CL-36 DATING OF VERY OLD GROUNDWATER .3. FURTHER-STUDIES IN THE GREAT ARTESIAN BASIN, AUSTRALIA

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 Cl-36 measurements from this study and that of Bentley et al. (1986a). The results indicate general agreement between the hydrologic model ages (Darcy's law) and the radioisotopic model ages suggesting that flow conditions over approximately 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 Cl-36 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 Cl-36 ages which integrate a history and evolution over the age of the groundwater.