PORTLAND — The Sierra Club and other environmental groups notified Portland General Electric on Tuesday they plan to file a federal lawsuit to force the utility to retrofit a coal-fired generating plant with pollution control equipment.

The environmentalists say the Boardman plant in Eastern Oregon violates the federal Clean Air Act and spreads pollution across at least 10 national parks and wilderness areas in the West, including the Columbia River Gorge National Scenic Area.

Sierra Club spokesman Nat Parker said Boardman is the largest stationary source of air pollution and global warming gas in Oregon and one of the nation’s oldest and dirtiest coal-fired power plants.

The pollutants include carbon dioxide — a greenhouse gas — along with sulfur dioxide, mercury and soot that contribute to smog in the region and to acid rain and fog in the gorge.

“It’s what we call a dirty dinosaur,” he said.

The plant, approved by the state in 1975, lacks modern pollution control equipment because operations were “grandfathered” — or allowed despite changes in the Clean Air Act and other environmental regulations, Parker said.

A spokesman for the Portland-based utility said it plans to invest $300 million to $400 million to reduce emissions. The plant supplies about 20 percent of PGE’s generating capacity.

“We’ve got an aggressive plan,” spokesman Steve Corson said. “We’ve done a lot of work on this.”

The conservation groups say the PGE plan does not go far enough, aiming for the lower of two standards for pollution control technology when the higher should apply.

“The plant has been operating illegally for more than two decades, and it is operating illegally today,” Parker said.

The Oregon Department of Environmental Quality, which is reviewing the plan, declined to comment, as did the U.S. Environmental Protection Agency, both citing pending legal action.

The Boardman plant burns low-sulfur coal from Wyoming to produce about 585 megawatts of electricity for PGE, enough to power about 280,000 homes, according to the utility.

Coal plants provide just over half the electricity generated nationally, while also producing about 2 billion tons of carbon dioxide, or about a third of the annual U.S. total.