Geochemical evolution of Lower William's Lake tailings basin, Elliot Lake, Ontario, Canada

Detailed site characterization studies of the Lower William's Lake tailings basin, Denison Mines, Elliot Lake, Ontario, Canada were conducted to assess its current vegetation and geochemical evolution status for evaluating long-term performance and stability of the site following prolonged inactivity and rehabilitation. The tailings at the site were deposited in the late fifties and early sixties as a result of an accidental spill at an upstream-located Upper William's Lake main tailing basin. The spilled tailings covered a previously existing small bog and wetland area. The site was rehabilitated by placement of a sand/gravel and till cover, up to 1.0 m in thickness, and revegetated during 1976 and 1977. The site supports lush vegetation of various planted agronomic and volunteered native species of grasses, shrubs and trees from both terrestrial and wetland habitats. The pond and water logged parts of the basin have been completely transformed into a natural wetland habitat. The applied sand/gravel and till cover has raised the water table in the basin and most of the tailings are saturated in a shallow water table configuration. Presence of organic matter and reducing conditions established within the saturated zone are contributing to net alkalinity generation within the site and no impact of the shallow zone oxidation is observed on the downstream water quality. Mineralogical characterization of the tailings established the occurrence of framboidal pyrite and precipitation of other secondary minerals. Aerobic and anaerobic oxidation of organic matter coupled with sulphate reduction is believed to be contributing to precipitation of secondary minerals and improvements in the water quality. (author)