干旱区生态环境知识资源中心

Open pit gold mining began in 2004 at Mupane's Tau Pit in Botswana and has since extended to a total of four pits, including Tholo, Kwena and Tawana. Tau and Tholo are currently not operational and pit lakes have formed in them. The two lakes are completely different hydrogeochemically in spite of being located within the same geological stratigraphy and located only 1 km apart. The Tau pit lake has a pH of 7-8 with elevated total arsenic concentrations exceeding the World Health Organisation (WHO) limits of 0.01 mg/L. The Tholo pit lake is highly acidic with a pH of 2-3 and high total metals including lead, zinc, iron and manganese, although arsenic is lower at 0.009 mg/L. Due to the arid climate and shortage of accessible surface water in the area, an understanding of the formation of the pit lakes on closure of the mine and the predictive long term water quality is required so that adequate mitigation measures can be undertaken to minimize the risks to the local ecology, currently using the lakes as a drinking water supply.Numerical groundwater flow modelling, a water balance, hydrogeochemical analysis including stable isotopes of groundwater, and modelling of the reactant rocks using PHREEQC indicate that there is sufficient buffering capacity in the groundwater to neutralize acid generated in the Tau pit. Conversely, the chemistry of the Tholo pit lake appears to be the result of extreme rainfall events rinsing precipitated efflorescent salts from the highwalls; and there is insufficient groundwater inflow into the pit to neutralize it.Mitigation measures could include addition of lime to raise the pH, and/or addition of organic carbon to enhance sulfate reduction. The depth of the pits should be taken into consideration to allow sufficient groundwater inflows to buffer the acidic runoff. Backfilling of the pits to the static water table depth could be considered to restrict access by fauna to the pit lakes.