Mendocino County, California

Arsenic: 9 of 71 sampled wells (13%) above the EPA limit of 10 µg/L. Nitrate: 1 of 128 sampled wells (1%) above the EPA limit of 10 mg/L as N. Samples 1960-08-31 to 2025-09-24; 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

9 of 71 sampled wells (13%) had a latest result above the EPA limit of 10 µg/L.

Median (latest per well): 0.13 µg/L
90th percentile: 17 µg/L
Highest: 85.2 µg/L
Non-detects: 27 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 109, 1982-05-19 to 2025-09-24
Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well, downloaded 2026-10-17

Nitrate

1 of 128 sampled wells (1%) had a latest result above the EPA limit of 10 mg/L as N.

Median (latest per well): 0.145 mg/L as N
90th percentile: 2 mg/L as N
Highest: 11 mg/L as N
Non-detects: 38 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 189, 1960-08-31 to 2025-09-24
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.