URANIUM-234/238 AND CHLORINE-36 AS TRACERS OF INTER-AQUIFER MIXING: OTWAY BASIN, SOUTH AUSTRALIA

摘要

Uranium isotopes and ~(36)Cl may be useful for the study of mixing of waters from different aquifers provided that the isotopes behave conservatively. This study applies these isotopes to investigate the location and amount of vertical transport of water between the two aquifer systems within the Otway Basin of South Australia - an unconfined karstic limestone aquifer (The Gambier aquifer) - and the confined Dilwyn sand aquifer. Downward transport to the confined sand aquifer can be clearly identified by waters with relatively high U concentrations and low ~(234)U/~(238)U activity ratios, in contrast to the much lower U concentrations and high activity ratios in the 'native' confined system. Despite the large variations in ~(238)U concentration (and redox conditions) between and within both aquifers, U isotope ratios appear to maintain their integrity, particularly in the northern part of the study area. Results for ~(36)Cl are consistent with a conceptual model of local recharge to the unconfined aquifer system using current estimates of atmospheric ~(36)Cl/Cl fallout in southeast Australia. Higher than expected ratios measured in some of the 'upgradient' parts of the system may be due to the palaeoclimatic effect from migration of the coastline during the last glacial maximum. Both ~(36)Cl data and U isotopes suggest a hydraulic discontinuity in the northern Dilwyn flow system.