Private wells
California well water: who's on a well and what to test for
People on private wells
2,185,200
Homes on wells
5.9%
Est. above arsenic limit
115,800
Every California county
Sorted by people on private wells. Arsenic and nitrate are modeled for typical wells in the county, not measured at yours; depth is the median of domestic well records.
| Riverside County | 227,200 | 9% | 7.5% | 2.8 | 295 ft |
| Fresno County | 149,800 | 16% | 8.3% | 9.3 | 230 ft |
| Los Angeles County | 113,500 | 1% | 16.3% | 1.6 | 300 ft |
| Sonoma County | 95,500 | 21% | 11.6% | 0.3 | 213 ft |
| San Diego County | 90,500 | 3% | 5.2% | 0.7 | 269 ft |
| Tulare County | 89,500 | 20% | 10.7% | 11.1 | 275 ft |
| San Bernardino County | 80,800 | 4% | 11.3% | 3.6 | 290 ft |
| Placer County | 79,200 | 19% | 3.1% | 2.3 | 200 ft |
| San Joaquin County | 74,900 | 10% | 8.4% | 4.0 | 255 ft |
| Stanislaus County | 68,100 | 12% | 8.4% | 11.8 | 190 ft |
| Kern County | 53,700 | 7% | 13.3% | 2.3 | 312 ft |
| El Dorado County | 53,500 | 28% | 1.6% | 0.2 | 250 ft |
| Merced County | 50,400 | 19% | 15.3% | 5.6 | 235 ft |
| Nevada County | 48,300 | 42% | 2.6% | 0.1 | 205 ft |
| Butte County | 47,300 | 23% | 5.8% | 1.1 | 165 ft |
| Madera County | 47,200 | 35% | 13.4% | 3.2 | 350 ft |
| Orange County | 45,800 | 2% | 27.1% | 2.0 | 208 ft |
| Monterey County | 43,600 | 11% | 4.7% | 1.9 | 290 ft |
| Sacramento County | 43,300 | 3% | 9.9% | 3.5 | 231 ft |
| Santa Cruz County | 40,900 | 17% | 3.1% | 0.2 | 280 ft |
| Yolo County | 40,500 | 18% | 13.3% | 1.4 | 215 ft |
| Tehama County | 39,000 | 59% | 5.9% | 2.1 | 190 ft |
| San Luis Obispo County | 38,800 | 12% | 5.9% | 1.1 | 300 ft |
| Mendocino County | 37,900 | 44% | 3.9% | 0.1 | 160 ft |
| Shasta County | 37,600 | 22% | 6.1% | 1.5 | 184 ft |
| Contra Costa County | 29,400 | 3% | 12.2% | 1.3 | 200 ft |
| Imperial County | 28,400 | 18% | 14.2% | 0.3 | 250 ft |
| Kings County | 27,000 | 19% | 26.3% | 1.0 | 246 ft |
| Yuba County | 25,200 | 33% | 3.9% | 3.1 | 180 ft |
| Santa Barbara County | 21,900 | 5% | 5.2% | 0.8 | 220 ft |
| Tuolumne County | 21,600 | 36% | 3.7% | 0.1 | 300 ft |
| Calaveras County | 21,600 | 50% | 2.5% | 0.1 | 300 ft |
| Napa County | 20,300 | 18% | 10.7% | 0.2 | 300 ft |
| Santa Clara County | 19,800 | 1% | 6.5% | 2.2 | 250 ft |
| Humboldt County | 18,000 | 13% | 6.1% | 0.1 | 140 ft |
| Siskiyou County | 17,900 | 41% | 3.3% | 0.3 | 160 ft |
| Amador County | 17,600 | 40% | 1.8% | 0.2 | 285 ft |
| Lake County | 16,400 | 25% | 3.4% | 0.3 | 133 ft |
| Sutter County | 16,200 | 18% | 10.6% | 3.4 | 112 ft |
| Solano County | 15,000 | 4% | 9.4% | 1.5 | 200 ft |
| Mariposa County | 12,200 | 70% | 2.8% | 0.3 | 250 ft |
| San Benito County | 11,600 | 19% | 7.7% | 2.0 | 230 ft |
| Lassen County | 11,200 | 44% | 14.2% | 0.5 | 160 ft |
| Marin County | 10,800 | 4% | 6.5% | 0.3 | 180 ft |
| Glenn County | 10,600 | 39% | 7.8% | 4.9 | 160 ft |
| Alameda County | 10,300 | 1% | 9.5% | 1.6 | 214 ft |
| Plumas County | 8,700 | 39% | 5.7% | 0.1 | 150 ft |
| San Mateo County | 8,600 | 1% | 3.4% | 0.1 | 170 ft |
| Trinity County | 8,600 | 52% | 4.9% | 0.1 | 195 ft |
| Del Norte County | 8,500 | 34% | 2.6% | 0.4 | 52 ft |
| Ventura County | 7,900 | 1% | 5.6% | 1.1 | 224 ft |
| Inyo County | 7,800 | 40% | 14.8% | 0.8 | 180 ft |
| Colusa County | 5,500 | 29% | 6.3% | 0.4 | 179 ft |
| Modoc County | 4,300 | 53% | 7.6% | 0.5 | 200 ft |
| Mono County | 3,400 | 17% | 9.2% | 0.4 | 207 ft |
| Sierra County | 1,200 | 37% | 5.3% | 0.1 | 175 ft |
| Alpine County | 840 | 43% | 11.9% | 0.1 | 240 ft |
| San Francisco County | 10 | 0% | 4.3% | 0.4 | n/a |
Shading: red is among the highest counties nationally, amber the middle, green the lower range. Click a column to sort.
Most well users above the arsenic limit
- Los Angeles County: about 17,200 people
- San Joaquin County: about 9,500 people
- Fresno County: about 8,000 people
- Merced County: about 7,500 people
- San Diego County: about 6,400 people
Highest modeled nitrate
- Stanislaus County: 11.8 mg/L average (limit 10)
- Tulare County: 11.1 mg/L average (limit 10)
- Fresno County: 9.3 mg/L average (limit 10)
- Merced County: 5.6 mg/L average (limit 10)
- Glenn County: 4.9 mg/L average (limit 10)
Test first, then treat what the test finds
We may earn a commission from links below, at no cost to you. Picks follow the testing order, not who pays. How we make money.
1. Certified lab well test
Start here, every year
A mail-in kit analyzed by a certified lab measures bacteria, nitrate, arsenic, metals and more. Test strips cannot measure arsenic or uranium reliably.
2. Sediment pre-filter
Sand, silt or cloudy water
A 5-micron cartridge after the pressure tank catches sand, silt and rust before they reach fixtures or other filters. Cartridges are cheap and swap every few months.
3. Iron and sulfur filter
Orange stains or rotten-egg smell
Air-injection or oxidizing-media filters remove orange staining and rotten-egg smell. Size one from a lab result for iron, manganese and hydrogen sulfide.
4. UV disinfection system
Coliform bacteria in a test
Inactivates bacteria and viruses; usually for private wells or after boil notices.
5. Under-sink reverse osmosis
Arsenic, nitrate or uranium in a test
The most thorough option for drinking water: PFAS, lead, nitrate, arsenic and more. Look for NSF/ANSI 58.
6. Whole-house well filter
Several problems at once
Multi-stage sediment and carbon for every tap. Add UV if bacteria shows up, and choose stages from your lab results, not a guess.
Questions
Which California counties rely most on wells?+
By share of homes: Mariposa County (70%), Tehama County (59%), Modoc County (53%), Trinity County (52%), Calaveras County (50%). By number of people: Riverside County (227,200), Fresno County (149,800), Los Angeles County (113,500).
Is arsenic a problem in California wells?+
USGS estimates about 115,800 California well users have water above the 10 microgram per liter arsenic limit that applies to public systems, the largest numbers in Los Angeles County, San Joaquin County, Fresno County. Arsenic has no taste or smell, so only a lab test finds it.
Do I have to test my well when I sell a house in California?+
We could not confirm a statewide rule for California. Check with your county health department. Buyers can still make a lab test a condition of the offer.
How often should I test my well?+
No federal law covers private wells. EPA's drinking water limits apply to public water systems, so testing and treating a private well is up to the owner. EPA and CDC recommend testing at least once a year for coliform bacteria, nitrate, pH and total dissolved solids, and testing for arsenic, uranium, radon and other local concerns based on where you live.
Well-user counts, arsenic and nitrate figures are each a modeled estimate built from national datasets, not a count of tested wells, and well depths summarize drilling records rather than every well. Your own well can be very different from the county average, which is why the only answer for your home is a lab test. Sources: EPA Office of Research and Development, Water Source 2020 (private well estimates by census block); CDC Environmental Public Health Tracking, Well Water Index (from USGS nitrate and arsenic models); USGS, county-level domestic well population with arsenic above 10 micrograms per liter (Ayotte and others, 2017); USGS Synthesized Domestic Well Database for the Conterminous United States (SynDWell-US), McCarthy and others, 2025 (depth, where a county has 30+ usable records).