Numerical modeling and environmental isotope methods in integrated mine-water management: a case study from the Witwatersrand basin, South Africa

Improved groundwater flow conceptualization was achieved using environmental stable isotope (ESI) and hydrochemical information to complete a numerical groundwater flow model with reasonable certainty. The study aimed to assess the source of excess water at a pumping shaft located near the town of Stilfontein, North West Province, South Africa. The results indicate that the water intercepted at Margaret Shaft comes largely from seepage of a nearby mine tailings dam (Dam 5) and from the upper dolomite aquifer. If pumping at the shaft continues at the current rate and Dam 5 is decommissioned, neighbouring shallow farm boreholes would dry up within approximately 10 years. Stable isotope data of shaft water indicate that up to 50 % of the pumped water from Margaret Shaft is recirculated, mainly from Dam 5. The results are supplemented by tritium data, demonstrating that recent recharge is taking place through open fractures as well as man-made underground workings, whereas hydrochemical data of fissure water samples from roughly 950 m below ground level exhibit mine-water signatures. Pumping at the shaft, which captures shallow groundwater as well as seepage from surface dams, is a highly recommended option for preventing flooding of downstream mines. The results of this research highlight the importance of additional methods (ESI and hydrochemical analyses) to improve flow conceptualization and numerical modelling.