Masters Degrees (Earth Sciences)

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Browsing Masters Degrees (Earth Sciences) by Subject "Acid mine drainage -- South Africa -- Witwatersrand Basin"

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    Numeric geochemical reaction modelling, incorporating systems theory and implications for sustainable development : study on East Rand basin acid mine drainage, Witwatersrand, South Africa
    (2014-12) Hansen, Robert Neill; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept of Earth Sciences.
    ENGLISH ABSTRACT: In recent years acid mine drainage (AMD) has become the focus on many mine sites throughout the world. The Witwatersrand gold mines have been the main focus of AMD in South Africa due to their extensive impact on especially groundwater resources. The Witwatersrand Basin is a regional geological feature containing the world-famous auriferous conglomerate horizons. It is divided into sub-basins and the East Rand Basin is one of them. Due to the regional scale of the East Rand Basin AMD issues, a systems approach is required to provide a useful tool to understand the pollution source term and fate and transport dynamics and to aid in environmental decision making and to evaluate the geochemical impact of mitigation measures and evaluate future scenarios. The numeric geochemical models, using a systems perspective, show that the mine waste facilities, specifically the tailings dams are significant contamination point sources in the East Rand Basin, specifically for acidity (low pH), SO4, Fe, Mn, U, Ni, Co, Al and Zn. When the AMD solution enters the soil beneath the tailings, ferrous and SO4 concentrations remain elevated, while Mn, U, Ni and Co and perhaps other metals are adsorbed. After ~50 years the pollution plume starts to break through the base of the soil profile and the concentration of the adsorbed metals increase in the discharging solution as the adsorption capacity of the soil becomes saturated. The pollution pulse then starts to migrate to the shallow groundwater where contamination of this resource occurs. Toe seepage from the tailings either first reacts with carbonate, where acidity is neutralised to a degree and some metals precipitated from solution, where after it reaches the surface water drainage, such as the Blesbokspruit, where it is diluted. Some evaporation can occur, but evaporation only leads to concentration of acidity and dissolved constituents, thereby effectively worsening the AMD solution quality. The mixing models have shown that the dilution factor is sufficient to mitigate much of the AMD, although seasonal variability in precipitation and evapotranspiration is expected to have some influence on the mixing ratio and some variability in the initial solution will also be reflected in variation in surface water quality. From a sustainability perspective, a basic cost benefit analysis shows that the costs for the operating mine and society in general is lower when mitigation measures are employed during operation. For a theoretical mine in the ERB with an operating life of 100 years, the cost of operational mitigation measures is ~R 31 billion. This value is 4% of turnover and 19% of profits over the time period. Post closure remediation costs are ~R 67 billion. This value is 8% of turnover and 41% of profit over the time period. Although the initial capital investment in mitigation measures is substantial, although some measures will be implemented during operation, it is a smaller percentage of profits than eventual post-closure mitigation.