Free testing, with limits
Free testing in California depends on where you live: 11 local domestic well programs offer it in their areas. Check which tests it includes before you count on it for radon in water.
California's private well pageRadon in California well water, by county
Radon has turned up often in 1 of the 30 California counties with 10 or more tested wells, but those results likely overstate it, because of who tested the wells, when or where. A county's share tells you how likely a problem is nearby. Only a test of your own well tells you what's in it.
How California compares with the rest of the country
Across California, about 1 in 30 tested wells had radon in water over 4,000 pCi/L, a level the EPA proposed (34 of 1,015). Across all tested wells in the US it's about 1 in 24. California is about average, so the county you live in tells you more than the state does.
Why it matters: It escapes into the air when you shower or run the tap, and breathing it raises the risk of lung cancer.
Radon is a gas given off by uranium in granite and similar rock.
Radon in tested wells, county by county
Share of each county's tested wells with at least one result over 4,000 pCi/L, a level the EPA proposed. Counties need 10 or more tested wells to be listed. Widespread means 15% or more, worth checking 5 to 15%, rare under 5%.
| County | Wells tested | Wells over the limit | Level |
|---|---|---|---|
| El Dorado County | 18 | Widespread | |
| Madera County | 39 | Worth checking | |
| San Diego County | 70 | Worth checking | |
| San Bernardino County | 83 | Worth checking | |
| Tulare County | 14 | Worth checking | |
| Lassen County | 18 | Worth checking | |
| Sonoma County | 47 | Rare | |
| Kern County | 42 | Rare | |
| Fresno County | 60 | Rare | |
| Riverside County | 66 | Rare | |
| Los Angeles County | 77 | Rare | |
| Monterey County | 53 | Rare | |
| Orange County | 36 | Rare | |
| Santa Cruz County | 31 | Rare | |
| Shasta County | 25 | Rare | |
| Stanislaus County | 23 | Rare | |
| Napa County | 21 | Rare | |
| Siskiyou County | 21 | Rare | |
| Mendocino County | 20 | Rare | |
| Santa Barbara County | 20 | Rare | |
| Santa Clara County | 19 | Rare | |
| San Luis Obispo County | 18 | Rare | |
| Humboldt County | 15 | Rare | |
| Nevada County | 14 | Rare | |
| Alameda County | 13 | Rare | |
| Butte County | 13 | Rare | |
| Sacramento County | 11 | Rare | |
| San Joaquin County | 11 | Rare | |
| Placer County | 10 | Rare | |
| Trinity County | 10 | Rare |
* Likely overstated. These shares come from a narrow set of wells, so they probably run high for the county as a whole:
- El Dorado County: All of these tests come from the US Geological Survey between 2007 and 2008 in one small part of the county, so this likely overstates it.
21 more California counties have 1 to 9 tested wells for radon in water, too few to rate.
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Where to get your well tested for radon in water in California
A lab California certifies
Ask for radon in water by name. Results from a certified lab count for a home sale or a loan.
Labs certified by CaliforniaA mail-in kit
Fill the bottles at your kitchen tap and mail them back. We mark the kits that include radon in water.
Compare kitsKeep reading about wells in California
- Well water testing in California: how to get tested and what turns up statewide
- Radon in well water: where it comes from, what it does and how to remove it
- Nitrate in California wells, by county
- Arsenic in California wells, by county
- Uranium in California wells, by county
- Manganese in California wells, by county
- Lead in California wells, by county
Questions people ask about radon in water in California
Is there radon in water in California well water?
Radon has turned up often in 1 of the 30 California counties with 10 or more tested wells, but those results likely overstate it, because of who tested the wells, when or where. Across California, about 1 in 30 tested wells had radon in water over 4,000 pCi/L, a level the EPA proposed (34 of 1,015). Across all tested wells in the US it's about 1 in 24. California is about average, so the county you live in tells you more than the state does.
Where can I get my well tested for radon in water in California?
Free testing in California depends on where you live: 11 local domestic well programs offer it in their areas. If that doesn't cover you, use a lab certified by California for drinking water, or a mail-in kit that includes radon in water from a certified lab.
See California's list of certified labs, or compare mail-in kits.
Where these numbers come from
- Lab tests: Water Quality Portal (USGS and EPA), well samples since 2000. A well counts as over if any of its tests was over 4,000 pCi/L, a level the EPA proposed. Wells tested for studies and monitoring, not a random sample of homes.
- Likely overstated: our own check of who tested the wells, when and where. How we check.
- State programs and links: State Water Resources Control Board.