Impact of quarry deepening on local groundwater users

A rise in demand for cement in South Africa led Pretoria Portland Cement to commission an ElA in 2007 to investigate the impacts of expanding one of its main limestone quarries. The quarry is located about 100km northeast of Cape Town, South Africa. SRK’s Cape Town Groundwater Department, led by Partner and Principal Hydrogeologist Peter Rosewarne, was appointed to assess the risk to downstream groundwater users due to the deepening of the quarry and to predict the resultant increase in groundwater inflow and the amount and extent of drawdown.

The quarry depth of 78m as of 2007 caused a zone of drawdown in groundwater levels in the surrounding shale aquifer, extending ~5km to the east but a smaller distance to the west, because of good recharge from the mountainous sandstone aquifer located there. Current groundwater inflow is ~200m3/day, with an electrical conductivity of 260 to 310mS/m.

Using numerical modelling techniques, SRK showed that after 50 years of expansion, with the quarry at 180m depth, existing boreholes on the neighbouring farm to the east could be expected to dry up as drawdowns of up to 60m develop. The modelling also showed that quarry inflows could more than double by the time the quarry reaches full depth development of 240m after 75 years, with drawdowns of up to 100m. However, the model indicated that only a relatively few boreholes would be so affected.

Some of the mitigation measures SRK proposed included drilling new production boreholes to greater depths than existing boreholes (>150m) to tap deeper groundwater resources, deepening existing boreholes, supplying compensation groundwater from the extra inflows made into the quarry and providing compensation water from the plant’s potable water supply feed.

Peter Rosewarne: prosewarne@srk.co.za



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