The state Department of Environmental Quality on Monday ordered Duke Energy to excavate all remaining coal ash ponds in North Carolina and move the toxic ash to lined landfills.

Secretary of Environmental Quality Michael Regan said his staff reached the conclusion that digging up and removing the ash was the only option to protect public health and the environment after a scientific review of Duke's proposals for the sites and meeting with affected communities.

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"Some of these ash ponds are over 200 acres, and some of these plumes are over 300 feet deep – these contamination plumes," Regan said. "So, what we did is we modeled it out. We looked at the science, and we had to make a determination of what's most protective of North Carolinians and our environment."

"We know for sure we have the science and the law to support our decision," he added.

The sites include the Mayo and Roxboro plants in Person County, the Marshall plant at Lake Norman, the Allen plant in Gaston County, the Cliffside/Rogers plant in Cleveland County and the Belews Creek plant in Stokes County.

Duke officials said all six sites were previously rated as low-risk by the state and that "science and engineering support a variety of closure methods [other than excavation] that all protect public health and the environment."

"We are making strong progress to permanently close every ash basin in North Carolina in ways that fully protect people and the environment, while keeping costs down as much as possible for our customers," company officials said in a statement.

Adding the sites to the excavation list means clearing out the coal ash could take decades,"stretching well beyond the current state and federal deadlines," officials said. It also could add $4 billion to $5 billion to the estimated $5.6 billion price for coal ash cleanup, they said.

The North Carolina Utilities Commission last year allowed Duke's two units that provide electricity across the state to recoup coal ash cleanup costs as they are incurred by passing those costs on to customers through higher rates.

Regan said it's up to the Utilities Commission and the Attorney General's Office to decide who should pay for the cleanup.

"The assumption that the public should pick that tab up, I don't think is one that we should make prematurely," he said.

Coal ash, what's left over after coal is burned for fuel, contains arsenic, lead, mercury and other toxins. Duke has, for decades, stored it in unlined, water-filled basins at its power plants.

Ash has been an environmental and health flashpoint in North Carolina since a major spill at Duke's Dan River plant in Rockingham County in February 2014 that dumped thousands of tons of the goop in the river.

Lawmakers passed bills in 2014 and again in 2016 to establish cleanup priorities and timelines and to have DEQ assess the risk at the 33 ash ponds at Duke's 14 plants.

Ash was ordered to be excavated from Duke plants in Wilmington, Asheville and Mount Holly, along with the Dan River site, in the 2014 legislation. Two years later, plants in Moncure, Goldsboro and Lumberton were added to the excavation list following court rulings after communities sued over nearby ash pits, and a settlement between the state and Rowan County residents added an eighth site to the list.

But the ash at the other six plants was allowed to be left in place if it was covered up and the water was drained from the ponds.

"We’ve seen the damage this pollution can do, including the families who had to live for years on bottled water until we were able to get them connected to permanent water solutions. Now, the cleanup of remaining coal ash needs to move ahead efficiently and effectively," Gov. Roy Cooper said in a statement.

Environmental advocates cheered Monday's announcement, calling it a historic day for the state.

"When the coal ash from all of these sites is finally removed, 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, senior attorney at the Southern Environmental Law Center, 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."

Duke was ordered to establish water supplies to residents within a half-mile of coal ash ponds to get them off well water under the 2016 legislation.

Holleman, who represented communities near coal ash ponds, has argued for years that the unlined ash dumps will continue to leach toxic compounds into surrounding water until it's all removed.

"That is probably one of the largest, if not the largest, source of toxic industrial pollution in our rivers and lakes in North Carolina, and that will be eliminated [by Monday's action]," he told WRAL News.

He added that he believes Duke is overestimating the cost and difficulty involved with excavating all of the sites.

"When the utilities are fighting and trying not to have to change they ways they've practiced for years, they greatly exaggerate the cost," he said.

"The debate about how to deal with Duke Energy's coal ash mess should be over," Dave Rogers, of the Sierra Club's Beyond Coal campaign, said in a statement. "Duke owes it to our communities to clean up its toxic mess and not try to force customers to foot the bill for it."

DEQ ordered Duke to submit final plans for excavating the six sites by Aug. 1. The plans will include a timeline and location for moving the coal ash.

South Carolina took this step years ago, while Virginia lawmakers voted last month to require their coal ash ponds to be excavated.