Hexavalent chromium (Cr(VI)) is a groundwater contaminant that is potentially harmful to human health. The co-occurrence of Cr(VI) and other oxyanion-forming elements like arsenic (As), vanadium (V), and uranium (U) in drinking water may pose combined and enhanced risks to human health beyond the effects of the single contaminants. Understanding the occurrence of Cr(VI) and other oxyanion-forming elements in groundwater resources is critical for evaluating their risks to human health. This NSF project aims to evaluate the occurrence of Cr(VI), As, V, and U in groundwater across North Carolina, and the factors that control their distribution in groundwater. The study is based on a large dataset (n=1494) of water quality including Cr(VI) and other oxyanion-forming elements concentrations in public, private, and monitoring wells from different aquifers across North Carolina. These water quality data come from new and previous measurements conducted at Duke University, as well as data reported by the U.S. Environmental Protection Agency, the North Carolina Department of Environmental Quality, and the U.S. Geological Survey. Through geospatial and geochemical analysis, this project evaluates the effect of aquifer lithology and geochemical conditions on the prevalence of the oxyanion-forming elements in groundwater. The data show that Cr(VI) is the predominant species of dissolved Cr and the oxyanion-forming elements occur mostly in groundwater from fractured igneous and metamorphic formations throughout the Piedmont and Blue Ridge regions, with different concentration distributions in different lithologies and higher concentrations than groundwater from the coastal plain. Additionally, groundwater pH and oxidation state were found to be important factors governing the occurrence of these redox sensitive elements in the groundwater. Though there is only one exceedance of the U.S. EPA Maximum Contaminant Level (100 ppb for total chromium) in the dataset, over half of all wells measured for Cr(VI) (470 out of 865) in the dataset exceeded the North Carolina Health Advisory Level of 0.07 ppb.  Using information from this dataset, the study explores three different approaches to predicting Cr(VI) in groundwater: (1) total chromium concentrations as a proxy for Cr(VI); (2) exceedance probabilities of health goals for groundwater from aquifers located in specific geologic areas; and (3) censored linear regression using commonly measured field parameters (pH, electrical conductivity, dissolved oxygen) with relationships to Cr(VI) as regressors. Combining these approaches, this study has identified several areas in the Piedmont region where Cr(VI) in drinking water wells is expected to be higher than the advisory level, which coincide with large population groundwater reliant populations. The study also found that rates of elements’ co-occurrence at concentrations deemed potentially harmful to human health varied by element combinations, with V and Cr co-occurring the most of any combination because of similarities in source rock and chemical behavior. The degree of co-occurrence presented in this study for different types of aquifer lithologies demonstrates the potential of additional risks to individuals living in the fast-growing Piedmont region of the eastern U.S. While this study focuses on North Carolina, the wide-spread occurrence of Cr(VI) and other toxic oxyanion-elements in groundwater at concentrations above health guidelines in aquifers of the Piedmont region could pose high human health risks to large populations in the eastern U.S. 

Last Modified: 11/07/2019
Modified by: Avner Vengosh