California Private Well Water Quality 2026

California’s Well Water Problem: More Complicated Than Most States

California is one of the wealthiest states in the country — and since 2012, it has formally recognised access to safe, clean water as a human right. Yet over 1.3 million Californians rely on private domestic wells that receive no mandatory testing and fall outside state drinking water regulations. For those users, the responsibility for water safety rests entirely with the homeowner.

The contamination picture is unusually complex. California has both natural geological sources of contamination — arsenic, hexavalent chromium, and uranium — and a heavy legacy of agricultural and industrial pollution including nitrate from fertiliser runoff, pesticide residues, and PFAS from multiple sources. Research published in the American Journal of Public Health found that of the approximately 370,000 Californians relying on water with elevated levels of arsenic, nitrate, or hexavalent chromium, 157,367 are served by unregulated domestic wells — 12.1% of all well users. That figure almost certainly understates the true scale, since PFAS and 1,2,3-trichloropropane (TCP) were not included in the study. A 2024 USGS report puts the total number of Californians relying on private domestic wells at over 2 million.

The contamination burden falls disproportionately on California’s agricultural communities, particularly in the San Joaquin Valley, where farmworker households are more likely to rely on private wells and face the highest combined risks from nitrate, arsenic, and pesticide-derived contaminants. As climate-driven drought deepens and water tables drop, concentrations of all these contaminants tend to increase in remaining groundwater.

PFAS in California Well Water: A New and Expanding Threat

PFAS contamination in California’s public water systems has been extensively documented. A 2021 NRDC analysis found that of 248 water systems tested, 65% had PFAS detected in the drinking water — serving more than 16 million people. Under the new federal 4 ppt threshold, the number of California wells with exceedances is expected to grow substantially. But the picture for private wells is considerably less clear, because private wells are not subject to the same monitoring requirements.

A 2024 USGS study of the eastern Sacramento Valley found PFAS detected in 29% of domestic groundwater resources tested, with 5% exceeding the new EPA MCLs. In Solano County, consultants testing soil where biosolids had been applied found PFAS at every single sampling location. Research published in late 2024 also identified PFAS contamination in rural farmland well water far from any known industrial, military, or airport source — with agricultural biosolids (sewage sludge spread as fertiliser) identified as the likely pathway, alongside PFAS-containing pesticides.

Known military and industrial PFAS sources add a further layer of risk. California has multiple major military installations with documented AFFF (aqueous film-forming foam) use histories, including Travis Air Force Base (Solano County), March Air Reserve Base (Riverside County), Edwards Air Force Base (Kern County), and Naval Air Station Lemoore (Kings County). State monitoring data show maximum total PFAS results for public supply wells as high as 1,380 ppt in Fresno County and 1,276 ppt in Riverside County — both vastly in excess of the 4 ppt federal limit, illustrating the severity of contamination possible in California groundwater. A 2021 NRDC analysis of the state’s own testing found that 65% of the 248 water systems tested had PFAS detected, serving more than 16 million people.

Nitrate: The Central Valley’s Chronic Well Water Crisis

Nitrate contamination from agricultural fertiliser runoff and dairy operations is California’s most longstanding well water problem, concentrated in the San Joaquin Valley. The Central Valley is estimated to have between 100,000 and 150,000 private domestic wells, many in areas where USGS modelling shows elevated nitrate at typical domestic well depths. Research from UC Davis found that confined animal feeding operations (dairies), citrus crops, and vegetable and berry cultivation had the highest nitrogen loading rates into groundwater.

Nitrate above 10 mg/L poses an acute risk for infants under six months, causing a condition called methaemoglobinaemia (blue baby syndrome). For adults, long-term exposure at elevated levels has been associated with increased cancer risk. Because the Central Valley aquifer nitrate problem has been building for decades, many wells in affected areas will remain contaminated regardless of changes to surface agricultural practices.

Arsenic and Hexavalent Chromium

Arsenic occurs naturally in California groundwater due to geology, and contamination is exacerbated when water tables drop — drought concentrates arsenic in remaining groundwater. The San Joaquin Valley and Imperial County/Mojave Desert region have the highest populations affected by arsenic in domestic wells. EGLE (in this case California’s State Water Resources Control Board, or SWRCB) cites volcanic activity and forest fires as additional natural arsenic sources.

Hexavalent chromium — the carcinogen made famous by the Hinkley, California case — is produced both naturally and by industrial activity. Sources include metal plating facilities around Los Angeles and the San Francisco Bay Area, natural geological redox processes in the Mojave Desert, and possible links to agricultural practices in the Central Valley. California adopted a hexavalent chromium MCL of 10 µg/L (10 ppb) in April 2024, effective October 1, 2024 — restoring a standard that had been struck down by courts in 2017 for inadequate economic analysis. The 2024 MCL is again facing legal challenge, but remains in force. This standard applies to public water systems only — private well owners receive no equivalent protection.

1,2,3-Trichloropropane (TCP): A Legacy Agricultural Pollutant

California adopted a drinking water MCL for TCP (a pesticide fumigant) of 5 parts per trillion in 2017 — the strictest in the nation. TCP was historically used in soil fumigants on vineyards and orchards throughout the Central Valley. It persists in groundwater and degrades slowly in oxygen-rich aquifer conditions. USGS research has linked areas with elevated TCP to those also affected by nitrate, meaning well owners in agricultural regions may face multiple simultaneous contaminants. Many private domestic wells in the San Joaquin Valley have never been tested for TCP.

Regulatory Situation for California Well Owners

Private domestic wells in California are not regulated under the Safe Drinking Water Act — neither the federal version nor California’s equivalent. The State Water Resources Control Board (SWRCB) and the Division of Drinking Water (DDW) regulate community water systems, not private wells. Testing and treatment of your own well remains entirely the homeowner’s responsibility.

For PFAS specifically, California is in the process of adopting its own state MCLs in line with the 2024 federal standards (4 ppt for PFOA and PFOS). The SWRCB identified PFAS MCLs as its highest priority drinking water standard for 2025. In October 2025, the state issued revised notification levels: PFOA at 4.0 ng/L, PFOS at 4.0 ng/L, PFHxS at 3.0 ng/L, and PFHxA at 1.0 µg/L. However, these regulatory standards apply only to public water systems — private well owners receive none of these protections.

Under Assembly Bill 178 (2022), the SWRCB is conducting a free water testing programme for approximately 3,600 drinking water wells in disadvantaged communities, running from 2024 through 2026. This testing covers 25 PFAS chemicals. If your well is in a qualifying disadvantaged community, you may be eligible — contact your county environmental health department for details.

For California city-by-city tap water data, see our California municipal water quality page and individual city pages for Los Angeles, San Francisco, San Diego, Sacramento, and Fresno. You can also use our live boil water notice tracker for active advisories statewide.

Known High-Risk Areas in California

If you live near any of the following locations, well water testing is urgent — not precautionary.

How to Test Your California Well Water — and What to Do Next

Given California’s contamination profile, every private well owner in the state should test their water — and Central Valley residents should treat testing as urgent rather than optional. PFAS, arsenic, and nitrate are all colourless and tasteless, so contamination at harmful levels produces no detectable sign. The only way to know your well is safe is to test it.

Use the SWRCB’s GAMA GIS mapping tool at gama.waterboards.ca.gov to check known groundwater quality data near your property before deciding which tests to prioritise. Contact your county environmental health department to find a certified laboratory. If you are in a disadvantaged community, ask about the AB 178 free PFAS testing programme, which is running through 2026.

For filter options, our well water filter guide covers reverse osmosis systems for PFAS, arsenic, nitrate, and hexavalent chromium, UV disinfection for bacteria, and whole-house well systems for comprehensive treatment. You can also browse our full water filter solutions page or check your ZIP code for local water quality context.

For other West Coast well water risks, see our pages on Oregon wells and Washington wells. For broader PFAS context, see our PFAS Protection Map 2026. Return to the private well water directory to find your state.