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privatewelldata / Counties / California

Contra Costa County, California

Nitrate: 8 of 70 sampled wells (11%) above the EPA limit of 10 mg/L as N. Samples 1957-02-04 to 1983-08-03; 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.

Nitrate

8 of 70 sampled wells (11%) had a latest result above the EPA limit of 10 mg/L as N.

Median (latest per well)
2.905 mg/L as N
90th percentile
10.843 mg/L as N
Highest
52 mg/L as N
Non-detects
4 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples
181, 1957-02-04 to 1983-08-03
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

  1. Test your own well. County numbers cannot tell you what is in your water. See which tests fit your well.
  2. Use a lab certified by California. Certified labs in California.
  3. 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.
  4. If a result is high: arsenic guide · nitrate guide · bacteria guide · salesperson checklist.
  5. Test again every year for bacteria and nitrate. Get a yearly reminder.