SUMMARY

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 from the Louvicourt Mine (Louvicourt, Abitibi, QC) 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 S H₂S, 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, there was no evidence for mobilization of Cu from tailings.

Unlike previous reports, which suggested that submerged tailings were effectively inert, our results show some alteration of the superficial layer over time. The observed Cd and Zn releases from the submerged tailings are however very small. For a typical disposal operation (a 1 km x 1.5 km impoundment with an average depth of 1 m, an overlying water volume of 1.5 x 10⁹ L, and an average water residence time in the impoundment of 1 year), calculations indicate that these releases would increase the overlying water Zn concentration by 47 nM (3 ppb) and the Cd concentration by 0.31 nM (0.04 ppb). The Cd and Zn fluxes from the tailings to the overlying water would thus have only minor impacts on the overlying water quality.

Last Modified: 2003-11-26		Important Notices

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