Conservationists and regulators are at odds over how much coal ash was swept away by flooding during Hurricane Matthew.
State inspectors say the ash would fit in the bed of a pickup truck, while a watchdog group argues it's a much larger spill.
Waterkeeper Alliance said it took its boats down the Neuse River earlier this week, collected samples and photographed thick gray muck in the water near the H.F. Lee plant near Goldsboro. They said it appears to be fly ash and is evidence of a large coal ash spill.
Duke Energy acknowledged that the material is a coal-burning byproduct carried off by the floodwaters that flowed over three inactive coal ash basins, but said it's a safe form of the residue.
Duke said it's working with the state to determine if cleanup is necessary.
The Associated Press contributed to this report.