Nitrate is worth checking in San Luis Obispo County wells.
1 worth checking, 6 rare out of 7 health checks.
From 54 wells tested in San Luis Obispo County since 2000; 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 7 people 38,848 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)
Possible here. A home air test is inexpensive.
Radon zones: EPA map
Here's how San Luis Obispo County compares with California and the counties around it.
Each result puts San Luis Obispo County next to the 4 counties that border it, on the chart's 0 to 50% scale.
This one is worth checking in San Luis Obispo County.
Nitrate7 of 54 wells over
These 6 are rare in San Luis Obispo County.
Manganese2 of 53 wells over
Uranium1 of 40 wells over
Arsenic1 of 41 wells over
Radon in water0 of 18 wells over
Too few tested to show: Kings.
Fluoride0 of 53 wells over
Lead0 of 40 wells over
None of the 4 San Luis Obispo County wells tested for bacteria had it, but every well needs a yearly test.
Coliform bacteria0 of 4 wells had it
E. coli0 of 4 wells had it
E. coli means human or animal waste is getting in, so if a test of your San Luis Obispo County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 6 of 53 tested wells, and it isn't a health risk.
Iron stains6 of 53 wells had it
About 4 in 10 of the 54 wells tested in San Luis Obispo County have a test from 2020 or later.
With 24 wells tested since 2020, age matters less here than who did the testing and why.
Most San Luis Obispo 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 3 things to check in San Luis Obispo County.
- Bacteria
- Nitrate
- 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 330 feet.
That comes from 3,514 household wells in California's drilling records. Most wells of every kind here are 148 to 480 feet deep. Wells as deep as San Luis Obispo 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 52 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 Luis Obispo County.
San Luis Obispo County is in the EPA's middle radon zone. An indoor air test is cheap and tells you whether to look at your water. Radon guide
Between 2023 and 2025 the EPA had 17 town water systems in San Luis Obispo County test for PFAS. 5 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 33 ng/L of PFOS. Only 4 wells in San Luis Obispo 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 Luis Obispo County
Is well water in San Luis Obispo County safe to drink?
7 of 54 wells tested in San Luis Obispo County had nitrate over the limit, about 1 in 8. Across California it's about 1 in 10. That's a bigger share than in 38 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 San Luis Obispo 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 and arsenic, the tests that matter most in San Luis Obispo County.
How deep are wells in San Luis Obispo County?
Half the household wells here are shallower than 330 feet. Typical household wells here go deeper than in 52 of the 56 other California counties with enough drilling records.
How does San Luis Obispo County compare with the counties around it?
For nitrate, the share of tested wells over the limit: 1 of the 4 neighboring counties with data had a higher share than San Luis Obispo County, at 13%. Monterey had the most, at 19%. Groundwater doesn't stop at county lines, so Monterey's 19% is worth knowing about if you live near it.
How often should I test my well in San Luis Obispo County?
None of the 4 wells tested for bacteria in San Luis Obispo County had it, but the CDC still says to test for bacteria and nitrate every year, and right away after a flood, a repair, or any change in taste or smell.
If you buy something through our links, we may earn a commission. We'd recommend the same thing either way, and we'll tell you when a free option exists. How we make money
Where the San Luis Obispo County numbers come from
- People on wells: 38,848 in San Luis Obispo County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 1,136 well water samples from 54 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2023.
- Well depths: 3,514 California drilling records for San Luis Obispo County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 4 counties that share a border with San Luis Obispo County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 17 town water systems in San Luis Obispo County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: San Luis Obispo County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.