Common tailings basin pollutants include mercury, copper, lead, arsenic, cadmium, selenium, zinc and nickel. “Water-borne pollutants (e.g., heavy metals) resulting from acid mine drainage can pollute communities well downstream of the mine” (source PDF).
This can occur from accidental spills or intentional releases. The draft Environmental Impact Statement (EIS) for the PolyMet mine project predicts that the West Pit will fill with water and overflow into the Partridge River about 45 years after the mine’s closure. The draft EIS noted that this discharge was predicted to exceed water quality standards for antimony for 550 to 2,000 years. Other contaminants that were cited as possibly exceeding water quality standards were nickel, sulfate, cobalt, copper and mercury (DEIS, pgs. 4.1-45, 4.1-64, 4.1-113, 4.1-123).