The U.S. Supreme Court on Tuesday let stand a decision that could mean costly changes at two Oklahoma Gas and Electric Co. power plants to increase visibility at federal parks.

Without comment, justices declined to review a case in which OG&E and Oklahoma Attorney General Scott Pruitt unsuccessfully challenged the Environmental Protection Agency’s plan for reducing regional haze. North Dakota’s appeal of a regional haze ruling was also rejected Tuesday by the high court.

OG&E warned last year that the EPA’s plan could require an investment of more than $1 billion to reduce emissions at coal units in Red Rock and Muskogee. OG&E spokesman Paul Renfrow said Tuesday that the utility would announce soon how it will comply with the EPA mandates.

“We are disappointed on behalf of our customers,” Renfrow said.

“We still believe that the Oklahoma State Implementation Plan would have enabled us to meet the Regional Haze requirements at a much lower cost. However, we accept the court’s ruling and now turn our attention to meeting the 55-month compliance deadline.”

The difference?

The Oklahoma plan called for the use of low-sulfur coal, but the EPA wants the utility to install scrubbers at their coal-fired plants to reduce pollution.

Whitney Pearson, with the Oklahoma chapter of the Sierra Club, said the Supreme Court’s decision not to review the case “means an end to the delays that have prevented Oklahomans from getting the cleaner, safer air they deserve.

“The highest courts in the land have affirmed and reaffirmed the need for OG&E to clean up its coal plants and have ruled against Attorney General Pruitt’s arguments three times,” Pearson said.

The federal Clean Air Act allows states to devise plans for reducing power plant pollutants that cause haze and damage air quality across state lines.

The EPA rejected Oklahoma’s plan, and the 10th U.S. Circuit Court of Appeals ruled 2-1 last year that the agency was within its authority to do so.

“We conclude that the EPA has authority to review the state’s plan and that it lawfully exercised that authority in rejecting it and promulgating its own,” two judges stated in the majority decision.

Judge Paul Kelly dissented from the decision, saying that the EPA plan would mean “no appreciable change in visibility. Moreover, there is no evidence this investment will have any effect whatsoever on air quality.”

‘A bad legal precedent’

Pruitt has been a harsh critic of President Barack Obama’s EPA, accusing the agency of ignoring the deference given to states under the Clean Air Act. He said Tuesday that the Supreme Court had let stand a “bad legal precedent that places the states in an inferior position to EPA’s ‘command and control’ approach to regional haze regulation.”

Pearson, with the Sierra Club, said Pruitt was fighting for OG&E rather than Oklahomans.

“OG&E can responsibly replace their coal plants with local clean energy solutions,” Pearson said.

“Wind is at its lowest cost in history and is saving customers money and cleaning the air.”