CO2 and Rn degassing from the natural analog of Campo de Calatrava (Spain): Implications for monitoring of CO2 storage sites

International

Journal

of

Greenhouse

Gas

Control

jo u r n al hom e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / i j g g c

CO

2

and

Rn

degassing

from

the

natural

analog

of

Campo

de

Calatrava

(Spain):

Implications

for

monitoring

of

CO

2

storage

sites

J.

Elío

_,

_M.F.

_Ortega

_,

_B.

_Nisi

_,

_L.F.

_Mazadiego

_,

_O.

_Vaselli

_,

_J.

_Caballero

_,

_F.

_Grandia

a_{UniversidadPolitécnicadeMadrid(UPM),Madrid,Spain} b_{CNR-IGGInstituteofGeosciencesandEarthResources,Pisa,Italy} c_{DepartmentofEarthSciences,Florence,Italy}

d_{CNR-IGGInstituteofGeosciencesandEarthResources,Florence,Italy} e_{AMPHOS21,Barcelona,Spain}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received8July2014

Receivedinrevisedform17October2014 Accepted23October2014

Availableonline13November2014 Keywords: CO2storage Geochemicalmonitoring CO2leakages Radonisotopes Naturalanalogs Soilgases

a

b

s

t

r

a

c

t

Naturalanalogsofferavaluableopportunitytoinvestigatethelong-termimpactsassociatedwiththe potentialleakageingeologicalstorageofCO2.

DegassingofCO2andradonisotopes(222Rn–220Rn)fromsoil,gasventsandthermalwaterdischarges

wasinvestigatedinthenaturalanalogofCampodeCalatravaVolcanicField(CCVF;CentralSpain)to determinetheCO2–RnrelationshipsandtoassesstheroleofCO2ascarriergasforradon.Furthermore,

radonmeasurementstodiscriminatebetweenshallowanddeepgassourceswereevaluatedunderthe perspectiveoftheirapplicabilityinmonitoringprogramsofcarbonstorageprojects.

CO2fluxashighas5000gm−2d−1and222Rnactivitiesupto430kBqm−3weremeasured;220Rn

activi-tieswereoneorderofmagnitudelowerthanthoseof222_Rn.The222_Rn/220_{Rnratioswereusedtoconstrain}

thesourceoftheCampodeCalatravasoilgasessinceapositivecorrelationbetweenradonisotopicratios andCO2 fluxeswasobserved.Thus,inagreementwithpreviousstudies,ourresultsindicateadeep

mantle-relatedoriginofCO2forbothfreeandsoilgases,suggestingthatcarbondioxideisanefficient

carrierforRn.Furthermore,itwasascertainedthattheincreaseof222_{Rninthesoilgaseswaslikely}

pro-ducedbytwomainprocesses:(i)directtransportbyacarriergas,i.e.,CO2and(ii)generationatshallow

levelduetothepresenceofrelativelyhighconcentrationsofdissolvedUandRainthethermalaquifer ofCampodeCalatrava.

ThediffuseCO2soilfluxandradonisotopicsurveyscarriedoutintheCampodeCalatravaVolcanic

FieldscanalsobeapplicabletogeochemicalmonitoringprogramsinCCS(CarbonCaptureandStorage) areasastheseparametersareusefulto:(i)constrainCO2leakagesoncedetectedand(ii)monitorboththe

evolutionoftheleakagesandtheeffectivenessofsubsequentremediationactivities.Thesemeasurements canalsoconvenientlybeusedtodetectdiffuseleakages.

©2014ElsevierLtd.Allrightsreserved.

1. Introduction

CarbonCaptureandStorage(CCS)isoneofthemostfeasible techniquestoreduceanthropogenicgreenhousegasemissionsinto theatmosphere,allowingfossilfuelscombustiontobe environ-mentallysustainable(IEA,2008;IPCC,2005).In2011theUnited Nations Framework Convention on Climate Change of Durban (UNFCCC,2011)definedCCSasacleandevelopmentmechanism. However,itscommercialapplicationstillrequiresfurther (theoret-icalandpractical)investigationsinordertomakethistechnology

∗ Correspondingauthor.Tel.:+34913366989;fax:+34913366948. E-mailaddress:javiereliomedina@gmail.com(J.Elío).

economicallyviableandenvironmentallysafe.Moreover,aproper evaluationofthecosts(Kühnetal.,2013;LupionandHerzog,2013; NatalyEchevarriaHuamanandXiuJun,2014;Romanaketal.,2013) isneededsinceCCScanbedifficulttosustainbyprivate compa-nieswithoutfinancialsupportbygovernmentalauthoritiesunless carbonsequestrationwithenhancedgasrecoveryandoil recov-eryisapplied(CSEGRandCSEOR;e.g.,Oldenburg,2003;Solomon etal.,2008).Eventually,asrecentlysuggested,e.g.,Oldenburg,2012 andreferencestherein,anthropogenicCO2canbeusedtoimprove theeconomicviabilitybyincreasingtheuseofthis greenhouse gasfordifferent(e.g.,industrial,agricultural,foodandbeverages, pharmaceutical,chemical,healthcare)purposes.Thus,ithasbeen suggestedtoincludethetermof“utilizationofCO2”intheCCS process,i.e.,carboncapture,utilizationandstorage(CCUS).

http://dx.doi.org/10.1016/j.ijggc.2014.10.014 1750-5836/©2014ElsevierLtd.Allrightsreserved.

ThestudyofnaturalsystemshostingCO2-richgasreservoirscan helptointerpretthedifferentstagesoftheCO2storageprocesses, whichnormallyrequirelongperiodsoftimeandcanonlypartially bereproducedinthelaboratory(Pearce,2006).Consequently,in therecentyearsalargenumberofstudiesaddressedtothe eval-uationofleakageprocessesfromCO2-richnaturalanalogsandthe implicationsforthegeologicalstorageofCO2hasbeenperformed (e.g.,Galetal.,2012;Hollowayetal.,2007;Jeandeletal.,2010; Lewickietal.,2007;Voltattornietal.,2009).

Surface monitoring methods (e.g., water and dissolved gas chemistryandsoil-gasinvestigations)arerelevantinthe moni-toring,verificationandaccounting(MVA)ofCO2geosequestration toassesstheenvironmentallysafestorage(Klusman,2011),during bothsitecharacterizationandinjectionandpost-injectionphases. Thesemethodshavebeenorientedtodetectandquantify possi-bleCO2 leakagestotheatmosphere and covera largerange of techniques(e.g.,IPPC,2006;NETL,2009).

Measurementsofradonactivity(222_Rn–220_{Rn)insoilgasesare} consideredusefulin monitoringprogramsof geologicalstorage fortwomainreasons:(i)CO2 isoftenregardedasthemain car-riergasforradon(e.g.,EtiopeandMartinelli,2002;Etiopeetal., 2005;Voltattornietal.,2009),andthenCO2 leakagesfromdeep sourcesmayproducesignificantanomaliesinradonactivity,and (ii)radon,similarlytoCO2,canbeusedtodetectfracture/fault sys-temssincetheyrepresentthepreferentialpathwaysforgasleakage (e.g.,Ioannidesetal.,2003;Waliaetal.,2010).Radon measure-mentsatthesoil-atmosphereinterfaceforleakagedetectionwere appliedinsomeprojectsofCO2geologicalstorage,suchastheIEA GHGWeyburnCO2 Monitoringand StorageProject (Riding and

Rochelle,2005;Struttetal.,2003;WilsonandMonea,2004)and theFrioBrinePilotProject(Nanceetal.,2005).

Thedetectionofradonemissionisalsosignificantintermsof itseffectonhumanhealth.USEPAhasestimatedthatradonisthe secondcauseoflungcanceraftersmoking(e.g.,EPA,2003;Field etal.,2000;RosarioandWichmann,2006).Thus,recognizing posi-tivecorrelationsbetweenRnandCO2anddefiningRnabundances frompermeablezonesarecriticalandwouldallowthedefinition ofpotentialhazardareastobetakenintoconsiderationintherisk assessmentofgeologicalstorage.

Thisresearcharticledescribesanddiscussestheresultsobtained onfree-andsoil-gasesfromtheCampodeCalatravaVolcanicField (CCVF)naturalanalog,whichislocatedintheNeogeneGranátula –MoraldeCalatravaBasin(centralSpain).Themaingoalsofthis studywereto(i)determinetherelationshipsbetweenCO2fluxes andactivityofradonisotopes(222_Rnand220_{Rn)inthesoilgasand} (ii)toassessthepossibleuseofradonmeasurementsasmonitoring toolinCCSprojects.

1.1. Radonphysical-chemicalbackground

Radonhasfourmainisotopes,218_Rn,219_Rn,220_Rnand222_Rn, whicharedecayproductsof218_At,223_Ra,224_Raand226_Ra, respec-tively,allbelongingtotheUandThdecayseries.Radonisotopes arecharacterizedbyrelativelyshorthalf-livesbeing222_Rn(t

1/2=3.8 days)and220_Rn(t

1/2=55seconds)withthelongesthalf-lives(e.g.,

Fleischer,1997).Thedecayproductsof220_{Rn(commonlynamed} thoron)and222_{Rnaretheradioactiveisotopesofpolonium,} bis-muth,leadandthallium.While220_{Rnhasnolong-livedprogeny,} 222_Rnhas214_Pb,214_Biand214_{Po(e.g.,RamachandanandSathish,}

2011).

Radon(222_Rn–220_{Rn)activityhasextensivelybeenusedinthe} earthsciencesastracerofUdeposits(e.g.,SuttonandSoonwala, 1975),precursorofseismicandvolcanicactivity(e.g.,Cox,1980; Due ˜nasandFernández,1987;FleischerandMogro-Campero,1978; Nielson,1978),tracerofsubsurfacefracturesystems(Waliaetal., 2010),oil/gasreservoirs(KlusmanandVoorhees,1983;Morseetal.,

1982; Mazadiego, 1994) and subsurface hydrocarbon pollution (Davis etal.,2003;García-Gonzálezet al.,2008;Schubertetal., 2001).Recently,radonisotopeshavebeenappliedtodistinguish CO2 emission fromdeepand shallow(biogenic)sources(Etiope

etal.,2005;EtiopeandMartinelli,2002;Giammancoetal.,2007; Michel-LePierresetal.,2010;Voltattornietal.,2009).

Aschematiccycleof220_Rnand222_{Rnisotopesisreportedin}

Fig.1.Themobilityofradonisotopeparents,includinguraniumand radium,islimited.Uranium(anelementwithlong-livedisotopes) is foundin minerals and chemical alterationfavors itstransfer tosurfaceandgroundwaters(e.g.,BonottoandAndrews,1999; Mazadiego,1994).Aqueousdissolutionofuraniumminerals(e.g., uraninite)canberepresentedbythefollowingreaction:

UO2(s)+2H++O2→UO22+(aq)+H2O

Additionally,ifwatercontainsHCO3−andCO32−themobilityof uraniumtendstoincreaseduetotheformationofcarbonate com-plexes(Casasetal.,1998;Gorman-Lewisetal.,2008;Majumdar etal.,2003).Underoxidizingconditions,uranium insolutionis preferentiallypresentasuranylion(UO22+),whereasU4+prevails inreducingconditionsandUO2canprecipitate(e.g.,Bonottoand

Andrews,1999;García-Gonzálezetal.,2008).

The uranyl ion may also be trapped in clay minerals and ironoxy-hydroxidesbyadsorption/absorptionprocesses(e.g.,Hsi andLangmuir,1985;Langmuir,1978).Conversely,radium(with shorter-livedisotopesthanuranium)isexclusivelypresentin solu-tionasRa2+_{(LangmuirandRiese,1985)andisratherrecalcitrantto} formsaltsunderreducingandoxidizingenvironments.Thus,in sur-faceandgroundwatersradiumismaintainedinsolution.Neutral oranionic(e.g.,SO4)complexesareformedforpHapproaching10 (LangmuirandRiese,1985).Nevertheless,accordingtoLangmuir andMelchoir(1985)andMartinandAkber(1999),Ra2+_can co-precipitate withbarite(BaSO4), witherite (BaCO3)and celestite (SrSO4).Consequently,springwatersandrelatedsaltsmay repre-sentasourceofradon(222_Rn–220_{Rn),whosevaluescanbehigher} thanthoseofthebackgroundsignals.

Inthevadosezone,Rnmayalsohaveadeeporiginalthough,due toitsrelativelyfastdecay,radonisexpectedtohavesmallmobility fromitssourcebydiffusionprocesses(CothernandSmith,1987; Martinelli,1998;Roseetal.,1979).Thus,undergroundlong migra-tionofradonfromdeepersourcestothesurfacerequiresadvective transportdrivenbymajorgasessuchasCO2andCH4(the“geogas” theory,EtiopeandMartinelli,2002).

AsRnmigrates, theradonisotopic ratioschangedue to sig-nificantdifferencesinthe222_{Rn and}220_{Rn half-lives.Thelatter} is thus preferentially foundclose to the parentisotope source (Huxoletal.,2012,2013).The222_Rn/220_{Rnratioscanbeusedto} definethe depthatwhich free- and soil-gases retrieved atthe surfacewereformed(e.g.,Giammancoetal.,2007).The relation-shipbetween222_Rnand220_{Rninsoilgasesisclearlydependent} ontheconcentrationoftheirparentsinthesubstrate(238_Uand 232_{Th,respectively).Thus,leakingofdeep-seatedgasesatthe} sur-faceisexpectedtoresultinreduced220_{Rnconcentrations,while} the222_Rn/220_{Rnratiosincrease.Relativelylow}222_Rn/220_Rnratios aregenerallyassociatedwiththeproductionofradoninshallow environments(e.g.,Giammancoetal.,2007).

Radonconcentrationsinsoilgasesareaffectedbyanumberof factors,suchasmeteorological,geologicaland pedological(e.g., De Jong et al., 1994; Vaupotic et al., 2007) and major seismic events(e.g.,KingandMinissale,1994;ToutainandBaubron,1999; VirkandWalia,2001).AccordingtoTanner(1980),radoncanbe releasedfromsoilsandrocksbymoleculardiffusion,directrecoil andmoleculardiffusionafterindirectrecoil.Molecularand convec-tionprocessesinfluencesoilradonactivityconcentrations,which

Fig.1.Sketchofradontransportinliquidandgasphase.

increasewithdepthuntilwhen anequilibriumconcentrationis achieved(NazaroffandNero,1988;Szabóetal.,2013).

Radon (222_{Rn) concentration (C}

Rn in [Bqm−3]) in soil gases dependson226_{Raconcentration(C}

Ra[Bqkg−1]),emanation coef-ficient(ε;adimensional),soildensity ([kgm−3]) andeffective porosity(n;adimensional)(GuerraandLombardi,2001;Schubert etal.,2001),asfollows:

CRn=CRa·ε··n−1 (1)

Soilwater(saturationinporespace,0≤SF≤1)increasesradon concentrationinsoilgas(Schubertetal.,2001).Suchanincrease isdefinedbyEq.(2)(Kw/air=0.25,radonpartitioningcoefficients betweenwaterandair)andplottedinFig.2.Itcanbeseenthatthe 222_{Rnconcentrationcanincreaseupto300%(S}

F=1)withrespect toradonequilibriumindrysoil(SF=0).

1

1−SF+SF·Kw/air (2)

2. Geologicaloutlines

The Campo de Calatrava Volcanic Field(CCVF) is located in the province of Ciudad Real (central Spain) (Ancochea, 1983; Hernández-Pacheco,1932;Fig.3a).Thestudyareaconsistsof a

Hercynian basement covered by late Cenozoic sediments. This basementis unconformablyoverlainbyupper Mioceneto Qua-ternaryfluvialandlacustrinesedimentsdepositedwithinaseries offault-boundedTertiary-Quaternarysedimentarybasinsofupto 200km2_{.Acomplexfracturepattern(E–WtoENE–WSW,NW–SE} andNE–SW)controlsthebasinsgeometries(Crespo,1992).Atleast threedifferentNeogenetectonicepisodeswererecognizedinthe CCVF(IGME,1988).AfirstextensionalphaseoflateMioceneage, whichresultedintheinitialopeningofthesedimentarybasins,was followedintimebytwoPliocenetectonicepisodes:theopening oftheso-calledLaManchabasinandafinalregional-scaleweak compressionalphase. TheMiocene-Quaternary CCVFconsistsof volcanicdepositsofalkalinecomposition(e.g.,Cebriáetal.,2009; CebriáandLópez-Ruiz,1995;López-Ruizetal.,2002).Theareais characterizedby severalgasand water discharges(e.g.,Melero Caba ˜nas,2007; Piedrabuena,1992; Vasellietal.,2013)thatare distributed almostthroughout thewholeCCVF. Itis interesting topointoutthatthegeochemicalandisotopicfeaturesofthegas seepsindicateahighpurityofCO2(upto98.5%byvol.),whilethe heliumisotopicratiosuggestsamantle-relatedsourcefor these gases (Pérez etal., 1996; Vaselli et al., 2013).The CCVF, along withLaSelva-Empordà(NESpain),likelyrepresentsthehighest CO2 dischargeareainthewholecontinentalSpain(Vasellietal.,

Fig.2. Waterinfluenceintheequilibriumradon(222_{Rn)concentrationofthesoil-gas.}

3. Materialsandmethods

3.1. Areaofstudy

Morethan25CO2 seepages fromCCVF,mainlyassociatedto fracturesystems,wereinvestigatedfromJune 2010toFebruary 2012.Gasleakageswerecommonlyfoundassmall(<1m2₎ leak-agepoints.ConsideringtheelevatednumberofCO2emissions,this studyfocusedontwoareasthatwereconsideredrepresentative oftheCO2seepageinCCVF,namelyLaSima(drygasvents)and JabalónRiver(degassingpoolsandsprings)(Fig.3a).

LaSimaisaCO2-richgasdischarge(upto2td−1),whichhas asurfaceofafewsquaremeters.Theemissionisrestrictedtoa small(≈5mindiameter)depression,where smalldeadanimals arefrequentlyfound.TheJabalónRiverareaislocated6kmfrom thevillageofGranátuladeCalatrava(Fig.3a).Foursubareaswere considered:(i)Jabalón,(ii)NH-1,(iii)Fontechaand(iv)NH-2.Inthe JabalónRiver,anelevatednumberofCO2-richmanifestations, bub-blingintowarmwater(<30◦C)springs,occur,likelyalignedalonga faultsystemparallel(NW–SE)totheriverbed(12emissionpoints inaprofileofapproximately2km).Thelocationofthestudiedsites whereCO2fluxandradonmeasurementswerecarriedoutisshown inFig.3.

3.2. CO2fluxmeasurements

Carbondioxidesoilfluxesweremeasuredin70stations(Table2, Fig.3)using theaccumulation chambermethod(e.g.,Cardellini etal.,2003;Chiodinietal.,1998),whichisbasedonthe continu-ousmeasurementoftheCO2concentrationswithtimebyusingan invertedchamberplacedontheground.Theaccumulation cham-berdeviceconsistsof:(i)ametalcylindricalchamberwithaninlet netareaandinnervolumeof3.14×10−2m2_and3.06×10−3_m3_, respectively,(ii)anInfra-Red(IR)Spectrophotometer(Licor® Li-820,infra-redsensordetector,measuringrangeof0–20,000ppm, accuracyof4%ofreadings),(iii)ananalog–digital(AD)converter, and(iv)apalmtopcomputer(PC).

3.3. Radonmeasurements

Therecordingoftheradonactivity(70pointsfor222_Rnand31 for220_{Rn,respectively;Table2,Fig.3)wasperformedwithtwo} SARAD® RTM-2100devicesand oneionizationchamberRADON V.O.S.,modelRM-2.Soilgaseswerecollectedwithastainless-steel hollowprobe,whichwashammeredintothesoil,downtoadepth of0.75–1.00minordertominimizetheinfluenceofatmospheric factors(e.g.,KingandMinissale,1994).AtLaSima,theprobewas

insertedatthedepthof0.15–0.20mduetothethinsoillayer,which coveredthebedrock.Theradonmonitorwasfittedwithacanister filledwithdrierite(97%CaSO4+3%CoCl2)andahydrophobicTeflon filtertominimizethehumiditycontentwithintheinstrumentand preventingfineparticlesenteringtheionizationchamber( García-Gonzálezetal.,2008).Inthebubblingsites,radonmeasurements werecarriedoutdirectlyintheairflow,withacollectorintowhich thebubblinggaswastrapped.

Theradon(222_Rn–220_{Rn)measurementswereperformedwith} a SARAD® RTM-2100byanalyzing theradondaughter isotopes (218_Po/214_Poand216_{Po).Thesubsequentradioactivedecayofthese} isotopeswasrecordedwithamultichannelanalyzer(alpha spec-trometry).Soilgasesweredrivenintotheionizationchamberby aninternal pump atthe rateof 3Lmin−1. Measurementswere carriedoutwiththeinstrumentin“thoron”modeandwith inte-grationtimeof1min.222_{Rnactivitywasassignedtothevalueof} “radon”after15minandthose ofthoronafter5min.Aftereach seriesofmeasurements,theinternalpumpwassetathighflow (3Lmin−1)for20–30mintocleantheionizationchamberbefore starting anewmeasurement. Thedetection limitof the instru-mentwas1kBqm−3,and thestatisticalerrorat1 wasdefined as100%/√N,whereNisthenumberofregisteredcountsofthe relevantnuclides(218_Po/214_Poand216_{Po)withinthetimeinterval.} ForanEquivalentRadonConcentrationof1kBqm−3orhigherthe statisticalerrorwasbelow10%.

TheRM-2systemwasdesignedforthedeterminationofsoil gasradon(222_{Rn)concentrations.Thedetectionprincipleconsists} inanionizationchamberwhereapotentialdifferencebetweena positivelychargedmetaloutershellandanelectrodelocatedalong thelongitudinalaxis(0V)wasapplied.Whenradonemitsalpha particles,ionsweredetectedbytheelectrode.Theioncurrentwas thenmeasuredandconvertedtoRnconcentration,expressedas Bqm−3. Thesoil gaswascollectedwitha 150mLsyringe, con-nectedtothetopofastainless-steelhollowprobeandtransferred intoapre-evacuatedionizationchamber(250mL).Pressureinside the chamber was equalized to the ambient atmospheric pres-surebyopeningthevalve.Measurementswerecarriedoutafter 15min.Thedelayof15minensuredthattheactivityof220_Rnwas practicallynegligibleduetoitsshorthalf-life.Thedetectionlimit was5kBqm−3.Theuncertaintyofradonconcentration(1␴)was 0.33(Crn)0.5,whereCrn is theradonconcentration(Crn±).The RM-2erroroftheradonconcentrationwasbelow15%.

Radonconcentrations(222_Rn[Bqm−3_{])obtainedwiththetwo} instrumentation(SARAD® RTM-2100and RM-2)didnot signifi-cantly differ(relative standard deviation –RSD – around 10%). Thus,the222_{Rnconcentrationswereanalyzedindependentlyofthe} instrument,andwillnotbediscussedfurther.

Fig.3.Samplingzones:(a)CCVFarea,(b)LaSima,(c)Jabalón,(d)NH-1,(e)Fontechaand(f)NH-2.

3.4. Soilsampling

Eightsoilsamples(Table3;Fig.3)werecollectedtomeasure theconcentrationoftheradonparentradionuclides(U–Ra) and toassesswhetherradonwasoriginatedbythesoilcoverorwas transferredfromdeepsourcesbyadvectivecirculation.Sampling wascarriedoutwithanEdelmanaugeratthesamedepthwhere thesoil-gasesweremeasured:twoatLaSimagasvent(insideand outsidethedegassingsite)andsixattheJabalónRiverarea(2close totheCO2seepage,i.e.,whereCO2 wasbubblinginwater,and4 atadistanceof10–30m).Sampleswereanalyzedbygamma-ray spectrometry(Quindósetal.,2006)attheDepartmentof Medi-calPhysics,FacultyofMedicine(UniversityofCantabria,Spain). The226_{Raconcentrationsinsoilsprovided thetheoretical}222_Rn concentrations,whichwereestimatedaccordingtoEq.(1).

Since ε, and nwerenot measured, thedistributionof the theoreticalconcentrationof222_{RnwasestimatedbyMonteCarlo} simulation,assumingthatε, andnwerenormallydistributed withthetypicalvaluesofgravel,sandandclay(Table1;Yuetal., 1993).Thetheoreticalconcentrationwasinterpretedasa geochem-icalthreshold.Hence,inthecasethatthesoilwastheonlysource

Table1

Typicalε,andnvaluesinsoil.

Parameter Minimum Mean Maximum

ε 0.18 0.29 0.40

(kgm−3_{) 1200 1350 1500}

n 0.01 0.30 0.50

of222_{Rn,whichwasrelatedtothepresenceof}226_{Ra,themeasured} activitywasexpectedtobesimilartothecalculatedcontent.If 222_{Rnactivitywashigherthanthisthresholdradonwasinterpreted} tobedeep-sourced.

4. Results

Toavoidpossibleinterpretationerrorsoriginatedbythe influ-ence of water to the activity of radon in the soil-gas, the geochemicalanalysesweredividedintotwotypesofemissions: (i)LaSima (drygasvent)and (ii)Jabalón River(CO2-richgases bubblinginwater).

4.1. LaSima

ThesoilCO2fluxrangedfrom0.20to5379gm−2d−1,although insidethemorphologicaldepressionofLaSimatheCO2fluxwas notmeasuredastheIRdetectorsaturatedinafewseconds(i.e., >20,000ppms−1 ofCO2).The222Rn activityinsoilalsoshowed a relatively wide range, between 0.64 and 138kBqm−3. The 220_{Rncontentsweremoreconstantandrangedbetween7.6and} 51.7kBqm−3(Table2).

Five out of the 23 CO2 flux measurements at La Sima (Table 2, Fig. 3b) were considered anomalous [higher than Q3+1.5×(Q3−Q1),beingQ1 andQ3thefirstandthethird quar-tile,respectively],i.e.,#13(45gm−2d−1),#12(196gm−2d−1),#8 (566gm−2d−1), #24 (3322gm−2d−1) and #1 (5379gm−2d−1). TheseanomalouspointswerelocatedinsidethedepressionofLa Sima(#1,8and24)orveryclose(#12)toit,suggestingthatthe CO2 seepagewaswellrestrictedtoverysmallarea(redzonein

Fig.3b).

Radium activity (226_{Ra) was similar inside and outside the} emissionsite,withanaveragevalueclusteringaround40Bqkg−3 (Table 3). From Eq. (1), the theoretical concentration of radon (222_{Rn),inside and outside theCO}

2 emission, wasestimatedto be55kBqm−3 (Table4),with95%ofthedatabelow82kBqm−3 (Table4).

4.2. JabalónRiver

Allstudied subareasalong theJabalónRiverwere relatedto spring water discharges(T<30◦C) where a CO2-rich gasphase wasgently-to-vigorouslybubbling.TheCO2fluxeswerefrom<0.2 (<D.L.)to1×106_gm−2_d−1_{.InsomecasestheCO2 fluxwasnot} measuredastheIRdetectorinstantlysaturated.Occasionally,high CO2 fluxes (up to 1tm−2d−1 or even higher) were measured, althoughatashortdistance,evenatthescaleoffewdecimeters, adramaticdecreasewasrecorded,andtheCO2fluxes,typicalof biologicrespiration(0.02–18gm−2d−1;Elíoetal.,2013and ref-erencetherein;Fig.4), weredetermined.Thelowest activityof 222_Rnwas2.3kBqm−3 _{whilstthehighestone was428kBqm}−3_. The concentrations of 220_{Rn ranged from 9.0 to 44.5kBqm}−3 (Table2).

Theresultsbygamma-rayspectrometry(Table3)showedthat therewasadifferenceinthe226_{Raconcentrationbetweenthesites} closetotheleakingpoints(#41and#45)and awayfromthem (#30,#42,#Aand#B).WhilenearthemainCO2-richsourcesthe 226_{Raconcentrationwasapproximately70Bqkg}−1_{,in thedistal} zonesitscontentabruptlydecreaseddownto30Bqkg−1.The dif-ferenceinradiumconcentrationproducedtwotheoretical222_Rn activitiesforthesoilgasduetothepresenceof226_{RaintheJabalón} Riverarea.The222_{Rnactivityforthosesoilgaslocatedfarfromthe} waterspringswasaround40kBqm−3,with95%ofthedatabelow 60kBqm−3.Thisvalueincreasedupto95kBqm−3,with95%ofthe databelow140kBqm−3,nearthewatersprings(Table4).

Fig.4.ExampleofveryhighCO2flowthroughverysmallspots.Totheleft,theIR

detectorsaturatedinafewseconds;afewcentimeterstotheright,theCO2soil

fluxshowedvalues,whichcanberelatedtoabackgroundemission,i.e.,biological activity.

5. Discussion 5.1. LaSima

5.1.1. Theoretical222_{Rnconcentrationinsoil-gas}

AtLaSima,most222_{Rnmeasurementswerebetween0.6and} 37.2kBqm−3,i.e.,slightly lowerthan thetheoreticalestimation (approximately55kBqm−3;Table4).Aspreviouslymentioned,the lackofadevelopedsoilcoverpartlypreventedtheradon measure-ment,whichwasmadeatthedepthof0.15–0.20cm,wherethe influenceoftheatmosphericparameterscansignificantlyaffectthe radonconcentrations.However,despitethediscrepancybetween thetheoreticalandmeasuredvaluesand theuncertaintyofthe fieldmeasurements(below10–15%)and thetheoretical estima-tions,itisreasonabletoassumethatthethresholdinthiszonewas of45–60kBqm−3.Suchhighconcentrationscannotbeexplained withthepresenceof226_{Rainthesoiland consequently,adeep} Rn source is required. This value was exceeded in four points (#12:63kBqm−3; #1: 117kBqm−3; #8:118kBqm−3 and #24: 138kBqm−3).

5.1.2. CorrelationbetweenCO2fluxandradon(222Rn–220Rn) activityinsoil-gas

Settingaside#13,whichhadaCO2fluxvalueof45gm−2d−1, four anomalous data were measured at La Sima, #12 (196gm−2d−1), #8 (566gm−2d−1), #24 (3322gm−2d−1) and #1 (5379gm−2d−1), likely associated with the occurrence of a deep-seated flow that affects the soil degassing in this area. Whencomparing theCO2 flux and the 222Rn activitydata, we observedthatthehighertheCO2fluxthehighertheRnactivities (#1,#24,#8and#12;Fig.5a).Thesesitesarethosewhose222_Rn concentrations cannot be explained without invoking a deep radonsource.Conversely,wherelowCO2 fluxesweremeasured the222_{Rnvaluescanbeassociatedwiththepresenceof}226_Rain thesoil.

The220_{RnconcentrationsareinverselycorrelatedwiththeCO} 2 fluxvalues(Fig.5b).Therelativelyhigh222_Rn/220_{Rnratios(upto} 20;Fig.5c)arelikelyassociatedwithadeeporiginofradon.This isalsosupportedbythe␦13_C–CO

2 and 3He/4Heisotopicvalues, whichsuggestadeep(mantle)rootfortheCCVFgasdischarges (Pérezetal.,1996;Vasellietal.,2013).

TherelationshipbetweenthehighestCO2fluxesand222Rn con-centrations is wellillustrated when plotting the W–E oriented

Site Data Sample Est North CO2flux(gm d ) Rn(kBqm ) Rn(kBqm ) Observation

LaSima June10 1 434,419 4,297,245 5379 116.8 7.6 Soil-gas

LaSima June10 2 434,413 4,297,238 0.4 0.64 Soil-gas

LaSima June10 3 434,408 4,297,236 0.3 1.2 Soil-gas

LaSima June10 4 434,428 4,297,243 4.3 1.7 Soil-gas

LaSima June10 5 434,434 4,297,234 2.1 6.6 Soil-gas

LaSima June10 6 434,411 4,297,252 1.3 9.7 Soil-gas

LaSima June10 7 434,408 4,297,263 1.5 5.7 Soil-gas

LaSima August10 8 434,418 4,297,242 567 118.4 13.3 Soil-gas

LaSima August10 9 434,423 4,297,242 5.4 3.1 31.1 Soil-gas

LaSima August10 10 434,443 4,297,242 0.2 11.1 34.0 Soil-gas

LaSima August10 11 434,453 4,297,242 0.6 6.6 26.7 Soil-gas

LaSima August10 12 434,415 4,297,242 196 63.2 10.4 Soil-gas

LaSima August10 13 434,404 4,297,243 45 10.5 Soil-gas

LaSima August10 14 434,394 4,297,243 7.1 10.2 23.0 Soil-gas

LaSima August10 15 434,385 4,297,245 0.2 13.0 32.7 Soil-gas

LaSima August10 16 434,375 4,297,243 0.5 11.5 51.7 Soil-gas

Fontecha May11 17 428,709 4,295,194 7.3 58.0 Soil-gas

Fontecha May11 18 428,701 4,295,186 192 428.0 Soil-gas

Fontecha May11 19 428,691 4,295,174 12.3 3.0 Soil-gas

Fontecha May11 20 428,687 4,295,192 17.9 39.7 Soil-gas

Fontecha May11 21 428,715 4,295,175 15.8 2.3 Soil-gas

LaSima May11 22 434,440 4,297,260 7.6 9.8 Soil-gas

LaSima May11 23 434,440 4,297,240 4.2 3.0 Soil-gas

LaSima May11 24 434,420 4,297,240 3322 138.0 Soil-gas

LaSima May11 25 434,400 4,297,240 5.5 37.2 Soil-gas

LaSima May11 26 434,410 4,297,230 5.7 16.7 Soil-gas

LaSima May11 27 434,403 4,297,222 18.0 34.0 Soil-gas

LaSima May11 28 434,409 4,297,249 3.1 11.5 Soil-gas

Jabalón September11 29 429,130 4,294,625 1.4 32.4 Soil-gas

Jabalón September11 30 429,170 4,294,625 44 83.1 27.0 Soil-gas

Jabalón September11 31 429,130 4,294,605 5.3 29.1 9.0 Soil-gas

Jabalón September11 32 429,190 4,294,605 21.9 28.2 Soil-gas

Jabalón September11 33 429,210 4,294,605 12.3 24.5 28.5 Soil-gas

Jabalón September11 34 429,110 4,294,585 <0.2 25.6 15.5 Soil-gas

Jabalón September11 35 429,150 4,294,585 5.1 12.2 17.8 Soil-gas

Jabalón September11 36 429,170 4,294,585 11.6 47.0 10.5 Soil-gas

Jabalón September11 37 429,210 4,294,585 9.6 49.7 Soil-gas

Jabalón September11 38 429,090 4,294,565 <0.2 18.6 11.2 Soil-gas

Jabalón September11 39 429,130 4,294,565 2.9 22.7 12.3 Soil-gas

Jabalón September11 40 429,150 4,294,565 25.7 34.2 30.8 Soil-gas

Jabalón September11 41 429,170 4,294,565 45 182.5 35.2 Soil-gas

Jabalón September11 42 429,190 4,294,565 102 91.1 31.6 Soil-gas

Jabalón September11 43 429,090 4,294,545 <0.2 24.2 19.4 Soil-gas

Jabalón September11 44 429,150 4,294,545 5.7 13.2 24.0 Soil-gas

Jabalón September11 45 429,170 4,294,545 141 183.5 18.4 Soil-gas

Jabalón September11 46 429,150 4,294,525 5.2 7.8 Soil-gas

Jabalón September11 47 429,170 4,294,525 0.5 17.3 13.9 Soil-gas

NH-1 February12 48.5 429,137.5 4,294,759 913,887 118.0 Bubblingpool

NH-1 February12 48 429,137 4,294,759 523,140 130.0 Bubblingpool

NH-1 February12 49 429,140 4,294,759 4.6 171.0 11.5 Soil-gas NH-1 February12 50 429,143 4,294,758 7.1 56.1 Soil-gas NH-1 February12 51 429,146 4,294,758 8.9 37.8 Soil-gas NH-1 February12 52 429,140 4,294,767 0.3 39.7 44.5 Soil-gas NH-1 February12 53 429,140 4,294,763 1.6 60.6 23.5 Soil-gas NH-1 February12 54 429,140 4,294,755 0.9 55.6 39.5 Soil-gas NH-1 February12 55 429,140 4,294,751 7.4 42.2 38.2 Soil-gas

Fontecha February12 56 429,184 4,294,614 3.2 23.2 Soil-gas

Fontecha February12 57 429,184 4,294,606 9.0 8.9 Soil-gas

Fontecha February12 58 429,184 4,294,602 1.4 59.0 Soil-gas

Fontecha February12 59 429,184 4,294,594 241 15.6 10.8 Soil-gas

Fontecha February12 60 428,700 4,295,187 1.2 75.1 Soil-gas

Fontecha February12 61 428,698 4,295,183 8.6 82.6 Soil-gas

Fontecha February12 62 428,701 4,295,180 6.6 421.9 27.9 Soil-gas

Fontecha February12 63 428,704 4,295,184 8.1 154.0 Soil-gas

Fontecha February12 64 428,700 4,295,184 7.1 218.0 Soil-gas

Fontecha February12 65 428,703 4,295,184 918,290 129.0 Bubblingpool

Fontecha February12 66 428,703 4,295,185 548,485 92.4 Bubblingpool

Fontecha February12 67 428,717 4,295,171 35,821 54.1 Bubblingpool

NH-2 February12 68 427,578 4,295,993 684 13.5 Bubblingpool

Table3

Resultsofgamma-rayspectrometryinsoilsamples.

Site Sample 238_U(234_Th) 226_Ra(214_Bi) 232_Th(228_Ac) 40_K 137_Cs

LaSima Outside 79.0±10.8 40.2±1.5 70.4±4.5 775.5±32.2 <D.L.(1.2) Inside 86.3±11.5 40.9±1.6 117.3±7.2 694.8±29.9 31.6±1.0 JabalónRiver 30 26.2±3.9 34.0±1.4 64.5±4.4 984.9±40.2 <D.L.(1.4) A <D.L.(25.4) 23.0±1.2 79.2±5.0 776.3±33.0 <D.L.(1.7) 42 39.5±8.5 26.5±1.3 71.1±4.9 871.8±35.6 <D.L.(1.8) 41 137.8±14.9 76.0±2.2 72.9±4.9 962.3±39.7 <D.L.(1.8) 45 <D.L.(27.6) 69.5±2.1 71.0±4.7 935.4±39.2 <D.L.(1.8) B <D.L.(24.9) 35.4±1.4 57.6±3.9 742.6±30.9 <D.L.(1.6)

ConcentrationsareinBqkg−1_{.PointsAandBnotshowninFig.3.}

Table4

Theoreticalconcentrationsof222_Rn(inBqm−3_{)insoilgasascalculatedby}226_{Radecayinthesoil.}

Site 226_Ra(Bqkg−1₎ 222_Rn(kBqm−3₎

Min. Q1 Median Mean Q3 95thpercentile Max.

LaSima 40 23 44 51 54 61 82 452

JabalónRiver(awaybubbling) 30 17 33 38 41 46 61 339

JabalónRiver(nearbubbling) 70 40 77 90 95 107 143 791

profile(Fig.6)asafunctionofthedistancefromLaSimagasvent.

Anotherrelevantresultwasthatat#8(distance0m)thehighest concentrationof222_{Rnwasrecordedwhilethatof}220_Rnwasamong thelowestmeasuredvalues.Thismightbeexplainedwiththe dilu-tionofa220_{Rn-freedeepgasasitrisestothesurface.Furthermore,} theamountof220_{Rngeneratedatthesurfacewaspromptlyreleased}

onceproducedbecauseoftheelevatedCO2-richgasfluxand, conse-quently,therecordeddataweresmallerthanthosereallyproduced. Thehighest222_Rn/220_{Rnratioswereat#8(LaSima)and#12,} sug-gesting a predominanceofthehighest half-life isotope(222_Rn). TheseresultsdemonstratethatCO2actsasacarriergasforradon inaregimeofadvectivetransport.

Fig.5. RelationshipbetweenCO2fluxand222Rn/220RnintheLaSimazone(Table2,Fig.3).

Fig.6. Spatialcorrelationbetween222_Rn/220_RnandCO

2fluxintheLaSimazone(Table2,Fig.3).Horizontalaxisisthedistance(m)fromthesamplepointtothecenterof

Fig.7.Equilibrium222_{Rnactivityofthesoil-gasintheareaofJabalónRiverversus}226_{Raactivity.Blackandbluelinesrepresentaradiumactivityof30Bqkg}−1_and70Bqkg−1_,

respectively.Continuouslinesarethemedianoftheoreticalradonconcentrationanddottedlinesarereferredtothefirst(Q1)andthethird(Q3)quantile(Table4).(For

interpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

5.2. JabalónRiver

5.2.1. Theoretical222_{Rnactivityinsoil-gas}

Theincreaseof226_{Raactivitybetweenthesiteslocatedaway} from thethermal waters (30Bqkg−1) and those close to them (70Bqkg−1)couldbeproduced byaprocessoftransportof dis-solvedUandRaandtheirprecipitationatthespring.Thus,two theoretical222_{Rnconcentrationswerecarriedout,althoughinthis} casetheinfluenceofsoilwatersaturation(SF;Eq.(2))aretobe takenintoaccountfortheinterpretationofthegas222_Rnorigin.The radonactivityequilibriumofthesoilgasisplottedinFig.7.Inorder toevaluatetheinfluenceontheradonmeasurementsproducedby thewatercontainedinthesoil,adistinctionbetweentheRa con-centrationswasmade.Inthosepointslocatedfarfromthewater springs,thesoilisexpectedtoberelativelydryorslightlymoist (SF≈0.1–0.5). Thiswould implythattheincreaseof222Rnfrom thewatercontainedinthesoilshouldnotexceedmorethan50% ofthetheoreticalconcentrationfordrysoil(≈70kBqm−3;black linesinFig.7).Thus,theconcentrationofRainthesoildoesnot accountforthe222_{Rnconcentrationsinsoilgaseswhentheyare} muchhigherthan40–70kBqm−3.Closetothewatersprings,where soilsareexpectedtoberelativelymoist(SF≈0.5–0.8),theradon concentrationcanincreaseof50–150%withrespecttodry condi-tions;thereforethetheoreticalactivityof222_{Rncouldbearound} 120kBqm−3 (blacklinesinFig.7).Ifthetransport ofradiumis drivenbygroundwater,the222_{Rncontentisexpectedtoincrease} upto270kBqm−3(bluelinesinFig.7).

Intheseareas,thepointslocatedatadistancefromthewater springswerecommonlycharacterizedbyradonactivityvaluesless than70kBqm−3,whichare inagreementwiththose calculated withthepresenceof226_{Raintherelativelydrysoil,e.g.,#17,#20,} #32and#35(Table2;Fig.3).However,thisthresholdwasexceeded attwopoints:#30(83kBqm−3)and#42(91kBqm−3),whichare unlikelyvaluesfordry/moderatelymoistsoils.Thiswouldimplya moreefficientadvectivetransportand/oradeepgassource.

In proximity to thewater springs (#18, #41, #45, #49–55, #60–64;Table2 andFig.3),the222_{Rnactivitywasbetween38} (#51)and428(#18)kBqm−3.Aspreviouslydescribed,theradon activitycanincreaseto120kBqm−3duetopresenceofwaterin thesoil.Actually,somehigh222_{Rnvalues(from56to83kBqm}−3_,

e.g.,#50,#53,#60and#61)couldberelatedtotheincreasein watersaturation.Nevertheless,veryhighvalueswerealso mea-suredat#63(154kBqm−3)#49(171kBqm−3),#41(182kBqm−3), #45(183kBqm−3),#64(218kBqm−3),#62(421kBqm−3)and#18 (428kBqm−3).Radonactivityat#63,#49,#41,#45and#64can beattributedtosoilmoisturewhentheradiumcontentareupto 70Bqkg−1 (bluelineinFig.7).However,radonactivitieshigher than270kBqm−3areunlikelywhenincreasesinradiumcontent andsoilmoistureareconsidered.Consequently,alsointhiscase, thecontributionbyadeepsourcecanbeinvoked(e.g.,#62and #18).Wemayspeculatethatinadditiontotheincreaseofradon concentrationbywatersaturationinthesoil,anexcessof222_Rn islikelyoccurringandfavoredbythepresenceofparent radionu-clides(UandRa)dissolvedinthegroundwatersystem.

5.2.2. CorrelationbetweenCO2fluxandradon(222Rn–220Rn) activityinsoil-gas

AtJabalónRiverthehighestactivitiesof222_RnandCO 2fluxes arenotapparentlycloselycorrelated(Fig.8a).Nevertheless,222_Rn contentshigherthanthoseestimatedbythepresenceof226_Rain thesoilweredetectedinsitesclosetothethermalsprings(e.g.,#18 and#62)andawayfromthem(e.g.,#30and#42),suggestingthe presenceofadeep222_{Rnsource.However,}222_{Rnactivitynearthe} springwaters(blue-colorednumbersinFig.8a)wasalwayshigher thanthosemeasuredinthedistalpoints(black-colorednumbersin Fig.8a),althoughCO2fluxeswererelativelylow(e.g.,#62and#64 versus#30,#42or#59).Aninversecorrelationbetween220_Rnand CO2wasnotobserved(Fig.8b).Ontheotherhand,220Rnhadasmall increase(Fig.9),whichcouldbeinterpretedintermsofshallow originofradon,atleastmoresuperficialthanthatofLaSima.This couldlikelybeaccountedforthetransportandprecipitationofU andRa.Inrelationtothe222_Rn/220_{Rnratio(Fig.8c)therewasnota} clearincreasewiththeCO2fluxincontrasttowhatobservedatLa Sima.Despitetherelativelyhighisotopicratio(e.g.,#49,#62,#45), nocorrelationwiththeCO2fluxwashighlightedsinceinsomecases the222_Rn/220_{Rnratiodecreased(e.g.,#59vs#36or#49).Similar} featurescanbeproducedbyeitherdeeporshalloworiginforradon andbythesoilwatersaturation.

Theseresultssuggestthattheincreaseofradonactivityinthe JabalónRiverwithrespecttothebackgroundislikelyrelatedtotwo

Fig.8.RelationshipbetweenCO2fluxand222Rn/220RnratiosintheJabalónRiver(bluetextsinAindicatethepointsnearwaterspring,Table2,Fig.3).(Forinterpretationof

thereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

contributions:(i)adeep-seatedsourcecarriedbyCO2,supported bythehigherconcentrationsof222_{Rnwithrespecttothose} theo-reticallyestimatedby226_{Rainsoil,and(ii)asourcerelatedtothe} decayofparentradionuclides(U–Ra),dissolvedandtransportedby groundwatersystemassupportedbythehighestradiumactivities whenapproachingthespringwaterdischarges.

TheprofilesoftheCO2fluxasafunctionofthedistancewith respecttotheCO2-richgasemissionsitesandthoseoftheRn iso-topicconcentrationand222_Rn/220_{RnratiosfortheJabalónRiverare} plottedinFigs.9and10.InFig.9aandb(profileN–SforJabalon, Fig.3c,Table2),thehighestvalueofCO2flux(#45,141gm−2d−1)is

coupledwiththatof222_Rn(184kBqm−3_{),althoughat#41theCO2} fluxsharplydecreasesdownto45gm−2d−1whiletheRnactivity wasfoundtobeconstant(182kBqm−3).Movingtothenorthof theCO2-bearingspring(#36)bothactivityof222RnandCO2flux (47kBqm−3 and 12gm−2d−1, respectively) decrease.Then, the 222_{Rnactivityachievesasecondarypeak(#30:83kBqm}−3_),which isnotrelatedtoanyincreaseof226_{Rainthesoil(Table3),whilethe} CO2 fluxincreases(44gm−2d−1).Incontrast,intheE–Wprofile (Fig.9candd)thehighestrecordedCO2flux(#42;102gm−2d−1) doesnotmatchwiththatofthe222_{Rnactivity(#41;182kBqm}−3_). Theactivityof220_{Rninbothprofilesshowsanincreaseassociated}

Fig.9.Spatialcorrelationbetween222_Rn/220_RnandCO

2fluxalongtheJabalónRiver(Table2,Fig.3;NF:noflow,soil-gascouldnotbesampledduetotheextremelylow

Fig.10.Spatialcorrelationbetween222_Rn/220_RnandCO

2fluxalongtheJabalónRiver(Table2,Fig.3).Horizontalaxisisthedistance(m)fromthesamplepointtothecenter

oftheCO2leak.

withtheCO2flux,althoughthehighest220Rnconcentration(#41) doesnotcompletelymatchwithrespecttothesitewherethe high-estCO2flux(#45inprofileN–Sand#42inprofileE–WinFig.9a andb)isfound.

InFig.10,thelackofcorrelationbetweenthehighestvaluesof 222_{RnconcentrationandCO}

2fluxforthesamespotismoreevident. Nevertheless,despitethefactthatalinearcorrelationbetweenCO2 and222_{Rnisnotdetected,thereisadirectinfluenceinradon(}222_Rn) activityassociatedwiththeCO2leakage.Thisissupportedbythe presenceof 222_{Rnconcentrations neartheseepage zonesmuch} higherthanthosecalculatedbyconsideringtheRaconcentrations inthesoilortheincreaserelatedtosoilwatersaturation.

InthespringswithbubblingCO2 (#48, #65to#68;Table2,

Fig.11)the222_{Rnactivityislowerwithrespecttothatmeasured} inthesurroundingsites.Thisdiscrepancycanlikelybeduetothe differentsamplingmethodssinceinthesespotsradon measure-mentswerecarriedoutdirectlyintheairflow,withacollectoron thebubblinggasphaseduetothepresenceofwater.Consequently, thesemeasurementscanberelatedtotheamountof222_Rncarried byCO2andnottothecontentofUandRainthethermalwaters orthepresenceofsoilwatersaturation(SF).Nevertheless,inthe soil-gasnearthedegassingsitesalleffectscouldbedetectedand thereforetheradonconcentrationinthegasishigher.Thepositive linearcorrelationbetweentheCO2fluxandconcentrationof222Rn inthegasventissupportingthishypothesis(Fig.11).

Anothersignificantobservationwasthatat LaSima theCO2 fluxesintheCO2-richgasemissionsiteswerehigherthanthose recordedalongtheJabalónRiver,withtheexceptionofthe water-degassingpoints,whereasthe222_{Rnactivitiesshowedanopposite} behavior. At La Sima the highest CO2 fluxes were from 3000 to5000gm−2d−1 and the222_{Rnconcentrationswereclustering}

around120kBqm−3(Table1;#1,#8a#24).Conversely,atJabalón RivertheCO2fluxwas200gm−2d−1(e.g.,#18and#59)andthe 222_{Rnconcentrationwasaround400kBqm}−3_{(e.g.,#18and#62).} Thiswouldsupportthehypothesisthat alongtheJabalon River a radon sourcederived from dissolved U and Ra adds to that

Fig.11.CorrelationbetweenCO2fluxand222Rninthedegassingpools(Table2,

transportedbyCO2.WhileatLaSimatheonlycausethatleadstothe increaseinradonistheCO2flux,intheJabalonRivercontributions fromsoilwater,UandRaandCO2fluxaretobeadded.Thus,this wouldexplainwhyatLaSimawemeasuredhighCO2fluxeswhile the222_{RnactivitieswererelativelylowerthanthatintheJabalon} Riverarea.

5.3. ImplicationformonitoringCO2storagesites

TheCO2-richemissionsatCCVFaremainlyassociatedwith frac-turesystemsandthegasdischargesareregardedaspoint-source seepage,beingoccasionallycharacterizedbyrelativelyhighflow: 1tm−2d−1orevenhigher.Inthevicinityofthesesites,theCO2 fluxesapproachvalues,whicharetypicalofsoilrespiration.Inthe perspectiveofassessinggasleakagesinareasselectedforgeological carbonstorage,theobservationofthepoint-sourceseepagereveals thatthedetectionofleakagesitescanposeseriousdifficultiesgiven thelargeareapotentiallyaffectedbytheleakageofinjectedCO2 (hundredskm2_{).ItisworthmentioningthattheamountofCO}

2 emittedfromthesepointscanbeoftheorderofmagnitudeofthe maximumleakage rateconsideredacceptable(0.1%ofthetotal amountinstorageperyear;VanderZwaanandSmekens,2006) foracommercialstorageproject(i.e.,approximately1Mtyear−1; suchasin Snоhvit:0.73Mtyear−1,Sleipner:0.98Mtyear−1 and InSalah:1.28Mtyear−1;Michaeletal.,2010).Thus,conventional approachesmightnotbeabletodetectsmall-scaleemissions.

Inthis regards,in theframework of CCSprojectssignificant variationsinterms ofsoildiffuseCO2 fluxmaybeoccurringat thescale of few meters or even centimeters and CO2 leakages cannot be detected until clear evidences (e.g., impacts on the vegetation,newlyformedbubblingsites)arevisibleinthefield, evenwhensmallsamplegrids,e.g.,10–20m(e.g.,Elíoetal.,2013 andreferencestherein)areconsidered.Besides,naturalanalogs (e.g.,geothermalorvolcanicareas)havegasfluxes,whichareby farhigherthanthose expectedfora sequestrationproject. Fur-thermore,inthecaseofseepagessmallgeographicchangesand moderateseasonalchangesintheCO2fluxesareexpectedtobe detectedwithrespecttothebackgroundlevels.Thismakesthe reconnaissanceofseepagesrealchallenging.

The application of other cost-effective monitoring/detection techniquesabletocover largerareas, suchasEddy covariance, open-pathlaserandremotesensing,needtobeimprovedand prop-erlyevaluatedfortheirapplicabilitytoCCSprojects(e.g.,Klusman, 2011;Lewickietal.,2009;Ortegaetal.,2014).Long-term continu-oussoilgassurveyscanhelptoidentifyleakages(e.g.,Schlömer et al., 2014), although these might also be undetectable when occurringinverylocalizedandsmallareas.

ConcerningradonmeasurementsintheCCVFareas,anincrease inthe222_{Rn soil-gasconcentrationwasdetectedandcorrelated} totheCO2flux.Theinfluenceofthis222Rnanomalyzone,though small,seemstobeoflargerextentwithrespecttothatofCO2. Consequently,radonisotopescanbeconsideredasgood tracers todiscriminateleakageareas,providingusefulinformationabout the(biologicalordeep-seated)originofCO2.Radonisotope mea-surements canovercome thelimitations of others geochemical methods,whicharecommonlyusedtointerprettheoriginofthe soilgases,e.g.,␦13_C–CO

2orheliumisotoperatios.Another advan-tageofradonmeasurementsisthattoavoid“falsepositives”.Radon activityinsoil-gasisindeedrelatedtolithologicalandgeostructural (faultsandfractures)features.Thismeansthatwhenanincrease “deep”CO2fluxeswithrespecttothebackgroundvaluesoccurs, radonactivityis alsoexpectedtoincrement.Conversely,ifCO2 fluxesincreaseinresponseofanenhancedbiologicalactivity,radon activityshouldnotsufferanyparticularvariation.

AtCCFV,the222_{Rnactivityinthesoilgaswasoccasionallyas} highas430kBqm−3,i.e.,much higherthanthethresholdvalue

(>50kBqm−3;CothernandSmith,1987)forwhichacertainsite isconsideredwithahigh-riskpotential.Suchhighconcentrations needtobetakenintoaccountwhenCCSriskassessmentplansareto bedesigned,particularlywhenCO2leakagesmayaffectresidential areasandworkingenvironments.ConsumptionofCO2-richwaters isalsoarisksincetheymaypotentiallycontainhighconcentrations ofdissolved222_{Rn,andUandRa.}

Inthis study,correlations between220_{Rn concentrationsand} CO2fluxeswerenotasclearasobservedforthoseof222RnandCO2. InthedrygasventofLaSima,asignificantreductionofthe220_Rn concentrationswasdetectedandlikelyrelatedtoanincreasein theCO2flux.However,intheemissionsitesassociatedwithwater springs222_RnandCO

2werenotcorrelatedwhileaslightincrease in220_{RnconcentrationswiththeCO}

2fluxwasrecorded.Thiseffect islikelyduetodifferentsourceof222_Rnand220_Rnisotopes.

6. Conclusions

TheCampodeCalatravaVolcanicFieldisarelevantnatural ana-logwheregasleakagesintheframeworkofgeologicalCO2storage projectscan beinvestigated.A largenumber ofCO2-rich emis-sions,predominantlyassociatedwithfracture/faultsystemsoccur, althoughtheymainlydischargeaspunctualmanifestations.The resultssuggestadeeporiginofCO2,inagreementwithprevious geochemicalandisotopicanalysesofthegasleaks,whichindicate thatCO2ismantlesourced.

Accordingtoourdata,theincreaseof222_{Rninthesoilgasesis} likelyproducedbytwomechanisms,(i)advectivetransportwhere CO2actsascarriergasand(ii)generationatshallowleveldueto thepresenceofUandRainboththermalwatersandsoils.

GeochemicalsurveysofCO2fluxandradonisotopic measure-mentsare very useful to: (i)define and quantify CO2 leakages oncedetectedand(ii)monitortheevolutionoftheleakagesand theeffectivenessofsubsequentremediationactivities.These tech-niquescanalsobesuccessfullyappliedtodetectdiffuseleakages. Acknowledgements

We wish tothank C. Jenkinsand NunziaVoltattorniand an anonymousreviewerwho greatlyimprovedanearlyversion of themanuscript.ThisworkwasfundedbytheCiudaddelaEnergía Foundation(CIUDEN),throughtheALM-08-006contract,and co-financedbytheEuropeanUnion(EuropeanEnergyProgrammefor Recovery).Thesoleresponsibilityofthispublicationlieswiththe authors.TheEuropeanUnionisnotresponsibleforanyusethat maybemadeoftheinformationcontainedtherein.

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