Abstract:
Gold mining in South Africa resulted in vast volumes of waste material, mainly in the form of tailings material. Poor management of most of the tailings dams resulted in the release of acid mine drainage that in some cases caused soil degradation and water contamination underneath and around these sites. Although many tailings dams have been partially or completely reclaimed, their contaminated footprints pose a serious threat to the water quality of the underlying aquifers (e.g. dolomitic aquifers). This study investigated the geotechnical, mineralogical and geochemical parameters of eleven selected partially or completely reclaimed sites situated near Johannesburg. The main objective of the field and laboratory experiments was to assess the pathway of contaminant migration resulting from acid mine drainage from tailings materials through the unsaturated zone into the groundwater system. Comparing extractable contaminant concentrations with a soil standard from literature represents the environmental short-term impact. In contrast, total element concentrations in the soil compared with background values were used to describe the long-term impact or worst-case scenario. Extraction tests have shown that only a minor portion of contaminants (i.e. Co, Ni and Zn) is mobile in acidic soils. This implies that plant growth could be limited because of phytotoxic elements occurring in the topsoils, complicating rehabilitation measures. In addition, the soils often contain anomalous trace element concentrations, providing a pool for future contamination. Buffer minerals will eventually be depleted and the subsequent acidification of the subsoil, could result in the remobilization of contaminants from the subsoil into the groundwater system in the long term. It is important to understand the parameters, which control the balance between retention and mobility of contaminants in soils. Therefore a risk assessment approach would be required for all tailings dams and reclaimed sites to identify those sites, which need rehabilitation and to define the type and extent of remedial measures. Minimum rehabilitation requirements at reclaimed sites could consist of soil management measures such as liming and the addition of organic material and fertilisers to minimise the contaminant migration from the topsoil into the subsoil and groundwater as well as to provide suitable conditions for vegetation growth and future land use. Removal of remaining tailings and excavation of those portions of the soil, which are excessively contaminated, are necessary. Tailings dams which pose a high risk to the environment would require a well-engineered soil and vegetation cover to limit rainfall infiltration into the impoundment, and thus to reduce the oxidation of sulphide-bearing minerals such as pyrite. Long-term monitoring is an absolute prerequisite to ensure the success of rehabilitation, and therefore the safe use of land and water.
