Nitrite in New Hampshire well water
In New Hampshire, <1% of 18,077 private wells tested between 2006 and 2020 had nitrite above the New Hampshire guidance level (1 mg/L), according to NH DHHS.
Nitrite is the short-lived form of nitrogen from septic systems and fertilizer. It causes the same infant blood disorder as nitrate at a tenth of the concentration, and usually points to fresh contamination.
By county, from homeowner tests
Share of private wells above the state guidance level (1 mg/L) in tests homeowners sent to the state lab, 2006 to 2020.
| County | Wells tested | Above state level | Highest | |
|---|---|---|---|---|
| Sullivan | 733 | <1% | 1.8 mg/L | |
| Merrimack | 4,873 | 0% | 0.5 mg/L | |
| Grafton | 2,686 | 0% | 0.4 mg/L | |
| Rockingham | 2,332 | 0% | 0.5 mg/L | |
| Hillsborough | 2,118 | 0% | 0.8 mg/L | |
| Carroll | 1,819 | 0% | 0.2 mg/L | |
| Belknap | 1,734 | 0% | 0.4 mg/L | |
| Cheshire | 637 | 0% | 0.1 mg/L | |
| Coos | 603 | 0% | 0.4 mg/L | |
| Strafford | 542 | 0% | 0.1 mg/L |
Towns with the highest share above the state level
- Washington, Sullivan County<1% of 111 wells
- Bow, Merrimack County0% of 594 wells
- Tuftonboro, Carroll County0% of 561 wells
- Hopkinton, Merrimack County0% of 393 wells
- Concord, Merrimack County0% of 388 wells
- Moultonborough, Carroll County0% of 332 wells
- Loudon, Merrimack County0% of 326 wells
- Holderness, Grafton County0% of 273 wells
- Weare, Hillsborough County0% of 273 wells
- Windham, Rockingham County0% of 268 wells
- Meredith, Belknap County0% of 267 wells
- Newbury, Merrimack County0% of 263 wells
- Alton, Belknap County0% of 238 wells
- Hillsborough, Hillsborough County0% of 222 wells
- Gilmanton, Belknap County0% of 219 wells
- New London, Merrimack County0% of 215 wells
- Sandwich, Carroll County0% of 211 wells
- Henniker, Merrimack County0% of 208 wells
- Canterbury, Merrimack County0% of 198 wells
- Sunapee, Sullivan County0% of 195 wells
- Chichester, Merrimack County0% of 195 wells
- Plaistow, Rockingham County0% of 195 wells
- Barnstead, Belknap County0% of 190 wells
- Andover, Merrimack County0% of 189 wells
- Campton, Grafton County0% of 186 wells
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.