Complex recharge mechanisms creating contradictory tracer signals in a large karst aquifer in northern Australia elucidated by a multi-tracer study.

Abstract:

The Beetaloo Sub-Basin in the Northern Territory of Australia has one of the largest Australian prospective resources of shale gas in the Velkerri formation at 2km to 2.5km depth. The shallowest aquifer covering this basin is the up to 300m thick Cambrian Limestone Aquifer (CLA), extending over more than 800km, with groundwater flow direction from the dry savanna climate in the southeast to the monsoon-dominated climate in the northwest. Both the pastoral industry of the region as well as shale gas industry will use water from the CLA and a robust water balance for this aquifer is needed.

Tracer findings in the karstified CLA are counter-intuitive: radiocarbon concentrations increase instead of decreasing in groundwater flow direction. Tritium is generally low, but detectable in the north, whereas modern gas tracers CFCs, SF₆ and H1301 are high throughout the aquifer, indicating high excess air and suggesting more recent recharge than tritium. Radiogenic helium is present in several bores, indicating upward fluid flow that cannot be further quantified due to a lack of observations in deeper formations. To shed further light on the recharge conditions, samples for ⁸⁵Kr and ³⁹Ar were taken. ⁸⁵Kr results indicate a wide range of concentrations from detection limit to modern.

The recharge model in best agreement with the data suggests a strong regional recharge gradient from south to north, further complicated locally by direct recharge into sinkholes. This creates water level fluctuations over wide areas, causing large excess air and allowing the anthropogenic gas tracers to exchange with the recent atmosphere in the unsaturated zone, while recharge is not actually happening. Tritium and ⁸⁵Kr are therefore considered as the most reliable recharge indicators, whereas CFCs, SF₆ and H1301 mainly indicate the gas exchange process.