Duke Energy and North Carolina regulators became aware that groundwater readings proved conclusive for toxins at dangerous levels back in 2011.

Residents claim Duke was not forced to clean up the area because the studies could not prove the toxins were directly caused by the coal ash. The studies suggested the presence of the toxins could have been naturally occurring.

Three pits containing a combined 6.1 million short tons of ash from the coal-fired boilers of Duke Energy's Buck Steam Station in Dukesville could well have been contaminating groundwater since the power station was built in 1926.

After the 2011 study found the groundwater was polluted those living near the plant said they were never warned and continued using their well water for drinking, bathing and cooking.

Following the massive coal ash spill at another Duke plant, state regulators, environmental activists and Duke officials have been testing the water supplies for some of the 150 homes surrounding the power station.

The state and Duke said their tests did not find any noteworthy problems, but environmental group Waterkeeper Alliance said its investigation showed levels of toxic substances above state standards.

Duke officials said they had seen no evidence that living near its ash pits posed any significant risk and insisted the contaminated groundwater was flowing away from neighbouring properties.

"If we had any indications that we see with concerns to your health, Duke Energy would be proactive," Duke spokesman Erin Culbert told residents at a meeting on Monday.