Knox County, Maine

Arsenic: 13 of 13 sampled wells (100%) above the EPA limit of 10 µg/L. Nitrate: 0 of 14 sampled wells (0%) above the EPA limit of 10 mg/L as N. Samples 1984-10-23 to 2007-07-11; 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

13 of 13 sampled wells (100%) had a latest result above the EPA limit of 10 µg/L.

Median (latest per well): 86 µg/L
90th percentile: 160 µg/L
Highest: 420 µg/L
Samples: 13, 2001-08-13 to 2007-07-11
Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well, downloaded 2026-10-18

Nitrate

0 of 14 sampled wells (0%) had a latest result above the EPA limit of 10 mg/L as N.

Median (latest per well): 0.065 mg/L as N
90th percentile: 0.7 mg/L as N
Highest: 1.3 mg/L as N
Non-detects: 6 wells' latest result was "not detected" with no detection limit reported; they are counted as below the limit.
Samples: 14, 1984-10-23 to 2007-07-11
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 Maine. Certified labs in Maine.
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.