The amount of metals in the mine water that ran into Colorado’s Animas River after contractors working for the Environmental Protection Agency (EPA) accidentally set off a spill at the abandoned Gold King mine was comparable to four to seven days of ongoing acid drainage from the mine, according to a report from the agency released in early January.

The August 2015 spill, triggered by an equipment operator inadvertently breaching a pile of debris with a backhoe, sent an estimated three million gallons of “acidic, mine-impacted waters” into the Animas River, turning it a vivid mustard colour as the iron and aluminum in the deluge reacted with the river water. The mine tailings crossed three state lines and three tribal lands over the course of nine days and thrust the problem of abandoned mines into the headlines.

The EPA’s final report on the spill examined the river conditions prior to the collapse, how the metals and mine water moved through the river system and the long-term impacts on the river.

The agency found that the total amount of metals that entered the river – 490,000 kilograms of mostly iron and aluminum, as well as manganese, lead, copper, arsenic, zinc, cadmium and mercury – during the nine-hour release period was comparable to the amount of metals carried by the river in one to two days of high spring runoff. But, it conceded, the concentrations of some of the metals in the plume were “higher than historical mine drainage.”

The report noted that mine waste had piled up outside the Gold King mine entrance for many years, and that acid mine waste has historically settled along the banks and sediment beds of the river system, which long captured the leaking mine waste from “hundreds of old and abandoned mines” in the region.

When the mine structure collapsed, the “initial load of metals contained in the [Gold King] release increased significantly as the mine water traveled down the hillslope and along Cement Creek, picking up additional metals from the waste pile and streambed along the way.”

The agency said there had been no reported fish deaths in the rivers affected by the spill, and surveys released by multiple organizations found that other aquatic life “does not appear to have suffered harmful short-term effects from the GKM plume.” It also said water quality in the affected rivers returned to pre-spill conditions within two weeks of the plume passing.

At least 73 claims were filed against the EPA after the collapse from people in Colorado and New Mexico, seeking a total of $1.2 billion in compensation, but the agency dismissed the claims after a legal analysis concluded it had “sovereign immunity,” according to the Denver Post. The Navajo Nation sought $160 million. The state of New Mexico filed suit against the EPA in May 2016, seeking reimbursement for the cleanup.

To date, the agency has dedicated more than $29 million to the incident, including more than $1 million to the Navajo Nation, and $1.75 million to New Mexico. Last September, the EPA designated the Bonita Peak Mining District in southwestern Colorado a Superfund site, making the Gold King mine and 46 other nearby sites including mines, tunnels and tailings impoundments potentially eligible for federal cleanup funding. The area had been mined for over a century before the last mine closed in 1991.