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.

• Cost of Analysis
  
• Origin
  
• Measurement Techniques
  
• Hydrological Applications
  
• References and further reading
• Internet resources

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)

Origin (return to top)

Cosmogenic
³⁹Ar is produced in the atmosphere by neutron bombardment:

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

³⁹Ar (t_½ = 269 years) undergoes beta decay back to ³⁹K.

Lithogenic
³⁷Ar (t_½ = 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)

³⁷Ar
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

WebElements.com

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