The geohydrology of the dolomite aquifers of the Malmani subgroup in the South-Western Transvaal, Republic of South Africa
Fleisher, J. N. E.
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The studied area includes the Plalmani Subgroup dolomite outcrops and sub-outcrops extending between the Klip River in the east and the town of Ventersdorp in the west. The composition, structures and nature of the aquifers are investigated. The area is divided into geohydrologic units (compartments) defined by hydrologic boundaries, and several selected units have been studied in detail. In part of the area the entire hydrologic set-up had been changed as a result of gold mining activities which often involve dewatering, artificial recharge and the run-off of effluents from the mines. In other parts semi-natural conditions prevail and groundwater discharges still take place through the natural outlet of springs. An analysis of springs and rainfall-discharge relationships enables the understanding of the natural recharge in these aquifers. The mechanism of the replenishment may be schematically viewed as consisting of two phases namely, an early nearly immediate intake, and a later delayed phase with a time lag of approximately 4 to 6 months. Annual outflows at springs observed over a succession of many years reveal the discharge to behave as a very strongly autoregressive process. The aquifer may he conceived as a reservoir which periodically, on the occasions of exceptionally high rainfall seasons, becomes over-filled, and henceforward for several years discharge is controlled more by this event of recharge than by the following moderate annuaI recharge increments. The application of a statistical model for prediction purposes simulated the discharge rather closely. The rate of natural replenishment as a percentage of annual rainfall was found to be between 13 and 27 per cent. The effective porosity derived from book-keeping water balances and a chemical mass balance is of the order of 1 to 3 per cent. Pumping tests indicate that the storage coefficient varies considerably and could locally be of the order of 10 -4. Sulphatic mine effluents of different concentrations and treated sewage waters are disposed of into the river courses resulting in the contamination of groundwater reservoirs and springs by artificial recharge. The interaction of this combined system of' surface water and groundwater has also been studied. Under undisturbed natural conditions the concentration of the sulphate ion in groundwaters of the investigated area is negligibly small, but on the other hand, groundwaters encountered in this study are unsaturated with regard to sulphate. This parameter is therefore used to identify contamination, as well as for quantitative storage calculations by means of chemical mass balance models.