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

California counties with modeled risk data.

People on private wells

area estimate

1,376,408

estimated across covered California counties.

Average arsenic risk

area estimate

average modeled chance a well exceeds 10 µg/L arsenic across covered counties.

California counties

Find a California-certified testing lab

Get your well tested at a California-certified laboratory. We list certification programs by state.

California certified labs

Already tested your well? Get a free plain-English reading

Upload your well water lab report (PDF or a photo) and our team will email you a full, plain-English reading of every result — free. Educational only, never medical advice.

Your upload also helps us keep local water data accurate — we only ever share anonymized, area-level summaries.

Upload my well test

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.

Need whole-house or professional water treatment?

For the problems a pitcher or under-sink filter cannot fix — scale, rust and black staining, sulfur smell, well contamination, or test results you cannot explain. Tell us what is happening and we will connect you with a local water-treatment professional.

Your name

Phone

ZIP code

What is happening with your water?

Where does your water come from?

Do you own or rent?

How soon do you want it fixed?

Budget (optional)

By submitting, I give my express written consent for TapWaterData and its partner water-treatment professionals to contact me about my inquiry at the phone number provided, including by phone call and text message (SMS), possibly using automated technology. Consent is not a condition of purchase. Message/data rates may apply. I can opt out at any time.

TapWaterData may be compensated when you are connected with a water-treatment professional.

All well-water states Well water testing near me — full guide Data & methodology Read a water test result

By TapWaterData Editorial