Private Well Water Testing in California
County-by-county USGS area-risk estimates for private wells across California — arsenic exceedance probability and private-well population — plus how to test your own well at a California-certified laboratory.
California private-well risk summary
Counties covered
58
California counties with modeled risk data.
People on private wells
area estimate1,376,408
estimated across covered California counties.
Average arsenic risk
area estimate8%
average modeled chance a well exceeds 10 µg/L arsenic across covered counties.
California counties
- Fresno County11% arsenic exceedance (area estimate)101,210 people on private wells
- Sonoma County7% arsenic exceedance (area estimate)73,801 people on private wells
- San Joaquin County10% arsenic exceedance (area estimate)71,848 people on private wells
- Tulare County8% arsenic exceedance (area estimate)64,398 people on private wells
- Riverside County11% arsenic exceedance (area estimate)59,570 people on private wells
- Stanislaus County8% arsenic exceedance (area estimate)54,435 people on private wells
- Kern County15% arsenic exceedance (area estimate)51,182 people on private wells
- Madera County9% arsenic exceedance (area estimate)45,858 people on private wells
- San Diego County9% arsenic exceedance (area estimate)42,637 people on private wells
- Merced County13% arsenic exceedance (area estimate)41,888 people on private wells
- Butte County6% arsenic exceedance (area estimate)39,954 people on private wells
- Sacramento County10% arsenic exceedance (area estimate)39,792 people on private wells
- San Bernardino County22% arsenic exceedance (area estimate)39,511 people on private wells
- Los Angeles County12% arsenic exceedance (area estimate)37,641 people on private wells
- Monterey County6% arsenic exceedance (area estimate)35,708 people on private wells
- Placer County3% arsenic exceedance (area estimate)31,639 people on private wells
- Tehama County6% arsenic exceedance (area estimate)31,186 people on private wells
- San Luis Obispo County6% arsenic exceedance (area estimate)30,844 people on private wells
- Shasta County5% arsenic exceedance (area estimate)28,279 people on private wells
- Nevada County3% arsenic exceedance (area estimate)27,683 people on private wells
- Santa Cruz County4% arsenic exceedance (area estimate)24,534 people on private wells
- El Dorado County2% arsenic exceedance (area estimate)24,186 people on private wells
- Mendocino County6% arsenic exceedance (area estimate)23,759 people on private wells
- Santa Clara County6% arsenic exceedance (area estimate)22,914 people on private wells
- Sutter County15% arsenic exceedance (area estimate)22,164 people on private wells
- Contra Costa County8% arsenic exceedance (area estimate)19,661 people on private wells
- Yuba County3% arsenic exceedance (area estimate)17,482 people on private wells
- Siskiyou County4% arsenic exceedance (area estimate)16,248 people on private wells
- Solano County11% arsenic exceedance (area estimate)15,999 people on private wells
- Yolo County11% arsenic exceedance (area estimate)15,743 people on private wells
- Kings County20% arsenic exceedance (area estimate)14,946 people on private wells
- Napa County7% arsenic exceedance (area estimate)14,377 people on private wells
- Calaveras County3% arsenic exceedance (area estimate)13,717 people on private wells
- Lake County3% arsenic exceedance (area estimate)13,675 people on private wells
- Humboldt County7% arsenic exceedance (area estimate)13,617 people on private wells
- Lassen County13% arsenic exceedance (area estimate)13,105 people on private wells
- Alameda County10% arsenic exceedance (area estimate)12,683 people on private wells
- Amador County2% arsenic exceedance (area estimate)12,334 people on private wells
- Mariposa County3% arsenic exceedance (area estimate)11,958 people on private wells
- Ventura County5% arsenic exceedance (area estimate)11,437 people on private wells
- Tuolumne County6% arsenic exceedance (area estimate)10,949 people on private wells
- Glenn County6% arsenic exceedance (area estimate)10,754 people on private wells
- Santa Barbara County6% arsenic exceedance (area estimate)8,824 people on private wells
- San Benito County6% arsenic exceedance (area estimate)8,252 people on private wells
- Del Norte County4% arsenic exceedance (area estimate)8,183 people on private wells
- Imperial County16% arsenic exceedance (area estimate)7,476 people on private wells
- Plumas County6% arsenic exceedance (area estimate)5,902 people on private wells
- San Mateo County5% arsenic exceedance (area estimate)5,784 people on private wells
- Orange County14% arsenic exceedance (area estimate)4,643 people on private wells
- Modoc County8% arsenic exceedance (area estimate)4,353 people on private wells
- Inyo County29% arsenic exceedance (area estimate)3,976 people on private wells
- Colusa County5% arsenic exceedance (area estimate)3,743 people on private wells
- Marin County8% arsenic exceedance (area estimate)3,239 people on private wells
- Trinity County7% arsenic exceedance (area estimate)2,871 people on private wells
- Mono County9% arsenic exceedance (area estimate)2,498 people on private wells
- Sierra County5% arsenic exceedance (area estimate)608 people on private wells
- San Francisco County6% arsenic exceedance (area estimate)504 people on private wells
- Alpine County12% arsenic exceedance (area estimate)246 people on private wells
Find a California-certified testing lab
Get your well tested at a California-certified laboratory. We list certification programs by state.
These figures are USGS area estimates: statistical groundwater models describing how likely elevated contaminant levels are across a county. They are not designed to predict the concentration in any single well. Only testing your own well reveals its water quality.
By TapWaterData Editorial
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