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Virginia Department of Game and Inland Fisheries biologist and U.S. Fish and Wildlife Service, Dept. of Interior biologist travel up the Dan River from Abreu-Grogan Park landing on Wednesday, Feb. 5, 2014, in Danville, VA. They will gather water and river bottom sediment samples from the coal ash spill at the Dan River Steam Station operated by Duke Energy upriver in Eden.

RALEIGH — North Carolina’s highest court is weighing whether state environmental rules require Duke Energy to clean up its leaky coal ash dumps immediately, or years down the road.

The North Carolina Supreme Court heard arguments Monday after Duke Energy and state regulators appealed last year’s ruling by Superior Court Judge Paul Ridgeway that the company is required to take “immediate action” to eliminate sources of groundwater pollution at its unlined ash pits.

Cape Fear River Watch and other groups sued after the state Environmental Management Commission determined Duke could satisfy the legal requirement for immediate action by taking steps to develop a plan for stopping its pollution in the future.

Judges ruled Monday the legislature was wrong last year in how it assembled the membership of the Coal Ash Management Commission, which is overseeing the closure of Duke Energy's coal ash dumps. The ruling also struck down the membership of two other panels set to start up this summer — the Oil and Gas Commission and the state Mining Commission.

Lawyers for Duke and the state commission claimed the issue is now moot since legislators passed a new law in the wake of last year’s massive Dan River spill that requires Duke to cap or remove all of its dumps by 2029. Four “high priority” sites are set to close by 2019.

Further, they argued, the term “immediate action” can rightly be construed as having multiple meanings other than requiring an immediate cleanup.

“The court erred in failing to give deference to the commission’s interpretation of its own rules,” said Special Deputy Attorney General Jennie Hauser, representing the state. “‘Immediate,’ in many contexts means immediate in terms of the timing. ... To ‘take immediate action’ means to initiate the action that’s required. That can be a prompt notification, or an assessment.”

D.J. Gerken, a senior attorney for the Southern Environmental Law Center, told the justices that the term should be applied literally, as the lower court had ruled.

“Immediate action is required to eliminate the source of the contamination,” Gerken said, summarizing the requirement.

Environmental groups have been pressing for years for state regulators to require Duke to stop its pollution, but gained little traction until last year’s big spill, which coated 70 miles of the Dan River in gray sludge. The ash, which is the waste left behind when coal is burned to generate electricity, contains such toxic heavy metals as arsenic, selenium, chromium and mercury.

Duke, the nation’s largest electricity company, adamantly denied any wrongdoing for years. But in December, the company conceded in regulatory filings that it had identified about 200 leaks and seeps at its 32 coal ash dumps statewide that together ooze out more than 3 million gallons of contaminated wastewater each day.

Last month, the U.S. Justice Department charged Duke with nine misdemeanor counts involving violations of the Clean Water Act for unlawful dumping at coal-fired power plants in Eden, Moncure, Asheville, Goldsboro and Mt. Holly. Duke says it has already negotiated a plea agreement under which it will admit guilt and pay $102 million in fines, restitution and community service.

Earlier this month, the state Department of Environment and Natural Resources fined Duke $25 million over pollution that has been seeping into groundwater for years from a Duke plant near Wilmington.

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