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.
Quick answer: Yes — arsenic occurs naturally in California groundwater. USGS modeling estimates about 115,817 California residents rely on private wells with arsenic above the EPA limit of 10 µg/L. The highest modeled exposure is in Los Angeles County (~17,244), San Joaquin County (~9,497), Fresno County (~8,027). For nitrate, about 10.4% of California's ~1,380,000 private-well residents are modeled above the 10 mg/L limit. These are area estimates, not a test of your well — arsenic has no taste or smell, so only a certified lab test tells you your level.
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.
Find out what's actually in your well
California has one of the higher modeled arsenic exposures in the country — but that number is an area estimate, not your well. Arsenic, nitrate and coliform bacteria are the three most common private-well problems, and none of them has a taste or smell. A certified lab test checks all of them at once; the CDC and EPA recommend testing every well at least once a year.
Basic screening only. Confirm arsenic, nitrate and bacteria with a certified lab.
Find a certified drinking-water labAmazon links are affiliate links — we may earn a commission at no extra cost to you.
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). Of these, 8 have measured USGS well samples: Alpine, Del Norte, Inyo, Marin, Plumas, San Francisco, Trinity, Tuolumne.
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Measured well tests by county
The USGS has published laboratory results from wells in 57 California counties (at least 5 wells each). These are real measurements of other wells — domestic, public-supply and monitoring wells — not a test of yours. The tables rank counties by the share of sampled wells over each federal threshold.
Nitrate (as N): counties with the most wells > 10 mg/L (EPA MCL)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Fresno County | 36% | 5.9 mg/L | 229 wells |
| San Francisco County | 29% | 6.7 mg/L | 14 wells |
| Tulare County | 25% | 5.2 mg/L | 89 wells |
| Merced County | 22% | 3.4 mg/L | 85 wells |
| Stanislaus County | 21% | 4.4 mg/L | 135 wells |
| Monterey County | 20% | 1.5 mg/L | 95 wells |
| Kings County | 19% | 0.3 mg/L | 81 wells |
| San Luis Obispo County | 18% | 1.3 mg/L | 60 wells |
| San Joaquin County | 17% | 0.8 mg/L | 99 wells |
| Riverside County | 15% | 2.5 mg/L | 421 wells |
Manganese: counties with the most wells > 300 µg/L (EPA health advisory)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Amador County | 73% | 10,050 µg/L | 22 wells |
| Kings County | 48% | 280 µg/L | 87 wells |
| San Benito County | 33% | 47 µg/L | 6 wells |
| Napa County | 30% | 93 µg/L | 47 wells |
| Yolo County | 28% | 21 µg/L | 18 wells |
| Santa Barbara County | 26% | 53 µg/L | 252 wells |
| Humboldt County | 25% | 25 µg/L | 16 wells |
| Sonoma County | 20% | 34 µg/L | 91 wells |
| San Mateo County | 19% | 69 µg/L | 26 wells |
| San Diego County | 18% | 40 µg/L | 201 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (manganese, pH, arsenic) and McMahon et al. (2019), Environ. Sci. Technol. 53:29–38 (nitrate, iron); aggregated by county with Census 2020 boundaries. USGS well samples; not a test of your well.
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California well water: FAQ
Is there arsenic in well water in California?
Yes. USGS modeling estimates about 115,817 people in California use private wells with arsenic above 10 µg/L, across 58 modeled counties. Arsenic comes from natural bedrock and varies well to well, even between neighbors, so a lab test is the only way to know your own level.
Which California counties have the most arsenic in well water?
By modeled number of well residents over the arsenic limit: Los Angeles County (~17,244), San Joaquin County (~9,497), Fresno County (~8,027). Counties with more private-well users rank higher, so a smaller county can still have a high share of affected wells.
How do I remove arsenic from well water in California?
Test first. If arsenic is present, a point-of-use reverse-osmosis system under the kitchen sink removes 90%+ of it; boiling does not remove arsenic (it concentrates it), and carbon pitchers and water softeners do not remove it either. Whole-house adsorptive arsenic media is used when levels are high. Re-test after installing and then annually.
Is nitrate a problem in California well water?
USGS models about 10.4% of California's private-well population (~144,000 people) above the 10 mg/L nitrate limit. Nitrate is highest in shallow wells near fertilized cropland, septic systems or livestock, and matters most for infants and pregnancy.
Do I need an acid neutralizer for my well in California?
Acidic well water is uncommon in California, but it does occur. In USGS lab results from 10,402 wells across 57 California counties, 2% had pH below 6.5. The most acidic sampled counties are Tuolumne County (64% of 14 wells), Amador County (53% of 30 wells), El Dorado County (33% of 61 wells). You only need a neutralizer if your own well tests below about 6.5 — signs include blue-green stains on fixtures and pinhole leaks in copper pipe. A calcite (or calcite/Corosex) neutralizer tank handles mildly acidic water (roughly pH 6–6.5); very acidic water usually needs soda-ash injection. Test first-draw lead and copper too, since acidic water dissolves both from plumbing.
High manganese in California well water — what should I do?
In USGS lab results from 5,013 wells across 54 California counties, 10% were above the EPA's 300 µg/L health advisory for manganese and 26% were above the 50 µg/L level where it stains fixtures and laundry black or brown. Sampled counties with the highest share over the health advisory: Amador County (73% of 22 wells), Kings County (48% of 87 wells), San Benito County (33% of 6 wells). If your own test is over 300 µg/L, don't use the water for infant formula and use another source for drinking and cooking until it is treated — high manganese is linked to neurological effects, especially in babies and children. Boiling does not remove it. For the whole house, an oxidizing filter (greensand, Birm or air-injection) handles manganese, usually alongside iron; a water softener can remove low levels. For drinking water, a point-of-use reverse osmosis system reduces manganese at the kitchen tap. Retest after treatment to confirm it worked.