Chemical and isotopic indicators of point-source recharge to a karst aquifer, South Australia

Abstract

Several hundred sinkholes, swamps and open drainage boreholes throughout the semi-arid to sub-humid areas of the southeast of South Australia are potential recharge sites to the upper part of the Gambier Limestone aquifer. This paper presents the results from a hydrochemical (Cl− and carbonate chemistry) and isotopic (δ2H, S 18O and 36Cl study used to estimate the importance of localised recharge from these point-source features to the karstic groundwater system. Results show that water recharging the groundwater via point-source features is detectable only on a local scale. Chemical and isotopic compositions of groundwaters more than 150 m from the larger of the point-source features are indistinguishable from regional groundwater compositions. Chloride concentrations, carbonate chemistry, and isotope data show that annual input of water via point sources contributes less than 10% of total recharge, with diffuse recharge providing the remainder to the study area as a whole. Point-source recharge is generally intermittent, and was observed to occur only for a few days at a time and only after a threshold of sustained rainfall had been exceeded (i.e. greater than 2.5 mm day−1 for more than 3 days).The stable isotopic composition of waters recharging via sinkholes tends to be enriched in 2H and 18O relative to regional groundwater and local precipitation. This is probably caused by a small degree of evaporation (less than 596 of water falling within the catchment) occurring during winter at high relative humidity (greater than 9596) before recharge. 36Cl from weapons testing is detectable in groundwaters near sinkholes and indicates significant retardation of Cl− within soils of the respective sub-drainage systems. Recharge has therefore occurred within the past 30 years, but cannot be quantified with any reliable precision from the 36Cl data.