NUMERICAL MODELLING OF MINE DEWATERING AND FLOODING IN THE EVANDER GOLD BASIN, SOUTH AFRICA

Abstract

The assessment and prediction of mine water rebound has become increasingly important for the gold mining industry in South Africa. The cessation of mine dewatering has led to large volumes of contaminated discharges in the western parts of the Witwatersrand Basin. Towards the eastern extremity of the Basin, the detached Evander Goldfield has been mined since the early 1950s at depths of between 400 m and 2 000 m below ground level, while overlain by shallower coal mining operations. The hydrogeology of the Evander Basin can be categorised by a shallow weathered-fractured rock aquifer comprising of the glacial and deltaic sediments of the Karoo Supergroup, while the deeper historically confined fractured bedrock aquifer consists predominantly of quartzite with subordinate lava, shale and conglomerate of the Witwatersrand Supergroup. The deep Witwatersrand aquifer has been actively dewatered for the past 60 years at a peak rate of 60 ML/d in the mid-to-late 1960s. Modelling the impacts of mine dewatering and flooding on a regional scale as for the Evander Goldfield presents many challenges including the appropriate discretisation of mine voids and the accurate modelling of layered aquifer systems. To predict the environmental impacts of both the historic and future deep-mining operations at Shaft 6, a detailed conceptual model of the aquifer systems and a 3-dimensional model of the mine voids were incorporated into a numerical groundwater model. This model was used to simulate the dewatering and post-closure rebound of the water tables in the vicinity of the mine. This model could serve as an example for the successful modelling of mine dewatering and flooding scenarios for other parts of the Witwatersrand Basin.