— Duke Energy said Wednesday that it will build lined landfills at two of its power plants to store coal ash.

The Charlotte-based utility is under a state mandate to close all 32 of its coal ash ponds across North Carolina by 2029, and officials said the new landfills will be able to handle more than 6 million tons of ash from the Dan River Steam Station in Eden and the Sutton Plant in Wilmington.

“This plan is a significant step forward in our strategy to close ash basins and manage coal ash across our service area,” John Elnitsky, Duke's senior vice president of ash basin strategy, said in a statement. “Our preference is to store coal ash at or near our plant sites, when possible. Siting these landfills on plant property minimizes impacts to the local community while maximizing the safe and efficient storage of coal ash at these locations.”

Coal ash is the material that's left over when coal is burned for fuel. While much of it is inert, it does contain toxic levels of certain substances, such as thallium, mercury, lead and other materials harmful to humans and wildlife.

For decades, the ash has been stored in unlined pits and mixed with water. The resulting ash ponds usually sit alongside the rivers that helped the power plants generate steam, and a pipe under the Dan River ponds ruptured last year, spilling about 39 tons sludge into the river and focusing state and federal attention on the issue.

State lawmakers passed legislation creating a state commission to oversee closure of Duke's ash ponds, and regulators fined the company $25.1 million for groundwater contamination near the Sutton Plant. Federal authorities have reached a deal for Duke to plead guilty to violations of the Clean Water Act for similar leaks at various ash ponds across the state and pay more than $100 million in penalties.

The Dan River and Sutton landfills will include several layers of synthetic and natural barriers beneath the ash and more lining on top to seal it from the surrounding soil and groundwater, Elnitsky said. Extensive groundwater monitoring will ensure the local environment is protected, he said.

"Our first priority is ensuring the safety of the public and our environment,” he said. “We will consolidate and contain coal ash at a greater distance from public waters and provide separation from surrounding soil and from groundwater. The excavation plans we have developed represent the best option for disposal of ash at these sites.”

Duke still plans to store about 2 million tons of the Sutton Plant ash in abandoned clay mines near Sanford, a plan that has prompted protests in both Chatham and Lee counties. The company is awaiting state permits for the move and hopes to begin the project in the fall, Elnitsky said.

"We must begin moving coal ash as soon as possible in order to comply with the strict timelines laid out in state law,” he said. “That’s why the Chatham County mine project is such an important part of our strategy for the Sutton Plant."

The company hopes to start construction on the lined landfills next year, so dry ash can be moved there beginning in late 2016 or early 2017, officials said. Both landfills will have the capacity to expand later, if needed, they said.