Contaminant Guide

Chromium in Well Water

Chromium exists in two primary forms in water: trivalent chromium (Cr-III), an essential mineral at trace levels, and hexavalent chromium (Cr-VI), a known human carcinogen. The federal MCL covers total chromium but was set before the carcinogenicity of ingested Cr-VI was established. California adopted a specific Cr-VI MCL of 10 µg/L in 2024, reflecting updated science.

What is chromium in well water?

Chromium is a naturally occurring metal found in soil and rock. Two forms matter in drinking water:

  • Trivalent chromium (Cr-III) — an essential trace mineral involved in glucose metabolism; much less toxic than Cr-VI
  • Hexavalent chromium (Cr-VI, chromate) — a known human carcinogen classified as IARC Group 1 for inhalation; classified as a probable human carcinogen (IARC Group 2A) via ingestion; associated with stomach cancer in high-exposure populations

Standard laboratory analysis for "total chromium" does not tell you which form is present. If your total chromium is elevated or you are near an industrial site, request hexavalent chromium analysis specifically.

Where is chromium most common?

Naturally occurring chromium is most common in California, Arizona, Nevada, Texas, and North Carolina — areas with chromite ore deposits or ultramafic rock. Industrial contamination (chrome plating shops, leather tanning, metal finishing, industrial cooling towers that used chromate corrosion inhibitors) has created point-source plumes in many states. The chromate plume in Hinkley, California — the subject of the Erin Brockovich case — was an industrial Cr-VI contamination.

Health effects

  • Stomach cancer from hexavalent chromium (Cr-VI) — Occupational inhalation exposure causes lung cancer (IARC Group 1). For ingestion, epidemiological studies in populations with high Cr-VI in drinking water show elevated stomach cancer risk. California's Office of Environmental Health Hazard Assessment established a public health goal of 0.02 µg/L for Cr-VI.

The EPA limit: MCL = 100 µg/L, MCLG = 100 µg/L

The federal MCL for total chromium is 100 µg/L — unchanged since 1991. The MCLG equals the MCL at 100 µg/L, set before the carcinogenicity of ingested Cr-VI was established. This MCL is widely considered scientifically outdated. California's Cr-VI-specific MCL of 10 µg/L (effective 2024) reflects the updated risk assessment and is ten times stricter than the federal total chromium standard.

Testing

Request both total chromium and hexavalent chromium analyses. Total chromium by ICP-MS (EPA Method 200.8); hexavalent chromium requires separate analysis by EPA Method 218.6 or 7196A. If total chromium is below 10 µg/L, the hexavalent fraction is not a significant concern. If total chromium is above 10 µg/L or you are near an industrial site, Cr-VI speciation is important.

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Treatment

  • Reduction to Cr-III then coagulation-filtration — standard approach for Cr-VI removal; requires chemical addition (reducing agent such as ferrous sulfate or sodium bisulfite) followed by precipitation
  • Strong-base anion exchange — highly effective for Cr-VI (chromate is an anion); used in point-of-entry systems
  • Reverse osmosis (RO) — removes both Cr-III and Cr-VI effectively at point of use
  • Activated alumina — less effective for Cr-VI than anion exchange

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Chromium in municipal water

Municipal water systems must comply with the federal total chromium MCL of 100 µg/L. California systems must now also comply with the Cr-VI-specific MCL of 10 µg/L. Your utility's Consumer Confidence Report (CCR) will show total chromium levels; Cr-VI may be reported separately in California.

If you are in an area with known Cr-VI groundwater contamination (southern California's San Gabriel Valley, parts of the San Joaquin Valley), check whether your utility has installed Cr-VI-specific treatment. If you have concerns, a point-of-use RO filter provides an additional treatment barrier for both total chromium and Cr-VI.

Learn how to read a Consumer Confidence Report

Regulatory framework

Federal MCL: 100 µg/L (total chromium). Federal MCLG: 100 µg/L. The MCLG=MCL is a legacy of the 1991 rulemaking, when ingested Cr-VI carcinogenicity was not established. EPA initiated a chromium MCL review in 2008 and 2011 but no revision has been finalized at the federal level.

California Cr-VI MCL: 10 µg/L (finalized 2023, effective 2024) — the first Cr-VI-specific drinking water regulation in the U.S., based on a public health goal of 0.02 µg/L derived from a NTP cancer bioassay. The MCL of 10 µg/L is set above the PHG at the feasibility limit.

Detection

Total chromium: ICP-MS (EPA Method 200.8) or ICP-OES (Method 200.7). Hexavalent chromium: colorimetric method using 1,5-diphenylcarbohydrazide (EPA Method 7196A or 218.6). Cr-VI must be analyzed separately; total chromium analysis cannot differentiate speciation. Field preservation with buffer required to prevent Cr-VI reduction during sample transport. Certified lab required.

Geochemistry

Chromium speciation is controlled by pH and redox conditions. Under oxidizing, alkaline conditions (common in arid-region groundwater), Cr-III is oxidized to Cr-VI by manganese oxides — the form mobilized in natural chromium occurrences in California. Under reducing conditions, Cr-VI is reduced to insoluble Cr-III, limiting mobility. Industrial Cr-VI contamination (chromate salts) bypasses this natural attenuation.

Data access

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References

  1. NTP (National Toxicology Program). (2008). Toxicology and carcinogenesis studies of sodium dichromate dihydrate in F344/N rats and B6C3F1 mice. NTP Technical Report Series 546. https://ntp.niehs.nih.gov/ntp/htdocs/lt_rpts/tr546.pdf
  2. Stern, A.H. (2010). A quantitative assessment of the carcinogenicity of hexavalent chromium by the oral route and its relevance to human exposure. Environmental Research, 110(8), 798-807. https://doi.org/10.1016/j.envres.2010.09.006