Photo Courtesy of Waterkeeper Alliance

A mix of coal ash and contaminated water spilled into the Dan River over several days.

A spill from a coal-ash pond at a retired Duke Energy coal plant near Eden, N.C., that turned the Dan River black, then gray, has been plugged, but environmental advocates worry the spill's ecological impact will be far-reaching.

Wet coal ash poured into the Dan River over a period of days, beginning on Feb. 2, as a result of a broken 48-in.-dia stormwater pipe under the plant's primary ash basin.

According to Duke Energy, a catch basin and a series of pipes at the end of the stormwater pipe reduced the amount of sludge that spilled into the river. The North Carolina Dept. of Environment and Natural Resources says that up to 82,000 tons of coal ash and 27 million gallons of contaminated water leaked into the river before a Duke emergency response team was able to successfully plug the pipe on Feb. 8 using concrete grout and a capping system.

State and municipal officials have said that the drinking water is safe, based on a test of the river and the drinking-water supply at the closest drinking-water utility, a Danville, Va., water treatment plant 30 miles away. But the Waterkeeper Alliance says its own testing at a site near the spill showed elevated levels of arsenic, lead and other contaminants that, cumulatively, could have a harmful effect on human health and wildlife.

On Feb. 9, the state environmental agency acknowledged that arsenic levels in two samples were above those considered safe for human health but said contaminant levels appeared to be decreasing.

James Pinkney, a regional spokesman for the U.S. Environmental Protection Agency, says that water-quality sampling by the agency, Duke and the state continues at the spill source and several locations and potable-water intakes downstream. He says EPA expects to have sample results available soon.

Duke Energy officials say they will "use all available resources" to restore the river. Charlie Gates, senior vice president of power generation and operations at Duke Energy, says, "Our next step is to continue to monitor the water quality of the river and to accelerate our planning for the best long-term solution at the site."

The specifics of the remediation effort have yet to be worked out. "Dredging is not impossible, but it would be a very difficult endeavor within that stretch," says Tiffany Haworth, executive director of the Dan River Basin Association. "It's definitely going to be a very long time, if ever, that the river returns to the way it was."