Minnesota's leading environmental groups are launching a statewide ad campaign to pressure Gov. Mark Dayton into requiring tougher environmental protections for the contentious copper-nickel mine that's been proposed for northeast Minnesota.

Speaking at a news conference Monday, they said a 3,000-page preliminary environmental impact statement commissioned by PolyMet Mining and now under review by the U.S. Environmental Protection Agency, Indian tribes and other agencies leaves significant questions unanswered.

Specifically, they are comparing PolyMet's proposed tailings basin, which will hold toxic waste mixed with water, to one that failed in British Columbia last year with disastrous consequences. The advertising campaign, sponsored by a coalition called Mining Truth, uses footage of the British Columbia incident showing contaminated water pouring through a breached basin wall and into the surrounding forest. The coalition says PolyMet's tailings basin will rely on similar risky technology, and urges Minnesotans to ask Dayton to require more advanced construction standards.

"Dayton should not accept the same design that could end in the same kind of disaster," said Aaron Klemz, communications director for the Friends of the BWCA.

Bruce Richardson, a PolyMet spokesman, said the designs for its tailings basin — which was used by a previous company for decades to hold waste from taconite mining — is considerably different, would meet all state and federal regulations and would be safe.

Dayton has remained silent on the $650 million PolyMet project since the environmental review began, and Monday a spokesman from his office said that will continue until it's complete.

If does win approval, the mine would create 300 to 350 permanent jobs, plus many more spinoff jobs in an area of the state that has recently been hit with major layoffs in the taconite industry.

But copper-nickel mining poses significantly higher environmental risks than Minnesota's venerable taconite industry.

Last week the Minnesota Department of Natural Resources (DNR) released what it called the Preliminary Final Environmental Impact Statement for the mining project, which has been under regulatory review since 2007. An earlier version of the document generated 58,000 comments from the public, more than any other environmental review in state history.

The most recent version, which addresses those comments, was described as a significant milestone by both the company and DNR commissioner Tom Landwehr.

But state regulators still have significant issues to resolve. Tribes and some other government agencies will have two months to review the study draft released last week. The public will then have 30 days to comment on the final version, which is expected to be complete later this year.

Under Minnesota law, Landwehr must make a final decision on whether the entire review has been "adequate," which he is likely to do by the end of the year. The EPA will make similar decisions.

After that, the state and the company must write a permit for the mine, another hugely complex process. That will include what is likely to be one of the most critical unanswered questions: the size of the damage deposit, or financial assurance, PolyMet must provide to pay for potential future environmental cleanup and to protect the state's taxpayers from costs should the company declare bankruptcy.

Putting off the financial assurance decision was highlighted by environmentalists as one of the "fatal flaws" in the state's latest review. Dayton and the DNR have enough information now to set final assurance levels for the mine, said Kathryn Hoffman, an attorney with the Minnesota Center for Environmental Advocacy, a nonprofit law firm.

"The longer we wait, the more likely that the damage deposit will be a backroom deal designed to protect PolyMet rather than a public discussion designed to protect Minnesota's clean water and its taxpayers," she said.

Previously, the state estimated that it will cost $200 million to close the mine, and $3.5 million to $6 million annually to treat the water — perhaps for hundreds of years.

The advocates also expressed disappointment that the newest version of the environmental review provided only a cursory nod to the regulatory changes in British Columbia inspired by the Mount Polley mine disaster.

The dam collapse occurred at the Imperial Metals Mount Polley gold and copper mine near the town of Likely, British Columbia. The dam's failure was catastrophic, allowing nearly the entire contents of the mine's tailings pond — an area the size of New York's Central Park, holding years worth of mining waste — to flow out into pristine creeks and lakes in the nearly untouched forest.

An independent panel that investigated the disaster concluded earlier this year that the dam failed because the wall was built on an unstable foundation. Under the massive weight of water and tailings behind it, it collapsed all at once. British Columbia has since tightened regulatory review of its mines, and now requires independent experts to weigh in on tailings basin construction.

Dave Chambers, a mining expert who analyzed the project for the Minnesota environmental groups, said the PolyMet tailings basin resembles Mount Polley in some ways, because part of its wall will also be built on unstable tailings from the previous taconite mine.

"Fundamentally, they still have [a] potentially unstable structure," he said.

Richardson said the PolyMet basin is much flatter than Mount Polley's, and added that the company plans to reinforce both the base and the bottom where the old tailings sit.

Nevertheless, Chambers said, no mine plan is foolproof. Environmental review and permitting can lay out strategies for mitigating risk, but they are not designed answer a critical question for the public, he said.

"Is it worth the risk?" he said.