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

The relationship between the distribution of ⁴He 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 ⁴He 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 ⁴He 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.