PFOS in well water
PFOS is a forever chemical with the same sources as PFOA. It is linked to immune suppression, thyroid disease, liver effects and cancer. Wells near airports, fire training areas, landfills and plating plants are most at risk.
- EPA limit
- 4 ng/L (EPA MCL (2024 PFAS rule); New Hampshire MCL is 15 ng/L)
- CAS number
- 1763-23-1
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 |
|---|---|---|---|---|
| Somerset County, Maine | 576 | 56% | 28% | 0.804 ng/L |
| Lincoln County, Maine | 110 | 52% | 15% | 0.504 ng/L |
| Washington County, Maine | 36 | 31% | 14% | 0 ng/L |
| Rockingham County, New Hampshire | 2,125 | 75% | 13% | 1.2 ng/L |
| Aroostook County, Maine | 113 | 35% | 12% | 0 ng/L |
| Kennebec County, Maine | 450 | 48% | 12% | 0 ng/L |
| Knox County, Maine | 36 | 44% | 11% | 0 ng/L |
| Hillsborough County, New Hampshire | 3,795 | 74% | 10% | 0.97 ng/L |
| Penobscot County, Maine | 294 | 41% | 10% | 0 ng/L |
| Cumberland County, Maine | 574 | 51% | 9% | 0.485 ng/L |
| York County, Maine | 386 | 48% | 6% | 0 ng/L |
| Waldo County, Maine | 401 | 33% | 6% | 0 ng/L |
| Androscoggin County, Maine | 215 | 35% | 6% | 0 ng/L |
| Hancock County, Maine | 58 | 36% | 5% | 0 ng/L |
| Franklin County, Maine | 44 | 25% | 5% | 0 ng/L |
How to test
Use a state-certified lab and a proper sampling bottle; hardware-store strips cannot measure pfos at the levels that matter. Collect from the kitchen tap before any treatment, and again after treatment if you install it.
How to treat
- Granular activated carbon whole-house
- Point-of-use reverse osmosis
- Anion exchange