Chromium-6 (Hexavalent Chromium) in Drinking Water
Chromium-6 (hexavalent chromium, or Cr-6) is a carcinogenic form of chromium that occurs both naturally from serpentinite rock weathering and from industrial contamination. It was brought to widespread public attention by the Hinkley, California case documented by Erin Brockovich. The EPA has no specific federal MCL for chromium-6 — only a combined 100 ppb limit for total chromium. California attempted to set a state-specific 10 ppb limit for Cr-6 before it was rescinded following industry litigation.
Quick Answer
Chromium exists in water in two primary forms: trivalent chromium (Cr-3), an essential trace nutrient, and hexavalent chromium (Cr-6), a known carcinogen at elevated concentrations. Chromium-6 is classified as a human carcinogen by the International Agency for Research on Cancer (IARC Group 1) when inhaled, and as a probable human carcinogen via ingestion. It enters water from natural weathering of chromite ore and serpentinite rock formations — widespread in California's Coast Ranges — and from industrial sources including chrome plating operations, leather tanning, and stainless steel manufacturing. Chromium-6 contamination from industrial sources can reach levels orders of magnitude above natural background.
Why Is Chromium-6 in Drinking Water a Concern?
Despite being a known carcinogen, chromium-6 has no specific federal drinking water limit in the U.S. The EPA's MCL covers total chromium at 100 ppb — a standard designed primarily around trivalent chromium. A 2008 National Toxicology Program study found dose-dependent increases in gastrointestinal tumors in rodents drinking chromium-6 water. The Environmental Working Group estimates that over 200 million Americans drink water containing detectable chromium-6, with 66 million drinking water above the level EWG considers protective (0.02 ppb). California's attempt to regulate Cr-6 at 10 ppb was struck down in 2017 after industry litigation challenged the cost-benefit analysis methodology; a revised regulation is in development.
Communities near chrome plating shops, leather tanneries, and stainless steel facilities face the highest industrial exposure. Natural exposure is highest in California (Coast Ranges, Central Valley, Mojave Desert), parts of New Mexico, Arizona, Nevada, and Oregon where serpentinite and chromite-bearing rock formations are common. Well owners near these formations or industrial sites are particularly at risk. See the [California well water guide](/well-water/california) and [Nevada well water guide](/well-water/nevada) for state-specific context.
Health Effects of Chromium-6 in Drinking Water
Gastrointestinal cancer risk with long-term ingestion — NTP rodent studies showed dose-dependent tumor formation
IARC Group 1 human carcinogen via inhalation; probable carcinogen via ingestion
Liver and kidney toxicity at high chronic exposure levels
Skin irritation and allergic dermatitis with direct contact (industrial settings)
No established safe ingestion threshold — the carcinogenic risk is considered dose-dependent with no zero-risk level
At levels found in naturally contaminated groundwater (1–100 ppb), long-term cancer risk is the primary concern
How Does Chromium-6 Get Into Drinking Water?
Natural weathering of serpentinite and chromite rock formations — primary source in California Coast Ranges, Central Valley, and parts of the Southwest
Chrome plating operations — historically a major source of industrial contamination in urban areas
Leather tanning facilities
Stainless steel manufacturing and welding
Wood preservation operations (legacy contamination)
Cooling tower water treatment using chromate corrosion inhibitors (largely phased out)
Regulatory Limit
EPA Maximum Contaminant Level (MCL)
100 ppb (total chromium)
The EPA's MCL of 100 ppb covers total chromium (all forms combined), not chromium-6 specifically. This standard was set in 1991 when trivalent chromium, not hexavalent chromium, was the primary concern. The MCL has not been updated to reflect the carcinogenicity evidence for Cr-6. In 2014, the EPA listed chromium-6 as a candidate for a standalone MCL under its regulatory agenda; the process has moved slowly. California set a Cr-6-specific MCL of 10 ppb in 2014, which was rescinded in 2017 after industry legal challenges. The current absence of a federal Cr-6 MCL means utilities can legally report 'no violation' for total chromium while having Cr-6 levels that some health researchers consider concerning.
How to Test for Chromium-6 in Your Water
Standard utility water tests measure total chromium — you must specifically request chromium-6 (hexavalent chromium) testing to get Cr-6 data. California utilities are required to test for and report Cr-6 separately. In other states, check whether your utility tests for Cr-6 specifically, or order a certified lab test that specifies hexavalent chromium analysis. The EPA's UCMR and ECHO databases contain monitoring data for public systems. Well owners in California, Nevada, and other high-risk states should include Cr-6 in comprehensive well testing panels.
How to Remove Chromium-6 from Drinking Water
These treatment methods have demonstrated effectiveness for Chromium-6 removal.
Frequently Asked Questions
Related Pages
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.
Quick Reference
Category
Heavy Metals
Risk Level
high
EPA Limit
100 ppb (total chromium)
Most at Risk
Communities near chrome plating shops, leather tanneries, and stainless steel facilities face the highest industrial exposure.
Well Water Relevant
Yes
Chromium-6 by State
Treatment Options