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Hydrogen and oxygen isotopes in groundwaterTopics in this class sectionn nIsotopic changes during transfer of water from precipitation to ground waters(recharge isotopic signatures)n nExamples of applying isotopic compositions of ground waters to hydrology problems such as hydrograph separationObjectivesn nHow do different recharge mechanisms affect the groundwater isotopic ratiosn nHow to use the different isotopic signatures of ground and surface waters for hydrological researchIsotope changes during rechargen nRecharge in temperate climatesn nRecharge in arid regionsTemperate climatesn nOnly 5 to 25%Only 5 to 25%of precipitation infiltrates to water of precipitation infiltrates to water tabletablen nThe other 75 to 95%of the precipitation goes to:The other 75 to 95%of the precipitation goes to:n nRunoff:no isotope effectRunoff:no isotope effectn nEvaporation from soils:isotope enrichment in soil waterEvaporation from soils:isotope enrichment in soil watern nTranspiration by vegetation:no isotope effect on soil waterTranspiration by vegetation:no isotope effect on soil waterSeasonality of rechargen nSpring:Recharge rates are the highestSpring:Recharge rates are the highestn nSoils are saturatedSoils are saturatedn nTemperature is low(low evaporation)Temperature is low(low evaporation)n nVegetation is inactive(not taking/transpiring water)Vegetation is inactive(not taking/transpiring water)n nSummer:recharge minimal,most precipitation Summer:recharge minimal,most precipitation transpired to the atmospheretranspired to the atmospheren nFall:recharge increases as photosynthesis shutdownFall:recharge increases as photosynthesis shutdownSeasonality of rechargen nWinter:little recharge due to frost.Precipitation stored as snow,often lost during spring runoffn nIsotopic composition of ground waters intermediate value of spring and fall rain mean annual precipitationAttenuation of seasonal variationsn nGroundwaters sampled below the water table generally have an isotopic value that is close to the weighted average of annual precipitationn nSeasonal variations in isotopes are attenuated during movement through the unsaturated zone,which is an important zone of mixingd dD attenuation at 5m depthd d18O attenuation in Quaternary gravelsCritical depthFine grained soils=3 to 5 mFractured rock(fast infiltration)10 m Attenuation in confined aquiferIsotopic changes in confined aquifer signifies mixing of different recharge watersShallow groundwater and precipitationEnriched precipitationGround water isotopic depletionn nMost precipitation occurs during the summer,but Most precipitation occurs during the summer,but does not contribute to recharge significantly due does not contribute to recharge significantly due to transpirationto transpirationn nMost recharge occurs in cooler months from Most recharge occurs in cooler months from October to MayOctober to Mayn nPrecipitation has more summer components,Precipitation has more summer components,whereas shallow ground water has more spring whereas shallow ground water has more spring componentscomponentsIsotopic values of snow and meltn nSnow is depleted in D and O-18,but the depletion is Snow is depleted in D and O-18,but the depletion is modulated due to surface sublimationmodulated due to surface sublimationn nSublimation from snow is more close to equilibrium Sublimation from snow is more close to equilibrium than evaporation from water,because of the high than evaporation from water,because of the high humidity in snow packhumidity in snow packn nThe slope of isotopic evolution is 5.75,steeper than The slope of isotopic evolution is 5.75,steeper than evaporation of water from an open surfaceevaporation of water from an open surfaceIsotopic effect during sublimationMixing of surface and inner snow melt Recharge in arid regionsn nGroundwater in arid regions more likely to get Groundwater in arid regions more likely to get isotopicallyisotopically enriched due to evaporation enriched due to evaporationn nSurface water evaporation prior to infiltrationSurface water evaporation prior to infiltrationn nEvaporation from unsaturated zoneEvaporation from unsaturated zonen nDirect evaporation from water tableDirect evaporation from water tablen nRecharge rates can be as low as 1%of precipitationRecharge rates can be as low as 1%of precipitationn nThe slope for The slope for d d1818O and O and d dD D plot vary between 3 and 5 plot vary between 3 and 5 evaporation slope evaporation slopeDifference in soil water and ground watern nWater in soil profiles is often Water in soil profiles is often isotopicallyisotopically enriched enrichedn nHowever,sometimes the ground water under the soil However,sometimes the ground water under the soil profiles do not show evaporative signal:GW still has profiles do not show evaporative signal:GW still has isotopic contents of mean precipitationisotopic contents of mean precipitationn nPresence of“fast channels”,Presence of“fast channels”,macroporesmacropores,preferential,preferential flow channels in the unsaturated zone:little mixing flow channels in the unsaturated zone:little mixing with enriched soil waterwith enriched soil waterSummer versus winter runoffEvaporative enrichment in summer runoffDeep versus alluvial ground watersEvaporative loss in alluvial GWn nIf evaporative slope=4.5 If evaporative slope=4.5 h=0.5 h=0.5n nD D 1818O Ov-blv-bl=-7.1;and =-7.1;and D D D Dv-blv-bl=-6.3 =-6.3 n nAt 30At 30o oC,C,totaltotal1818O=O=d d1818O Ov v d d1818O Ol l=1818O Ov-lv-l+D D 1818O Ov-blv-bl=-8.9-7.1=-16.0=-8.9-7.1=-16.0 totaltotalD D=d d2 2H Hv v d d2 2H Hl l=2 2H Hv-lv-l+D D 2 2H Hv-blv-bl=-6.3-71 =-6.3-71=-77.3=-77.3 n nD D1818O=O=d d1818O Ogwgw d d1818O Oo o=totaltotal1818O O lnflnf=4 =4 f=22%loss of water via evaporation(other loss by f=22%loss of water via evaporation(other loss by runoff)runoff)Direct infiltration on sand desertsSandy dunes in Saudi ArabiaVery low slope!Estimating the recharge ratesDisplacement caused by differing rates of rechargeDisplacement Recharge Isotope displacement recharge rateWater loss by evaporation vs.transpirationn nIn irrigated agricultural areas,salt often builds up in In irrigated agricultural areas,salt often builds up in the soil water due to water lossthe soil water due to water lossn nIt is important to know if loss of water is due to It is important to know if loss of water is due to evaporation or to transpiration by crops,so that right evaporation or to transpiration by crops,so that right measures can be taken to stop salinity increasemeasures can be taken to stop salinity increasen nEvaporation enriches O-18 and D in residue water,Evaporation enriches O-18 and D in residue water,but transpiration does not,so the culprit is:but transpiration does not,so the culprit is:n nEvaporation,if the isotopic ratios increaseEvaporation,if the isotopic ratios increasen nCrop transpiration,if isotopic increase is small relative to Crop transpiration,if isotopic increase is small relative to the amount of water lossthe amount of water lossPan experimentn nEstimate isotopic changes at given water loss due to Estimate isotopic changes at given water loss due to evaporation at local conditions(humidity,evaporation at local conditions(humidity,temperature and wind shear)temperature and wind shear)n nFor example,Nile Delta,Pan experiment shows For example,Nile Delta,Pan experiment shows d dD D increases by 0.65 per 1%water evaporation,or increases by 0.65 per 1%water evaporation,or 0.185 0.185 d d1818O increases per 1%water evaporationO increases per 1%water evaporationn nThe agricultural drainage water has The agricultural drainage water has d dD D 25,25,whereas irrigation water(Nile river water)has whereas irrigation water(Nile river water)has d dD D 28.3 28.3 n nEvaporation vs.crop transpirationSalt buildup mainly due to agricultural practice,which has to be changed to stop the soil salinity increasesSalinity goes up,little change in dDRecharge from river-connected aquifersn nMany alluvial aquifers rely on river recharge(e.g.,along the river Nile,the Tigris in Irag,Euphrates,Indus river in Pakistan)n nAlluvial aquifer helps purify the river water into drinkable water(e.g.,Danube in Europe)n nTracer experiment useful but controversial to usen nStable isotopes are natural tracers,safeTime series isotopic monitoring in river connected aquifer H II mainly recharged from IIIer River,but has strong seasonal contribution from Weihung RiverRecharge from the Nile RiverOld GW,before DamNew GW,After damSource water of NileHydrographHydrograph is a graph of the flow in a stream over a period of time.is a graph of the flow in a stream over a period of time.Above is a picture of a hydrograph,with stream flow(discharge)in cubic Above is a picture of a hydrograph,with stream flow(discharge)in cubic feet per second on the y-axis and time in months on the x-axis.Peaks in feet per second on the y-axis and time in months on the x-axis.Peaks in the hydrograph are usually a result of precipitation events,while troughs the hydrograph are usually a result of precipitation events,while troughs represent drier timesrepresent drier times.“Flashy”hydrograph=those with sharp.“Flashy”hydrograph=those with sharp jumps followed by rapid drops,like the one abovejumps followed by rapid drops,like the one aboveHydrograph separation in catchment studiesn nThe recharge,storage and discharge behavior of a The recharge,storage and discharge behavior of a watershed is revealed during rainfall and watershed is revealed during rainfall and baseflowbaseflow (I.e.flow between storms)(I.e.flow between storms)n nEngineers need to quantify surface runoff during Engineers need to quantify surface runoff during storm events,to design drainage systemsstorm events,to design drainage systemsn nIn basin budget studies,it is important to assess the In basin budget studies,it is important to assess the proportion of precipitation that actually charges proportion of precipitation that actually charges groundwater,and%lost by surface runoffgroundwater,and%lost by surface runoffn nImportant to separate the hydrograph into Important to separate the hydrograph into baseflowbaseflow and storm runoffand storm runoffTwo component separationTotal stream discharge=Storm flow+GroundwaterTwo component hydrograph separationn nQ Qt t=total steam discharge;=total steam discharge;Q Qgwgw=prestormprestorm baseflowbaseflow;Q Qr r=Storm runoff(flow)=Storm runoff(flow)Q Qt t=Q Qgwgw+Q Qr r(1)(1)Q Qt t d dt t=Q Qgwgw d dgwgw+Q Qr r d dr r(2)(2)Conditions for two component separationn nThere are only two major components of water input There are only two major components of water input to stream during a storm,pre-storm to stream during a storm,pre-storm baseflowbaseflow and and storm flowstorm flown nThe isotopic compositions of the two components are The isotopic compositions of the two components are different(often)different(often)n nIf there is another component of water input(e.g.,soil If there is another component of water input(e.g.,soil water),another conservative tracer(e.g.,dissolved water),another conservative tracer(e.g.,dissolved SiOSiO2 2,ClCl-,electric conductivity)is needed to perform,electric conductivity)is needed to perform three component hydrograph separationthree component hydrograph separationThree component separationQ Qt t=Q Qgwgw+Q+Qs s+Q Qr r(1)(1)Q Qt t d dt t=Q Qgwgw d dgwgw+Q Qs s d ds s+Q Qr r d dr r(2)(2)Q Qt tSiSit t=Q QgwgwSiSigwgw+Q Qs sSiSis s+Q Qr rSiSir r(3)(3)Three component hydrograph separationPre-event ground water contribution to total dischargeIt is pre-event groundwater,not storm water that contributes the greater amount to stream flow during precipitation events(except urban rainfall)Example of the Big Otter Creek Basin,OntarioPhase I:Simple mixing between pre-storm base flow(BF)and rain falling on the channel Phase II:Increasing influence of direct runoff which has d18O of rain,but higher E.C.due to flow over the ground from some distance from the channelPhase III:Decrease in the direct runoff/baseflow ratio and establishment of a new base flow,which includes the influence of the storm rechargeSeparation using D/H and Cl-Cl-separation show lower runoff contribution,which is explained due to accumulation of salt on the surface under the semi-arid climateGround water mixingdsample=fdA+(1-f)dBTernary ground water mixingGroundwater mixing in karst systems
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