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Nitrate in California well water, by county

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

Nitrate is widespread, over the limit in 15% or more of tested wells, in 7 of the 47 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 a nitrate test See every county

How California compares with the rest of the country

Across California, about 1 in 10 tested wells had nitrate over the EPA limit of 10 mg/L (461 of 4,733). Across all tested wells in the US it's about 1 in 12. California is about average, so the county you live in tells you more than the state does.

Why it matters: It's dangerous for babies under 6 months, because it keeps their blood from carrying enough oxygen.

Nitrate soaks down into groundwater from fertilizer, manure and septic systems.

Nitrate in tested wells, county by county

Share of each county's tested wells with at least one result over the EPA limit of 10 mg/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
Tulare County 120
37% 44 of 120
 Widespread
Merced County 88
30% 26 of 88
 Widespread
Fresno County 149
29% 43 of 149
 Widespread
Stanislaus County 152
23% 35 of 152
 Widespread
Riverside County 375
21% 78 of 375
 Widespread
Monterey County 80
19% 15 of 80
 Widespread
San Joaquin County 126
16% 20 of 126
 Widespread
San Benito County 35
14% 5 of 35
 Worth checking
San Luis Obispo County 54
13% 7 of 54
 Worth checking
Santa Barbara County 241
10% 25 of 241
 Worth checking
Ventura County 59
10% 6 of 59
 Worth checking
Madera County 86
9% 8 of 86
 Worth checking
San Bernardino County 984
9% 88 of 984
 Worth checking
Yolo County 13
8% 1 of 13
 Worth checking
Kern County 185
7% 13 of 185
 Worth checking
San Diego County 212
7% 14 of 212
 Worth checking
Orange County 79
6% 5 of 79
 Worth checking
Sutter County 19
5% 1 of 19
 Worth checking
Lassen County 20
5% 1 of 20
 Worth checking
Humboldt County 21
5% 1 of 21
 Rare
Alameda County 57
4% 2 of 57
 Rare
Los Angeles County 438
3% 14 of 438
 Rare
Kings County 33
3% 1 of 33
 Rare
Napa County 50
2% 1 of 50
 Rare
Placer County 52
2% 1 of 52
 Rare
El Dorado County 115
2% 2 of 115
 Rare
Santa Clara County 78
1% 1 of 78
 Rare
Sonoma County 115
1% 1 of 115
 Rare
Sacramento County 87
0% 0 of 87
 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
Imperial County 51
0% 0 of 51
 Rare
Butte County 48
0% 0 of 48
 Rare
Santa Cruz County 38
0% 0 of 38
 Rare
Mendocino County 30
0% 0 of 30
 Rare
Mono County 28
0% 0 of 28
 Rare
Siskiyou County 26
0% 0 of 26
 Rare
Inyo County 23
0% 0 of 23
 Rare
Yuba County 21
0% 0 of 21
 Rare
Glenn County 18
0% 0 of 18
 Rare
Lake County 17
0% 0 of 17
 Rare
Modoc County 17
0% 0 of 17
 Rare
Amador County 16
0% 0 of 16
 Rare
San Mateo County 16
0% 0 of 16
 Rare
Calaveras County 13
0% 0 of 13
 Rare
Trinity County 10
0% 0 of 10
 Rare

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

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Where to get your well tested for nitrate 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 nitrate.

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 nitrate.

Compare kits

Keep reading about wells in California

Questions people ask about nitrate in California

Is there nitrate in California well water?

Nitrate is widespread, over the limit in 15% or more of tested wells, in 7 of the 47 California counties with 10 or more tested wells. Across California, about 1 in 10 tested wells had nitrate over the EPA limit of 10 mg/L (461 of 4,733). Across all tested wells in the US it's about 1 in 12. 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 nitrate 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 nitrate 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 10 mg/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.