Colusa County, California

Arsenic: 11 of 136 sampled wells (8%) above the EPA limit of 10 µg/L. Nitrate: 3 of 164 sampled wells (2%) above the EPA limit of 10 mg/L as N. Samples 1956-03-01 to 2021-07-14; 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

11 of 136 sampled wells (8%) had a latest result above the EPA limit of 10 µg/L.

Median (latest per well): 2.05 µg/L
90th percentile: 10 µg/L
Highest: 80.6 µg/L
Non-detects: 9 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 183, 1960-06-13 to 2021-07-14
Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well, downloaded 2026-10-17

Nitrate

3 of 164 sampled wells (2%) had a latest result above the EPA limit of 10 mg/L as N.

Median (latest per well): 1.3 mg/L as N
90th percentile: 4.744 mg/L as N
Highest: 27.108 mg/L as N
Non-detects: 11 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 301, 1956-03-01 to 2021-07-14
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.