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A geochemical investigation of ground water and soils affected by evaporation pond seepage, at the Namakwa Sands Mineral Seperation Plant(MSP)

Evaporation ponds are used for treating wastewaters in arid regions worldwide. Namakwa Sands an Anglo American pIc operation, mines heavy minerals along the semi arid West Coast of South Africa. An acid effluent is generated during treatment of these heavy minerals. The effluent is neutralized and disposed of into unlined evaporation ponds. Seepage from the ponds affects the surrounding environment as well as the subsurface waters. The main objective of this study was to investigate the effects of the seepage on the groundwater resources and potential implications for contaminant migration in the subsurface environment. Three main areas were investigated, namely the evaporation ponds, the surrounding soils and the groundwater. The effluent entering the evaporation ponds is Na2S04 dominant whereas the natural environment is NaCI dominant, therefore sulphate can be used as a tracer of effluent movement. Analyses of trace element concentration in evaporation pond samples revealed that the raw effluent entering the evaporation ponds contains high concentrations of Th (10.4 mglL), U (475 pglL), As (217 pglL) and Se (52.7 pglL). The following elements were also high in comparison with the background boreholes, Fe (2887 mglL) and AI (1414 mglL). The neutralization process seems to be ineffective since the pH of the effluent in these ponds is 2.4. However, the water that accumulates in an adjacent seepage pond that formed naturally down slope of the evaporation ponds has a pH of 8.3 meaning that the soil acts welt as a neutralising medium. The soil sampled around the ponds is moderately alkaline. Therefore it will act as a good buffer for the acidic seepage. The soils can also generally be classified as saline-sodic. Trace elements of concern are Th and U, they exhibit elevated levels in comparison with the background samples. The groundwater sampled from the boreholes around the evaporation ponds can be described as neutral but highly saline. Trace elements of concern in the groundwater are Uranium (0.61 pg/L 29.13 pglL), Arsenic (5.90 pglL 87.4 pglL), and Selenium (33.35 pglL 177 pglL), they exhibit elevated levels in relation to the boreholes used as control samples. Based on the findings in this study the effluent entering evaporation ponds does affect the surrounding environment and subsurface groundwater. The water quality is of such a nature that it cannot be used for domestic, livestock watering or for irrigation purposes. Un iv rsi ty of Ca pe T w