Chromium-6 in Southern California Tap Water: What the Data Shows and How to Remove It

Chromium-6 (hexavalent chromium) is detectable in tap water across Los Angeles, Orange County, San Diego, Riverside, San Bernardino, Ventura, and Imperial counties, according to utility Consumer Confidence Reports and the Environmental Working Group Tap Water Database, which aggregates monitoring records submitted to the California State Water Resources Control Board. California re-established its enforceable Maximum Contaminant Level at 10 parts per billion in October 2024, but the state public health goal set by the Office of Environmental Health Hazard Assessment is 0.02 ppb, a level 500 times lower than the legal limit. Most SoCal utilities report detections above that health benchmark while remaining within the legal limit. Reverse osmosis systems certified to NSF/ANSI Standard 58 are the only residential technology with independent certification for hexavalent chromium removal, achieving 90 to 97 percent reduction in independent testing at concentrations typical of SoCal tap water.

Published August 24, 2026  ·  Data sourced from public regulatory filings  ·  Southern California Water Treatment

What Is Chromium-6 and Where Does It Come From?

Chromium is a naturally occurring metal found in rocks, soil, and groundwater. Its hexavalent form, chromium-6, is the form associated with cancer risk and is classified as a known human carcinogen by the International Agency for Research on Cancer (IARC Monograph Volume 49, 1990). It gained widespread public attention through the Hinkley, California contamination case documented in the 1990s, though the San Gabriel Basin Superfund complex in Los Angeles County represents a significantly larger affected population.

In Southern California, chromium-6 enters drinking water through two distinct pathways. The first is industrial: aerospace and defense manufacturing, chrome plating, metal finishing, and leather tanning facilities that operated across the Los Angeles Basin and Inland Empire discharged chromium compounds into soil and groundwater throughout the mid-20th century. The U.S. Environmental Protection Agency Superfund program lists multiple active remediation sites in San Bernardino and Riverside counties addressing industrial chromium-6 plumes, some still requiring ongoing cleanup.

The second pathway is geological. Chromium occurs in ultramafic rock formations widespread across California's Coast Ranges. Weathering releases the metal into groundwater and surface water, which enters municipal supplies. Colorado River water, imported by the Metropolitan Water District to serve roughly two-thirds of Southern California's population, carries naturally occurring chromium-6 from geological sources along its corridor. This means utilities with no significant industrial history still show detections.

Chromium-6 Levels in Southern California: What the Data Shows

The EPA's Unregulated Contaminant Monitoring Rule Round 3 (UCMR3), conducted from 2013 to 2015, required large public water systems to test for chromium-6 for the first time at the national level. The results confirmed what California regulators had suspected: hexavalent chromium was detectable across most of the state, including densely populated Southern California.

The EWG Tap Water Database, which aggregates California State Water Resources Control Board monitoring records, documents hexavalent chromium detections above the 0.02 ppb public health goal in the majority of Southern California utilities. The Los Angeles Department of Water and Power, which serves approximately 4 million customers, reported chromium-6 detections ranging from 0.09 to 0.42 ppb across its service territory in its 2023 Consumer Confidence Report. That places most LADWP service areas four to 21 times above the OEHHA health goal, while remaining compliant with the 10 ppb MCL.

The San Gabriel Valley shows the highest documented ranges in the region. EPA UCMR3 monitoring recorded chromium-6 detections from 0.15 to 0.60 ppb in San Gabriel Valley utility samples, driven by industrial-era contamination from the San Gabriel Basin Superfund sites. San Diego County, drawing primarily on Colorado River and State Water Project imports, reports chromium-6 in the 0.04 to 0.14 ppb range: above the health goal but far below industrial-plume levels.

Private well users face a separate and often unquantified risk. California does not require private wells to meet Safe Drinking Water Act standards, and well owners bear full responsibility for testing. In Riverside County and San Bernardino County, where private wells draw from aquifers near documented industrial contamination sites, testing is the only way to establish a household-specific baseline.

California's Chromium-6 Standard: The Gap Between Legal and Safe

California set the nation's first chromium-6 specific drinking water standard in 2014, placing the Maximum Contaminant Level at 10 ppb, according to the State Water Resources Control Board. The rule was challenged in court by water utilities and chemical industry groups. A Sacramento Superior Court ruled in 2017 that the SWRCB had not adequately analyzed economic feasibility, and enforcement was suspended. After years of revised rulemaking, California reinstated the 10 ppb MCL in October 2024.

The practical consequence of the gap between the 10 ppb legal limit and the 0.02 ppb health goal is that a household's water can be fully compliant with California law while still exceeding the level that OEHHA's scientists identified as the no-known-risk threshold for lifetime exposure. Legal compliance and health-goal compliance are not the same measurement.

Private well users are outside this regulatory framework entirely. California's MCL applies only to public water systems serving 25 or more people. Wells serving individual homes are unregulated for chromium-6 regardless of contamination levels.

Which Water Treatment Methods Remove Chromium-6?

Not all water filtration removes chromium-6. The contaminant exists as a dissolved ionic species (chromate or dichromate) in water, which means physical filtration that targets particulates and bacteria has no effect on it. The following comparison covers the technologies with documented efficacy and the ones commonly marketed for water quality that provide no chromium-6 benefit.

Sources: NSF International certified product registry (nsf.org); EPA Drinking Water Treatment Technology Unit Cost Report; AWWA Water Quality and Treatment, 6th ed. Removal rates reflect performance under controlled conditions at concentrations typical of SoCal municipal supplies. Actual performance depends on feedwater chemistry, system sizing, and maintenance.

What Southern California Homeowners Should Do Next

1. Request your Consumer Confidence Report. Every public water utility in the United States is required to publish an annual CCR listing detected contaminants. Search your utility name plus "Consumer Confidence Report" or access the EPA's CCR lookup at epa.gov/ccr. The report includes chromium-6 detection ranges and your utility's compliance status relative to California's 10 ppb MCL.
2. Test your specific tap. CCRs show system-wide averages; your tap may read higher or lower depending on pipe age, service area, and seasonal source blending. A certified water test from a California ELAP-certified laboratory gives you a household-specific chromium-6 number. Request testing for hexavalent chromium specifically, not total chromium, since total chromium results may not distinguish the toxic hexavalent form.
3. Know your well status. If your home draws from a private well, you are outside the public water system framework and there is no regulatory monitoring covering your tap. Riverside County and San Bernardino County have documented industrial chromium-6 plumes in portions of their groundwater basins. Testing is the only way to establish your baseline.
4. Install NSF-certified treatment if your numbers warrant it. If your water test shows chromium-6 above your comfort level, an RO system carrying NSF/ANSI 58 certification for chromium-6 reduction is the evidence-backed residential solution. Confirm the specific contaminant is listed on the certification, not just the product category, as not all NSF/ANSI 58 products are tested for chromium-6.
5. Cross-check EWG and SWRCB data. The EWG Tap Water Database and California Drinking Water Watch at waterboards.ca.gov provide utility-level monitoring records. EWG may lag state records by one to two reporting cycles; cross-check against your utility's current CCR.