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A proposed copper-nickel mine in northeast Minnesota would generate water pollution for up to 500 years and require billions of dollars in long-term cleanup costs, state regulators have concluded as they near a key stage in the project’s review.

The mine would require what critics say is essentially perpetual water treatment – a first in Minnesota’s long history of mining – to remove pollutants and heavy metals that would otherwise flow into nearby streams and rivers and eventually Lake Superior, according to a draft environmental impact statement.

The analysis, which regulators expect to release for public review in November, was prepared as part of the state’s review of a mining complex proposed by PolyMet Mining Corp., at a site near Hoyt Lakes.

The prospect of centuries of water treatment illustrates the scope of the environmental challenges facing what would be Minnesota’s first copper-nickel mine – and why it has generated intense environmental scrutiny and divided communities on the Iron Range. PolyMet is the first of many companies lining up to tap into one of the world’s largest copper-nickel deposits. The deposits offer the promise of a new era of mining for Minnesota, but one that comes with significant ecological risks for the wildest and most treasured corner of the state.  

“What they are saying is we have to treat in perpetuity,” said Dave Chambers, a geophysicist with the Center for Science in Public Participation, a Montana consulting nonprofit that has examined the PolyMet review. “And you can make mistakes. Those mistakes can and have cost a lot of money.”

A spokeswoman for PolyMet said long-term water treatment systems are a common part of modern mining operations, as operators comply with mandatory environmental standards. PolyMet expects to meet them as well, she said.