Regional recharge to a karst aquifer estimated from chemical and isotopic composition of diffuse and localised recharge, South Australia

The relative importance of different recharge mechanisms has been investigated for a karstic area in a subhumid to semi-arid region of South Australia. The chemical and isotopic signature (δ²H and δ¹⁸O) of regional groundwater, sampled from shallow bores in the study area, as well as water from sinkholes, swamps, drainage bores and water extracted from soil in the unsaturated zone has been measured. Point estimates of recharge, based on data from the unsaturated zone, have been combined with temporal and spatial trends in the isotopic and chemical signature of the groundwater to provide a regional estimate for recharge for different land elements. For approximately half of the study area, which has predominantly clay soils, recharge is low (< 10 mm year^-1) and the groundwaters within the study area are derived from lateral flow from the east. For the remaining area, recharge has significantly increased since agricultural development, and has affected the groundwater system. Groundwater within the vicinity of irrigation areas has shown marked increases in salinity over the past 30 years owing to the impact of irrigation drainage waters. Other areas not under irrigation, but cleared of native vegetation and replaced by shallow rooting pastures, also show increased salinity in parts where there are sandy soils. In these areas, saline soil water from the unsaturated zone has been displaced by the increased recharge since clearing. The effect of clearing on areas of clay surface soils has not increased recharge to the same extent and groundwater salinity levels are largely unchanged. Areas where salinity levels are rising correlate well with increased ¹⁴C activity and depleted δ¹³C concentrations of the groundwater suggesting that these isotopes may be an early indicator of areas of enhanced recharge.