Nitrate in well water
The county where Nitrate is found above the limit most often is Aroostook County, Maine: 4% of 1,087 private wells tested, according to Maine Tracking Network private well summaries.
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.
- EPA limit
- 10 mg/L (EPA MCL as nitrogen (40 CFR 141.62))
- New Hampshire level
- 10 mg/L
- CAS number
- 14797-55-8
Where nitrate is worst, by homeowner tests
Counties ranked by the share of private wells above the state guidance level in tests homeowners sent to the state lab. Shares combine town summaries, weighted by wells tested.
| County | Wells tested | Above state level | Highest | |
|---|---|---|---|---|
| Aroostook County, Maine | 1,087 | 4% | ||
| Rockingham County, New Hampshire | 2,331 | 1% | 42 mg/L | |
| Somerset County, Maine | 1,097 | 1% | ||
| Franklin County, Vermont | 3,060 | <1% | 19 mg/L | |
| Penobscot County, Maine | 3,022 | <1% | ||
| Strafford County, New Hampshire | 542 | <1% | 11 mg/L | |
| Hillsborough County, New Hampshire | 2,119 | <1% | 34 mg/L | |
| York County, Maine | 2,517 | <1% | ||
| Washington County, Vermont | 2,979 | <1% | 26 mg/L | |
| Franklin County, Maine | 937 | <1% | ||
| Addison County, Vermont | 2,022 | <1% | 17 mg/L | |
| Caledonia County, Vermont | 960 | <1% | 15 mg/L | |
| Cumberland County, Maine | 4,301 | <1% | ||
| Orleans County, Vermont | 1,306 | <1% | 32.8 mg/L | |
| Grand Isle County, Vermont | 445 | <1% | 13.4 mg/L |
By individual samples
Counties with at least 30 individual samples in public monitoring data, against the EPA limit.
| County | Samples | Detected | Above EPA limit | Median |
|---|---|---|---|---|
| Cumberland County, Maine | 508 | 89% | 18% | 1.55 mg/L |
| Penobscot County, Maine | 42 | 88% | 10% | 1.2 mg/L |
| York County, Maine | 94 | 72% | 9% | 0.073 mg/L |
| Aroostook County, Maine | 58 | 95% | 7% | 1.75 mg/L |
| Oxford County, Maine | 126 | 86% | 6% | 0.445 mg/L |
| Androscoggin County, Maine | 57 | 70% | 5% | 0.09 mg/L |
| Caledonia County, Vermont | 39 | 54% | 5% | 0.11 mg/L |
| Orange County, Vermont | 157 | 93% | 5% | 0.12 mg/L |
| Strafford County, New Hampshire | 96 | 56% | 4% | 0.2 mg/L |
| Kennebec County, Maine | 84 | 51% | 4% | 0 mg/L |
| Cheshire County, New Hampshire | 90 | 57% | 3% | 0 mg/L |
| Sullivan County, New Hampshire | 62 | 66% | 2% | 0.215 mg/L |
| Carroll County, New Hampshire | 355 | 61% | 1% | 0.1 mg/L |
| Merrimack County, New Hampshire | 564 | 40% | 1% | 0 mg/L |
| Rockingham County, New Hampshire | 294 | 45% | <1% | 0 mg/L |
How to test
Use a state-certified lab and a proper sampling bottle; hardware-store strips cannot measure nitrate at the levels that matter. Collect from the kitchen tap before any treatment, and again after treatment if you install it.
How to treat
- Point-of-use reverse osmosis
- Anion exchange
- Distillation