Impact of a base metal slimes dam on water systems, Madziwa Mine, Zimbabwe

Abstract

The Mazowe Valley contains several of Zimbabwe’s largest current mining operations, is densely populated and is also a major agricultural area. The urban areas of Bindura, Goromonzi, Shamva, Marondera, Murehwa and Mutoko all draw water from within the Mazowe Valley. Irrigation of commercial crops is also a major water user. Accordingly, managing the impact of mining operations on water quality in the Mazowe Valley must be a major priority for sustainable development in this area. Madziwa Mine, 150 km north-east of Harare, is a case in point. Mining took place between 1966 and 2001. The main sulphides were chalcopyrite, pyrrhotite, pentlandite and pyrite. Waste from the mine’s plant has been disposed of via a tailings dam, the focus of this study. Surface water samples were collected at 12 sites around the slimes dam and groundwater samples were collected from six boreholes. The samples were analysed for dissolved metals using atomic absorption spectrometry and for anions using gravimetry and titration. The surface water chemical analyses showed that acidic effluent with high concentrations of iron, nickel and sulphate emanates from the tailings dam. Concentrations of metals are lower after the water has passed through natural wetlands. Chemical analysis of groundwater showed similarly high levels of acidity, sulphate and metal. These findings show that acid mine drainage is seeping from the tailings dam. Efforts are being made to reduce the effects of the acid mine drainage. For surface seepage from the dumps these efforts include diverting acidic effluent from the dump into natural wetlands that neutralise the acidity. To reduce drainage into the groundwater efforts are being made to plant trees with high evaporation rates to minimise the amount of water that can cause acid mine drainage.