Abstract

The Malmani dolomite aquifer underlies large urban and rural areas to the north of Johannesburg and it is a strategic water source for the Metropolitan Municipality of Tshwane. The East and West Fountains springs supply approximately 45 Mltrs/d to the city while abstraction from boreholes accounts for only 13% of average groundwater throughput. The occurrence of sinkholes is a recognised risk in the dolomite and groundwater lowering by pumping from boreholes can increase the likelihood of sinkholes forming. Pumping tends to increase during periods of drought, when there is a natural recession in water levels, so that separation of the two components of drawdown requires information on either abstraction rates or recharge. The Cumulative Rainfall Departure (CRD) method was used with considerable success to separate the effects of pumping during a period of drought from the lowering of water levels due to reduced recharge.

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