Duke Energy had removed coal ash from the Dan River near the location in the days after the February incident but on Monday the company began the clean-up of 2500 short tons of coal ash and sediment near the Schoolfield Dam, which it said was the largest deposit found to date.

Monday's work upstream of the dam marked the beginning of major clean-up operations further along the river.

The Dan River Basin Association said it was a lost opportunity.

“We know there are coal ash deposits at Town Creek and down here at Schoolfield Dam,” association program manager Brian Williams told TWC News.

“We also now know it's below this, too, south of Danville.

“The problem is it's not in giant deposits like it used to be, so it took so long to even get this process started that now you've got coal ash spread out 70 miles down the river.”

A Duke Energy spokesman said: “The first order of business after the spill was to stop the leak and ensure drinking water was safe. Then came the lengthy job of surveying the river.”

Up to 39,000t of coal ash spilled into the river but Duke said not all of it could be removed because of the potential risk to fish and wildlife.

Duke expected the clean-up at Schoolfield Dam to wrap up at the end of June but pledged to monitor the entire stretch of the river for years to come.