Drinking water is reported safe, however, and no arsenic or toxins have been detected in initial samples and no downstream problems have been identified, according to local and company officials; the tributary supplies water for Danville, Virginia, 25 miles downriver.

Discharge intensity, meanwhile, declined by Feb. 4 as the 27-acre retired plant pond continued emptying and fluctuating from rain and repairs, Meghan Musgrave, a company spokeswoman, told Bloomberg News.

Crews are burrowing into a ruptured, 48-inch reinforced concrete storm water pipe beneath the pond, the source of the water and ash, to plug it before a permanent cleanup, said Duke. Among Duke’s 14 North Carolina ash ponds, this is the only one with such a pipe beneath it, according to officials quoted by The Charlotte Observer.

It remains unclear why the pipe broke Duke engineers told The Observer. A temporary, inflatable bladder that stoppered the pipe was removed so cameras could be guided inside to view the damage. “This was unexpected,” Issa Zarzar, a head of Duke’s plant demolition and retirement programs told the paper. “The break of the reinforced concrete pipe was definitely unexpected.”

The pond leak source is Duke’s Dan River Steam Station, opened in 1949 and shuttered in 2012, as the company began replacing coal-fired plants with natural gas-run stations ahead of tighter restrictions by the U.S. Environmental Protection Agency (EPA).

Duke Energy told the Charlotte Business Journal on that the ash ponds it has at eight retired plant sites statewide – including the Dan River and Riverbend locations – are outdated and the company plans to change the way ash is stored at its retired plants.

Along with North Carolina officials, Danville has collected its own samples and sent them for analysis, Barry Dunkley, the local Director of Water and Wastewater Treatment also told The Observer. Danville authorities said water treatment workers have successfully removed ash particles that turned river gray and EPA regulators are onsite.

Officials at North Carolina’s Department of Environment and Natural Resources said drinking water remains unaffected. “Water treated by nearby facilities is safe,” said Department Secretary John Skvarla in a statement.

Duke is nonetheless under heavy fire from environmentalists. “Duke has done nothing to contain the spill,” said Donna Lisenby, a coordinator for the environmental group Waterkeeper Alliance, who visited the site. “There were no hard assets deployed to clean the ash, contain the ash, or stop the spill.”

Frank Holleman, senior attorney with the Southern Environmental Law Center, meanwhile, said the incident will energize a lawsuit already filed against Duke by the Charlotte-based Catawba Riverkeeper Foundation over its ash use. “It’s a failure of Duke’s own mistakes and engineering,” he told The Observer.

In 2012 Duke agreed to a $100,000 settlement with state claims that ash ponds at two of its coal-fired plants polluted groundwater, said a U.S. Securities and Exchange Commission filing quoted by Bloomberg News. Duke is currently working with North Carolina officials for an ash site plan for closed plants, said company officials.