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 [SO42-]). 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.