Environmental groups are suing the U.S. Environmental Protection Agency to force Xcel Energy to take potentially expensive steps to clean up emissions at its Sherco coal-fired power plant — emissions another government agency says obscure the scenery over two national parks in Minnesota and Michigan.

The suit, filed Wednesday, Dec. 5, in U.S. District Court in St. Paul, asks the court to make the EPA act on a 2009 finding by the U.S. Department of the Interior that identified the massive coal-fired plant in Becker, Minn., as a source of the haze floating over the Voyageurs and Isle Royale parks.

The emissions responsible for the haze also cause lung and heart disease throughout the state, said Kevin Reuther, legal director for the Minnesota Center for Environmental Advocacy, a St. Paul nonprofit organization that is one of the plaintiffs in the suit.

“It’s time for the EPA to step up and tell Xcel to clean up that plant,” he said.

The groups believe the haze also is harming the Boundary Waters Canoe Area Wilderness, but the lawsuit does not cover that area because the Interior Department has jurisdiction only over national parks, Reuther said.

Joining the lawsuit were the National Parks Conservation Association, Friends of the Boundary Waters Wilderness, Voyageurs National Park Association, Fresh Energy and the Sierra Club.

The EPA regional office in Chicago, which oversees Minnesota, did not respond to a message for comment.

An official with Minneapolis-based Xcel Energy said Wednesday that Sherco, located 45 miles northwest of the Twin Cities, is too far from the wilderness areas to be the source of the haze.

“Sherco is not causing or contributing to visibility impairment in the distant national parks and wilderness areas in northern Minnesota,” Frank Prager, Xcel Energy vice president of environmental policy and services, said in a statement.

Sherco, or officially the Sherburne County Generating Station, which has the capacity to generate 2,225 megawatts of electricity from its three coal-fired units, is the backbone of Xcel’s fleet of plants. Xcel is the primary power utility serving Minnesota.

The utility said it is investing $50 million at Units 1 and 2, Sherco’s older units, that should cut sulfur dioxide and nitrogen oxide emissions — the sources of haze — by half.

“These emissions reductions at Sherco have already been found adequate to comply with Regional Haze regulations, the Clean Air Act’s primary program for protecting visibility in our national parks and wilderness areas,” Prager said.

The differences of opinion boil down to two clean air standards. The environmental groups are using a standard from the 1980s called Reasonably Attributable Visibility Impairment, or RAVI. It allows the EPA to focus on individual sources of pollution, such as a power plant like Sherco.

The RAVI standard calls for more expensive retrofitting technologies than Xcel wants to use that could cost hundreds of millions of dollars, Reuther said.

Or Xcel could replace the two older units with less polluting alternatives, such as natural gas or renewable energy, but in either case, it would require additional investment, the environmental lawyer said.

Xcel and the Minnesota Pollution Control Agency favored a newer standard under the Regional Haze Program, said Catherine Neuschler, state implementation plan coordinator for the MPCA.

The Regional Haze standard doesn’t focus on individual sources of pollution but tries to figure out how to cut haze emissions throughout a region, and utilities can get credit for work at other plants, Neuschler said.

Xcel wants credit for its $1.2 billion Metro Emissions Reduction Program through which it converted two aging coal-fired plants in St. Paul and Minneapolis to natural gas and retrofit a third plant in Oak Park Heights with better emission controls several years ago. The utility also plans to retire its Black Dog coal-fired units in Burnsville in 2015.

“The MPCA took a stance under the newer, broader Regional Haze program, but Xcel is also subject to the older RAVI standard under the EPA, and EPA has not acted,” Neuschler said.

Xcel is evaluating the future operations of its older units at Sherco, and it expects to submit its study to the Minnesota Public Utilities Commission in July.

Leslie Brooks Suzukamo can be reached at 651-228-5475.

Follow him at twitter.com/suzukamo.