A proposed copper-nickel mine in northern Minnesota is drawing new criticism for the possibility that polluted water from the site could move north toward the Boundary Waters Canoe Area. The agencies preparing an environmental impact statement for PolyMet's NorthMet mine project are discussing that possibility.

Refresher on the PolyMet NorthMet project:

PolyMet has been working for 10 years to get approval for an open pit copper-nickel mine near Hoyt Lakes. The company would extract the metals from sulfide ore. Right now, the Minnesota DNR, the U.S. Forest Service and the U.S. Army Corps of Engineers are all looking at potential impacts on the environment, from air quality to wetlands to endangered species.

But environmentalists are most concerned about the potential for contaminated water to escape and pollute both groundwater and nearby streams and rivers. Those concerns generated thousands of comments to a draft environmental impact statement issued in 2013.

What did the draft environmental impact statement say about water pollution?

It said that PolyMet's plan would capture more than 90 percent of the polluted water and run it through a wastewater treatment plant. Polluted water that does escape the site would flow into the St. Louis River watershed, according to the analysis, leading to Lake Superior and not the Boundary Waters.

A few months ago, the Great Lakes Indian Fish and Wildlife Commission, which is working on behalf of Indian tribes to evaluate the project, came forward with a new analysis that raises the possibility of water flowing north toward the Boundary Waters after the mine closes. The commission's scientists go as far to call that outcome probable.

How have environmental groups reacted to commission's analysis?

They are obviously concerned. Environmental groups are also skeptical of the environmental impact statement's conclusion that most of the contaminants will be captured. And they've questioned the assumptions made about how much water is moving through the site and how fast. These new questions about what direction potential contaminants will flow is yet another concern.

What do the DNR and other agencies say about the possibility of water flowing north toward the Boundary Waters?

The DNR, U.S. Forest Service and U.S. Army Corps of Engineers have said in letters to the other agencies that it's very unlikely water would flow northward, but officials did say there's a "theoretical possibility." DNR officials issued a statement saying they're investigating the possibility and will respond with their findings in the final environmental impact statement.

What does PolyMet say about it?

Company officials also say it's very unlikely that water would flow northward, based on all the data that's been collected to complete the environmental review. Officials also say the mine plan calls for groundwater-monitoring wells that would detect the potential for water to move northward. If the monitoring wells detected that, the company could respond by adjusting water levels at the site to prevent any contaminated water from flowing into the watershed that contains the Boundary Waters.

This field and lake are built on top of old tailings from the LTV Taconite mine at PolyMet near Aurora, Minn. Plans would cover this field with new tailings and waste rock from a new mining operation. Derek Montgomery | For MPR News 2013

How big of a deal could this dispute over water pollution be for the project?

We don't know yet. It could be that the discussions among the DNR, the tribal agencies and others could reach a consensus on this issue. But the Great Lakes Indian Fish and Wildlife Commission has been an outspoken critic of the project and the environmental analysis, and this is only one issue the commission's scientists have raised.

Water modeling is a very common point of disagreement on these types of projects, and the analyses have often been subject to lawsuits. The reason is that you're predicting outcomes far into the future, so there's automatically some uncertainty involved.

PolyMet's project has always been controversial, and environmentalists have kept up their pressure throughout the process. If the project moves forward, we could see a lawsuit like those filed in many other mining cases.