Hydrogeochemistry and geothermometry of thermal groundwaters from the Birdsville Track Ridge, Great Artesian Basin, South Australia

The hydrogeochemistry of thermal artesian groundwaters flowing from 12 stock bores along the Birdsville Track Ridge in northeast South Australia has been examined. The Na-HCO3-Cl type groundwater composition has provided a basis for the application of chemical geothermometers to estimating aquifer temperatures and has allowed comparisons of various silica and cation geothermometers. Aquifer and bore penetration depth decrease between Birdsville and Marree from 1220 m to 170 m. A corresponding decrease in measured emergence temperature is also observed (94-31°C). Chalcedony geotemperature estimates ranging from 110°C to 41°C between Birdsville and Marree are considered the most accurate of the various geothermometers tested. Log(Q/K) versus T diagrams have also been evaluated to determine likely aquifer mineral assemblages and reservoir temperatures (111-39°C). The Birdsville Track Ridge acts as a conduit for low salinity groundwater (total dissolved solids range from 640 mg/l at Birdsville to 1900 mg/l at Marree) in the Great Artesian Basin. Old, slowly migrating groundwater from the deeper basins on either side of the ridge is characterised by higher emergence temperature and higher total dissolved solids. This old groundwater is inferred to have mixed with the younger, lower temperature, lower salinity groundwater that is migrating relatively rapidly along the Birdsville Track Ridge axis. Since the various geothermometers provide distinct types of information, evidence for the mixing is provided by the interpretation of the different temperature estimates. Silica equilibration temperatures reflect aquifer temperatures along the ridge axis, whereas cation geotemperatures partly preserve higher temperatures from the deeper, flanking basins. This study demonstrates how the thermal regime and hydrodynamics of an area can be characterised using a sparse dataset, thus representing a novel and effective methodology for regions anomalous to this central Australian example.