Application of Tracers inAridZane Hydrology (Proceedings of the Vienna Symposium, August 1994). IAHS Pub!, no. 232, 1995. 15 Chlorine-36 in hypersaline palaeochannel ground- waters of Western Australia J. V. TURNER CSIRO Division of Water Resources, Private Bag, PO Wembley, Western Australia 6014 M. R. ROSEN Institute of Geological and Nuclear Science Ltd., Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand L. K. ÎTFTELD & G. L. ALLAN Department of Nuclear Physics, The Australian National University, GPO Box 4, Canberra, Australian Capital Territory 2601 Abstract Comparative application of the environmental tracers 36 C1, 14 C, ô 2 H and ô 18 0 in addition to chloride mass balance and hydrogeological information has been carried out in order to evaluate the groundwater recharge-discharge process in hypersaline palaeochannels of the Eastern Goldfields of Western Australia. Groundwater and chloride residence times are also estimated. The groundwater travel time through one section of palaeochannel was estimated via 14 C to be approximately 20 000 years. Possible interpretations of 36 C1 concentrations are strongly dependent on the reliability of estimates of chloride and 36 C1 input into the groundwater system and the selection of an input 36 C1/C1 ratio, Rj. A slight decrease in the 36 C1/C1 ratio with distance along the flow direction of the palaeo- channel was observed that provided the only suggestion of 36 C1 decay. In general however, the concentration of 36 C1 in the hypersaline ground- waters is determined by the processes of concentration and dilution. Groundwater and chloride residence times are short compared to the half- life of 36 C1. Because the accumulation time for chloride is of the order of 10 5 years, significant radioactive decay of 36 C1 is not expected. INTRODUCTION The conjunctive application of environmental isotope techniques, hydrogeological assessments and groundwater modelling is now well recognized as a powerful approach to solving water resources problems in arid regions. In this paper we focus on the iso- tope hydrology of a system of hypersaline, palaeochannel aquifers, located in the arid Eastern Goldfields region of Western Australia near the town of Kalgoorlie. In parti- cular, attention is given to the interpretation of 36 C1 data in relation to the recharge and discharge process of groundwater, the groundwater residence time and the residence time of chloride in the aquifer system. Use is made of the variable distribution of hyper- saline groundwater across the study region, where chloride is the dominant anion, in addition to carbon-14 and stable isotope data on groundwaters and results from hydro-