Arsenic and manganese are worth checking in Mendocino County wells.
2 worth checking, 5 rare out of 7 health checks.
From 33 wells tested in Mendocino County since 2009; 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 2 people 37,855 people
Nobody else tests these wells. Only you do.
People on wells: EPA researchers' 2020 estimate
How deep wells go here
Middle depth: some shelter from the surface, not complete.
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 Mendocino County compares with California and the counties around it.
Each result puts Mendocino County next to the 6 counties that border it, on the chart's 0 to 50% scale.
These 2 are worth checking in Mendocino County.
Arsenic4 of 32 wells over
Manganese4 of 32 wells over
Too few tested to show: Trinity.
These 5 are rare in Mendocino County.
Nitrate0 of 30 wells over
Uranium0 of 26 wells over
Too few tested to show: Lake.
Radon in water0 of 20 wells over
Too few tested to show: Glenn, Lake, Tehama.
Fluoride0 of 26 wells over
Lead0 of 27 wells over
Too few tested to show: Trinity.
Coliform bacteria turned up in 10 of 23 tested Mendocino County wells, so test for it every year.
Coliform bacteria10 of 23 wells had it
Too few tested to show: Glenn.
E. coli3 of 23 wells had it
Too few tested to show: Glenn.
E. coli means human or animal waste is getting in, so if a test of your Mendocino County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 8 of 30 tested wells, and it isn't a health risk.
Iron stains8 of 30 wells had it
Mendocino County has 33 tested wells, and the busiest decade for testing was the 2000s.
Only 8 of the 33 wells have a test from 2020 or later, so a fresh test of your own well tells you more than any of these.
Most Mendocino 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 Mendocino County.
- Bacteria
- Nitrate
- Arsenic
- Manganese
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 140 feet.
That comes from 9,894 household wells in California's drilling records. Most wells of every kind here are 80 to 200 feet deep. Wells as deep as Mendocino 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 50 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 Mendocino County.
Mendocino 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 8 town water systems in Mendocino County test for PFAS. 1 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 7.4 ng/L of PFOS. Only 6 wells in Mendocino 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 Mendocino County
Is well water in Mendocino County safe to drink?
4 of 32 wells tested in Mendocino County had arsenic over the limit, about 1 in 8. Across California it's about 1 in 7. That's a bigger share than in 34 of the 46 other California counties with 10 or more tested wells. So test your own well for arsenic before you count on it.
Can I get my well tested for free in Mendocino 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, arsenic and manganese, the tests that matter most in Mendocino County.
How deep are wells in Mendocino County?
Half the household wells here are shallower than 140 feet. Typical household wells here are shallower than in 50 of the 56 other California counties with enough drilling records.
How does Mendocino County compare with the counties around it?
For arsenic, the share of tested wells over the limit: Mendocino County, at 12%, had a higher share than all 6 neighboring counties with data. Groundwater doesn't stop at county lines, so Sonoma's 11% is worth knowing about if you live near it.
How often should I test my well in Mendocino County?
In Mendocino County, 10 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 Mendocino County numbers come from
- People on wells: 37,855 in Mendocino County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 448 well water samples from 33 wells in the Water Quality Portal, run by the USGS and EPA, 2009 to 2026.
- Well depths: 9,894 California drilling records for Mendocino County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 6 counties that share a border with Mendocino County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 8 town water systems in Mendocino County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Mendocino County is zone 3 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.