Well Water Notebook
Information only, not medical advice. We never say whether your water is safe to drink. Ask a state-certified lab or your local health department. Disclaimer

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Well water testing in Sonoma County, California

About one in seven Sonoma County residents supplied their own household water in USGS's 2015 estimates. Here's who permits wells, what the county says to test and how often, the well ordinance rules that can affect a building project or a home purchase, and where to find a lab, with a source for every fact.

This page can't tell you whether your water is safe to drink. Only a test of your well can do that.

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How many people here use a private well

USGS estimates that about 77,000 people in Sonoma County supplied their own household water in 2015, about 15% of the county's population.1

Who regulates wells in Sonoma County

Sonoma County Environmental Health says private well water isn't regulated by any government agency; it's the property owner's responsibility to have the well tested by a state-certified lab. It also says landlords are required to provide potable water to their tenants.2 Phone: (707) 565-6565.3

Permit Sonoma issues well permits under Sonoma County Code Chapter 25B. Amendments to the well ordinance took effect May 18, 2023. Permit Sonoma says:4

Permit Sonoma's January 2025 well permitting workshop mentions a court-ordered suspension of non-emergency well permits.5 Ask Permit Sonoma about current status before you plan a new well.

Dry weather well testing

Permit Sonoma requires a dry weather well test (a pump test of well yield) when you build a new or replacement dwelling in a Class 4 groundwater availability area, or an ADU in a Class 3 or 4 area, or when staff require it.4 The testing season usually runs from July 15 to October 1.6 If you're buying land to build on, check the groundwater availability class first.

What to test and how often

Sonoma County Environmental Health's recommended schedule:3

Test How often (county recommendation)
Bacteria (total and fecal coliform) Twice a year, or after a flood or other disaster that may have affected the well
Nitrate Every five years
Minerals Every 5 years, or if taste or color changes
Petroleum, solvents, pesticides Every three to five years
VOCs and SVOCs After wildfire damage to the water system, or suspected industrial contamination

The State Water Board's statewide guide recommends testing total coliform, nitrate and electrical conductivity every year.7 Where the two differ, the more frequent schedule costs more but catches changes sooner.

PFAS: the county says PFAS sampling needs specialized collection procedures because samples are easily contaminated, so owners may want to work with experienced professionals or trained sample collectors, and that one sample shows only one point in time.3

There are no free domestic well testing programs listed for Sonoma County in the State Water Board's June 2026 directory.8

Buying a home with a well in Sonoma County

We didn't find a statewide rule requiring a well test at sale; the Transfer Disclosure Statement asks whether the water supply is a well.9 Ask for the well completion report, a water test and a flow test, and ask Permit Sonoma about permits on file. See buying a home with a well.

Get quotes from local well pros

Need a well drilled, a pump fixed, your water tested or treated, or a well inspected for a home sale in Sonoma County, California? Tell us what you need. With your permission, we'll try to pass your request to up to 3 local licensed or certified providers. It's free. You can also look up licensed pros yourself in our directory.

Well Water Notebook is a free information site, not a contractor, and we can't guarantee anyone will contact you. We don't sell your details or add you to a mailing list. See our privacy policy.

Where to get tested and treatment options

Labs: see accredited labs in Sonoma County, from the State Water Board's ELAP data. Sonoma County's testing page also lists ELAP-accredited labs.3 Confirm the lab is accredited for each test you need.

Drillers: check a C-57 license with CSLB's Instant License Check.

Dry well? Report it on DWR's Dry Well Reporting System.10

If a result is high:

  1. Dieter, C.A., and others, 2018, Estimated Use of Water in the United States County-Level Data for 2015 (ver. 2.0), U.S. Geological Survey data release. https://doi.org/10.5066/F7TB15V5 ↩

  2. County of Sonoma Department of Health Services, Environmental Health, "Well Water Quality." https://sonomacounty.gov/health-and-human-services/health-services/divisions/public-health/environmental-health/programs-and-services/well-water-quality ↩

  3. County of Sonoma Department of Health Services, Environmental Health, "Well Water Testing." https://sonomacounty.gov/health-and-human-services/health-services/divisions/public-health/environmental-health/programs-and-services/well-water-quality/well-water-testing ↩↩↩↩

  4. Permit Sonoma, "Water Wells." https://permitsonoma.org/divisions/engineeringandconstruction/wellandsepticsystems/waterwells ↩↩

  5. Permit Sonoma, "General Well Permitting Information." https://permitsonoma.org/wellpermittinginfo ↩

  6. Permit Sonoma, "Dry Weather Well Testing." https://permitsonoma.org/divisions/engineeringandconstruction/wellandsepticsystems/waterwells/dryweathertesting ↩

  7. California State Water Resources Control Board, GAMA Program, A Guide for Private Domestic Well Owners (June 2025). https://www.waterboards.ca.gov/water_issues/programs/gama/docs/wellowner_guide.pdf ↩

  8. California State Water Resources Control Board, Well Testing Program Directory (spreadsheet, last updated June 2026). https://www.waterboards.ca.gov/drinking_water/certlic/drinkingwater/docs/well-testing-program-directory.xlsx ↩

  9. California Department of Real Estate, Disclosures in Real Property Transactions (RE 6). https://www.dre.ca.gov/files/pdf/re6.pdf ↩

  10. California Department of Water Resources, "Dry Well Reporting System." https://mydrywell.water.ca.gov/report/ ↩