Sonoma County, California

Arsenic: 10 of 123 sampled wells (8%) above the EPA limit of 10 µg/L. Nitrate: 1 of 133 sampled wells (1%) above the EPA limit of 10 mg/L as N. Samples 1971-05-13 to 2022-06-06; updated 2026-10-17. Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well.

What wells in this county have measured for arsenic and nitrate. A local picture only: test your own well to know your water.

Arsenic

10 of 123 sampled wells (8%) had a latest result above the EPA limit of 10 µg/L.

Median (latest per well): 1.6 µg/L
90th percentile: 9 µg/L
Highest: 58.1 µg/L
Non-detects: 6 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 240, 1976-03-08 to 2022-06-06
Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well, downloaded 2026-10-17

Nitrate

1 of 133 sampled wells (1%) had a latest result above the EPA limit of 10 mg/L as N.

Median (latest per well): 0.1 mg/L as N
90th percentile: 2.395 mg/L as N
Highest: 10.9 mg/L as N
Non-detects: 49 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 350, 1971-05-13 to 2022-06-06
Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well, downloaded 2026-10-17

Includes monitoring, research and private wells reported to the Water Quality Portal. Non-detects count only when the lab's detection limit is at or below the EPA limit. Method: data rules.

What to do next

Test your own well. County numbers cannot tell you what is in your water. See which tests fit your well.
Use a lab certified by California. Certified labs in California.
Compare your results with the EPA limits (arsenic 10 µg/L, nitrate 10 mg/L as N). If a result is above a limit, contact your county or state health department.
If a result is high: arsenic guide · nitrate guide · bacteria guide · salesperson checklist.
Test again every year for bacteria and nitrate. Get a yearly reminder.