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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
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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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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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Geology and Hydrogeology
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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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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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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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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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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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
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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
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