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Accelerator mass spectrometry in hydrology
Nuclear Instruments and Methods in Physics Research B 92 (1994) 367-375 North-Holland
Accelerator
mass spectrometry
k!lOM B
Beam Interactions with Materials % Atoms
in hydrology
Jean-Charles Fontes a*?and John N. Andrews b** a Laboratoire d’Hydroiogie et Gkxhimie Isotopique, Universitk de Paris-&d,
Orsay, F91405 Cedex, France ’ Postgraduate Research Institute for Sedimentology, University of Reading, Reading RG6 2AB, UK
. hydrology using the AMS technique are reviewed. The small amounts of carbon needed for Applications of 14C,36Cland rz9I m AMS measurement of r4C have facilitated new areas of hydrological research. Uncertainties in the natural fall-out of 36C1and the dissolution of 36Cl-labelled chloride during groundwater migration are limitations for groundwater dating with this nuclide. Estimation of recharge by measurements of the advance of the 36CI bomb peak through the unsaturated zone is of increasing importance in hydrofogy. AMS measurements of “‘1 will nrovide an environmental monitor for nuclear pollution plumes. Some directions for future studies are suggested.
1. Introduction
Accelerator mass spectrometry (AMS) has applications in hydrological sciences either because of its capabili~ for measuring radioisotopes (e.g., 36Cl and iz91) which could not be assayed with adequate accuracy by nuclear counting techniques or because of its potential for making measurements on relatively small samples. In the case of i4C, for example, sample size may be up to 1000 times smaller than that required for ~-counting.
2. Rsdiocarbon
in groundwaters
2.1. Advantages of AMS fur carbon-I# ~eas~re~en~ uqaifers
in
Groundwater dating by measurement of the 14C activity of total dissolved inorganic carbon (TDIC) was introduced in the late 1950s and has subsequently been much applied by hydrologists. For @-counting under standard conditions, a sample containing = 2 g of carbon is typically required either for liquid scintillation counting or for assay with gas proportional counters. Such an amount of carbon is generally provided by the TDIC in about 100 litres of groundwater which must be obtained from a pumped well or borehole. The carbon is pre-concentrated by addition of BaCI, or SrCI, and precipitation of the corresponding carbonate at pH = 10. In contrast, the AMS technique requires I 1 mg of carbon for the target of the ion * corresponding author. Tel. +44 734 318684, fax +44 734 310279. ’ Deceased.
source. This large reduction in the sample size is an obvious advantage for simplification of the field protocol for TDIC recovery but, more importantly, it makes it possible to investigate the age stratification in open boreholes which are subject only to natural flow conditions. This may be accomplished by careful down-hole sampling without any disturbance or mixing in the water column due to pumping. There have, however, so far been few attempts to evaluate this application. Two samples from a vertical profile in an old ( = 20 ka) unconfined groundwater system in northern Mali [l] did reveal an unexpected age inversion of about 1.5 ka in the vertical age distribution. This could be attributed to either more rapid flow in a more permeable zone at depth or to an upwards leakage induced by the very high evaporation of the region. Similar studies on the TDIC in small soil water samples could be used to evaluate low recharge rates in arid regions. 2.2. Dissolved organic carbon The dilution of the r4C activity of TDIC due to dissolution of, or exchange with, solid carbonates along the flow path still presents a problem for the interpretation of groundwater t4C ages. Although many correction models have been developed (see ref. [2]), there is still doubt about the validity of corrected ages. For this reason, there has been much interest in the use of dissolved organic carbon (DOC) for estimating 14C ages. The AMS technique has made possible the investigation of systems with low concentrations of DOC. Several studies 13-71 have investigated the use of the humic and fulvic acid fractions of DOC. These and other high molecular weight organic compounds are believed to result from biological activity in the soil zone and not to be affected by isotopic dilution during
Old-583X/94/$07.~ 0 1994 - EIsevier Science B.V. Ah rights reserved SSDI 0168-583X(94)00143-J
VI. HYDROLOGY
techniques to measuring the r4C activity of the fractions of DOC should contribute to a better standing of the behaviour of soil-derived DOC effect of solution of diagenetic or fossil DOC 14C dating methodolo~.
groundwater migration. Low mofeeufar weight organic compounds are the result of diagenetic pracesses in which high molecular weight compounds are degraded, for example, in the formation of hydrocarbons. Studies of groundwaters from the fractured granites at Stripa [3,6] and in the sediments of the Milk River aquifer [6,7] have confirmed that Iow molecular weight fraction of DOC has relatively old 14C ages reflecting such a diagenetic origin and these compounds must not, therefore, be used for groundwater dating. Ages which were in acceptable agreement with the hydrological situations were obtained from the humic and fulvic acid components of the DOC. Subsequent studies in Canada [S] and in Germany IS], however, have shown that the humic fraction of DOC may also produce ages. which are too old, suggesting that the humic fraction can also be preserved in fossil organic deposits from which it may be leached, unrelated to the more recent groundwater flow system. The application of AMS \
\
\
‘\
\
\
\\
$y----
\
;: f ; 3+
VI
$4
RI
increases
3. Chlorine-34 3.2. Get&remicaf cycle for ~~~~~u~ 36Cl Fig, 1 illustrates the 36CI and Cl- sources and mixing processes which occur in the hydrological cycle. Various aspects of this cycle are discussed below. X1.1. Cosmic ~~o~u~tio~ anb fa&out of ~~~~~~~-~6 chlorine-36 is formed in the atmosphere by the spallation reaction 40Ar(p, ntwj3?21, 40% of production occurring within the troposphere and the remainder in \
\
\
\
\
cos\mogenic \ 36Cl(t$*=301 ko!
F?li-out’\of
f?----R,
‘i t
\
\
\\
various underand the on the
\ ‘.
a------
OS! I v
3:,3? ct
I v
d@creo3es
Marine aerosol
Oceanic
laccotith
+
+
mixing
+
+++++++
Ro (no Cl-uddition) L---r Fig. 1, A
Re
L or Em--,
schematic cycle for natural 36C1.Marine Cl- is depleted in 3hCIbecause its oceanic residence time is much greater than
the 36C1half-life (301 ka). The dilution of cosmogenic 3”Cl by 35C1and 37Cl from marine aerosols varies with the distance from the sea and on the atmospheric transport pattern. The evolution of 36CI in groundwater is controlled by: (Case 1) in situ activation of “Cl in U-rich (high neutron flux) environments; (Case 2) decay of the cosmic input along the flowpath for closed Cl systems in U-depleted sediments; and (Case 3) admixture of 36C1from Cl--rich confining beds.
J.-C. Fontes, J.N. Andrews / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 367-375
the stratosphere. La1 and Peters [9] estimated the average global production rate to be 11 atoms m -* s- ’ and Onufriev [lo] suggested that there should be an additional production of 5 atomsm-2s-1 because of the neutron activation reaction 36Ar(n, P)~?I. Recent cross section measurements for the latter reaction [ll], however, suggest that it is much less significant and the rarity of 36Ar and neutrons in the high atmosphere suggest that it remains negligble in comparison with the spallation reaction. The dominance of the stratospheric production results in a strong dependence of the fall-out rate on geomagnetic latitude with a marked maximum corresponding to injection at mid latitudes via the tropopause. A corrected curve based on the spallation production rate [ll] and on the latitude dependence suggested by La1 [12] was proposed by Andrews and Fontes [13]. It is difficult to estimate the uncertainties in these estimates which involve production rates, injection from the stratosphere and tropospheric mixing. It is important for hydrological purposes that such errors are assessed and we suggest an uncertainty of at least * 10% for values at particular geomagnetic latitudes. Measurements of 36CI in precipitation could be used to test the distribution of fall-out with latitude but there are very few such determinations presently available. Some data for the United States [14] and Australia [15] correspond to fall-out rates which are two to three times that estimated from the theoretical curve. A possible expianation for this discrepancy is that the tail of the nuclear peak has not yet returned to the pre-bomb level as suggested by determinations of 36C1 in a Greenland ice core [16]. However, it is also possible that the theoretical curve [9,12,13] underestimates
369
36C1fall-out. There is a clear need for global monitoring of the 36C1 fall-out and rain-out to resolve this problem. The fall-out of 36CI- mixes with Cl- in marine aerosols in the troposphere for both species is about 36C1/(35C1
where the residence one week. The ratio,
time R =
+
37C1),generally expressed as atoms of 36C1 per 10” atoms of stable Cl, is dependent upon the local Cl- fall-out which is in turn dependent on distance from the sea and atmospheric circulation. The value of R is independent of evaporation and evapotranspiration before and during recharge. However, these processes affect the concentration of 36C1 (and Cl-) in the infiltrating groundwater. The concentration, A, of j6C1 in groundwater is expressed as atoms per kg of groundwater. The following mass balance relationships are readily derived between precipitation P (mma-‘1, fall-out F (atomsm-* s-l), evapotranspiration E (mm a-‘), stable Cl- concentration in precipitation [Cl] (ppm) and the ratio R and 36CI concentration A: A=3.15x107F/{p(P-E)}, R = A x 35.45/(
6.02 x
1O23x 0.001 x [Cl]),
where p is the density (kg]-‘1 of the groundwater solution. Eqs. (1) and (2) show that for old groundwaters, the palaeoinfiltration (P-E) and the Cl- content of palaeoprecipitation, [CI], can be deduced from measurements of A and R. 3.1.2. Chlorine-36 praduction in the epigene The interaction of cosmic rays and cosmic neutrons in the shallow horizons of the Earth’s crust can cause 36Cl production. This is due to the spallation reactions
Table 1
^, In situ neutron production, neutron flux and equilibrium ““Cl/Cl ratio for various rock types Rock
Ultramafic Basalt Granite Stripa SW England Sandstone GAB UK Trias Limestone Clay/shale Sea water Evaporite
[gcm-31
[ppml Li
B
Gd
U
Th
2.8 2.6 2.6 2.6 2.6 2.6 2.4 2.6 2.6 2.6 1.028 2.2
0.5 17.0 40.0 11.0 258.0 15.0 31.3 15.0 5.0 15.0 0.2 6.2
1.0 5.0 15.0 4.0 231.0 35.0 30.0 35.0 20.0 35.0 4.6 136.5
0.0 5.3 9.0 12.9 4.4 10.0 4.3 10.0 1.3 10.0 0.0 0.0
0.003 1.0 3.5 44.0 13.9 0.4 1.7 1.9 2.2 0.4 0.003 0.09
0.005 4.0 18.0 33.0 19.5 1.7 6.1 8.7 1.7 1.7 0.0001 0.001
P
N,
pl7
[m-3s-l]
f&1]
$5,
1.26 1.95 1.82 1.74 4.26 1.60 1.48 1.60 1.01 1.60 1.23 77.80
0.001 0.30 1.14 6.72 2.36 0.07 0.34 0.34 0.20 0.07 0.009 0.06
0.001 0.15 0.63 3.86 0.55 0.05 0.23 0.21 0.20 0.05 0.007 0.001
0.0 6.9 28.3 174.0 25.0 2.1 10.3 9.6 8.8 2.1 0.3 0.04
[m-i]
N,= Macroscopic neutron absorption cross section for rock; P, = neutron production in rock; 4, = in situ neutron flux: p = specific gravity of rock; R, = equilibrium 36CI/Cl ratio due to in situ production of 36Cl. Unspecified rock types are average compositions from ref. [53]. Evaporite composition is that of a completely evaporate seawater. Note The following production reactions were considered: (i) “Clfn, y)s6CI; (ii) 39Kfn, aIshCI; (iii) 40Ca(n, pcr).“CI. Approximately 98% of 36C1production is due to reaction (i) for all rock types. VI. HYDROLOGY
370
J.-C. Fontes, J.N. Andrews/Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 367-375
39K(p, s~)~~Cl; 40Ca(p, SP)~~CI and the neutron-induced reactions 39K(n, (w)~~C~and 35Cl(n, y)36C1 of which the last reaction is the most important in hydrological situations. The production decreases exponentially with depth and is generally insigificant below a few metres depth. Evaporation and evapotranspiration may cause chloride accumulation in the soil zone of arid or semi-arid environments. The reaction of cosmic neutrons with such chloride can cause significant 36C1 inputs to groundwater during subsequent rainy periods. Similarly, outcrop zones of confining beds such as marine clays may contain sufficient Cl- to make the 35Cl(n, y13?Z1 reaction significant and thus introduce both stable Cl and 36Cl to the adjacent aquifer via surface run-off. Transport or diffusion of minor amounts of such epigene-derived 36C1 into the aquifer can produce substantial increases in the ratio R, as may be the case for the Milk River aquifer [13,17]. 3.1.3. In situ production of chlorine-36 The in situ neutron flux is produced by (a, n> reactions which are generated in most formations because of a-particle emission by U, Th and their decay products. Formations with higher U and Th contents therefore have Cl- which has a significant 36C1/C1 ratio, R. The R value reaches 97% of equilibrium after five half-lives of 36C1,that is about 1.5 Ma. The equilibrium R values, R,, have been calculated for various formations by methods previously described [18] and data for the neutron absorbing properties, neutron emission, neutron flux and R are summarised in Table 1. Granitic rocks have the highest radioelement contents of igneous rocks and correspondingly the highest R, values for Cl- exposed to the in-situ neutron flux. The variation of R between different granites is largely caused by the variability of their radioelement contents, but is also affected by their Li, B and Gd contents because of their high neutron absorption cross-sections. In contrast, the neutron flux and equilibrium R, value within sedimentary formations (other than U ore deposits) are lower than for granites; the R, value ranging from 2 to 10 in sandstones and limestones. The higher radioelement contents of clays and shales (aquitards) produce a somewhat higher equilibrium R value than in other sedimentary formations. It may also be noted that the R, values within sedimentary formations (Table 1) are generally less than those expected for meteoric input. 3.2. Natural 36Cl in hydrological studies The very long half-life, 301 f 2 ka [19], of 36C1 makes it a unique natural tracer with potential for dating very old groundwaters. The ubiquity of cosmogenie 36C1 in recharge waters and the identical chemistry to that of common chloride suggest that it should be widely applicable. The advantages of chloride chem-
istry for groundwater studies are i) the strong hydrophilic nature and high solubility of the Cl- form, ii) the absence of metabolic processes which would fractionate chlorine isotopes, iii) Cl- does not react with any mineral phases in the aquifer. For these reasons, Cl- and 36C1 are conservative in groundwater, that is they remain in solution after having been dissolved. If the cosmic fallout of 36C1depends only on geomagnetic latitude and can be considered as constant over the last million years, chloride ages can be derived from the effect of 36C1decay on the isotopic ratio 36C1/C1. This ratio can be affected by any additional supply of chloride in the aquifer and age estimates must be appropriately corrected as discussed in section 3.2.2. 3.2.1. Chlorine-36 in basinal groundwaters The possibility of using 36C1 as a hydrological chronometer was recognised about 40 years ago [20] and AMS measurement techniques were developed during the late 1970s [21]. The first attempts to apply the method for “Dating of Very Old Groundwater” were published in 1986 for the Great Artesian Basin of Australia (GAB) [22,23], one of the largest confined aquifers in the world, and for the Milk River Aquifer (MRA) in Canada [23,24]. Further measurements were reported for MRA in 1990-1991 [25,26] and for GAB in 1991 [27]. Other large aquifers have been investigated in S. America [23] and N. America [23,28] and in Australia [26,27]. In all cases, ages were derived for the apparent decay of 36C1 down-gradient relative to an assessed value for the 36C1/C1 ratio in the recharge zone. For example, in the GAB these ratios decreased from = 110 to = 6 and ages up to several hundred thousand years were proposed. However, in every case, the Cl- content increased downstream in the aquifer, indicating an input of Cl- by dissolution from the aquitards. Because of the high cross-section of 35C1for neutron capture, such chloride is always labelled with 36C1 which has been produced through irradiation of stable chloride by the in situ neutron flux. 3.2.2. Correction of 36Cl ratio for dilution by aquifer chloride The equilibrium R value is independent of the amount of Cl- in the formation and large numbers of 36C1 atoms may enter groundwater by chloride diffusion from the aquicludes. This must have occurred if the down-gradient groundwaters become more saline. Such Cl- supply is also frequently indicated by a change in the Cl/Br ratio in the groundwater. In the studies mentioned above, groundwater ages were based on the decrease of the R value down-gradient in the aquifer. Corrections were made for mixing between the cosmic input of 36C1 and that which was derived from the aquifer due to solution of Cl- at secular equilibrium. A correction procedure was described by Bentley
L-C.
Fontes, J.N. Andrews
/ Nucl.
IFI&
371
and Meth. in Phys.Res. B 92 (‘1994)367-375
et al. [22] who derived the following equation for the time, t, since infiltration of the cosmic j6C1 + aerosol Cl- mixture: t= [ln{Ci(Ri-R,)}
-lnfC,(R,-R,)}]/h,
(1)
where C is the Cl- concentration and the subscripts t, i and e refer to the time since groundwater infiltration, initial (or infiltration) time and equilibrium (with the aquifer matrix), respectively, and h is the decay constant for 3”C1. The total error in t, ii,, may be derived by differentiation of Eq. (1) from which: 6,~ [(sC,/CO
W/(R,
1.0
0.8
0.5
+ (SCi/‘Ci) + SR,/‘(R*-R,)
+(aRi)/(Ri-R,) +
1.3
- &)]/A,
0.3
+ 6R,/‘tRi-R,) (2)
where 6 indicates the absolute error in each parameter. The error, SC,, in the chloride content of the groundwater is in most cases negligible and SR, is based on the counting statistics (usually expressed at 66% confidence level). The values of Xi and 6Ri are difficult to assess as both Cj and Ri are somewhat subjective judgements of the conditions at groundwater recharge. Both depend upon the palaeoclimatic conditions which existed at recharge (the value of Ci being dependent upon the amount of marine aerosol, distance from the sea and evaporative concentration) and it cannot be assumed that modern recharge conditions are the same as those which existed in the past. The uncertainties in composition and spatial variability of the aquifer and its aquicludes contribute to the error in the equilibrium ratio, R, The error term &RJR, RJ increases as R, approaches R, and the term 6R,/(R, -R,) is a constant which depends on estimated values for R, and R,.Even assigning relatively small uncertainties of + 10% to the various unknowns, it is evident that the overall errors for age estimation are considerably greater than those based solely on counting statistics. In the case of the GAB, Andrews and Fontes [31] have shown that in an R versus Cldiagram, the instantaneous mixing curve between two end members (Ri,Ci and R,, C,,,) could not be distinguished from the curve for decay of the initial input with a gradual Cl- addition. It is thus not possible to deduce groundwater age if there is significant Cl- dissolution. In basinal groundwater systems 36Cf content is an indicator of climatic change and mixing processes rather than a chronometer for very old groundwaters [31,32]. An application of 36C1 to study climatic change in the south-western United States has recently been reported [33]. 3.2.3. The effect of change in the in situ pr~ucti~~ rate The equilibrium “Cl ratio, R,, for groundwater chloride changes if the groundwater enters a formation with a higher in situ neutron flux due to the presence
0.0
1950
1960
1970
1960
Fig. 2. Comparison of the “?ZI and 3H fall-out from nuclear weapon tests in the atmosphere. Full-circles indicate the 36C1 fallout deduced from the Greenland ice profile (Synal et al. [16]f; the solid line shows the 3H content of precipitation for Valentia, Ireland and the dashed line shows that for Vienna, Austria (from IAEA database).
of high concentrations of U and Th. In such a situation, the residence time of the groundwater in the formation with the high neutron flux may be calculated from the extent to which equilibrium has been attained. For this purpose, the equilibrium R value in the high neutron flux environment must be known. It may either be calculated from a detailed geochemical analysis of the rock matrix or be dete~ined from direct 36Cl or neutron flux measurements. This approach was used for groundwaters in the high radioelement content granites at Stripa, Sweden [34] and for waters in granites of northern Switzerland [35]. If the Cl- content remains constant, this is the most soundly based 36C1 method for determining Cl- (and possibly groundwater) residence time in the high flux environment. It is, of course, only applicable in the relatively rare situations where groundwater migrates from a low to a high neutron flux environment. 3.3. Man-made chlorite-36 Chlorine-36 is produced in nuclear power plants by the 36Ar(n, pg6C1 reaction. Some localised environmental contamination by nuclear production has been observed in the vicinity of nuclear reprocessing plants 136,371. The hydrophilic character of Cl- accompanies water movement without retardation due to adsorption effects. Measurements of the dispersion of 36Cl may be used to determine groundwater velocities and preferential flow directions from such contamination sites. Such data may be used to assess the potential future VI. HYDROLOGY
J.-C. Fontes, J.N. Andrews/Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 367-375
372
These advantages make 36C1 a more useful tracer for infiltration through the unsaturated zone than is 3H which, in contrast, was assymmetrically distributed between the hemispheres and has now substantially decayed because of its short half-life (12.4 a [19]). For future studies of unsaturated zone processes 36C1 will become the only available radiometric method because of this rapid 3H decay.
hazard due to the slower migration of radionuclides which are significantly retarded. Chlorine-36 which was produced during aerial detonations of nuclear bombs by neutron irradiation of 35C1, has a much greater potential for hydrological applications than such localised releases. The advantages of thermonuclear 36C1 for hydrological investigations are: i) it was abundantly produced during a limited time span, mainly between 1954 and 1958 (Fig. 2), and resulted in an integrated fallout of 2 to 3 x 1012 atoms m P2 s-l [16,38,39], ii) it was injected into the stratosphere by these explosions and hence was subsequently distributed evenly in both hemispheres due to the near equatorial location of the test sites, iii) any fractionation of 36C1relative to stable isotopes of chlorine is negligible over the time and distance scales involved for infiltration studies, and iv) decay is negligible for times less than or equal to a few centuries.
\
\
\ \
‘*
\
\ \
\
\ Al
\
\ \
\ Al
3.3.1. Estimation of recharge rate from 36Cl profiles The rate of advance of the 36C1bomb peak through the unsaturated zone may be used to estimate the rate of mass transport of water through the soil zone. In a shallow unconfined groundwater system in Ontario, a 36Cl peak was found at a depth of about 17 m and the integrated peak was comparable with estimates based on the yield of nuclear explosions [38]. This suggests that infiltration may be considered to be dominantly \
\
Fall-aut’\,of 4 ‘4
\
\
cos,mogenic ‘vtl,~=l5~7p) \ 3 ;Y 4
\
‘\ 4
Marine
0I
Decay of 1291 with no addition of 1291or 1271
flow rate dL, Ro
Ro
aerosol
02
‘-aquitord Yimestone
238U (s.f.) 1291in sandstone
‘4
rom orgamc-rich
shale
Re
increases Ro
admixture of 1291 no addition of 1271
Re
Fig. 3. A schematic cycle for natural lz91. Marine I (I- + IO;) has a constant atomic ratio, R = ‘291/‘271, because the oceanic residence time of I isotopes is much smaller than the lz91 half-life (15.7 Ma). The ocean consequently forms the main reservoir for lz91 and the atomic ratio R in aerosols is not locally influenced by the cosmic fall-out. The evolution of lz91 in groundwater is controlled by: (Case 1) admixture of ‘29I derived by spontaneous fission of 238U in U-rich sediments: (Case 2) decay of the aerosol input along the flowpath for closed I systems in almost-stagnant groundwater; and (Case 3) admixture of lz91 and 12’1 in variable proportions from U-rich and/or I--rich confining beds.
J.-C. Fontes, J.N. Andrews /Nucl.
Instr. and Meth. in Phys. Res. B 92 (1994) 367-375
vertical and although the dispersion was comparable with that present in the 36Cl record in ice cores 116,391, no estimates of infiltration rate were made because of uncertainties in the groundwater flow pattern, In the unsaturated zone, water movement must be truly vertical, in an upwards direction for evaporative loss and downwards for a net infiltration. Norris et al. [40] found the thermonuclear 36Cl peak at a depth of 0.5 m in unsaturated tuffs in Nevada and showed that no bomb 36Cl penetrated to depths below 2 m under the arid climate. Studies of sandy soils at a desert site in New Mexico [41] identified bomb 36Cl peaks for three soil profiles and estimated recharge rates of 2.3 to 3.0 mm a-l, comparable with chloride mass balance estimates. However, the thermonuclear 3H peak, which occurred about eight years after the 36Cl peak, was found at greater depths than the 36Cl peak. No entirely satisfactory explanation for this phenomenon was advanced but it was suggested that it was associated with gaseous diffusion of the 3H-labelled aqueous phase and the complexity of water and solute transport in desert soils. A recharge rate of 1.4 mma-’ was deduced from the moisture content and the bomb 36Cl peak depth (0.5 ml in the arid zone of Texas [42]. Experiments in a soil profile in Southern Australia [43] showed very convincing agreement between recharge rates assessed from 36Cl and choride mass balance. Recharge of as little as 0.1 mma-’ could be measured and an increase in recharge rate was identified with the decrease in evapotranspiration coincident with deforestation of the area. The method can only be applied to profiles where the 36Cl peak has migrated beyond the root zone.
373
situ 1291in a groundwater, the efficiency of its release from the rock matrix must be assessed. The release efficiency is specific for each rock and is very difficult to assess as it depends upon both the distribution of U in the rock surface at the rock-water interface as well as upon the specific surface of this interface. 4.2. Atmospheric fall-out of 129I There has been significant anthropogenic production of 1291 by induced neutron fission of 235U and 239Pu through nuclear weapon testing and by release on reprocessing of nuclear fuels. Modern inputs of 1291 at aquifer recharge has therefore been variable and generally has much exceeded the natural input. The latter has had to be estimated from the 129I/I ratio for marine sediments which have derived iodine from the ocean surface waters prior to anthropogenic contamination [45]. The main source of lz91 for aquifer input is atmospheric but the main reservoir is the ocean (Fig. 3). This occurs because the residence time of iodine in the atmosphere is = 2 weeks whereas that in the oceans is = 40000 years [46]. The residence time in the ocean is long compared with oceanic circulation times but short compared with the half-life of 1291so that the atmospheric input to the ocean is uniformly mixed in the ocean. Formation of aerosol from sea spray and volatilisation of iodine species from the ocean then buffers the atmospheric 129I content and the atmospheric residence time is sufficiently long to ensure that atmospheric circulation maintains an identical fall-out in continental locations [47]. 4.3. Estimation of groundwater residence time with 129I
4. Iodine-129 4.1. Cosmic and in-situ production of 129I
Long-lived lz91 (half-life = 15.7 Ma [19]) is produced naturally by irradiation of atmospheric xenon by cosmic neutrons via the reaction lz8Xe(n, ~1~~~1and in the sub-surface by spontaneous fission of 238U (spontaneous fission half-life = 8.15 x 1015 a [44]). Spontaneous fission is a rare decay mode of 238U but because of the wide distribution of U in the earth’s crust, causes a significant production of lz91 in many aquifer rock matrices. The release of such in situ produced 1291 into the groundwater depends upon the proximity of U to the aqueous phase and upon the porosity of the aquifer. The isotopic ratio, 129I/I in the groundwater, is influenced by release of in situ 1291but the sources of such 129I and natural I are not related. This is in sharp contrast to in situ production of 36Cl for which a characteristic ‘?Zl ratio can be calculated for each aquifer system. For estimation of the contribution of in
Some problems with the use of 1291for groundwater dating are i) the absolute age estimate is dependent upon the value adopted for the initial 129I/I ratio (this is common with other radiometric chronometers but it may also be noted that it is a consequence of the very long half-life for 1291 that a 10% uncertainty in the 129I/I ratio corresponds to an uncertainty of 2.4 Ma in decay time); ii) the effect of in situ production and 1291 release on the 129I/I ratio; and iii) the decrease of the 129I/I ratio by dissolution of ‘291-dead natural iodide may be derived from organic-rich sedimentary formations. For these reasons, the use of 1291for groundwater dating will be limited to systems which approach closed conditions (Fig. 3). The age of brine inclusions in salt domes in Louisiana, which could be considered as closed systems, has been estimated to be = 8 f 4 Ma from the decay of the atmospheric ‘29I/I input [47]. In this case, in situ production of 129Iwas negligible because of the very low U content of the salt. For groundwaters of the VI. HYDROLOGY
374
J.-C. Fontes, J.N. Andrews /Nucl.
Instr. and Meth. in Phys. Rex B 92 (1994) 367-375
Great Artesian Basin (Australia), no age estimation was possible because of the dominant effect of in situ production 1471. In situ production was also shown to be very significant in the high U-content (44 ppm) granites at Stripa, Sweden [48]. Investigations of mixing between old formation waters and circulating younger waters have also assessed in situ production [49]. The lz91/I ratio in crude oils is greater than the marine ‘29I/I ratio and has been used to estimate the age of the oil [50]. The associated saline waters have the same age as the formation [51]. Anthropogenic lz91 could provide an excellent tracer for nuclear pollution plumes in the marine and terrestrial environments. The principal source which mainabout 100 tains the atmospheric iz91 concentration times the natural level is the release from nuclear fuel reprocessing plants. It should be possible to trace the migration of such lz91 at both the local and regional scale. Large releases such as the Chernobyl reactor accident may increase the atmospheric level by about 10 times 1521. Iodine-129 could be considered as a global tracer similar to %r if the record of iz91 release could be reconstructed.
5. Conclusions
The AMS technique, because of its very small sample requirement, will permit the extension of i4C groundwater dating to new concepts such as the investigation of age stratification in groundwater. The 36Cl ingrowth in high neutron flux environments, where applicable, is a more reliable dating method than decay of the cosmic input. Uncertainty in the latter is increased by the necessity of correction for in situ production and dissolution of aquifer chloride. In basinal aquifers, %l may provide an interesting method for investigation of palaeorecharge conditions. Unsaturated zone studies using the thermonuclear 36Cl input is a significant advance compared with other methods for determination of recharge rate. It requires large numbers of measurements and further applications would be encouraged if the anaIytica1 costs could be reduced. The use of 12’1 for groundwater dating is even more complex than that of 36Cl because of the different origin of in situ 1291and stable I in the aquifer and the release processes for these separate species require further investigation. Anthropogenic 36C1 and lz91 can both be used as tracers for dispersion of radioactive contaminants from nuclear facilities.
Acknowledgements
This work was supported by CNRS grant URA 723 (J-Ch.F); by NERC grant RG/7857 (JNA) and by a British Council/ Ministlre des Affaires Ctrangeres “Alliance” grant (JNA and J-Ch.F). The paper was finalised during a period by JNA as Visiting Professor at Universite de Paris-Sud.
References [l] J-Ch. Fontes, J.N. Andrews, W.M. Edmunds, A. Guerre and Y. Travi, Water Resour. Res. 27 (19911 199. [Z] L.N. Plummer, E.G. Prestemon and D.L. Parkhurst, U.S. Geological Survey, Water Resources Investigations Report 91-4078, Reston, Virginia, 1991, 227 pp. [3] E.M. Murphy, S.N. Davis, A. Long, D. Donahue and A.J.T. Jull, Nature 337 (1989) 153. [4] A. Long, E.M. Murphy, S.N. Davis and R.M. Kalin, in: Radiocarbon After Four Decades, eds. R.E. Taylor, A. Long and R.S. Kra (Springer, New York, 1992) p. 288. [S] L.I. Wassenaar, R. Aravena and P. Fritz, in: Isotope Techniques in Water Resources Development (International Atomic Energy Agency, Vienna, 1992) p. 143. [6] E.M. Murphy, S.N. Davis, A. Long, D. Donahue and A.J.T. Jull, Water Resour. Res. 25 (19891 1893. [7] R.J. Drimmie, R. Aravena, L.J. Wassenaar, P. Fritz, M.J. Hendry and G. Hut, Appl. Geochem. 6 (IQ911 381. [8] S. Geyer, M. Wolf, P. Fritz, G. Buckau and J-I. Kim, in: Applications of Isotope Techniques in Studying Past and Current Environmental Changes in the Hydrosphere and the Atmosphere (International Atomic Energy Agency, Vienna, 1993) p. 359. [9] D. La1 and S. Peters, in: Handbuch der Physik, 46/2, ed. K. Sitte (Springer, Berlin, 19671 p. 551. [lo] V.G. Onufriev, Yad. Geofiz. Geokhim. Isot. Metogy Geol. (1968) 364, in Russian. [ll] S.S. Jiang, T.K Hemmick, P.W. Kubik, D. Elmore, H.E. Gove and S. Tullai-Fitzpatrick, Nucl. Instr. and Meth. B 52 (1990) 608. 1121 D. Lal, in: Earth Science and Meteorites, eds. 3. Geiss and E.D. Goldberg (Noah-Holland, Amsterdam, 19631p. 115. [13] J.N. Andrews and J-Ch. Fontes, in: Isotope Techniques in Water Resources Development (International Atomic Energy Agency, Vienna, 19921 p. 245. 1141 L. Hainsworth, A.C. Mignerey, G.R. Helz, P. Sharma and P.W. Kubik, these Proceedings (6th Int. Conf. on Accelerator Mass S~ctromet~ (AMS-61, Canberra-Sydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994). [15] D. Knies, D. Elmore, S. Vogt, R. Li, M. Lipschutz, G. Petty, E. Agee, J. Farrell, S. Fritz and M. Monaghan, these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61, Canberra-Sydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (19941. [16] H.A. Synal, J. Beer, G. Bonani, M. Suter and W. Wolfli, Nucl. Instr. and Meth. B 52 (19901483. [17] J.N. Andrews, T. Fiorkowski, B.E. Lehmann and H.H. Loosli, Appl. Geochem. 6 (19911425.
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Instr. and Meth. in Phys. Res. B 92 (1994) 367-375
[18] J.N. Andrews, S. Davis, J. Fabryka-Martin, J-Ch. Fontes, B.E. Lehmann, H.H. Loosli, J-L. Michelot, H. Moser, B. Smith and M. Wolf, Geochim. Cosmochim. Acta 53 (1989) 1803. [19] E. Browne and R.B. Firestone, Table of Radioactive Isotopes (Wiley Interscience, New York, 1986). [20] R. Davis, Jr. and O.A. Schaeffer, Ann. N.Y. Acad. Sci. 62 (1955) 105. [21] D. Elmore, B.R. Fulton, M.R. Glover, J.R. Marsden, H.E. Gove, H. Naylor, K.H. Purser, L.R. Kilius, R.P. Beukens and A.E. Litherland, Nature 277 (1979) 22. [22] H.W. Bentley, F.M. Phillips, S.N. Davis, M.A. Habermehl, P.L. Airey, G.E. Calf, D. Elmore, H.E. Gove and T. Torgersen, Water Resour. Res. 22 (1986) 1991. [23] H.W. Bentley, F.M. Phillips and S.N. Davis, in: Handbook of Environmental Isotope Geochemistry, vol. 2, The Terrestrial Environment, eds. P. Fritz and J-Ch. Fontes (Elsevier, Amsterdam, 1986) p. 427. [24] F.M. Phillips, H.W. Bentley, S.N. Davis, D. Elmore and G.B. Swanick, Water Resour. Res. 22 (1986) 1991. [25] E. Nolte, P. Krauthan, U. Heim and G. Korschinek, Nucl. Instr. and Meth. B 52 (1990) 477. [26] E. Nolte, P. Krauthan, G. Korschinek, P. Maloszewski, P. Fritz and M. Wolf, Appl. Geochem. 6 (1991) 435. [27] T. Torgersen, M.A. Habermehl, F.M. Phillips, D. Elmore, P. Kubik, B. Geoffrey Jones, T. Hemmick and H.E. Gove, Water Resour. Res. 22 (1991) 3201. [28] C.B. Purdy, A.C. Mignerey, G.R. Helz, D.R. Drummond, P.W. Kubik, D. Elmore and T. Hemmick, Nucl. Instr. and Meth. B 29 (1987) 372. [29] J.R. Bird, G.E. Calf, R.F. Davie, L.K. Fifield, T.R. Ophel, W.R. Evans, J.R. Kellet and M.A. Habermehl, Radiocarbon 31 (1989) 877. [30] R.F. Davie, J.R. Kellet, L.K. Fifield, W.R. Evans, G.E. Calf, J.R. Bird, S. Topham and T.R. Opel, BMR J. Aust. Geol. Geophys. 11 (1989) 261. [31] J.N. Andrews and J-Ch. Fontes, Water Resour. Res. 29 (1993) 1871. [32] J-Ch. Fontes and J.N. Andrews, Appl. Geochem. 8 (19931 663. [33] M. Stute, J.F. Clark, F.M. Phillips and D. Elmore, in Isotope techniques in studying past and current environmental changes in the hydrosphere and the atmosphere, Intern. Atomic En. Agency, Vienna, (1993) pp. 259-270. [34] J.N. Andrews, J-Ch. Fontes, J-Th. Michelot and D.E. Elmore, Earth Planet. Sci. Lett. 77 (1986) 49. [35] F.J. Pearson, W. Balderer, H.H. Loosli, B.E. Lehmann,
375
A. Matter, T. Peters, H. Schmassmann, A. Gautschi et al., Applied Isotope Hydrogeology - A Case Study in Northern Switzerland (Elsevier, Amsterdam, 1991) 460 [36] FM. Beasley, D. Elmore, P.W Kubik and P. Sharma, Ground Water 30 (1992) 539. [37] T.M. Beasley, R.D. Cecil, P. Sharma, P.W. Kubik, U. Fehn, L.J. Mann and H.E. Gove, Ground Water 31 (1993) 302. [38] H.W. Bentley, F.M. Phillips, S.N. Davis, S. Gifford, D. Elmore, L.E. Tubbs and H.E. Gove, Nature 300 (1982) 737. [39] D. Elmore, Th.E. Tubbs, D. Newman, X.Z. Ma, R. Finkel, K. Nishiizumi, J. Beer, H. Oeschger and M. Andree, Nature 300 (1982) 735. [40] A.E. Norris, K. Wolfsberg, SK. Gifford, H.W. Bentley and D. Elmore, Nucl. Instr. and Meth. B 29 (1987) 376. [41] F. Phillips, J. Mattick, T. Duval, D. Elmore and P.W. Kubik, Water Resour. Res. 24 (1988) 1877. [42] B.R. Scanlon, P.W. Kubik, P. Sharma, B.C. Richter and H.E Gove, Nucl. Instr. and Meth. B 52 (1990) 489. [43] G.R. Walker, I.D. Jolly, M.H. Stadter, F.W. Leaney, R.F. Davie, L.K. Fifield, T.R. Ophel and J.R. Bird, in: Isotope Techniques in: Water Resources Development, (International Atomic Energy Agency, Vienna, 1992) p. 19. 1441H.G. DeCarvalho, J.B. Martins, E.L. Medeiros and. O.A.P. Tavares, Nucl. Instr. and Meth. 197 (1982) 417. [45] U. Fehn, G.R. Holdren, D. Elmore, T. Brunelle, R. Teng and P.W. Kubik, Geophys. Res. Lett. 13 (1986) 137. [46] W.S. Broecker and T.H. Peng, Tracers in the Sea (Eldigio, 1982). [47] J. Fabryka-Martin, H. Bentley, D. Elmore and P.L. Airey, Geochim. Cosmochim. Acta 49 (1985) 337. [48] J. Fabryka-Martin, S.N Davis, D. Elmore and P.W. Kubik, Geochim. Cosmochim. Acta 53 (1989) 1817. [49] U. Fehn, E.K. Peters, S. Tullai-Fitzpatrick, P.W. Sharma, R.T.D. Teng, H.E. Gove and D. Elmore, Geochim. Cosmochim. Acta 56 (1992) 2069. [50] U. Fehn, S. Tullai-Fitzpatrick, R.T.D. Teng, H.E. Gove, P.W. Kubik, P. Sharma and D. Elmore, Nucl. Instr. and Meth. B 52 (1990) 446. [51] U. Fehn, G.R. Holdren, D. Elmore, T. Brunelle, R. Teng and P.W. Kubik, Nucl. Instr. and Meth. B 29 (1987) 380. [52] M. Paul, D. Fink, G. Hellos, A. Kaufman, W. Kutschera and M. Margaritz, Nucl. Instr. and Meth. 29 (1987) 341. [53] R.L. Parker, U.S. Geol. Surv., Prof. Paper. 440-D, 1967.
VI. HYDROLOGY
Accelerator
mass spectrometry
k!lOM B
Beam Interactions with Materials % Atoms
in hydrology
Jean-Charles Fontes a*?and John N. Andrews b** a Laboratoire d’Hydroiogie et Gkxhimie Isotopique, Universitk de Paris-&d,
Orsay, F91405 Cedex, France ’ Postgraduate Research Institute for Sedimentology, University of Reading, Reading RG6 2AB, UK
. hydrology using the AMS technique are reviewed. The small amounts of carbon needed for Applications of 14C,36Cland rz9I m AMS measurement of r4C have facilitated new areas of hydrological research. Uncertainties in the natural fall-out of 36C1and the dissolution of 36Cl-labelled chloride during groundwater migration are limitations for groundwater dating with this nuclide. Estimation of recharge by measurements of the advance of the 36CI bomb peak through the unsaturated zone is of increasing importance in hydrofogy. AMS measurements of “‘1 will nrovide an environmental monitor for nuclear pollution plumes. Some directions for future studies are suggested.
1. Introduction
Accelerator mass spectrometry (AMS) has applications in hydrological sciences either because of its capabili~ for measuring radioisotopes (e.g., 36Cl and iz91) which could not be assayed with adequate accuracy by nuclear counting techniques or because of its potential for making measurements on relatively small samples. In the case of i4C, for example, sample size may be up to 1000 times smaller than that required for ~-counting.
2. Rsdiocarbon
in groundwaters
2.1. Advantages of AMS fur carbon-I# ~eas~re~en~ uqaifers
in
Groundwater dating by measurement of the 14C activity of total dissolved inorganic carbon (TDIC) was introduced in the late 1950s and has subsequently been much applied by hydrologists. For @-counting under standard conditions, a sample containing = 2 g of carbon is typically required either for liquid scintillation counting or for assay with gas proportional counters. Such an amount of carbon is generally provided by the TDIC in about 100 litres of groundwater which must be obtained from a pumped well or borehole. The carbon is pre-concentrated by addition of BaCI, or SrCI, and precipitation of the corresponding carbonate at pH = 10. In contrast, the AMS technique requires I 1 mg of carbon for the target of the ion * corresponding author. Tel. +44 734 318684, fax +44 734 310279. ’ Deceased.
source. This large reduction in the sample size is an obvious advantage for simplification of the field protocol for TDIC recovery but, more importantly, it makes it possible to investigate the age stratification in open boreholes which are subject only to natural flow conditions. This may be accomplished by careful down-hole sampling without any disturbance or mixing in the water column due to pumping. There have, however, so far been few attempts to evaluate this application. Two samples from a vertical profile in an old ( = 20 ka) unconfined groundwater system in northern Mali [l] did reveal an unexpected age inversion of about 1.5 ka in the vertical age distribution. This could be attributed to either more rapid flow in a more permeable zone at depth or to an upwards leakage induced by the very high evaporation of the region. Similar studies on the TDIC in small soil water samples could be used to evaluate low recharge rates in arid regions. 2.2. Dissolved organic carbon The dilution of the r4C activity of TDIC due to dissolution of, or exchange with, solid carbonates along the flow path still presents a problem for the interpretation of groundwater t4C ages. Although many correction models have been developed (see ref. [2]), there is still doubt about the validity of corrected ages. For this reason, there has been much interest in the use of dissolved organic carbon (DOC) for estimating 14C ages. The AMS technique has made possible the investigation of systems with low concentrations of DOC. Several studies 13-71 have investigated the use of the humic and fulvic acid fractions of DOC. These and other high molecular weight organic compounds are believed to result from biological activity in the soil zone and not to be affected by isotopic dilution during
Old-583X/94/$07.~ 0 1994 - EIsevier Science B.V. Ah rights reserved SSDI 0168-583X(94)00143-J
VI. HYDROLOGY
techniques to measuring the r4C activity of the fractions of DOC should contribute to a better standing of the behaviour of soil-derived DOC effect of solution of diagenetic or fossil DOC 14C dating methodolo~.
groundwater migration. Low mofeeufar weight organic compounds are the result of diagenetic pracesses in which high molecular weight compounds are degraded, for example, in the formation of hydrocarbons. Studies of groundwaters from the fractured granites at Stripa [3,6] and in the sediments of the Milk River aquifer [6,7] have confirmed that Iow molecular weight fraction of DOC has relatively old 14C ages reflecting such a diagenetic origin and these compounds must not, therefore, be used for groundwater dating. Ages which were in acceptable agreement with the hydrological situations were obtained from the humic and fulvic acid components of the DOC. Subsequent studies in Canada [S] and in Germany IS], however, have shown that the humic fraction of DOC may also produce ages. which are too old, suggesting that the humic fraction can also be preserved in fossil organic deposits from which it may be leached, unrelated to the more recent groundwater flow system. The application of AMS \
\
\
‘\
\
\
\\
$y----
\
;: f ; 3+
VI
$4
RI
increases
3. Chlorine-34 3.2. Get&remicaf cycle for ~~~~~u~ 36Cl Fig, 1 illustrates the 36CI and Cl- sources and mixing processes which occur in the hydrological cycle. Various aspects of this cycle are discussed below. X1.1. Cosmic ~~o~u~tio~ anb fa&out of ~~~~~~~-~6 chlorine-36 is formed in the atmosphere by the spallation reaction 40Ar(p, ntwj3?21, 40% of production occurring within the troposphere and the remainder in \
\
\
\
\
cos\mogenic \ 36Cl(t$*=301 ko!
F?li-out’\of
f?----R,
‘i t
\
\
\\
various underand the on the
\ ‘.
a------
OS! I v
3:,3? ct
I v
d@creo3es
Marine aerosol
Oceanic
laccotith
+
+
mixing
+
+++++++
Ro (no Cl-uddition) L---r Fig. 1, A
Re
L or Em--,
schematic cycle for natural 36C1.Marine Cl- is depleted in 3hCIbecause its oceanic residence time is much greater than
the 36C1half-life (301 ka). The dilution of cosmogenic 3”Cl by 35C1and 37Cl from marine aerosols varies with the distance from the sea and on the atmospheric transport pattern. The evolution of 36CI in groundwater is controlled by: (Case 1) in situ activation of “Cl in U-rich (high neutron flux) environments; (Case 2) decay of the cosmic input along the flowpath for closed Cl systems in U-depleted sediments; and (Case 3) admixture of 36C1from Cl--rich confining beds.
J.-C. Fontes, J.N. Andrews / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 367-375
the stratosphere. La1 and Peters [9] estimated the average global production rate to be 11 atoms m -* s- ’ and Onufriev [lo] suggested that there should be an additional production of 5 atomsm-2s-1 because of the neutron activation reaction 36Ar(n, P)~?I. Recent cross section measurements for the latter reaction [ll], however, suggest that it is much less significant and the rarity of 36Ar and neutrons in the high atmosphere suggest that it remains negligble in comparison with the spallation reaction. The dominance of the stratospheric production results in a strong dependence of the fall-out rate on geomagnetic latitude with a marked maximum corresponding to injection at mid latitudes via the tropopause. A corrected curve based on the spallation production rate [ll] and on the latitude dependence suggested by La1 [12] was proposed by Andrews and Fontes [13]. It is difficult to estimate the uncertainties in these estimates which involve production rates, injection from the stratosphere and tropospheric mixing. It is important for hydrological purposes that such errors are assessed and we suggest an uncertainty of at least * 10% for values at particular geomagnetic latitudes. Measurements of 36CI in precipitation could be used to test the distribution of fall-out with latitude but there are very few such determinations presently available. Some data for the United States [14] and Australia [15] correspond to fall-out rates which are two to three times that estimated from the theoretical curve. A possible expianation for this discrepancy is that the tail of the nuclear peak has not yet returned to the pre-bomb level as suggested by determinations of 36C1 in a Greenland ice core [16]. However, it is also possible that the theoretical curve [9,12,13] underestimates
369
36C1fall-out. There is a clear need for global monitoring of the 36C1 fall-out and rain-out to resolve this problem. The fall-out of 36CI- mixes with Cl- in marine aerosols in the troposphere for both species is about 36C1/(35C1
where the residence one week. The ratio,
time R =
+
37C1),generally expressed as atoms of 36C1 per 10” atoms of stable Cl, is dependent upon the local Cl- fall-out which is in turn dependent on distance from the sea and atmospheric circulation. The value of R is independent of evaporation and evapotranspiration before and during recharge. However, these processes affect the concentration of 36C1 (and Cl-) in the infiltrating groundwater. The concentration, A, of j6C1 in groundwater is expressed as atoms per kg of groundwater. The following mass balance relationships are readily derived between precipitation P (mma-‘1, fall-out F (atomsm-* s-l), evapotranspiration E (mm a-‘), stable Cl- concentration in precipitation [Cl] (ppm) and the ratio R and 36CI concentration A: A=3.15x107F/{p(P-E)}, R = A x 35.45/(
6.02 x
1O23x 0.001 x [Cl]),
where p is the density (kg]-‘1 of the groundwater solution. Eqs. (1) and (2) show that for old groundwaters, the palaeoinfiltration (P-E) and the Cl- content of palaeoprecipitation, [CI], can be deduced from measurements of A and R. 3.1.2. Chlorine-36 praduction in the epigene The interaction of cosmic rays and cosmic neutrons in the shallow horizons of the Earth’s crust can cause 36Cl production. This is due to the spallation reactions
Table 1
^, In situ neutron production, neutron flux and equilibrium ““Cl/Cl ratio for various rock types Rock
Ultramafic Basalt Granite Stripa SW England Sandstone GAB UK Trias Limestone Clay/shale Sea water Evaporite
[gcm-31
[ppml Li
B
Gd
U
Th
2.8 2.6 2.6 2.6 2.6 2.6 2.4 2.6 2.6 2.6 1.028 2.2
0.5 17.0 40.0 11.0 258.0 15.0 31.3 15.0 5.0 15.0 0.2 6.2
1.0 5.0 15.0 4.0 231.0 35.0 30.0 35.0 20.0 35.0 4.6 136.5
0.0 5.3 9.0 12.9 4.4 10.0 4.3 10.0 1.3 10.0 0.0 0.0
0.003 1.0 3.5 44.0 13.9 0.4 1.7 1.9 2.2 0.4 0.003 0.09
0.005 4.0 18.0 33.0 19.5 1.7 6.1 8.7 1.7 1.7 0.0001 0.001
P
N,
pl7
[m-3s-l]
f&1]
$5,
1.26 1.95 1.82 1.74 4.26 1.60 1.48 1.60 1.01 1.60 1.23 77.80
0.001 0.30 1.14 6.72 2.36 0.07 0.34 0.34 0.20 0.07 0.009 0.06
0.001 0.15 0.63 3.86 0.55 0.05 0.23 0.21 0.20 0.05 0.007 0.001
0.0 6.9 28.3 174.0 25.0 2.1 10.3 9.6 8.8 2.1 0.3 0.04
[m-i]
N,= Macroscopic neutron absorption cross section for rock; P, = neutron production in rock; 4, = in situ neutron flux: p = specific gravity of rock; R, = equilibrium 36CI/Cl ratio due to in situ production of 36Cl. Unspecified rock types are average compositions from ref. [53]. Evaporite composition is that of a completely evaporate seawater. Note The following production reactions were considered: (i) “Clfn, y)s6CI; (ii) 39Kfn, aIshCI; (iii) 40Ca(n, pcr).“CI. Approximately 98% of 36C1production is due to reaction (i) for all rock types. VI. HYDROLOGY
370
J.-C. Fontes, J.N. Andrews/Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 367-375
39K(p, s~)~~Cl; 40Ca(p, SP)~~CI and the neutron-induced reactions 39K(n, (w)~~C~and 35Cl(n, y)36C1 of which the last reaction is the most important in hydrological situations. The production decreases exponentially with depth and is generally insigificant below a few metres depth. Evaporation and evapotranspiration may cause chloride accumulation in the soil zone of arid or semi-arid environments. The reaction of cosmic neutrons with such chloride can cause significant 36C1 inputs to groundwater during subsequent rainy periods. Similarly, outcrop zones of confining beds such as marine clays may contain sufficient Cl- to make the 35Cl(n, y13?Z1 reaction significant and thus introduce both stable Cl and 36Cl to the adjacent aquifer via surface run-off. Transport or diffusion of minor amounts of such epigene-derived 36C1 into the aquifer can produce substantial increases in the ratio R, as may be the case for the Milk River aquifer [13,17]. 3.1.3. In situ production of chlorine-36 The in situ neutron flux is produced by (a, n> reactions which are generated in most formations because of a-particle emission by U, Th and their decay products. Formations with higher U and Th contents therefore have Cl- which has a significant 36C1/C1 ratio, R. The R value reaches 97% of equilibrium after five half-lives of 36C1,that is about 1.5 Ma. The equilibrium R values, R,, have been calculated for various formations by methods previously described [18] and data for the neutron absorbing properties, neutron emission, neutron flux and R are summarised in Table 1. Granitic rocks have the highest radioelement contents of igneous rocks and correspondingly the highest R, values for Cl- exposed to the in-situ neutron flux. The variation of R between different granites is largely caused by the variability of their radioelement contents, but is also affected by their Li, B and Gd contents because of their high neutron absorption cross-sections. In contrast, the neutron flux and equilibrium R, value within sedimentary formations (other than U ore deposits) are lower than for granites; the R, value ranging from 2 to 10 in sandstones and limestones. The higher radioelement contents of clays and shales (aquitards) produce a somewhat higher equilibrium R value than in other sedimentary formations. It may also be noted that the R, values within sedimentary formations (Table 1) are generally less than those expected for meteoric input. 3.2. Natural 36Cl in hydrological studies The very long half-life, 301 f 2 ka [19], of 36C1 makes it a unique natural tracer with potential for dating very old groundwaters. The ubiquity of cosmogenie 36C1 in recharge waters and the identical chemistry to that of common chloride suggest that it should be widely applicable. The advantages of chloride chem-
istry for groundwater studies are i) the strong hydrophilic nature and high solubility of the Cl- form, ii) the absence of metabolic processes which would fractionate chlorine isotopes, iii) Cl- does not react with any mineral phases in the aquifer. For these reasons, Cl- and 36C1 are conservative in groundwater, that is they remain in solution after having been dissolved. If the cosmic fallout of 36C1depends only on geomagnetic latitude and can be considered as constant over the last million years, chloride ages can be derived from the effect of 36C1decay on the isotopic ratio 36C1/C1. This ratio can be affected by any additional supply of chloride in the aquifer and age estimates must be appropriately corrected as discussed in section 3.2.2. 3.2.1. Chlorine-36 in basinal groundwaters The possibility of using 36C1 as a hydrological chronometer was recognised about 40 years ago [20] and AMS measurement techniques were developed during the late 1970s [21]. The first attempts to apply the method for “Dating of Very Old Groundwater” were published in 1986 for the Great Artesian Basin of Australia (GAB) [22,23], one of the largest confined aquifers in the world, and for the Milk River Aquifer (MRA) in Canada [23,24]. Further measurements were reported for MRA in 1990-1991 [25,26] and for GAB in 1991 [27]. Other large aquifers have been investigated in S. America [23] and N. America [23,28] and in Australia [26,27]. In all cases, ages were derived for the apparent decay of 36C1 down-gradient relative to an assessed value for the 36C1/C1 ratio in the recharge zone. For example, in the GAB these ratios decreased from = 110 to = 6 and ages up to several hundred thousand years were proposed. However, in every case, the Cl- content increased downstream in the aquifer, indicating an input of Cl- by dissolution from the aquitards. Because of the high cross-section of 35C1for neutron capture, such chloride is always labelled with 36C1 which has been produced through irradiation of stable chloride by the in situ neutron flux. 3.2.2. Correction of 36Cl ratio for dilution by aquifer chloride The equilibrium R value is independent of the amount of Cl- in the formation and large numbers of 36C1 atoms may enter groundwater by chloride diffusion from the aquicludes. This must have occurred if the down-gradient groundwaters become more saline. Such Cl- supply is also frequently indicated by a change in the Cl/Br ratio in the groundwater. In the studies mentioned above, groundwater ages were based on the decrease of the R value down-gradient in the aquifer. Corrections were made for mixing between the cosmic input of 36C1 and that which was derived from the aquifer due to solution of Cl- at secular equilibrium. A correction procedure was described by Bentley
L-C.
Fontes, J.N. Andrews
/ Nucl.
IFI&
371
and Meth. in Phys.Res. B 92 (‘1994)367-375
et al. [22] who derived the following equation for the time, t, since infiltration of the cosmic j6C1 + aerosol Cl- mixture: t= [ln{Ci(Ri-R,)}
-lnfC,(R,-R,)}]/h,
(1)
where C is the Cl- concentration and the subscripts t, i and e refer to the time since groundwater infiltration, initial (or infiltration) time and equilibrium (with the aquifer matrix), respectively, and h is the decay constant for 3”C1. The total error in t, ii,, may be derived by differentiation of Eq. (1) from which: 6,~ [(sC,/CO
W/(R,
1.0
0.8
0.5
+ (SCi/‘Ci) + SR,/‘(R*-R,)
+(aRi)/(Ri-R,) +
1.3
- &)]/A,
0.3
+ 6R,/‘tRi-R,) (2)
where 6 indicates the absolute error in each parameter. The error, SC,, in the chloride content of the groundwater is in most cases negligible and SR, is based on the counting statistics (usually expressed at 66% confidence level). The values of Xi and 6Ri are difficult to assess as both Cj and Ri are somewhat subjective judgements of the conditions at groundwater recharge. Both depend upon the palaeoclimatic conditions which existed at recharge (the value of Ci being dependent upon the amount of marine aerosol, distance from the sea and evaporative concentration) and it cannot be assumed that modern recharge conditions are the same as those which existed in the past. The uncertainties in composition and spatial variability of the aquifer and its aquicludes contribute to the error in the equilibrium ratio, R, The error term &RJR, RJ increases as R, approaches R, and the term 6R,/(R, -R,) is a constant which depends on estimated values for R, and R,.Even assigning relatively small uncertainties of + 10% to the various unknowns, it is evident that the overall errors for age estimation are considerably greater than those based solely on counting statistics. In the case of the GAB, Andrews and Fontes [31] have shown that in an R versus Cldiagram, the instantaneous mixing curve between two end members (Ri,Ci and R,, C,,,) could not be distinguished from the curve for decay of the initial input with a gradual Cl- addition. It is thus not possible to deduce groundwater age if there is significant Cl- dissolution. In basinal groundwater systems 36Cf content is an indicator of climatic change and mixing processes rather than a chronometer for very old groundwaters [31,32]. An application of 36C1 to study climatic change in the south-western United States has recently been reported [33]. 3.2.3. The effect of change in the in situ pr~ucti~~ rate The equilibrium “Cl ratio, R,, for groundwater chloride changes if the groundwater enters a formation with a higher in situ neutron flux due to the presence
0.0
1950
1960
1970
1960
Fig. 2. Comparison of the “?ZI and 3H fall-out from nuclear weapon tests in the atmosphere. Full-circles indicate the 36C1 fallout deduced from the Greenland ice profile (Synal et al. [16]f; the solid line shows the 3H content of precipitation for Valentia, Ireland and the dashed line shows that for Vienna, Austria (from IAEA database).
of high concentrations of U and Th. In such a situation, the residence time of the groundwater in the formation with the high neutron flux may be calculated from the extent to which equilibrium has been attained. For this purpose, the equilibrium R value in the high neutron flux environment must be known. It may either be calculated from a detailed geochemical analysis of the rock matrix or be dete~ined from direct 36Cl or neutron flux measurements. This approach was used for groundwaters in the high radioelement content granites at Stripa, Sweden [34] and for waters in granites of northern Switzerland [35]. If the Cl- content remains constant, this is the most soundly based 36C1 method for determining Cl- (and possibly groundwater) residence time in the high flux environment. It is, of course, only applicable in the relatively rare situations where groundwater migrates from a low to a high neutron flux environment. 3.3. Man-made chlorite-36 Chlorine-36 is produced in nuclear power plants by the 36Ar(n, pg6C1 reaction. Some localised environmental contamination by nuclear production has been observed in the vicinity of nuclear reprocessing plants 136,371. The hydrophilic character of Cl- accompanies water movement without retardation due to adsorption effects. Measurements of the dispersion of 36Cl may be used to determine groundwater velocities and preferential flow directions from such contamination sites. Such data may be used to assess the potential future VI. HYDROLOGY
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372
These advantages make 36C1 a more useful tracer for infiltration through the unsaturated zone than is 3H which, in contrast, was assymmetrically distributed between the hemispheres and has now substantially decayed because of its short half-life (12.4 a [19]). For future studies of unsaturated zone processes 36C1 will become the only available radiometric method because of this rapid 3H decay.
hazard due to the slower migration of radionuclides which are significantly retarded. Chlorine-36 which was produced during aerial detonations of nuclear bombs by neutron irradiation of 35C1, has a much greater potential for hydrological applications than such localised releases. The advantages of thermonuclear 36C1 for hydrological investigations are: i) it was abundantly produced during a limited time span, mainly between 1954 and 1958 (Fig. 2), and resulted in an integrated fallout of 2 to 3 x 1012 atoms m P2 s-l [16,38,39], ii) it was injected into the stratosphere by these explosions and hence was subsequently distributed evenly in both hemispheres due to the near equatorial location of the test sites, iii) any fractionation of 36C1relative to stable isotopes of chlorine is negligible over the time and distance scales involved for infiltration studies, and iv) decay is negligible for times less than or equal to a few centuries.
\
\
\ \
‘*
\
\ \
\
\ Al
\
\ \
\ Al
3.3.1. Estimation of recharge rate from 36Cl profiles The rate of advance of the 36C1bomb peak through the unsaturated zone may be used to estimate the rate of mass transport of water through the soil zone. In a shallow unconfined groundwater system in Ontario, a 36Cl peak was found at a depth of about 17 m and the integrated peak was comparable with estimates based on the yield of nuclear explosions [38]. This suggests that infiltration may be considered to be dominantly \
\
Fall-aut’\,of 4 ‘4
\
\
cos,mogenic ‘vtl,~=l5~7p) \ 3 ;Y 4
\
‘\ 4
Marine
0I
Decay of 1291 with no addition of 1291or 1271
flow rate dL, Ro
Ro
aerosol
02
‘-aquitord Yimestone
238U (s.f.) 1291in sandstone
‘4
rom orgamc-rich
shale
Re
increases Ro
admixture of 1291 no addition of 1271
Re
Fig. 3. A schematic cycle for natural lz91. Marine I (I- + IO;) has a constant atomic ratio, R = ‘291/‘271, because the oceanic residence time of I isotopes is much smaller than the lz91 half-life (15.7 Ma). The ocean consequently forms the main reservoir for lz91 and the atomic ratio R in aerosols is not locally influenced by the cosmic fall-out. The evolution of lz91 in groundwater is controlled by: (Case 1) admixture of ‘29I derived by spontaneous fission of 238U in U-rich sediments: (Case 2) decay of the aerosol input along the flowpath for closed I systems in almost-stagnant groundwater; and (Case 3) admixture of lz91 and 12’1 in variable proportions from U-rich and/or I--rich confining beds.
J.-C. Fontes, J.N. Andrews /Nucl.
Instr. and Meth. in Phys. Res. B 92 (1994) 367-375
vertical and although the dispersion was comparable with that present in the 36Cl record in ice cores 116,391, no estimates of infiltration rate were made because of uncertainties in the groundwater flow pattern, In the unsaturated zone, water movement must be truly vertical, in an upwards direction for evaporative loss and downwards for a net infiltration. Norris et al. [40] found the thermonuclear 36Cl peak at a depth of 0.5 m in unsaturated tuffs in Nevada and showed that no bomb 36Cl penetrated to depths below 2 m under the arid climate. Studies of sandy soils at a desert site in New Mexico [41] identified bomb 36Cl peaks for three soil profiles and estimated recharge rates of 2.3 to 3.0 mm a-l, comparable with chloride mass balance estimates. However, the thermonuclear 3H peak, which occurred about eight years after the 36Cl peak, was found at greater depths than the 36Cl peak. No entirely satisfactory explanation for this phenomenon was advanced but it was suggested that it was associated with gaseous diffusion of the 3H-labelled aqueous phase and the complexity of water and solute transport in desert soils. A recharge rate of 1.4 mma-’ was deduced from the moisture content and the bomb 36Cl peak depth (0.5 ml in the arid zone of Texas [42]. Experiments in a soil profile in Southern Australia [43] showed very convincing agreement between recharge rates assessed from 36Cl and choride mass balance. Recharge of as little as 0.1 mma-’ could be measured and an increase in recharge rate was identified with the decrease in evapotranspiration coincident with deforestation of the area. The method can only be applied to profiles where the 36Cl peak has migrated beyond the root zone.
373
situ 1291in a groundwater, the efficiency of its release from the rock matrix must be assessed. The release efficiency is specific for each rock and is very difficult to assess as it depends upon both the distribution of U in the rock surface at the rock-water interface as well as upon the specific surface of this interface. 4.2. Atmospheric fall-out of 129I There has been significant anthropogenic production of 1291 by induced neutron fission of 235U and 239Pu through nuclear weapon testing and by release on reprocessing of nuclear fuels. Modern inputs of 1291 at aquifer recharge has therefore been variable and generally has much exceeded the natural input. The latter has had to be estimated from the 129I/I ratio for marine sediments which have derived iodine from the ocean surface waters prior to anthropogenic contamination [45]. The main source of lz91 for aquifer input is atmospheric but the main reservoir is the ocean (Fig. 3). This occurs because the residence time of iodine in the atmosphere is = 2 weeks whereas that in the oceans is = 40000 years [46]. The residence time in the ocean is long compared with oceanic circulation times but short compared with the half-life of 1291so that the atmospheric input to the ocean is uniformly mixed in the ocean. Formation of aerosol from sea spray and volatilisation of iodine species from the ocean then buffers the atmospheric 129I content and the atmospheric residence time is sufficiently long to ensure that atmospheric circulation maintains an identical fall-out in continental locations [47]. 4.3. Estimation of groundwater residence time with 129I
4. Iodine-129 4.1. Cosmic and in-situ production of 129I
Long-lived lz91 (half-life = 15.7 Ma [19]) is produced naturally by irradiation of atmospheric xenon by cosmic neutrons via the reaction lz8Xe(n, ~1~~~1and in the sub-surface by spontaneous fission of 238U (spontaneous fission half-life = 8.15 x 1015 a [44]). Spontaneous fission is a rare decay mode of 238U but because of the wide distribution of U in the earth’s crust, causes a significant production of lz91 in many aquifer rock matrices. The release of such in situ produced 1291 into the groundwater depends upon the proximity of U to the aqueous phase and upon the porosity of the aquifer. The isotopic ratio, 129I/I in the groundwater, is influenced by release of in situ 1291but the sources of such 129I and natural I are not related. This is in sharp contrast to in situ production of 36Cl for which a characteristic ‘?Zl ratio can be calculated for each aquifer system. For estimation of the contribution of in
Some problems with the use of 1291for groundwater dating are i) the absolute age estimate is dependent upon the value adopted for the initial 129I/I ratio (this is common with other radiometric chronometers but it may also be noted that it is a consequence of the very long half-life for 1291 that a 10% uncertainty in the 129I/I ratio corresponds to an uncertainty of 2.4 Ma in decay time); ii) the effect of in situ production and 1291 release on the 129I/I ratio; and iii) the decrease of the 129I/I ratio by dissolution of ‘291-dead natural iodide may be derived from organic-rich sedimentary formations. For these reasons, the use of 1291for groundwater dating will be limited to systems which approach closed conditions (Fig. 3). The age of brine inclusions in salt domes in Louisiana, which could be considered as closed systems, has been estimated to be = 8 f 4 Ma from the decay of the atmospheric ‘29I/I input [47]. In this case, in situ production of 129Iwas negligible because of the very low U content of the salt. For groundwaters of the VI. HYDROLOGY
374
J.-C. Fontes, J.N. Andrews /Nucl.
Instr. and Meth. in Phys. Rex B 92 (1994) 367-375
Great Artesian Basin (Australia), no age estimation was possible because of the dominant effect of in situ production 1471. In situ production was also shown to be very significant in the high U-content (44 ppm) granites at Stripa, Sweden [48]. Investigations of mixing between old formation waters and circulating younger waters have also assessed in situ production [49]. The lz91/I ratio in crude oils is greater than the marine ‘29I/I ratio and has been used to estimate the age of the oil [50]. The associated saline waters have the same age as the formation [51]. Anthropogenic lz91 could provide an excellent tracer for nuclear pollution plumes in the marine and terrestrial environments. The principal source which mainabout 100 tains the atmospheric iz91 concentration times the natural level is the release from nuclear fuel reprocessing plants. It should be possible to trace the migration of such lz91 at both the local and regional scale. Large releases such as the Chernobyl reactor accident may increase the atmospheric level by about 10 times 1521. Iodine-129 could be considered as a global tracer similar to %r if the record of iz91 release could be reconstructed.
5. Conclusions
The AMS technique, because of its very small sample requirement, will permit the extension of i4C groundwater dating to new concepts such as the investigation of age stratification in groundwater. The 36Cl ingrowth in high neutron flux environments, where applicable, is a more reliable dating method than decay of the cosmic input. Uncertainty in the latter is increased by the necessity of correction for in situ production and dissolution of aquifer chloride. In basinal aquifers, %l may provide an interesting method for investigation of palaeorecharge conditions. Unsaturated zone studies using the thermonuclear 36Cl input is a significant advance compared with other methods for determination of recharge rate. It requires large numbers of measurements and further applications would be encouraged if the anaIytica1 costs could be reduced. The use of 12’1 for groundwater dating is even more complex than that of 36Cl because of the different origin of in situ 1291and stable I in the aquifer and the release processes for these separate species require further investigation. Anthropogenic 36C1 and lz91 can both be used as tracers for dispersion of radioactive contaminants from nuclear facilities.
Acknowledgements
This work was supported by CNRS grant URA 723 (J-Ch.F); by NERC grant RG/7857 (JNA) and by a British Council/ Ministlre des Affaires Ctrangeres “Alliance” grant (JNA and J-Ch.F). The paper was finalised during a period by JNA as Visiting Professor at Universite de Paris-Sud.
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VI. HYDROLOGY