Refers to the water stored on-site to wash ore and receive waste tailings as a residue. This provides both a water supply and a sediment trap. Two serious environmental problems ensue. The water leaches out metals and other dangerous elements that were formerly trapped underground in a reducing (non-oxidizing) environment; acidic wastes amplify the leaching. And though usually stable, some dams have burst during flood stages, unleashing a torrent of toxic materials and posing a combined hazard of mudflow and flooding.