A contaminant at the center of a months-long furor over coal ash and polluted wells doesn’t come from ash after all, Duke University scientists report in a study published Wednesday.
Hexavalent chromium turns out to be surprisingly widespread in North Carolina’s Piedmont, the study says, and occurs naturally.
The toxic form of chromium was detected last year in private wells near Duke Energy’s coal-fired power plants and suspected of coming from the ash stored there. Hundreds of well owners were warned not to use their wells, in part because hexavalent chromium in drinking water might cause cancer.
The study published in the journal Environmental Science and Technology Letters may lay only part of the controversy to rest. Hexavalent chromium in water remains a threat, experts say, no matter the source. About half of North Carolina’s 10 million residents rely on wells for drinking water.
Duke Energy, which has insisted that data shows its coal ash has not harmed private wells, claimed vindication.
“When combined with previous research, there is overwhelming evidence that coal ash basins are not impacting water quality in neighbor wells,” said Harry Sideris, senior vice president of environmental, health and safety. “This study is an extraordinary development, particularly for hundreds of plant neighbors who have been needlessly concerned that ash basins contributed hexavalent chromium or other substances to their wells.”
Duke said it will still abide by a state law that requires it to provide alternative water sources to owners of contaminated wells near its plants.
Hexavalent chromium rose to public notice with the 2000 movie “Erin Brockovich,” a dramatization of a real case in which contaminants released by a California utility polluted the groundwater under a small town.
Industrial releases were long thought to be a major source of hexavalent chromium in the environment. But that’s beginning to change. The American Water Works Association, which represents water utilities, says chromium in geologic formations is the major source in drinking water.
The Duke study found that hexavalent chromium leaches from volcanic rock into groundwater under much of North Carolina’s Piedmont. Because similar formations are found across the Southeast, said lead author Avner Vengosh, millions of people outside North Carolina could be at risk without knowing it.
The researchers collected water from 376 wells both near and far from coal ash ponds. Hexavalent chromium was found in about 90 percent of the wells, some at levels considered unsafe in drinking water.
Then they used tracers developed by Vengosh’s team to identify geochemical “fingerprints” that can trace contaminants to their source.
Contaminants leaking from coal ash present a distinct geochemical profile of elements such as boron, strontium and arsenic, Vengosh said. When his team found water with hexavalent chromium in it, he said, “we see a totally different chemistry.”
Finding high levels of the contaminant over a large area, regardless of how close to ash ponds, supports the conclusion that ash isn’t the source, he said.
The findings don’t mean ash ponds are benign, Vengosh said. Previous research has found other contaminants including arsenic, a carcinogen, and selenium leaking from them.
North Carolina’s Department of Environmental Quality has not explicitly stated whether the hexavalent chromium detected near Duke’s power plants came from natural sources, but offered hints last year.
DEQ tested 24 private wells that are near Duke’s plants but too far away to be affected by its ash ponds. Elevated levels of hexavalent chromium were found in 12 of the wells, and the agency recommended that seven others be retested for the chemical. Duke Energy tested about 200 of its employees’ wells with similar results.
Duke Energy said the “clear evidence” from multiple sources shows that “it’s time to move forward with safely closing ash basins in ways that protect people, the environment and wallets.”
Vengosh said the research also points to the need for states to set water safety standards for hexavalent chromium. Only California, where it also occurs naturally, has a state standard.
Arsenic, which is found in ash, also occurs naturally in the Carolina Slate Belt, a geologic formation that arcs across the Piedmont. North Carolina has a groundwater standard for arsenic but includes hexavalent chromium in the standard for total chromium.
“From a public health standpoint, it doesn’t matter whether it’s naturally occurring or from a man-made source. The toxicity risk is the same and the cancer risk is the same,” state toxicologist Kenneth Rudo said of the two contaminants.
The federal water standard for chromium also includes its hexavalent form, but the Environmental Protection Agency is reviewing health studies to decide whether hexavalent chromium merits a separate standard.
North Carolina’s DEQ said it will adopt new or revised water standards when the federal government does. The department said it will continue groundwater tests to establish concentrations of hexavalent chromium that represent natural conditions.