California has a substantial private-well population, concentrated heavily in the agricultural Central Valley as well as rural areas throughout the state. Private wells are not regulated under the federal Safe Drinking Water Act, and California — like every state — leaves testing up to the well owner rather than routinely testing wells on the owner’s behalf.
Why test
Well water can look and taste normal while carrying contaminants that only a lab test can detect — nitrate, arsenic, and bacteria in particular give no reliable taste or smell warning. Agricultural land use, well depth, and local geology all affect what’s likely to be present, and conditions can shift over time.
What to test for
A baseline panel for a California well should include:
- Total coliform bacteria and E. coli — a standard check for whether the well may be vulnerable to contamination.
- Nitrate — a well-documented concern in parts of the Central Valley and other agricultural regions, tied to fertilizer and manure application; especially important if anyone in the household prepares infant formula with well water.
- Arsenic — naturally occurring arsenic has been documented in California groundwater in various regions; it has no taste, odor, or color at concerning levels.
- Hexavalent chromium — California set its own state-specific drinking water limit for hexavalent chromium (10 ppb) in 2024, stricter than the federal total-chromium limit; if your area has documented industrial history or naturally elevated chromium in groundwater, this is worth including.
- Lead — relevant mainly for household plumbing rather than the source water, but worth a baseline check.
How often to test
EPA guidance recommends testing private wells for bacteria and nitrate at least annually, and again any time water quality noticeably changes, after well or plumbing work, or after flooding or nearby land disturbance.
Where to find a certified lab
California’s State Water Resources Control Board runs the Environmental Laboratory Accreditation Program (ELAP), which accredits labs for drinking-water and other environmental testing statewide — see the source link above for the program’s lab-search resources.
After you test
If a result comes back above a health-based guideline, see our filter guides for which technologies are actually certified to remove the specific contaminant found, rather than guessing at a fix.