Arsenic in well water
The county where Arsenic is found above the limit most often is Strafford County, New Hampshire: 40% of 530 private wells tested, according to NH DHHS.
Arsenic occurs naturally in New England bedrock and dissolves into well water. Long-term exposure raises the risk of bladder, lung and skin cancers, and is linked to heart disease and lower IQ in children. It has no taste or smell, so testing is the only way to know.
- EPA limit
- 10 ug/L (EPA MCL (40 CFR 141.62))
- New Hampshire level
- 5 ug/L
- CAS number
- 7440-38-2
Where arsenic 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 | |
|---|---|---|---|---|
| Strafford County, New Hampshire | 530 | 40% | 183 ug/L | |
| Rockingham County, New Hampshire | 2,990 | 34% | 766 ug/L | |
| Merrimack County, New Hampshire | 4,603 | 33% | 1,040 ug/L | |
| Kennebec County, Maine | 4,743 | 28% | ||
| Carroll County, New Hampshire | 1,741 | 26% | 489 ug/L | |
| Hillsborough County, New Hampshire | 3,558 | 24% | 523 ug/L | |
| York County, Maine | 2,174 | 23% | ||
| Belknap County, New Hampshire | 1,532 | 22% | 753 ug/L | |
| Hancock County, Maine | 2,622 | 22% | ||
| Somerset County, Maine | 952 | 18% | ||
| Cumberland County, Maine | 4,050 | 17% | ||
| Washington County, Maine | 1,210 | 17% | ||
| Knox County, Maine | 798 | 17% | ||
| Androscoggin County, Maine | 1,293 | 15% | ||
| Rutland County, Vermont | 1,551 | 13% | 173 ug/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 |
|---|---|---|---|---|
| Kennebec County, Maine | 114 | 99% | 90% | 49.4 ug/L |
| York County, Maine | 48 | 98% | 69% | 55.5 ug/L |
| Cumberland County, Maine | 73 | 71% | 52% | 24 ug/L |
| Hancock County, Maine | 64 | 84% | 50% | 10.1 ug/L |
| Rockingham County, New Hampshire | 322 | 75% | 39% | 4.2 ug/L |
| Androscoggin County, Maine | 33 | 91% | 36% | 2.7 ug/L |
| Somerset County, Maine | 31 | 94% | 35% | 1.9 ug/L |
| Washington County, Maine | 61 | 98% | 28% | 1 ug/L |
| Merrimack County, New Hampshire | 206 | 61% | 25% | 0.65 ug/L |
| Hillsborough County, New Hampshire | 225 | 58% | 20% | 1 ug/L |
| Oxford County, Maine | 31 | 74% | 16% | 0.1 ug/L |
| Strafford County, New Hampshire | 71 | 56% | 4% | 1 ug/L |
| Sullivan County, New Hampshire | 32 | 47% | 3% | 0 ug/L |
| Grafton County, New Hampshire | 67 | 33% | 1% | 0 ug/L |
| Carroll County, New Hampshire | 79 | 20% | 1% | 0 ug/L |
How to test
Use a state-certified lab and a proper sampling bottle; hardware-store strips cannot measure arsenic 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
- Adsorptive media (iron-based) whole-house
- Anion exchange