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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
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complicated and often questionable application of radiocarbon dating. The reason for this is to be found
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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\
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origin was discovered in atmospheric CO2LQ E\:LOODUG)/LEE\ZKRGHWHUPLQHGDKDOIOLIHRI
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UDGLRFDUERQGDWLQJRIRUJDQLFFDUERQRIELRORJLFDORULJLQZKLFKUHYROXWLRQL]HGUHVHDUFKLQDQXPEHU
RI ¿HOGV LQFOXGLQJ DUFKDHRORJ\ DQG TXDWHUQDU\ JHRORJ\FOLPDWRORJ\ E\ HVWDEOLVKLQJ DJHV DQG
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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
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cycles. The atmospheric mixing ratio of CO2 is balanced by the rate of cosmogenic production,
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&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
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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
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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
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WR FDOHQGDU \HDUV )RU H[DPSOH WKH ODVW JODFLDO PD[LPXP /*0 RFFXUUHG DERXW ND UDGLRFDUERQ
\HDUVDJRZKLFKFRUUHVSRQGVWRDERXWNDFDOHQGDU\HDUV %DUGHWDO >@ +RZHYHUUHFHQW
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)DLUEDQNV >@
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JURXQGZDWHU 0QQLFK >@ 0QQLFK >@ 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
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)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[WRIJHRFKHPLFDOPRGHOOLQJ WKDWLVWKHVWXG\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
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processes occurring in aquifers, a groundwater sample is a collection of water molecules, each of which
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system, or of stationary waters entrapped in a geological formation, the concept of groundwater
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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
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This chapter reviews some of the past radiocarbon adjustment models, the conditions under
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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
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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
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(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 SRVW V ZDWHU:DWHU
IURP WKH SRVW V ERPE HUD KDV C amounts that are greater than the historic values that existed
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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.
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needs to be given to the palaeoclimatic conditions corresponding to the time an old, geochemically
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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
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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
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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
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3H concentrations for waters from recharge areas of the Middle Rio Grande Basin aquifer system
and
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content of & RI ',& LQ UHFKDUJH DUHDV WR WKH JURXQGZDWHU V\VWHP ZDV QHDU SPF &DUERQ
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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,
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