The Corps said the permit did not comply with the water quality standards set by the Fond du Lac Band of Lake Superior Chippewa, whose reservation on the St. Louis River is downstream from the mine and processing plant sites.

The vast but untapped reserves of buried copper, nickel and precious metals in northeastern Minnesota are locked up in sulfide minerals that can leach sulfuric acid and other pollutants when exposed to air and water.

Court discoveries from that lawsuit and open-records requests by Minnesota-based nonprofit WaterLegacy unearthed documents showing that state regulators had pressured the EPA to withhold its concerns about the mine from public comments.

“Whistleblowers, Freedom of Information Act lawsuits, and the district court hearing helped us learn that the MPCA used a corrupt process to keep EPA’s criticisms of the PolyMet permit secret,” WaterLegacy Advocacy Director Paula Maccabee said.

The other justices also joined a concurrent opinion written by McKeig that more strongly criticized the Minnesota Pollution Control Agency for ignoring and disrespecting the Fond du Lac Band throughout the permitting process.