The North Carolina Department of Environmental Quality has ordered Duke Energy LLC and Duke Energy Progress LLC to excavate all remaining coal ash impoundments in North Carolina.

Duke has nine ash impoundments at six sites across the state.

Parent company Duke Energy Corp. estimates the cost of closing all remaining impoundments by excavation to be $4 billion to $5 billion more than the current project cost estimate of $5.6 billion.

Excavation would likely take decades, extending well beyond the required state and federal deadlines for impoundment closure.

According to a statement released by Duke Energy, the company is “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.”

After coal is burned for energy, it produces coal ash. This waste can contain various toxins, including lead and mercury, which Duke Energy has been storing in unlined, water filled ash basins at its power plants for decades.

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

“Today’s action sends another clear message that protecting public health and natural resources is a top priority of the Cooper administration,” he added.

Duke Energy must submit final excavation closure plans to state regulators by Aug. 1, 2019. In those plans, Duke must propose where excavated coal ash will reside and estimate how long that process will take.

Duke Energy stock closed at $89.28 Monday, down 0.8 percent from Friday’s close of $90.

The release can be found here.