1 Implications of dewatering on the dolomite compartments
The term dewatering tends to create a concept of completely dry mines or aquifers. This is not the case as dewatering is a state where the inflow to the mine is reduced by removing water that is in storage in the dolomites. When the compartment is dewatered the mine still experiences a continuous inflow, often very significant, with inflow rate equal to the recharge rate.
Dewatering is achieved when
Pumping rate = Recharge rate
Thus even when a compartment is dewatered, substantial volumes of water must be pumped from the mines.
For example: In the Oberholzer compartment, dewatering began in September 1955 and was accomplished in April 1973. During this period maximum pumping rates reached 170 ML/day while the steady state pumping, after dewatering, is 50 ML/day. The advantage of dewatering is that under steady state conditions the water inflow to the mines is controlled and predictable. Sudden catastrophic inflows are not likely to happen.
Bredenkamp (1993), shows that the Gemsbokfontein compartment has been dewatered by WAGM since 1986, while in the Bekkersdal compartment north of the Gemsbokfontein compartment and separated by the Panvlakte dyke, water levels are apparently unaffected. The eastern Gemsbokfontein compartment leaks through the Magazine dyke, to prevent drawdown from occurring to a level where sinkholes may develop. This compartment is artificially recharged with dewatered water. The Zuurbekom compartment is affected by dewatering.
Bredenkamp’s (1993) study confirms leakage from Gemsbokfontein east Compartment and suggests that a substantial amount of recirculation is taking place. The recharge is estimated to be 24% of the average rainfall of the preceding 12 months. Water levels in the Gemsbokfontein West Compartment are declining at 0.375 m/month.
2 Pumping rates and water levels
The available data on pumping rates has been recorded in a database allowing digital evaluation of the data. Despite gaps in the data, as some mines kept more complete records than others, the flooding of West Driefontein in 1968 is evident, and the stabilisation in pumping can be seen in Figure 5-39, for the mines when dewatering was achieved.
The mine dewatering pumping is equivalent to a large scale pumping test, the evaluation of which could yield crucial information about the aquifer parameters including hydraulic conductivity and storage characteristics. The response of water levels or pumping rate to rainfall also gives indications of the recharge characteristics. The results of such evaluation are important for making predictions about future water levels, rates of water rise and when considering management options for the system. Unfortunately not all the mines have kept suitable records and indirect techniques have to be used.
Dewatering rates at different mines over time.
Comparison between pumping rate and rainfall is an indirect indication of the water-level response to rainfall and shows the type of recharge that may be happening. Evaluation of the data in this way gave no clear relationships, although other investigators show a lag time of between two and four months on the West Rand mines, and six months in the Bank compartment (Fleisher 1981 and Krantz 1997). These investigators suggest that the lag time show two methods of recharge.
1. Immediate recharge happens when the mines have direct connection to the surface.
2. Delayed recharge happens when water is derived via recharge of aquifers and from these, flow to mine workings via fractures.
The mining companies, particularly GFSA, have monitored groundwater levels. Monitoring sites include: shafts, monitoring bores adjacent to shafts and monitoring and production bores in other parts of the catchment. The main influence is a steep drawdown near...
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