Environmental groups implored Gov. Mark Dayton on Monday to push for changes to PolyMet Mining's plans to mine copper, nickel and precious metals near the Boundary Waters.

They claim the changes will help prevent a potentially catastrophic accident in a water-rich and ecologically sensitive region of the state.

The Minnesota Department of Natural Resources last week sent a 3,100-page preliminary environmental report on the project to the federal Environmental Protection Agency and three Minnesota Indian tribes for review. The DNR intends to release the study for public comment later this fall.

But the environmental groups, calling themselves Mining Truth, want Dayton to step in now. They say the report acknowledges that water quality treatment will be needed "indefinitely" after the mine closes to prevent water pollution from escaping.

And they say PolyMet's plan to store mine waste does not take into account recommendations made in the wake of a major mine accident last year in British Columbia, Canada, when a tailings dam broke at the Mount Polley copper and gold mine, flooding nearby waterways with over 1 billion gallons of waste. (PolyMet does not own Mount Polley.)

Aaron Klemz, right, communications director of Friends of the Boundary Waters, and Kathryn Hoffman, staff attorney at the Minnesota Center for Environmental Advocacy, at a press conference Monday. Christina Ayodele | MPR News

"Since the Legislature has failed to act, and the Department of Natural Resources has not required PolyMet to implement the recommendations of the expert review panel at Mount Polley," said Aaron Klemz with Friends of the Boundary Waters Wilderness, "it's up to Gov. Dayton to do what's needed to protect Minnesota's clean water."

The groups also announced the launch of a statewide ad campaign using footage of the Mount Polley accident, asking viewers to contact Governor Dayton and tell him tell him "not to accept PolyMet's high-risk plan."

PolyMet officials say the preliminary document released last week shows the company can meet all state and federal environmental standards.

"The kind of tailings basin disaster that happened at Mount Polley isn't acceptable, to PolyMet or anyone here in Minnesota," said the company's Brad Moore.

PolyMet plans to reuse an existing tailings basin that covers four square miles and has stored taconite waste for 40 years. An extensive analysis shows the basin is safe, Moore said. He said the company plans additional improvements to further strengthen the facility.

The environmental review of PolyMet's mining proposal has now exceeded 10 years, as the company has modified its plans to respond to concerns of state and federal regulators and Minnesota citizens. The public submitted 58,000 comments that were incorporated into the preliminary document released last week.

The fact that environmental groups are attacking the plan, said PolyMet's Moore, is normal. "Every single mine developed in the United States has opposition," he said. "We expected it."

The Minnesota DNR plans to release a final environmental impact statement later this fall, at which point the public will have another opportunity to comment. The state is not expected to make a decision on the adequacy of the environmental review until early next year.

In a statement, a Dayton spokesperson said until that happens, he "is remaining neutral and undecided about the proposed PolyMet project. He has instructed his cabinet to do the same."