Opponents of the proposed Donlin gold mine in western Alaska won a key victory Monday when a federal judge ruled that regulators who issued the permit needed to build the project failed to adequately consider the risk of a catastrophic release of mine waste.

U.S. District Court Judge Sharon Gleason agreed with tribal plaintiffs who argued that the environmental study that led to the federal permit for the Donlin mine illegally omitted analysis of the effects of the failure of a large tailings dam.

“I think this is a great victory for the tribes and the region,” said Hannah Foster, a lawyer with the environmental group Earthjustice, which represents tribal governments fighting the mine.

At issue was the environmental impact statement that the U.S. Army Corps of Engineers relied on when it issued a key permit in 2018 to build a gold mine, one of the largest in the world.

The environmental impact statement needed to fully analyze the possibility of a breach of the 471-foot-high dam, which is designed to hold hundreds of millions of tons of tailings, which are crushed pieces of rock left over from mineral extraction.

The environmental impact statement indicated that the risk of catastrophic failure was 0.5% per year, or 2% over 20 years. However, the Corps failed to take into account the outcome of such a spill, an omission that Gleason argues violates the National Environmental Policy Act and the Alaska National Lands Protection Act.

“To borrow an analogy from the plaintiffs and put these numbers into perspective, the same odds of risking a plane crash would likely discourage almost anyone from flying. “Because a tailings leak greater than 0.5% of the total tailings volume is reasonably foreseeable, a larger leak should have been included in the (final environmental impact statement),” Gleason said in her ruling.

Gleason’s ruling did not result in a complete invalidation of the permit, as the plaintiffs had requested. Instead, it gave both sides the opportunity to further discuss how to respond to shortcomings in the environmental impact statement.

Representatives of the six tribal governments that filed suit to overturn the Corps’ permit welcomed the court’s ruling, even though it was only a partial victory.

“The threat from the Donlin gold mine has been hanging over us for many years, posing unacceptable risks to the health of our lands, waters, fish, wildlife and our people. We are relieved and happy that the judge is requiring a closer look at the mine’s impacts and tailings failure risks, Walter Jim, chairman of the Orutsararmiut Indigenous Council, said in a statement.