The Department of Environment and Natural Resources believes much of the danger from the spill has passed after 39,000 tons spewed from one of Duke’s retention ponds on February 2, 2014 – despite conceding that less than 10% of it had been removed.

"Certainly, the indications from the testing indicate that things have come back remarkably quickly, and we'll continue the monitoring to make sure that, in fact, is happening," DENR Secretary Donald van der Vaart said, according to wral.com.

The rest of the coal ash remaining is spread over a 120km stretching from Eden, winding across the Virginia-North Carolina border four times before reaching Milton. Coal ash, the waste from coal burned to generate electricity, contains toxic arsenic, selenium, chromium and mercury.

Locals have reported seeing a sliver slick after the spill that obscured the Dan River's normal light brown colour for days. That ash has been sandwiched between a layer of clay and sediment already on the bottom of the river and a new layer of sediment that has settled since the spill.

Although it is now covered, the coal ash is not hard to find with a simple probe.

Duke was issued notices for violations after regulators found issues at other of its coal ash dumps across the state after North Carolina environmental officials initially issued the company two formal notices for violating wastewater and storm water regulations.

How much Duke ends up paying will depend on what, if any, long-range damage was done by the ash spilled into the river – which officials say is on the mend despite barely 10% of it being recovered.

"I am glad to say that the river is thriving, and there's hard science that tells us so," Duke Energy's president for North Carolina operations, Paul Newton, said last week.

The high readings for arsenic, lead and aluminum revealed from tests on water samples collected downstream of the spill immediately after the spill dropped within days after the ash settled to the river bottom in drifts as thick as five feet.

The Feds have got involved, with US Environmental Protection Agency supervising Duke-hired contractors as they vacuum out the largest pockets, recovering less than 8% of the ash before stopping in July.

North Caroline Division of Water Resources determined in October that the size and diversity of the populations both upstream and downstream from the spill were similar after sampling aquatic insects and other small animals in the river.

And while a second study by an environmental consulting firm that Duke hired, said in November that the river's mussels were flourishing, other scientists and environmentalists have warned it could take years to know the true long-range impact on the river.

The Dan River is home to two federally listed endangered species, the Roanoke logperch fish and the James spinymussel.