Origin of the groundwater salinization in La Aldea coastal aquifer (Gran Canaria, Canary Islands)

Proceedings J,ne21-2612010 SWI M21 Azores· Portugal 21st Sa~ Water Intrusion Meeting OriginofthegroundwatersalinizationinLaAldeacoastalaquifer (GranCanaria,CanaryIslands) T.Cruz1,M.C.Cabrera1andJ.Heredia2  1[email protected]&mca[email protected] DepartmentofPhysics,UniversityofLasPalmasdeGranCanaria,35017 LasPalmasdeGranCanaria(Spain) 2[email protected] GeologicalSurveyofSpain,RíosRosas,23.28003Madrid(Spain) ABSTRACT GroundwaterchemistryinLaAldeaaquifer(GranCanaria,CanaryIslands)showshigh contentsofchlorideandnitrateions.Thesalinizationprocesshasbeenmodelledusing thegeochemicaldata,takingintoaccounttheresultsofapreviousflowmodel.The resultsallowtoidentifythesalinityoftherechargefromtherainfallunderaridity conditionsandtheirrigationreturnslikethemaincausesofthegroundwater salinization. INTRODUCTION LaAldearavine,locatedonthewesternsideofGranCanaria(CanaryIslands,Spain),presentsa flatbottomsurroundedbyhighmountains(withheightsvaryingfromsealevelto1,415m).The climateisdrysubtropicalwithhightemperaturesthroughouttheyear.Intensivegreenhouse horticulture(mainlytomatoesandcucumbers)ispracticedwithacultivationsurfaceof645ha andirrigationwaterismainlysuppliedbythreedamssituatedupstream,althoughmorethan 370large‐diameterwellsalsoprovidecropwaterrequirementsinthedryseasons,diminishing thequalityofgroundwater.  Thegeologyoftheareashowsasedimentaryunit,heterogeneous,composedofalluvialand screesdeposits(consistingofdetriticalmaterialwithphonolites‐trachytes,ignimbritesand basaltsboulders)andavolcanicunit,alsoheterogeneous,consistingofMiocenebasaltslocated beneaththesedimentaryunitandoccupiestheentiresurfacewithalowerpermeabilitythan thealluvialdeposits(Figure1).Thereisaresidualrelieflocatedeastofthestudyareabetween FurelandLaAldearavines(LasTabladas)withacomplexgeology,whereslidhydrothermal deposits(Azulejos)cropsout.  GroundwaterflowsmainlythroughalluvialmaterialsfromeasttowestalongtheLaaldea alluvial,thatrepresentsadischargeareafromtheaquifertothesea.Thereisalsoanimportant flowfrommountainousareaslocatednorthandsouthtowardstheLaAldeaalluvial.Water balancehasbeendoneforthehydrologicalyears1991‐1999(Cruz,2008)andshowsthatthe rechargeisaresultofirrigationreturns(60%)andrainfall(20%).  Thehydrogeochemistryoftheareahasbeenstudiedsince1992duringseveralresearchprojects inordertoqualitativelycharacterizeLaAldeaaquifer(Muñozetal.,1996;Muñoz,2005;Cruz; 2008).Thehydrogeochemicalconceptualmodelwasestablishedandsuggeststhatawaterflow frombasaltstothealluvialexists,andthereforetheextractedwaterrepresentamixingofwater frombothunits. Azores, Portugal SWIM21 - 21st Salt Water Intrusion Meeting June 21 - 26, 2010 35 Groundwaterintheareapresentshighsalinityratios(Clcontentsupto7000mg∙L‐1andN contentsupto700mg∙L‐1)insomeplaces,andtheoriginofsalinityistheaimofthepresent communication.    Figure1.LocalizationofLaAldeaaquiferinGranCanaria(CanaryIsland,Spain)andgeologicmap showingthemaingeologicalunits.  METHODS Inthiswork,thespatialandtemporaldistributionofchlorideandnitrateionsandchemical characteristicsofthedifferentsourcesofrechargehavebeenstudiedtoquantifytheinfluence ofthedifferentsourcesofsalinity(mainlysalinityofrainfallduetoaridityandirrigation returns).Theaquifersalinityhasbeenmodeledalongaflowline(withthreerepresentative wells)intheaquifer(Figure1)usingPHREEQC.Themodelhassimulatedmixingprocesses betweenrechargewater(rainandirrigationreturns)andgroundwaterandchemicalreactions thatoccurinsoilandrocksindifferentareasoftheaquifer.  RESULTS Figure2showsthespatialandtemporalevolutionofchloridefortheyears1992and1999.The highestconcentrationsareobservedinLasTabladasfootandresultsfromtheleachingofthe Azulejos.ThelowerconcentrationswerefoundintheMiocenebasaltsinthehighestlevels.In general,thereiscoincidencebetweenthedirectionofgroundwaterflowandtheincreasein chlorideconcentration.Comparingbothyears,anincreaseinchlorideconcentrationsis observed.Theaverageconcentrationofchlorideinthestudyareais711mg∙L‐1for1992and 1110mg∙L‐1for1999.  Theaverageconcentrationofnitratesis88mg∙L‐1for1992and175mg∙L‐1for1999which representsasubstantialincreaseintheconcentrationforthistimeperiod,reachingmaximum valuesof484mg∙L‐1for1992and708mg∙L‐11999.Figure2showsthespatialandtemporal evolutionofnitrateionshowingthatmaximumvaluesarelocatedatthemouthofthe Azores, Portugal SWIM21 - 21st Salt Water Intrusion Meeting June 21 - 26, 2010 36 TocodománravineandthesouthernofLaAldearavinein1992andin1999highvaluesof nitratesoccupymostofthealluviallocatedthatiswheremostcropsarelocated.     Figure2.Isoconcentrationofchlorideandnitratefor1992and1999.  DISCUSSIONANDCONCLUSIONS Differenthydrogeochemicalprocessesthatcontrolgroundwatergeochemistryhavebeen identifiedandquantified.Rainwaterrepresentsthe20%ofrechargethatjoinstheaquiferwith extrasalinizationduetosalinityoftherechargefromtherainfallunderaridityconditions (Custodio,1993).Irrigationreturnsrepresents60%oftherechargeandcausesincreasedsalinity duetothehighevapotranspirationrates.Finally,othergeologicalinputofhighsalinitywatersis locatedatLasTabladashasbeenattributedtoleachingofthedepositsknownasAzulejos.  Chlorideconcentrationinrainwaterrangesfrom30mg∙L‐1and450mg∙L‐1inareasfartherand nearerthesearespectively(Munoz,2005;Alcala,2005,Cruz2008).Rainwaterevaporationhas beencalculatedbetween60%and70%(Cruz,2008)producinganincreaseinthesalinityofthe Azores, Portugal SWIM21 - 21st Salt Water Intrusion Meeting June 21 - 26, 2010 37 recharge.Irrigationreturnswatershaveanitrateconcentrationofmorethan650mg∙L‐1(Cruz, 2008)TheAzulejosinfluencehasbeenstudiedfromthechemistryofsomeoozeslocatedinLas Tabladaswithwatersthathavechlorideconcentrationsofmorethan8000mg∙L‐1andnitrate concentrationabove500mg∙L‐1.  Themodelinghasbeencarriedoutreproducingthechlorideandnitrateconcentrationintwo wellslocatedinthemiddleandlowpartofLaAldearavine(0779‐TPand0218‐O1,respectively) fromthewellwater2714‐TPlocatedatheader(Figure1)assumingthatallnitratecomesfrom irrigationreturnsandchloriderechargebyrainfall.Thestudyofthechemicalevolutionof groundwaterintheflowline(2714‐TPto0779‐TPand2714‐TPto0218‐O1)showsthatthewell 0779‐TPpresents2%ofmarinewaterrespectto2714‐TPandthewell0218‐O1presents4%of marineinfluencerespectto2714‐TP.Thismeansamarinecontributionof0.5%perkminthe flowlinefor0779‐TP,rangingto0.7%for0218‐O1.Thisincreaseintherateofmarinespray contributionof0.2%in0218‐TPshaftwithrespecttothewell0779‐TPimpliesanincreaseinthe contributionasweapproachedthecoast,whichalsoconfirmrainfallchemicaldata.  Attendingtothemodelresults,thegroundwaterreceives51%ofirrigationreturnsinthewell 0779‐TPand89%inthewell0218‐O1.Thisdemonstratestheimportanceofirrigationreturns waterasinputtothesystemandasasourceofcontaminationofgroundwaterandconfirms thatgroundwaterpresentsmoreinfluenceofirrigationreturnsasmovinginthedirectionof flow.  REFERENCES Alcalá,F.J.2005.Recargadelosacuíferosespañolesmediantebalancehidrogeoquímico.Doctoral Thesis.TechnicalUniversityofCatalonia.  CruzT.,2008.Contribucióndelamodelaciónnuméricaehidroquímicaalconocimientodelacuífero deLaAldea(GranCanaria,IslasCanarias).DoctoralThesisUniversityofLasPalmasdeGran Canaria.  Custodio,E.,1993.Coastalaquifersalinizationasaconsequenceofaridity:thecaseofAmurga phonoliticmassif,GranCanariaIsland.InE.CustodioandA.Galofré(eds.):Studyand modelingofsaltwaterintrusionintoAquifers.Ed.CIHS/CIMNE,Barcelona,81‐98.  Muñoz,J.,2005.FuncionamientohidrogeológicodelAcuíferodeLaAldea(GranCanaria).Doctoral ThesisUniversityofLasPalmasdeGranCanaria.  Muñoz,R.,Cabrera,M.C.,Hernández,F.andSocorro,A.R.1996.DevelopmentofAnalyticaland SamplingMethodsforPriorityPesticidesandRelevantTransformationProductsinAquifers. FinalProjectReport.   ContactInformation:TatianaCruz,UniversityofLasPalmasdeGranCanaria.Departmentof Physics,UniversityofLasPalmasdeGranCanaria,35017LasPalmasdeGranCanaria(Spain),Phone: +34928458080,Fax:+34928452922,Email:[email protected]  Azores, Portugal SWIM21 - 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