California well water by county: arsenic and nitrate
Modelled arsenic and nitrate for private-well users in every California county, what to test for, and where to find a certified lab. Private wells aren’t covered by the Safe Drinking Water Act.
All 58 counties
- Fresno CountyWell users 101,100Arsenic about 8 in 100Nitrate 9.3 mg/L
- Sonoma CountyWell users 73,700Arsenic about 12 in 100Nitrate 0.3 mg/L
- San Joaquin CountyWell users 71,800Arsenic about 8 in 100Nitrate 4.0 mg/L
- Tulare CountyWell users 64,700Arsenic about 11 in 100Nitrate 11.1 mg/L
- Riverside CountyWell users 59,600Arsenic about 7 in 100Nitrate 2.8 mg/L
- Stanislaus CountyWell users 54,400Arsenic about 8 in 100Nitrate 11.8 mg/L
- Kern CountyWell users 51,000Arsenic about 13 in 100Nitrate 2.3 mg/L
- Madera CountyWell users 45,900Arsenic about 13 in 100Nitrate 3.2 mg/L
- San Diego CountyWell users 42,700Arsenic about 5 in 100Nitrate 0.7 mg/L
- Merced CountyWell users 42,000Arsenic about 15 in 100Nitrate 5.6 mg/L
- Butte CountyWell users 39,900Arsenic about 6 in 100Nitrate 1.1 mg/L
- Sacramento CountyWell users 39,800Arsenic about 10 in 100Nitrate 3.5 mg/L
- San Bernardino CountyWell users 39,600Arsenic about 11 in 100Nitrate 3.6 mg/L
- Los Angeles CountyWell users 37,600Arsenic about 16 in 100Nitrate 1.6 mg/L
- Monterey CountyWell users 35,700Arsenic about 5 in 100Nitrate 1.9 mg/L
- Placer CountyWell users 32,500Arsenic about 3 in 100Nitrate 2.3 mg/L
- Tehama CountyWell users 31,300Arsenic about 6 in 100Nitrate 2.1 mg/L
- San Luis Obispo CountyWell users 30,800Arsenic about 6 in 100Nitrate 1.1 mg/L
- Shasta CountyWell users 28,200Arsenic about 6 in 100Nitrate 1.5 mg/L
- Nevada CountyWell users 27,700Arsenic about 3 in 100Nitrate 0.1 mg/L
- Santa Cruz CountyWell users 24,400Arsenic about 3 in 100Nitrate 0.2 mg/L
- El Dorado CountyWell users 24,100Arsenic about 2 in 100Nitrate 0.2 mg/L
- Mendocino CountyWell users 23,500Arsenic about 4 in 100Nitrate 0.1 mg/L
- Santa Clara CountyWell users 22,900Arsenic about 7 in 100Nitrate 2.2 mg/L
- Sutter CountyWell users 22,100Arsenic about 11 in 100Nitrate 3.4 mg/L
- Contra Costa CountyWell users 19,700Arsenic about 12 in 100Nitrate 1.3 mg/L
- Lassen CountyWell users 19,500Arsenic about 14 in 100Nitrate 0.5 mg/L
- Yuba CountyWell users 18,400Arsenic about 4 in 100Nitrate 3.1 mg/L
- Siskiyou CountyWell users 16,300Arsenic about 3 in 100Nitrate 0.4 mg/L
- Solano CountyWell users 16,000Arsenic about 9 in 100Nitrate 1.5 mg/L
- Yolo CountyWell users 15,800Arsenic about 13 in 100Nitrate 1.4 mg/L
- Kings CountyWell users 14,900Arsenic about 26 in 100Nitrate 1.0 mg/L
- Napa CountyWell users 14,300Arsenic about 11 in 100Nitrate 0.2 mg/L
- Lake CountyWell users 13,800Arsenic about 3 in 100Nitrate 0.3 mg/L
- Calaveras CountyWell users 13,700Arsenic about 2 in 100Nitrate 0.1 mg/L
- Humboldt CountyWell users 13,500Arsenic about 6 in 100Nitrate 0.1 mg/L
- Alameda CountyWell users 12,700Arsenic about 9 in 100Nitrate 1.6 mg/L
- Amador CountyWell users 12,400Arsenic about 2 in 100Nitrate 0.2 mg/L
- Mariposa CountyWell users 12,000Arsenic about 3 in 100Nitrate 0.2 mg/L
- Ventura CountyWell users 11,400Arsenic about 6 in 100Nitrate 1.1 mg/L
- Tuolumne CountyWell users 10,900Arsenic about 4 in 100Nitrate 0.1 mg/L
- Glenn CountyWell users 10,700Arsenic about 8 in 100Nitrate 4.9 mg/L
- Santa Barbara CountyWell users 8,800Arsenic about 5 in 100Nitrate 0.8 mg/L
- San Benito CountyWell users 8,200Arsenic about 8 in 100Nitrate 2.0 mg/L
- Del Norte CountyWell users 8,100Arsenic about 3 in 100Nitrate 0.4 mg/L
- Imperial CountyWell users 7,500Arsenic about 14 in 100Nitrate 0.3 mg/L
- Plumas CountyWell users 5,900Arsenic about 6 in 100Nitrate 0.1 mg/L
- San Mateo CountyWell users 5,600Arsenic about 3 in 100Nitrate 0.2 mg/L
- Orange CountyWell users 4,600Arsenic about 27 in 100Nitrate 2.0 mg/L
- Modoc CountyWell users 4,400Arsenic about 8 in 100Nitrate 0.5 mg/L
- Inyo CountyWell users 4,000Arsenic about 15 in 100Nitrate 0.8 mg/L
- Colusa CountyWell users 3,800Arsenic about 6 in 100Nitrate 0.4 mg/L
- Marin CountyWell users 3,200Arsenic about 7 in 100Nitrate 0.3 mg/L
- Trinity CountyWell users 2,900Arsenic about 5 in 100Nitrate 0.1 mg/L
- Mono CountyWell users 2,500Arsenic about 9 in 100Nitrate 0.4 mg/L
- Sierra CountyWell users 580Arsenic about 5 in 100Nitrate 0.1 mg/L
- San Francisco CountyWell users 500Arsenic about 4 in 100Nitrate 0.4 mg/L
- Alpine CountyWell users 270Arsenic about 12 in 100Nitrate 0.1 mg/L
Each mark = 1 in 100 well users the model places above 10 µg/L arsenic (scale 0–10)10 mg/L nitrate: the federal limit (MCL) for public systems
- These figures describe the area, not your well. Only a lab test shows what’s in your water.
- CDC says the Well Water Index figures “do not describe risk to individual wells.” (CDC Tracking Network)
- The arsenic model is better at spotting low-arsenic areas than at confirming high ones.
- Well-user counts are 2010 USGS estimates; tract values are averages over each census tract.
- Total coliform bacteriaEvery year
- NitrateEvery year
- Total dissolved solidsEvery year
- pHEvery year
- ArsenicAsk your local health department
EPA: Private Drinking Water Wells · CDC: Guidelines for Testing Well Water
Use a state-certified lab
California State Water Resources Control Board lists the laboratories certified to test drinking water in California.
Screening tells you if something is worth a closer look
A lab result is the number you can act on.
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Lab-certified results
ETR Labs Basic Water Quality Test Kit
ETR Laboratories
50+ substances, including lead, copper, arsenic, nitrate, hardness and coliform bacteria
- Analyzed by Environmental Testing and Research Laboratories (ETR Labs), Leominster, MA · ISO/IEC 17025:2017, ANAB certificate AT-2638
- Turnaround: 2–4 business days after the lab receives it
What it can't tell you: It does not test PFAS, and not every result on the panel falls under the lab's accredited scope.
Screening: quick check, not lab-grade
Watersafe Drinking Water Test Kit
Silver Lake Research
Lead, bacteria, nitrate, copper, iron, chlorine, hardness and pH
- Turnaround: Minutes; the bacteria test can take up to 48 hours
What it can't tell you: Each test gives a yes/no or a rough range, not a lab number, and it cannot detect PFAS or arsenic.
A mail-in kit’s lab is accredited (ISO/IEC 17025) but may not be certified by your state. For a home sale, a loan, or a health-department follow-up, use a state-certified lab.
California well water
Sources: CDC Well Water Index (2024) · USGS arsenic and nitrate models · USGS domestic well populations · EPA and CDC testing guidance. Data last updated 2026-10-08. All sources →
How many people use private wells in California?
About 1.4 million people across 58 California counties, by the USGS 2010 estimate. USGS models place about 118,700 of them in areas where wells are likely to be above the 10 µg/L arsenic limit. These are area estimates, not well results.
Are private wells regulated in California?
The quality and safety of drinking water from private domestic wells are not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws. (EPA) Testing and treatment are the owner's responsibility.
Where can I get my well tested in California?
California State Water Resources Control Board keeps the list of laboratories certified to test drinking water in California. Only use laboratories that are certified to do drinking water testing. (EPA)