PFOS in well water
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private wells tested in Middlesex County, Massachusetts had pfos above the limit, the highest share of any county, according to MassDEP Private Well PFAS Sampling Program.
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)
- Massachusetts level
- 4 ng/L
- CAS number
- 1763-23-1
Check your PFOS number
Type the value from your lab report. It is compared to the EPA limit, the same way the full interpretation does it.
Test your own well for PFOS
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
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Systems that remove PFOS
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Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.
For PFOS
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For PFOS
Whole-house PFAS carbon system
whole-house, at the point of entry, about $3,000
Granular activated carbon certified to NSF/ANSI 53 for PFOA and PFOS; replace media on schedule.
For PFOS
Aquasana SmartFlow reverse osmosis
under-sink or countertop, about $225
Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.
Rather have it installed and sized for your well? Book a free in-home water test with a licensed local company.
Where pfos 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 | Above state level | Wells tested | Highest |
|---|---|---|---|
| Middlesex County, Massachusetts | 274 | 610 ng/L | |
| Kent County, Michigan | 1,595 | 83,000 ng/L | |
| Essex County, Massachusetts | 67 | 29.8 ng/L | |
| Nantucket County, Massachusetts | 48 | 11.5 ng/L | |
| Passaic County, New Jersey | 931 | ||
| Sussex County, New Jersey | 2,246 | ||
| Plymouth County, Massachusetts | 110 | 19 ng/L | |
| Mercer County, New Jersey | 302 | ||
| Bristol County, Massachusetts | 144 | 15.9 ng/L | |
| Middlesex County, New Jersey | 131 | ||
| Morris County, New Jersey | 1,851 | ||
| Hampshire County, Massachusetts | 174 | 310 ng/L | |
| Barnstable County, Massachusetts | 83 | 22.2 ng/L | |
| Cape May County, New Jersey | 696 | ||
| Somerset County, New Jersey | 911 |
By individual samples
Counties with at least 30 individual samples in public monitoring data, against the EPA limit.
| County | Above EPA limit | Samples | Detected | Median |
|---|---|---|---|---|
| Barnstable County, Massachusetts | 113 | 83% | 120 ng/L | |
| Nassau County, New York | 93 | 57% | 2.2 ng/L | |
| Suffolk County, New York | 60 | 53% | 1.45 ng/L | |
| Somerset County, Maine | 576 | 56% | 0.804 ng/L | |
| Burlington County, New Jersey | 43 | 37% | 0 ng/L | |
| Washington County, Minnesota | 648 | 21% | 0 ng/L | |
| Salt Lake County, Utah | 38 | 39% | 0 ng/L | |
| Lincoln County, Maine | 110 | 52% | 0.504 ng/L | |
| Los Angeles County, California | 55 | 25% | 0 ng/L | |
| Washington County, Maine | 36 | 31% | 0 ng/L | |
| Rockingham County, New Hampshire | 2,125 | 75% | 1.2 ng/L | |
| Aroostook County, Maine | 113 | 35% | 0 ng/L | |
| Kennebec County, Maine | 450 | 48% | 0 ng/L | |
| Knox County, Maine | 36 | 44% | 0 ng/L | |
| Hillsborough County, New Hampshire | 3,795 | 74% | 0.97 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
Analysis: PFAS in private wells: known sources versus routine testing, from the same public records.