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The oxidation of sulphide-rich mine tailings stored on land by acidophilic bacteria is frequently associated with severe environmental degradation, as metal-rich acid drainage from the tailings contaminates groundwater and surface runoff. Permanent storage of tailings underwater may prove to be more chemically benign if oxidation is inhibited sufficiently. To assess this possibility, the chemistry of interstitial waters and the associated solid phases in tailings and organic-rich (20–30 wt.% C) natural sediments in Anderson Lake, Manitoba, is examined. The lake has been used as a receiving basin for sulphide-rich tailings since 1979, and is contaminated with metals as a result of the input of acid mine drainage along the north shore and the discharge of mill process water with the tailings. Three cores were collected and processed under nitrogen to extract interstitial waters. At all sites, the occurrence of high concentrations of dissolved iron in shallow pore waters indicates that the deposits are anoxic within several millimetres of the sediment–water interface. Despite high dissolved metal inventories in the lake, the concentrations of Zn, Cu, Pb, and Cd decrease abruptly with depth in the pore waters. 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. Dissolved sulphate data and the widespread occurrence of framboidal pyrite in surface sediments suggest that the metals are being precipitated as sulphide phases at shallow depths.

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Sulfide-rich zinc-, copper-, and lead-bearing tailings have been deposited via floating pipeline into shallow(< 8 m) Anderson Lake in north-central Manitoba since 1979. The lake water contains elevated levels of dissolved metals which are derived mainly from an acid-generating roadway along the north shore. Water-column sampling and sediment coring were carried out through the ice on Anderson Lake in April 1993. Two sites were occupied, one proximal to the tailings discharge (Station B, the "pure tailings" site) and the other about 2 km away (Station A, the "natural sediments" site). Interstitial waters were extracted from duplicate cores collected at each location. The water column was strongly stratified at both locations, exhibiting four layers at the natural sediments site (two of which were dysaerobic or anoxic), and two layers near the tailings outfall, the lower being dysaerobic. The unusual multiple layering probably reflects the influence of laterally variable advective processes. High concentrations of dissolved iron in shallow pore waters indicate that the sediments at both locations were anoxic at shallow subsurface depths. Concentrations of Zn, Cu, Pb and Cd were very low or undetectable in the pore waters. There was no evidence of release of Cu, Cd or Pb from the deposited tailings, although there was evidence of possible minor release of Zn from surface sediments in one of the tailings cores. Metals were removed from pore waters below the upper 10 cm at both sites, which is attributed to precipitation of authigenic sulfide phases.