The Tar Creek Superfund Site (the 'Site') is located in the northeastern portion of Ottawa County, Oklahoma. The Site is a former lead and zinc mining area. The Site includes the Oklahoma portion of the Tri-State mining district of northeastern Oklahoma, southeastern Kansas and southwestern Missouri. Mining began in Ottawa County in the early 1900's and continued until the 1970's. The Boone Formation was the source of the metal ore. The Boone Formation is also an aquifer. Due to the presence of the aquifer in the ore-producing Boone Formation, the mining companies were forced to pump large volumes of water from the extensive underground mine workings. Pumping continued until the mining ceased, at which time the aquifer, and hence the mines, began refilling. As water filled the mines, the native sulfide minerals, which had been oxidized by exposure to air, dissolved, creating acid mine water. By 1979, water levels had increased to the point that the acid mine water began discharging at the surface from several locations, severely impacting Tar Creek.