Jul. 3--A federal judge in Asheville on Thursday dismissed an environmental lawsuit challenging Duke Energy's construction of a $2.4 billion addition to its Cliffside coal-fired power plant.

Judge Lacy Thornburg's order leaves remaining legal opposition to the plant, which is more than one-third complete, before a state administrative court.

Five environmental groups filed the federal suit last year, claiming Duke illegally began work on the plant before a full review of the stringency of its pollution controls. Cliffside is 60 miles west of Charlotte.

The groups maintained that, contrary to a ruling by state regulators, Cliffside will be a major source of toxic pollutants, such as mercury, and so is required to install the most effective controls available.

Thornburg, last December, agreed that the environmentalists could be right. A trial had been scheduled for Aug. 17.

But the judge noted Thursday that both federal and state courts are being asked to decide the same issue. "This is obviously an unwise use of scarce judicial resources," he wrote in removing himself from the case.

The ruling validates the "exhaustive emissions review" Duke and state regulators did of the new Cliffside boiler, Duke spokesman Jason Walls said.

"The minor-source designation by the Division of Air Quality confirms that Unit6 will be one of the cleanest, most efficient units in the country when it comes online in 2012," he said.

Thornburg's ruling noted that the environmental groups could return to federal court after the state case is resolved if they still believe Duke is breaking federal clean-air law.

"I think he's kind of holding the case open until a later day," said John Suttles, a Southern Environmental Law Center attorney representing the environmental groups. "The court in no way said our concerns were invalid or improper."

The law center is also involved in the four lawsuits before the N.C. Office of Administrative Hearings. Such cases typically take months or years to decide.

Cliffside has drawn intense opposition since construction began in January 2008. It was a major focus of a march on Duke's Charlotte headquarters by 250 protesters in April, where more than 40 people were arrested.

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