Numeric geochemical reaction modelling, incorporating systems theory and implications for sustainable development : study on East Rand basin acid mine drainage, Witwatersrand, South Africa

[摘要] 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 extensiveimpact on especially groundwater resources. The Witwatersrand Basin is a regional geological featurecontaining the world-famous auriferous conglomerate horizons. It is divided into sub-basins and the EastRand Basin is one of them. Due to the regional scale of the East Rand Basin AMD issues, a systemsapproach is required to provide a useful tool to understand the pollution source term and fate and transportdynamics and to aid in environmental decision making and to evaluate the geochemical impact of mitigationmeasures 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, specificallyfor acidity (low pH), SO4, Fe, Mn, U, Ni, Co, Al and Zn. When the AMD solution enters the soil beneath thetailings, ferrous and SO4 concentrations remain elevated, while Mn, U, Ni and Co and perhaps other metalsare adsorbed. After ~50 years the pollution plume starts to break through the base of the soil profile and theconcentration of the adsorbed metals increase in the discharging solution as the adsorption capacity of thesoil becomes saturated. The pollution pulse then starts to migrate to the shallow groundwater wherecontamination of this resource occurs.Toe seepage from the tailings either first reacts with carbonate, where acidity is neutralised to a degree andsome metals precipitated from solution, where after it reaches the surface water drainage, such as theBlesbokspruit, where it is diluted. Some evaporation can occur, but evaporation only leads to concentrationof acidity and dissolved constituents, thereby effectively worsening the AMD solution quality. The mixingmodels have shown that the dilution factor is sufficient to mitigate much of the AMD, although seasonalvariability in precipitation and evapotranspiration is expected to have some influence on the mixing ratio andsome 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 mineand society in general is lower when mitigation measures are employed during operation. For a theoreticalmine in the ERB with an operating life of 100 years, the cost of operational mitigation measures is ~R 31billion. This value is 4% of turnover and 19% of profits over the time period. Post closure remediation costsare ~R 67 billion. This value is 8% of turnover and 41% of profit over the time period. Although the initialcapital investment in mitigation measures is substantial, although some measures will be implementedduring operation, it is a smaller percentage of profits than eventual post-closure mitigation.

[发布机构] Stellenbosch University