CHLORINE STABLE ISOTOPE STUDIES OF OLD GROUNDWATER, SOUTHWESTERN GREAT ARTESIAN BASIN, AUSTRALIA

Sources of chloride and mechanisms for chlorine isotopic fractionation may be determined based on the relationship between d³⁷Cl and chloride concentration. Predictive models can be constructed that are representative of various processes (e.g. mixing). Comparison of the predictive models with the measured correlation may allow the underlying mechanism to be inferred. This study aims to understand the origin of chloride and the mechanisms by which chloride is transported in the southwest flow systems of the Great Artesian Basin, Australia.

The d³⁷Cl values in groundwater range from –2.5 to 0.1‰, becoming more depleted along flow paths. The chlorine isotopic compositions show a negative correlation with the chloride concentrations measured. Chlorine isotopic compositions were also measured for leachates of rock samples. Compared to the groundwater samples taken at the same location, the rock leachates are more depleted in d³⁷Cl.

Deposition of marine aerosols by rainfall is likely responsible for d³⁷Cl values close to Standard Mean Ocean Chloride (SMOC) measured near inferred recharge area. Mixing between recharge water and an old groundwater in the aquifer is one possible mechanism that would result in the observed increase in chloride concentration along the flow direction. However, the simulated mixing curve does not match the d³⁷Cl-Cl relationship measured. Alternatively, the chloride may have originated from the overlying aquitard (the Bulldog shale). Theoretical chlorine isotopic signatures of aquitard rocks were calculated, and yield good agreement with the d³⁷Cl measured in the rock leachates, verifying that the aquitard is the source of chloride. Pore waters likely contribute to the highly depleted chlorine isotopic compositions measured in the leachates. The decrease in d³⁷Cl in pore water along the flow direction may be related to the mixing of rock pore waters with infiltration water. It seems reasonable that such mixing is more significant near the recharge areas, and decreases toward the interior of the groundwater system.