By a 4-1 vote, the State Air Pollution Control Board approved a permit for Competitive Power Ventures Inc. of Silver Spring, Md.

The independent power company plans to start construction on the plant near Front Royal within a year and sell electricity to Dominion Virginia Power, project director Thomas E. Eiden said.

"The state of Virginia has just approved the cleanest power plant ever approved in the state or the United States," an ecstatic Eiden said after the vote.

The vote followed a five-hour meeting in the General Assembly Building in which environmentalists fiercely fought the proposal.

Shenandoah park is a "national treasure" yet "one of the most polluted parks in the country," said Jeff Gleason, deputy director of the Southern Environmental Law Center.

"Four miles is too close" for a power plant, he said.

The park, along the Blue Ridge between Waynesboro and Front Royal, suffers from smog, haze and acid rain. Sources of the pollution include power plants as far away as the Ohio River Valley and cars on Interstate 81.

Dan Holmes of the Piedmont Environmental Council said the state needs to look at the cumulative effects of new power plants.

Over the past five years, developers have proposed about 30 plants in Virginia. Several projects have fallen through, but about nine plants are running or under construction, Holmes said.

"We believe these facilities have a major effect on Virginia's environment and public health," he said.

But even the environmentalists conceded the Competitive Power plant would be fairly clean. Their problem was the site, in Warren County just above the park's northern tip.

"That really is the issue - location, location, location," said the chairman of the air board, Gary H. Baise of McLean.

The 580-megawatt plant will burn natural gas and be equipped with sophisticated pollution controls.

The medium-size plant will release, among other things, about 153 tons a year of nitrogen oxides, pollutants linked to smog, haze and acid rain.

Emissions from a coal-fired plant the same size would probably be 10 to 20 times greater, said Sharon Foley, air permit manager for the state Department of Environmental Quality's Shenandoah Valley region.

"Our analysis has shown that the impact of this plant on air quality is fairly insignificant," Foley said in an interview.

Representatives of Shenandoah park did not raise major objections to the plant.

In approving the permit, the air board required the plant's owners to obtain "offsets" that will reduce more tons of nitrogen oxides than the plant releases.

For example, if the plant releases 100 tons a year, it would have to reduce the pollution by 115 tons - say, by paying for pollution controls at other industries in the region.

Before the final vote, a motion to deny the permit fell 3-2, with Vivian E. Thomson of Charlottesville and Smita Siddhanti of McLean voting against the plant.