Groundwater dating and subsurface processes investigated by noble gas isotopes (~(37)Ar, ~(39)Ar, ~(85)Kr, ~(222)Rn, ~4He)

摘要

Dating of groundwater is often based on radionuclides. Their concentration in water however may be changed by rock-water interaction. This limits on one hand the reliability of dating methods (e.g. ~(14)C, ~(39)Ar); but subsurface produced nuclides can on the other hand be used to better understand a flow system (e.g. by applying ~4He, ~(222)Rn, and ~(37)Ar). The combination of different isotope methods completed with model calculations is discussed for two water systems in Switzerland. For the first application subsurface produced radionuclides with different half-lives are combined to investigate the flow path of the groundwater during the corresponding time periods: ~(222)Rn allows conclusions about the last few days, ~(37)Ar describes the last months and ~(39)Ar the last centuries before the water reaches the surface. In the second case study measured ~4He concentrations in pore water are compared with results of a numerical diffusion model. Conclusions are possible about the minimal accumulation times under the consideration of diffusive loss of ~4He from greater depths to the atmosphere. Estimations are also possible for the average spacing between conductive fractures where advective removal of helium may occur.