like sulfur. At site B lead increases upward in both profiles (Figure 23
and Figure 24) which could indicate association with organic matter but the mass balance
(Figure 40 and Figure 41) suggest an association with iron hydroxides in the oxidation layer.
Profile B1 (Figure 23) shows accumulation of Zn, Co and Al in the TZ, mass balance (Figure
42
40) instead shows liberation of these elements although Zn and Co still follow the Al trend. In
profile 2 at site B (Figure 24), Co and Cr follow sulfur while the HREE are accumulated in the
TZ. The mass balance (Figure 41) suggests that Co and Cr follow iron and that the HREE
leaches through the entire profile instead. In profile A2 lead is slightly accumulated in the TZ
but in mass balance (Figure 39) it increases upward in the profile. In profile A1 (Figure 34), Mn
and Cu show a slight accumulation in the transition zone and V increases upward but the mass
balance (Figure 38) suggest that they all are leached out through the entire profile.
In profile 1 at site C (Figure 32), the HREE have a trend with increasing concentrations upward
which is according to the mass balance (Figure 42) instead an accumulation in the TZ. In profile
2, the HREE are instead thought to be leached only from the TZ and Co to be relatively stable
through the entire profile. According to (Figure 43) the HREE are leaching through the entire
profile and Co follows iron with leaching only in the TZ. Mass balance is a good way of
correcting errors that are probably related to interference with organic matter since the ratio is
based here on zirconium concentration that is thought to be immobile. This despite the fact
that zircons have been found that are clearly affected by weathering in the low pH.
4.6 Classification
Since potential acid sulfate soil (PASS) contains iron sulfides that easily oxidize when exposed
to oxygen, pH drops fast. This effect can be seen in Table 1 when comparing field pH with pH
after oxidation. The pH in the oxidation zone in profile 1 at site D was taken on a different
depth than the actual sample was taken and therefore not possible to compare.
Table 1: Table over pH measured in the field and after oxidation in the lab.
If comparing sulfur content to where the pH has changed the most in each profile one can see
that at sites A, C and profile B1 the pH has been lowered the most where sulfur content is
highest. This probably follows directly to sulfide content in the profiles especially since all
samples were collected at different dates and have therefore been oxidized for a different time.
Profiles B2, C2 and D2 have a trend where the pH shows a large drop even though the sulfur
content is low. This could mean that the remaining sulfur is included in iron monosulfides that
oxidizes within hours when exposed to oxygen. Profile D1 shows a decreasing pH trend with
depth which is not correlating with the sulfur trend. This could mean that sulfur minerals with
slower oxidation rate than iron monosulfides occur in the transition zone rather than in the
reduced layer. This since pH has dropped faster in the reduced layer.
43
When classifying each sample according to the SGU-standard (section 2.6) the OZ from
profiles B1-D1 are classified as AASS (field pH< 4). The OZ from profile A1 and the TZ from
profiles A2 and D1 are classified as semi-acid while the TZ and the RZ of profiles A1-B1 are
classified as reduced. The RZ of profiles A2 and C2, the TZ of profile C1 and the OZ of profiles
C2-D2 also classified as reduced. Some layers did not reach pH<4 after oxidation and are
therefore not classified as acidifying but are most likely sulfidic due to large pH drops. When
classifying each profile according to the GTK-standard presented by (Edén et al., 2012) where
starting depth of PASS, minimum field pH and also the maximum sulfur content are taken
into account the result follows in Table 2.
Table 2: Classification according to GTK system.
Class 1 means the PASS horizon starts within 1 m and class 2 within 1-1.5 m from the ground
surface. When classifying according to minimum field-pH class D represent a pH > 4.5, class
B a pH between 3.5-3.9 and class A pH < 3.5. Total sulfur content is taken for the uppermost
40 cm of the PASS horizon where (I) are representing a total sulfur content > 1 % and (III) a
content between 0.2-0.6 %. Pousette et al. (2007) propose classifying the acidic potential of
samples with Fe/S ratio, leading to this study’s acid potential presented in Table 3.
Table 3: Acidic potential classification.
If the Fe/S ratio is < 3 the soil is very acidifying and > 60 non-sulfidic. This means that the
transition zone at site C and profile D1 have the potential of being very acidifying and site A
close of being. Many of the reduced layers are also showing a high potential of being
acidifying. All oxidation layers are showing a low potential of being acidifying which is since
most of the sulfur has already leached out from them.
44
5 Conclusion Framboidal pyrite was observed at both site B and C in all samples except for the oxidation
zone samples from profile 1. This is due to extensive oxidation that leads to a low pH. In these
samples secondary iron hydroxides precipitate. In profile 1 at site B the secondary mineral
jarosite was observed amongst the hydroxides but was not observed at site C although the
sample had the potential of forming it. The low sulfur concentration is thought to prohibit this
formation at site C. Barite was only observed at site B (profile 1 TZ-RZ and profile 2 OZ) but
if site C had been further investigated with SEM it might have been found also there. Elemental
sulfur was only found in the TZ at site C and although site B was thoroughly investigated for
this specie no other observation was achieved. This may indicate either the formation or
oxidation of iron sulfides. Different morphologies and stages of evolution of framboidal pyrite
were observed in the samples, where most variation occurred at site B.
A combination of iron sulfides is thought to be present at site B even though the RZ looked
pyrite dominated and although no iron monosulfides were observed they cannot be excluded.
This also gives rise to the debate about whether iron monosulfides gives the black color or just
pyrite since the black lines were filled with framboidal pyrites. The waterlogged profiles retain
the pH high enough for framboidal pyrite to still be intact and in profile 2 at site D it would be
interesting to see if the sulfur concentration increases further with depth. According to the
oxidation rate, site B oxidizes faster than site C on a small scale which could indicate different
sulfide mineralogy. At site B and in profile A1 iron is retained due to hydroxide precipitation
in the OZ while it leaches out at site C and in profile D1. This could indicate a lower activity
of oxidation bacteria at these two sites that prevents precipitation of iron hydroxides. Sulfur
leaches out from sites A and B while accumulating in the TZ at site C and in profile D1. This
is directly reflected on the amount of framboidal pyrites observed at site B and C. The usually
related sulfide elements Zn, Co and Ni show either accumulation in the transition zone like in
other research (Åström, 1998), relation to iron or total liberation through most profiles.
Elements that could be associated with iron hydroxides at different sites are As, Pb, Cu, V, Zn,
Co, Ni and Mo. Many of the elements show the same trend in both profiles at a location. At
site B and C where pH is low most elements not associated with hydroxides, heavy minerals
or organic matter are liberated with large intensity. Aluminum is in many profiles most likely
associated with organic matter. Elements that could be transported and accumulated
downstream are Co, Ni, Zn and Cr and at some of the profiles also Cu and V.
5.1 Future work
In the future, monitoring geochemical parameters and continual groundwater sampling to
measure mobile element concentrations are recommended to increase the knowledge of mass
fluxes in the system. Addition of tracers to the groundwater is proposed to study if elements
travel downstream to the more waterlogged part and conclude if it accumulates further out.
Other work could be to investigate elemental sulfur closer since it has been observed in SEM
at site C and therefore may do some more experimental studies to investigate its stability and
evolution further. Further studies are needed to investigate the parent material in order to
understand if elements are liberated from either sulfides or the silicate matrix.
45
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7 Appendix
7.1 Appendix 1: Profile descriptions
Table 4: Coordinates from profile sites.
Table 5: Profile description site A profile 1.
Table 6: Profile description site A profile 2.
Table 7: Profile description site B profile 1.
Table 8: Profile description site B profile 2.
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Table 9: Profile description site C profile 1.
Table 10: Profile description site C profile 2.
Table 11: Profile description site D profile 1.
Table 12: Profile description site D profile 2.
53
7.2 Appendix 2: Mass balance
Figure 38: Mass balance site A profile 1.
Figure 39: Mass balance site A profile 2.
54
Figure 40: Mass balance site B profile 1.
Figure 41: Mass balance site B profile 2.
55
Figure 42: Mass balance site C profile 1.
Figure 43: Mass balance site C profile 2.
56
Figure 44: Mass balance site D profile 1.
Figure 45: Mass balance site D profile 2.
57
7.3 Appendix 3: Immobile element vertical distribution
Figure 46: Zirconium and titanium distribution site A profile 2.
Figure 47: Zirconium and titanium distribution site D profile 2.
7.4 Appendix 4: Whole rock geochemical data
58
Tab
le 13: Sam
ple ch
emistry
from
total d
igestio
n an
d m
easured
with
ICP
-SF
MS
. Green
are recom
men
ded
valu
es by
AL
S to
use fo
r interp
retation
.
59
Tab
le 14: Sam
ple ch
emistry
from
fusio
n an
alysis an
d m
easured
with
ICP
-SF
MS
. Green
are recom
men
ded
valu
es by
AL
S to
use fo
r interp
retation
.