布朗大学同位素地球化学Hydrogenandoxygenisotopesingroundwater

HydrogenandoxygenisotopesingroundwaterHydrogenandoxygenisotopesi1TopicsinthisclasssectionIsotopicchangesduringtransferofwaterfromprecipitationtogroundwaters(rechargeisotopicsignatures)ExamplesofapplyingisotopiccompositionsofgroundwaterstohydrologyproblemssuchashydrographseparationTopicsinthisclasssectionIs2ObjectivesHowdodifferentrechargemechanismsaffectthegroundwaterisotopicratiosHowtousethedifferentisotopicsignaturesofgroundandsurfacewatersforhydrologicalresearchObjectivesHowdodifferentrec3IsotopechangesduringrechargeRechargeintemperateclimatesRechargeinaridregionsIsotopechangesduringrecharg4TemperateclimatesOnly5to25%ofprecipitationinfiltratestowatertableTheother75to95%oftheprecipitationgoesto:Runoff:noisotopeeffectEvaporationfromsoils:isotopeenrichmentinsoilwaterTranspirationbyvegetation:noisotopeeffectonsoilwaterTemperateclimatesOnly5to255SeasonalityofrechargeSpring:RechargeratesarethehighestSoilsaresaturatedTemperatureislow(lowevaporation)Vegetationisinactive(nottaking/transpiringwater)Summer:rechargeminimal,mostprecipitationtranspiredtotheatmosphereFall:rechargeincreasesasphotosynthesisshutdownSeasonalityofrechargeSpring:6SeasonalityofrechargeWinter:littlerechargeduetofrost.Precipitationstoredassnow,oftenlostduringspringrunoffIsotopiccompositionofgroundwatersintermediatevalueofspringandfallrainmeanannualprecipitationSeasonalityofrechargeWinter:7AttenuationofseasonalvariationsGroundwaterssampledbelowthewatertablegenerallyhaveanisotopicvaluethatisclosetotheweightedaverageofannualprecipitationSeasonalvariationsinisotopesareattenuatedduringmovementthroughtheunsaturatedzone,whichisanimportantzoneofmixingAttenuationofseasonalvariat8dDattenuationat5mdepthdDattenuationat5mdepth9d18OattenuationinQuaternarygravelsd18OattenuationinQuaternary10CriticaldepthFinegrainedsoils=3to5mFracturedrock(fastinfiltration)>10m

CriticaldepthFinegrainedsoi11AttenuationinconfinedaquiferIsotopicchangesinconfinedaquifersignifiesmixingofdifferentrechargewatersAttenuationinconfinedaquife12ShallowgroundwaterandprecipitationEnrichedprecipitationShallowgroundwaterandprecip13GroundwaterisotopicdepletionMostprecipitationoccursduringthesummer,butdoesnotcontributetorechargesignificantlyduetotranspirationMostrechargeoccursincoolermonthsfromOctobertoMayPrecipitationhasmoresummercomponents,whereasshallowgroundwaterhasmorespringcomponentsGroundwaterisotopicdepletio14IsotopicvaluesofsnowandmeltSnowisdepletedinDandO-18,butthedepletionismodulatedduetosurfacesublimationSublimationfromsnowismoreclosetoequilibriumthanevaporationfromwater,becauseofthehighhumidityinsnowpackTheslopeofisotopicevolutionis~5.75,steeperthanevaporationofwaterfromanopensurfaceIsotopicvaluesofsnowandme15IsotopiceffectduringsublimationIsotopiceffectduringsublima16Mixingofsurfaceandinnersnowmelt

Mixingofsurfaceandinnersn17RechargeinaridregionsGroundwaterinaridregionsmorelikelytogetisotopicallyenrichedduetoevaporationSurfacewaterevaporationpriortoinfiltrationEvaporationfromunsaturatedzoneDirectevaporationfromwatertableRechargeratescanbeaslowas1%ofprecipitationTheslopeford18OanddDplotvarybetween3and5–evaporationslopeRechargeinaridregionsGround18DifferenceinsoilwaterandgroundwaterWaterinsoilprofilesisoftenisotopicallyenrichedHowever,sometimesthegroundwaterunderthesoilprofilesdonotshowevaporativesignal:GWstillhasisotopiccontentsofmeanprecipitationPresenceof“fastchannels”,macropores,preferentialflowchannelsintheunsaturatedzone:littlemixingwithenrichedsoilwaterDifferenceinsoilwaterandg19SummerversuswinterrunoffEvaporativeenrichmentinsummerrunoffSummerversuswinterrunoffEva20DeepversusalluvialgroundwatersDeepversusalluvialgroundwa21EvaporativelossinalluvialGWIfevaporativeslope=4.5→h=0.5Dε18Ov-bl=-7.1‰;andDεDv-bl=-6.3‰At30oC,εtotal18O=d18Ov–d18Ol=

18Ov-l+D

18Ov-bl =-8.9-7.1=-16.0‰

εtotalD=d2Hv–d2Hl=

2Hv-l+D

2Hv-bl=-6.3-71 =-77.3‰D18O=d18Ogw–d18Oo=εtotal18Olnf=4‰f=22%lossofwaterviaevaporation(otherlossbyrunoff)EvaporativelossinalluvialG22DirectinfiltrationonsanddesertsDirectinfiltrationonsandde23SandydunesinSaudiArabiaVerylowslope!SandydunesinSaudiArabiaVer24EstimatingtherechargeratesEstimatingtherechargerates25DisplacementcausedbydifferingratesofrechargeDisplacement↑Recharge↓Displacementcausedbydifferi26Isotopedisplacement↔rechargerateIsotopedisplacement↔recharg27Waterlossbyevaporationvs.transpirationInirrigatedagriculturalareas,saltoftenbuildsupinthesoilwaterduetowaterlossItisimportanttoknowiflossofwaterisduetoevaporationortotranspirationbycrops,sothatrightmeasurescanbetakentostopsalinityincreaseEvaporationenrichesO-18andDinresiduewater,buttranspirationdoesnot,sotheculpritis:Evaporation,iftheisotopicratiosincreaseCroptranspiration,ifisotopicincreaseissmallrelativetotheamountofwaterlossWaterlossbyevaporationvs.28PanexperimentEstimateisotopicchangesatgivenwaterlossduetoevaporationatlocalconditions(humidity,temperatureandwindshear)Forexample,NileDelta,PanexperimentshowsdDincreasesby~0.65‰per1%waterevaporation,or0.185‰d18Oincreasesper1%waterevaporationTheagriculturaldrainagewaterhasdD–25‰,whereasirrigationwater(Nileriverwater)hasdD–28.3‰

PanexperimentEstimateisotopi29Evaporationvs.croptranspirationSaltbuildupmainlyduetoagriculturalpractice,whichhastobechangedtostopthesoilsalinityincreasesSalinitygoesup,littlechangeindDEvaporationvs.croptranspira30Rechargefromriver-connectedaquifersManyalluvialaquifersrelyonriverrecharge(e.g.,alongtheriverNile,theTigrisinIrag,Euphrates,IndusriverinPakistan)Alluvialaquiferhelpspurifytheriverwaterintodrinkablewater(e.g.,DanubeinEurope)TracerexperimentusefulbutcontroversialtouseStableisotopesarenaturaltracers,safeRechargefromriver-connected31Timeseriesisotopicmonitoringinriverconnectedaquifer

HIImainlyrechargedfromIIIerRiver,buthasstrongseasonalcontributionfromWeihungRiverTimeseriesisotopicmonitorin32RechargefromtheNileRiverOldGW,beforeDamNewGW,AfterdamSourcewaterofNileRechargefromtheNileRiverOl33 Hydrographisagraphoftheflowinastreamoveraperiodoftime.Aboveisapictureofahydrograph,withstreamflow(discharge)incubicfeetpersecondonthey-axisandtimeinmonthsonthex-axis.Peaksinthehydrographareusuallyaresultofprecipitationevents,whiletroughsrepresentdriertimes.“Flashy”hydrograph=thosewithsharpjumpsfollowedbyrapiddrops,liketheoneabove Hydrographisagraphofthe34HydrographseparationincatchmentstudiesTherecharge,storageanddischargebehaviorofawatershedisrevealedduringrainfallandbaseflow(I.e.flowbetweenstorms)Engineersneedtoquantifysurfacerunoffduringstormevents,todesigndrainagesystemsInbasinbudgetstudies,itisimportanttoassesstheproportionofprecipitationthatactuallychargesgroundwater,and%lostbysurfacerunoffImportanttoseparatethehydrographintobaseflowandstormrunoffHydrographseparationincatch35TwocomponentseparationTotalstreamdischarge=Stormflow+GroundwaterTwocomponentseparationTotal36TwocomponenthydrographseparationQt=totalsteamdischarge;Qgw=prestormbaseflow; Qr=Stormrunoff(flow)Qt=Qgw+Qr (1)Qt•dt=Qgw•dgw+Qr•dr (2)

Twocomponenthydrographsepar37ConditionsfortwocomponentseparationThereareonlytwomajorcomponentsofwaterinputtostreamduringastorm,pre-stormbaseflowandstormflowTheisotopiccompositionsofthetwocomponentsaredifferent(often)Ifthereisanothercomponentofwaterinput(e.g.,soilwater),anotherconservativetracer(e.g.,dissolvedSiO2,Cl-,electricconductivity)isneededtoperformthreecomponenthydrographseparationConditionsfortwocomponents38ThreecomponentseparationQt=Qgw+Qs+Qr (1)Qt•dt=Qgw•dgw+Qs•ds+Qr•dr (2) Qt•Sit=Qgw•Sigw+Qs•Sis+Qr•Sir (3)ThreecomponentseparationQt=39ThreecomponenthydrographseparationThreecomponenthydrographsep40Pre-eventgroundwatercontributiontototaldischargeItispre-eventgroundwater,notstormwaterthatcontributesthegreateramount