Arsenic, nitrate and manganese are worth checking in Yolo County wells.
3 worth checking, 3 rare out of 6 health checks.
From 13 wells tested in Yolo County since 2005; your own well needs its own test. How to read this
Not charted, because fewer than 10 wells were tested: radon (7).
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 40,521 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 Yolo County compares with California and the counties around it.
Each result puts Yolo County next to the 6 counties that border it, on the chart's 0 to 50% scale.
These 3 are worth checking in Yolo County.
Arsenic1 of 13 wells over
Too few tested to show: Colusa, Solano.
Nitrate1 of 13 wells over
Too few tested to show: Colusa, Solano.
Manganese1 of 13 wells over
Too few tested to show: Colusa, Solano.
These 3 are rare in Yolo County.
Uranium0 of 13 wells over
Too few tested to show: Colusa, Lake, Solano.
Fluoride0 of 13 wells over
Too few tested to show: Colusa, Lake, Solano.
Lead0 of 12 wells over
Too few tested to show: Colusa, Solano.
Too few Yolo County wells were tested for this one.
Radon in water7 wells tested
Coliform bacteria turned up in 1 of 7 tested Yolo County wells, so test for it every year.
Coliform bacteria1 of 7 wells had it
E. coli0 of 7 wells had it
E. coli means human or animal waste is getting in, so if a test of your Yolo County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 1 of 13 tested wells, and it isn't a health risk.
Iron stains1 of 13 wells had it
Too few tested to show: Colusa, Solano.
Most of the 13 wells tested in Yolo County were last tested before 2010.
An old result shows the water as it was then, so treat these 13 wells as a starting point and test your own.
Most Yolo 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 Yolo 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 197 feet.
That comes from 3,786 household wells in California's drilling records. Most wells of every kind here are 126 to 360 feet deep. Wells as deep as Yolo 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 32 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 Yolo County.
Yolo 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 5 town water systems in Yolo County test for PFAS. 1 found some PFAS, all below the new limits. Only 4 wells in Yolo 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 Yolo County
Is well water in Yolo County safe to drink?
1 of 13 wells tested in Yolo County had nitrate over the limit, about 1 in 13. Across California it's about 1 in 10. That's a bigger share than in 33 of the 46 other California counties with 10 or more tested wells. So test your own well for nitrate before you count on it.
Can I get my well tested for free in Yolo 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 Yolo County.
How deep are wells in Yolo County?
Half the household wells here are shallower than 197 feet. Typical household wells here are shallower than in 32 of the 56 other California counties with enough drilling records.
How does Yolo County compare with the counties around it?
For arsenic, the share of tested wells over the limit: 4 of the 4 neighboring counties with data had a higher share than Yolo County, at 8%. Napa had the most, at 27%. Groundwater doesn't stop at county lines, so Napa's 27% is worth knowing about if you live near it.
How often should I test my well in Yolo County?
In Yolo County, 1 of 7 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 Yolo County numbers come from
- People on wells: 40,521 in Yolo County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 173 well water samples from 13 wells in the Water Quality Portal, run by the USGS and EPA, 2005 to 2021.
- Well depths: 3,786 California drilling records for Yolo County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 6 counties that share a border with Yolo County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 5 town water systems in Yolo County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Yolo County is zone 3 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.