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.