Assessment of recharge to groundwater systems in the arid southwestern part of Northern Territory, Australia, using chlorine-36 |
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Authors: | Richard Cresswell John Wischusen Gerry Jacobson Keith Fifield |
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Institution: | (1) Department of Nuclear Physics, Australian National University Canberra, ACT, Australia. 0200 Fax: +61-2-6249-0748 e-mail: richard.cresswell@anu.edu.au, AU;(2) Australian Geological Survey Organisation, Box 378 Canberra, ACT, Australia. 2601, AU |
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Abstract: | 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 (36Cl). These aquifers include fractured sandstones of the Ngalia Basin, fractured metamorphic rocks and Cainozoic sands and
gravels. 36Cl/Cl ratios for these shallow, regional groundwaters exhibit a bimodal distribution with peaks at 205 (±7) and 170 (±7)×10–15. 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 36Cl "age", or mean residence time, of 80–100 ka, 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
36Cl/Cl ratio measured near playa lakes are affected by addition of chloride from remobilised salts. Finite carbon-14 (14C) data for the groundwaters are at variance with the 36Cl results, but a depth profile suggests low recharge, allowing diffusion of recent atmospheric carbon to the water table.
The 36Cl 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.
Received, September 1997 · Revised, August 1998, March 1999 · Accepted, March 1999 |
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Keywords: | groundwater recharge/water budget radioactive isotopes chlorine-36 Australia arid regions |
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