San Joaquin County, California

Arsenic: 56 of 302 sampled wells (19%) above the EPA limit of 10 µg/L. Nitrate: 42 of 520 sampled wells (8%) above the EPA limit of 10 mg/L as N. Samples 1943-01-01 to 2023-07-17; 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

56 of 302 sampled wells (19%) had a latest result above the EPA limit of 10 µg/L.

Median (latest per well): 3.3 µg/L
90th percentile: 19 µg/L
Highest: 81.6 µg/L
Non-detects: 9 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 417, 1970-07-22 to 2023-07-17
Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well, downloaded 2026-10-17

Nitrate

42 of 520 sampled wells (8%) had a latest result above the EPA limit of 10 mg/L as N.

Median (latest per well): 1.804 mg/L as N
90th percentile: 9.3 mg/L as N
Highest: 63.025 mg/L as N
Non-detects: 51 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 760, 1943-01-01 to 2020-06-16
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.