Radioisotope dynamics — the origin and fate of nuclides in groundwater

The use of radionuclides as clocks for groundwater dating and as probes to investigate the geometry and spatial extent of the contact area between rocks and water is reviewed. Subsurface production rates for222Rn37Ar,85Kr,39Ar,36CI,3He,4He and40Ar in various rock types are listed. Measured Rn fluxes from the surface of sandstone grains and from pieces of granite point to scale-dependent diffusion coefficients. The temporal evolution of subsurface-produced222Rn-,37Ar-,85Kr- and39Ar-activities in groundwaters yields radionuclide escape factors between 0.1 % and 9% for the Stripa granite (Sweden) and between 1% and 4% for the Milk River sandstone (Canada). The combination of3H,85Kr,39Ar,14C,36CI,4He in the UK Triassic sandstone aquifer allows groundwater dating up to 40 000 a. Very old groundwaters can be studied using Cl,36Cl and4He evolution as demonstrated in the Milk River aquifer in Canada.