Environmental groups filed a federal lawsuit Wednesday seeking stricter regulation of an Xcel Energy coal-fired power plant whose smokestack emissions are linked to reduced visibility over Voyageurs National Park.

The National Parks Conservation Association and five other groups allege the U.S. Environmental Protection Agency hasn't enforced longstanding rules under the U.S. Clean Air Act to reduce haze over Voyageurs and the Boundary Waters Canoe Area Wilderness in northern Minnesota and Isle Royale National Park in Michigan.

The lawsuit, filed in U.S. District Court in Minnesota, seeks an injunction forcing the EPA to set stricter limits on haze-causing emissions from two coal-buring units at Xcel's Sherco power plant in Becker, Minn. The plant is the largest electric generator in Minnesota.

An EPA spokeswoman said the agency does not comment on pending litigation.

Xcel isn't named as a defendant, but the lawsuit could directly affect the Minneapolis-based power company that serves 1.2 million customers in the state. Although Xcel is upgrading pollution controls at the two 1970s-era generators, the lawsuit, if successful, could force the company to install the most-advanced technology, called Selective Catalytic Reduction, at an estimated cost of $340 million.

"Sherco is a huge emissions source," said Kevin Reuther, legal director for the Minnesota Center for Environmental Advocacy, a nonprofit group in St. Paul leading the court challenge. "The plume is just enormous. People don't realize that when they are way up in the Boundary Waters they are breathing emissions from the plant in Sherburne County, but they are."

Three years ago, the U.S. Interior Department concluded that Sherco units 1 and 2 contributed to haze, or reduced visibility, over Voyageurs and Isle Royale. That finding, the lawsuit contends, required the EPA to take regulatory steps, but the agency didn't. The suit seeks to have a judge mandate and oversee EPA's actions under the haze rule.

Frank Prager, Xcel vice president of environmental policy and services, would not comment directly on the lawsuit, but said in a statement Wednesday that Sherco is not causing visibility problems in the parks and wilderness.

"We are currently investing $50 million in improvements at Sherco Units 1 and 2 that will cut sulfur dioxide and nitrogen oxide emissions by about half," he said.

Those reductions, Prager added, will comply with separate regional haze regulations under the Clean Air Act. Xcel also is studying the future of the Sherco plant, including whether the older units should be upgraded or replaced. That study will be submitted to the state Public Utilities Commission next July.

"We believe this is a better venue to discuss these issues than in the courtroom," Prager said.

One outcome of the lawsuit is that it may add weight to clean air groups' arguments to retire the Sherco plant. Reuther said the advocacy group, which also intervenes in Xcel's state regulatory matters, supports replacing the two units with renewable energy and generators burning natural gas.

Other groups behind the lawsuit are the Friends of the Boundary Waters, Voyageurs National Park Association, Fresh Energy and Sierra Club.

The haze rule has been on the books for decades. Congress added special protections for parks and wilderness areas to the Clean Air Act in 1997. In South Dakota, the coal-burning Big Stone power plant operated by Otter Tail Power Co., based in Fergus Falls, is installing advanced pollution controls to protect the Minnesota natural areas.

Reed Zars, a Laramie, Wyo., attorney who represents two of the groups in the Minnesota case, said he knows of just two similar lawsuits to protect parks or wilderness areas. In 1993, a federal appellate court upheld a requirement to add scrubbers to an Arizona coal-fired power plant to protect Grand Canyon National Park, Zars said. Another case resulted in a 1996 settlement to upgrade two Colorado coal-burning generators, including one owned by Xcel, to protect the Mount Zirkel Wilderness in that state.

David Shaffer • 612-673-7090