Hinkley, California: Chromium-6 in Drinking Water and What Came After Erin Brockovich

In 1952, Pacific Gas and Electric began operating a natural gas compressor station near the small Mojave Desert community of Hinkley, California. To prevent corrosion in its cooling tower water, PG&E used hexavalent chromium — chromium-6 — the industrially useful but biologically hazardous form of the metal. From 1952 to 1966, Cr(VI)-contaminated wastewater was discharged into unlined ponds that allowed it to percolate into the groundwater.

By the early 1960s, the plume of Cr(VI)-contaminated groundwater had reached the wells serving Hinkley residents. PG&E knew. Internal documents later produced in litigation showed the company was aware of the contamination and its potential health implications. Residents were not told.

The contamination continued until 1966, when the unlined ponds were replaced — but by then, the plume had spread. And for the next three decades, residents of Hinkley drank from wells that contained hexavalent chromium at concentrations that the EPA and California regulators would later identify as significantly elevated above safety guidelines.

Erin Brockovich was a legal clerk at a California law firm when she began investigating PG&E's potential liability for the Hinkley contamination in the early 1990s. Her persistence in gathering documents and interviewing residents — and her ability to build relationships of trust with a community that had been dismissed by authorities — was the foundation of one of the largest environmental tort settlements in US history.

The case — Anderson v. Pacific Gas and Electric Company — was settled in 1996 for $333 million. Approximately 333 Hinkley plaintiffs received compensation, making it the largest settlement ever paid in a direct action lawsuit in US history at that time.

The film and its legacy The 2000 film "Erin Brockovich," starring Julia Roberts, made the case a household name and contributed enormously to public awareness of hexavalent chromium as a health concern. The film was accurate in its broad strokes — the contamination, the cover-up, and the community's fight — while simplifying the scientific questions and the legal complexity.

The scientific controversy The scientific question that the film left ambiguous — whether the Cr(VI) concentrations in Hinkley's water were sufficient to cause the health effects claimed — has been the subject of continued research and controversy. Follow-up studies of Hinkley's population have not consistently found the elevated cancer rates that were alleged in the litigation. This has led some researchers to question whether the scientific claims in the case were overstated, while others point to the difficulty of detecting excess cancer in a small population over limited follow-up time.

Hexavalent chromium — Cr(VI) — is the carcinogenic form of chromium. It is distinct from trivalent chromium (Cr(III)), which is an essential nutrient required in trace quantities for glucose metabolism.

The carcinogenicity evidence IARC classifies Cr(VI) compounds as Group 1 human carcinogens, based on strong evidence from occupational studies. Workers in chromate production, chrome plating, and leather tanning with high Cr(VI) exposure have documented elevated rates of lung cancer — the most consistently associated malignancy.

The evidence for Cr(VI) as a carcinogen through drinking water exposure is more limited. The occupational evidence involves primarily inhalation; the oral route of exposure at the concentrations found in drinking water creates different dose and metabolic considerations. A 2008 National Toxicology Program study found that drinking water Cr(VI) caused gastrointestinal tumours in rodents, providing the animal evidence that strengthened concern about drinking water exposure.

The mechanism Cr(VI) readily crosses cell membranes, where it is reduced to Cr(III) and Cr(II) in a process that generates reactive intermediates capable of forming DNA adducts. The carcinogenic mechanism involves DNA damage and oxidative stress in affected cells.

The current regulatory situation Despite the Hinkley case, the national media attention, and the NTP rodent data, the US EPA has not established a federal MCL for hexavalent chromium specifically — the existing MCL of 100 ppb covers total chromium. California established a state MCL for Cr(VI) of 10 ppb in 2014, making it one of the few jurisdictions in the world with a specific hexavalent chromium drinking water standard.

The absence of a federal drinking water standard for hexavalent chromium — over three decades after Hinkley made it a household name — illustrates how regulatory inaction can persist despite significant public awareness and scientific concern.

The regulatory history The EPA proposed a Cr(VI) MCL in 2011 following the NTP rodent study. The proposed rule faced industry opposition — chromate production and use is industrially significant — and the EPA withdrew the proposed rule in 2020, citing the need for additional cancer dose-response modelling.

The 2023 PFAS rulemaking demonstrated that EPA can move expeditiously on drinking water contaminants when political will is present. The continued absence of a federal Cr(VI) MCL reflects the persistent industrial opposition and the less prominent public advocacy for this standard compared to PFAS.

Current exposures A 2010 Environmental Working Group analysis of tap water data found detectable Cr(VI) in the drinking water of 31 states, with elevated concentrations in several regions beyond Hinkley. Arizona, California, and Oklahoma had the highest Cr(VI) readings in public water systems.

Testing and mitigation The primary mitigation option for Cr(VI) in tap water is reverse osmosis, which effectively removes it. For households in areas with chromate industrial facilities, mining, or natural geologic sources of chromium, Cr(VI) testing of tap water provides the information needed to decide whether filtration is warranted.

PollutionProfile's Water Quality feature includes total chromium data where available from utility monitoring, and flags proximity to industrial chromate sources in the TRI database — providing the starting point for investigating whether your specific water supply warrants Cr(VI) testing.