Origin of Hexavalent Chromium in Drinking Water Wells from the Piedmont Aquifers of North Carolina

Hexavalent chromium [Cr­(VI)] is a known pulmonary carcinogen. Recent detection of Cr­(VI) in drinking water wells in North Carolina has raised public concern about contamination of drinking water wells by nearby coal ash ponds. Here we report, for the first time, the prevalence of Cr and Cr­(VI) in drinking water wells from the Piedmont region of central North Carolina, combined with a geochemical analysis to determine the source of the elevated Cr­(VI) levels. We show that Cr­(VI) is the predominant species of dissolved Cr in groundwater and elevated levels of Cr and Cr­(VI) are found in wells located both near and far (>30 km) from coal ash ponds. The geochemical characteristics, including the overall chemistry, boron to chromium ratios, and strontium isotope (⁸⁷Sr/⁸⁶Sr) variations in groundwater with elevated Cr­(IV) levels, are different from those of coal ash leachates. Alternatively, the groundwater chemistry and Sr isotope variations are consistent with water–rock interactions as the major source for Cr­(VI) in groundwater. Our results indicate that Cr­(VI) is most likely naturally occurring and ubiquitous in groundwater from the Piedmont region in the eastern United States, which could pose health risks to residents in the region who consume well water as a major drinking water source.