understanding hydrochemical processes for resources …€¦ · kai-iwi bore 1figure 1: location...

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Understanding Hydrochemical Understanding Hydrochemical Processes for Resource Evaluation Processes for Resource Evaluation and Planning in a Limestone Aquiferand Planning in a Limestone Aquifer

Barnaby HardingHydrogeologistPattle Delamore Partners LtdAuckland

NZHS 2009: Waters for the Future –Balancing its Values

Colin HoveyWanganui District Council

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Presentation Outline

IntroductionGeology and HydrogeologyBorehole ChemistryHydrochemical ProcessesFuture use of the Resource

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Introduction

Soft water supply for Wanganui CityGroundwater - sole water source for public supplyNukumaru Limestone aquifer – high hardness in placesSome wells show low hardnessAmmonia increases in areas of low hardnessIdeal composition 100-120 mg/l (as CaCO3) and ammonia <0.3 mg/l (as N)Future development of resource requires understanding of hydrochemical processes

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Figure 1: Location PlanKai-iwi Bore 1Kai-iwi Bore 2

Kai-iwi Bore 3

Western Line

BlueskinLithgow

Aramaho

TunnelHarding

Bristol

Taylor

Lenihan

Reservoirs

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Geology and Hydrogeology

South to south-west dipVarious lithologies (mudst, siltst, sandst, limest.)E to W orientated outcrop, younging to southNorth-east to south-west faultsBlock bounded by Upokongaro Fault and Nukumaru Fault zone

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Layered aquifer/aquitard sequencePrimary aquifer

Nukumaru Formation (in Nukumaru Group)Coarse micaceous brown sands and Limestone layersTargeted by WDC boreholes80 to 120 m thickCrops out in north, 600-700 m deep beneath WanganuiKh = 8 x 10-5 m/s

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Recharge over outcrop, confined to the southTotal estimated volume approx 200,000 m3/d (SKM Model)

Anticlockwise movement of groundwaterDischarge:

to sea (upward leakage)to Whanganui River (deeply incised)Wells (approx 20,000 m3/d)Fault upwelling

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Borehole Chemistry

Hardness varies 11 to 191. Most boreholes are moderately hard (75-150) or hard (150-300).Ammonia - <0.01 to 1.61 mg/lSodium - 16.4 to 260 mg/lChloride - 19.1 to 30.6 mg/lSulphate – 17.1 to 82.9 mg/l

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Borehole Chemistry

Trends indicate evolution along flowpathDecrease in calcium concentration and hardnessRise in sodium concentrationRise in sulphate concentrationRise in ammonia concentrationIncrease in temperature

Relatively sharp boundary close to Aramaho, Harding and Tunnel bores

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Nukumaru ChemistryNukumaru aquifer properties v distance from recharge

0

100

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3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000

distance from recharge m

alka

linity

,sod

ium

, TD

S (g

/m3)

, pH

(*10

0)

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Ca,

SO

4, M

g, C

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ide

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ammonia sodium TDS pH*100

sulphate calcium magnesium chloride

Reservoirs BristolHardingWestern Line Blueskin RoadLithgow

Tunnel

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Borehole Chemistry

Durov plot indicates two main water types + possible intermediateFalling hardness accompanied by increased ammoniaFuture use – “sweet spot”

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Hydrochemical Processes

Similarities with Lincolnshire LimestoneClassic case of hydrochemical zoningSimilar processes suspectedKey processes

Calcite dissolutionRedox boundary in confined aquiferIon exchangeSulphate reductionMixing – new recharge with connate saline

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Hydrochemical Zones

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Hydrochemical Processes

Division into 5 zones – on basis of hydrochemical process (Downing and Williams)Division into 3 water types (Lloyd et al)

Type I – Recent recharge (hard)Type II – Mixing zoneType III – Connate (saline)

Hydrochemical Zones in the Lincolnshire Limestone (after Downing and Williams) Zone Approx

width Redox Potential

Hydrochemical Process Dominant Ions

1 As outcrop Positive Congruent CaCO3 dissolution, solution of sulphates and nitrates, sulphide oxidation, oxidation of organic material

Ca2+, HCO3-

2 1.5 to 11 km Positive Incongruent CaCO3 dissolution, nitrate reduction, organic oxidation

Ca2+, HCO3-

3 0.6 to 4 km Transition – Redox Barrier

Sulphate reduction, incongruent CaCO3 dissolution

Ca2+, HCO3-

4 0.35 to 2.5 km

Negative Ion exchange (Ca/Na), mixing of fresh and saline waters, sulphate reduction

Ca2+, Na+, HCO3- Na+, HCO3- Na+, Cl-, HCO3-

5 >2.5 to >18 km

Negative Water mixing with saline end-member, sulphate reduction, methane fermentation

Na+, Cl-

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Lincolnshire Limestone Chemistry

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Carbonate Dissolution (congruent)

CO2 + H2O + CaCO3 = Ca2+ + H2CO3* + HCO3- + CO3

2-

Carbon dioxide + water + Calcite (Limestone) = Calcium (ion) + carbonic acid + bicarbonate (ion) carbonate (ion)CO2 from root zoneOpen system (water in contact with gaseous phase)Closed system (no contact with gaseous phase)Calcite saturation achieved rapidly close to recharge zone

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Congruent CaCO3 Dissolution

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Carbonate Dissolution (incongruent)

Solution and concomitant dissolution of carbonateE.g. Dolomite – solution of CaMg(CO3)2 with precipitation of CaCO3Increased concentrations of Sr and Mg in Lincs Limestone

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Ion Exchange

Exchange of calcium for sodiumCa2+ = 2Na+

Exchange sites:Clay minerals, marine depositionFinely disseminated ferrous minerals and organic material

Initiation of ion exchange:Redox conditions?Mixing?Lack of saline end-member in Nukumaru Lst suggests former

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Sulphate

Lincs Limestone - reduction in concentration down flowpathNukumaru – increase in concentration down flowpathEarlier studies suggest SRB actionLink between decreasing Ca and Sulphate reduction

1SO4 = 2HCO3 = 2CaMore recent work suggests agrochemical source

Not enough H2S for SRB actionReduction rate expected to be too slowNot supported by modelling

Sulphate can increase beyond redox barrier (e.g. Triassic Sandstone in the UK, gypsum/anhydrite dissolution)

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Redox Barrier

Drop of around 300 mVAll studies show correlation between location of redox barrier and onset of ion exchangeIn addition to marine clays, finely disseminated ferrous minerals and organic material may act as exchange site under certain redox conditionsRedox condition may therefore be the trigger for ion exchange in Nukumaru Limestone – Additional data required

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Future Use of the Resource

Location:Sweet spot – low hardness and low ammoniaThin transition zoneAge-related - not perpendicular to flowpath but parallel to isochronsMost favourable location is north-east to south-west zone to north-west of Aramaho, Tunnel, Harding, and Bristol bores

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Figure 7: Target Zone for Future Development

Kai-iwi Bore 1Kai-iwi Bore 2

Kai-iwi Bore 3

Western Line

BlueskinLithgow

Aramaho

TunnelHarding

Bristol

Taylor

Lenihan

Reservoirs

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Future Use of the Resource

Sustainability:Ion exchange capacity of aquiferSlowly migrating front? Mining?Abstraction approx 0.1 x recharge

Indications from Lincs LimestoneReview of changes over 10 years

Over-abstractionChange from sharp to diffuse redox boundaryDowngradient migration of high calcium and sulphate concs (7km)Increased chloride concentration close to recharge zone

Indicates decreased effectiveness of ion exchange and inability to keep up with abstraction

What does this mean for the Nukumaru aquifer?

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Conclusion

In the Nukumaru limestone aquifer hydrochemical processes are important to understand for future use of the resource.

Understanding these processes allows better planning with respect to reduced treatment requirements, future locations, overall abstraction volumes, and the risks


©PDP 2008

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StratigraphyChronostratigraphy Lithostratigraphic

Unit Dominant Lithologies Depositional Environment

Holocene Epoch Lahar, dune, river terrace and undifferentiated deposits

Mainly non-marine

Undifferentiated Gravels Mainly non-marine

Papaiti Alluvium Coarse greywacke and andesite gravel, sand and silt Mainly non-marine

St Johns Alluvium Fine gravel, sand, and silt Mainly non-marine

Rapanui Fmn Basal conglomerate, marine sand, volcanic sand and lignite bands

Partly marine

Brunswick Fmn Greywacke, quartzite and andesite gravel, marine sand, pumice, clay and sand

Partly marine

Hawera Series Kaiatea Fmn Quartzite, greywacke and argillite gravel, volcanic ash and sand

Partly marine

Shakespeare Group Mudstone, silts, and sands Shallow marine

Kai-Iwi Group Sands silts and shelly layers Marine and shallow marine

Castelcliffian Stage

Okehu Group Shelly conglomerate and grit, tuff Marine

Maxwell Group Various including lignite layers Deltaic and Estuarine

Wanganui Series

Nukumaruan Stage

Nukumaru Group Coarse sand and interbedded limestone Marine and shallow marine

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Figure 1: Location PlanKai-iwi Bore 1Kai-iwi Bore 2

Kai-iwi Bore 3

Western Line

BlueskinLithgow

Aramaho

TunnelHarding

Bristol

Taylor

Lenihan

Reservoirs

©PDP 2009


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understanding hydrochemical processes for resources …€¦ · kai-iwi bore 1figure 1: location...

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