Vertical carbon-14 profiles for resolving spatial variability in recharge in arid environments

Groundwater age tracers are often measured to help constrain estimates of groundwater recharge, especially in arid environments where other methods are unsuitable. However multiple processes can influence the shape of vertical tracer profiles in an aquifer including (1) variation in tracer input concentrations from the unsaturated zone, (2) the role of diffusion in transporting tracer into the aquifer when fluxes are low and (3) spatial variability in recharge. This study demonstrates the influence of spatially variable recharge and spatially variable carbon-14 (¹⁴C) activities in the unsaturated zone on vertical ¹⁴C profiles in groundwater. Through groundwater flow and solute transport modelling, we demonstrate that recharge estimated from single point measurements of ¹⁴C may be wrong more than an order of magnitude when unsaturated zone ¹⁴C activities and recharge vary spatially. We then present a case study from the Ti Tree Basin in arid central Australia, where detailed profiles of ¹⁴C activity in unsaturated zone gas and groundwater have been measured, and spatial variability in unsaturated zone ¹⁴C is observed (ranging from 54 to 106pMC above the watertable). Through modelling our data, we show that when unsaturated zone ¹⁴C activities are known, measurement of the ¹⁴C profile can help constrain estimates of recharge and its spatial variability. This approach improves our understanding of groundwater flow in the Ti Tree Basin, by showing mountain front recharge to be an important mechanism.