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Noble Gas Isotopes in Hydrology and Climatic Studies

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Noble Gas Isotopes in Hydrology and Climatic Studies WATER & ENVIRONMENT NEWS XA9953205 NOBLE GAS ISOTOPES IN HYDROLOGY AND CLIMATIC STUDIES Noble Gas Isotopes estimated from atmospheric noble helium cannot be directly included in gas concentrations and then dis- this calculation, because helium Noble gas isotopes are very well cuss the use of helium isotopes for concentrations are often dominated suited to serve as tracers for physi- dating. by non-atmospheric sources (see cal processes in the environment below). Based on the noble gases because their concentrations are Temperature Estimation Based Ne, Ar, Kr and Xe, which are mostly not influenced by chemical or bio- on He, Ne, Ar, Kr, Xe of atmospheric origin, the method logical transformations. The ele- described above was applied to re- mental abundance and isotopic Reconstructing surface water tem- construct atmospheric cooling dur- composition of noble gases in wa- perature from noble gas concentra- ing the fast ice age (e.g. Stute et al. ter have been employed to study tions assumes that at the time when 1995 and Figure 1). Typically, the paleo and recent temperature the water was in contact with the precision of the noble-gas tempera- changes, to date ground water in atmosphere the concentrations in tures is in the order of 1°C. the time range from a few days to the water were in equilibrium with several million years and to quan- those in the atmosphere. Since the A major limitation of the noble-gas tify mixing in lakes and oceans. equilibrium concentrations depend temperature method lies in the com- on temperature T and pressure p, position of the excess of noble- The main reservoir of noble gases these parameters can directly be gases. In some ground waters the on the Earth is the atmosphere, inferred from the observed noble excess air component seems to be where their elemental abundance gas concentrations. However, in fractionated in favor of the heavier is well known and seems to be many cases and in particular in noble gases. This effect is com- constant at ieast on times scales ground waters, the measured con- monly explained by partial de- up to a few million years. The centrations are significantly higher gassing of excess air during concentrations of noble gases in than those expected for equilibrium recharge. However, neither the gen- water are primarily controlled by conditions. This phenomenon is eration of excess air nor the reason air-water exchange. The solubility commonly explained as caused by for the fractionation of noble gases of noble gases in surface waters the complete dissolution of air within the excess air component is depends mainly on temperature (excess air). well understood. Thus, in ground and pressure, but also to a minor waters with strong fractionation of extent on the chemical composi- Temperature and the entrainment the excess noble gases, recharge tion of the water. Thus, noble gas of excess air have a different effect temperature derived from noble gas concentrations in the water provide on the concentration of each of the concentrations is not well con- information on temperature and noble gases. Therefore, the simul- strained (Aeschbach-Hertig et al. pressure conditions prevailing dur- taneous measurement of the five 1998). ing gas exchange. noble gases allows to distinguish between excess air and the equilib- Dating Using Helium isotopes Some noble gas isotopes (e.g. rium concentrations. Thus, the pa- 3 3 3He, "He, ^Ar, 40Ar, MKr) are pro- rameters T and p, as well as the H- He apparent water age 3 duced by radioactive decay or amount of excess air, can be ob- He is produced by radioactive de- 3 other nuclear reactions. The accu- tained from the noble gas concen- cay of tritium H (half life 12.38 yr). mulation of the stable isotopes trations. Note, that in many cases As long as a water parcel is in such as 3He, 4He and 40Ar, as well as the decay of the radioactive Table 1. Time scales of dating methods by noble gas isotopes. noble gas isotopes such as MAr (half life. 270 yr) and 85Kr (half life Tracer Time scale Source Detection 222 10.5 yr), can be employed to date Rn days to weeks 238U a counting surface and ground waters (see 37Ar months spallation (S) low level counting Table 1). The dating methods pro- (LLC) vide a means to estimate resi- 3H-3He months to S, nuclear bomb test- mass spectrometry dence times and flow velocities in decades ing (MS) aquifer systems and to quantify 85Kr months to nuclear power plants LLC vertical exchange in the deep part decades of surface water bodies. 39Ar ltfyr S LLC 4 3 8 Herad 10 -10 yr U,Th MS In the following we first describe >10syr MS how recharge temperature can be Quarterly No. 5, October 1998 - Page 15 WATER & ENVIRONMENT NEWS contact with the atmosphere, 3He ter bodies such as the Temperature produced by tritium decay (3He,n) is Caspian Sea (Figure 2). rapidly removed from the water to the atmosphere by gas exchange. The major limitation of dating Thus, the concentration of dis- with the 3H-3He apparent wa- solved 3He in the water parcel re- ter age is the terrigenic input mains near the atmospheric equi- of 3He. It must either be librium concentration. As soon as small or well known that 3Hetrf the water parcel is isolated from can be calculated with suffi- the atmosphere the concentration cient precision. of 3He in the water increases be- 0 MOW 20000 30000 Califeratecf X-age <yeats B.P.) cause the produced 3He can no *He dating longer escape by gas exchange. 4He is produced by a-decay Figure 1. Recharge temperatures derived from the noble The ratio between 3He,n and 3H is a gases Ne, Ar.KrandXeat different locations along the flow path of isotopes within the ura- in the Glatt Valley aquifer, Switzerland. The data suggests that measure for the apparent water nium and thorium series. the temperature prevailing at recharge of ground water was about age which is the time elapsed This 4He is released from 4to5°C lower during the last ice-age than it is today (Beyerte et since the water was last in contact a/. 1998). 3 3 rock and minerals and accu- with the atmosphere. The H- He mulates in water saturated apparent water age is given by regions. If the natural flux of Caspian Sea (Torgersen 1977) 4He into the ground water is 4 known the He concentra- 200-- 1 f 3He,. tions are a direct measure for r = - In ' 1 & (1) the residence time of the wa- A, ter. —$— Southern Basm 1995 «oo-: —O*" Central Basin 1995 809-- —»~~ Soytficrli Basin IVib Unfortunately, in many - O Ceolral Basin J<J°6 where X (= 0.05599 yr1) is the ground waters the accumula- 1000-- 3 4 decay constant of H. To obtain tion rate of He is much larger 0 5 10 15 2ft 25 3 s 3Hetn from the measured 3He con- than the in-situ 4He produc- H- He water age {yr] 3 centration, He of atmospheric ori- tion expected from the ura- Figure 2. H-He water age in the southern and central basin of gin and 3He introduced from the nium and thorium concentra- the Caspian Sea. In 1995 and in 1996 the 3H-3He water age in geological setting (terrigenic 3He) tions in the aquifer matrix. the central and southern basin is the same at equivalent depth. 3 4 This suggest that the mixing rates must be similar in both basins. have to be subtracted. The He of Because the additional He The time required to replace the deep water below 200 m by atmospheric origin is comprised of flux often contributes signifi- water from above 200 m corresponds to the difference of the two components: 1) the 3He equi- cantly to the overall "He con- mean water age in both regions. This time of internal mixing is librium concentration which can be centration in the ground wa- about 15 years which is significantly smaller than the refill time of the Caspian Sea (about 200 years) (Peeters et al. 1998). calculated from the solubility of ter, 4He dating can only be helium and the fractionation of 3He 4 3 used as a qualitative indicator of ted 1998. Analysis of deep-water exchange in and He during solution and 2) He the Caspian Sea based on environmental residence time. However, because tracers. Deep-Sea Res. introduced by excess air which can 4 He dating covers residence times Schtosser, P., M. Stute, C. Dor, C. Sonntag and be calculated from the Ne excess. 3 8 3 from 10 up to 10 yrs, it is the most K.O. MOnnich, 1988. TritiumfiHe-dating of The terrigenic He component can shallow groundwater. Earth Planet. Sci. Lett. 4 reliable method available to date be estimated from the He excess 89: 353-362. 3 4 very old and stagnant ground wa- if the He/ He ratio of the terrigenic Stute, M., M. Forster, H. Frischkom, A. Semjo, ters such as those found in the J.F. Clark, P. Schlosser, W.S. Bnoeckerand component is known. Note that in G. Bonani, 1995. Cooling of tropical Brazil (5 Great Artesian Basin in Australia. contrast to the classical tritium dat- °C) during the Last Glacial Maximum. Sci- 3 3 ence. 269: 379-383. ing the calculation of H- He appar- Torgersen, T., Z. Top, W.B. Clarke, W.J. Jenk- ent water age does not depend on References ins and W.S. Broecker, 1977. A new method the knowledge of the local input for physical limnology - tritium-hetium-3 ages Aeschbach-Hertig, W., F.
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