RALEIGH, N.C. (CN) – North Carolina officials on Monday ordered Charlotte-based Duke Energy to excavate and relocate coal ash from storage ponds at six power plants.

Crews with Duke Energy remove coal ash from old ponds at the Sutton Plant in Wilmington, N.C. (Ken Blevins /The Star-News via AP, File)

The coal will be removed from wet basins at six facilities and placed in dry, lined landfills as part of Duke Energy’s compliance with the order from the North Carolina Department of Environmental Quality.

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

Final plans for where the ash will end up and an excavation timeline needs to be submitted by Duke by Aug. 1.

Concern about the wet storage of the coal-combustion byproduct heightened in 2014 when a spill at one Duke facility exposed the Dan River and nearby groundwater to heavy metals found in the material.

Groups represented by the Southern Environmental Law Center brought a series of lawsuits against the utility in the following years, demanding it remove coal ash from unlined basins.

Frank Holleman, a senior attorney at the Southern Environmental Law Center, celebrated the order in a statement Monday.

“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,” Holleman said. “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 made repairs to risky retaining dams after the 2014 spill and has provided some communities with public water services to replace potentially contaminated well water.

A state law required the company to phase out its ash ponds across North Carolina following the Dan River incident, giving Duke the option to use different basin closure methods based on a contamination risk level assigned to each plant.

Two coal plants are classified as low-risk locations, meaning the possibility of ash ponds breaching or contacting surrounding water is considered less likely.

Before Monday, Duke had the option to just drain water from the low-risk ponds and put a cap on the leftover material. But this option is no longer acceptable for any of the six locations, state officials said Monday, finding that excavation is the most appropriate closure method because its removes the primary source of groundwater contamination.

The utility giant said Monday’s order to excavate the basins instead of capping them could take decades and will add approximately $4 to $5 billion to the current estimate of $5.6 billion for ash pond closures in the Carolinas.

“With respect to the final six sites—which NCDEQ has ruled are low-risk—science and engineering support a variety of closure methods including capping in place and hybrid cap-in-place as appropriate solutions that all protect public health and the environment,” Duke said in a statement on Monday.

Hundreds of other basins that are expected to close around the country are cleared to be capped in place, which is the cheaper and quicker option, according to Duke.

Facilities containing basins to be excavated include the Marshall plant on Lake Norman near Charlotte, the Mayo and Roxboro plants in Person County, Gaston County’s Allen facility, the Cliffside plant in Cleveland County, and the Belews Creek plant in Stokes County.

These six plants, which have a total of nine basins, are added to eight that Duke had previously agreed to excavate.

In its final basin closure plan, Duke will have the opportunity to propose a plan to recycle the material— a solution the company says is the only way to prevent permanent coal ash storage.

Duke recycled 63 percent of the coal ash it produced in 2015, according to its safe basin closure progress report, and says it will continue to pursue additional opportunities for recycling.

The company also said it is transitioning its operations to cleaner fuel sources that do not produce coal ash.