Regulators in North Carolina are requiring the nation’s largest energy company to change how it stores pools of toxic residue that result from burning coal.

Duke Energy must now excavate its existing coal ash ponds and move the substance to lined pits, North Carolina’s environmental agency said Monday.

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The order from the state's Department of Environmental Quality (DEQ) requires Duke Energy to store coal ash in lined pits, rebuffing the company’s request to cover the pits in order to keep out rainwater and prevent the pits from overflowing.

“The science points us clearly to excavation as the only way to protect public health and the environment,” DEQ Secretary Michael Regan said in a statement.

Coal ash is often mixed with water to stop the substance from becoming airborne, but when stored in unlined pits, coal ash can leach into the ground, depositing arsenic, lead and mercury into the water supply.

Environmental groups praised the decision.

“North Carolina’s rivers will be cleaner, North Carolina’s drinking water will be safer, and North Carolina’s communities will be more secure,” Frank Holleman, a senior attorney at the Southern Environmental Law Center (SELC), said in a statement. “We will no longer have to hold our breath every time a storm, a flood, or a hurricane hits a community with unlined coal ash pits sitting on the banks of waterways.”

SELC represented numerous communities suing Duke Energy to close the coal ash pits.

Hurricane Florence in 2018 damaged a coal ash pond near the Cape Fear River, dumping coal ash into the water supply for Wilmington, N.C.

DEQ has already ordered the closure of eight other coal ash disposal sites in North Carolina. This order will close the remaining six, and Duke Energy has until August to propose its closure plans.

Duke Energy said it has committed to closing all of its coal ash pits but estimated that moving the substance to lined pits will take longer and cost as much as $5 billion more than other options.