It has been appreciated since the first geological report by Johnson, Hiern and Steel (RB 51/00136) that the filling of the proposed Chowilla Dam would cause some flushing downstream of saline groundwater present in sands beneath the Murray River...
It has been appreciated since the first geological report by Johnson, Hiern and Steel (RB 51/00136) that the filling of the proposed Chowilla Dam would cause some flushing downstream of saline groundwater present in sands beneath the Murray River alluvial flat. This potential problem has been studied in detail during design of the dam. For the immediate dam area, and for about half a mile downstream, provision is made in the design to collect most saline water displaced during the filling operation. Recent field studies in the Tilmy Flat area, and office studies of existing groundwater levels and gradients in the Murray Basin suggest that saline groundwater is entering the Murray River in many places both upstream and downstream from the dam site. In the first systematic geological examination of the reservoir rim, Boucaut (RB 63/00107) drew attention to numerous exposures of Parilla and Loxton sands bordering the reservoir site in the first few miles upstream from the proposed dam location. Due to the highly permeable nature of these sands and the relatively low water table, it seems certain that water will flow into them from the completed reservoir. This will form an elevated wedge-shaped layer of fresh water, sloping away from the filled reservoir rim, the slope angle or angles depending upon the sand permeabilities, and the shape of the present water table. Assuming that this likely loss from dam storage does not represent a serious problem, there is still the probability that over a number of years this water will tend to flow downstream around the margins of the reservoir, and eventually back into the Murray River somewhere downstream from the dam. It is possible that this water could (a) become saline by dissolving salt out of the sands or (b) become mixed with highly saline groundwater, or (c) that it could cause saline groundwater to be flushed downstream. The rate at which this downstream flow could occur, and the quantities of fresh lake water and saline groundwater which could enter the Murray downstream, cannot be predicted at this stage.
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