Investigation into the groundwater interaction between a deep coal mine and a deeper lying gold mine

Abstract

After the closure of mines, it is expected that water in the mined-out areas will flow along preferred pathways and accumulate in lower-lying areas (Grobbelaar, 2001). Over time, these man-made voids will fill up with water and hydraulic gradients will be exerted on peripheral areas within mines, resulting in groundwater flow between mines and possibly surface. This flow is referred to as intermine flow (Grobbelaar, 2001). Due to the potential long-term impact of intermine flow in terms of water quantities and qualities, the Department of Water Affairs and Forestry regards intermine flow as one of the most important challenges in the mining industry. Hydro-chemical and stable environmental isotope sampling were used to determine the interaction between the Karoo aquifer and the Witwatersrand aquifer. Comparison between the results showed that there is interaction between the Karoo and Witwatersrand aquifer, both where coal mining is present and where coal mining is not present. As the coal mine in future will cover most of the gold mining area except for the central part of the study area, it is expected that groundwater affected by the coal mining in the Karoo aquifer will move along naturally occurring structures from the base of the Karoo aquifer into the gold mine workings if preferential pathways are encountered. This interaction can directly link to the number of structures that are water-bearing in the study area and are covered by coal mining activities. From the field work done in the underground gold mine, the number of structures that are water-bearing are the exception rather than the norm. Therefore the potential for groundwater interaction from future coal mining into the gold mine will be low. 1 Based on current inflow rates, the gold mine workings will take approximately 406 years to flood and reach the bottom of the coal mine workings. It is highly unlikely that this flooding will decant onto the surface through the coal mine, as the shallowest coal mine workings are at 39 m below surface in the study area. In the unlikely event of decanting, polluted water originating from the mining cavities will seep out as normal unpolluted springs at low points.