Quantifying the influence of surface water–groundwater interaction on nutrient flux in a lowland karst catchment


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Quantifying the influence of surface water–groundwater interaction on nutrient flux in a lowland karst catchment

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Title:
Quantifying the influence of surface water–groundwater interaction on nutrient flux in a lowland karst catchment
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Hydrology and Earth System Sciences
Creator:
McCormack, T.
Naughton, O.
Johnston, P. M.
Gill, L. W.
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English

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Ground Water ( local )
Nutrient Flux ( local )
Lowland Karst Catchment ( local )
Alkalinity Sampling ( local )
Hydrological Model ( local )
Turloughs ( local )
Surface Water ( local )
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serial ( sobekcm )

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Nutrient contamination of surface waters and groundwaters is an issue of growing importance as the risks associated with agricultural run-off escalate due to increasing demands on global food production. In this study, the influence of surface water–groundwater interaction on the nutrient flux in a lowland karst catchment was investigated with the aid of alkalinity sampling and a hydrological model. The objective of the study was to determine the impact of ephemeral karst lakes (turloughs) on the surface water–groundwater nutrient flux, and whether these lakes act as sources or sinks of nutrients within the groundwater flow system. Water samples were tested from a variety of rivers, turloughs, boreholes and springs at monthly intervals over 3 years. Alkalinity sampling was used to elucidate the contrasting hydrological functioning between different turloughs. Such disparate hydrological functioning was further investigated with the aid of a hydrological model which allowed for an estimate of allogenically and autogenically derived nutrient loading into the karst system. The model also allowed for an investigation of mixing within the turloughs, comparing observed behaviours with the hypothetical conservative behaviour allowed for by the model. Within the turloughs, recorded nutrient concentrations were found to reduce over the flooded period, even though the turloughs hydrological functioning (and the hydrological model) suggested this would not occur under conservative conditions. As such, it was determined that nutrient loss processes were occurring within the system. Denitrification during stable flooded periods (typically 3–4 months per year) was deemed to be the main process reducing nitrogen concentrations within the turloughs, whereas phosphorus loss is thought to occur mostly via sedimentation and subsequent soil deposition. The results from this study suggest that, in stable conditions, ephemeral lakes can impart considerable nutrient losses on a karst groundwater system.
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Hydrology and Earth System Sciences, Vol. 20 (2016-06-01).

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Hydrol.EarthSyst.Sci.,20,2119–2133,2016 www.hydrol-earth-syst-sci.net/20/2119/2016/ doi:10.5194/hess-20-2119-2016 Authors2016.CCAttribution3.0License. Quantifyingtheinuenceofsurfacewater–groundwaterinteraction onnutrientuxinalowlandkarstcatchment T.McCormack 1 ,O.Naughton 2 ,P.M.Johnston 1 ,andL.W.Gill 1 1 DepartmentofCivil,StructuralandEnvironmentalEngineering,TrinityCollegeDublin,Dublin2,Ireland 2 CivilEngineering,CollegeofEngineeringandInformatics,NationalUniversityofIreland,Galway,Ireland Correspondenceto: T.McCormackmccormte@tcd.ie Received:18August2015–PublishedinHydrol.EarthSyst.Sci.Discuss.:9October2015 Revised:13May2016–Accepted:13May2016–Published:1June2016 Abstract. Nutrientcontaminationofsurfacewatersand groundwatersisanissueofgrowingimportanceastherisks associatedwithagriculturalrun-offescalateduetoincreasingdemandsonglobalfoodproduction.Inthisstudy,the inuenceofsurfacewater–groundwaterinteractiononthe nutrientuxinalowlandkarstcatchmentwasinvestigated withtheaidofalkalinitysamplingandahydrologicalmodel. Theobjectiveofthestudywastodeterminetheimpact ofephemeralkarstlakesturloughsonthesurfacewater– groundwaternutrientux,andwhethertheselakesactas sourcesorsinksofnutrientswithinthegroundwaterow system.Watersamplesweretestedfromavarietyofrivers, turloughs,boreholesandspringsatmonthlyintervalsover3 years.Alkalinitysamplingwasusedtoelucidatethecontrastinghydrologicalfunctioningbetweendifferentturloughs. Suchdisparatehydrologicalfunctioningwasfurtherinvestigatedwiththeaidofahydrologicalmodelwhichallowed foranestimateofallogenicallyandautogenicallyderivednutrientloadingintothekarstsystem.Themodelalsoallowed foraninvestigationofmixingwithintheturloughs,comparingobservedbehaviourswiththehypotheticalconservative behaviourallowedforbythemodel.Withintheturloughs, recordednutrientconcentrationswerefoundtoreduceover theoodedperiod,eventhoughtheturloughshydrologicalfunctioningandthehydrologicalmodelsuggestedthis wouldnotoccurunderconservativeconditions.Assuch,it wasdeterminedthatnutrientlossprocesseswereoccurring withinthesystem.Denitricationduringstableoodedperiodstypically3monthsperyearwasdeemedtobe themainprocessreducingnitrogenconcentrationswithin theturloughs,whereasphosphoruslossisthoughttooccurmostlyviasedimentationandsubsequentsoildeposition. Theresultsfromthisstudysuggestthat,instableconditions, ephemerallakescanimpartconsiderablenutrientlossesona karstgroundwatersystem. 1Introduction Globalfoodproductionispredictedtoincreasebyapproximately60%by2050AlexandratosandBruinsma,2012, therebyincreasingthecontaminationrisksassociatedwith agriculturalrun-offofraisednutrientconcentrationsinsensitivegroundwaterandsurfacewaters.Nutrientcontaminationofgroundwaterhasbeenreportedacrosstheworld– forexample,ChinaZhangetal.,1996,TurkeyDavrazet al.,2009,IndiaRaoandPrasad,1997andtheUSDomagalskiandJohnson,2012;Hudak,2000–withsuchevidencecontributingtowardstheintroductionoftheEUNitrates/676/EECandGroundwater/118/ECdirectives. Innon-carbonateaquifers,nitrogenNandphosphorusParesubjecttoseparatetransportdynamics.NitrateNO 3 isoftenfoundtobeconservativelytransported duetoitshighsolubilityandmobilitycharacteristics,while PisretainedduetoitsafnitytoparticulatematterWeiskel andHowes,1992.Incarbonateaquifershowever,theexistenceofpointrechargefeatures,suchasswalloworsinkholes,providesdirectaccesspointsforNandPintothe aquifer.Thisallowscontaminantstobypasstheprotective soilcoverassociatedwithmostdiffuserechargeandenter thekarstfracture/conduitnetworkwithlittleornoattenuationCoxon,2011.Withintheconduitsystem,acontaminantcanthenberapidlytransmittedthroughanaquiferin PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.

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2120T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux ecologicallysignicantquantitieswithverylittleattenuation orchemicalbreakdown. IntheRepublicofIreland,carboniferouslimestonecovers approximatelyhalfofthelandsurfaceandisoftenheavily karstied.Mostofthislimestoneislowlandandcoincides withproductiveagriculturallandDrew,2008,and,assuch, theinuenceofagriculturalpracticesandnutrientloadingon karstisofparticularimportance.CurrentresearchintonutrientcontaminationinIrelandisofadditionalsignicanceas manycatchmentshavefailedtoachievetheobjectivessetout bytherstmanagementcycleoftheEUWaterFramework Directive/60/EC,wherebyallwaterbodieswereto haveachievedatleast“good”waterstatusby2015inlight ofthis,theobjectivesofthedirectivearetobereassessedfor thesecondandthirdmanagementcycles,whichendin2021 and2027respectively. Whilethehydrochemicalprocessesinpermanentlakeshas beenthesubjectofmuchresearch,relativelylittleworkhas beencarriedoutintothenutrientuxwithinephemerallakes andtheirinuenceatacatchmentscale.Ephemerallakes, knownas turloughs ,areacharacteristicfeatureoftheIrish karstlandscape.Theiroodingresultsfromacombination ofhighrainfallandconsequentlyhighgroundwaterlevelsin topographicdepressionsinkarst.Floodingtypicallyoccurs throughundergroundconduitsandspringsinautumn,formingalakeforseveralmonthsinwinterwhichthenempties viaswallowholesorestavellesinthespringtimeSheehy Skefngtonetal.,2006.Thisoodingpromotesabiodiversehabitatasspecieshavetoadapttosurvivetheoscillationbetweenterrestrialtoaquaticconditions.Theturlough habitatisprotectedundertheEUWaterFrameworkDirective/60/ECanddesignatedasapriorityhabitatunder Annex1oftheEUHabitatsDirective/43/EEC.Numeroussitessupportingecologicalcommunitiesofnationaland internationalimportancehavebeendesignatedasSpecialAreasofConservationSACandaffordedthehighestlevelof protectionavailableunderEUconservationallaw. Duetotheprotectedstatusofturloughswithinthestudy areaofthisproject,aswellastheprotectedstatusoftheir eventualoutletatKinvaraBaypartofGalwayBaycomplexSAC,itisimportanttounderstandthenutrientprocesseswhichareoccurringintheregion.Theseprocesses areespeciallyimportantinthecontextofthelikelyfuture pressuresonthecatchment.FoodHarvest2020isthestrategicplantodeveloptheIrishAgriculturalSectorandisexpectedtoleadtoa33%increaseinprimaryoutputacross thecountry,comparedto2007averagesDepartment ofAgricultureFisheriesandFood,2010.Suchaplanwould leadtosubstantialescalationinnutrientloadingfromagriculturalsourcesandthusposesasignicantchallengetoIrelandmeetingthegoalsassetoutbytheWaterFramework Directive.Theproblemisexacerbatedfurtherwiththelikely increasesinrainfallintensityandfrequencyofstormevents duetoclimatechange,whichmayencouragenutrientstobypasstheprotectivesoilcoverandenterthekarstaquifervia pointsourcefeatures.Hence,theobjectiveofthisresearch istodeterminewhethertheseprotectedtemporarylakesare subjecttothesametransformationprocessesasfoundinpermanentlakesandtoassesstheimpactoftheturloughsonthe nutrientuxwithinthewidercatchment;i.e.dotheturloughs operateassourcesorsinksofnutrientstothecatchment? 2Areadescriptionandbackground TheGortLowlandsisa480km 2 catchmentlocatedin CountyGalwayinthewestofIreland.Theeasternportionof thecatchmentisdominatedbytheSlieveAughtyMountains andunderlainbyDevonianOldRedSandstoneFig.1.The westernportionofthecatchmentismostlyatandunderlainbypurecarboniferouslimestone.Similartothemajority ofkarsticregionsfoundwithinIreland,thecatchmentisprimarilylowlandrarelyrisingabove30m,and,assuch,the regionissubjecttoconsiderableinteractionbetweenground andsurfacewaters. AsdemonstratedbyprevioustracerstudiesDrew,2003, theprevailingdrainagedirectioninthecatchmentiseastto west,withrechargefromthenon-carbonateSlieveAughty Mountainshereafterreferredtoasjust“mountains”owingacrossthelowlandkarsttowardsamajorintertidalspring atKinvaraBayknownasKinvaraWest.Thissignicant contributionofallogenicrechargeintothekarstaquiferimpartsthecatchmentwithadistinctivehydrochemicaluxas wellasuniquehydrologicalandecologicalcharacteristics. Threemainriversrundownthemountainsandintothe carboniferouslowlands:theOwenshreeSA1;theBallycahalanSA2;andtheOwendalulleeghSA3,whichgoes ontofeedtheBeaghRiverSA4.Theriverssupplychemicallyaggressiveacidicwatersderivedfromthepeatynoncarbonatecatchmentsofthemountainsintothelowlands whichhaverapidlyinuencedkarstdevelopmentintheregionandthedevelopmentofacomplexnetworkofsinking streams,conduitsandturloughs. IntheGortLowlandsFig.1,turloughsformakeycomponentofthehydrologicalregime,offeringazoneoftemporarystorageforwatersurchargingoutoftheactiveconduitnetwork.Numerousturloughsarepresentwithinthe GortLowlands,butveturloughsinparticularBlackrock, Coy,Coole,GarrylandandCaherglassaunareknowntobe highlyinuentialupontheactiveconduitnetworkGillet al.,2013b.Thechemicallyaggressiveallogenicrechargeenteringthelowlandshascontributedtothedevelopmentofa complexconduitnetworkwithrelativelyhighowrates.The veturloughswithinthenetworkareallrelativelyeutrophic anddeepincomparisontootherturloughsaroundIreland andareunderlainbynon-alluvialmineralsoiltypesofrelativelylowCaCO 3 concentrationcomparedtotheorganic andmarlysoiltypesgenerallyassociatedwithturloughsof longerperiodsofinundationKimberleyetal.,2012. Hydrol.EarthSyst.Sci.,20,2119–2133,2016www.hydrol-earth-syst-sci.net/20/2119/2016/

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T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux2121 Figure1. Geologyofstudyareadisplayingturloughs,raingauges,Kinvara,rivergaugingstations,thedirectionofundergroundconduitow andtheKinvaraspringscatchment. Figure2. Conceptualisationofdiffuseow-through,riverow-throughandsurchargetankturloughsystemsmodiedfromGilletal., 2013a. Turloughscanbedividedconceptuallyintothreegroups: diffuseow-through,riverow-throughandsurchargetank systemsFig.2Naughtonetal.,2012;Gilletal.,2013a. ThemajorityofturloughsinIrelandarethoughttobehaveas diffuseow-throughsystemswiththeuxofwaterthrough theturloughfromthesurroundingepikarstenteringandexitingrelativelyslowlyFig.2a.IntheGortLowlandshowever,thedevelopedconduitsystemresultsinturloughsoperatingmoreakinto riverow-through and surchargetank systemsGilletal.,2013,b.In riverow-through systems Fig.2b,waterisalsoconstantlyowingthroughtheturloughsimilarto diffuseow-through systemsFig.2a;howeverwatervolumestendtobelargerwithhigherdischarge rates.Theseturloughsalsotendtoshowmuchmore“ashy” oodingbehaviourastheyaredirectlylinkedtoariver– BlackrockandCooleturloughsseeFig.1beingexamples ofsuchtypes.Insurchargetanksystemstheturloughcanbe viewedasapressurereleasepointalonganundergroundpipe network,providingoverowstoragefortheexcessgroundwaterthatcannotbeaccommodatedduetoinsufcienthydrauliccapacityoftheconduitnetwork–Coy,Garryland andCaherglassaunbeingexamplesofthistypeofsystem. Surchargetanksystemsthushaveanegligibleow-through componentandcanbeconsideredtoremainrelativelyundilutedoncetheyhaveooded.Thishasbeenconrmedby apreviousstudyMcCormack,2014whichusedmultiple electricalconductivityECloggersplacedwithinCoyturloughandfoundECtospikeduringinoweventsbutremain constantforthemajorityoftheoodedseason. Mostofthenutrientloadingwithinthecatchmentisderivedfromagriculturalandforestrysources.Nutrientsentertheaquiferviaallogenicpointsources,suchasthethree riversdrainingthemountains,orbyautogenicdiffusemechanismswithinthelowlands.Eachmechanismprovidinga hydrochemicallydistinctinput.Allogenicrechargeischaracterisedbyrelativelylow-alkalinitywaterduetothenoncarbonatebedrockandmoderatenutrientconcentrationsbecauseoftherelativelylow-intensityagricultureintheuplands.Inthelowlands,thecarbonatebedrockresultsinmuch higheralkalinitylevels,andthehigheragriculturalintensity www.hydrol-earth-syst-sci.net/20/2119/2016/Hydrol.EarthSyst.Sci.,20,2119–2133,2016

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2122T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux Figure3. Pipenetworkmodelschematicandsamplinglocationsofturloughs,riversandboreholes.Note:theadditional“U”and“L”labels ontherivernamesrefertoupperandlowersamplinglocationsrespectively. mainlypastureforcattlecausescorrespondinghighernutrientconcentrationsparticularlyforNwithinthediffuse groundwater. TheGortLowlandscatchmenthasbeenhydrologicallymodelledusingInfoworksCSWallingfordSoftware, Wallingford,UK,ahydraulicmodellingpackagemoreoftenusedtomodelurbandrainagenetworks.Themodelsimulatesthehydraulicbehaviourofapipenetworkundervarying conditionsofrainfall,landuse,inowsetc.andrepresents thecatchmentasacomplexnetworkofpipesconduits, tanksturloughsandsubcatchmentsdiffuse/epikarst.Internalstoragewithinthesystemwasrepresentedusingve pondswiththesamestage–volumecharacteristicsasthesurveyedturloughs.ThemodelwasoriginallycalibratedbyGill etal.aandwassubsequentlyrecalibratedduetothe availabilityofadditionaldataMcCormacketal.,2014. Fortherecalibratedmodelwhichwasusedforthiscurrent study,themodelefciency,or r 2 ,wasassessedoverthe period2010usingtheNash–Sutcliffecriterionbased onthevolumesineachturlough.Valuesofr 2 forallturloughswerecalculatedas0.81,0.89,0.96,0.97and0.96for Blackrock,Coy,Coole,GarrylandandCaherglassaunrespectively.TheuseofthismodeltopredictcatchmenthydrodynamicsandsubmarinegroundwaterdischargehasbeendiscussedpreviouslybyGilletal.aandMcCormacket al..Forthisstudy,themodelwasadaptedtosimulate themovementofnutrientswithinthesystemseeFig.3fora schematicillustrationofthemodel. 3Methodology Theobjectiveofthestudywastodeterminetheimpact oftemporarykarstlakesturloughsonthesurfacewater– groundwaternutrientux,andwhethertheselakesactas sourcesorsinksofnutrientswithinthegroundwaterowsystem.Thiswascarriedoutusingthefollowingstrategy: – Alkalinityausefulindicatorofrechargeoriginwas usedasahydrochemicalmethodtovalidatethehydraulicconceptualisationoftheturloughssurcharge tank,ow-throughetc.. – Followingthis,thehydraulicmodelwasusedtosimulatethebehaviourofnutrientspassingthroughthe karstsystemassumingconservativeconditions.Modelledandobservednutrientbehaviourwithintheturloughswerethencomparedandanydifferencestakenas indicativeofnon-conservativenutrientprocesseswithin theturloughs; – Ifaturloughisfoundtobehavenon-conservatively,the variouspossibleprocessese.g.dilution,sedimentation, denitricationetc.areassessedtodiscernthelikely cause. 3.1Hydrometry TurloughwaterlevelsweremonitoredusingMini-Diver DI501andDI502monitorsSchlumbergerWaterServices placedatthelowestpointineachturlough.Compensation forthevariationinprevailingairpressurewasmadeusing aBaroDiver DI500whichwasinstalledatgroundlevel Hydrol.EarthSyst.Sci.,20,2119–2133,2016www.hydrol-earth-syst-sci.net/20/2119/2016/

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T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux2123 nearCoyturlough.Thelocationsofthediverplatformswere surveyedviaGPS,whichallowedthewaterdepthreadings tobereferencedagainstordnancedatum. TwotippingbucketARG100raingaugesEnvironmental MeasurementLtd.,NorthShields,UKwereinstalledatthe upperendofthecatchmentatKilchreestmAOD:metresaboveordinancedatumandFrancisGapmAOD. Inaddition,hourlyrainfallandevapotranspirationdatawere obtainedfromtheAthenrysynopticweatherstation 20km fromtheGortLowlandsrunbythenationalweatherservice, Metireann. Rivergaugingstationswerelocatedonthethreeprimary riversdrainingoffthemountains–SA1,SA2andSA3–with anadditionalstationlocatedonSA4neartheoutletofLough Cutra.Thegaugesconsistedofapressuretransducerembeddedintotheriverwiththedataloggerssettocollectdata at15mintimesteps.Ratingcurvesweredevelopedforeach gaugingstationFig.3usingthemid-sectionvelocitydepth surveyingmethodShaw,2011. 3.2Hydrochemistry Monthlysamplingwascarriedoutatturloughs,rivers, springsandtwouplandsitesFandPEbetweenMarch2010 andMarch2013,inadditiontogroundwatersamplesfrom boreholesandwellswithinthecarboniferousaquifersurroundingtheturloughnetworkFig.3.Watersampleswere testedwithin24hofcollection.SamplesweretestedforalkalinitybasedonStandardMethodsAPHA,1999;total nitrogenTN,nitrateNO 3 –N,nitriteNO 2 andammoniumNH 4 wereanalysedusingaMerckSpectroquantNova 60spectrophotometerandassociatedreagentkits.Quality controlQCwascarriedoutusingMerckCombicheckstandardsforeachbatchofmonthlysamples.TotalphosphorusTPconcentrationsweredeterminedbyacidicpersulphatedigestionofsamplesat120 Candsubsequentmeasurementofphosphatebycolorimetryinaccordancewiththe StandardMethodsAPHA,1999.TotaldissolvedphosphorusTDPconcentrationswereobtainedsimilarlybutwith theaddedstepofltrationdirectlyaftersamplingusinga 45micronlter.QCwascarriedoutforPbyrunningaQC sample.025mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TPwitheachbatchofPanalyses.All resultswerebaseduponduplicatesamplesthatwerecollectedandtestedseparatelytoruleoutsamplingerror. The1-monthsamplingintervalprovidedanapproximation oftheirmeanandmaximumobservednutrientconcentrationswithintherivers.Intheturloughs,the1-monthinterval wasdeemedappropriatebasedonthendingsofprevious studiessuchasGill,who,inanattempttooptimise samplingmethodologies,evaluatednutrientconcentrations bothspatiallyandtemporallyandfound1monthtobean adequatesamplinginterval.This1-monthintervalhasalso beenusedasanestablishedsamplingtechniqueforavarietyofotherecohydrologicalturloughstudiesCunhaPereira, 2011;CunhaPereiraetal.,2010;KimberleyandWaldren, 2012;Porstetal.,2012;Waldren,2015. 3.3Modelling AlongwithmodellingthehydraulicprocessesofapipenetworkGilletal.,2013a,InfoworksCSalsoincorporatesa waterqualitymodelwhichwasusedinordertoevaluatethe nutrienttransportprocesseswithintheGortLowlands.The waterqualitymodeleffectivelyrunsinparallelwiththehydraulicmodel;thecalculatedowsfromthehydraulicmodel areusedtocalculatetheassociatedoutputfromthewater qualitymodelateachtimestep.Eachhydrochemicalspecies canbemodelledasbeingentirelydissolvedorpartiallyattachedtosediment,withthepollutantsbeingtreatedasfully conservative.Nointeractionbetweenpollutantsandtheirenvironmentwassimulated,norbetweenonepollutantandanother.Thewaterqualitymodelforthetransportofdissolved nutrientscarriedoutitscalculationsintwostagesforeach timestep. 1.The NetworkModel calculatestheconcentrationofdissolvedpollutantsatallnodesusingthefollowingconservationofmassequation: d M J d t D X i Q i C i C d M s J d t )]TJ/F29 9.9626 Tf 9.431 9.464 Td [(X o Q o C o ; where M J ismassofdissolvedpollutantinnode J kg, Q i isowintonode J fromlink i m 3 s )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 , C i isconcentrationintheowintonode J fromlink i kgm )]TJ/F69 7.5716 Tf 5.906 0 Td [(3 , M s J isadditionalmassenteringnode J fromexternal sourceskg, Q o isowfromnode J tolink o m 3 s )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 and C o isconcentrationintheowfromnode J tolink o kgm )]TJ/F69 7.5716 Tf 5.905 0 Td [(3 . 2.The ConduitModel calculatestheconcentrationofdissolvedpollutantsalongeachconduitrepresentedasa conceptuallinkofdenedlengthbetweentwonodes inthenetwork.Thegoverningequationdescribingthe transportofdissolvedpollutantbasedontheconservationofmassisthefollowing: d C d t C u d C d x D 0 ; where C isconcentrationkgm )]TJ/F69 7.5716 Tf 5.906 0 Td [(3 , u isowvelocity ms )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 , t istimesand x isthespatialco-ordinatem. 4Results Theresultsofalkalinity,totalnitrogen,nitrate,totalphosphorusandtotaldissolvedphosphorusarepresentedinTable1. www.hydrol-earth-syst-sci.net/20/2119/2016/Hydrol.EarthSyst.Sci.,20,2119–2133,2016

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2124T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux Table1. RangesandmeanvaluesforalkalinitynitrateNO 3 ,totalnitrogenTN,totalphosphorusTPandtotaldissolvedphosphorusTDPforturloughs,groundwater,selectedriversandKinvaracombinedvaluesdisplayedinboldtextfortheperiodbetween March2010andMarch2013. Alkalinity mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 CaCO 3 TNmgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 NO 3 mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 NO 3 –NTPmgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TDPmgL )]TJ/F69 7.5716 Tf 5.905 0 Td [(1 RangeMeanRangeMeanRangeMeanRangeMeanRangeMean Rivers148.50.91.010.10.710.0037.1210.0260.0660.018 SA115148.10.2.41.030.1.50.550.014.1130.0270.006.0440.016 SA21268.20.11.120.1.60.680.015.1020.0320.008.0640.024 SA3238.80.1.51.090.1.10.660.012.0870.0390.007.0490.020 F116.50.70.870.40.530.005.0210.0110.0140.007 PE110.20.2.20.8900.840.004.0550.0130.0420.009 Turloughs42131.80.1.31.120.40.660.014.11533.700.006.0610.021 Blackrock46138.40.31.320.3.50.810.022.1150.0470.013.00610.029 Coy58150.30.31.1100.570.025.0640.0420.006.00460.021 Coole42114.40.3.21.100.2.70.660.024.0450.0300.009.00320.020 Garryland77134.60.3.70.950.1.70.600.014.0340.0210.005.00250.016 Caherglassaun77121.30.1.31.110.40.650.019.0360.0270.008.00280.020 Groundwater104365.10.2.42.300.31.510.580.0310.484.90.021 BH3135387.80.4.92.450.1.61.600.003.050.0130.0350.008 BH5246307.70.3.11.390.40.480.009.580.0720.005.4850.052 BH7308366.90.4.43.310.32.790.007.0530.0150.005.0140.008 BH10104357.00.1.22.170.1.61.520.0130.0050.0006.0100.004 BH11269313.60.2.41.240.90.690.005.130.0330.004.0210.007 BH12123375.50.3.22.950.2.81.970.008.0470.0190.002.0290.007 BH14162375.51.12.910.41.750.033.0820.0530.031.0650.042 BH15369425.70.2.11.310.1.90.680.031.080.0520.029.0580.042 BH16316376.00.3.52.960.2.32.150.011.0390.0190.009.0340.017 KinvaraWestKW96155.60.4.31.050.1.50.660.009.0330.0230.008.1.0220.017 4.1Alkalinity IntheGortLowlands,alkalinityisparticularlybenecialas anindicatorofrechargeoriginduetothesubstantialinputof undersaturatedallogenicrecharge.Byexploitingthedistinct contrastbetweenthelow-alkalinityallogenicrechargeand thesaturated,high-alkalinityautogenicrecharge,insightscan bemadeintothelikelysourceofwaterwithinthecatchment. 4.1.1Surfacewater Alkalinityconcentrationswithintheturloughswerefoundto bequitevariable.Thepredominantprocesscontrollingtheir alkalinitiesistheirhydrologicalfunctioningandtheinux ofwaterfromconduitordiffusesources.Otherprocesses thatarelikelytoalteraturlough'sCaCO 3 concentration,althoughtoalesserdegree,includecarbonateprecipitationand dissolution. BlackrockandCoyturloughshadmeanalkalinitiesof 138.4and150.3mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 CaCO 3 respectively.Theseconcentrationsreectthealkalinityoftheirprimarysourceof water,SA1,whichhadameanalkalinityof148.1mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 CaCO 3 .ThealkalinitiesofCoole,GarrylandandCaherglassaunturloughswereslightlylower.4,134.6and 121.3mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 CaCO 3 ,reectingthelowerconcentration contributionsofSA2.2mgL )]TJ/F69 7.5716 Tf 5.905 0 Td [(1 CaCO 3 andSA3 .8mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 CaCO 3 rivers.However,theseturloughshave noticeablyhigherconcentrationsthanwouldbeexpected fromaweightedmeanalkalinitybasedonthepercentage owcontributionfromthethreeriversmgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 CaCO 3 . Theirincreasedalkalinity,relativetowhatwouldbeexpectedfromtheriverinputs,canbeattributedtothreefactors.Firstly,theseturloughsreceiveaminorinuxofwaterfromthemorealkalineCloonteenRivercatchmenttothe southoftheGortLowlandsseeFig.1,mostsignicantly atGarrylandturlough.Secondly,asSA2andSA3riversenterthelimestonesystemundersaturatedindissolvedCaCO 3 , theirwaterischemicallyaggressiveandhasahighdissolutionpotential.Thisislikelytocauseconsiderablesolutionof thelimestonebedrockastheyowtowardsCoole.Thirdly,as theriver/conduitwatermovesthroughthecatchmenttowards thelowerthreeturloughs,itisbeingdilutedbytheaddition ofhigh-alkalinityrechargefromthediffusegroundwater. Coy,GarrylandandCaherglassaunareknowntooperate hydraulicallyassurchargetankturloughsfedviaasingleestavellewithadegreeofisolationfromthemainkarstows throughthesystemGilletal.,2013b.Theirhydrochemistrysuggeststhatthelow-alkalinitywaterbroughtinfrom theinitialoodingeventremainswithintheturloughsand onlyslowlybecomesenrichedinbicarbonateovertime,most likelyduetogradualrechargefromthesurroundingepikarst, asshowninFig.4.BlackrockandCooleturloughs,onthe Hydrol.EarthSyst.Sci.,20,2119–2133,2016www.hydrol-earth-syst-sci.net/20/2119/2016/

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T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux2125 Figure4. TurloughstageandalkalinityresultsforBlackrock,Coy,Coole,GarrylandandCaherglassaunturloughsoverthe2012oodingseason. otherhand,areseentobedirectlyinuencedbyriverconcentrations,evenduringoodedperiods,withdramaticreductionsinalkalinityinresponsetoaoodingevent.This patternsuggeststhattheseturloughscanreceiveasignicantamountofnewlow-alkalinitywaterfromtheirsurface inputswhiledrainingawaytheolderhigheralkalinitywaterthroughtheirestavelles,i.e.actingpredominantlyasriver ow-throughsystems,asopposedtothediffuseow-through fromthesurroundingepikarst. Thetrendofincreasingalkalinityovertheoodingseasonasseeninthesurchargetanksystemsisunusualfor turloughs.Typicalautogenicallyrechargedturloughstendto havemuchhigheralkalinitylevels,duetotheCaCO 3 -rich watersthatfeedthem,whichdoesnotincreaseovertime astheyaresaturatedbuttendstodecreaseasobservedby CunhaPereira,2011.SuchlossesinCaCO 3 fromturloughs havebeenattributedtotheinuxofwatersaturatedwith CO 2 whichcomesintocontactwiththeairandgradually losesitsCO 2 totheatmosphere,primarilyfromphysiochemicalprocessesbutalsopossiblybiogenicprocessesCoxon, 1994. 4.1.2Groundwater Groundwateralkalinitymeasuredacrossthecatchmentgenerallyvariedbetween300and400mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 CaCO 3 butoverallwasfoundtobequiteconsistentSDstandarddeviation 40mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 ,withameanvalueof365.1mgL )]TJ/F69 7.5716 Tf 5.905 0 Td [(1 CaCO 3 .Thebroadagreementandlackofvariationbetween mostgroundwatersamplesindicatesthepresenceofalarge diffuse/epikarsttypeaquiferwithlowtransmissivitywhich surroundstheactiveconduitnetworkMcCormacketal., 2014. 4.2Nutrients TheresultsoftheNO 3 ,TN,totalTDPandTPsampleanalysisareshowninTable1.NO 2 andNH 4 wereinitiallymeasuredbutwereoftenneartoorbelowdetectionlimits,and, assuch,theirmeasurementwasceased. 4.2.1Surfacewater:rivers ValuesforTNinallriversrangedbetween0and3.9mgL )]TJ/F69 7.5716 Tf 5.905 0 Td [(1 withameanof1.01mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 ,whileTPconcentrations rangedbetween0and0.12mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 withameanvalueof 0.026mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 .Nutrientconcentrationsintheriversshowed ahighdegreeofvariation,althoughaseasonaltrendwas apparent,withNandPhighestinsummer,whereaslowest concentrationswereinthewinterforNandthespringfor P.Contrastingsource/transportdynamicsbetweenNandP areapparentintherivernutrientconcentrations.MeanvaluesofTNforeachriverwerequitesimilar,rangingbetween www.hydrol-earth-syst-sci.net/20/2119/2016/Hydrol.EarthSyst.Sci.,20,2119–2133,2016

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2126T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux Figure5. DailyrainfallandTNconcentrationsattheupperUand lowerLsamplinglocationsontheSA1River. 0.87and1.12mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 ,whereasforTPtheriversshoweda widerangeofmeanvaluesbetween0.011and0.032mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 Table1. ThelackofvariationforNbetweenallsamplinglocations,andthelackofvariationforbothNandPbetween theupperandlowerriversamplinglocations,indicatesthat thereisaminorbutconstantadditionofnutrientstothe riversfromagriculturalandforestryland-usepracticesas theytraveldownthroughtheircatchments.Figures5and6 showexamplesofnutrientvariationfortheSA1riverupper andlowersamplinglocations.ThepeakinPinJuly2012 Fig.6whichwasalsoseentoalesserextentintheother tworiversoccursduringthetypicalforestryfertilisationseasonofApriltoAugustNutrientrequirementsinTeagasc, 2015andcoincideswithaperiodofheavyrainfall.Kilroy andCoxonsuggestthataresponsesuchasthiscould possiblyreectahydrologicalswitchwherethecatchments changefromasoilmoisturedecittoasoilmoisturesurplus situation. Nutrientloadquantitiesintheriverswereestimatedby combiningthemeasurednutrientconcentrationdatawith theobservedowdata.MaximumobservedTNloadingfor theSA1,SA2,SA3andSA4riverswasfoundtobe46, 34,23.9and35.2kgh )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 respectively,whileTPloadingwas foundtobe1.2,4.1,2.1and3.3kgh )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 respectively. 4.2.2Surfacewater:turloughs MeanTNandTPconcentrationsfortheturloughswere 1.12and0.034mgL )]TJ/F69 7.5716 Tf 5.905 0 Td [(1 ,withhighestconcentrationsrecorded of4.3mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TNand0.115mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TP.Itshouldbenoted thatthemeanTPconcentrationliesjustbelow0.035mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 , theOECDthresholdforTPineutrophiclakesOECD, 1982.Generally,theuppertwoturloughsBlackrockand CoyshowedslightlyhigherNconcentrationsandsignicantlyhigherPconcentrationsthanthelowerthreeturloughs. ThisisasexpectedconsideringthatthecatchmentoftheuppertwoturloughsencompassesagreaterproportionofagriculturallandthanthelowerthreeturloughsCunhaPereira, 2011.TheupperturloughsalsotendedtoshowmeanconcentrationsgreaterthanthoseoftheSA1Riverfeedingthem, Figure6. DailyrainfallandTPconcentrationsattheupperUand lowerLsamplinglocationsontheSA1River. whichsuggeststhattheseturloughsaregainingnutrients fromadditionalsourcesseeDiscussionsection. ThelowermeannutrientconcentrationsinCoole,GarrylandandCaherglassaunturloughstendedtoreecttheconcentrationsoftheriversfeedingthem.Forexample,mean concentrationsofTNandTPinCooleturloughwerewithin 1%oftheirprimarysourceofwater,theSA3river.NutrientconcentrationsinCaherglassaunshowsimilarvalues toCoole,indicatingadirectrelationshipbetweentheseturloughs.However,Garrylandturloughdisplayslowernutrient concentrations,mostlikelyduetotheinuxofwaterfrom thesouthernCloonteencatchmentasdiscussedpreviously. Figure7showsthetimeseriesofnutrientconcentrationdata andturloughvolumedataacrossthe2011season.For purposesofclarity,andastheoodingpatternsintheve turloughsarequitesimilar,onlytheaveragevolumeofthe veturloughsisshownasapercentageofmaximumvolumeratherthantheveindividualtimeseriesforindividualoodingpatterns,seeFig.4.Whilethenutrientconcentrationsintheseplotsareshowntobequitevariable,a trendcanbeseenwherebynutrientconcentrationsappear todecreaseovertheoodedperiodbetweenDecemberand February/March.ThispatternisseenclearlyseenforTN, NO 3 andTDP.However,ahighTPconcentrationinBlackrockduringJanuary2012doesnotconformtothetrend,the reasonforwhichisunclear.Onehypothesisisthatthesamplewasinuencedbypointsourcecontaminationfroman abattoironthesoutheasternedgeofBlackrockturlough.NutrientsareseentoincreasesignicantlyinthecaseofTN afterthemainoodvolumeshaverecededbutwithasmall quantityofwaterstillremaining.Thesespikescouldbedue totheincreasedsensitivityoftheturloughstotheirriverinputsduringsuchdryperiods. 4.2.3Groundwater Theprimarylanduseinthecatchmentisagriculture,and, assuch,therearesignicantadditionalsourcesofNand P.Meangroundwaterconcentrationsacrossthecatchment wererecordedas2.30and0.031mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 forTNandTPrespectivelyTable1,withoverallmeanNconcentrationsbeHydrol.EarthSyst.Sci.,20,2119–2133,2016www.hydrol-earth-syst-sci.net/20/2119/2016/

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T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux2127 Figure7. ConcentrationsofTN,NO 3 –N,TPandTDPinBlackrock,Coy,Coole,GarrylandandCaherglassaunturloughsplottedtogether withaveragepercentagevolumeoftheveturloughsi.e.volumeasapercentageofthemaxvolumeovertheperiodshown. ingalmostdoublethoseofsurfacewaterbodies,whilethe overallmeanPconcentrationsoftheturloughsandgroundwaterwereshowntobesimilar.Theresultsobtainedfrom boreholeswithintheGortLowlandsshowedawiderangeof recordedresultsforN.2.4mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TNwithastandard deviationof0.92mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TN,althoughthemeanconcentrationsateachboreholeacrossthecatchmentarewithinasimilarrangebetween1.2and3.3mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 .Pshowedagreater rangeofmeasuredresults.58mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TPwithastandarddeviationof0.027mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TP,but,moresignicantly, themeanconcentrationsateachboreholeshowedlargedifferencesbetween0.005and0.072mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TP.TheseresultsindicatethatNwasabletoreachthegroundwaterrelativelyeasilyduetothehighmobilityandsolubilityofNO 3 , andmoreorlessequalisedacrossthecatchment.P,onthe otherhand,beingmuchlessmobilewouldonlybelikely toenterthegroundwaterinareasofextremevulnerability i.e.throughshallowand/orpermeablesubsoils.Thus,while thesourcesofPcouldbeequallyaswidespread,itsability toreachthegroundwaterishighlyvariable.However,once intheconduitsystem,PisknowntobetransportedconservativelywithnegligibleattenuationMellanderetal.,2013; KilroyandCoxon,2005. 4.2.4Kinvarasprings MeanTNconcentrationforKWwasmeasuredas 1.05mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 ,whichreectedthemeanconcentrationsofthe turloughs.12mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TN.Pconcentrationsatthesprings wereamongthelowestmeanconcentrationsfoundwithin thecatchment.023mgL )]TJ/F69 7.5716 Tf 5.905 0 Td [(1 / ,suggestingthelossofPas watermovesthroughthekarstsystem.ThesenutrientconcentrationsareinaccordancewiththendingsofSmithand Cave,whosuggestthatKinvaraBayisasourceofN tothegreaterGalwayBay. ThenutrientloadsleavingtheGortLowlandssystemand dischargingintotheseawerecalculatedusingKWdischarge andnutrientconcentrationdataobtainedbysamplingatKW. ThesimulateddischargeatKWwasestimatedusingthehydrologicalmodelwhichaccountedfortemporaltidaleffects anddidnotincludeanyadditionaldischargefromtheunmodelledsouthernCloonteencatchmentseeMcCormacket al.forfurtherdetail.Usingthismethodology,theaveragedailyTNloadwascalculatedas788kgday )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 ,while theaveragedailyTPloadwas17.3kgday )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 . 4.3Nutrientmodelling Thehydraulicmodelwasusedtosimulatethebehaviourof nutrientspassingthroughthekarstsystemactingasconservativetracers.Theseresultshavethenbeencomparedagainst theeldsamplingresultsfromtheturloughsfromwhichinsightshavebeenmadeastothemobilityandattenuationbehaviourofthesenutrients. www.hydrol-earth-syst-sci.net/20/2119/2016/Hydrol.EarthSyst.Sci.,20,2119–2133,2016

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2128T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux 4.3.1Nutrientretention Nutrienttransportthroughahighlykarstiedcatchmentsuch astheGortLowlandscanbereasonablyassumedtoactconservativelyoncethenutrientshaveenteredtheconduitsystem.Assuch,makingacomparisonbetweenmodelledand observednutrientbehaviourswithintheturloughsisauseful techniquetoascertainthemagnitudeofanynon-conservative nutrientmechanismstakingplaceinthesegroundwaterdependentecosystems. Duetothelimitationsofmonthlysamplingintherivers, arepresentativetimeseriesofobservednutrientconcentrationscouldnotbeestablished.Assuch,ahypotheticalnutrientplumewasusedasaninputsignal.Thepurposeofthis pulseinputsignalistopredicthownutrients/contaminants wouldbehaveafterenteringriverow-throughandasurchargetankturloughs.Thehypotheticalinputsignal,presentedinFig.8,consistsofmeanobservedTDPvaluesinthe riversandapulseofTDPoccurringintheSA1Riveratthe onsetoftheoodingseason.Whilethesimulationsusingthis hypotheticalnutrientplumecannotbecompareddirectlyto observedbehaviour,acomparisonofnormalisedsimulation resultswithnormalisedobservedresultscouldbeapplied. BlackrockturloughFig.9,ariverow-throughturlough, showsanutrientconcentrationpeak–recessiontypepattern wheretheconcentrationdropsastheturloughisstilllling. Thispatternisexhibitedinbothsimulatedandobservedresultsandindicatesaconstantuxofwaterthroughtheturloughwhetheritisoodingoremptying.ThesimulatedresponseofCoy,thesurchargetanktypeturlough,isdistinctly differenttothatofBlackrock.Oncethecontaminanthasenteredthewaterbody,themodelledconcentrationremainsrelativelyunchangedthesmalldropofconcentrationseenearly onisduetothepresenceofasecondswallowholewhich onlyinuencestheturloughatadepthabove10m.Crucially however,theobservedresultsinCoyalsoshowapatternof reducingconcentrationssimilartothatofBlackrock.Thefact thatthisow-throughpattern–whichisalsoseeninthetwo othersurchargetanksystems,GarrylandandCaherglassuan Fig.7–isoccurringinaturloughwhichisknowntohavea minorow-throughcomponentthussuggeststhatsomenonconservativenutrientremoval/transformationprocessesmust beoccurringwithintheseturloughs. 4.3.2Diffusecontribution Thecontributionofmodelleddiffuseowtotheconduitnetworkaddedapproximately35%tothedischargefromthe catchment.Bycombininggroundwaterconcentrationswith theestimateddiffuseowfromeachsubcatchment,aloadingrateforeachsubcatchmentwasdetermined.Diffuseinuxaddedbetween48and112%basedonameangroundwaterconcentrationof2.3mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TN,SDof0.92mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 . ForP,theinuxwaslowerbutconsiderablymorevariable, addingbetween5and65%basedonameangroundwater Figure8. ModelinputsignaltosimulateapulseofTDPoccurring inSA1.SA2andSA3areinputtedasconstantsignalsbasedon meanobservedTDPconcentrations. concentrationof0.0031mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TN,SD0.027mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 TN. Whiletheestimateofdischargefromthesubcatchmentsmay besufcientlyaccuratetopredicthydrologicalprocesses,the signicantvariabilityofobservednutrientconcentrationsin groundwaterhindersanypreciseestimationofnutrientloadingfromdiffusesources,particularlyforP. 5Discussion Hydrologically,turloughssitwithinaspectrumofdifferenttypesrangingfromdiffuse-ow-through-dominated toconduit-dominatedephemerallakes.Theturloughsof theGortLowlandspredominantlyfallundertheconduitdominatedcategoryandareknowntooperateasriverowthroughsystemsBlackrockandCooleorsurchargesystems Coy,GarrylandandCaherglassaun.Conceptually,results fromtheow-throughturloughsreectthehydrochemistry oftheirfeedingrivers,whereasthesurchargetankturloughs canbeisolatedfromanynutrientinputdependingonthe oodconditions. 5.1Alkalinity Alkalinityresultssupportedtheconceptualhydraulicmodels forthecatchment.BlackrockandCooleturloughsshowed signsofow-throughbehaviourasevidencedbyquickdrops inalkalinityduringaoodedperiod.Coy,GarrylandandCaherglassaun,ontheotherhand,showednosuchbehaviour whichwouldconformtotheirconceptualmodelsassurchargetanks.Themostnoticeabletrend,particularlyfor thesurchargetankturloughs,wastheincreaseinalkalinity acrosstheoodingseason.AsmentionedinSect.4.1.1,this couldbeattributedtogradualrechargefromthesurroundingepikarstduringrecessionduetoahydrologicalgradient betweentheturloughanditssurroundingepikarst. 5.2Nitrogen ThetypicalpatternofNintheturloughsispeakconcentrationsoccurringinearlywintercoincidingwithpeaksornear peaksinwaterlevelsfollowedbyareductioninconcenHydrol.EarthSyst.Sci.,20,2119–2133,2016www.hydrol-earth-syst-sci.net/20/2119/2016/

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T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux2129 Figure9. Observedbluepointsandsimulatedredlinenormalisedresultsofanutrientplumeinaow-throughsystemBlackrockanda surchargetanksystemCoy. trationsandloadthroughoutthespringandsummer.This patternisalsoreportedinnumerouspermanentwaterbodiesinIreland,suchasLoughBunnyPybusetal.,2003and LoughCarraKingandChamp,2000,aswellasinScotlandPetryetal.,2002andWalesReynoldsetal.,1992. Thetrendisusuallyexplainedbyreducedeffectiverainfall andincreasedplantandmicrobialNuptakeinthecatchments duringthegrowingseasonlatespringtoearlyautumnand thereverseprocessoccurringinthelateautumnandwinter CunhaPereira,2011;Kasteetal.,2003.Thispatternwould thusbeexpectedofBlackrockandCooleturloughsasthey shouldreecttheNofthewaterfeedingthem,andindeed theresultsgenerallysupportedthis.Interestinglyhowever, thetrendcanalsobeseeninCoy,GarrylandandCaherglassaunturloughs.ThissuggeststhatNisbeinglostfromthese turloughsbyalternativeprocesses. LossesofNfromlakesaretypicallyexplainedbythree mainprocesses:anetlosswithoutowingwateri.e.owthrough,bpermanentlossofinorganicandorganic nitrogen-containingcompoundstothesediments,andcreductionofNO 3 toN 2 bybacterialdenitricationandsubsequentreturnofN 2 totheatmosphereWetzel,2001.These processesareofadditionalimportancewithintheGortLowlandsasthelimitingnutrientintheseturloughshasbeen showntobeNratherthanPCunhaPereiraetal.,2010. AnadditionalcomplicationforNcyclinginturloughsisthe shiftfromoodedanddryphaseswhichresultinuctuation betweenaerobicandanaerobicsoilconditions. FormanyturloughsinIreland,whichoperatemoreasdiffuseow-throughsystems,themostlikelyexplanationfor adeclineofNconcentrationisduetoanequivalentdecline inNconcentrationfromtheinowingwater.Massbalance calculationscarriedoutbyNaughtonshowedthat,in orderfordilutiontobethemainprocessresponsibleforloweringTNconcentrations,excessivelyhighlevelsofturnover wererequiredduringtherecessionperiod.Whilesomedegreeofow-throughbehaviourmustinevitablybeoccurring,otherNreductionprocessesarealsolikelytobetakingplace.Thisoutow–dilutionconceptisasuitablepartial explanationforthebehaviourofBlackrockandCooleturloughs,whicharecloselyrelatedtotheirrespectiveriverinputs.Thisconcepthoweverdoesnotexplainthereduction ofNinthesurchargetankturloughs.Whilethesesurcharge tankturloughsdoexperiencesomedilutionfromdiffusewaterasshownbyalkalinitymeasurements,theincomingwaterwouldbemorelikelytoincreaseNconcentrationrather thanreduceit.Thus,internalreductionprocessesmustalso betakingplacewithintheseturloughs. Inmanypermanentlakes,sedimentationcanbeamajor sourceofNlossasaresultofpermanentinternmentofpartiallydecomposedbiotaandinorganicandorganicnitrogen compoundsadsorbedtoorganicparticulatematterinthesedimentsWetzel,2001.However,itisprimarilyorganicnitrogenthatislosttosedimentsasdissolvedformsofNsuchas ammoniumandnitratearehardlyadsorbedbysedimentparticlesanddonotnormallyprecipitatetoinsolubleformsin thesedimentScheffer,1998.WithinturloughsNinthewatercolumnisprimarilyfoundinaninorganicform.Assuch, theeffectofsedimentationontheGortLowlandsturloughs shouldbelimited. DenitricationcancausesignicantlossofNinlakes.For ittotakeplace,thekeyconditionrequiredisanoxicconditions.Duetothiscondition,denitricationisanunlikely causeofNlossinmostturloughsastheytendtoshowdissolvedoxygenDOlevelsnearsaturation > 10mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 CunhaPereira,2011.Asmostturloughsareshallowwith averagedepthsbetween1and3mNaughton,2011,DO levelscanbeassumedtoremainhighthroughouttheturloughwatercolumn.TheturloughsoftheGortLowlands howeveraredeeper,typicallyreachingdepthsgreaterthan 10m.Theseturloughsarealsomoreeutrophic,whichwould encouragea“clinograde”oxygenprolewherebyDOlevels reducewithdepthduetooxidativeprocesses.Inlakeswhere thisclinogradeoxygenproleoccurs,oxygenconsumption ismostintenseatthesediment–waterinterface,wherethe accumulationoforganicmatterandbacterialmetabolismare greatestWetzel,2001.Thusthesedimentsurfaceisthe mostimportantsitefordenitricationScheffer,1998.AnalysisofsoilsamplesfromtheGortLowlandturloughsby KimberleyandWaldrenfoundthatelevatedconcentrationsofavailableformsofNandPinthelowerturlough zonesmaybetheresultofanaerobicconditions,whichsuggeststhatdenitricationcouldoccurwithintheturloughsof theGortLowlands. www.hydrol-earth-syst-sci.net/20/2119/2016/Hydrol.EarthSyst.Sci.,20,2119–2133,2016

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2130T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux ReddyandDeLaunestatethatdenitricationrates inlakesvarybetween34and57mgNm )]TJ/F69 7.5716 Tf 5.906 0 Td [(2 day )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 .Lookingat theexampleofCaherglassaunoverthe2011ooding season,thatwouldsuggestaremovalof755kgNvia denitricationbetweensamplingpointsAandBmonth apart,highlightedinFig.10.TheactualamountofNremovedcanbecalculatedasfollows: – NloadatpointAis3121kg.1mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 2837295 m 3 .NloadatpointBis1724kg. – SupposingthatNwasremovedbyoutowonly,theconcentrationshouldstayat1.1mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 ,whilethevolume reducesto2463700m 3 .SotheNloadatpointBwould be2710kg. – Thus986kgNhasbeenremovedbynonconservativeprocesses. ThisvaluekgsitscomfortablybetweenthedenitricationvaluesaspredictedforCaherglassaunbasedonthedenitricationratesofReddyandDeLaune,whichsuggeststhatdenitricationisaplausiblecauseofNremoval duringthisperiod.Itshouldbenotedthatthiscalculationis onlymadepossiblebythefactthattheturloughwasinrecessionfortheentireperiodbetweenpointsAandB.This allowedforthetransformationprocessestobeisolatedfrom anydilutioneffectsastheturloughdidnotreceivesignicant inowduringthisperiod. Whenthissamecalculationiscarriedoutoverthe sameperiodforGarrylandturlough,theReddyandDeLauneremovalratepredictionisbetween356and 597kgN,butonly151kgisremovedfromGarryland.This lesserremovalrateinGarrylandmayberelatedtothefact thattheturloughisoccasionallylinkedwiththesouthern CloonteencatchmentaswellasCooleturloughdependingon waterlevels,whichwoulddiscouragethestableconditions requiredfordenitrication.ThisissimilartoCoy,whichover thisparticularperiodappearstoshownodenitricationatall. Againthismaybelinkedtoinstabilityatcertainwaterlevels asCoyisknowntohaveanelevatedswallowholewhichacts asanoverowathighwaterlevels.Asriverow-throughturloughs,BlackrockandCoolewerenotconsideredforcalculationastheyareknowntobeunstableoveroodedperiods. ThusCaherglassaun,whichisthedeepestandlastturloughin thenetwork,andconsequentlythemoststable,ispredictably foundtobethemostlikelysitefordenitricationtooccur withinthesystem. Thesurchargetankturloughs,particularlyCaherglassaun, canthereforebeconsideredassinksofNduringthefew monthstypically3monthsinwhichtheyaredeep enoughandstableenoughfordenitricationtotakeplace. Theow-throughturloughs,ontheotherhand,arepredominantlyinuencedbydilutionandtendtoreecttheirinput. Incertainsituationshowever,theseow-throughturloughs canseeminglyoperateasnutrientsources.Meanobserved Figure10. Denitricationexample,Caherglassaun.Denitrication occurringbetweenpointsAandB.Standarddeviationofduplicate samplesshownbyerrorbars. concentrationsinBlackrock.32mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 exceededthose measuredintheriverfeedingit.03mgL )]TJ/F69 7.5716 Tf 5.906 0 Td [(1 overthestudy period,whichsuggestsapossibleinternalsourceofnutrients suchasfromgrazinganimalsorthenearbyabattoir. 5.3Phosphorus ThemajorsourceofPtotheturloughsisviariverinputs.For thelowerthreeturloughsCoole,GarrylandandCaherglassaun,meanturloughPconcentrationswereaclearreectionoftheirriverinput.Theuppertwoturloughs,however, showedPlevelsinexcessoftheirwatersourceSA1,which suggeststhattheseturloughsactasasourceofPorperhaps BlackrockisthesourceandCoyPconcentrationsareonly elevatedbyinuxofBlackrockoutow.SimilartothediscussiononN,thiselevatedPcouldbeduetothepresence ofanabattoirlocatednexttoBlackrockorduetograzing duringdryperiodsonbothturloughs,whichwouldleadtoincreasednutrientconcentrationsattheonsetofoodingdueto thereleaseofsolublePfrommanuredeposition.Anotherimportantfactorcouldbeanartefactofthetemporalresolution ofsampling.Monthlysamplingofturloughswasdeemedto beadequatetocharacterisethesystemaswateristypically retainedintheturloughsforlongperiods.However,forthe rivers,monthlysamplingonlyoffersasnapshotofconcentrationsatthetimeofsampling.Thusanypotentialplumes ofpointsourcecontaminationintheriverscouldbemissed bytheriversamplesbutwouldlikelybeaccountedforinthe turloughsamples. Intermsoftemporalvariation,theturloughsappearto besimilarlyinuencedbylossmechanismsforPasfor NFig.7.UnlikeN,thePcycleinlakeshasnogaseous lossmechanism;thusanyPaddedtothesurchargetank turloughsshouldremainwithinthesystemuntildrainage, butnotnecessarilythewatercolumnReddyandDeLaune, 2008.OneofthepredominantmechanismsbywhichP istransformed/removedfromlakesystemsissedimentation andsubsequentaccumulationandsoildeposition.IfPhas beensorbedontoparticulatematter,itcansettleandaccumulateatthebaseoftheturlough,thusreducingthetotalPTP Hydrol.EarthSyst.Sci.,20,2119–2133,2016www.hydrol-earth-syst-sci.net/20/2119/2016/

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T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux2131 concentrationofthewatercolumn;i.e.theuxofparticulate matterisgenerallyfromthewatercolumntosoil.Thiswas conrmedbyKeane,whofoundthatturloughsoils donotre-releasesignicantPamountsbackintothewater column.Also,turloughswithmineralsoilssuchastheGort LowlandsturloughsaremorelikelytoaccumulatePthan turloughswithorganicsoilsWaldren,2015.AsthePisretainedinthesoil,itcantransferfromavailablePpoolsinto muchlargerimmobilePpoolsandthuskeepaccumulating inthesoil,awell-documentedphenomenoninordinaryagriculturalsoils.Thesedimentationprocesswouldresultina reductionofbothTPandTDPspecies,ascanbeseenoccurringintheturloughs.Indeed,thisprocesswassomewhat evidencedbyKimberleyandWaldren,whofoundelevatedPconcentrationsinsoilsamplestakenfromthemore saturatedlowerzonesofturloughs.Thus,thepatternofreducingPwithintheturloughscouldbepartiallyduetoadsorptionofPfromadissolvedformtoaparticulateformand subsequentsedimentationoutofthewatercolumnintothe soil.However,furtherresearchisrequiredintothemineralogyofturloughsoilsandtherelativeimportanceofdifferent Premovalmechanismsadsorption,precipitationunderthe prevailinghydrochemicalconditionsinthiskarstareacoveredbyglaciallyderivedcalcareouslimestonetill. Asideforthetrendofreducingconcentrationsoverthe oodedperiod,anotherpatterncanbeseenwherebynutrients NandPareseentoincreasesignicantlyforTNoncethe oodhasreceded.Thesespikescouldbeduetotheincreased sensitivityoftheturloughstotheirriverinputsduringdryperiods.Duringtheseperiods,theturloughshavelesscapacity todiluteanyincomingnutrientplumes,andsospikesinnutrientconcentrationsshouldbeexpected.Alternatively,ithas beensuggestedCunhaPereira,2011thatsuchspikesmight beduetothepossiblereleaseofnutrientsandorganicmatter tothewatercolumnowingtotheincreasedsoil–waterinteractions. 6Conclusions Thenutrientuxwithinalowlandkarstcatchmenthas beenmonitoredovera3-yearperiod.Theallogenicnature ofthiscatchmentprovidesdistincthydrochemicalcharacteristics,asdemonstratedbyalkalinityresults.Theallogenicallyfedriver–conduit–turloughnetworkdisplaysrelativelylow-alkalinityconcentrationscomparedtothemore autogenicslow-movingwaterfoundwithinthesurroundingepikarst/diffuseaquifer.Withintheturloughs,alkalinitywasabletoeasilydistinguishbetweentheow-through turloughsBlackrock,CooleandthesurchargetankturloughsCoy,Garryland,Caherglassaun.Flow-throughturloughsdisplayedadistinctpatternwherebyasignicantinuxoffreshwatercouldcauseanoticeablechangeinhydrochemistryovertime.Thisisincontrasttothesurchargetank turloughswhichshowedstablealkalinityconcentrationswith aslowincreaseovertimeduetotheinuxofdiffuserecharge fromthesurroundingaquifer. Unlikealkalinity,nutrientconcentrationswithinthecatchmentareprimarilyinuencedbyanthropogenicprocesses, i.e.agriculture.Asaresult,thenutrientuxwithinthecatchmentdisplayedagreaterdegreeofcomplexity,particularly asaresultofthecontrastingmobilitytraitsofNandP.By combiningthehydraulicmodelwithconservativenutrient concentrations,insightsweregainedintohowtheturloughs shouldconceptuallyoperate.Thisshowedthat,whilethe ow-throughturloughsbehavedasexpectedwithrespectto nutrients,thesurchargetankturloughscanbehavesimilarly topermanentlakesundercertainconditions.Undersuchconditionslong,deepandstableooding,theturloughscan operateasnutrientsinkswithinthecatchment.Thesenutrientlossesi.e.non-conservativebehaviourwereattributed tobemostlikelyduetotheprocessofdenitricationforN andsedimentationforP. Aswellasbeingnutrientsinks,theturloughsmayalso operateasnutrientsourcesduetomanuredepositionfrom grazinganimalsduringdryperiodsinthesummerorvia otherpointsourcessuchastheabattoirlocatednearBlackrockturlough,aswellason-sitewastewatertreatmentsystems,slurrytanksetc..Thesesourcescanbepresentinmost turloughs,andresultsfromthisstudysuggestthatsometurloughsmayhavegainedconsiderablenutrientloadsbysuch processesoverthestudyperiod.However,astheseinputscan occurveryrapidly,itisdifculttoquantifywithouthigherfrequencysamplingoftheturloughsandtheirinputs.Itcan thusbeconcludedthat,whilenoteveryturloughhasthepotentialtoactasanutrientsinkeveryyear,everyturloughdoes havethepotentialtoactasnutrientsourceeveryyear. Acknowledgements. Thefundingforthisresearchwasprovided bytheAXAResearchFund.Wealsowishtothankthefarmers ofsouthGalwaywhoallowedaccesstotheirlandthroughoutthe researchprojectandtheOfceofPublicWorksforprovidingriver owdata. Editedby:G.H.deRooij References Alexandratos,N.,andBruinsma,J.:Worldagriculturetowards2030/2050:the2012revision,ESAWorkingpaperNo.1203,FAO,Rome,154pp.,2012. APHA:StandardMethodsforExaminationofWater&Wastewater, 20thEdn.,AmericanPublicHealthAssociation,Washington, D.C.,1325pp.,1999. Coxon,C.E.:CarbonateDepositioninTurloughsSeasonalLakesontheWesternLimestoneLowlandsofIreland.I:Presentdayprocesses,IrishGeogr.,27,14, doi:10.1080/00750779409478695,1994. Coxon,C.E.:AgricultureandKarst,in:KarstManagement,edited by:Beynen,P.E.,Springer,Dordrecht,103,2011. www.hydrol-earth-syst-sci.net/20/2119/2016/Hydrol.EarthSyst.Sci.,20,2119–2133,2016

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2132T.McCormacketal.:Quantifyingtheinuenceofsurfacewater–groundwaterinteractiononnutrientux CunhaPereira,H.:HydrochemistryandAlgalCommunitiesofTurloughsKarstSeasonalLakes,CentrefortheEnvironment,TrinityCollegeDublin,Dublin,230pp.,2011. CunhaPereira,H.,Allott,N.,andCoxon,C.E.:Areseasonallakes asproductiveaspermanentlakes?AcasestudyfromIreland, Can.J.Fish.Aquat.Sci.,67,1291,doi:10.1139/F10-062, 2010. Davraz,A.,Karaguzel,R.,Soyaslan,I.,Sener,E.,Seyman,F.,and Sener,S.:HydrogeologyofkarstaquifersystemsinSWTurkey andanassessmentofwaterqualityandcontaminationproblems, Environ.Geol.,58,973-9-88,doi:10.1007/s00254-008-1577-5, 2009. DepartmentofAgricultureFisheriesandFood:FoodHarvest2020 –AVisionforIrishAgri-FoodandFisheries,AgricultureHouse, KildareSt.Dublin2,Ireland,60pp.,2010. Domagalski,J.L.andJohnson,H.:PhosphorusandGroundwater: EstablishingLinksBetweenAgriculturalUseandTransportto Streams,USGeologicalSurveyFactSheet2012-3004,USGelogicalSurvey,Reston,4pp.,2012. 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