Assessment of recharge to groundwater systems in the arid southwestern part of Northern Territory, Australia, using chlorine-36

The sustainability of community water supplies drawn from shallow aquifers in the arid southwest of the Northern Territory has been evaluated using the radioactive isotope chlorine-36 (^sup 36^Cl). These aquifers include fractured sandstones of the Ngalia Basin, fractured metamorphic rocks and Cainozoic sands and gravels. ^sup 36^Cl/Cl ratios for these shallow, regional groundwaters exhibit a bimodal distribution with peaks at 205 (±7) and 170 (±7)×10^sup -15^. The higher ratio probably represents modern (Holocene) recharge, diluted with windblown salts from local playa lakes, and occurs mostly around the margin of the basin. The lower ratio corresponds to a ^sup 36^Cl "age", or mean residence time, of 80-100ka, implying that the last major recharge occurred during the last interglacial interval (Oxygen Isotope Stage 5). These values are mainly observed in the interior of the Ngalia Basin. Lower values of the ^sup 36^Cl/Cl ratio measured near playa lakes are affected by addition of chloride from remobilised salts. Finite carbon-14 (^sup 14^C) data for the groundwaters are at variance with the ^sup 36^Cl results, but a depth profile suggests low recharge, allowing diffusion of recent atmospheric carbon to the water table. The ^sup 36^Cl results have important implications for groundwater management in this region, with substantial recharge only occurring during favourable, wet, interglacial climatic regimes; most community water supplies are dependent on these "old" waters.[PUBLICATION ABSTRACT]