The Rio Tinto Mine site is an abandoned copper mine and associated mill, heap leach pads and tailings located on 280 acres, south of Mountain City in Elko County in northern Nevada. Anaconda operated the Mine using underground mining techniques and a flotation mill from 1932 to 1947. During this period, Anaconda established tailings ponds, and deposited tailings and waste rock in the Mill Creek Watershed, and realigned Mill Creek. In 1966, G.M. Wallace and Company and Cleveland Cliffs, formerly Cliffs Copper Corporation, performed acid leaching activities and constructed an additional tailings pond in the Mill Creek Watershed. From 1972 to 1975, in order to remove water accumulation in underground mining areas, a 6700-foot deep injection well and a wastewater treatment plant were installed. The treatment plant discharged wastewater to Mill Creek and produced sludge. A sludge pond was developed to dispose of this sludge. Also, drainage ditches were created in the tailings. From 1986 to 1987, Rio Tinto Copper, Inc. owned and operated an in-situ acid leaching operation.

From the time Anaconda established tailings ponds, surface and ground water flows have occurred, and continue to occur, through tailings, waste rock, and treatment ponds causing acid mine drainage (AMD), and the release of hazardous substances and contaminants to Mill Creek and the Owyhee River. Releases of AMD, and hazardous substances and pollutants are documented from the Mine since at least 1958, and have been occurring since sometime after construction of the tailings ponds, and tailings and waste rock deposition began in 1932 in the Mill Creek Watershed. As a result of Mine operations, water inflows to the Mine were regularly discharged to Mill Creek. Hazardous substances releases including acid mine drainage, metals, and trace elements to Mill Creek continue to date. Additionally, catastrophic failures of impoundments in the Mill Creek channel have caused acute releases of hazardous substances and contaminants to Mill Creek. From 1958 to 1975, seven documented fish kills occurred in the Owyhee River downstream from its confluence with Mill Creek which is about one mile downstream of the Mine.

Natural resources potentially affected by AMD, and hazardous substances and contaminants from the Mine include surface water, ground water, soils, sediments, fish, birds, mammals, amphibians, invertebrates, and instream and riparian habitats of locations to include, but are not limited to, Mill Creek, the Owyhee River, and several reservoirs. Tribal and Bureau of Indian Affairs (BIA) trust resources are located about five miles downstream from the Mine. The Owyhee River is the primary water source for surface and ground water recharge, and for Tribal livestock, agriculture, and fisheries in Duck Valley. Cultural resource values including lost use, have been affected by releases from the Mine.