Available evidence suggests that the post-depositional (diagenetic) chemical reactivity of mine tailings is inhibited by storage under water, and implies that such storage may provide a preferred long-term disposal option for such wastes. To assess this, we examine in this report the chemistry of interstitial waters and the associated solid phases in tailings and natural sediments in Anderson Lake, near Snow Lake, Manitoba. Anderson Lake has been used as a receiving basin for tailings since 1979. Tailings are areally widespread in the lake, despite discharge being confined to the western side.
Three cores were collected in June 1990, and processed under nitrogen to extract interstitial waters. Solid-phase chemical measurements indicate that two of the cores penetrated though metal- or tailings-rich zones into underlying, premine, organic-rich (20 to 30 wt. % C) natural deposits; the third consisted essentially of pure tailings. At all sites, dissolved iron measurements made on the pore water samples indicate that the deposits are anoxic at very shallow depths, within several millimetres of the sediment-water interface. This reflects the high oxidant demand of the sediments at all locations, and/or the physical effect of rapid accumulation of tailings which limits the penetration depth Of 02 via downward-diffusion, given the short residence time of a specific horizon near the interface.
High dissolved Zn, Cu and Pb concentrations in supernatant water confirm measurements made previously that Anderson Lake water is elevated with respect to these metals. The concentrations of all three elements decrease abruptly with depth in the cores at all three locations, however. This cannot be due to non-steady-state effects and indicates unequivocally that both the tailings and natural sediments are acting as sinks for metals, rather than as sources. Presumably, the metals are being precipitated as sulphide phases at shallow depths. Sulphate reductions with concomitant production of HS- is expected, given the evidence for very shall anoxia in the deposits.
It is clear that the deposited tailings are not releasing metals to the overlying lake water at present. In the post-discharge future, the tailings will be progressively covered by a blanket of organic-rich natural sediments which will act to preserve the anoxic conditions, now extant, essentially in perpetuity. Thus, it is unlikely that the tailings on the lake floor will ever support a benthic efflux of metals to Anderson Lake water.
