RALEIGH, N.C. - University researchers say a cancer-causing heavy metal found in water wells near coal ash pits and other industrial sites is much more widespread and naturally occurring than previously thought.

Duke University geochemistry professor Avner Vengosh said Wednesday the presence of hexavalent chromium is more related to a kind of volcanic rock found in North Carolina and nearby states than the pits used to store the waste left after burning coal.

Vengosh says the findings mean it's urgent that the U.S. Environmental Protection Agency set safety limits for hexavalent chromium in drinking water.

The chemical is at the centre of concerns about whether massive coal ash basins are polluting nearby groundwater. Vengosh says the study doesn't mean other chemicals aren't leaking from coal ash dumps.