CONTROLS ON THE DISTRIBUTION AND ISOTOPIC COMPOSITION OF HELIUM IN DEEP GROUNDWATER FLOWS

The distribution and isotopic composition of helium in sedimentary bas ins can be used to interpret the ages of very old ground waters. The p iston-flow model commonly used in such interpretation, however, does n ot account for several important factors and as such works well only i n very simple flow regimes. In this study of helium transport in a hyp othetical sedimentary basin, we develop a numerical model that account s for the magnitude and distribution of the basal helium flux, hydrody namic dispersion, and complexities in flow regimes such as subregional flow cells. The modeling shows that these factors exert strong contro ls on the helium distribution and isotopic composition. The simulation s may provide a basis for more accurate interpretations of observed he lium concentrations and isotopic ratios in sedimentary basins.