Duluth, MN (Learfield News Service) -  A new study suggests the proposed PolyMet copper-nickel mine in northeastern Minnesota poses a future spill risk.

The report by Duluth for Clean Water says communities downstream could expect to see an average of five spills per decade. The analysis also says there's a 99.5-percent likelihood the tailings dam would fail during its projected 500-year lifetime.

Bridget Holcomb of Duluth for Clean Water said, "our wet climate increases the risk that spills and dam failures would flow into our lakes and rivers." Environmental groups are challenging the state permits issued for the project.

PolyMet proposes storing 225-million tons of copper sulfide mining waste upstream of the St. Louis River and Lake Superior.