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 36 Cl 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 36 Cl , secular variations in the rate of production of 36 Cl , and its subsequent fallout as well as rates of diffusion of stable Cl from adjacent aquitards mask the interpretation of rates of decay of 36 Cl/Cl along hydraulic gradients and can preclude estimating absolute groundwater ages. However, mean flow velocities calculated from absolute Cl 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 δ 18 O concentrations along the head gradient.