Mechanisms for noble gas dissolution by groundwaters

Groundwaters acquire dissolved gases because of air equilibration and excess air entrainment within the unsaturated zone and from other sources during their migration. The latter include the diffusion of stored radiogenic helium from within the rock matrix or from depth along fluid filled fractures, the release of radiogenic 40Ar by mineral alteration or thermal processes and the production of biogenic or thermogenic CH4 and N2 from organic matter. 3He/4He isotopic measurements may be used to identify the source rock of radiogenic helium that dilutes atmospheric helium in groundwaters during their migration. Groundwater residence times may be estimated from their 4He contents and the radiogenic 4He flux into the aquifer. The production rate of radiogenic 4He within the aquifer is generally much less significant than such diffusive flux of 4He. The noble gas content is related to groundwater recharge temperatures, which can be used to place groundwater age in its paleoclimatic context. (author). 68 refs, 7 figs, 4 tabs