A combined-method approach to trace submarine groundwater discharge from a coastal karst aquifer in Ireland


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A combined-method approach to trace submarine groundwater discharge from a coastal karst aquifer in Ireland

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Title:
A combined-method approach to trace submarine groundwater discharge from a coastal karst aquifer in Ireland
Creator:
Schuler, Philip
Stoeckl, L.
Schnegg, P.-A.
Bunce, C.
Gil, L.
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English

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Coastal Aquifer ( local )
Tracer Test ( local )
Submarine Groundwater Discharge ( local )
Remote Sensing ( local )
Ireland ( local )
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serial ( sobekcm )

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Abstract:
Knowledge about the hydraulic connections between submarine groundwater discharge (SGD) and its terrestrial coastal catchment is relevant with regard to the management of marine and coastal waters in karst areas. This study applies different methods and monitoring approaches to trace SGD between the Burren Limestone Plateau and Galway Bay in western Ireland, via an excavated sinkhole shaft and deep conduit. Areas of potential SGD were first delineated based on sea surface temperature anomalies using Landsat satellite images. Two fluorescent dyes and solid wood chips were then used as tracers. Solid wood chips were tested as potential means to circumvent the problem of high dispersion in the sea, impacting on the fluorescent dyes to yield readings below the detection limits. Sampling was conducted at 10 different terrestrial locations and in the sea at Galway Bay. Offshore sampling was conducted in transects over a period of four successive days onboard of a vessel using an automated field fluorometer and a conductivity-temperature-depth sensor. No wood chips were recovered in the sea but both fluorescent dyes were successfully sampled. The estimated travel times are in the order of 100 to 354 m/h, and localised tracer readings correlate well in space and time with low conductivity readings. By confirming hydraulic connections between the two karst features and Galway Bay, the study substantiates the hypothesised importance of Variscan veins with regard to regional groundwater flow in the region.
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Vol. 28 (2019-12-09).

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This item is licensed with the Creative Commons Attribution License. This license lets others distribute, remix, tweak, and build upon this work, even commercially, as long as they credit the author for the original creation.
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K26-05115 ( USFLDC: LOCAL DOI )
k26.5115 ( USFLDC: LOCAL Handle )

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PAPER Acombined-methodapproachtotracesubmarinegroundwaterdischargefromacoastalkarstaquiferinIrelandPhilipSchuler1&L.Stoeckl2&P.-A.Schnegg3&C.Bunce4&L.Gill1Received:10December2018/Accepted:11November2019#TheAuthor(s)2019AbstractKnowledgeaboutthehydraulicconnectionsbetweensubmarinegroundwaterdischarge(SGD)anditsterrestrialcoastalcatch-mentisrelevantwithregardtothemanagementofmarineandcoastalwatersinkarstareas.ThisstudyappliesdifferentmethodsandmonitoringapproachestotraceSGDbetweentheBurrenLimestonePlateauandGalwayBayinwesternIreland,viaanexcavatedsinkholeshaftanddeepconduit.AreasofpotentialSGDwerefirstdelineatedbasedonseasurfacetemperatureanomaliesusingLandsatsatelliteimages.Twofluorescentdyesandsolidwoodchipswerethenusedastracers.Solidwoodchipsweretestedaspotentialmeanstocircumventtheproblemofhighdispersioninthesea,impactingonthefluorescentdyestoyieldreadingsbelowthedetectionlimits.Samplingwasconductedat10differentterrestriallocationsandintheseaatGalwayBay.Offshoresamplingwasconductedintransectsoveraperiodoffoursuccessivedaysonboardofavesselusinganautomatedfieldfluorometerandaconductivity-temperature-depthsensor.Nowoodchipswererecoveredintheseabutbothfluorescentdyesweresuccessfullysampled.Theestimatedtraveltimesareintheorderof100to354m/h,andlocalisedtracerreadingscorrelatewellinspaceandtimewithlowconductivityreadings.ByconfirminghydraulicconnectionsbetweenthetwokarstfeaturesandGalwayBay,thestudysubstantiatesthehypothesisedimportanceofVariscanveinswithregardtoregionalground-waterflowintheregion.KeywordsCoastalaquifer.Tracertest.Submarinegroundwaterdischarge.Remotesensing.IrelandIntroductionArtificialtracertestsarecommonmethodsinkarsthydro-geology(Kaess1998;Benischkeetal.2007)tostudyconduitparameters(Geyeretal.2007;Luhmannetal.2012),thetransportationcharacteristicsofpotentialcon-taminants(FlynnandSinreich2010),ormoregenerallytoestablishhydraulicconnectionsandestimatetransittimes(LauberandGoldscheider2014;Marganeetal.2018).Fluorescentdyesarecommonlyusedsuchasuranine(so-diumfluorescein),rhodaminesoropticalbrightener,asarephysico/chemicaltracerssuchaschlorideandtemperature(Luhmannetal.2012)orparticulatebacteriophages(SinreichandFlynn2006;Mauriceetal.2010).Artificialtracersareusuallyappliedbetweenadefinedinjectionsite(e.g.sinkhole,undergroundriver,orbore-hole)andadiscretesamplingsite,usuallyaspring.Tracerstudiesaremostcommonlyexecutedwithinterrestrialcatchments.Manykarstcatchmentsarecoastal,dischargingviasubma-rinesprings.Eustaticsealevelvariationsdownto120mbelowmodernsealevelresultinarangeoflowerbaselevelsincoastalaquifersglobally.OfftheRepublicofIreland,sealevelsareestimatedtohavedroppedby60–100mbelowthepresentlevelattheIrishcoastandGalwayBaybetween15and26kaago(EdwardsandCraven2017;O’ConnellandMolloy2017).Asaresult,karstmorphologyextendsbeyondtheshoreintothesea,asisthecasefortheBurrenPlateau(KozichandSautter2018). *PhilipSchulerschulerp@tcd.ie1DepartmentofCivil,StructuralandEnvironmentalEngineering,UniversityofDublinTrinityCollege,Dublin2,Ireland2DepartmentforGroundwaterResources–QualityandDynamics,FederalInstituteforGeosciencesandNaturalResources(BGR),Stilleweg2,30655Hannover,Germany3AlbilliaCo.,CH2000Neuchâtel,Switzerland4BurrenOutdoorandEducationCentre(BOEC),Turlough,Clare,Irelandhttps://doi.org/10.1007/s10040-019-02082-0 HydrogeologyJournal– /Publishedonline:9December2019 (2020)28:561– 577

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Submarineandintertidalgroundwaterdischarge(SiGD)orpurelysubmarinegroundwaterdischarge(SGD)hasreceivedincreasedattentionovertherecentdecades,andthesefluxesareinternationallyrecognizedasanimportantpathwayfortransportintothecoastalenvironment,particularlyinkarstareas(Taniguchietal.2002;Burnettetal.2006;Dimovaetal.2011;Montieletal.2018).Knowledgeabouttheoccur-renceandquantificationofSiGD/SGDismostlyrelevantintermsofunderstandingthehydraulicsandhydrologyofcatch-ments(SmithandNield2003;Petersonetal.2008)and/ortoestimatenutrientfluxesfromthelandintothesea(Santosetal.2008;Leeetal.2012;McCormacketal.2014;Nulletal.2014),potentiallycausingadverseimpactssuchasalgalblooms(Silkeetal.2005;Greenetal.2014;Lietal.2017)togetherwithnegativeimpactsontheecologyorthemaricul-tureindustry(Larocheetal.1997).Equally,suchfluxesmayattractspecificecology,forexample,theoccurrenceoftheendangeredspeciesundulateray(Rajaundulata)isassociatedwithfreshwaterinputsintothesea,includingalongtheIrishcoast(Ellisetal.2012);speciesprotectionthereforecanalsobelinkedtoonshorecatchmentdynamics.LocatingdiffuseorpointSGDandfurtherdeterminingtheassociatedterrestrialcatchmentisachallengeinitselfduetotheinherentsignificantspatialandtemporalvari-abilityintheflux(BurnettandDulaiova2003).Naturalenvironmentaltracerssuchassalinity,temperatureorra-donmaybesampledinsitu(Schubertetal.2014),whilesometracers,liketemperature,maybealsoremotelysensedfromspace(Zektseretal.2007;Johnsonetal.2008;WilsonandRocha2012;Tamborskietal.2015).Inallcases,theuseofsuchparametersisconstrainedbyoceanmixinganddilution,therebyweakeningandspatial-lyintegratingthesignalpotentialtowardsalimitofdetec-tionorbelow(Breieretal.2005).Withinthecontextofkarst,abundantstudiesexistrelatingto:(1)thedetectionand/orquantificationofSGD,and(2)theuseofartificialtracermethodsforonshorecatchmenthydro-geology.Yet,untilpresent,totheknowledgeoftheauthors,thereisnosystematicapproachwhichlinksthestudyofSGDwithartificialtracertests,despitetheprevalenceoftracerstud-iesinonshorecatchmenthydrogeology(Benischkeetal.2007),andtheclearneedtolinktheSGDtoitscatchment.Thisresearchusesacombined-methodapproachto:(1)locateoffshoreareasofSGDusingremotesensing,and(2)usedif-ferentartificialandnaturaltracerstoevaluatehydrauliccon-nectionsbetweenterrestrialinjectionpointsandmarineSGDlocations.Morespecifically,thisstudypresentsanapproachfortracingSGDtoonshorelocationsofacoastaltelogenetickarstaquiferincombinationwithremotesensingtechniques.Suchknowledgecanthenbeusedtomoreeffectivelylinkthemanagementoflandusewithcoastalwaterquality.MaterialsandmethodsStudyareaGeography,geology,structureThegroundwatercatchmentofBellHarbour(Fig.1)—aspre-viouslyadoptedbyMcCormacketal.(2017);Schuleretal.(2018)—islocatedinthenorth-easternpartoftheBurrenLimestonePlateauinthewestoftheRepublicofIreland.TheBurren,includingtheuplandcatchmentofBellHarbour,isatemperateglaciokarstlandscape,whichhasbeensubjecttorepeatedglaciationduringthePleistocene,showingfeaturestypicalofglaciationsuchasice-pluckedcrags,scouredrocksurfaces,limestonepavementsanderraticboul-ders(Simms2014).Themeanannualairtemperatureis13.6°Candtheannualrainfallwas1,386and1,560mmforthehydrologicalyears2017and2018,respectively,asmea-suredattheweatherstationC1(Fig.1).Multiplevalleysin-tersectthestudyareathatrangeinelevationbetweensealevelinthenorthandupto340mabovesealevel(masl)alongtheescarpments.Alongtheescarpmentsthebareoutcropshowshighdegreesofkarstification.TheentireareaisunderlainbyLowerCarboniferouswell-beddedandpurelimestonesrangingbetweentheTubberforma-tion(earlyViséan),theLowerBurrenandUpperBurrenforma-tion(mid-Viséan,Asbian),andtheSlievenaglashaFormation(lateViséan,Brigantian).Thepresenceofirregularlimestonesurfacesisinterpretedaspalaeokarst(Prachtetal.2015).Thetotalthicknessoflimestonereachesatleast510m.Thestratadipgenerallytothesouth,rangingbetween2and3°.ThedeformationofCarboniferousrocksinIrelandislarge-lyattributedtotheVariscanorogenyandtheassociatednorth/northwest-orientedcompression(Graham2009).TheVariscancontractionaldeformationcausedtheformationofcalciteveinsofafewmicronsto0.5mwidth,whicharelaterallyandverticallyconsistentacrossbeddingdiscontinu-ities(Gillespieetal.2001).Asuiteofveinswithmoreexoticmineralisation,includingfluorite,quartzandoccasionalsulphides,occursina~10-kmwidenorth–southzonecentrednearCarran(seeFig.1).Thesegenerallywiderveinsrangefrom0.05to1m,andarepartofastructurewhichextends20kmacrossallstratigraphicse-quencesoftheBurren(J.Walshetal.,UniversityCollegeDublin,unpublishedpaper,2019).Theveinsarevisibleinoutcrop,andhavebeenobservedatmorethan100mdepth,forexamplealongtheverticallyorientedcavePollGonzo(Bunce2010).Thehorizontalpersistencyofveinsalongstrikesspansover7km,interconnectingcaves(MacSharry2006).AnotherexampleistherecentlydiscoveredsinkholeDeelinPot,whichisa18mdeepand5-m-wideexcavated HydrogeolJ(2020)28:561– 577 562

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Bell HarbourBallyvaughanKinvaraCarranKillnaboyAughinishBostonC1BHBBH1SiGD1FR25015010050150100200100150100 00 150200100505050502001005025020015015030025015010015015010015015025020020025020020010050200150200502002001501501002001501501001501001001001005050505050 050505 100150150150150150100100100100150150BallyvaughanEastBallyvaughanWestCorranrooWestKinvaraEastKinvaraWestPouldoodyToberbreenDeelinPotPollGonzo-25-15-15-5-5-5-5-5-5-5-5-5-5-5-5 523000 526000 529000 532000 535000 538000 692000 696000 700000 704000 708000 712000 716000 Atlantic Ocean / Galway BayBell Harbour Bay LegendGW catchment Bell Harbour Catchment boundaries Bell Harbour Bay outletSampling sites")Weather station C1")Open borehole BH1#*Intertidal spring SiGD1")Buoy sampler BHB!(Fergus River FR 50 m contourTerrestrial springSubmarine spring#Karst feature Fergus River Minor stream Fault Mineral vein GSI recovered tracer test ShoreGeological formations Tubber Form. Lower Burren Form. Upper Burren Form. Slievenaglasha Form. Namurian sandstoneBathymetry [masl] 10 m bathymetry 0 -25.7 Republic of IrelandUK (Northern Ireland)Atlantic OceanStudy area(main map) Fig.1SiteareaofBellHarbourinthewestofIreland,CountyClare:topography,geology(MacDermotetal.2003),structure,terrestrial(GSI1999)andsubmarine(O’Connelletal.2018)springs,successfulgroundwatertraces(GSI2017),andsamplinglocationsoftheprojectusedinthisstudy HydrogeolJ(2020)28:561– 577 563

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shaftorientedalongnorth–southorientedcalciteveinscon-tainingalsosilicaveining,previouslyfilledwithglacialdebris(BunceandDrew2017).Furthermore,slickensideveinswererecordedatseveraldepthsina490-mdeepboreholedrilledbytheGeologicalSurveyIreland(GSI)atlocationBH1(Fig.1).Theveinsmaybedissolvedoutorstillintact.Whereintact,thecontactbetweenlimestonehostrockandthe(calcite)veinmaterialmayactasaninceptionhorizon(LoweandWaters2014)andpathwayofpreferentialdissolutionofthelimestonerock.Thepresenceofsulphidemineralsinfillprovidesasourceofsulphurfortheformationofsulphuricacidstoin-creasetherateofdissolution.Therefore,veinsmustbecon-sideredimportantforregionalgroundwaterflow,astheirlat-eralandverticalextentisbelievedtohavelargelycontributedtonorth-southgroundwaterflowpaths(MooreandWalsh2013)and(J.Walshetal.,UniversityCollegeDublin,unpub-lishedpaper,2019).Thejointspatternisveryprominent,anditisknowntoextendseveralkilometresoff-shore,atleastto80mbelowsealevel(mbsl;KozichandSautter2018).OffshoresinkholesareknowntoexistwithinBellHarbourBay.Theuseofmarineelectricalresistivitytomography(ERT)confirmedthatthesesinkholesarepartlyhydraulicallyactivated,hence,actingassubmarinesprings.Thesesinkholesaregenerallyfilledwithsediments,yet,therearesingleexam-plesthatindicatetheabsenceofasedimentinfill(O’Connelletal.2018).ThegeologyandstructureofthetelogenetickarstBurrenPlateauextendsseveralkilometresoff-shorewestoftheBurren(GillespieandSautter2018),andthereforesink-holesmayaccompanythesestructures,yet,totheknowledgeoftheauthors,nodetailedstudieshaveyetconfirmedthis.GroundwaterflowandprevioustracertestsApproximately60%oftheBurrenPlateaudrainsdirectlytothesea,andsignificantquantitiesofgroundwaterdischargeoccursalongtheshoreviaintertidalsprings(Drew2018).Theintertidalspringsthatarerelevanttothisstudywerepre-viouslytracedsuccessfully(Table1)andareillustratedinFig.1.Alltracertestswerecarriedoutqualitatively;hence,theaverageflowvelocitiesareapproximatedandrangebetween21and460m/h.FortheBellHarbourcatchment,onlyonesuccessfultracertestwascarriedoutoveradistanceof2.2km.KnownlocationsofSGDareeitherinconjunctionwithintertidalspringsalongthewesternshoreoftheBurrenPlateauorlimitedtoBellHarbourandKinvaraBay(Drew2003;O’Connelletal.2018)withestimateddischargeratesrangingbetween0and4.3m3/s,and5–16m3/srespectively(McCormacketal.2014;Schuleretal.2018).Onalargerscale,significantareasofsea-surface-temperatureanomaliesinterpretedasSGDweredetectedonthewesternsideoftheBurrenPlateaulinkedtostructuralgeology(WilsonandRocha2012).Duetothescaleofthesetemperatureanomalies,therateofSGDmayexceedtheabovementionednumbers.WithintheBellHarbourcatchment,thegroundwaterflowisaconduit-dominatedupdipfromsouthtonorth(McCormacketal.2017).DischargeofthestudyareaoccursviaSiGD,includingtheintertidalspringPouldoody,intoBellHarbourBay.Ashallownorth–southconduitof~2mdiame-terisarguedtolinkshallowflowwithinthecatchmentandspringsinBellHarbourBay(McCormacketal.2017;Fig.1).TheSiGDregimefluctuatesseasonallycorrespondingtotheoverallpiezometricstateoftheaquiferdrivinganoverflowmechanism.Previoustracertestssuggestthatgroundwaterflowvelocitiesmayreachupto460m/h(Table1).Verticalgroundwaterflowwasmeasuredupto176mbslinBH1reachingupto97m/h.ThisdeepflowwashypothesisedtodrainthecoastalaquiferasSGDintoGalwayBayasanaddi-tionaloutlettotheSiGDintoBellHarbourBay(Schuleretal.2018),withonegoalofthisstudytoprovideconfirmationbyartificialtracertests.Thehighdegreeofkarstificationlimitssurface-waterfea-turestoshortreachesofephemeralstreams(Drew1990).SouthofthestudyareaandtheBurrenPlateau,theFergusRiverdrainslargeareasfromthesouthernpartoftheBurren.Rechargeinthestudyareaoriginatesfromrainfall,percolatingrapidlytowardsthephreaticzonebasedonthe Table1Summaryofsuccessfultracertestspreviouslycarriedouttoinvestigateintertidalgroundwaterdischarge;sourceofdata:Croninetal.(1999)andGSI(2017).NotethatfluorescentdyesweresampledqualitativelyusingactivatedcharcoalforuranineandrhodamineWT,andcottonwoolforleucophorIntertidalspringNo.oftestsDate/yearTracerusedRangeofflowvelocitiesBallyvaughanWest51999Leucophor,RhodamineWT,Uranine21–150m/hBallyvaughanEast51999Leucophor,RhodamineWT,Uranine21–150m/hBellHarbour12003Uranine–CorranrooWest221Nov1996Bacteriophages(Psf2andH40)111–126m/hToberbreen11Oct1996Leucophor13–26m/hKinvaraWestandCentral61and18Oct1996;21Nov1996;Bacteriophages(H4,H40,T7),Leucophor,RhodamineWT,Uranine47–460m/hKinvaraEast5Jul,AugandSep1990;18Oct1996;21Nov1996;4Dec1996Bacteriophages(Psf2),Leucophor,RhodamineWT150–460m/h HydrogeolJ(2020)28:561– 577 564

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responseofboreholehydrographs(Schuleretal.2018).Rechargeisassumedtogetconcentratedalongpreferentialflowpaths,asforexamplewithinPollGonzo,whereanun-dergroundstreamrapidlyfalls40muntilreachingtheperma-nentlocalwatertableat~31masl(Bunce2010).Twotracerstudieswereconducted,in2011and2015,using5kgofuranineand4kgofrhodamineWT,respectively,totraceflowpathsfromPollGonzo,butwithoutsuccess.InadditiontothetracertestsillustratedinFig.1,11tracertestshavebeencar-riedoutintheuplandsoftheBurrenPlateau.Fourofthesetestsinvestigatedahydraulicconnectiontotheshore,yet,noneprovedconfirmatory(BunceandDrew2017).Relativelyfast,concentratedandmainlydeepgroundwaterflowmayhavebypassedanytracerfromshallowsamplinglocations.Furthermore,itisassumedthatlargedissolutionfeaturessuchasDeelinPotmustbewellconnectedtosuchdeepconduitnetwork.HydroclimaticsamplinganddataWithinthestudyarea,differentsamplingsitesweresetuptorecordhydroclimaticdata.AfullweatherstationatC1(38masl,Fig.1)recordsrainfallin15-minintervalsusingaCELtipping-bucketraingauge(Casella,Bedford,UK)attachedtoaRainloggerModel3002(SolinstCanadaLtd.,Ontario,Canada)datalogger,windspeedin15-minintervalsusinganultrasonicwindsensor(GillInstrumentsLtd)attachedtoaCR800datalogger(CampbellScientificLtd.,Shepshed,UK),andatmo-sphericpressurein1-hintervalsusinganINWPT2Xsensor(Seametrics,Kent,WA,USA).ThewaterdepthoftheFergusRiveratFR(~23masl,Fig.1)wasmonitoredat1-hintervalsusingaMini-Diver502(SchlumbergerWaterServices,BritishColombia,Canada).Astage-dischargecurvewasestablishedthrough13spotdischargemeasurementsusinganOTTacoustic-digital-currentmeter(OTTHydrometGmbH,Kempten,Germany).Temperature,levelandelectricalconduc-tivity(temperaturecompensated)ofintertidalgroundwaterdis-chargeatPouldoodyspringatSiGD1(0.1masl,Fig.1)weremeasuredin1-hintervalsusingaSolinstLTCsensor(SolinstCanadaLtd.,Canada).TheLTCsampleratSiGDwasfixedonaconcreteplatformplacedintothenear-shoresediments1-2mawayand~1mbelowthespringoutlet.AsecondSolinstLTCwasusedtomonitortemperature,levelandelectricalconductiv-ity(temperaturecompensated)intheseaonboardofavessel.Bathymetrydatausedinthisstudycomprisea5-and10-mgridprovidedbytheGSI.LandtopographyisbasedontheShuttleRadarTopographyMission(SRTM;USGS2014).RemotesensingSatelliteimagerywasusedtoidentifyoffshoresamplingareasbasedonexpectedtemperaturedifferencesbetweentheseaandemergingSGD(Roxburgh1985;Schubertetal.2014).WatersurfacetemperatureanomalieswereidentifiedusingLandsat-8OperationalLandImager(OLI)andThermalInfraredSensor(TIRS)imageswith<20%cloudcoverfromtheUSGeologicalSurvey.Landsat-8isanopticalsensorwithaspatialresolutionof15m(band8),30m(band1–7and9)and100m(band10–11,TIRS).Surfacetemperaturegridsin°CweregeneratedinthreestepsinArcMap(version10.1,ESRI,Redlands,USA)usingatoolboxforautomatedmapping(Walawenderetal.2012):(1)conversionofthethermalinfraredbandintobrightnesstemperature(temperatureofablackbodyinthermalequilib-riumwithitssurroundings);(2)computationofthelandsur-faceemissivityvianormaliseddifferencedvegetationindex(NDVI)usingtheredbandandthenearinfraredband;and(3)calculationofthelandsurfacetemperatureusingthepre-viousoutputsalongwithatmosphericcorrectionparameterstoaccountforatmospherictransmissivity,up-wellingatmo-sphericradiance,anddown-wellingatmosphericradiance.Atmosphericcorrectionparametersweregeneratedusingtheweb-basedAtmosphericCorrectionParameterCalculator(Barsietal.2003).FourLandsatimageswerechosenasmostsuitabletoestimatetheseasurfacetemperaturesdating2January2017,8April2017,11July2013and24November2016.OnlyFig.2bincludedanyvisibledistortionbycloudcoveroverthewatersurface.TemperaturegridswerevisualisedinArcMapusingmulti-colourstretchedpatternstofacilitatethevisualidentificationoflocalisedtemperatureanomalies.ArtificialtracertestsSoluteandsolidtracersTwotypesoftracerswereusedforthisstudy:(1)floatingbiodegradablewoodchips,and(2)fluorescentdyesintheformoftheconservativetraceruranine(AcidYellow73)andthenonconservativetracerrhodamineWT(AcidRed;Fieldetal.1995;Leibundgutetal.2009).Samplingoffluo-rescentdyesfollowstheprincipleofexcitationofawatersampleandmeasuringtheensuingemissionintensity.ThefluorescenceofadyetracerispH-dependent,andnotconsid-eredtobeafactorinthisenvironmentwherethepHisap-proximately7.Solubilityis300g/Lforuraninecomparedto3-20g/LforrhodamineWT(Leibundgutetal.2009).Excitationandemissionmaximaforuranineare491and516nmrespectively,and561and586nmforrhodamineWT.Thesorptionbehaviouroftracersisparticularlyimportantwithregardtothequantitativeanalysis.Ingeneral,thehigherthesolubilityinwater,thelowerthesorptioncapacityofthesubstance.RhodamineWTshowsastrongsorptionbehaviour,whileuraninehasaverylowsorptiontendency(Field2002)whichappliestothegivenenvironment. HydrogeolJ(2020)28:561– 577 565

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GiventheuncertaintyofSGDlocationsinthesea,thesuitabilityoffloatingwoodchipsasanaturalandbiodegrad-abletracerwastested.Thereasoningforusingwoodchipswastomitigatetheproblemofspatialintegration(dispersion)relatedtosolutetracers:thereisnodetectionlimitasthewoodchipsaremeasuredbyvisualobservation.Hence,asingledetectedfloatingwoodchipmaybesufficienttocon-firmthesuccessofthetracertest.Theidealsolidtracerwouldhaveadensitybetweenfreshwater(1kg/m3)andseawater(~1.035kg/m3)inordertobeneutrallybuoyantwithinthefreshwaterundergroundriverandconduitsoftheaquiferbe-forebeingdischargedintotheseawhereitwouldfloatuptotheseasurfaceandtherebybevisuallydetectable.Thebuoyancyoffivedifferenttypesofwoodwastestedinastirringfreshwatertankovertheperiodof10days,rangingindensityfrom660to960kg/m3(TRADA2018).Thechipstestedwereofthesize4×40×50mm.ThebestperformanceintermsofneutralbuoyancyinfreshwaterwasachievedbyKeruing(Dipterocarpusspp.).Whensalinitywasincreased,thebuoyancyincreased.Atotalof250-LofKeruingchipsweremanuallyproduced,soakedinfreshwaterandpackedin10×30Lcavingbags.Theconcentrationofwoodchips>2mmgrainsizewascountedas>1,000partsper50ml—ergo,thetotalamountofchips(>2mm)forthevolumeof250lwasestimatedat5.71million.Thegeophysicalanalysisofthesubmarinesinkholescon-sideredaspotentialSGDlocations,showboththepresenceandabsenceofasedimentinfill(O’Connelletal.2018).Suchsedimentsmightplayaroleinblockingorattenuatingthesolidwood,whichneedstobeconsideredintheinterpretationofresults.InjectionsThetwofluorescentdyesandwoodchipswereinjectedatthreedifferentsiteson14April2018:twoinjectionsiteslo-catedwithinthecavePollGonzowiththeentrance7.5kmsouthofBellHarbourBayandtheopenlyaccessiblesinkholeDeelinPot5.9kmsouthofBellHarbourBay.AccesstoPollGonzorequiredateamofsixcaverstoabseilthreepitchesof~20meachonfixedropes.Theuraninewasdissolvedin-situwithina100-Linflatableswimmingpoolandreleasedintotheundergroundstreamupstreamofawaterfallat~70masland56mbelowgroundlevel(Table2,site1a).The250Lofwoodchipswerereleasedatthebottomofthecaveat~31maslatthelocalwatertable,wherethree !.!.!.AughinishBallyvaughanBell Harbour 522500 525000 527500 530000 532500 535000 710000 712500 715000 717500 08 Apr 2017 !.!.!.AughinishBallyvaughanBell Harbour 522500 525000 527500 530000 532500 535000 710000 712500 715000 717500 24 Nov 2016Cloud cover T anomalies ShoreºCHigh : 21.7Low : 11.4 !.!.!.AughinishBallyvaughanBell Harbour 522500 525000 527500 530000 532500 535000 710000 712500 715000 717500 02 Jan 2017 T anomalies ShoreºC High : 10.3 Low : 2.1 T anomalies ShoreºC High : 12.5 Low : 2.3d)a)b) !.!.!.AughinishBallyvaughanBell Harbour 522500 525000 527500 530000 532500 535000 710000 712500 715000 717500 11Jul 2013 T anomalies ShoreºC High : 31.5 Low : 17.7c)LandLandLandLandInflowfrom Kinvara Bay? Surface water inflow? Page 28 of 38 Fig.2SeasurfacetemperaturesderivedfromLandsat8OLI,indicatinglocalisedtemperatureanomaliesindicativeforsubmarinegroundwaterdischarge(SGD)orderedbymonthoftheyearona2January2017,b8April2017,c11July2013andd24November2016.Notethattemperatureisscaleddifferentlyineachscene HydrogeolJ(2020)28:561– 577 566

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experiencedspeleologistsabseileddownanotherpitchwithdifficultaccess(Table2,site1b).RhodamineWTwasinjectedintothebottomofDeelinPot1.67kmnorth-northwestofPollGonzo,wherea1m3carbon-fibretankwithareleasevalveatitsbottomwasinstalledtomixthedyein-situ(Table2,site2).Thetotalvolumeofwaterusedfordilutionandflushingwas2.7m3.MonitoringofsoluteandsolidtracersSamplingforthetracersatpotentialoutletswasconductedatterrestriallocations,aswellasoffshorealongtransectstoac-countforthenumerouspotentialSGDlocationsinGalwayBay.Theterrestrialoutletsweresampledusingafluorometerand/oractivatedcharcoalkeptinplacefortheperiodofobser-vation.Detectionoffluorescenttracerswasdonequalitativelyusingpermeablenylonbagsofactivatedcharcoalpellets(technicalgrade,AppliChemPanreac).Eachbagwasfilledwith~20-gpellets.Foroff-shoresampling,GGUN-FL30fieldfluorometers(AlbilliaCo.,Switzerland)wereusedto(semi-)quantitativelyanalysethefluorescence.GGUN-FLfieldfluorometerscanmeasurethreedistinctdyesinparallelaswellasturbidityatasamplinginterval10s.ReadingsinmVareconvertedintoppbusingalinearcalibrationtothreetracerstandards(1,10and100ppb)foreachtracermadeusinglocalseawateranddyefromthebatchinjected;hence,backgroundfluorescenceintheseaisbroadlyaccountedfor.Theminimumdetectionlimitforuranineinclearwateris0.02ppb,and0.2ppbforrhodamineWT(Schnegg2002).Visualmonitoringforwoodchipswasconductedoffshorefromonboardofthevessel.Stationarysamplingwasconductedat10differentsites(Table3;Fig.3).AfieldfluorometerwasinstalledattheFergusRivertoassesstheunlikelyscenarioofsouthwardstransport.AsecondfieldfluorometerwasinstalledatPouldoodyspring,alongwithaconductivity-temperature-depth(CTD)sensortorecordthetidalfluctuation,temperatureandconductivityasindicatorsfortheoccurrenceofSiGDatPouldoodyspring.Qualitativesamplingmethodsusingactivatedcharcoalbagsvariedbysite.Withintheopenborehole(BH1),fourbagsweretiedona200-m-longbottom-weightedropehungintheboreholetosampledepthsrangingfrom15to170masl.Thesixknownintertidalcoastalspringsweremonitoredwithindividualbagstiedtoconcreteslabsattheintertidaloutlets:BallyvaughanWestandEast,CorranrooWest,ToberbreenandKinvaraWestandEast.Furthermore,twocharcoalbagswereattachedtoafloatingbuoyanchoredintheseabedinthecentreoftheoutletofBellHarbourBay(Fig.1),samplingthetopseawatercolumn.ThehightidalrangelargelyemptiesBellHarbourBayduringebbtide,suchthatfluorescencemayhavebeendetectedatthatsite.Thecharcoalbagswerecollectedon19Aprilandthere-placementsetcollectedon25April.Mobilesamplingofseasurfacefluorescenceandconduc-tivityovertransectswasconductedover21honfoursucces-sivedaysfromavesselintheseatargetingtheareasoftem-peratureanomaliespreviouslyidentifiedbyremotesensing.Samplingfocusedonthecentralandwesternarea,asthetem-peratureanomalieswereconsideredtobemoreexplicitthanthoseoftheeast,whichmaybeduetofreshwaterdilutionsfromKinvaraBay/surfacewater.Basedonestimatedtraveltimes(Table1),samplingstarted36hafterinjectionandlastedfor4days.Thefluorometerwastiedonropespulledbythevessel,whilealsocabledtothedataloggerandacomputeronboard.Theropeswereweightedtomakesurethefluorometerdidnotfloat,samplingbetween0.03and5.77m(average1.01m)belowtheseasurface.Inaddition,thespeedofthevesselwasveryslowat3–5km/htoreduceperturbationanderroneousreadingsduetoairbubblesfromthepropeller.ACTDsensorwasattachedclosetothefluorometertosamplefordepth,temperature,andconductivityasindicatorforSGD.Ahand-heldglobalpositioningsystem(GPS;GPSmap60CSx,Garmin,KS,USA)wassynchronisedwiththefluorometerandCTDsensor,recordingthegeographicpositionofthevesselin3-sintervals.AnalysisofsoluteandsolidtracerresultsCharcoalbagswerecollectedon19and25April2018,5and11daysaftertheinjections,respectively.Allsamplebagswereindividuallysealedandkeptprotectedfromsunlightinaclosedcontainer.Thecharcoalsampleswere Table2Injectionsitecoordinates,elevation,tracermassandtype,injectiontimeon14April2018andflushingvolumeInjectionsiteCoordinatesElevation[masl]TracerandmassInjectiontimeFlushingvolumeSite1aPollGonzowaterfall53.055°;9.073°~70Uranine,25kg13:00~30L/sSite1bPollGonzobottomsump53.055°;9.073°~30Woodchips,250L12:00~30L/sSite2DeelinPot53.070°;9.080°~84RhodamineWT,25kg17:002.7m3 HydrogeolJ(2020)28:561– 577 567

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analysedwithindaysfollowingcollection.TheSmartSolutionwasusedforrhodamineWTconsistingof1-propanol,deionizedwaterand30%NH4OHintheratio5:3:2(SmartandSimpson2002).Foruranine,theeluantwasamixtureof70%isopropylalcohol,30%highpuritydeionizedwaterand10g/Lsodiumhydroxide.Theanal-ysiswascarriedoutusingaCaryEclipseFluorescenceSpectrophotometer(AgilentTechnologies,SantaClara,CA,USA)basedintheenvironmentallaboratoryoftheGSIinDublin,Ireland.Thesampleswereanalysedinsequence;hence,theelutiontimeofacharcoalsamplerrangedfromatleastone,toamaximumof2h.Incontrasttotherathercleancontinuoussignalachiev-ablewithterrestrialstationarymonitoring,samplingintheseaisimpactedbyperturbationthroughcurrents,dragoftheboat,andairbubblescausingpotentiallyerroneousreadings.Hence,potentiallyerroneousreadingswerefil-teredfollowingdataacquisition.ThefiltercomparestheratioofobservedmVsignalsofgiventracerswiththeratioofmVsignalsofthesametracersasestablishedduringthecalibrationprocedureusingthevaluesofthecorrespondingphotodetectorswhichwereL1andL2forthetwotracersrhodamineanduranine,onlytheratiosofL1andL2wererecordedintherawfiles.First,theratioofmeasuredmVsignalsforatracerTnofinterestisestablishedusingthemeasuredsignalsofL1duringcalibrationL1cal[mV]andL2duringcalibrationL2cal[mV].Thecalibrationratior0isthengivenbyEq.(1),r0;Tn¼ L2cal;TnL2cal;WL1cal;TnL1cal;Wð1ÞwiththeassociatedsignalforpurewaterW[mV]andthetracerwithn=1foruranineandn=2forrhodamine.Theresultisacharacteristicratioforeachofthethreetracersthatcanbemonitored,correctedbysubtractingthebackgroundsignalofwater.Theactualobservedratiobetweenphotodetector1L1obs[mV]andphotodetector2L2obs[mV]attimesteptisestablishedtoyieldrtfollowingEq.(2)rt¼ L2obsL2cal;WL1obsL1cal;Wð2Þagain,correctedforthebackgroundvalueofwater.Ideally,rt¼r0;Tnbutthisisneverexactlythecase.SinceL2ismeasured100msafterL1,themeasurementisrejectedifalargedeviationofrtfromr0;Tnisobserved,indicatingtheoccur-renceofairbubblesduringthemeasurementofL1orL2.Here,anobservationwasconsideredreliableifitsrtrangesbetweenthelowerandupperlimitoftheratiosofthetwotracersconsidered,whicharer1=0.035foruranineandr2=2.597forrhodamine.Inaddition,athresholddeviationof±20%wasdefinedasaccept-ablesothat0:8r0;T1
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C1BHBBH1SiGD1FRBell HarbourBallyvaughanKinvaraCarranKillnaboyAughinishBoston250250150300100150100150100200150150200150505050250200150100200250200150100200250150150150100150150200200250200100200100200150100200501501501501002002001505050505050505050505050505050100100100100150100150150100100100100150150100150150100150100100BallyvaughanEastBallyvaughanWestCorranrooWestKinvaraEastKinvaraWestPouldoodyToberbreenDeelinPotPollGonzocharcoalcharcoalcharcoalcharcoalcharcoalcharcoalcharcoalfluofluocharcoal-25-15-5-5-5-5-5-15-5-15-5-5-5-5-5-5 520000 523000 526000 529000 532000 535000 538000 692000 696000 700000 704000 708000 712000 716000 LegendGW catchment Bell Harbour Catchment boundaries Bell Harbour Bay outletSampling sites")Weather station C1")Open borehole BH1#*Intertidal spring SiGD1")Buoy sampler BHB!(Fergus River FR 50 m contourTerrestrial springSubmarine spring#Karst feature Fergus River Fault Mineral vein Shore T anomalyOffshore monitoring East West TotalLinear trace line Rhodamine UranineGeological formations Tubber Form. Lower Burren Form. Upper Burren Form. Slievenaglasha Form. Namurian sandstoneBathymetry [masl] 10 m bathymetry 0 -25.7 Atlantic Ocean / Galway BayBell Harbour BayAtlantic Ocean Fig.3Overviewofthetracerstudyincludingstationaryonshoremonitoringofintertidalsprings,theoutletofBellHarbourBay(BHB),theFergusRiver(FR)andtheborehole(BH1)usingfieldfluorometersand/oractivatedcharcoalsamplers,aswellasmobileoffshoremonitoringtransectsacrossareasoftemperatureanomaliesusingafieldfluorometerandCTDsensor HydrogeolJ(2020)28:561– 577 569

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Table4Meanandstandarddeviation(SD)oftemperatures(T)intheeasternpartofGalwayBayandwithintemperatureanomalypolygonsin°C,aswellastheirdifference,orderedbymonthoftheyearImagedateTGalwayBay[°C]Tanomalies[°C]Difference[°C]MeanSDMeanSDMeanSD2January20177.50.895.00.372.50.528April201712.60.6912.00.200.60.4911July201320.71.1019.20.221.500.8824November201610.70.808.800.231.90.57 HydrogeolJ(2020)28:561– 577 570 Becauseofitsnature,themethodiscalled‘quotientmeth-od’.Thisapproachwasconsideredtobeveryconservativeinordertoreliablydefinethevalidityofasingletracerreadingfollowingatransparentfilteringsequence.ResultsRemotesensingThecalculatedtemperaturesaredisplayedinFig.2a–d.Theaimistohighlighttherelativetemperaturedifferenceasthisistheparameterofinterestforthisanalysis.ThewatersurfaceofGalwayBayshowsarelativelyconsistenttemperature(exceptFig.2).LocalisedcoldtemperatureanomalieswereinterpretedaspotentialinfluenceofSGD,whichchangeinpositionoverthe4monthsoftheyearshownintheimages.ThismaybecausedbythepresenceofmultipleSGDlocations,currentsanddifferentmixingdynamicsintheseaovertheyear,andbetweenyears.Anotheraspecttobeconsiderediswaterinflowfromtheshore:Fig.2ashowstwolargetemperatureanomaliesintheeast,whicharelikelyrelatedtocooldischargefromKinvaraBay(Gilletal.2013;Schubertetal.2015)andsurface-waterinflow.Table4givesanoverviewaboutthemeanandstandarddeviationsofsurfacetemperaturesofeast-ernGalwayBay(extentofthemapinFig.2)andidentifiedtemperatureanomalies,aswellastheirdifferences.Thesetem-peratureanomalieswereconsideredtobepotentiallyrelatedtoSGD.TemperatureanomaliesdifferthemostbetweentheJanuary2017andNovember2016images;therefore,theseareaswereconsideredasmostrelevantforsampling.ArtificialtracertestsFigure3outlinestheconceivedpathwaysbetweenthetwoinjectionsitesDeelinPotandPollGonzoandpotentialSGDoutletsinGalwayBay,aswellasthestationaryterrestrialmonitoringsitesatintertidalsprings,aboreholeandtheFergusRiverusingactivatedcharcoalsamplersorfieldfluo-rometers.ThelineardistancesbetweeninjectionsitesandsamplinglocationsareinTable3.Basedonthetemperatureanomaliesidentifiedinsection‘Remotesensing’,transectsweredrawnthatwereusedasreferencetonavigatethevesseliterativelythroughouttheareasofpotentialSGD.Sincethetemperatureanomaliesinthewesternpartweremorepronounced,samplingfocussedintheseareasbydefiningawesternmonitoringtransect.Stationarysampling(fluorometersandcharcoalbags)Noneoftheemissionspectraoftheeluantfromcharcoalsam-plescollectedon19and25Aprilshowemissionpeaksof516nmforuranine(Fig.4a,b),noranemissionpeakof586nmforrhodamine(Fig.4c,d)samples.Allcharcoalsam-plesareinterpretedtobefreeofanyofthetwodyes.Thetimeseriesfluorometry(Fig.5a)showstheresultsobtainedfromPouldoodyspring,alongwithwaterlevelfluctuationsmeasuredatthemonitoringsite.Theobvioustwice-diurnalwater-levelfluctuationisafunctionofthetidalcycles.Further,thefluctuationofconductivityindi-catesactivedischargeofthespringwithnotableminimumvaluesofupto2.9mS/cmduringlowwaterlevelsstartingon17April.Groundwaterdischargeisatmaxi-mumwhentheobservedECisatitsminimumasaresultofthelowwaterlevel(ebbtide).Atsuchtimes,there-cordedconcentrationsofrhodamineanduraninepeak<0.3ppbandslightlyabove1ppb,respectively.Theim-pactofthetidalfluctuationonthereadingsofbothdyesisbelievedtobetheresultofchangingchemistryandcom-positionofthewateratthesite,probablydissolvedorgan-icmatter(DOM).Asaconsequenceoftheselowconcen-trations,theresultsareinterpretedasnegative.Figure5bshowstheresultofthefieldfluorometerinstalledattheFergusRivertothesouthofthetracerinjectionpoints.Theriverflowexhibitsaclearresponsetothetwoprecedingrainfalleventsof11.6mm(15April3:00to16April1:00)and14mm(16April14:00to17April4:00).Asaresult,dis-chargeincreasesfrom1.6to12.8m3/salongwithconcurrentincreasesinturbidity.Uranineandrhodamineshowbothacorrelationwithturbiditypeakingat2and3ppbrespectively.Correlationbetweenfluorescentdyesandturbiditywithinthisrangeiscommon(Schnegg2002).Inaddition,measuredfluo-rescenceexhibitsanegativecorrelationwiththetemperatureinthestream;hence,altogether,theobserveddyeconcentra-tionsareinterpretedasamixtureofnaturalbackgroundand/oranthropogenicsources.

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100101102Intensity [a.u.]Ballyvaughan EastBallyvaughan WestBH1-1BH1-2BH1-3BH1-4BHBCorranrooKinvaraEastKinvaraWestToberbreenpositive result100101102Intensity [a.u.]580600620640660680700Wavelength [nm]100101102520540560580600Wavelength [nm]100101102c)a)d)b) Fig.4Emissionspectraofactivatedcharcoalsamplesforuraninecollectedona19Aprilandb25Aprilandrhodaminecollectedonc19Aprilandd25April Fig.5aConcentrationofuranineandrhodamineinppb,turbidityinNTUandwaterlevelinmobservedattheintertidalspringPouldoodybetween14Apriland20April;andbreadingsofuranineandrhodamineinppb,turbidityinNTUanddischargeinm3/sobservedattheFergusRiverbetween14Apriland25April,aswellashourlyrainfallmeasuredatC1.Thegreen-andred-dashedverticallinesindicateinjectiontimesofuranineandrhodamine,respectively HydrogeolJ(2020)28:561– 577 571

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MobilesamplingFigure6a,bshowstheconcentrationsofuranineandrho-damineforthefiltereddatainsemi-logscalealongwithmeasuredconductivityinthesea,depthofsamplingandhourlyrainfall.Conductivityrecordsaremissingforday1(16April).Monitoringwaslimitedtotheeasterntransectonday1duetoveryharshweatherconditionsandaredweatherwarningforGalwayBayissuedbytheIrishme-teorologicalserviceMetEireann.Duringdays2–3,sam-plingfocussedtothecentreandthewesterntransect,forthereasonspreviouslydiscussed,andthatmoreelevatedtracerreadingswerenoticedinreal-timeinthecentreandthewestonday2.Therawdatashow4,998readingswithmaximumvaluesforrhodamineanduraninereaching208.6,457.8ppbrespectively(Fig.6b).Thequotientmethodfiltered12.7%potentiallyerroneousreadings,leaving4,365readingsor87.3%tobeconsidered(Fig.6b).Asaresult,themaximumconcentrationsofrhodamineanduranineconsideredreliableare29.3and50.7ppbrespectively.Bothmaximumdyeconcentrationswererecordedonsamplingday2(17April)whichcorrelatewiththelowestconductivity<5mS/cmvaluesalsorecordedonday2.Periodsoflowconductivityandhightracerreadingsarehighlightedinpinkrectangulardashedboxes(day2:15:05–15:20;day4:11:15–11:28).Importantly,thefluo-rometerandCTDsensorwereveryshallowduringthesetwoperiods(0.18,0.23m).Theaveragesamplingdepthsondays1,2and3are0.81,1.20and0.96m,respectively(averageof1.01m).Lowconductivitylevelsmaybelinkedtothetworainfalleventsbetween15Apriland17Aprilwithatotalof22.6mm.ItishypothesisedthattherapidrechargeincreasedSGDwiththeresultantlow-eringofconductivityintheseaattheselocations.Incom-parison,theconductivityonthe18and19ofApril,dur-ingwhichtherewasnorainfall,aremuchmorehomoge-neousandrelativelyconstant.Itisassumedthatwithoutrainfall,therateofSGDislower.Nocorrelationbetweenconductivityandtracerconcentrationoccurredonday3withtheconductivityreadingsremaininghighat50mS/cm,withexceptions.ThefactthathightracerreadingscorrelateintimeandspacewithlowECreadingssuggestconsistencyofthemethod.Overall,itwasassumedthatalargeamountofdilutionweakensandspatiallyintegratesthesignal,andthereforeaclearcorrelationcannotbeexpected.Inordertoevaluatethespatialpatternofrecordedvalues,thegeoreferencedandfilteredppbreadingsforrhodamineanduranineweremapped,alongwithmonitoredconduc-tivityoftheseawaterandthetotaltrackrecordedbytheGPS(Fig.7).Bothdyesareplottedforvalues>2ppbinordertonotoverloadthemapwithlow(andpresumablynotmeaningful)concentrations.Ingeneral,tracerwasrecoveredinarelativelylargegeographicalarea,mostlyinthreeclusters:cluster1inthewest,cluster2inthecentreandcluster3intheeast. Fig.6aHourlyrainfallandelectricalconductivity(EC)inmS/cmrecordedoffshore;bFilteredrecordsofconcentrationsofrhodamineanduranineinppbplottedwiththedepthoftheCTDsensorandfieldfluorometerinmbelowsurface.Theblackbarsonthetopshowthedurationofsamplingforeachday.Thepink-dashedrectangles*1and*2highlightshortperiodsoflowconductivityandhightracerreadings(day2:15:05–15:20;day4:11:15–11:28) HydrogeolJ(2020)28:561– 577 572

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Inallclusters,bothdyeswererecovered.Altogether,aspatialcorrelationbetweendyereadingsandconductivityoccurred.Cluster1containsrhodamineanduranineread-ingsofdays2–4.Thisclustercontainsthemaximumob-servationsforrhodamineanduraninereaching29.3and50.7ppbrespectively,associatedwithlowconductivityreadingsdownto3.4mS/cmonday2(Fig.6,*1)corre-spondingtotraveltimesof~160and165m/hforrhoda-mineanduranine.Also,Cluster2relatesmainlytothehighlycorrelatedlowconductivityandhightracerread-ingsobservedonDay2(Fig.6,*1)withassociatedtraveltimesof~>181and194m/hforrhodamineanduranine.ThesetwoareasareconsideredasSGD‘hotspots’.Thelongestdistancebetweeninjectionsitesandsamplinglocationsoccurredincluster3,whereuraninewassampled15.8kmawayfromPollGonzoandrhodamine14.6kmawayfromDeelinPot,whichcorrespondtotraveltimesof~331and339m/h.Nowoodchipswereobservedduringoffshoremoni-toring.Duringthefirst3daysofsampling,theseawasveryroughduetostrongwindswithmaximumaveragehourlywindspeedsof8–11m/s.Theseconditionsmadetheworkonboarddifficultandmonitoringofwoodchipsontheseasurfacechallenging.Onday4,theseabecameverycalm,butnowoodchipswereseenorsampled;therefore,theresultofthetracertestusingwoodchipsisconsideredasinconclusive/negative.DiscussionPotentialSGDareaswereidentifiedusingLandsat8OLIim-ages.TheresultingtemperatureanomalymapsshowdifferentareasofpotentialSGDintime.Thereasonforthedifferentspatiotemporalpatternisbelievedtobethemultipleoutletsanddifferentmixingpatternsinthesea,modifyingthesignalofSGDdispersionintheseaandassociatedtemperaturepat-terns(Breieretal.2005).Accordingly,theuseofdifferenttemperaturemapsprovedtobeofuseindesigningtheoff-shoresamplingstrategy.Samplingoffluorescenceintheseausingafieldfluorom-eterisachallenge.Differentfactors,particularlytheweatherconditions,createdlimitationsinsampling.Potentiallyerro-neousreadingswereobserved,mostlikelyrelatedtoairbub-blescausingperturbationproblems.Potentiallyerroneousdatawerefilteredoutusinganewdevelopedquotientmethod, Aughinish50150100505050Pouldoody-5-25--5-5-5-5-15-5-5-155220005240005260005280005300005320007100007120007140007160001)2)3)CatchmentboundariesBellHarbourBayoutlet50mcontourTerrestrialspringSubmarinespringTanomalyGeologicalformationsTubberForm.LowerBurrenForm.UpperBurrenForm.Bathymetry[masl]Uranine[ppb]>2-3>3-10>10-50.7Rhodamine[ppb]>2-3>3-10>10-29.3ECmS/cm3.4-25>25-35>35-45>45-51.4!Trackcovered0-25.7ShoreLegend10mbathymetry Fig.7Resultsofoffshoresamplingbetween16and19April2018:measuredandcorrectedtracerconcentrationsofrhodamineanduranine(>2ppb)alongwithmeasuredelectricalconductivity(EC).Thetrackofthevesselisplottedbutcoveredbyconductivityreadingswhereavailable.Previouslyidentifiedtemperatureanomaliesareplotted.Threeareas,showninhashedboxesnos.1,2and3,showconcentratedtracerreadings.Inareas1and2,tracerreadingscorrelateverywellwithconductivityreadings HydrogeolJ(2020)28:561– 577 573

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leaving87%oftheobservationpointsforanalysis.Thequo-tientmethodisconsideredtobeconservative,withpositiveresultsconsideredtobevalid.Inthefuturesuchproblemscouldbemitigatedbyusingabetter,bubble-freelocationofthefluorometer,e.g.underthevessel.Similarly,thisappliestotheconductivityreadings,whichmaybeimpactedbyairbub-bles;however,inthisstudy,theconductivityreadingsdidnotshowanyevidenceoferroneousreadings.Themajorityofreadingswereconsistentlycloseto50mS/cm,withlowerreadingsoccurringsystematicallyinsomelocations,andalsocorrelatingwithelevatedtracerreadingsondays2and4.Thecombinationofconsistentreadingsandcorrelationwithtracerreadingsintimeandspaceareinterpretedasastrongindicatorforthevalidityofthetracerandconductivitytimeseries.Noneofthecharcoalsamplesplacedattheintertidalspringsalongtheshore,theborehole(BH1)andtheoutletofBellHarbourBay(BHB)showedevidenceofadsorbeddye.Hence,theanalysesofthecharcoalsampleswereinterpretedasnegative,theresultsbeingconsistentwithprevioustracerstudiescarriedoutintheuplandsinthearea(BunceandDrew2017).Factorssuchastheperiodofelutionandthecomposi-tionofthesampledwater,includingthepresenceoforganics,generallyimpactontheintensityoftheanalysedspectra(SmartandSimpson2002;Wernli2011);hence,“watertrac-inginkarstisaninexactscience”(BunceandDrew2017).Ahydraulicconnectionbetweenthetwoinjectionsitesandthesamplingsitesfortheperiodofthetracertestcouldneitherbeconfirmednorrejected.Submarinegroundwaterdischarge(SGD)inGalwayBayislikelytooccurviasinkholesthatarefilledbysediments.Suchsedimentinfillmayhavetrappedthesolidwoodchips,providingoneexplanationforthenegativetraceusingwoodchips.Alternatively,thewoodchipscouldbetrappedinconduitsorcavitiesenroute,or,theymayhavebeendischargedatsea,buttheycouldnotbeobservedduetotheparticularlyroughweatherconditionsoverthefirst3days.Further,thetotalpartsofvisiblewoodchips(>2mm)is5.17million,whichisinsignificantlysmallcomparedtothetotalpartsoffluorescentdyesinjected(25kgofbothuranineandrhodamine)withrespecttotheirdetectionlevels<1ppb.Giventhesecircumstances,thechanceofdetectingwoodchipsvisuallyduringthe4daysofsamplingcanbeconsideredmuchlowerthandetectingthefluorescentdyesusingthefieldfluorometer.Besidesthenegativeresults,thetwofluorescentdyesweresuccessfullysampledinthesea.Singleobservation-basedtraveltimesrangebetween100and354m/h.Thesetimesareintheorderoftheestimatedtraveltimespreviouslycon-ductedinthearea(Table1).ThedepthoftheCTDsensorandfluorometerrangedfrom0.03to5.8mbelowthesurfaceduringtheseatransects,largelyimpactedbythespeedofthevessel.Itisnotablethatthehighestcorrelationbetweentracerreadingsandlowconductivitylevelswasrecordedatrelative-lyshallowdepths(0.18–0.23m).ThisindicatesthatfreshwaterSGDfloatsasalayerontheseasurface;therefore,forafutureoffshoremonitoringcampaign,itisrecommendedtofixthefluorometerandCTDsensorsatshallowdepthsof<0.25morlessbelowtheseasurface.Thesampledtracerintheseaonlypartlyoverlapswiththeareaspreviouslyidentifiedonthetemperatureanomalymaps.ThisobservationhighlightsthecomplexityofSGDstudieswithregardtotheexistenceofpotentiallymultipleoutletsandsubsequentmixinganddispersiondynamicsinthesea.Atthesametime,variablesthatinfluencethedynamicsofSGDsuchastides,wavesandcurrents(Gilletal.2013;Parraetal.2016)werenotconsideredinthedesignofthestudyandinterpretationoftheresults,buttheirunderstandingmayimprovetheunderstandingoftheSGDregimes.Giventhepositiveresultsintheseaandnegativeresultsalongtheshore,thestudysuggeststhatthetwodyesmayhavebypassedthesampledintertidalsprings,potentiallyatdeeperlevels.Suchflowpathswillhavedevelopedduringperiodsoflowersealevelstiedtoglobaleustaticsealevelvariations(Shennanetal.2018).Theoccurrenceofsignificantdeepgroundwatercirculationincludingabypassingeffectofinter-tidalspringshadbeenproposedbasedonlong-termmonitor-ing,quantificationofSiGD,waterbalancecalculationsandnumericalmodelling(BunceandDrew2017;Schuleretal.2018).Suchdeepflowishypothesisedtobelinkedtoprefer-entialflowpathsalongnorth/north–north–west-trendingVariscanveins(MooreandWalsh2013);hence,thisstudysupportstherelevanceofthesestructuralfeatureswithregardtoregionalgroundwaterflow.ConclusionSubmarinegroundwaterdischarge(SGD)isrecognizedasanimportantpathwayforcontaminanttransportintothecoastalenvironment;hence,itisofrelevanceinthecontextofcoastalkarstcatchments.AgoodunderstandingofSGDdynamicslinkedtothedrainageofcoastalkarstaquiferisthereforenecessary.Thisstudycombinesmethodsto:(1)locatepotentialareasofSGDusingfirstremotesensingto;(2)applyasetofdifferentartificialtracers,includingfluorescentdyesaswellasfloatingwoodchips,and(3)evaluatehydraulicconnectionsbetweenterrestrialinjectionpointsandoffshoresubmarinedischargelo-cations.ThemethodwassuccessfullyappliedinthestudyareaofthecoastalaquiferofBellHarbourinwesternIreland.Noneofthewoodchipswererecoveredfromthesea.Differentfactorsmayhavepreventedapositiveresult,includ-ingthepotentialsedimentinfillofsubmarinesprings,trappingthesesolidparticles.Bothofthefluorescentdyeswererecoveredinthesea,andupto15.8kmawayfromaninjectionpoint.Offshoresam-plingwasconductedintransectsintheseaoverfour HydrogeolJ(2020)28:561– 577 574

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HydrogeolJ(2020)28:561– 577 575 successivedaysonboardavessel.Theestimatedtraveltimesareintheorder160–339m/horless.Fluorescencepeakscorrelatewithlowerconductivityvaluesinthesea,indeedindicatingthedischargeoffresh/brackishgroundwater.TwomainareasofSGDwereidentified,whereasthespatialpatternofSGDisbelievedtoreflectmultiple-pointSGDlocations.Itishypothesisedthatthemainoutletsofthecatchmentarelocatedinclusters1and2;however,additionalstudiesareneededtoconfirmthisconclusion.Hydraulicconnectionswereestablishedbetweentwostruc-turalkarstfeaturesoftheuplandoftheBurrenPlateauandGalwayBay.Bothstructuralfeaturesincludemineralveins,supportingthehypothesizedrelevanceofVariscanveinswithregardtoregionalgroundwaterflowcontrol(MooreandWalsh2013;J.Walshetal.,UniversityCollegeDublin,unpublishedpaper,2019).AcknowledgementsWewishtoexpresssincerestthankstothecaversRalphDoyleandTimO’ConnellfortheirsupportincarryingoutthetracerinjectionsinPollGonzo,andtoJamesLinnaneforprovidinglandaccessandthenecessaryinfrastructureatDeelinPot.Wefurtherwishtothankthethree(anonymous)reviewersforexcellentandverydetailedcomments,whichtremendouslyimprovedthequalityofthispaper.FundinginformationThisresearchwaspartfundedbytheGeologicalSurveyIrelandResearchProgramme,grantnumber2017-SC-001,anditwasconductedwithintheIrishCentreforResearchinAppliedGeosciences(ICRAG)supportedinpartbyaresearchgrantfromScienceFoundationIreland(SFI)underGrantNumber13/RC/2092,co-fundedundertheEuropeanRegionalDevelopmentFundandbyICRAGindustrypartners.OpenAccessThisarticleislicensedunderaCreativeCommonsAttribution4.0InternationalLicense,whichpermitsuse,sharing,adap-tation,distributionandreproductioninanymediumorformat,aslongasyougiveappropriatecredittotheoriginalauthor(s)andthesource,pro-videalinktotheCreativeCommonslicence,andindicateifchangesweremade.Theimagesorotherthirdpartymaterialinthisarticleareincludedinthearticle’sCreativeCommonslicence,unlessindicatedotherwiseinacreditlinetothematerial.Ifmaterialisnotincludedinthearticle’sCreativeCommonslicenceandyourintendeduseisnotpermittedbystatutoryregulationorexceedsthepermitteduse,youwillneedtoobtainpermissiondirectlyfromthecopyrightholder.Toviewacopyofthislicence,visithttp://creativecommons.org/licenses/by/4.0/.ReferencesBarsiJA,BarkerJL,SchottJR(2003)AnAtmosphericCorrectionParameterCalculatorforaSingleThermalBandEarth-SensingInstrument.PaperpresentedattheGeoscienceandRemoteSensingSymposium,Toulouse,France,21–25July2003BenischkeR,GoldscheiderN,SmartC(2007)Tracertechniques.In:GoldscheiderN,DrewD(eds)MethodsinKarsthydrogeology.IAHInternationalContributionstoHydrogeology26,IAH,Wallingford,UK,pp147–170BreierJA,BreierCF,EdmondsHN(2005)Detectingsubmarineground-waterdischargewithsynopticsurveysofsedimentresistivity,radium,andsalinity.GeophysResLett32.https://doi.org/10.1029/2005GL024639BunceC(2010)PollGonzo.IrishSpeleol19:16–21BunceC,DrewD(2017)WatertracingintheCarrondepressionandRiverFergusValley,Burren,CoClare.IrishGroundwaterNewsletter.GeologicalSurveyIreland,Dublin55:13–18BurnettWC,DulaiovaH(2003)Estimatingthedynamicsofgroundwaterinputintothecoastalzoneviacontinuousradon-222measurements.JEnvironRadioact69:21–35.https://doi.org/10.1016/S0265-931X(03)00084-5BurnettWC,AggarwalPK,AureliA,BokuniewiczH,CableJE,CharetteMA,KontarE,KrupaS,KulkarniKM,LovelessA,MooreWS,OberdorferJA,OliveiraJ,OzyurtN,PovinecP,PriviteraAMG,RajarR,RamessurRT,ScholtenJ,StieglitzT,TaniguchiM,TurnerJV(2006)Quantifyingsubmarinegroundwaterdischargeinthecoastalzoneviamultiplemethods.SciTotalEnviron367:498–543.https://doi.org/10.1016/j.scitotenv.2006.05.009CroninC,DalyD,DeakinJ,KellyD(1999)Ballyvaughanpublicsupplygroundwatersourceprotectionzones.GSIreport,ClareCountyCouncil,Ennis,Ireland,11ppDimovaNT,BurnettWC,SpeerK(2011)AnaturaltracerinvestigationofthehydrologicalregimeofSpringCreeksprings,thelargestsub-marinespringsysteminFlorida.ContShelfRes31:731–738.https://doi.org/10.1016/j.csr.2011.01.010DrewD(1990)ThehydrologyoftheBurren,CountyClare.IrGeogr23:69–89DrewD(2003)TheHydrologyoftheBurrenandoftheClareandGalwayLowlands.In:MullanG(ed)CavesofCountyClareandSouthGalway.UniversityofBristolSpelaeolog,Bristol,UK,pp31-43DrewD(2018)KarstofIreland.GeologicalSurveyIreland,Dublin,IrelandEdwardsR,CravenK(2017)Relativesea-levelchangearoundtheIrishCoast.In:CoxonP,McCarronS,MitchellF(eds)AdvancesinIrishQuaternarystudies.Springer,Heidelberg,Germany,pp181–215EllisJR,McCullySR,BrownMJ(2012)AnoverviewofthebiologyandstatusofundulaterayRajaundulatainthenorth-eastAtlanticOcean.JFishBiol80:1057–1074.https://doi.org/10.1111/j.1095-8649.2011.03211.xFieldMS(2002)TheQtracer2programfortracer-breakthroughcurveanalysisfortracertestsinkarsticaquifersandotherhydrologicsys-tems.USEnvironmentalProtectionAgency,Washington,DC,179ppFieldMS,WilhelmRG,QuinlanJF,AleyTJ(1995)Anassessmentofthepotentialadversepropertiesoffluorescenttracerdyesusedforgroundwatertracing.EnvironMonitAssess38:75–96.https://doi.org/10.1007/BF00547128FlynnRM,SinreichM(2010)Characterisationofvirustransportandattenuationinepikarstusingshortpulseandprolongedinjectionmulti-tr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