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privatewelldata / States / California

California well water testing

135 labs are listed on California’s official certified-lab list; 14% of 8,314 sampled wells were above the arsenic limit across 53 counties. Updated 2026-10-17.

Official lab list for California

California State Water Resources Control BoardThis state agency certifies the labs that may test drinking water. Use its list to pick a certified lab for your well sample.waterboards.maps.arcgis.com · checked 2026-10-18 · link via EPA

Coverage map

Map of California: 129 listed labs open to private wells shown as dots, and 53 counties shaded by the share of sampled wells above the arsenic limit.
● 129 listed labs open to private wells (placed by town or postal code). Counties shaded by share of sampled wells above 10 µg/L arsenic: 0 1–5% 5–10% 10–25% 25–50% 50%+ grey = no data. Sources: official state list; USGS/EPA Water Quality Portal; US Census boundaries · privatewelldata.com

Where the labs are

90 rows · showing 1–25

Listed labs by town. Source: official state list · privatewelldata.com
TownLabs
San Diego8
Sacramento5
Signal Hill5
Tustin4
Bakersfield3
Fresno3
Pomona3
San Luis Obispo3
Ventura3
Anaheim2
Burlingame2
Cerritos2
El Dorado Hills2
Fairfield2
Irvine2
Long Beach2
Los Angeles2
Modesto2
Orange2
Rancho Cordova2
San Jose2
Santa Rosa2
Stockton2
Turlock2
Vista2
West Sacramento2
Anderson1
Arcata1
Brawley1
Carson1
Chico1
Chino1
Chino Hills1
City of Industry1
Clovis1
Commerce1
Compton1
Costa Mesa1
Cypress1
Davis1
El Centro1
El Segundo1
Elk Grove1
Emeryville1
Escalon1
Folsom1
Fremont1
Gilroy1
Glendale1
Grand Terrace1
Grass Valley1
Hayward1
Hesperia1
Huntington Beach1
Jackson1
Kettleman City1
Laguna Hills1
Lake Forest1
Livermore1
Lodi1
Lompoc1
Milpitas1
Monterey1
Morgan Hill1
Napa1
National City1
Oakland1
Ontario1
Petaluma1
Pittsburg1
Rancho Dominguez1
Redding1
Riverside1
Salinas1
San Bernardino1
San Carlos1
San Fernando1
San Francisco1
San Leandro1
San Mateo1
Santa Fe Springs1
Santa Maria1
Santa Paula1
South Pasadena1
Thousand Oaks1
Torrance1
Tulare1
Twain Harte1
Ukiah1
Visalia1

Well water results by county

53 rows · showing 1–25

Share of sampled wells above the EPA limit. Source: USGS/EPA Water Quality Portal · privatewelldata.com
CountyArsenic > 10 µg/LNitrate > 10 mg/L
Alameda County8% of 8812% of 267
Amador County5% of 2110% of 21
Butte County3% of 1307% of 202
Calaveras County7% of 150% of 15
Colusa County8% of 1362% of 164
Contra Costa County–11% of 70
El Dorado County6% of 861% of 145
Fresno County4% of 55631% of 970
Glenn County3% of 881% of 200
Humboldt County5% of 223% of 38
Imperial County20% of 813% of 224
Inyo County17% of 875% of 143
Kern County29% of 5056% of 1,349
Kings County36% of 14915% of 228
Lake County7% of 150% of 22
Lassen County12% of 425% of 63
Los Angeles County13% of 5972% of 870
Madera County8% of 1186% of 245
Mariposa County0% of 320% of 49
Mendocino County13% of 711% of 128
Merced County12% of 23614% of 497
Modoc County0% of 170% of 19
Mono County57% of 632% of 52
Monterey County6% of 29016% of 293
Napa County25% of 552% of 53
Nevada County5% of 620% of 64
Orange County8% of 663% of 486
Placer County5% of 1102% of 144
Plumas County0% of 100% of 10
Riverside County9% of 54614% of 1,370
Sacramento County19% of 2340% of 270
San Benito County0% of 3016% of 37
San Bernardino County25% of 1,44110% of 2,260
San Diego County8% of 2798% of 843
San Francisco County–30% of 46
San Joaquin County19% of 3028% of 520
San Luis Obispo County2% of 5916% of 140
San Mateo County17% of 122% of 45
Santa Barbara County7% of 22611% of 1,260
Santa Clara County0% of 825% of 113
Santa Cruz County2% of 1159% of 178
Shasta County3% of 990% of 151
Siskiyou County7% of 290% of 45
Solano County4% of 488% of 112
Sonoma County8% of 1231% of 133
Stanislaus County5% of 23813% of 411
Sutter County32% of 943% of 158
Tehama County3% of 1441% of 222
Trinity County0% of 120% of 12
Tulare County19% of 18219% of 353
Ventura County1% of 767% of 292
Yolo County6% of 1363% of 231
Yuba County2% of 592% of 111

Nitrate: 10% of 16,344 sampled wells statewide. All California counties →

See all 398 California lab listings →

How to choose a lab

  1. Pick a lab on the official list above that is certified for each test you need (bacteria and chemistry are often separate certifications).
  2. Ask for the lab’s own sample bottles and instructions; bacteria samples usually must reach the lab within 24–30 hours.
  3. Compare prices for a basic package (bacteria, nitrate, pH/TDS) and add arsenic or other metals where local data shows a risk.
  4. Be wary of free in-home tests from treatment-system sellers — see our salesperson checklist.

Based on CDC and EPA private-well guidance.

Questions

Where can I get my well water tested in California?

Use a lab on California’s official certified-lab list. We count 135 listed labs (excluding those marked as serving public water systems only), most in San Diego, Sacramento, Signal Hill. Confirm the lab is certified for the exact tests you need before you send a sample.

What should I test my well for in California?

Test every year for bacteria (total coliform and E. coli), nitrate, pH and total dissolved solids (CDC). In California, 14% of sampled wells with arsenic data were above the EPA limit, so include arsenic. Use our test helper for a list that fits your home.

Is there well water data for my county in California?

Yes for 53 counties: we show the share of sampled wells above the EPA arsenic and nitrate limits, from the Water Quality Portal (USGS/EPA). These are monitoring wells as well as private wells, so test your own well.

Get a test list for your home → · If a result is high