A laboratory evaluation of metal release and transport in flooded pre-oxidized mine tailings

A potential problem in implementing water covers over preoxidized tailings impoundments is the dissolution of oxidation products following flooding, which may result in high concentrations of metals in both tailings pore water and the water cover. To examine metal release phenomena under controlled conditions, a laboratory study consisting of four column experiments of flooded partially oxidized tailings was performed over 2 years. In two of the columns enough water was added to establish a 1-m water cover, while the other two columns were only filled to the surface of the tailings. The columns were kept stagnant for the first half of the study (“static phase”) and flushed under constant flow during the second half (“kinetic phase”). Samples of pore water and water cover were collected and analyzed for metals, SO4, acidity, pH, redox, and dissolved O2. Mass balance calculations were performed to assess metal release rates. At the end of the experiments the quality of the pore-water (pH 6) within the water covered tailings was better than that of the water cover itself (pH 3). While mineral dissolution released large amounts of Fe during the static phase in all experiments, no substantial dissolution or removal of Fe was detected after 300 days. In the water-covered tailings, Zn was removed from solution during both the static and kinetic phases, apparently through co-precipitation or adsorption with Fe(III) hydroxides. Metal and SO4 release rates measured in the laboratory were similar to those observed at full-scale field sites. The laboratory observations suggest that while implementing a water cover over pre-oxidized tailings does substantially reduce metal release, long-term treatment of pond effluent and seepage may still be required in some cases. In the field, however, metal concentrations would be influenced by other phenomena, such as dilution due to surface water flow and sediment resuspension through wind, which were not simulated in the laboratory. Thus, the laboratory results cannot be directly extrapolated to the field.