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Argon is a noble gas with three stable isotopes. Natural variation in the abundance of these isotopes can be used to determine the paleo-recharge temperature of groundwater.


The three stable isotopes, 36Ar, 38Ar, and 40Ar, are considered in the discussion of the Noble Gases. Of the seven radioactive argon isotopes, 37Ar, and 39Ar are also used in hydrologic studies and discussed below. Use of the 40Ar/36Ar ratio in hydrological applications is also discussed.

Cost of Analysis (return to top)

There are no labs currently performing argon isotope analysis for the public.

See the USGS Reston Chlorofluorocarbon Laboratory for more information)

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39Ar is produced in the atmosphere by neutron bombardment:

However, in groundwater 39Ar can be produced in situ by the following reactions:

39Ar (t½ = 269 years) undergoes beta decay back to 39K.

37Ar (t½ = 35 days) is constantly produced underground from 40Ca(n,a)37Ar reactions in the rock matrix.

Measurement Techniques (return to top)

Gas proportional counting

Argon analysis is purely research-based at present because of the very small concentration of argon in water. Argon represents less than one percent of the total gases in the atmosphere. Equilibrium with the atmosphere therefore produces minute concentrations of dissolved argon in water. Water sample sizes range from 2 liters (if vacuum degassing is used) to 15 cubic meters (if the sample is to be boiled) (Clark & Fritz 1997). Samples are analyzed by high pressure gas proportional counting. Since the activity of 39Ar is very low (around 2 counts per hour), approximately 1 month is required for analysis (Cook and Herczeg 2000).

(See the decay counting page for more information on the GPC process).

Hydrological Applications (return to top)

Because of the short half life of 37Ar, almost no groundwater has cosmogenically produced forms of this isotope present. However, subsurface production is common [40Ca(n,a)37Ar]. By measuring the amount of 37Ar present, hydrogeologists can determine the subsurface production rate (which in turn can help determine the neutron flux) and also the efficiency of mineral to water transfer (Cook and Herczeg 2000). Both of these values are useful for constraining the use of other lithogenic isotopes in hydrologic applications.

39Ar in Groundwater Dating
39Ar dating has been mainly used in dating groundwater in conjunction with other isotopes. Its half-life of 269 years allows for comparison of ages with the high end of the tritium range, and the low end of the 14C range. 39Ar is useful for dating submodern groundwater (~40 to ~1000 years B.P.) because it fills this gap of uncertainty between the most widely applied isotopes in groundwater dating (3H and14C).

Advantages and Disadvantages of Using 39Ar to Date Water
There are numerous advantages and disadvantages to using 39Ar to date water. On the positive side, argon is a noble gas and therefore inert. There are no complications of side reactions and its conservative behavior makes it an excellent hydrologic tracer. Secondly, 39Ar production did not increase as a result of thermonuclear bomb testing. Its activity has remained almost constant for at least the past 1000 years.

However, in groundwater areas where uranium and thorium are present, in situ production of 39Ar can be substantial. Since 39Ar concentrations are very low in groundwater, in situ production can produce concentrations of 39Ar that drown out the atmospheric concentrations in the water. Other disadvantages stem from the sampling and analytical techniques for 39Ar, mainly the sample size and measurement time.

Other Applications of 39Ar
39Ar can be used to date water masses in the ocean. The application here is very similar to groundwater dating, except that the in situ production of 39Ar is negligible. 39Ar is also used in ice coring.

40Ar/36Ar Ratio in Groundwater Dating
The 40Ar/36Ar ratio has also been used to aid groundwater dating. This ratio has a constant value in the atmosphere of 295.5. Most aquifers contain potassium-bearing minerals. 40K (with a half-life of 125 x 109 years) beta decays to 40Ar and thus, over time this ratio becomes larger. If the production rate of 40Ar is known, this ratio can be used to age date very old groundwater. However, this ratio may be considerably compromised and elevated by the transport of radiogenic 40Ar from neighboring rock strata outside an aquifer. A more quantitative evaluation of 40Ar/36Ar ratios for analysis of water residence times will require improved understanding of rock weathering processes and the role of fluid inclusion (Rauber et al. 1991).

References and further reading (return to top)

  • Andrews, J.N., et al, The in situ production of radioisotopes in rock matrices with particular reference to the Stripa granite, Geochimica et Cosmochimica Acta, 53, 1803-1815, 1989.

  • Clark, I., and P. Fritz, Environmental Isotopes in Hydrogeology, Lewis Publishers, Boca Raton, 1997.

  • Cook. P.G., and A.L. Herczeg, editors, Environmental Tracers in Subsurface Hydrology, Kluwer Academic Publishers, Boston, 2000.

  • Lehmann, B.E. et al, Atmospheric and subsurface sources of stable and radioactive nuclides used for groundwater dating, Water Resour. Res. 29(7), 2027-2040, 1993.

  • Loosli, H.H., A dating method with 39Ar, Earth and Planetary Science Letters, 63, 51-62, 1983.

  • Loosli, H.H., and H. Oeschger, Argon-39, carbon-14 and krypton-85 measurements in groundwater samples, in Isotope Hydrology 1978, vol. 2, 931-997, International Atomic Energy Agency, Vienna, 1979.

  • Pearson, F.J., Applied Isotope Hydrogeology: A Case Study In Northern Switzerland, Elsevier, New York, 1991.

  • Rauber, D., H. H. Loosli, and B.E. Lehmann, 40Ar/36Ar ratios, in chapter 6 of Applied Isotope Hydrogeology: A Case Study in Northern Switzerland, Elsevier, Amsterdam, 1991.

  • Scholtis, A., et al, Integration of environmental isotopes, hydrochemical and mineralogical data to characterize groundwaters from a potential repository site in central Switzerland, in Isotopes in Water Resource Management, pp. 263-280, International Atomic Energy Agency, Vienna, 1996.

Internet resources (return to top)

USGS Periodic Table - Argon


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