Well water testing in Marin County, California

What to test for in Marin County

Private wells are not regulated like public water: EPA advises owners to test their well every year for total coliform bacteria, nitrates, total dissolved solids and pH. Your state's own schedule:

• The State Water Board's well-owner guide recommends testing once a year. At a minimum: total coliform bacteria, nitrate and electrical conductivity every year. A minerals panel (arsenic, lead, manganese, iron and other metals) every 5 to 10 years, or sooner if conductivity, taste, color, odor or nearby land use changes. source

Arsenic. USGS estimates that about 53 people using private wells in Marin County had arsenic above 10 µg/L, EPA's limit for public water (2010 base year; 95% range 37 to 73). USGS describes this kind of estimate as useful "for broad screening of the population at risk", so it says nothing certain about any one well. Arsenic has no taste or smell: test for it by name.

Radon. EPA puts Marin County in Zone 3: low potential (average indoor radon may be less than 2 pCi/L). That map is about radon in indoor air, and EPA says it should not be used to decide whether an individual home needs testing. If radon is found in your air and you have a well, EPA suggests testing the water too: see radon in well water.

For anything specific to your home (farms nearby, old plumbing, a new baby, a smell or stain), use the which-tests checklist. When the results arrive, check your numbers against EPA's limits.

PFAS in public water near Marin County (public supply, not your well)

Under EPA’s fifth Unregulated Contaminant Monitoring Rule (UCMR 5), public water systems sampled their water for 29 PFAS between Jan 3, 2023 and Jun 17, 2026. 5 public systems that serve ZIP codes mainly in Marin County reported results. 0 of them had at least one sample with PFOA or PFOS at or above 4.0 ng/L, and 0 had at least one of the 29 PFAS measured at or above its reporting level.

4.0 ng/L (parts per trillion) is both EPA’s UCMR 5 reporting level for PFOA and PFOS and the number in EPA’s 2024 limit for public water. That limit is judged on a running annual average, so one sample at that level is not a violation. EPA proposed changes to its PFAS rules in May 2026: see PFAS in well water for the current status and who should test a well. To test your own well, find a lab certified for PFAS.

Method: a system counts for this county when it reports serving a ZIP code whose largest land part is in this county, so a system serving several counties counts in each. Source: U.S. EPA, UCMR 5 occurrence data (by state), file dated Oct 15, 2026; downloaded Nov 1, 2026.

Testing programs and help in California

• Free well testing is offered in some areas through domestic well programs funded or regulated by the State Water Board or a Regional Water Board. The Board counts 11 such programs. Services and eligibility differ by program, and some also provide replacement drinking water. source
• The Board's Well Testing Program Directory (updated June 2026) lists well testing programs in Fresno, Kern, Kings, Madera, Mariposa, Merced, Monterey, San Benito, San Joaquin, San Luis Obispo, Santa Barbara, Santa Cruz, Stanislaus and Tulare counties, and in parts of Riverside (eastern Coachella Valley), Yolo (Yolo groundwater basin), San Mateo, Santa Clara and Ventura counties. Many cover only a groundwater basin or part of a county, so check the service area. source
• The state does not regulate water quality in most private domestic wells, but counties or local agencies may have their own rules. For example, some wells on farmland may have to be tested for nitrate. Local environmental health agencies may help you read your results. source

Buying a home with a well in Marin County

State-certified labs in Marin County

Labs on the California list whose own websites say they take homeowner samples: Abalone Coast Analytical, Inc. (San Luis Obispo), Analytical Environmental Lab, LLC (San Diego), Antelope Valley-East Kern Water Agency (Palmdale), Brelje and Race Laboratories, Inc. (Santa Rosa), Caltest Analytical Laboratory (Napa), Clinical Laboratory of San Bernardino II (Lompoc), Clinical Laboratory of San Bernardino, Inc. (Grand Terrace), Coachella Valley Water District (Palm Desert) and 14 more. Use Check my ZIP to sort labs by distance.

See all California labs in the finder and filter by the test you need and by labs that take homeowner samples.