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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.

Mining regulators from the Minnesota Department of Natural Resources (DNR) declined to discuss the issue in detail because the environmental impact statement is not finished. But they said the analysis calls for “long-term” — not perpetual — treatment, potentially a crucial distinction. Moreover, final decisions on water treatment will be made when PolyMet applies for a permit, probably sometime next year, said Jess Richards, the DNR’s director of lands and minerals.

How taxpayers would be protected from any long-term cleanup costs remains an open question and one of the most contentious issues in the ongoing debate over copper-nickel mining’s future in Minnesota.

State law requires mining companies to put up financial instruments, such as bonds or insurance, in advance to pay for mine closures and any ensuing cleanups. The DNR says it will require adequate “financial assurance” before granting PolyMet a permit, and that the appropriate time to decide specifics is when the company applies for a permit.

But the U.S. Environmental Protection Agency (EPA) and other groups have urged state regulators to address that issue now, as part of the environmental review, so the public has a chance to understand what’s at stake. The release of the final environmental impact statement in November will be followed by hearings and a public comment period.

“It is a critical question for the environmental impact of this project,” said Kathryn Hoffman, an attorney with the Minnesota Center for Environmental Advocacy, a nonprofit law firm. No corporation is likely to exist for hundreds of years, Hoffman said, and accurately predicting the costs of water treatment for centuries is difficult if not impossible.

Chambers, who has studied other mining projects across the country, said even those that start with financial safeguards can end up costing taxpayers millions of dollars. The Zortman-Landusky gold mine in Montana, he said, is an example. When it went bankrupt in 1999, the operator turned off its water treatment plant and left. Some financial assurance had been built into the project, but the state had underestimated the volume of water needing treatment, and state taxpayers had to create a $34 million trust fund to pay for it.

“If you are going to agree to take that risk, then all the risk takers should be involved,” Chambers said.

Sulfide byproducts

PolyMet, a Vancouver-based company that is one-fourth owned by Swiss commodities giant Glencore Plc., has proposed an open-pit mine near Babbitt and an ore processing center at the old LTV taconite mine north of Hoyt Lakes.

The $650 million project would create about 350 jobs for the duration of the 20-year-mine, plus spinoff jobs — a prospect that has won support among a number of elected officials on the Iron Range.

In addition to an open pit mine, the project would include a state-of-the-art metallurgical processing plant that would extract small amounts of precious metals from tons of rock. The metals are in high demand for computers, smartphones, wind turbines and many other technologies related to green energy.

But unlike Minnesota’s familiar iron ore deposits, the sulfide-bearing waste rock and tailings from copper-nickel mining produce acid when exposed to air. That can then leach heavy metals, change the acidity of surface waters, and damage fish and other aquatic life.