Geochemical changes in sulfidic mine tailings stored under a shallow water cover.

The objective of this study was to assess the effectiveness of an engineered shallow water cover in reducing the oxidation of sulfidic mine tailings and thus preventing the development of acid rock drainage. Fresh tailings were submerged under a 0.3-m water cover in experimental field cells. From 1996 to 1998, we followed the chemistry of the interstitial water near the tailings-overlying water interface using in situ dialysis, and determined pH and dissolved oxygen (DO) profiles across the tailing water interface using micro-electrodes. Penetration of DO into the tailings was limited to <7 mm, even in the presence of DO produced by benthic periphyton. Anoxia in the tailings was further demonstrated by the appearance of dissolved sigmaH2S, Fe and Mn in pore water at depths -1.5 cm below the interface. However, there was clear evidence of surface oxidation of the mine tailings at the mm scale (i.e., DO depletion, coupled with localized increases in [H+] and [SO4(2-)]). Mobilization of Cd and Zn from this surface layer was indicated by the presence of sub-surface peaks in the concentrations of these two metals in the tailings interstitial water and by a change in their solid phase partitioning from refractory to more labile fractions. In contrast, mobilization of Cu from tailings was less evident. Unlike previous reports, which suggested that submerged tailings were effectively inert, our results show alteration of the superficial layer over time.