Sources of Hexavalent Chromium in California Groundwater: Natural or Anthropogenic?

February 7, 2019

Hexavalent chromium (chrome-6) has been conventionally assumed to be an anthropogenic contaminant. However, recently published research shows that natural geochemical processes are responsible for chrome-6 in drinking water in many parts of California. Although the maximum contaminant level (MCL) for chrome-6 in California was invalidated in 2017, the State Water Resources Control Board (State Board) plans to "establish a new MCL for chrome-6, which could be at the same level as the invalidated MCL." With new regulations looming, it is important for water purveyors and companies with potential liability to be aware of the impacts of these new scientific findings.

Chromium is a naturally occurring element typically found in the environment in trivalent chemical form (chrome-3); chrome-3 is an essential nutrient for humans and other organisms and is sparingly soluble in water. Chrome-6 on the other hand is often associated with industrial sites because of its widespread use in chrome plating, paints and dyes, leather tanning, and as a corrosion inhibitor, and is a problematic environmental contaminant due to its high solubility in water. The California Environmental Protection Agency designated chrome-6 as a potential oral carcinogen in 1989, while the U.S. Environmental Protection Agency continues to review the developing scientific evidence on pharmacokinetics and mode of action of chrome-6.

Chrome-6 in California groundwater

There are many examples of chrome-6 from industrial sources in groundwater beneath urban areas — e.g., the widespread historical use of chromium has caused regional-scale contamination of alluvial aquifers in the Los Angeles metropolitan area. An often-overlooked source of chrome-6 to groundwater, however, is the natural environment.

Naturally occurring chrome-3 can be oxidized to chrome-6 under specific geochemical conditions common along the west side of the Central Valley and the Coastal Ranges, and naturally occurring chrome-6 has been widely reported across parts of California. Recent studies have identified California drinking water supplies impacted by chrome-6 resulting from anthropogenic, agricultural, and natural sources.

Hausladen (2018) surveyed publicly available data and identified chrome-6 from mostly non-industrial sources in concentrations greater than 10 µg/L in 780 groundwater supply wells (about 7% of 10,642 surveyed wells) across California. In contrast, environmental monitoring wells, which are typically representative of anthropogenic sources, had chrome-6 concentrations as high as 2.9 g/L (2,900,000 µg/L), and 1,335 wells (26% of 5,073 wells) had chrome-6 concentrations above 10 µg/L. Under a scenario where California reinstates a 10 µg/L chrome-6 MCL, these results show that operators of nearly 800 groundwater supply wells would be in violation as a result of natural sources, and over 1,300 monitoring wells would likely require additional review and potential regulatory oversight.

Hausladen also found that concentrations of chrome-6 and chlorinated solvents, including their byproducts and solvent stabilizers, were correlated in monitoring wells, suggesting pollution from metal manufacturing processes. In some cases, data suggested that remediation of chlorinated solvents by in situ chemical oxidation techniques had rapidly increased oxidation rates of naturally occurring chrome-3 to chrome-6. These findings show the importance of considering local geochemistry in the development of in situ remediation approaches to avoid worsening site conditions and suggest that additional monitoring be considered at sites where chrome-3 oxidation to chrome-6 is a potential concern.

In sum, chrome-6 is a multi-faceted issue in California. As new science emerges regarding its carcinogenicity, the MCL in California, and potentially on the federal level, may be lowered to near natural background levels, far below the current total chromium MCL of 50 µg/L. As implied by the Superior Court of Sacramento County's ruling, this may place an undue financial burden on groundwater supply well operators across the state for increased water treatment costs, create additional complications for responsible parties at environmental monitoring sites, and influence remediation alternative feasibility at cleanup sites. As the financial burden is realized following the reinstatement of a chrome-6 MCL, questions of chrome-6 sources to the environment and identification of responsible parties will be front and center.

How Can Exponent Help

Exponent's environmental scientists and engineers can evaluate the contribution of different sources of chromium to the environment, conduct fate and transport evaluations, and assess remediation and water treatment methods. Exponent's health scientists can perform exposure and risk assessments and analyze toxicological and epidemiologic data. In addition, Exponent can assist in evaluating data and information related to rulemaking, regulatory, and permit processes.