The coal ash poured out of a broken pipe into the Dan River, turning water into dark muck.
It took nearly a week to stem the spill, which sent millions of gallons of sludge from a retired power plant into a river that supplies drinking water to communities in North Carolina and neighboring Virginia.
Workers stopped the spill by plugging the broken pipe with concrete this weekend. Now government scientists and the United States' largest electric utility face a daunting task: cleaning it up.
Tests since the spill have turned up higher levels of harmful chemicals such as arsenic in the river. But so far, officials say tap water is safe to drink.
Some environmental activists in the area say they aren't so sure. They fear the consequences for wildlife and say that the situation shows state regulators haven't done enough to crack down on Duke Energy.
The utility has apologized for the spill and vowed to clean up any damage.
"We're committed to the Dan River and the communities that it serves," Charlie Gates, the company's senior vice president of power generation operations, said in a statement Saturday. "We are accountable for what has happened and have plenty of work ahead of us."
Concerns over drinking water, wildlife
Duke Energy announced last week that it found the leak in a 48-inch stormwater pipe at the retired Dan River Steam Station in Eden, North Carolina, on February 2.
On Saturday, six days later, the company said it had plugged the broken pipe that was causing the spill and was working with officials on developing a cleanup plan.
Coal ash, the material that remains after burning coal for electricity, contains metals such as arsenic, selenium and cadmium.
Tests of the river last week revealed levels of copper, aluminum, iron and arsenic above state standards for surface water, state environmental officials said.
It's unclear what that could mean for wildlife in the area, said Jamie Kritzer, a spokesman for the North Carolina Department of Environment and Natural Resources.
"It's certainly cause for concern for the long-term impacts of this coal ash spill on the health of the Dan River," he said.
Environmental advocates warn that the damage could be significant, potentially harming fish in the river and impacting the food chain.
"You have a cleanup effort that is going to be difficult," said Sam Perkins, who works for the Catawba Riverkeeper Foundation, a nonprofit advocacy group dedicated to protecting waterways in the area. "This shows even a small spill has an impact on the ecosystem."
Some activists accuse authorities of deliberately playing down the danger of the situation and taking too long to notify the public.
"This is another shameless attempt by (environmental officials) to downplay the risks facing the communities along the Dan River downstream," Peter Harrison of the Waterkeeper Alliance advocacy group said in a written statement to CNN on Friday.
"Are we supposed to feel good that there are only four hazardous toxins, including a carcinogen, in our drinking water supply?"
Samples taken by the Waterkeeper Alliance last week contained "extremely high levels of arsenic, chromium, iron, lead and other toxic metals," the group said in a statement.