Several environmental groups are trying to force the federal government to impose pollution controls at Xcel's Sherburne County power plant.
The U.S. Department of the Interior says the plant contributes to polluted air in Voyageurs National Park.
That determination means the Environmental Protection agency should require pollution controls on the plant, said Kevin Reuther of the Minnesota Center for Environmental Advocacy.
"They've had nearly three years to determine what the appropriate pollution control should be on Sherco. Meanwhile the air isn't getting any better," Reuther said.
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Sherco burns three trainloads of coal daily, producing enough electricity to power nearly two million homes.
Reuther's group and five others warn the EPA that if it takes no action within six months, they will sue the agency.
In a prepared statement, Xcel says Sherco meets the requirements of the Clean Air Act and does not impair visibility in Minnesota's national parks.
The company is investing $50 million to cut haze-forming pollutants by about 50 percent. However, environmental groups say more expensive technology could achieve 90 percent-reduction of some pollutants.