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

Several hundred sinkholes, swamps and open drainage boreholes throughout the semi-arid to subhumid 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, δ¹⁸O and ³⁶Cl) study used to estimate the importance of localized 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 shown 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 5% of water falling within the catchment) occurring during winter at high relative humidity (greater than 95%) 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.