Nitrate in New Hampshire well water
In New Hampshire, <1% of 18,081 private wells tested between 2006 and 2020 had nitrate above the New Hampshire guidance level (10 mg/L), according to NH DHHS.
Nitrate reaches wells from fertilizer, septic systems and manure. Above the limit it can cause methemoglobinemia (blue baby syndrome) in infants under six months, and long-term exposure is associated with some cancers. Shallow and dug wells near farmland or dense septic systems are most at risk.
By county, from homeowner tests
Share of private wells above the state guidance level (10 mg/L) in tests homeowners sent to the state lab, 2006 to 2020.
| County | Wells tested | Above state level | Highest | |
|---|---|---|---|---|
| Rockingham | 2,331 | 1% | 42 mg/L | |
| Strafford | 542 | <1% | 11 mg/L | |
| Hillsborough | 2,119 | <1% | 34 mg/L | |
| Merrimack | 4,875 | <1% | 21 mg/L | |
| Carroll | 1,819 | <1% | 40 mg/L | |
| Cheshire | 637 | <1% | 14 mg/L | |
| Grafton | 2,687 | <1% | 20 mg/L | |
| Coos | 603 | <1% | 18 mg/L | |
| Sullivan | 733 | <1% | 10 mg/L | |
| Belknap | 1,735 | <1% | 15 mg/L |
Towns with the highest share above the state level
- Newton, Rockingham County8% of 51 wells
- Salem, Rockingham County4% of 72 wells
- Ossipee, Carroll County4% of 144 wells
- Hillsborough, Hillsborough County3% of 222 wells
- Windham, Rockingham County3% of 268 wells
- Stratford, Coos County3% of 34 wells
- Danville, Rockingham County3% of 36 wells
- Kingston, Rockingham County3% of 75 wells
- Fremont, Rockingham County2% of 44 wells
- Durham, Strafford County2% of 43 wells
- Derry, Rockingham County2% of 91 wells
- Francestown, Hillsborough County2% of 45 wells
- Madbury, Strafford County2% of 49 wells
- Stoddard, Cheshire County2% of 54 wells
- Bethlehem, Grafton County2% of 56 wells
- Chester, Rockingham County2% of 58 wells
- Plaistow, Rockingham County2% of 194 wells
- Canterbury, Merrimack County2% of 198 wells
- Webster, Merrimack County1% of 147 wells
- Pittsfield, Merrimack County1% of 71 wells
- Temple, Hillsborough County1% of 69 wells
- Barrington, Strafford County1% of 76 wells
- Amherst, Hillsborough County1% of 75 wells
- Hooksett, Merrimack County1% of 171 wells
- Thornton, Grafton County1% of 160 wells
By county, from individual samples
Public monitoring samples against the EPA limit (10 mg/L); counties with at least 30 samples.
| County | Samples | Detected | Above EPA limit | Median | Highest |
|---|---|---|---|---|---|
| Strafford | 96 | 56% | 4% | 0.2 | 19 mg/L |
| Cheshire | 90 | 57% | 3% | 0 | 19.9 mg/L |
| Sullivan | 62 | 66% | 2% | 0.215 | 37.2 mg/L |
| Carroll | 355 | 61% | 1% | 0.1 | 14 mg/L |
| Merrimack | 564 | 40% | 1% | 0 | 24.5 mg/L |
| Rockingham | 294 | 45% | <1% | 0 | 19 mg/L |
| Coos | 210 | 54% | <1% | 0.063 | 11.8 mg/L |
| Grafton | 283 | 48% | <1% | 0 | 11.7 mg/L |
| Hillsborough | 336 | 69% | <1% | 0.1 | 16 mg/L |
| Belknap | 198 | 32% | 0% | 0 | 4.69 mg/L |
Sources
- NH DHHS WISDOM private well summaries: Town summaries of private well tests run by the New Hampshire Public Health Laboratories, published by NH DHHS through the WISDOM environmental public health tracking portal. Exceedance shares use New Hampshire guidance levels, which are stricter than EPA for arsenic (5 ug/L), manganese (0.3 mg/L), radon (2,000 pCi/L) and sodium (20 mg/L). Statistics for small counts are suppressed (*). Retrieved September 3, 2026.
- Individual samples from the USGS Water Quality Portal and state monitoring, summarized per geography. nitrate counts and shares are listed on each county and town page with their sources.