What Is Chromium and Where Does It Come From?

Chromium is a naturally occurring element found in rocks, soil, volcanic emissions, and natural waters. It exists in several oxidation states, but the two most significant in water are trivalent chromium (Cr III) and hexavalent chromium (Cr VI). These two forms have vastly different chemical properties, toxicity, and environmental behavior.

Trivalent chromium (Cr III) is relatively insoluble in water at neutral pH and is the predominant form in most natural waters. It is an essential trace nutrient that plays a role in glucose metabolism. Hexavalent chromium (Cr VI) is more soluble, more mobile in groundwater, and significantly more toxic than the trivalent form.

Natural sources of chromium in water include the weathering of chromite (FeCr2O4) ore deposits, which are found in ultramafic and serpentine geological formations. In California, natural hexavalent chromium in groundwater occurs in areas underlain by these formations.

Industrial sources are the primary contributors to hexavalent chromium contamination. These include chrome electroplating, stainless steel and alloy manufacturing, leather tanning (which uses chromium sulfate), wood preservation using chromated copper arsenate (CCA), pigment production, and cooling tower water treatment (which historically used chromate-based corrosion inhibitors). The Hinkley, California contamination case, brought to public attention through the advocacy of Erin Brockovich, involved hexavalent chromium from a natural gas compressor station that contaminated local groundwater.

Health Effects of Chromium in Water

The health effects of chromium depend critically on the oxidation state. Trivalent chromium is considered an essential nutrient with low toxicity. Hexavalent chromium, however, is a known human carcinogen when inhaled (classified as Group 1 by IARC) and is considered a likely carcinogen when ingested based on animal studies and mechanistic evidence.

Ingestion of hexavalent chromium in drinking water has been associated with increased incidence of gastrointestinal tumors in laboratory animals. The National Toxicology Program (NTP) found clear evidence of carcinogenic activity of hexavalent chromium in drinking water in rats and mice.

Non-cancer health effects of hexavalent chromium exposure include gastrointestinal irritation, liver and kidney damage, and dermatitis. Hexavalent chromium is a potent skin sensitizer, and contact with chromium-contaminated water can cause allergic dermatitis in sensitized individuals.

The California Environmental Protection Agency's Office of Environmental Health Hazard Assessment (OEHHA) established a Public Health Goal (PHG) of 0.02 micrograms per liter (0.00002 mg/L) for hexavalent chromium in drinking water based on cancer risk assessment.

Regulatory Limits for Chromium in Drinking Water

The EPA MCL of 0.1 mg/L for total chromium was established in 1991. The EPA has been reviewing whether to establish a separate standard for hexavalent chromium based on growing evidence of its oral carcinogenicity. California is the only U.S. state with a specific hexavalent chromium MCL.

How to Test for Chromium in Water

Total chromium is measured by EPA Method 200.8 (ICP-MS) or EPA Method 200.7 (ICP-OES). Hexavalent chromium requires a separate analysis using EPA Method 218.6 (ion chromatography) or EPA Method 218.7, which specifically quantifies the Cr(VI) species.

It is important to request hexavalent chromium analysis specifically, as standard total chromium tests do not distinguish between the trivalent and hexavalent forms. Samples for Cr(VI) analysis require specific preservation protocols (pH adjustment with ammonium sulfate buffer) to prevent oxidation-state changes during transport and storage.

Treatment Methods for Chromium Removal

Reverse Osmosis

Reverse osmosis systems effectively reject both hexavalent and trivalent chromium, achieving removal rates of 90-98%. RO is one of the most reliable technologies for hexavalent chromium removal and is suitable for both point-of-use and large-scale applications.

Ion Exchange

Strong base anion exchange resins effectively remove hexavalent chromium (present as chromate or dichromate anions) from water. Ion exchange systems can achieve effluent Cr(VI) concentrations well below 0.010 mg/L. Regeneration with sodium chloride brine produces a concentrate stream that requires proper management.

Reduction and Precipitation

Chemical reduction of hexavalent chromium to trivalent chromium using ferrous sulfate, sodium bisulfite, or other reducing agents, followed by precipitation at elevated pH and filtration, is the standard approach for industrial wastewater treatment. Treatment chemicals for chromium reduction are available for industrial applications.

Adsorptive Media

Iron-based adsorptive media can remove hexavalent chromium through a combination of reduction to Cr(III) and adsorption onto the media surface. This technology is used in point-of-use and point-of-entry applications and does not require chemical regeneration.

Stannous Chloride Reduction

Stannous chloride (tin(II) chloride) can reduce hexavalent chromium to trivalent chromium at the point of entry. This approach has been demonstrated in California water systems and is being evaluated as a compliance technology for hexavalent chromium standards.

Frequently Asked Questions

What is the difference between chromium-3 and chromium-6?

Trivalent chromium (chromium-3 or Cr III) is an essential trace nutrient found naturally in foods and is relatively non-toxic. Hexavalent chromium (chromium-6 or Cr VI) is the toxic form, classified as a known human carcinogen when inhaled and a likely carcinogen when ingested. The EPA MCL of 0.1 mg/L applies to total chromium (both forms combined).

What is the EPA limit for chromium in drinking water?

The EPA MCL for total chromium in drinking water is 0.1 mg/L (100 ppb). This standard covers both trivalent and hexavalent forms combined. There is currently no separate federal MCL for hexavalent chromium, though California established a state MCL of 0.010 mg/L (10 ppb) specifically for hexavalent chromium.

How widespread is hexavalent chromium contamination?

Hexavalent chromium is more widespread than previously recognized. A nationwide study found hexavalent chromium in drinking water serving over 200 million Americans. Natural sources include the weathering of chromite ore deposits. Industrial contamination from chrome plating, tanneries, and chemical manufacturing has created localized hotspots. The case in Hinkley, California, made famous by Erin Brockovich, involved hexavalent chromium contamination from a natural gas compressor station.

Need to Remove Chromium from Your Water?

ForeverPure provides commercial and industrial chromium removal systems, including reverse osmosis units, anion exchange systems, and chemical reduction-precipitation treatment trains. Our engineering team designs solutions for both total chromium and hexavalent chromium compliance based on your water analysis and regulatory requirements.

Contact ForeverPure for a customized chromium removal solution.