Quick answer
About 1,384,000 people in California (3.7% of residents) got their water from private wells in 2010, according to USGS estimates. California State Water Resources Control Board runs the state’s private well guidance. Free domestic well testing programs funded or regulated by the State or Regional Water Boards operate in specific areas, and the State Water Board lists them in a Well Testing Program Directory.
- People on private wells
- 1,384,131
- Share of residents
- 3.7%
- Rank among states
- 12th
- Well users modeled above 10 µg/L arsenic
- 8.6%
Where to get your well tested in California
- California State Water Resources Control Board: GAMA - Domestic Well Owners, the state’s guidance for private well owners.
- Well water testing in California: what to test for and how to collect a sample.
- Certified laboratories in California. Use a lab certified for drinking water, so results are reliable.
Free or low-cost testing
Free domestic well testing programs funded or regulated by the State or Regional Water Boards operate in specific areas, and the State Water Board lists them in a Well Testing Program Directory. (source)
Mail-in lab kits are another option when no local lab is convenient. See well water test kits for what home kits can and can’t tell you.
What to test for
- Every year: total coliform bacteria and nitrate.
- Before buying equipment: pH, hardness, iron, manganese and, if there’s a smell, hydrogen sulfide.
- Based on location: arsenic, uranium and radon, PFAS, and lead in older plumbing.
USGS modeling estimates that about 8.6% of California’s private well users have water above the 10 µg/L arsenic limit. Arsenic has no taste or smell, so include it in your first test.
Private wells by county
Estimated people relying on private wells in each California county in 2010, the share of residents that represents, and the modeled share of well users with arsenic above 10 µg/L. These are USGS model estimates (2010 population), not measurements of any well. Arsenic is hidden for counties with fewer than 500 well users.
| County | People on wells | Share | Arsenic >10 µg/L |
|---|---|---|---|
| Fresno County | 101,117 | 10.9% | 8.2% |
| Sonoma County | 73,745 | 15.2% | 11.5% |
| San Joaquin County | 71,769 | 10.5% | 8.4% |
| Tulare County | 64,703 | 14.6% | 10.8% |
| Riverside County | 59,550 | 2.7% | 7.4% |
| Stanislaus County | 54,427 | 10.6% | 8.4% |
| Kern County | 51,015 | 6.1% | 13.1% |
| Madera County | 45,912 | 30.4% | 13.3% |
| San Diego County | 42,657 | 1.4% | 5.2% |
| Merced County | 41,986 | 16.4% | 15.3% |
| Butte County | 39,938 | 18.2% | 5.8% |
| Sacramento County | 39,820 | 2.8% | 9.9% |
Show all 58 counties
| San Bernardino County | 39,587 | 1.9% | 11.2% |
| Los Angeles County | 37,642 | 0.4% | 16.3% |
| Monterey County | 35,730 | 8.6% | 4.7% |
| Placer County | 32,548 | 9.3% | 3.2% |
| Tehama County | 31,316 | 49.3% | 6.0% |
| San Luis Obispo County | 30,816 | 11.4% | 5.8% |
| Shasta County | 28,215 | 15.9% | 6.0% |
| Nevada County | 27,680 | 28.0% | 2.5% |
| Santa Cruz County | 24,448 | 9.3% | 3.1% |
| El Dorado County | 24,131 | 13.3% | 1.7% |
| Mendocino County | 23,502 | 26.8% | 3.9% |
| Santa Clara County | 22,930 | 1.3% | 6.5% |
| Sutter County | 22,079 | 23.3% | 10.6% |
| Contra Costa County | 19,686 | 1.9% | 12.2% |
| Lassen County | 19,541 | 56.0% | 14.2% |
| Yuba County | 18,385 | 25.5% | 4.0% |
| Siskiyou County | 16,260 | 36.2% | 3.3% |
| Solano County | 16,028 | 3.9% | 9.3% |
| Yolo County | 15,750 | 7.8% | 13.6% |
| Kings County | 14,937 | 9.8% | 26.4% |
| Napa County | 14,278 | 10.5% | 10.7% |
| Lake County | 13,771 | 21.3% | 3.5% |
| Calaveras County | 13,714 | 30.1% | 2.5% |
| Humboldt County | 13,472 | 10.0% | 6.2% |
| Alameda County | 12,731 | 0.8% | 9.5% |
| Amador County | 12,362 | 32.5% | 1.9% |
| Mariposa County | 12,008 | 65.8% | 2.5% |
| Ventura County | 11,354 | 1.4% | 5.5% |
| Tuolumne County | 10,915 | 19.7% | 3.8% |
| Glenn County | 10,724 | 38.1% | 7.9% |
| Santa Barbara County | 8,776 | 2.1% | 5.2% |
| San Benito County | 8,223 | 14.9% | 7.9% |
| Del Norte County | 8,124 | 28.4% | 2.4% |
| Imperial County | 7,516 | 4.3% | 14.3% |
| Plumas County | 5,930 | 29.6% | 5.7% |
| San Mateo County | 5,594 | 0.8% | 3.5% |
| Orange County | 4,627 | 0.2% | 27.1% |
| Modoc County | 4,402 | 45.4% | 7.5% |
| Inyo County | 4,028 | 21.7% | 14.4% |
| Colusa County | 3,802 | 17.8% | 6.3% |
| Marin County | 3,168 | 1.3% | 6.8% |
| Trinity County | 2,883 | 20.9% | 5.0% |
| Mono County | 2,536 | 17.9% | 9.3% |
| Sierra County | 576 | 17.8% | 5.0% |
| San Francisco County | 501 | 0.1% | 4.3% |
| Alpine County | 266 | 22.7% | – |
Counties with the highest modeled arsenic share: Orange County (27.1%), Kings County (26.4%), Los Angeles County (16.3%), Merced County (15.3%), Inyo County (14.4%).
After you get results
Enter your numbers in the test results tool to see what needs treating and in what order. The guide to reading a well water test explains units and limits.
Common questions
Does California offer free well water testing?
Free domestic well testing programs funded or regulated by the State or Regional Water Boards operate in specific areas, and the State Water Board lists them in a Well Testing Program Directory.
Who oversees private wells in California?
California State Water Resources Control Board (GAMA - Domestic Well Owners) publishes guidance for private well owners. Private wells aren’t covered by the federal Safe Drinking Water Act, so testing and treatment are up to the owner.
How many people use private wells in California?
USGS estimates about 1,384,000 people, or 3.7% of the population in 2010. That ranks 12th of the states covered by the estimate.
What should I test my well for in California?
Test every year for total coliform bacteria and nitrate, plus pH, hardness, iron and manganese before buying equipment. Arsenic is a known concern in parts of California, so add it to the first test.
Sources
- California State Water Resources Control Board: GAMA - Domestic Well Owners
- California State Water Resources Control Board: well water testing
- California certified laboratories
- California free or low-cost testing
- USGS data release: domestic well locations and populations served, 2000 and 2010 (Johnson, Belitz and Lombard, 2019)
- CDC Environmental Public Health Tracking: private well water data (from USGS)