HEXAVALENT CHROMIUM DISTRIBUTION AND CO-OCCURRENCE WITH OTHER HEALTH RELEVANT OXYANIONS IN GROUNDWATER FROM THE PIEDMONT REGION OF NORTH CAROLINA

This study investigates the distribution and mobilization of hexavalent chromium in groundwater from North Carolina’s Piedmont region. The study is based on data from 889 wells in a variety of lithologies throughout the region. Over half of all wells measured in the Piedmont violated the North Carolina Department of Health and Human Services (NCDHHS) screening level of 0.07 µg/L for Cr(VI). Within the igneous and metamorphic rocks of the Piedmont itself, most mobilization of Cr(VI) occurs in formations with mafic or intermediate character, which is consistent with observations in other locations. However, we also find several instances of significant mobilization (Cr concentrations up to 8 µg/L) from granitic and other aquifers of more felsic character. Non-radiogenic ⁸⁷Sr/⁸⁶Sr (0.703607 to 0.706847) ratios suggest that in most of these cases, either the aquifer has more mafic character locally than geologic mapping indicate, or that the water moved into the area after interacting with mafic material elsewhere.

Additionally, we examine the co-occurrence of hexavalent chromium with arsenic, vanadium and uranium. Aquifers from the Piedmont region contain significantly higher concentrations of all of the investigated health-relevant oxyanions than groundwater from semi-consolidated and unconsolidated sand and gravel aquifers from the coastal plain. The co-occurrence of oxyanions in groundwater may pose human health risks beyond the effect of individual contaminants. Uranium is found at health relevant concentrations in North Carolina almost exclusively in aquifers composed of granitic rocks which are unlikely to bear significant amounts of Cr, As, or V. While Cr and As are often found at health relevant levels in the same lithology, notably in the Carolina Slate Belt, they do not often co-occur at levels beyond health guidelines, due largely to redox factors. Our data show that vanadium and Cr are most mobile in similar redox environments and are derived from similar source rocks. Though the relationship is not linear, V and Cr frequently co-occur in groundwater at health relevant concentrations. Given the large numbers of homeowners reliant on well water, the elevated concentrations of oxyanions beyond health screening levels and their co-occurrence in certain aquifers could pose risks for millions in the Piedmont region of the U.S.