Arsenic, nitrate and manganese are widespread in San Joaquin County wells.
3 widespread, 4 rare out of 7 health checks.
From 137 wells tested in San Joaquin County since 2001; 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 10 people 74,887 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 San Joaquin County compares with California and the counties around it.
Each result puts San Joaquin County next to the 6 counties that border it, on the chart's 0 to 50% scale.
These 3 are widespread in San Joaquin County wells.
Arsenic40 of 137 wells over
Too few tested to show: Contra Costa.
Nitrate20 of 126 wells over
Too few tested to show: Contra Costa.
Manganese21 of 137 wells over
Too few tested to show: Contra Costa.
These 4 are rare in San Joaquin County.
Uranium4 of 89 wells over
Too few tested to show: Calaveras, Contra Costa.
Radon in water0 of 11 wells over
Too few tested to show: Amador, Calaveras, Contra Costa.
Fluoride0 of 136 wells over
Too few tested to show: Contra Costa.
Lead0 of 57 wells over
Too few tested to show: Calaveras, Contra Costa.
Coliform bacteria turned up in 7 of 43 tested San Joaquin County wells, so test for it every year.
Coliform bacteria7 of 43 wells had it
Too few tested to show: Alameda, Calaveras, Contra Costa.
E. coli0 of 43 wells had it
Too few tested to show: Alameda, Calaveras, Contra Costa.
E. coli means human or animal waste is getting in, so if a test of your San Joaquin County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 11 of 137 tested wells, and it isn't a health risk.
Iron stains11 of 137 wells had it
Too few tested to show: Contra Costa.
San Joaquin County has 137 tested wells, and the busiest decade for testing was the 2000s.
Only 9 of the 137 wells have a test from 2020 or later, so a fresh test of your own well tells you more than any of these.
Most San Joaquin 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 San Joaquin 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 220 feet.
That comes from 10,961 household wells in California's drilling records. Most wells of every kind here are 150 to 300 feet deep. Wells as deep as San Joaquin 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 go deeper than in 33 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 San Joaquin County.
San Joaquin 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 13 town water systems in San Joaquin County test for PFAS. 7 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 28 ng/L of PFOS. Only 35 wells in San Joaquin 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 San Joaquin County
Is well water in San Joaquin County safe to drink?
40 of 137 wells tested in San Joaquin County had arsenic over the limit, about 3 in 10. Across California it's about 1 in 7. That's a bigger share than in 44 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 San Joaquin 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 San Joaquin County.
How deep are wells in San Joaquin County?
Half the household wells here are shallower than 220 feet. Typical household wells here go deeper than in 33 of the 56 other California counties with enough drilling records.
How does San Joaquin County compare with the counties around it?
For arsenic, the share of tested wells over the limit: San Joaquin County, at 29%, had a higher share than all 5 neighboring counties with data. Groundwater doesn't stop at county lines, so Sacramento's 18% is worth knowing about if you live near it.
How often should I test my well in San Joaquin County?
In San Joaquin County, 7 of 43 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 San Joaquin County numbers come from
- People on wells: 74,887 in San Joaquin County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 1,384 well water samples from 137 wells in the Water Quality Portal, run by the USGS and EPA, 2001 to 2023.
- Well depths: 10,961 California drilling records for San Joaquin County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 6 counties that share a border with San Joaquin County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 13 town water systems in San Joaquin County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: San Joaquin County is zone 3 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.