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.