Argon-39, argon-37 and krypton-85 isotopes in Stripa groundwaters

39Ar, 37Ar and 85Kr activities in groundwater samples from three boreholes in the Stripa granite are compared with 222Rn activities and with data from a granitic groundwater in Switzerland. In situ production rates in bulk granite and in fracture filling minerals are calculated. The measured isotopes in groundwater have been produced by nuclear reactions in the rock matrix. It was reported earlier that a large fraction of the in situ produced 222Rn can escape to the water phase due to the unusual microscopic distribution of U which seems to be in close contact with the water phase. The same conclusion can be drawn for 85Kr with escape factors between 5% and 25%. The 39Ar values in Stripa groundwaters are the highest measured so far but consistent with the unusually high neutron-flux in Stripa as compared to granites with lower U- and Th-contents. The 37Ar concentrations appear to be quite high, but since it is the first time that this isotope has been identified in groundwater the basis for a comparison with different rock formations is still too small. It seems possible that calcite fracture fillings in Stripa have a major influence on the concentration of 37Ar in groundwater