Nitrate is widespread in Riverside County wells.
1 widespread, 3 worth checking, 3 rare out of 7 health checks.
From 379 wells tested in Riverside 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 11 people 227,221 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 Riverside County compares with California and the counties around it.
Each result puts Riverside County next to the 5 counties that border it, on the chart's 0 to 50% scale.
This one is widespread in Riverside County wells.
Nitrate78 of 375 wells over
These 3 are worth checking in Riverside County.
Arsenic40 of 343 wells over
Fluoride23 of 370 wells over
Uranium17 of 283 wells over
Too few tested to show: La Paz, AZ.
These 3 are rare in Riverside County.
Lead8 of 223 wells over
Manganese10 of 347 wells over
Radon in water1 of 66 wells over
Too few tested to show: Imperial.
Coliform bacteria turned up in 15 of 68 tested Riverside County wells, so test for it every year.
Coliform bacteria15 of 68 wells had it
Too few tested to show: Imperial, La Paz, AZ, San Diego.
E. coli10 of 68 wells had it
Too few tested to show: Imperial, La Paz, AZ.
E. coli means human or animal waste is getting in, so if a test of your Riverside County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 14 of 369 tested wells, and it isn't a health risk.
Iron stains14 of 369 wells had it
About 4 in 10 of the 379 wells tested in Riverside County have a test from 2020 or later.
With 142 wells tested since 2020, age matters less here than who did the testing and why.
Most Riverside 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 5 things to check in Riverside County.
- Bacteria
- Nitrate
- Arsenic
- Fluoride
- Uranium
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 280 feet.
That comes from 15,169 household wells in California's drilling records. Most wells of every kind here are 160 to 465 feet deep. Wells as deep as Riverside 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 45 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 Riverside County.
Riverside 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 33 town water systems in Riverside 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 31 ng/L of PFOS. Only 49 wells in Riverside 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 Riverside County
Is well water in Riverside County safe to drink?
78 of 375 wells tested in Riverside County had nitrate over the limit, about 2 in 10. Across California it's about 1 in 10. That's a bigger share than in 42 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 Riverside 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, fluoride and uranium, the tests that matter most in Riverside County.
How deep are wells in Riverside County?
Half the household wells here are shallower than 280 feet. Typical household wells here go deeper than in 45 of the 56 other California counties with enough drilling records.
How does Riverside County compare with the counties around it?
For nitrate, the share of tested wells over the limit: 1 of the 5 neighboring counties with data had a higher share than Riverside County, at 21%. La Paz, AZ had the most, at 33%. Groundwater doesn't stop at county lines, so La Paz, AZ's 33% is worth knowing about if you live near it.
How often should I test my well in Riverside County?
In Riverside County, 15 of 68 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 Riverside County numbers come from
- People on wells: 227,221 in Riverside County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 5,511 well water samples from 379 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2026.
- Well depths: 15,169 California drilling records for Riverside County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 5 counties that share a border with Riverside County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 33 town water systems in Riverside County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Riverside County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.