When Environmental Protection Agency personnel, along with workers for Environmental Restoration LLC (a Missouri company under EPA contract to mitigate pollutants from the closed mine), caused the release of toxic waste water into the Animas River watershed.
They caused the accident while attempting to drain ponded water near the entrance of the mine on August 5.
Contractors accidentally destroyed the plug holding water trapped inside the mine, which caused an overflow of the pond, spilling three million US gallons (eleven megalitres) of mine waste water and tailings, including heavy metals such as cadmium and lead, and other toxic elements, such as arsenic, The EPA was criticized for not warning Colorado and New Mexico about the operation until the day after the waste water spilled, despite the fact the EPA employee "in charge of Gold King Mine knew of blowout risk." The EPA has taken responsibility for the incident, but refused to pay for any damages claims filed after the accident on grounds of sovereign immunity, pending special authorization from Congress or re-filing of lawsuits in federal court.
Governor of Colorado John Hickenlooper declared the affected area a disaster zone.
The spill affects waterways of municipalities in the states of Colorado, New Mexico, and Utah, as well as the Navajo Nation.
As of August 11, 2015, acidic water continued to spill at a rate of 500–700 US gal/min (1.9–2.6 m Prior to the spill, the Upper Animas water basin had already become devoid of fish, because of the adverse environmental impacts of regional mines such as Gold King, when contaminants entered the water system.
The chemical processes involved in acid mine drainage are common around the world: where subsurface mining exposes metal sulfide minerals such as pyrite to water and air, this water must be carefully managed to prevent harm to riparian ecology.
At the time of the accident, the EPA was working at the Gold King Mine to stem the leaking mine water going into Cement Creek.
Water was accumulating behind a plug at the mine's entrance.
They planned to add pipes that would allow the slow release and treatment of that water before it backed up enough to blow out.
Unknown to the crew, the mine tunnel behind the plug was already full of pressurized water.
It burst through the plug soon after excavation began.