Application of mathematical modelling techniques to the alluvial aquifer system near Wagga Wagga,New South Wales |
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| Authors: | F R Kalf D R Woolley |
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| Institution: | Water Resources Commission , P.O. Box 952, North Sydney, N.S.W., 2060 |
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| Abstract: | The central part of the Murrumbidgee River meanders across an alluvial plain which is 1.5 km to 5 km wide, and confined between low hills composed of granite and folded lower Palaeozoic rocks. Deposition commenced under humid and reducing conditions during the Miocene, and continued during Pliocene time. Oligomict quartz‐sand and gravel beds are dominant in the sequence, and are interlayered with grey to black clay and silty clay, which contain a prolific pollen fauna. During Pleistocene time, a marked reduction in rainfall and consequent change in vegetation resulted in the deposition of a widespread coarse polymict poorly‐sorted gravel associated with yellow and brown clay and silt. The quartz‐sand and gravel beds within the Pliocene deposits constitute a major aquifer system and contain low salinity water suitable for irrigation and town water‐supply purposes. Since 1968, increasing demand has been made on this aquifer system, and wells sunk for various State and Local Government bodies have a total installed capacity of approximately 1000 litres/sec. The most concentrated exploration and development has been effected east of Wagga Wagga, where a well field was constructed to provide water for a regional water supply scheme. Following a refraction seismic survey, test drilling, and mathematical simulation, four production wells were constructed. Mathematical simulation models developed on a mini‐computer were designed primarily to study the behaviour of the aquifer system in response to pumping from the Pliocene aquifers. An analytical model was used to assist in the location of production wells, and a numerical model was used, after construction of the wells, to determine what additional losses from the Murrumbidgee River would be induced by pumping them. These losses were found to correspond to approximately 90% of the pumping rate during long term pumping. |
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