Radon in California well water
In California, radon was detected in 100% of 1,621 tested private well samples.
Radon is a radioactive gas released from bedrock into well water. It escapes into household air during showers and washing, where inhaling it raises lung cancer risk. EPA has no enforceable water MCL; New Hampshire recommends treatment above 2,000 pCi/L, and many states use 4,000 pCi/L.
Check your radon number
Type the value from your lab report. It is compared to the EPA limit, the same way the full interpretation does it.
Test your California well for radon
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
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Getting it fixed
Book a free in-home water test with a licensed local company, or see the treatment guide for what removes radon.
Where to get a well tested for radon in California: certified labs, state programs and prices.
By county, from individual samples
Public monitoring samples against the EPA limit; counties with at least 30 samples.
| County | Above EPA limit | Samples | Detected | Median | Highest |
|---|---|---|---|---|---|
| Tulare | no limit | 30 | 100% | 655 | 5,450 pCi/L |
| Fresno | no limit | 117 | 100% | 580 | 6,700 pCi/L |
| Kern | no limit | 95 | 100% | 540 | 4,670 pCi/L |
| Los Angeles | no limit | 134 | 100% | 360 | 3,350 pCi/L |
| Madera | no limit | 51 | 100% | 670 | 40,700 pCi/L |
| Monterey | no limit | 76 | 100% | 530 | 3,900 pCi/L |
| Orange | no limit | 63 | 100% | 420 | 1,430 pCi/L |
| Riverside | no limit | 106 | 100% | 345 | 4,200 pCi/L |
| San Bernardino | no limit | 121 | 100% | 520 | 19,300 pCi/L |
| San Diego | no limit | 84 | 100% | 740 | 17,200 pCi/L |
| Santa Cruz | no limit | 37 | 100% | 300 | 1,920 pCi/L |
| El Dorado | no limit | 68 | 100% | 1,290 | 12,000 pCi/L |
| Stanislaus | no limit | 51 | 100% | 630 | 1,830 pCi/L |
| Sonoma | no limit | 56 | 98% | 505 | 16,700 pCi/L |
- Anaheim-Santa Ana-Garden Grove CCD, Orange County100% detected of 33
- North Antelope Valley CCD, Los Angeles County100% detected of 59
- South Antelope Valley CCD, Los Angeles County100% detected of 41
- Fresno CCD, Fresno County100% detected of 46
- San Diego CCD, San Diego County100% detected of 31
- South Lake Tahoe CCD, El Dorado County100% detected of 66
- East Kern CCD, Kern County100% detected of 65
Sources
- Individual samples from the USGS Water Quality Portal and state monitoring, summarized per geography. radon counts and shares are listed on each county and town page with their sources.