Groundwater flow and the 4 He distribution in the Great Artesian Basin of Australia

Abstract

The relationship between the distribution of 4 He and the pattern of groundwater flow within the Jurassic aquifer of the Great Artesian Basin has been a source of controversy. According to one explanation, a large and possibly heterogeneous flux of helium passes from the crystalline crust into basin strata, accumulating in groundwater flowing through the aquifer. A second explanation holds that contrary to current understanding of the basin's hydrology, groundwater in the aquifer is stagnant, allowing helium produced in situ to accumulate over geologic time. We use a numerical model to study the relationship between groundwater flow and helium transport in the basin. The calculations assume a moderate and homogenous flux of helium and uniform hydraulic conductivities of sandstone (aquifers) and fine‐grained sediments (aquitards). We show that the 4 He concentration along the top of the Jurassic aquifer is controlled not only by the lateral flow of groundwater but by its descent and upwelling. Our simulations reproduce the pattern of 4 He content across the basin, showing that the basin's isotope distribution and groundwater hydrology can be reconciled in a quantitative sense without calling on large or uneven helium fluxes or on stagnant flow conditions. These results further show that isotopic study can give important information about three‐dimensional patterns of groundwater flow within sedimentary basins.