Geochemistry and potential environmental impact of the mine tailingsat Rosh Pinah, southern Namibia

Abstract

Mine tailings at Rosh Pinah located in semiarid southern Namibia were investigated by the combinationof mineralogical methods and leaching using water and simulated gastric solution. They are well-neutral-ized with leachate pH > 7 and neutralization potential ratios (NPR) up to 4. Neutralization is mainly dueto abundant Mn-rich dolomite in the matrix. Concentrations of released contaminants in water leachatefollow the order Zn > Pb > Cu > As. Relatively high leached concentrations of Zn and partly also of Pb arecaused by their link to relatively soluble carbonates and Mn-oxyhydroxides. In contrast, As is almostimmobile by binding into Fe-oxyhydroxides, which are resistant to dissolution. Barium is released bythe dissolution of Ba-carbonate (norsethite) and precipitates in sulfate-rich pore water as barite.Dissolved concentrations in neutral mine drainage water collected in the southern pond are low, butwhen total concentrations including colloidal fraction are taken into account, more than 70% of Zn isin colloidal form. Groundwater upgradient of the mine tailings is of poor quality and there seems tobe no negative impact on groundwater downgradient from mine tailings.Contaminant concentrations in simulated gastric leachates are in the order Ba > Pb > Zn > Cu > As witha maximum gastric bioaccessibility of 86.6% for Ba and a minimum of 3.3% for As. These results demon-strate that total contaminant content and toxicity in the solid phase are poor predictors of risk, and there-fore mineralogical and bioavailability/bioaccessibility studies are necessary for evaluation ofcontaminant environmental impact.