(WRAL) -- Environmental activists say they have proof of a coal ash leak into North Carolina's Neuse River during the flooding after Hurricane Matthew, but Duke Energy says the substance visible on the water is not coal ash and is not toxic.

Pete Harrison of the Waterkeeper Alliance has been collecting evidence along the river near three inactive coal ash pits at the HF Lee Steam Plant in Goldsboro, where a white, powdery substance coats trees and still waters near the river's banks.

"I think what this shows us is it's inherently dangerous to store coal ash in unlined pits in the flood zones of major rivers," Harrison says.

Duke spokesman Jeff Brooks says the white substance ash at all. It is what's called "cenospheres."

"They freely exist on top of our ash basins. They are harvested on a regular basis from the ash basins so they are not inherently toxic by nature," he said.

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