The U.S. ­Environmental Protection Agency has agreed to reassess whether coal burned at Minnesota’s largest power plant is reducing visi­bility at national parks in Minnesota and Michigan.

If the environmental agency decides that emissions from Xcel Energy’s Sherco power plant in Becker, Minn., cause haze, it could mean costly pollution control upgrades or early retirement of two 1970s-era coal-burning units there.

Under an agreement filed Tuesday to settle a federal lawsuit by environmental groups, the EPA pledged to study the power plant’s emissions and their potential effects on visibility in Voyageurs National Park in Minnesota and Isle Royale National Park in Lake Superior.

Six environmental groups had sued the EPA in U.S. District Court in Minnesota alleging that the agency wasn’t enforcing the Clean Air Act’s provisions to protect air quality in pristine natural areas. The groups contend that visi­bility in the Boundary Waters Canoe Area Wilderness also is hurt by Sherco smokestack emissions. The settlement must be approved by a judge after a comment period.

The U.S. Interior Department, parent agency of the National Park Service, declared in 2009 that Sherco units 1 and 2 are the source of airborne dust and smoke affecting the parks. But the EPA and Xcel had disputed that finding.

The best technology to reduce such pollution is known as selective catalytic reduction and would cost Xcel an estimated $350 million for the two units.

“It is an expensive tech­nology,” said Kevin Reuther, an attorney for the Minnesota Center for Environmental Advocacy, a St. Paul ­nonprofit that joined five other groups to sue the EPA. “So you get to the point of ‘Do we want to make the investment in these old plants or is it time to look at retiring them?’ ”

In a statement, Xcel spokeswoman Patti Nystuen said the Minneapolis-based utility is reviewing the consent decree and assessing its options. She said Xcel is waiting for an appellate court ruling on whether Xcel can be a part of the litigation. The company is “disappointed the other parties are trying to move forward at this time,” she said.

U.S. Justice Department spokesman Wyn Hornbuckle said it is too early to say how EPA will reassess the Sherco units’ potential effects on the parks. Reuther said it likely would require computer modeling of air pollutants.

The power plant is 260 miles south of Voyageurs National Park on the U.S.-Canada border, and 360 miles southwest of Isle Royale National Park, which is 22 miles by ferry from Grand Portage on Minnesota’s North Shore.

The Sherco power plant, 45 miles northwest of the Twin Cities, burns three trainloads of western coal a day in three large generating units. The largest unit, completed in 1987 and not at issue in the national park litigation, is 41 percent owned by the Southern Minnesota Municipal Power Agency. Xcel solely owns the two older units.

In Xcel’s next long-range plan, to be filed next January with the state Public Utilities Commission, the ­utility is expected to address whether to retain or retire the two older units. That’s about a month before the EPA plans to issue its findings on the units’ effects on the parks.

The five other environmental groups that brought the lawsuit are the National Parks Conservation Assoc­iation, Friends of the Boundary Waters, Voyageurs National Park Association, Fresh Energy and the Sierra Club.