North Carolina’s Department of Environmental Quality issued an order to Duke Energy Corp. to excavate coal ash from all of its North Carolina power plant sites. The Associated Press reported that the order was given with the intention of slashing the risk of toxic chemicals leaking into water supplies. However, the order could potentially add billions of dollars to the costs consumers pay.

Duke Energy said Monday the expanded excavation requirement could double its costs to about $10 billion. It had previously estimated the task would take up to 30 years to complete at all 14 current and former coal-burning plants in the state.

The company had proposed covering some storage pits with a waterproof cap, saying that would prevent rain from passing through and carrying chemicals through the unlined bottoms and would provide a quicker and cheaper option.

The decision affects six coal-burning plants still operating in North Carolina. Pits at eight other power plants around the state had previously been ordered excavated, with the ash to be stored away from waterways.

“We did a thorough analysis of the six sites and it wasn't a decision that was made by other reasons than the science,” state Department of Environmental Quality Secretary Michael Regan said in an interview. “We’re making these decisions so that they are most protective of public health and the environment.”

The move means North Carolina joins Virginia and South Carolina in ordering its major electric utilities to move their coal ash out of unlined storage.

Monitoring data that U.S. electric utilities were required to report for the first time last year showed that of the 265 power plants reporting, nine out of 10 had contaminated ground water with toxic substances at levels outstripping federal safety standards, Earthjustice and the Environmental Integrity Project found. The advocacy groups ranked Duke Energy's Allen power plant in Belmont the second-most contaminated in the county.

A 2008 spill in Tennessee drew national attention to coal ash storage. Cleanup became a priority in North Carolina after a 2014 leak from a Duke Energy site left coal ash coating 110 km (70 miles) of the Dan River on the state's border with Virginia.

Duke Energy pleaded guilty in 2015 to federal environmental crimes after an investigation found the company allowed coal ash dumps at five power plants to leak toxic waste into water supplies. The company agreed to pay $102 million in fines and restitution.

Last year, Duke Energy agreed to pay a $156,000 penalty for similar state environmental violations at three other power plants after pollution entered groundwater and the adjoining Catawba and Broad rivers.

The company said it is reviewing the agency's decision that it must fully excavate its North Carolina sites "and will continue to support solutions that protect our customers and the environment."

State utilities regulators last year set a precedent in deciding that Duke Energy's two North Carolina divisions could charge ratepayers the first $778 million chunk of cleanup costs.

The same North Carolina Utilities Commission assessed the company a $70 million mismanagement penalty, finding that Duke Energy's "irresponsible management" of its ash pits" resulted in cost increases greater than those necessary to adequately maintain and operate its facilities."

State Attorney General Josh Stein said last year he will try to stop Duke Energy from passing along its cleanup costs to 3.4 million North Carolina power customers, arguing that corporate mismanagement increased costs that shareholders should also be forced to bear.