Manganese is widespread in Sonoma County wells.
1 widespread, 1 worth checking, 5 rare out of 7 health checks.
From 116 wells tested in Sonoma County since 2002; your own well needs its own test. How to read this
Rare: under 5%Worth checking: 5 to 15%Widespread: 15%+
Lab results: Water Quality Portal (USGS and EPA)
Who drinks from a well here
1 in 5 people 95,450 people
Nobody else tests these wells. Only you do.
People on wells: EPA researchers' 2020 estimate
How deep wells go here
Deep wells: fewer surface germs, more minerals from the rock.
Well depths: US Groundwater Well Database
Radon risk (a gas from rock)
Less likely, but the EPA still says test your air.
Radon zones: EPA map
Here's how Sonoma County compares with California and the counties around it.
Each result puts Sonoma County next to the 6 counties that border it, on the chart's 0 to 50% scale.
This one is widespread in Sonoma County wells.
Manganese23 of 109 wells over
Too few tested to show: Contra Costa, Marin, Solano.
This one is worth checking in Sonoma County.
Arsenic13 of 114 wells over
Too few tested to show: Contra Costa, Marin, Solano.
These 5 are rare in Sonoma County.
Radon in water2 of 47 wells over
Too few tested to show: Contra Costa, Lake, Marin, Solano.
Fluoride2 of 113 wells over
Too few tested to show: Contra Costa, Lake, Marin, Solano.
Nitrate1 of 115 wells over
Too few tested to show: Contra Costa, Marin, Solano.
Uranium0 of 103 wells over
Too few tested to show: Contra Costa, Lake, Marin, Solano.
Lead0 of 83 wells over
Too few tested to show: Contra Costa, Marin, Solano.
Coliform bacteria turned up in 2 of 23 tested Sonoma County wells, so test for it every year.
Coliform bacteria2 of 23 wells had it
Too few tested to show: Contra Costa, Marin, Napa, Solano.
E. coli1 of 24 wells had it
Too few tested to show: Contra Costa, Marin, Napa, Solano.
E. coli means human or animal waste is getting in, so if a test of your Sonoma County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 22 of 111 tested wells, and it isn't a health risk.
Iron stains22 of 111 wells had it
Too few tested to show: Contra Costa, Marin, Solano.
Sonoma County has 116 tested wells, and the busiest decade for testing was the 2010s.
Only 7 of the 116 wells have a test from 2020 or later, so a fresh test of your own well tells you more than any of these.
Most Sonoma County wells in these records were tested for research or because someone suspected a problem, which can push the shares above what a typical home well would show (how we read the numbers).
One mail-in lab test covers the 4 things to check in Sonoma County.
- Bacteria
- Nitrate
- Manganese
- Arsenic
What California offers well owners
- Free testingThe State and Regional Water Boards fund or regulate 11 domestic well programs that offer free well testing in their service areas. See the program
- Certified labsCalifornia's list of certified labs
- Well recordsCalifornia's well record search
Half the household wells here are shallower than 190 feet.
That comes from 22,616 household wells in California's drilling records. Most wells of every kind here are 92 to 282 feet deep. Wells as deep as Sonoma County's are usually better protected from fertilizer and bacteria at the surface, but they can pick up more of what's in the rock, like arsenic and uranium. Typical household wells here are shallower than in 34 of the 56 other California counties with enough drilling records.
Look up your own well's depth in California's well record search, or see how to find it.
Well depths: US Groundwater Well Database
Here's what we know about radon and PFAS in Sonoma County.
Sonoma County is in the EPA's lowest radon zone, so radon from well water is unlikely to be a problem. Radon guide
Between 2023 and 2025 the EPA had 17 town water systems in Sonoma County test for PFAS. 3 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 6.2 ng/L of PFOA. Only 15 wells in Sonoma County have public PFAS results, so the town systems are the best clue to what's in the groundwater nearby. PFAS guide
Radon zones: EPA Map of Radon Zones. PFAS: EPA tests of town water systems, 2023 to 2025
Questions people ask about well water in Sonoma County
Is well water in Sonoma County safe to drink?
23 of 109 wells tested in Sonoma County had manganese over the limit, about 2 in 10. Across California it's about 1 in 12. That's a bigger share than in 38 of the 45 other California counties with 10 or more tested wells. So test your own well for manganese before you count on it.
Can I get my well tested for free in Sonoma County?
The State and Regional Water Boards fund or regulate 11 domestic well programs that offer free well testing in their service areas. If not, use a lab California has certified for drinking water. Ask for bacteria, nitrate, manganese and arsenic, the tests that matter most in Sonoma County.
How deep are wells in Sonoma County?
Half the household wells here are shallower than 190 feet. Typical household wells here are shallower than in 34 of the 56 other California counties with enough drilling records.
How does Sonoma County compare with the counties around it?
For manganese, the share of tested wells over the limit: 1 of the 3 neighboring counties with data had a higher share than Sonoma County, at 21%. Napa had the most, at 29%. Groundwater doesn't stop at county lines, so Napa's 29% is worth knowing about if you live near it.
How often should I test my well in Sonoma County?
In Sonoma County, 2 of 23 tested wells had coliform bacteria, so the CDC's advice matters here: test for bacteria and nitrate every year, and right away after a flood, a repair, or any change in taste or smell.
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Where the Sonoma County numbers come from
- People on wells: 95,450 in Sonoma County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 1,364 well water samples from 116 wells in the Water Quality Portal, run by the USGS and EPA, 2002 to 2022.
- Well depths: 22,616 California drilling records for Sonoma County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 6 counties that share a border with Sonoma County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 17 town water systems in Sonoma County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Sonoma County is zone 3 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.