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 Cl-36 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 Cl-36, secular variations in the rate of production of Cl-36, 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 Cl-36/Cl along hydraulic gradients and can preclude estimating absolute groundwater ages. However, mean flow velocities calculated from absolute Cl-36 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 delta(18)O concentrations along the head gradient.