Water quality before and after reclamation at the abandoned valzinco Zn-Pb mine site, Spotsylvania County, Virginia

Abstract

The Valzinco deposit was a massive sulfide ore body, dominated by pyrite, sphalerite, galena, and chalcopyrite. It was mined intermittently in the first half of the 20th century by underground methods. The host rocks were metamorphosed felsic volcanic rocks. Mine wastes were dominated by flotation tailings deposited in the stream channel, which were subsequently partially transported downstream by fluvial processes. Acid-base accounting results indicate that the tailings are net acid (net neutralizing potential = -268 to -138 kg CaCO 3/tonne) and leach significant quantities of Fe, Al, Zn, Pb, Cu, Cd, and SO 42-. Pre-reclamation quarterly sampling of the stream below the site documented low-pH waters having elevated concentrations of Fe, Al, Zn, Pb, Cu, Cd, and SO 42-. Reclamation began in 2001 and major construction was completed in 2002. Reclamation included the removal and encapsulation on site of tailings, application of bactericide, installation of limestone drains, construction of wetlands, and revegetation. Results of recent sampling of the site (June 2007) indicate an increase in pH from 3.4 to 5.1, an increase in hardness of 37 %, and decreases in total dissolved solids (68 %), Fe (94 %), Al (98 %), Zn (77 %), Pb (99.5 %), Cu (97 %), Cd (94 %), and SO 42- (81 %) relative to mean pre-reclamation values. Even though significant reductions in dissolved metals have been achieved and positive biologic indicators of ecosystem health are present, the concentrations of Cu and Zn remain above hardness-based acute ecosystem toxicity criteria. The elevated concentrations, in part, appear to be a natural, pre-mining characteristic of the watershed. The elevated concentrations also highlight the sensitivity of calculated hardness-based toxicity criteria for soft waters. The hardness-based criteria do not account for the mitigating effect of dissolved organic carbon on metal toxicity in wetland areas such as at Valzinco.