Groundwater dating with helium isotopes

The use of 3He and of 4He for groundwater dating is described in the literature as problematic. 3He is sensitive to diffusive losses and to the background produced in the lithosphere. The paper presents a methodical tool to separate the component of bomb tritium from the background. This tool was tested on three aquifers with different sizes of helium backgrounds. No notable diffusive loss was found, maybe because the investigated aquifers were all confined. Consequently, 3He seems to be in most cases a useful tracer for groundwater systems. In an aquifer with very high lithospheric helium production the helium isotope ratio identifies the lithospherically produced 3H equilibrium content in groundwater. Frequently 4He accumulation and 4He flux age determinations are not applicable because a helium flux reaches the groundwater, and because this flux is not clearly defined. Only if the groundwater sample represents the whole depth of the flow system, does the 4He excess lie in the flux range expected. Vice versa, in deeper flow systems the 4He content can be investigated in addition to 14C and 39Ar model ages. (author). 23 refs, 6 figs, 6 tabs