Mercer County, New Jersey

Arsenic: 4 of 92 sampled wells (4%) above the EPA limit of 10 µg/L. Nitrate: 13 of 147 sampled wells (9%) above the EPA limit of 10 mg/L as N. Samples 1925-08-12 to 2022-06-15; updated 2026-10-18. 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

4 of 92 sampled wells (4%) had a latest result above the EPA limit of 10 µg/L.

Median (latest per well): 0.27 µg/L
90th percentile: 5 µg/L
Highest: 23 µg/L
Non-detects: 39 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 151, 1983-07-05 to 2022-06-15
Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well, downloaded 2026-10-18

Nitrate

13 of 147 sampled wells (9%) had a latest result above the EPA limit of 10 mg/L as N.

Median (latest per well): 2 mg/L as N
90th percentile: 8.9 mg/L as N
Highest: 72.4 mg/L as N
Non-detects: 16 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 230, 1925-08-12 to 2022-06-15
Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well, downloaded 2026-10-18

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 New Jersey. Certified labs in New Jersey.
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.