Drinking water wells in central NC, including Wake and Mecklenburg counties, at increased risk of containing cancer-causing contaminant

The regions in orange and red have the greatest probability of groundwater contaminated with hexavalent chromium. The dots represent the concentrations in individual wells. Although hexavalent chromium is found in coal ash, it is also naturally occurring. The geology of central and some areas of western North Carolina contributes to the element’s presence in groundwater. (Map: Science of the Total Environment)

More than half of the drinking water wells in central North Carolina could contain levels of hexavalent chromium above the state’s health advisory level, according to a new study by Duke University professor and scientist Avner Vengosh.

And wells in Wake and Mecklenburg counties are at heightened risk of containing the known carcinogen.

Although hexavalent chromium — also known as Chromium 6 — is often found near coal ash ponds, it is also naturally occurring within certain geologic belts.

Of 865 wells that Duke researchers tested, 470 exceeded 0.07 parts per billion for hexavalent chromium, the state’s drinking water advisory level. That level is based on a 1-in-1-million cancer risk over a 70-year lifetime. While not enforceable, the state’s benchmark is more stringent than the EPA’s, which is 100 ppb.

Researchers then used this data to develop a computer model that predicted the occurrence of exceedances in drinking water wells based on the their location, including an area’s geology. The model also factored in the acidity, oxygen content, and salinity of the groundwater.

 “The areas where we see the largest number of groundwater users, like Wake and Mecklenburg Counties, coincide with some of the highest probabilities for the occurrence of hexavalent chromium above the health advisory level,” said Rachel Coyte, a doctoral student in Vengosh’s lab and lead author on the study.

Roughly 4 million people in North Carolina rely on groundwater for drinking, cooking and bathing. Private drinking water wells are not regulated by the federal Safe Drinking Water Act. State law requires water from wells drilled within the last 10 years to be tested initially, but there is no requirement for subsequent monitoring. Older wells aren’t required to be tested at all.

“Unlike public groundwater systems, private wells have no testing requirements and therefore many private well owners do not know the concentration of hexavalent chromium in their well water,” Vengosh said. “Since we show that total chromium can be a good proxy for the presence of hexavalent chromium, homeowners can test total chromium, which is more common and cheaper.”

Under-the-sink reverse osmosis systems can remove hexavalent chromium. They cost $150 to $200 per sink.

Some well owners are hesitant to test their wells. Naeema Muhammad, co-director of the NC Environmental Justice Network, recently told an audience at UNC that some low-income households “don’t want to know” what’s in their water because they can’t afford treatment systems on their wells.

“They’re afraid if they can’t treat their water, they’ll have to move” — impractical or even impossible for low-income people, Muhammad said.

Vengosh and Coyte’s findings were published in the journal Science of the Total Environment; the study was funded by a grant from the National Science Foundation.

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