Residence times of groundwater along a flow path in the Great Artesian Basin determined by ⁸¹Kr, ³⁶Cl and ⁴He: Implications for palaeo hydrogeology

Understanding the age distribution of groundwater can provide information on both the recharge history as well as the geochemical evolution of groundwater flow systems. Of the few candidates available that can be used to date old groundwater, ⁸¹Kr shows the most promise because its input function is constant through time and there are less sources and sinks to complicate the dating procedure in comparison to traditional tracers such as ³⁶Cl and ⁴He. In this paper we use ⁸¹Kr in a large groundwater basin to obtain a better understanding of the residence time distribution of an unconfined-confined aquifer system. A suite of environmental tracers along a groundwater flow path in the south–west Great Artesian Basin of Australia have been sampled. All age tracers (⁸⁵Kr, ³⁹Ar ¹⁴C, ⁸¹Kr, ³⁶Cl and ⁴He) display a consistent increase in groundwater age with distance from the recharge area indicating the presence of a connected flow path. Assuming that ⁸¹Kr is the most accurate dating technique the ³⁶Cl/Cl systematics was unravelled to reveal information on recharge mechanism and chloride concentration at the time of recharge. Current-day recharge occurs via ephemeral river recharge beneath the Finke River, while diffuse recharge is minor in the young groundwaters. Towards the end of the transect the influence of ephemeral recharge is less while diffuse recharge and the initial chloride concentration at recharge were higher.