California has some of the most stringent water quality regulations in the US. It also has some of the most documented contamination problems. Both things are true at the same time, and residents need to understand both.
Chromium-6: California’s 2024 Standard
In April 2024, California became the first state in the US to set a specific maximum contaminant level (MCL) for hexavalent chromium (chromium-6) at 10 parts per billion. The EPA has no federal MCL for chromium-6 specifically, only a 100 ppb limit for total chromium.
This matters because the EWG estimates that chromium-6 concentrations above 0.02 ppb may increase cancer risk over a lifetime of exposure, which is far below both California’s standard and the EPA’s total chromium limit. Central Valley agricultural areas, communities with industrial history, and parts of Southern California have shown the highest concentrations.
Chromium-6 is a known carcinogen at elevated levels. It became widely known through the Hinkley, California contamination case (the story behind the Erin Brockovich litigation), where Pacific Gas and Electric contaminated local groundwater. That was an extreme case, but lower-level detections exist across the state.
Reverse osmosis certified to NSF/ANSI 58 removes hexavalent chromium effectively. Standard carbon filters do not.
PFAS Contamination
Dozens of California water systems have detected PFAS above the EPA’s 2024 MCLs of 4 parts per trillion for PFOA and PFOS. California’s contamination footprint follows a predictable pattern: wherever military aviation existed and wherever AFFF (aqueous film-forming foam) was used for fire training, PFAS contaminated soil and groundwater.
Documented contamination areas include:
- Camp Pendleton (San Diego County)
- Travis Air Force Base (Solano County)
- Former McClellan Air Force Base (Sacramento area)
- Airports across the state where AFFF was used for crash response training
The Inland Empire (San Bernardino and Riverside counties) has some of the densest PFAS contamination in California due to a combination of military, industrial, and manufacturing sources. Parts of the Bay Area and the Central Valley have also reported PFAS detections in municipal systems.
The only filters that remove PFAS are NSF/ANSI 58-certified reverse osmosis systems and NSF/ANSI 53-certified activated carbon at high contact times. Pitcher filters vary considerably. See our PFAS contaminant guide for the full breakdown on treatment.
Nitrates in the Central Valley
Agricultural runoff from the San Joaquin and Sacramento valleys has contaminated groundwater with nitrates for decades. Small rural communities relying on domestic wells face the most exposure. The EPA MCL for nitrates is 10 mg/L, and the EWG’s database shows multiple small system violations in Tulare, Kings, and Fresno counties.
The fundamental problem: nitrogen fertilizer applied to crops dissolves in irrigation and rain water, leaches through sandy soils, and ends up in shallow aquifers. Communities whose wells draw from those aquifers see elevated nitrates.
If you’re in a rural Central Valley community, test your well for nitrates annually, particularly in spring after irrigation season begins. And critically, if you have an infant under 6 months old and your well hasn’t been tested, use certified bottled or filtered water for formula until you have results. Nitrates above 10 mg/L can cause methemoglobinemia in infants. See our full nitrates contaminant page for the science.
Boiling does not remove nitrates. Reverse osmosis (NSF 58) and ion exchange with anion resin do.
Arsenic in Southern California and the Central Valley
Naturally occurring arsenic shows up in parts of Imperial County, Fresno County, and some communities in the Mojave Desert. Several small water systems have faced compliance challenges meeting the 10 ppb federal MCL. This is a geology issue: specific rock formations release arsenic into groundwater over time.
The MCLG for arsenic is 0, meaning no safe exposure level has been established. Well owners in Imperial County, parts of Fresno County, and high desert communities should test specifically for arsenic. An NSF 58 RO system or activated alumina filter handles arsenic at the point of use.
Urban Water: The Large Utilities
Los Angeles (LADWP), San Francisco (SFPUC), East Bay (East Bay MUD), and San Diego (Water Authority) consistently meet federal standards and publish detailed quality reports. These are large, well-funded utilities with treatment capacity and monitoring programs.
The SFPUC sources primarily from Hetch Hetchy reservoir in the Sierra Nevada, which produces consistently low-contamination water. LADWP blends Colorado River water with local groundwater and reclaimed sources, resulting in harder water with higher total dissolved solids than San Francisco’s.
Meeting federal standards isn’t the same as meeting the EWG’s stricter health guidelines, and both things are worth knowing. Check the EWG Tap Water Database for your specific utility’s results.
What to Do
Start with the EWG Tap Water Database or your utility’s Consumer Confidence Report. Those two sources cover most of the state’s documented contamination.
If you’re in the Central Valley near agricultural areas, test for nitrates annually. If you’re near a former military installation or in the Inland Empire, test specifically for PFAS. If you’re in a rural area with a private well in a region with arsenic geology, include arsenic in your panel.
For confirmed chromium-6, PFAS, or nitrate problems, an NSF 58-certified under-sink RO system is the most reliable point-of-use solution. One filter type handles all three, which makes the decision relatively straightforward.