Geochemical evaluation of soils and groundwater affected by infiltrating effluent from evaporation ponds of a heavy mineral processing facility, West Coast, South Africa

Abstract

At a heavy mineral separation plant situated in the West Coast of South Africa, process waters, with a low pH and high SO42− and trace element contents, are neutralized and disposed of in evaporation ponds. Because of the permeability of the ponds, the waste water infiltrates soils and groundwater and poses a serious threat to the ecosystem. This study assesses the mobility of process water-derived major and trace elements in the surrounding soils and their evolution in the adjacent aquifer, using a combination of geochemical modeling and multivariate analyses. Although the samples collected around the ponds had high concentrations of SO42− and a suite of trace elements compared to the control samples, liquid–solid partitioning coefficients suggested that Se, As, Cu and Zn were the most mobile elements in the soils. The strong negative values of the first principal component (PC), that explains 62% of the variance, were attributed to the dilution effects of the pond effluent entering the aquifer, whereas those of the second and third PCs may be due to the geochemical evolution of the elements under oxidizing conditions. The association between Na+, Cl− and SO42− on the principal component score plot identified the ponds as a common source of these elements. However, the closeness of Na/Cl ratios of the groundwater to those of seawater and process waters suggested that Na+ and Cl− had pond and marine origins, whereas SO42− was primarily derived from the mineral processing facilities. The plot of factor loadings grouped the samples according to the similarities in their elemental compositions and their proximity to the ponds. The low Na/Cl of the groundwater relative to the waste water was attributed to base-exchange reactions. Surface complexation modeling identified ferrihydrite as the main sink for dissolved Cu and, to a lesser extent, Zn and Ni in the aquifer. Although precipitating carbonate species seemed to reduce dissolved Th concentrations in the aquifer, U species were likely to remain in solution by carbonate complexes. This study demonstrated that the process waters are likely to contaminate distant water systems and enter food web.