Madera County, California

Arsenic: 10 of 118 sampled wells (8%) above the EPA limit of 10 µg/L. Nitrate: 14 of 245 sampled wells (6%) above the EPA limit of 10 mg/L as N. Samples 1955-06-09 to 2023-05-11; 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 118 sampled wells (8%) had a latest result above the EPA limit of 10 µg/L.

Median (latest per well): 2 µg/L
90th percentile: 8 µg/L
Highest: 76 µg/L
Non-detects: 1 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 188, 1987-06-03 to 2023-05-11
Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well, downloaded 2026-10-17

Nitrate

14 of 245 sampled wells (6%) had a latest result above the EPA limit of 10 mg/L as N.

Median (latest per well): 1.99 mg/L as N
90th percentile: 7.68 mg/L as N
Highest: 51.053 mg/L as N
Non-detects: 24 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 315, 1955-06-09 to 2023-05-11
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.