California Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in California. They describe the area's modeled risk — not any single well.
This is a modeled probability for the area, not a measurement of your well. USGS statistical models estimate how many private wells in this area are likely to exceed the federal limit — they cannot tell you what is in your water. Two neighbors can have very different results. The only way to know your well is to test it.
Nitrate
10.4%
of the private-well population is modeled above the 10 mg/L nitrate limit
~144,000
people on wells estimated above the limit
~1,380,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Arsenic
Across California, an estimated ~115,817 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Los Angeles County (~17,244 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Los Angeles County | ~17,244 | 12,915–22,850 |
| San Joaquin County | ~9,497 | 8,029–11,222 |
| Fresno County | ~8,027 | 6,705–9,627 |
| Merced County | ~7,499 | 6,248–8,969 |
| San Diego County | ~6,352 | 4,786–8,414 |
| Tulare County | ~5,344 | 4,469–6,420 |
| Stanislaus County | ~4,253 | 3,612–4,985 |
| San Luis Obispo County | ~3,676 | 2,758–4,844 |
| Kern County | ~3,647 | 2,951–4,490 |
| Contra Costa County | ~3,370 | 2,789–3,995 |
| Madera County | ~3,323 | 2,800–3,952 |
| Sonoma County | ~3,099 | 2,203–4,422 |
| Lassen County | ~2,919 | 2,256–3,690 |
| San Bernardino County | ~2,878 | 2,363–3,483 |
| Monterey County | ~2,775 | 2,099–3,644 |
| Riverside County | ~2,677 | 2,076–3,432 |
| Santa Barbara County | ~2,623 | 1,893–3,527 |
| Imperial County | ~2,526 | 2,124–2,944 |
| Santa Clara County | ~2,392 | 1,850–3,046 |
| Kings County | ~2,218 | 1,889–2,593 |
| Orange County | ~1,975 | 1,591–2,430 |
| Alameda County | ~1,941 | 1,453–2,583 |
| Tehama County | ~1,477 | 1,167–1,878 |
| Napa County | ~1,372 | 957–1,880 |
| Sacramento County | ~1,339 | 1,141–1,570 |
| Ventura County | ~1,206 | 868–1,679 |
| Placer County | ~1,193 | 920–1,541 |
| Yolo County | ~1,133 | 941–1,365 |
| Butte County | ~666 | 527–847 |
| Nevada County | ~656 | 481–882 |
| San Benito County | ~652 | 498–843 |
| Glenn County | ~575 | 478–694 |
| Santa Cruz County | ~527 | 342–822 |
| Yuba County | ~514 | 428–622 |
| Lake County | ~453 | 326–626 |
| Mariposa County | ~431 | 326–570 |
| Sutter County | ~350 | 285–439 |
| Solano County | ~326 | 251–427 |
| Modoc County | ~311 | 237–405 |
| San Mateo County | ~307 | 198–453 |
| Siskiyou County | ~280 | 212–370 |
| Mendocino County | ~265 | 201–347 |
| Calaveras County | ~254 | 178–355 |
| Shasta County | ~224 | 175–295 |
| El Dorado County | ~184 | 139–244 |
| Colusa County | ~172 | 140–212 |
| Humboldt County | ~129 | 88–180 |
| Mono County | ~108 | 78–150 |
| Amador County | ~101 | 73–138 |
9 additional California counties have lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit).
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Treating nitrate in California
Nitrate is not removed by carbon pitchers or faucet filters. Reverse osmosis is the treatment that brings it below the federal limit at your tap.
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