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Uranium in California well water, by county

By the Tested Water teamLab data from the USGS and EPA Water Quality Portal, 2000 to 2024Updated November 8, 2026

Uranium is widespread, over the limit in 15% or more of tested wells, in 7 of the 44 California counties with 10 or more tested wells. 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.

Find an uranium test See every county

How California compares with the rest of the country

Across California, about 1 in 13 tested wells had uranium over the EPA limit of 30 µg/L (254 of 3,360). Across all tested wells in the US it's about 1 in 22. So uranium turns up somewhat more often in California than in the country as a whole. Check your county below.

Why it matters: Drinking it for years can harm your kidneys and raises the risk of cancer.

Uranium comes out of the rock and sediment the wells draw from.

Uranium in tested wells, county by county

Share of each county's tested wells with at least one result over the EPA limit of 30 µg/L. Counties need 10 or more tested wells to be listed. Widespread means 15% or more, worth checking 5 to 15%, rare under 5%.

CountyWells testedWells over the limitLevel
Mono County 14
36% 5 of 14
 Widespread
Fresno County 147
25% 37 of 147
 Widespread
Orange County 52
23% 12 of 52
 Widespread
Kings County 33
21% 7 of 33
 Widespread
San Diego County 70
20% 14 of 70
 Widespread
Inyo County 24
17% 4 of 24
 Widespread
Stanislaus County 140
15% 21 of 140
 Widespread
Tulare County 119
14% 17 of 119
 Worth checking
Madera County 87
14% 12 of 87
 Worth checking
Kern County 179
12% 21 of 179
 Worth checking
Merced County 74
11% 8 of 74
 Worth checking
Lassen County 19
11% 2 of 19
 Worth checking
San Bernardino County 569
10% 59 of 569
 Worth checking
Monterey County 76
7% 5 of 76
 Worth checking
Riverside County 283
6% 17 of 283
 Worth checking
San Joaquin County 89
4% 4 of 89
 Rare
Alameda County 56
4% 2 of 56
 Rare
El Dorado County 68
3% 2 of 68
 Rare
San Benito County 35
3% 1 of 35
 Rare
San Luis Obispo County 40
2% 1 of 40
 Rare
Sacramento County 87
1% 1 of 87
 Rare
Santa Barbara County 95
1% 1 of 95
 Rare
Los Angeles County 180
0% 0 of 180
 Rare
Sonoma County 103
0% 0 of 103
 Rare
Tehama County 66
0% 0 of 66
 Rare
Shasta County 60
0% 0 of 60
 Rare
Nevada County 58
0% 0 of 58
 Rare
Santa Clara County 47
0% 0 of 47
 Rare
Ventura County 47
0% 0 of 47
 Rare
Butte County 46
0% 0 of 46
 Rare
Placer County 41
0% 0 of 41
 Rare
Santa Cruz County 38
0% 0 of 38
 Rare
Napa County 37
0% 0 of 37
 Rare
Imperial County 30
0% 0 of 30
 Rare
Mendocino County 26
0% 0 of 26
 Rare
Siskiyou County 22
0% 0 of 22
 Rare
Yuba County 21
0% 0 of 21
 Rare
Humboldt County 19
0% 0 of 19
 Rare
Sutter County 19
0% 0 of 19
 Rare
Glenn County 17
0% 0 of 17
 Rare
Amador County 16
0% 0 of 16
 Rare
Yolo County 13
0% 0 of 13
 Rare
San Mateo County 12
0% 0 of 12
 Rare
Trinity County 10
0% 0 of 10
 Rare

12 more California counties have 1 to 9 tested wells for uranium, too few to rate.

Download this table (CSV) · Free to reuse under CC BY 4.0

Where to get your well tested for uranium in California

1

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 uranium.

California's private well page
3

A mail-in kit

Fill the bottles at your kitchen tap and mail them back. We mark the kits that include uranium.

Compare kits

Keep reading about wells in California

Questions people ask about uranium in California

Is there uranium in California well water?

Uranium is widespread, over the limit in 15% or more of tested wells, in 7 of the 44 California counties with 10 or more tested wells. Across California, about 1 in 13 tested wells had uranium over the EPA limit of 30 µg/L (254 of 3,360). Across all tested wells in the US it's about 1 in 22. So uranium turns up somewhat more often in California than in the country as a whole. Check your county below.

Where can I get my well tested for uranium 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 uranium 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 the EPA limit of 30 µg/L. 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.