The Navajo Nation announced Monday plans to file a lawsuit against the Environmental Protection Agency over damage stemming from last year’s Gold King Mine spill, calling it an “unprecedented environmental disaster.”

The New Mexico-based nation is scheduled to hold a press conference Tuesday on its reservation near the San Juan River, which was contaminated after an EPA-led crew accidentally uncorked 3 million gallons of toxic sludge from an inactive mine upstream near Silverton, Colorado.

The plume moved downstream from the Animas River to the San Juan, turning the rivers orange, forcing local officials to cut off water supplies for residents, livestock and farms, and requiring the tribe to bring in truckloads of clean water.

The Navajo lawsuit comes as the latest litigation stemming from the August 2015 contamination: In May, New Mexico Attorney General Hector Balderas filed a complaint in federal court against the mine’s owners and the EPA, arguing that the agency’s response was inadequate.

Mr. Balderas also sued the state of Colorado for what he described as its “reckless” management and lax oversight of the mine after negotiations between the states failed.

So far, Colorado Attorney General Cynthia H. Coffman has taken no legal action against the EPA.

The Navajo tribal government is represented by the Los Angeles-based law firm Hueston Hennigan.

The EPA already has awarded $602,000 in reimbursements to the Navajo Nation for costs incurred for “various activities associated with the release response, including field evaluations, water quality sampling, laboratory analyses, and personnel,” according to an Aug. 6 update.

“EPA continues to evaluate state, tribal and local response costs and has reimbursed approximately $3 million to date through cooperative agreements established with partners,” said the agency.

An Interior Department technical review attributed the spill to the failure of the crew to gauge the water level behind debris before starting excavation, triggering the release of contaminated drainage into the Animas River.

EPA Administrator Gina McCarthy promised to take responsibility for the clean-up shortly after the spill, although House Republicans have accused the agency of holding itself to a lower standard than that expected of private companies.

LoRenzo Bates, speaker of the Navajo Nation, called on the EPA to expedite its response at a forum last week hosted by Rep. Ben Ray Lujan, New Mexico Democrat.

“We shouldn’t expect this to go away,” Mr. Bates said in a press release. “We have to understand that there will be long-term impacts and we all have our own resources so we need to determine how these multiple jurisdictions will work together in the long-term.”

In its Aug. 6 update, the EPA said that contamination from the 160,000 abandoned mines in Western states continues to pose costly and complex challenges for the region’s states, tribes and communities.”

“EPA is collaborating with partners on the best practices and lessons to address the legacy of abandoned mines,” said the agency.

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