Noble Gas Radioisotopes: 37Ar, 85Kr, 39Ar, 81Kr

Noble gases are chemically inert and therefore at least some of the complications that are common in interpreting isotope data are absent when working with such tracers. The dependence of their solubility on temperature makes it possible to determine the recharge temperature of recent and paleo groundwaters (Stute and Schlosser; Chapter 11). In this chapter we focus on specific radioactive isotopes of two noble gases, argon and krypton, which occur in trace quantities. These isotopes provide information on when recharge took place, because of their known source functions and decay rates. Four nuclides exist that have been measured in groundwater: 37Ar (half-life 35 days), 85Kr (half-life 10.76 years), 39Ar (half-life 269 years) and 81Kr (half-life 229 000 years). Unfortunately, the concentrations of all four isotopes are very small in subsurface waters and consequently analytical procedures are rather complicated. Nevertheless, very valuable results have been achieved in a number of groundwater studies. Since the number of dating tools is quite limited in hydrogeology, any additional information is welcome even if the analytical effort is considerably higher than for some other tracers. In particular, noble gas radioisotopes have been used in combination with more conventional groundwater dating methods, to investigate mixing of waters of different ages: 3H and 85Kr are applicable for young groundwaters (<40 yrs) (this combination is especially useful since the two input functions are different), 39Ar and 14C are appropriate for older groundwaters (50–20 000 yrs) and 8lKr and 36C1 for very old groundwaters (up to 106 yrs). Where such mixing occurs, the use of a single tracer will never be sufficient to characterise the age distribution. In general, as many tracers as possible should be used to cover the time ranges considered to be adequate for a specific hydrogeological situation.