Identifying fluvial recharge and artesian upwards leakage contributions to arid zone shallow, unconfined groundwater

Abstract

Evaporation losses from unconfined groundwater can form an important part of the water balance of arid zone groundwater systems. However, in artesian groundwater systems, the discrimination between artesian leakage and surface recharge contributions to the unconfined water table is required. We use hydrochemical analysis techniques and isotopic data to investigate the provenance of unconfined groundwater in zones of artesian discharge along the margin of the Great Artesian Basin (GAB) of Australia. Forward modelling of evapoconcentration and use of trilinear plots identified unconfined groundwater that was largely derived from fluvial recharge, compared to upwards artesian leakage, particularly in areas where the artesian groundwater had a Na-HCO3-Cl composition in contrast to the Na-Cl-SO4 composition more typical of arid zone meteoric recharge. Mixing models, combined with forward evapoconcentration models, confirmed that the contribution of artesian groundwater was minimal in areas of Na-Cl-SO4 unconfined groundwater underlain by Na-HCO3-Cl artesian groundwater. However, the evaporation modelling was still useful in identifying probable artesian versus meteoric contributions from Na-Cl-SO4 composition artesian groundwater Stable isotope data were consistent with the unconfined groundwater being evapoconcentrated from the artesian groundwater end-member and were unable to discriminate between contributions from fluvial recharge and artesian discharge. The hydrochemical analysis techniques can assist in water balance studies of aquifers with Na-HCO3-Cl composition groundwater discharging into semi-arid and arid environments, in addition to identifying areas of local fluvial recharge.