Chlorine 36 Dating of Very Old Groundwater: 3. Further Studies in the Great Artesian Basin, Australia

Abstract

The Great Artesian Basin (GAB), Australia, contains several groundwater types within the Lower Cretaceous‐Jurassic confined aquifer system with distinctly different chemistry. The groundwater flow pattern of the GAB has been analyzed with over 40 36 Cl measurements from this study and that of Bentley et al. (1986 a ). The results indicate general agreement between the hydrologie model ages (Darcy's law) and the radioisotopic model ages suggesting that flow conditions over ∼1 m.y. have remained largely unchanged. The results confirm that the chemically distinct western GAB operates as a separate flow system with its own recharge area and its own geochemical evolution. The results indicate an area of groundwater with relatively young 36 Cl ages in the central portion of the basin which cannot be explained by flow or mixing along the presently known flow lines. It is hypothesized that an additional source of young water originated from the basin margin region to the north and northwest between the Simpson Desert and Mount Isa. This additional recharge area, active more strongly over the last 500 kyr than at present, is delineated by 36 Cl ages which integrate a history and evolution over the age of the groundwater.