Free testing, with limits
Free testing in California depends on where you live: 11 local domestic well programs offer it in their areas. Check which tests it includes before you count on it for arsenic.
California's private well pageArsenic in California well water, by county
Arsenic is widespread, over the limit in 15% or more of tested wells, in 10 of the 47 California counties with 10 or more tested wells. A county's share tells you how likely a problem is nearby. Only a test of your own well tells you what's in it.
How California compares with the rest of the country
Across California, about 1 in 7 tested wells had arsenic over the EPA limit of 10 µg/L (652 of 4,520). Across all tested wells in the US it's about 1 in 9. California is about average, so the county you live in tells you more than the state does.
Why it matters: Drinking it for years raises the risk of skin, bladder and lung cancer.
Arsenic dissolves out of certain rocks and old sediments, so it can be high even where nobody has polluted anything.
Arsenic in tested wells, county by county
Share of each county's tested wells with at least one result over the EPA limit of 10 µg/L. Counties need 10 or more tested wells to be listed. Widespread means 15% or more, worth checking 5 to 15%, rare under 5%.
| County | Wells tested | Wells over the limit | Level |
|---|---|---|---|
| Mono County | 35 | Widespread | |
| Kings County | 33 | Widespread | |
| San Joaquin County | 137 | Widespread | |
| Napa County | 52 | Widespread | |
| Sutter County | 19 | Widespread | |
| Kern County | 207 | Widespread | |
| San Bernardino County | 984 | Widespread | |
| Sacramento County | 87 | Widespread | |
| Inyo County | 23 | Widespread | |
| San Mateo County | 12 | Widespread | |
| Merced County | 74 | Worth checking | |
| Los Angeles County | 472 | Worth checking | |
| Mendocino County | 32 | Worth checking | |
| San Diego County | 207 | Worth checking | |
| Riverside County | 343 | Worth checking | |
| Sonoma County | 114 | Worth checking | |
| Madera County | 87 | Worth checking | |
| Tulare County | 120 | Worth checking | |
| Lassen County | 21 | Worth checking | |
| Imperial County | 34 | Worth checking | |
| Orange County | 60 | Worth checking | |
| Lake County | 12 | Worth checking | |
| Siskiyou County | 26 | Worth checking | |
| Calaveras County | 13 | Worth checking | |
| Yolo County | 13 | Worth checking | |
| Fresno County | 148 | Worth checking | |
| El Dorado County | 68 | Worth checking | |
| Alameda County | 58 | Worth checking | |
| Santa Barbara County | 152 | Worth checking | |
| Amador County | 16 | Worth checking | |
| Modoc County | 17 | Worth checking | |
| Stanislaus County | 139 | Worth checking | |
| Santa Cruz County | 37 | Worth checking | |
| Nevada County | 58 | Worth checking | |
| Placer County | 41 | Rare | |
| Humboldt County | 21 | Rare | |
| Tehama County | 66 | Rare | |
| San Luis Obispo County | 41 | Rare | |
| Butte County | 46 | Rare | |
| Ventura County | 65 | Rare | |
| Monterey County | 80 | Rare | |
| Santa Clara County | 60 | Rare | |
| Shasta County | 60 | Rare | |
| San Benito County | 28 | Rare | |
| Yuba County | 21 | Rare | |
| Glenn County | 17 | Rare | |
| Trinity County | 10 | Rare |
9 more California counties have 1 to 9 tested wells for arsenic, too few to rate.
Download this table (CSV) · Free to reuse under CC BY 4.0
Where to get your well tested for arsenic in California
A lab California certifies
Ask for arsenic by name. Results from a certified lab count for a home sale or a loan.
Labs certified by CaliforniaA mail-in kit
Fill the bottles at your kitchen tap and mail them back. We mark the kits that include arsenic.
Compare kitsKeep reading about wells in California
- Well water testing in California: how to get tested and what turns up statewide
- Arsenic in well water: where it comes from, what it does and how to remove it
- Nitrate in California wells, by county
- Uranium in California wells, by county
- Manganese in California wells, by county
- Lead in California wells, by county
- Radon in California wells, by county
Questions people ask about arsenic in California
Is there arsenic in California well water?
Arsenic is widespread, over the limit in 15% or more of tested wells, in 10 of the 47 California counties with 10 or more tested wells. Across California, about 1 in 7 tested wells had arsenic over the EPA limit of 10 µg/L (652 of 4,520). Across all tested wells in the US it's about 1 in 9. California is about average, so the county you live in tells you more than the state does.
Where can I get my well tested for arsenic in California?
Free testing in California depends on where you live: 11 local domestic well programs offer it in their areas. If that doesn't cover you, use a lab certified by California for drinking water, or a mail-in kit that includes arsenic from a certified lab.
See California's list of certified labs, or compare mail-in kits.
Where these numbers come from
- Lab tests: Water Quality Portal (USGS and EPA), well samples since 2000. A well counts as over if any of its tests was over the EPA limit of 10 µg/L. Wells tested for studies and monitoring, not a random sample of homes.
- Likely overstated: our own check of who tested the wells, when and where. How we check.
- State programs and links: State Water Resources Control Board.