Hexavalent Chromium Sources and Distribution in California Groundwater

Groundwater resources in California represent a confluence of high-risk factors for hexavalent chromium contamination as a result of industrial activities, natural geology, and, potentially, land use. Here, we examine state-wide links in California between groundwater Cr(VI) concentrations and chemicals that provide signatures for source attribution. In environmental monitoring wells, Cr(VI) had the highest co-occurrence and also clustered with 1,4-dioxane and several chlorinated hydrocarbons indicative of the metal plating industry. Additionally, hotspots of Cr(VI) co-occurring with bromoform result from volatile organic compound remediation using in situ chemical oxidation that inadvertently oxidizes naturally occurring Cr(III). In groundwater supply wells, which are typically free of industrial inputs, Cr(VI) correlates with dichlorodiphenyldichloroethylene (DDE), vanadium, and ammonia and clusters with nitrate and dissolved oxygen, suggesting potential links between agricultural activities and Cr(VI). Specific controls on Cr(VI) vary substantially by region: from the metal plating industry around Los Angeles and the San Francisco Bay areas to natural redox conditions along flow paths in the Mojave Desert and to correlations with agricultural practices in the Central Valley of California. While industrial uses of Cr lead to the most acute cases of groundwater Cr(VI) contamination, oxidation of naturally occurring Cr affects a larger area, more wells, and a greater number of people throughout California.