New Mexico is suing the state of Colorado over the Gold King Mine spill in 2015, saying Colorado should be held responsible for the contamination caused by the spill as well as decades of toxic drainage from mines near the headwaters of the Animas River.

The lawsuit stems from the results of recent soil samples taken north of Durango, CO, where discolored sediment was visible at residential properties. The results showed lead at concentrations far above the risk level established by EPA.

The Denver Post reported that the lawsuit also outlines the business and regulatory history of the Sunnyside Gold Mine, where operators were allowed to install plugs — or bulkheads — that eventually caused wastewater to back up and fill the mine and its workings. That led to problems at nearby mines.

The shuttering of a water treatment plant for mine discharge further aggravated the situation, according to New Mexico officials.

By 2011, acidic drainage from four inactive mine sites at and above the former treatment plant — including at Gold King Mine — was pouring into Cement Creek, a tributary of the Animas River, at a rate of nearly 850 gallons per minute, according to the lawsuit.

“The Gold King Mine release was the coup de grace of two decades of disastrous environmental decision-making by Colorado, for which New Mexico and its citizens are now paying the price,” the lawsuit states.

It is the second major legal salvo fired by New Mexico in the wake of the August 2015 spill. New Mexico is also suing the U.S. Environmental Protection Agency (EPA) and the owners of two mines.

“We had hoped EPA and Colorado would try to work with us and come up with solutions,” New Mexico Environment Secretary Ryan Flynn told The Associated Press. “But the state of Colorado, its leadership, seems intent on defending EPA at every turn and is unwilling to work with us to move forward in a meaningful manner.”

The EPA has declined to comment on the litigation, but it has said repeatedly that it takes responsibility for the cleanup.

Colorado officials previously declined to comment on New Mexico’s claims, citing possible litigation.

An EPA contractor triggered the spill. The wastewater made its way into the Animas and eventually down to the San Juan River, setting off a major response by government agencies and private groups.

During the spill, water utilities shut down intake valves and farmers stopped drawing from the rivers as the plume moved downstream.

The EPA said water quality quickly returned to pre-spill levels. But New Mexico officials and others continue to warn about heavy metals collecting in the sediment and getting stirred up each time rain or snowmelt results in runoff.

The state is seeking damages and demands that Colorado address the problems at the mines.

New Mexico and Colorado officials had been in talks for months, and Flynn said the state is still open to discussions in hopes of settling with the EPA and Colorado.

“We’re here out of necessity,” Flynn said. “I would prefer to spend time and resources resolving this rather than duking it out in court.”