Simplified geological map of the Carletonville mining district, west Wits line, showing the relative positions of Western Deep Levels (sampled in 1996), West Driefontein, East Driefontein, and Kloof mines. East Driefontein, Shaft #5 is indicated in pink. Pink line is strike of crossection plotted in following figure. The geological contact is between the 2.2 Ga Transvaal Dolomite (brick gray) and the overlying Pretoria Group (tan) terrigenous sediments with the contact dipping 7 degrees to the south. The Transvaal Dolomite is the second major source of water for the mines. It is partitioned into "compartments" by 1.4 Ga syenite dikes radiating from the Pilansberg Complex (within the Bushveld Complex) to the north. Water from the dolomite enters into the underlying Witwatersrand beds primarily through fissures arranged en echelon to these dike aquicludes. The Witwatersrand units beneath vary from being "wet" mines, like West Driefontein, to "dry" mines like Doornfontein (Wolmarans, 1986). This is not controlled by the natural recharge rate into the dolomite, which is uniformly quite high. The Oberholzer Compartment averages 56 megaliters/day.

The degree of "wetness" appears to be controlled by the type of "fissures". At West Driefontein and Venterspost Mine the fissures are "dirty" or highly porous and permeable; whereas, at Doornfontein mine the fissures tend to be mylonitic and much less permeable. The Venterspost mine pumps upwards to 50 megaliters/day from their subsurface operations. The vertical hydraulic conductivity may also be related to the relative orientations of the regional principal stress (arrow in figure) and the en echelon fissures (Gay and Jager, 1986).

The "wet" mines also appear to be "hot" or express a higher geothermal gradient than "dry" mines. The lowest geothermal gradient measured, 9¡C/km, was at East Driefontein Mine. The highest geothermal gradient, 15¡C/km, was measured at Kloof mine. The correlation of high geotherms with wet conditions, however, implies that a component of deeper, hotter, fissure water may be flowing upwards through the Witwatersrand units.

Since mining operations began in this area the water table has been lowered from 50 to 100 meters. Originally the water table dipped to the west, but today it is flat with cones of depression centered around "pump chambers". The water originally flowed through the cavernous portions of the Transvaal dolomite, but today the water table is well below the cavernous zone into a lower porosity zone.

References:

Gay, N.C. and Jager, A.J. (1986) The influence of geological features on problems of rock mechanics in Witwatersrand
    Mines. In Mineral Deposits of Southern Africa (Anhaeusser, C.R. and Maske, S, eds.)
    Africa vols. 1&II, Geol. Soc. S. Afr., Johannesburg, 753-772.

Wolmarans, J.F. (1986) Some engineering-geological and hydrological aspects of mining on the West Wits line.
    In Mineral Deposits of Southern Africa (Anhaeusser, C.R. and Maske, S, eds)
    vols. 1&II, Geol. Soc. S. Afr., Johannesburg, 791-796.