Plummer-Glynn-IAEA-ch4-2013.pdf - Chapter 4 RADIOCARBON DATING IN GROUNDWATER SYSTEMS L.N PLUMMER P.D GLYNN United States Geological Survey Reston

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Unformatted text preview: Chapter 4 RADIOCARBON DATING IN GROUNDWATER SYSTEMS L.N. PLUMMER, P.D. GLYNN United States Geological Survey, Reston, Virginia, United States of America ³5DGLRFDUERQ GDWLQJ RI JURXQGZDWHU ZKLFK VWDUWHG ZLWK WKH RULJLQDO ZRUN RI WKH +HLGHOEHUJ 5DGLRFDUERQ /DERUDWRU\ EDFN LQ WKH ODWH V LV FHUWDLQO\ RQH RI WKH PRVW ² LI QRW WKH PRVW ² complicated and often questionable application of radiocarbon dating. The reason for this is to be found LQWKHDTXHRXVJHRFKHPLVWU\RIFDUERQLQWKHXQVDWXUDWHGDQGVDWXUDWHGRQHV3DUWRIWKHFDUERQDQG radiocarbon content of dissolved carbon in groundwater is of inorganic and part is of organic origin.” 0RRN  >@   INTRODUCTION The radioactive isotope of carbon, radiocarbon (&  ZDV ¿UVW SURGXFHG DUWL¿FLDOO\ LQ  E\ 0DUWLQ.DPHQDQG6DP5XEHQZKRERPEDUGHGJUDSKLWHLQDF\FORWURQDWWKH5DGLDWLRQ/DERUDWRU\DW %HUNHOH\&$LQDQDWWHPSWWRSURGXFHDUDGLRDFWLYHLVRWRSHRIFDUERQWKDWFRXOGEHXVHGDVDWUDFHULQ ELRORJLFDOV\VWHPV .DPHQ  >@5XEHQDQG.DPHQ  >@ &DUERQRIFRVPRJHQLF origin was discovered in atmospheric CO2LQE\:LOODUG)/LEE\ZKRGHWHUPLQHGDKDOIOLIHRI D/LEE\DQGKLVFRZRUNHUV $QGHUVRQHWDO  >@/LEE\HWDO  >@ GHYHORSHG UDGLRFDUERQGDWLQJRIRUJDQLFFDUERQRIELRORJLFDORULJLQZKLFKUHYROXWLRQL]HGUHVHDUFKLQDQXPEHU RI ¿HOGV LQFOXGLQJ DUFKDHRORJ\ DQG TXDWHUQDU\ JHRORJ\FOLPDWRORJ\ E\ HVWDEOLVKLQJ DJHV DQG FKURQRORJLHVRIHYHQWVWKDWKDYHRFFXUUHGRYHUWKHSDVWDSSUR[LPDWHO\ND Cosmogenic & LV SURGXFHG LQ WKH XSSHU DWPRVSKHUH E\ WKH QXFOHDU UHDFWLRQ 0DFND\   >@0DNHWDO  >@   1Qĺ&S  where &R[LGL]HVWRCO and then CO2, and mixes with the atmosphere. CO2 is absorbed by SODQWV GXULQJ SKRWRV\QWKHVLV DQG EHFRPHV LQFRUSRUDWHG LQWR WKH (DUWK¶V ELRORJLFDO DQG K\GURORJLFDO cycles. The atmospheric mixing ratio of CO2 is balanced by the rate of cosmogenic production, XSWDNHE\WKHYDULRXVFDUERQUHVHUYRLUVRQ(DUWK DWPRVSKHUHRFHDQVPDULQHRUJDQLVPVSODQWV DQG UDGLRDFWLYHGHFD\&DUERQGHFD\VDFFRUGLQJWR  &ĺN + ßí  ZLWK D JHQHUDOO\ DFFHSWHG FRQVHQVXV  KDOIOLIH RI  “  D *RGZLQ   >@  &KLX HW DO   >@ UHYLHZ WKH YDULRXV PHDVXUHPHQWV RI & KDOIOLIH DQG VXJJHVW WKDW WKH PRGHUQ KDOIOLIH may be underestimated by approximately 300 a. The conventional radiocarbon age, tLQ\HDUVLVE\GH¿QLWLRQ t W ln Ao  A  33 where IJ LVWKH/LEE\PHDQOLIH DOQZKHUHLVWKH/LEE\KDOIOLIH  is the initial &VSHFL¿FDFWLYLW\ LQ%TNJRUP%TJ%T GLVLQWHJUDWLRQSHUVHFRQG  Ao A is the measured &VSHFL¿FDFWLYLW\ %\ LQWHUQDWLRQDO FRQYHQWLRQ VSHFL¿F DFWLYLWLHV DUH FRPSDUHG WR D VWDQGDUG DFWLYLW\ Aox, where Aox WLPHVWKHVSHFL¿FDFWLYLW\RI1%6R[DOLFDFLG îGLVLQWHJUDWLRQVSHUPLQXWHSHU gram of carbon (dpm/g C) in the year 1950 A.D.). The initial &VSHFL¿FDFWLYLW\Ao, and the measured &VSHFL¿FDFWLYLW\RIDVDPSOHA, can be expressed as a percentage of this standard activity in per cent modern carbon (pmc) where pmc = (A/Aox î 0RRN  >@ 7KHPRGHUQSUHQXFOHDU detonation atmospheric C content is, by convention, 100 pmc, corresponding to 13.56 dpm/g C in WKH\HDU$' 6WXLYHUDQG3RODFK  >@ &RQYHQWLRQDOUDGLRFDUERQDJHVFRQWLQXHWREH UHSRUWHGEDVHGRQWKH/LEE\KDOIOLIHVRDVQRWWRFRQÀLFWZLWKHDUOLHUVWXGLHVDQG C model ages are WKHQ GH¿QHG LQ µUDGLRFDUERQ \HDUV¶$ IXUWKHU FRPSOLFDWLRQ LQ UDGLRFDUERQ GDWLQJ ZDV WKH GLVFRYHU\ that the amount of CO2 in the atmosphere has not been constant over time. Past variations in the solar ZLQG DQG WKH JHRPDJQHWLF ¿HOGV RI WKH VXQ DQG (DUWK KDYH FDXVHG YDULDWLRQV LQ WKH ÀX[ RI FRVPLF rays reaching the Earth, resulting in variations in the atmospheric concentration of CO2 .DOLQ   >@  ,W LV OLNHO\ WKDW WKH DWPRVSKHULF C content has also changed in response to changes in the residence time of the major global reservoirs (terrestrial, biosphere and ocean). The discovery of past variations in the amount of atmospheric & KDV OHG WR DQRWKHU PDMRU ¿HOG RI UDGLRFDUERQ LQYHVWLJDWLRQVLQGHWHUPLQLQJDQGUH¿QLQJUDGLRFDUERQFDOLEUDWLRQVFDOHVWRFRQYHUWUDGLRFDUERQ\HDUV WR FDOHQGDU \HDUV )RU H[DPSOH WKH ODVW JODFLDO PD[LPXP /*0  RFFXUUHG DERXW  ND UDGLRFDUERQ \HDUVDJRZKLFKFRUUHVSRQGVWRDERXWNDFDOHQGDU\HDUV %DUGHWDO  >@ +RZHYHUUHFHQW DQDO\VLVRIWKH%DUEDGRVVHDOHYHOUHFRUGSODFHVWKH/*0DWDERXWNDFDOHQGDU\HDUV 3HOWLHUDQG )DLUEDQNV  >@  0QQLFK ZDV WKH ¿UVW WR H[WHQG UDGLRFDUERQ GDWLQJ WR GLVVROYHG LQRUJDQLF FDUERQ ',&  LQ JURXQGZDWHU 0QQLFK   >@ 0QQLFK   >@  2YHU WKH SDVW  \HDUV DQ H[WHQVLYH literature of investigations and applications of radiocarbon in hydrological systems has followed. Many advances in collection and analysis of C have also followed and now C content is almost routinely determined on carbon samples as small as 1 mg by using accelerator mass spectrometry (AMS). Many of the original studies were reported in proceedings of symposia sponsored by the Isotope Hydrology 6HFWLRQRIWKH,$($$QXPEHURIUHYLHZVVXPPDUL]HVRPHRIWKHDGYDQFHVSULQFLSOHVDQGSUREOHPV LQUDGLRFDUERQGDWLQJRI',&LQJURXQGZDWHU )RQWHV  >@)RQWHVDQG*DUQLHU  >@ )RQWHV  >@*H\K  >@.DOLQ  >@0RRN  >@0RRN  >@  Numerous studies have applied radiocarbon dating to establish chronologies of the (approximately) ±NDWLPHVFDOHLQK\GURORJLFDOV\VWHPVWRHVWLPDWHPRGHUQDQGSDODHRUHFKDUJHUDWHVWRDTXLIHUV WRUHFRJQL]HQRQUHQHZDEOHSDODHRZDWHUVWRH[WUDFWSDODHRFOLPDWLFLQIRUPDWLRQIURPWKHJURXQGZDWHU DUFKLYHWRFDOLEUDWHJURXQGZDWHUÀRZPRGHOVDQGWRLQYHVWLJDWHWKHDYDLODELOLW\DQGVXVWDLQDELOLW\RI groundwater resources in areas of rapid population growth. It is beyond the scope of this chapter to review these many studies. In spite of the many advances in collection, analysis and application of radiocarbon in the hydrological sciences, interpretation of the radiocarbon model age of dissolved carbon in groundwater is still limited by many uncertainties in determining the initial C content of dissolved carbon in recharge areas to aquifers and in accounting for the many chemical and physical processes that alter the &FRQWHQWDORQJÀRZSDWKVLQDTXLIHUV7KHSXUSRVHKHUHLVWRVXPPDUL]HWKHFXUUHQWVWDWH of methods used to interpret C model age from measurements of C in DIC and dissolved organic carbon (DOC) in groundwater. Historically, hydrologists and geochemists have resorted to simplifying assumptions regarding JHRFKHPLFDODGMXVWPHQWVRIUDGLRFDUERQLQJURXQGZDWHUV\VWHPVRIWHQZLWKRXWVXI¿FLHQWGDWDWRNQRZ whether additional processes are needed to accurately date the DIC in groundwater systems. As many  geochemical interactions and hydrological processes in groundwater systems can affect radiocarbon content in aquifers, modern approaches to radiocarbon dating in groundwater systems are often treated ZLWKLQWKHFRQWH[WRIJHRFKHPLFDOPRGHOOLQJWKDWLVWKHVWXG\RIJHRFKHPLFDOHYROXWLRQRIZDWHU±URFN systems. Relatively large uncertainties in the C model age of DIC in groundwater systems remain, however, and as a result, although radiocarbon calibration is commonly applied to dating of biological carbon, such calibration is rarely warranted in radiocarbon dating of DIC in groundwater due to WKHPDQ\XQNQRZQJHRFKHPLFDODQGSK\VLFDOSURFHVVHVDIIHFWLQJWKHC content of DIC. %HWKNH DQG -RKQVRQ   >@ UHFHQWO\ GLVWLQJXLVKHG EHWZHHQ L  VDPSOH DJH FDOFXODWHG DFFRUGLQJ WR D JHRFKHPLFDO DJHGDWLQJ WHFKQLTXH DV FRPPRQO\ DSSOLHG LQ µWUDGLWLRQDO¶ UDGLRFDUERQ DGMXVWPHQWPRGHOVDQG LL SLVWRQÀRZDJHWKHWLPHUHTXLUHGWRWUDYHUVHDÀRZOLQHIURPWKHUHFKDUJH SRLQW WR D ORFDWLRQ LQ WKH VXEVXUIDFH )XUWKHUPRUH WKH\ UHFRJQL]HG WKDW EHFDXVH RI K\GURG\QDPLF processes occurring in aquifers, a groundwater sample is a collection of water molecules, each of which KDVLWVRZQDJH6LPLODUO\)RQWHV  >@ZURWH ³2ZLQJ WR GLVSHUVLRQ WKH µDJH¶ RI D JURXQGZDWHU VDPSOH FRUUHVSRQGV JHQHUDOO\ WR D WLPH GLVWULEXWLRQRIPDQ\HOHPHQWDU\ÀRZV7KXVH[FHSWLQWKHWKHRUHWLFDOFDVHRIDSXUHSLVWRQÀRZ system, or of stationary waters entrapped in a geological formation, the concept of groundwater DJHKDVOLWWOHVLJQL¿FDQFH´ 7KHVHDJHFRQFHSWVDUHGLVFXVVHGLQ&KDSWHULQWKHFRQWH[WRIWKLVERRN $OWKRXJK K\GURG\QDPLF SURFHVVHV FDQ FDXVH GLI¿FXOWLHV LQ WKH LQWHUSUHWDWLRQ RI WUDFHU PRGHO age, many hydrogeological settings have been investigated where radiocarbon dating has provided XVHIXOLQIRUPDWLRQRQÀRZDQGUHFKDUJHUDWHVDQGC model ages have been partially corroborated by FRQFRUGDQFHZLWKRWKHULVRWRSLFDQGHQYLURQPHQWDOWUDFHUVRQWKH±NDWLPHVFDOH7KHLQWHUSUHWLRQ of the C model age is provided below, where the model age is the &SLVWRQÀRZDJHFDOFXODWHGE\D µWUDGLWLRQDO¶JHRFKHPLFDODGMXVWPHQWPHWKRGWKDWLVDSSOLHGWR',&7KHQVRPHRIWKHPRUHDGYDQFHG JHRFKHPLFDO PRGHOOLQJ WHFKQLTXHV WKDW FDQ KHOS UH¿QH D C model age and quantify hydrodynamic mixing on the basis of solution chemistry and isotopic compositions will be discussed. A discussion of advances in radiocarbon dating of DOC follows. Finally, the complexities of assessing the effects of GLIIXVLYHSURFHVVHVDQGOHDNDJHIURPRUWKURXJKFRQ¿QLQJXQLWVZLOOEHH[DPLQHG This chapter reviews some of the past radiocarbon adjustment models, the conditions under ZKLFK WKH\ DSSO\ DQG VXPPDUL]HV WKH JHRFKHPLFDO PRGHOOLQJ DSSURDFK WR UDGLRFDUERQ GDWLQJ DV LPSOHPHQWHG LQ WKH LQYHUVH JHRFKHPLFDO PRGHOOLQJ FRGH 1(73$7+ 3OXPPHU HW DO   >@  which applies radiocarbon dating to the total dissolved carbon (TDC) system (DIC + DOC + CH). In an effort not to obscure the presentation with too many equations and details, these have been SODFHGLQWKH$SSHQGL[WR&KDSWHUDORQJZLWKVRPHH[DPSOHFDOFXODWLRQV8QLWVXVHGLQUHSRUWLQJ UDGLRFDUERQPHDVXUHPHQWVDQGFRQYHQWLRQDOUDGLRFDUERQDJHDUHGH¿QHGLQWKH$SSHQGL[WR&KDSWHU (TXDWLRQVGHVFULELQJLVRWRSLFIUDFWLRQDWLRQLQWKHFDUERQDWHV\VWHPDUHVXPPDUL]HGLQWKH$SSHQGL[ WR&KDSWHUZKHUHWKH\DUHJHQHUDOL]HGWRV\VWHPVRI7'& ',&'2&&+). The Appendix to &KDSWHUDOVRSURYLGHVVRPHJXLGDQFHRQUDGLRFDUERQFDOLEUDWLRQWRFDOHQGDU\HDUV'HWDLOVSHUWDLQLQJ WR¿HOGVDPSOLQJDUHSURYLGHGDVZHOODVUHIHUHQFHWRDYDLODEOHVRIWZDUHXVHGLQUDGLRFDUERQGDWLQJRI ',&LQJURXQGZDWHU)LQDOO\WKH$SSHQGL[WR&KDSWHUVXSSOLHVDUHIHUHQFHWRVHOHFWHG$06IDFLOLWLHV providing radiocarbon determinations.  INTERPRETATION OF RADIOCARBON AGE OF DISSOLVED INORGANIC CARBON IN GROUNDWATER &DUERQ RI FRVPRJHQLF RULJLQ LV LQFRUSRUDWHG LQ JURXQGZDWHU GXULQJ UHFKDUJH E\ LQWHUDFWLRQ RI LQ¿OWUDWLQJ ZDWHU ZLWK VRLO &22 from plant root respiration and microbial degradation of soil RUJDQLF PDWWHU VHH IRU H[DPSOH .DOLQ   >@  )ROORZLQJ UHFKDUJH ',& EHFRPHV LVRODWHG 35 from the modern C plant–soil gas–air reservoir and decays with time. Many physical and chemical processes can affect the C content of DIC in groundwater, beyond that of radioactive decay, and must be considered to interpret radiocarbon model ages and their uncertainties. The most important considerations in radiocarbon dating of DIC in groundwater can be grouped under four general topics: (a) Determination of the initial C content, Ao, of DIC in groundwater recharge, at the point where LQ¿OWUDWLQJZDWHULVLVRODWHGIURPWKHXQVDWXUDWHGRQHC reservoir; (b) Determination of the extent of geochemical reactions that occur within the aquifer following LVRODWLRQIURPWKHXQVDWXUDWHGRQHDQGWKHHIIHFWRIJHRFKHPLFDOUHDFWLRQVRQC content; (c) Evaluation of the extent to which physical processes alter the C content (such as mixing of old and young water in samples pumped from wells; hydrodynamic dispersion along hydrological ÀRZSDWKVPDWUL[GLIIXVLRQDQGRUGLIIXVLYHH[FKDQJHZLWKFRQ¿QLQJOD\HUVOHDNDJHIURPRWKHU DTXLIHUVRUVXU¿FLDOZDWHUVLQVLWXSURGXFWLRQ  (d) When warranted, correction for historical variations in atmospheric C content, through application of radiocarbon calibration scales. These four topics are discussed below. 4.2.1. Determination of initial 14C in recharge water, Ao ,Q WKH XQVDWXUDWHG ]RQH &22 partial pressure is typically substantially higher than that in the atmosphere (about 10–3.5) as a result of biological activity, soil moisture and, often, higher WHPSHUDWXUH %URRN HW DO   >@  $V LQ¿OWUDWLQJ ZDWHU PRYHV WKURXJK WKH XQVDWXUDWHG ]RQH the CO2LQLQ¿OWUDWLQJZDWHULVDXJPHQWHGE\VRLORQH&22. The dissolved CO2 reacts with carbonate and silicate minerals in the soil and sediment of the recharge area, resulting in increased concentrations RIGLVVROYHGFDUERQ ',&DQG'2& LQWKHLQ¿OWUDWLQJZDWHU The term Ao refers to the initial C content of DIC in groundwater that occurs following recharge and isolation of the water from the modern &UHVHUYRLURIXQVDWXUDWHGRQH&22. AoPXVWEHNQRZQRU estimated to date the C of dissolved carbon in groundwater in hydrological systems. In the following, WKHWHUPµSPF¶LVXVHGWRH[SUHVVWKHC content as a per cent of the modern standard (see above). 4.2.1.1. Estimation of Ao from measurements in recharge areas Ideally, measurements of the radiocarbon content of DIC and DOC in groundwater from WKH UHFKDUJH DUHDV RI DTXLIHUV FDQ EH XVHG WR GH¿QH Ao, but this approach has two complications. First, many of the waters in recharge areas of aquifers today contain tritium and/or CFCs, which are indications of potential contamination of &IURPSRVWQXFOHDUGHWRQDWLRQ SRVWV ZDWHU:DWHU IURP WKH SRVWV ERPE HUD KDV C amounts that are greater than the historic values that existed LQ SUHV UHFKDUJH DUHDV DQG LI WKHVH REVHUYHG YDOXHV IURP UHFKDUJH DUHDV DUH XVHG LQ GDWLQJ radiocarbon model ages will be biased old. Waters from recharge areas of aquifers can also be mixtures RISUHDQGSRVWERPEHUDZDWHUVDJDLQOHDGLQJWRDQROGELDVLQWKH C model age. The issue here LV QRW VR PXFK WKDW RI FRQWDPLQDWLRQ EXW UDWKHU RI LQVXI¿FLHQW NQRZOHGJH RI WKH FRQWDPLQDWLRQ WR adequately calculate the effective AoYDOXHWKDWZRXOGLQFRUSRUDWHRQO\QDWXUDOSUHERPE C dilution processes, and would have applied at the time of recharge of the old groundwater under investigation. 6HFRQGO\ HYHQ LI SUHERPE ZDWHUV FDQ EH LGHQWL¿HG LQ WKH UHFKDUJH DUHD WRGD\ FRQVLGHUDWLRQ needs to be given to the palaeoclimatic conditions corresponding to the time an old, geochemically HYROYHG ZDWHU VDPSOH ZDV UHFKDUJHG $V VKRZQ EHORZ XVLQJ VRPH RI WKH ZHOO NQRZQ DGMXVWPHQW models, modelled values of Ao can be sensitive to the G13C of soil gas CO2. Further, the G13C of soil gas CO2 FDQ FKDQJH VLJQL¿FDQWO\ RYHU WLPH LQ UHFKDUJH DUHDV LQ UHVSRQVH WR FOLPDWLF YDULDWLRQV WKDW FDXVHFKDQJHVLQWKHUHODWLYHSURSRUWLRQVRISODQWVXWLOL]LQJWKH&3 and C photosynthetic pathways. In addition, the extent to which recharge waters evolve in isotopic equilibrium with soil gas (open system 36 HYROXWLRQ  RU UHDFW ZLWK FDUERQDWHV IROORZLQJ UHFKDUJH FORVHG V\VWHP HYROXWLRQ  &ODUN DQG )ULW]  >@'HLQHVHWDO  >@ FDQOHDGWRXQFHUWDLQWLHVLQ C model age of old groundwater as much as a full & KDOIOLIH ,VRWRSLF IUDFWLRQDWLRQ LQ RSHQ DQG FORVHG V\VWHPV LV GLVFXVVHG LQ WKH $SSHQGL[WR&KDSWHU(YHQLILWFDQEHGHWHUPLQHGZKHWKHUUHFKDUJHZDWHUVSUHVHQWO\HYROYHXQGHU RSHQRUFORVHGV\VWHPFRQGLWLRQVLWLVQRWNQRZQZKHWKHUPRGHUQFRQGLWLRQVSUHYDLOHGZKHQWKHROG groundwater was recharged. Another assumption that is commonly made, and which is applicable in most but perhaps not all FDVHVLVWKDWWKHµUHFKDUJHDUHD¶RIROGJURXQGZDWHUZDVWKHVDPHDVWKDWREVHUYHGWRGD\,QVRPHFDVHV differences in regional climate may have caused differences in the relative amounts of recharge from different areas over time. Furthermore, modern water resource management activities can also affect the distribution of modern recharge. Changes in aridity and other climatic factors in a recharge area over the timescale of an aquifer can cause changes in the distribution of C to C3 in plants, which can result in changes in the value of G13C of soil gas CO2. Some of the models used to estimate values of Ao in recharge areas (see below) are quite sensitive to the value of G13C of soil gas CO2. Finally, although JHRORJLFDODQGWHFWRQLFSURFHVVHVRIWHQKDYHQRWKDGHQRXJKWLPHWRVLJQL¿FDQWO\DOWHUWKHODQGVFDSHRI groundwater recharge over the C timescale, exceptions do occur. Still, if a set of groundwater samples can be obtained from the modern recharge area, or along DÀRZSDWKH[DPLQDWLRQRIWKH&FRQWHQWRI',&LQUHODWLRQWRWULWLXP *H\K  >@.DOLQ   >@ 9HUKDJHQ   >@ 9HUKDJHQ HW DO   >@  RU RWKHU DQWKURSRJHQLF HQYLURQPHQWDO WUDFHUVVXFKDVWULWLXPDQG&)&V ,$($  >@3OXPPHUHWDO  >@3OXPPHUHWDO   >@3OXPPHUDQG%XVHQEHUJ  >@ RULQUHODWLRQWRGLVWDQFHRIÀRZLQDTXLIHUV *H\K   >@9RJHO  >@ FDQEHXVHGWRHVWLPDWHWKHPRGHUQSUHERPEFRQWHQWRIC. )LJXUH  FRPSDUHV & VSHFL¿F DFWLYLWLHV RI ',& H[SUHVVHG DV SPF DV D IXQFWLRQ RI &)& 3H concentrations for waters from recharge areas of the Middle Rio Grande Basin aquifer system and RI1HZ0H[LFR86$ 3OXPPHUHWDO  >@ 7KHGDWDVXJJHVWWKDWWKHSUHQXFOHDUGHWRQDWLRQ content of & RI ',& LQ UHFKDUJH DUHDV WR WKH JURXQGZDWHU V\VWHP ZDV QHDU  SPF &DUERQ YDOXHV!SPFKDYH&)&SLVWRQÀRZDJHVIURPWKHPLGVIRUJURXQGZDWHUWKDWLQ¿OWUDWHG ),*  &DUERQ FRQWHQW H[SUHVVHG DV SPF SHU FHQW RI WKH PRGHUQ VWDQGDUG  RI ',& LQ JURXQGZDWHU from the Middle Rio Grande Basin, NM, USA, as a function of (a) CFC-12 concentration (see graph in (b) for H[SODQDWLRQRIV\PEROV DQG E WULWLXP 37 FIG. 4.2. Concentrations of 14C and CFC-12 measured in groundwater from the Middle Rio Grande Basin, NM, 86$ UHGWULDQJOHV LQUHODWLRQWRFRQFHQWUDWLRQVH[SHFWHGIRUZDWHUFRQWDLQLQJPRGHUQ',&QRWGLOXWHGZLWKROG ',&7KHEOXHOLQHUHSUHVHQWVWKHDWPRVSKHULFLQSXWRIUDGLRFDUERQ /HYLQDQG.URPHU  >@/HYLQHW DO  >@ DQG&)& KWWSZDWHUXVJVJRYODE  XQPL[HGSLVWRQÀRZ 7KHOLJKWGDVKHGOLQHVUHSUHVHQW K\SRWKHWLFDO PL[LQJ RI ROG ZDWHU ZLWK ZDWHU IURP    DQG  PRGL¿HG IURP 3OXPPHU HW DO  >@  IURPWKH5LR*UDQGH FHQWUDORQH DQGSRVWDJHVIRUZDWHUVZKLFKLQ¿OWUDWHGDORQJWKHHDVWHUQ PRXQWDLQ IURQW WKH ODWWHU FRPSULVHG PL[WXUHV RU VDPSOHV FRQWDLQLQJ D IUDFWLRQ RI QRQDWPRVSKHULF &)& )LJ D 7KH KLJKHVW & YDOXH LQ VDPSOHV ORZ LQ WULWLXP DQG &)& LV QHDU  SPF )LJV D DQG E FRQVLVWHQWZLWKRSHQV\VWHPHYROXWLRQIURPWKLVVHPLDULGUHJLRQRIWKHVRXWKZHVW 86$ PRGL¿HGIURP3OXPPHUHWDO  >@  :DWHUVLQWKHQRUWKHUQPRXQWDLQIURQWQRUWKZHVWHUQDQGHDVWHUQPRXQWDLQIURQWK\GURFKHPLFDORQHV ZHUH UHFKDUJHG DORQJ PRXQWDLQ IURQWV WKDW ERUGHU WKH EDVLQ WR WKH QRUWK DQG HDVW:DWHU IURP WKHZHVW±FHQWUDORQHLVWKRXJKWWRKDYHUHFKDUJHGLQKLJKHOHYDWLRQDUHDVQRUWKRIWKHEDV...
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