Hexavalent Chromium in Drinking Water Continues to Cause Concern
In 1996, the town of Hinkley, California sued Pacific Gas & Electric, alleging that contamination of drinking water with hexavalent chromium (Cr-6) from a cooling tower caused widespread illness in residents. The case was settled for $333 million, the largest settlement paid in a direct-action lawsuit to that point—and the basis for the Oscar-winning film "Erin Brockovich."
As of 2019, average Cr-6 levels for well water in Hinkley were still peaking at 100 times the maximum contaminant level (MCL) allowed in California, the only state with a standard for chromium. However, due to studies showing that drinking water may cause cancer, concerns about Cr-6 have escalated.
Let's take a closer look at chromium, the history of its assessment in drinking water, and the standards that may result from further evaluation of Cr-6 exposure.
A Primer on Chromium
Chromium occurs naturally in various states, including trivalent chromium, an essential nutrient. But in the environment, Cr-6 is rare unless manmade and associated with industrial processes. For example, the Cr-6 in Hinkley occurred at a compressor station where wastewater discharged to unlined ponds percolated into the groundwater.
Cr-6 is an established human carcinogen in specific occupational settings as a result of inhalation exposure, and compounds have also been found in drinking water. In 1998, the Environmental Protection Agency (EPA) released an Integrated Risk Information System (IRIS) toxicological assessment of Cr-6. Ten years later, the National Toxicology Program found a significant increase in stomach and intestinal tumors in rats and mice that consumed Cr-6 in drinking water.
The EPA completed a draft health assessment in 2000 and concluded that relatively low doses of Cr-6 could increase cancer risk. More recently, the agency provided a systematic review of hundreds of studies on cancer and multiple noncancer health effects of inhalation and oral exposure to Cr-6. And in October 2023, EPA's Science Advisory Board released a final report, confirming Cr-6 in drinking water was likely carcinogenic.
History of Chromium in Drinking Water
In 1991, the national primary drinking water regulation (NPDWR) that established the MCL for total chromium of 0.1 mg/l or 100 parts per billion (ppb) was promulgated. But that's based on the potential for skin irritation only, not cancer. Cr-6 and Cr-3 are jointly covered under the total chromium drinking water standard because these forms of chromium can convert back and forth in water and in the human body, depending on environmental conditions.
The Safe Drinking Water Act requires EPA to periodically review the NPDWR for each contaminant and revise the regulation, if appropriate. In March 2010, EPA noted that it had initiated a reassessment of the chromium exposure risk but felt it inappropriate to revise the NPDWR while that effort was in process.
To learn more about the levels of Cr-6 in drinking water, EPA required select water systems to perform Cr-6 monitoring for one year between 2013 and 2015 under the Third Unregulated Contaminant Monitoring Rule (UCMR 3). The UCMR 3 required many but not all public water systems to monitor Cr-6 to better understand its impact on drinking water. The work anticipated a lower, more stringent regulatory level for chromium.
Extent of Chromium Contamination
An Environmental Working Group analysis of federal data from nationwide drinking water tests showed that Cr-6 contaminates water supplies for more than 200 million Americans in all 50 states, Puerto Rico, the Northern Mariana Islands, and Guam. States reporting levels above health guidelines include California, Florida, Illinois, Texas, and New York.
In 2011, California's Office of Health Hazard Assessment set a public health goal for Cr-6 in tap water of 0.02 ppb—the level expected to cause no more than one case of cancer in one million people who drink it for a lifetime. California drafted an MCL for chromium-6 in drinking water at 10 ppb, which is 500 times the state's public health goal. This MCL was struck down in court in 2017, and the state's water board is preparing a new regulation.
Until a national drinking water standard for Cr-6 is implemented, consumers can learn about the total chromium in their drinking water by requesting a copy of the Consumer Confidence Report (CCR). The CCR will also have information for water systems that were UCMR-selected to monitor Cr-6 precisely. In addition, water quality test labs play a crucial role in ensuring health by testing levels of Cr-6 in drinking water and recommending filtration where levels exceed public health goals.
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