ABSTRACT
ABSTRACT: Groundwater dating is one of the most promising methods to evaluate very slow groundwater flow. In this study, 36Cl and 4He dating were carried out in the Great Artesian Basin, Australia. Groundwater samples were collected from 77 flowing artesian waterbores tapping aquifers in the Cadna-owie Formation and Hooray Sandstone, the uppermost confined aquifers in the Lower Cretaceous-Jurassic sedimentary sequence. Major ions, stable isotopes (δD, δ18O and δ13C), radioactive isotopes (14C, 36Cl) and noble gases (4He, 3He/4He, Ne) were analysed. The results show that 36Cl/Cl ratios decrease from the recharge areas to the discharge areas and 4He values increase with the distances from the recharge areas. The 36Cl ages have a close relationship with the distances from the recharge areas, as well as those from earlier studies. These relationships of 36Cl concentrations reflect the groundwater residence times. Concentrations of 4He increase with 36Cl ages near the recharge areas. This result indicates that 4He concentrations also reflect the groundwater residence times. However, 4He concentrations vary due to sampling conditions, such as depressurisation and high temperatures in waterbores. These results confirm that 36Cl and 4He are useful indicators of groundwater residence times of very old groundwater.
