A vortex of coal ash swirls in the gray waters of the Dan River at Danville, Viginia, downstream from the Duke Energy Steam Station in Eden, NC. John D. Simmons/Charlotte Observer/MCT via Getty Images

Duke Energy’s recent agreement to clean up a toxic coal ash spill in North Carolina and Virginia doesn’t do enough to address questions about the utility’s broader coal-related pollution problems, clean water advocates say.

Duke, the nation’s largest power company, signed an agreement on Monday with environmental and wildlife officials from both states requiring the utility to pay any “reasonable” cost associated with a Feb. 2 sludge spill at its power plant near Eden, N.C., the Associated Press reported. The accident sent coal ash, a byproduct of coal generation, floating down 70 miles of the Dan River, which also flows into Virginia. The agreement also requires Duke to foot the bill for continued monitoring by government agencies of the spill’s impact on the area's aquatic ecosystem.

But it doesn’t shed any light on what steps Duke might take to prevent future coal ash spills or to address current groundwater contamination at its other coal facilities in North Carolina. “There’s still the possibility that North Carolinians will be on the hook for the remainder of the cleanup,” Amy Adams, the North Carolina coordinator at Appalachian Voices, told International Business Times.

Duke operates 33 ash dumps at 14 power plants across the state, which, together, contain more than 100 million tons of the byproduct. All of the facilities are located near rivers and lakes that supply towns with drinking water and all of Duke’s unlined waste pits are contaminating groundwater, according to state environmental officials. Coal ash, which looks like a thick gray sludge, contains toxic heavy metals like arsenic, mercury and lead.

The utility said it spent $15 million to contain the Dan River spill and its immediate damages, and it told investors in an SEC filing that it doesn’t expect the remaining cleanup expenses to be large, the Charlotte Business Journal noted. But Duke said it could cost up to $10 billion to change the way it stores and processes coal waste at all of its North Carolina facilities, an expense Duke could pass on to its electricity customers. Regulators are still assessing how coal ash is stored at the 14 plants -- five of which received violation notices this spring for failing to secure storm water permits.

The agreement also leaves questions about where Duke will store all the coal ash it intends to remove from its other facilities, John Runkle, an attorney at NC WARN, a Durham, N.C.-based climate advocacy group, said. “We’re very concerned about any plans for the ultimate storage of that [waste],” he said. “We want to make sure that we’re not just passing the problem on.”

Advocates said they were, however, generally encouraged by Duke’s cleanup agreement, largely because it continues to loop federal agencies like the U.S. Fish and Wildlife Service and the Environmental Protection Agency into the response efforts. The utility and its state regulator, the North Carolina Department of Environment and Natural Resources (DENR), have both come under intense scrutiny after the Dan River spill raised concerns over their allegedly chummy ties. Just weeks after the accident, federal prosecutors opened a criminal investigation into DENR's relationship with Duke regarding its coal ash dumps.

“It gives at least some level of comfort that we’re not just dealing with the same two entities that created this situation,” Adams said. She added that agencies “are going to be looking at more than just the removal of the ash. They’ll be taking a much more holistic approach. [The agreement] is a good move for the health of the river.”

Jeff Brooks, a spokesman for Duke Energy, said the agreement is “a positive step forward that will provide benefits to the Dan River basin.” In response to advocates’ concerns that the agreement leaves questions unanswered, he explained that it was never intended to address them.

“It doesn’t encompass any action that will take place at our other facilities or encompass how the other coal ash basins will be managed,” Brooks said. “It’s tied to assessing any damages along the [Dan] River and developing a cooperative plan for trying to remediate that.”

He added that Duke is taking a “scientific, deliberate approach” to evaluating its next steps for each of its coal ash facilities in North Carolina.

Peter Harrison, a staff attorney at the Waterkeeper Alliance, said that Monday’s agreement “is a good start, but we’ll have to see where it goes. I think there’s still cause for skepticism” given the utility’s recent string of coal ash violations.

He noted that a separate coal ash decision was also reached this week. A federal court ruled that conservation groups represented by the Southern Environmental Law Center could proceed with their lawsuit against Duke, which accuses the utility of violating the Clean Water Act and polluting Sutton Lake, a public fishery near Wilmington, N.C. The law center commissioned a study last year of the lake’s fish that found deformities consistent with elevated levels of selenium, a toxic element in coal ash.