The Environmental Protection Agency’s (EPA’s) controversial Mercury and Air Toxics Standards (MATS) survived another day as the U.S. Supreme Court on June 13 declined to review the D.C. Circuit’s decision to leave the rule in place while the EPA revises it to comply with a 2015 Supreme Court ruling.

The 2015 decision sent MATS back to the EPA, saying the agency had failed to properly consider costs during its rulemaking, but did not vacate the rule itself. The court held that the EPA was required to take costs into consideration when deciding whether regulating power plants was “appropriate and necessary” under the Clean Air Act.

After the ruling, the EPA proposed a supplemental finding stating that it would have reached the same conclusions by following the Supreme Court’s approach and proposed to leave the rule as-is. After the finding was released, a coalition of states argued in the D.C. Circuit that MATS should be vacated and the agency required to conduct a new rulemaking. In December 2015, the D.C. Circuit disagreed, saying it would allow the agency to complete a final finding. The states asked the Supreme Court to overturn that ruling, and the court on Monday declined to hear the appeal.

The original rule, finalized in 2012, is intended to curb emissions of mercury and other pollutants from U.S. power plants. The compliance deadline was in April 2015, meaning most affected plants have already either upgraded their emissions control systems or shut down. Only a few plants that received one-year extensions might have been saved by a different ruling in the D.C. Circuit.

Though the final finding was expected earlier this spring, the EPA has not yet released it. Further litigation over that finding is a near certainty.

—Thomas W. Overton JD is a POWER associate editor (@thomas_overton, @POWERmagazine).