To date, South Africa has mined approximately 3.2 billion tons of coal from a number of different coal reserves located in various parts of the country. A large number of the mines have reached the end of their productive life, resulting in numerous mine closures. With closures, groundwater levels have rebounded, resulting in decant of mine water into the environment. This paper describes a case study of a closed underground coal mine, the rebound of water levels, the evolution of the groundwater quality and the impact it has had on the management of the potential decant.
On closure of the Ermelo Mines in 1992, initial water quality monitoring indicated that a water treatment plant would be required to treat the mine decant. However, as the groundwater levels in the mine rebounded, the water quality in the mine void evolved from sulphate type water to sodium type water. The evolution of the water quality can be attributed to sulphate reducing bacteria, vertical recharge from the hanging aquifer and stratification. Water level and quality monitoring have shown that the water in the old mine void will not decant to surface due to the depth of the mine void, hydrogeological conditions, a "hanging aquifer" and the recharge mechanisms. As a result, no water treatment will be required and the mine will not impact on the surface water. The main applications from this paper are:
- Design of a correct monitoring procedure to allow for monitoring of water quality stratification in rebounding mines.
- Identifying the role of sulphate reducing bacteria in the evolution of groundwater quality in a methane rich coal mine void.
- The role of a hanging aquifer in recharging of a coal mine void and resultant stratification.
- Designing of a mine taking into consideration mine closure.
The main contribution of this paper is the use of hydrogeological information in design of a coal mine so as not to decant on closure.