Argon-37 and Argon-39: Measured concentrations in groundwater compared with calculated concentrations in rock

Measured 37Ar and 39Ar activities in groundwaters from various formations are compared with calculated equilibrium ratios in rock, which originate from subsurface production. For a selected set of samples the 39Ar/37Ar ratios measured in water and the 39Ar/39Ar equilibrium ratios calculated in rock appear to be of the same order of magnitude. Since the half-lives of the two nuclides differ by more than three orders of magnitude, it is concluded that the transfer from rock to water is most likely not controlled by diffusion. For porous media such as sandstone and carbonate rock, the escaping fraction is of the order of 0.5-3% for both nuclides. Under the assumption that recoil is the dominant process, specific surfaces between 1000 and 5000 cm2/cm3 are calculated. Groundwater dating using atmospheric 39Ar seems possible for sedimentary aquifers; ingrowth of subsurface produced 39Ar can help to estimate water residence times in crystalline aquifers. (author). 16 refs, 4 figs, 2 tabs