RECHARGE IN KARST AND DUNE ELEMENTS OF A SEMI-ARID LANDSCAPE AS INDICATED BY NATURAL ISOTOPES AND CHLORIDE

The rates and mechanisms of local recharge in a semi-arid environment have been investigated beneath two major landscape settings in the Murray Basin in South Australia; these were calcrete flats with sinkholes and sand dunes in adjacent landscape settings. Five minor settings were investigated and identified as undisturbed calcrete, primary (older) sinkholes, secondary (younger) sinkholes, sand dunes with native vegetation and sand dunes with introduced pasture. Data from a limited series of boreholes were interpreted to suggest that recharge varied from in excess of 100 mm yr. ** minus **1 for secondary sinkholes to less than 0. 1 mm yr. ** minus **1 beneath sand dunes with native vegetation. The data suggest that recharge under sand dunes has increased by more than two orders of magnitude following clearing of the native vegetation 50 years ago. At sites where the inferred recharge is lowest, profiles of the chloride concentration of soil water are consistent with the hypothesis that there is in excess of 50,000-yr. recharge stored in the thick unsaturated zone. Some evidence for changes in recharge rate over this time scale is seen. Refs.