Rates and sources of⁴He accumulation in groundwater

Data are presented for the 4He and 222Rn contents and 14C ages of groundwater from two artesian aquifers in southern Africa. The relative importance of helium derived from the decay of uranium and thorium in the aquifers is assessed, as well as “weathering” release of stored helium, fluxes into the aquifer, and diffusive losses. It is demonstrated that a 4He flux originating from rocks below the aquifer is the most important source. The magnitude of this flux is relatively constant over the sampled intervals and is qualitatively estimated to be in the range 1 x 10−3 to 3 x 10−2 m1nHe m−2year−1 (9 x 104 to 3 x 106 atoms He cm−2s−1). The 4He data cannot be used to calculate the absolute age of the groundwater, but may be used to estimate the relative age and flow direction of very old groundwater if the 4He flux rate is assumed, or known to be reasonably constant. Examination of the 222Rn data suggests that they may have a useful application in the qualitative determination of the uranium content and porosity of aquifers.