CHEMICAL AND ISOTOPIC INDICATORS OF POINT-SOURCE RECHARGE TO A KARST AQUIFER, SOUTH AUSTRALIA

Several hundred sinkholes, swamps and open drainage boreholes througho ut the semi-arid to sub-humid areas of the southeast of South Australi a are potential recharge sites to the upper part of the Gambler Limest one aquifer. This paper presents the results from a hydrochemical (Cl- and carbonate chemistry) and isotopic (delta(2)H, delta(18)O and Cl-3 6) study used to estimate the importance of localised recharge from th ese point-source features to the karstic groundwater system. Results s how 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 fe atures are indistinguishable from regional groundwater compositions. C hloride concentrations, carbonate chemistry, and isotope data show tha t 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 aft er a threshold of sustained rainfall had been exceeded (i.e. greater t han 2.5 mm day(-1) for more than 3 days). The stable isotopic composit ion of waters recharging via sinkholes tends to be enriched in H-2 and O-18 relative to regional groundwater and local precipitation. This i s probably caused by a small degree of evaporation (less than 5% of wa ter falling within the catchment) occurring during winter at high rela tive humidity (greater than 95%) before recharge, Cl-36 from weapons t esting is detectable in groundwaters near sinkholes and indicates sign ificant retardation of Cl- within soils of the respective sub-drainage systems. Recharge has therefore occurred within the past 30 years, bu t cannot be quantified with any reliable precision from the Cl-36 data . (C) 1997 Elsevier Science B.V.