The Cambrian Limestone Aquifer (CLA) in the Beetaloo Sub-Basin is the main water supply for domestic use and pastoral agriculture, and provides baseflow to unique environmental assets at headwaters of local rivers. In competition, it is a potential resource for fluids to be used in unconventional gas extraction as well as a potential receptor of contamination. The CLA, whose recharge has not been quantified to date, is a complex and highly vulnerable system because it is karstic and only partially confined.
Multiple campaigns have been undertaken, targeting 500 km transects from Mataranka (NT) southwards, collecting samples from 33 bores. The measured tracers include major and minor ions, stable isotopes (18O&2H of H2O, 18O&34S of SO4, 13C, 87Sr/86Sr), tritium, SF6, halon-1301, radiocarbon, and noble gases. Cluster analysis on hydrochemical data were used to identify different hydrogeological groups.
Radiocarbon activity increased along the flow path, contrary to expectations from the northward flow direction. Modern water was also evident across the sampling area from halon-1301 and SF6. A two-component mixing model indicated “old” water at the beginning of the investigated flow path with only 2% in radiocarbon and increasingly modern recharge with >100% in radiocarbon further along the flow path. Some samples show elevated 4He, which may be indicative of older water from layers below the CLA.
Noble Gas inferred temperatures were 37-41°C for samples from the Gum Ridge Formation and 32-37°C for the Tindall Limestone. These differences likely result from different recharge characteristics in the North and South. Strontium isotopes differentiate the Gum Ridge Formation and Anthony Lagoon Beds from the Tindall Limestone.
Tracer analysis indicates an increased rate of recharge northwards, consistent with the rainfall gradient.