Sources of chloride and implications for ³⁶Cl dating of old groundwater, southwestern Great Artesian Basin, Australia

We investigate the relative importance of a number of processes that affect Cl and ³⁶Cl distribution in the southwestern flow systems of the Great Artesian Basin, Australia, to provide more precise estimates of age and flow and recharge rates. In situ production of ³⁶Cl, secular variations in the rate of production of ³⁶Cl, and it's subsequent fallout as well as rates of diffusion of stable Cl from adjacent aquitards mask the interpretation of rates of decay of ³⁶Cl/Cl along hydraulic gradients and can preclude estimating absolute groundwater ages. However, mean flow velocities calculated from absolute ³⁶Cl concentrations are 0.24 ± 0.03 m/yr and are a more reliable parameter than calculated ages because fewer assumptions and approximations are required. Recharge rates to the main Cadna-owie Formation and Algebuckina Sandstone aquifer of the Great Artesian Basin estimated from Cl mass balance in the unconfined aquifers are 0.16 ± 0.08 mm/yr and were probably up to a factor of 3 higher > 30 kyr ago as indicated by decreasing Cl and δ¹⁸O concentrations along the head gradient.