Geochemical changes in sulfidic mine tailings stored under a shallow watercover

The objective of this study was to assess the effectiveness of an engineere d shallow water cover in reducing the oxidation of sulfidic mine tailings a nd thus preventing the development of acid rock drainage. Fresh tailings we re submerged under a 0.3-m water cover in experimental field cells. From 19 96 to 1998, we followed the chemistry of the interstitial water near the ta ilings-overlying water interface using dr 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 <Sigma>H2S , Fe and Mn in pore water at depths - 1.5cm below the interface. However, t here 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 [S O42-]). Mobilization of Cd and Zn from this surface layer was indicated hv 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 pa rtitioning from refractory to more labile fractions. In contrast. mobilizat ion of Cu from tailings was less evident. Unlike previous reports, which su ggested that submerged tailings were effectively inert, our results show al teration of the superficial layer over time. (C) 2001 Elsevier Science Ltd. All rights reserved.