United States Environmental Protection Agency personnel accidentally caused a spillage (aftermath pictured) of 3 million gallons (11 million litres) of mine waste water and tailings trapped inside the Gold King Mine near Silverton, Colorado.

The Environmental Protection Agency (EPA) is an independent agency of the United States government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it began operation on December 2, 1970, after Nixon signed an executive order. The order establishing the EPA was ratified by committee hearings in the House and Senate.

The 2015 Gold King Mine waste water spill was an environmental disaster that began at the Gold King Mine near Silverton, Colorado, when Environmental Protection Agency (EPA) personnel, along with workers for Environmental Restoration LLC, caused the release of toxic waste water into the Animas River watershed. They caused the accident by breaching a tailings dam while attempting to drain ponded water near the entrance of the mine on August 5. After the spill, the Silverton Board of Trustees and the San Juan County Commission approved a joint resolution seeking Superfund money.

Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock drainage (ARD) is the outflow of acidic water from metal mines and coal mines.

In mining, tailings or tails are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are different from overburden, which is the waste rock or other material that overlies an ore or mineral body and is displaced during mining without being processed. Waste valorization is the evaluation of waste and residues from an economic process in order to determine their value in reuse or recycling, as what was gangue at the time of separation may increase with time or more sophisticated recovery processes.