When we started digging into Cohasset's own water system (not the Weir River system that serves North Cohasset — see below), we found something we didn't expect: two credible sources that don't agree.
The Cohasset Water Department's own 2023 Consumer Confidence Report shows the system's PFAS6 level (the sum of six compounds regulated under the Massachusetts standard) tested quarterly at a range of 5.5–16.1 parts per trillion (ppt), averaging 12.08 ppt across the year — comfortably within the state's 20 ppt limit, with no violation recorded.
A separate compilation of Massachusetts DEP data, published by the Sierra Club's Massachusetts chapter and covering readings since 2018 through mid-2022, lists Cohasset's system at a peak of 25.8 ppt — above that same 20 ppt standard. That source is careful to note its numbers "combine historical raw and finished water" and don't necessarily reflect current tap water quality, and we couldn't independently pin down the exact date or whether it was a raw source-water sample rather than treated water reaching homes.
Our read: both facts are real, and we're not going to bury either one. The most current, most directly sourced number — the town's own 2023 testing — shows compliance. But residents deserve to know the same system has, per third-party compiled state data, shown a reading above the standard at some point in the several years before that. See the full breakdown and our reasoning on the Water data page.
On May 18, 2026, EPA announced two proposals affecting the federal PFAS rule finalized in April 2024. The first would let water systems request a two-year extension — from 2029 to 2031 — to comply with the enforceable limits for PFOA and PFOS. The second would rescind the individual limits for three other PFAS compounds (PFHxS, PFNA, and HFPO-DA/GenX) and the combined "Hazard Index" limit for mixtures of those plus PFBS, on procedural grounds under the Safe Drinking Water Act.
What doesn't change: the 4 ppt limits for PFOA and PFOS individually are not part of either rescission proposal. What's uncertain is timing — a system could legally take until 2031 rather than 2029 to act.
For Cohasset specifically, this federal rule matters less directly than it does for some neighboring systems, because the town's own public reporting only publishes the combined PFAS6 sum, not an individual PFOA/PFOS breakdown we could compare against this federal number. We're flagging that as a genuine gap in what we can tell you, not glossing over it.
EPA held a public hearing on July 7, 2026, and the comment period on both proposals is scheduled to close July 20, 2026. Nothing here is final; treat the 2024 rule as the current baseline until EPA actually finalizes a change.
See the full regulatory timeline for how this fits with the 2020 state standard and the 2024 federal rule.
Until April 2024, there was no federal limit on PFAS in drinking water at all — only the Massachusetts state standard set in 2020. That changed when EPA finalized its National Primary Drinking Water Regulation (NPDWR) for PFAS: the first time PFAS compounds have been individually, enforceably regulated at the federal level.
The rule set limits of 4 parts per trillion (ppt) each for PFOA and PFOS, 10 ppt each for three additional compounds (PFHxS, PFNA, and HFPO-DA), and a combined "Hazard Index" limit for mixtures of those plus PFBS. Water systems nationwide were given until 2027 to complete initial monitoring and until 2029 to come into full compliance.
Here's where Cohasset's situation actually differs from some neighboring towns: because the Cohasset Water Department's public reporting only shows the combined PFAS6 sum (5.5–16.1 ppt in 2023) rather than a compound-by-compound breakdown, we don't have a sourced individual PFOA or PFOS number for this system to compare directly against this 4 ppt federal limit. We're telling you that gap exists rather than assuming compliance or exceedance either way.
Source: Federal Register — PFAS National Primary Drinking Water Regulation.
Long before there was a federal PFAS rule, there was a Massachusetts one. In October 2020, MassDEP finalized an enforceable drinking water standard — a Maximum Contaminant Level, or MCL — of 20 parts per trillion for the combined total of six PFAS compounds, a grouping the state calls "PFAS6": PFOS, PFOA, PFHxS, PFNA, PFHpA, and PFDA.
At the time, this made Massachusetts one of a small number of states with any enforceable PFAS standard at all. The federal government wouldn't set its own limits for another three and a half years. That gap is part of why state-level standards like this one matter: they can move faster than federal rulemaking, and they're what actually governs the Cohasset Water Department's PFAS obligations today — its quarterly testing is measured directly against this 20 ppt number, not a federal one.
This 20 ppt combined standard is the operative rule for Cohasset's own system. It's also the standard that, per a third-party compilation of state data, this same system exceeded at some point between 2018 and mid-2022, before returning to compliance by the time of the town's 2023 testing. See our Water data page for the full picture.
Source: Mass.gov — Massachusetts PFAS Drinking Water Standard (MCL).
If you've searched for water quality information about Cohasset before, you may have found coverage of the Weir River Water System, which serves the North Cohasset section of town jointly with Hingham and Hull. That's a real and separate system, well covered elsewhere — but it isn't the water most of Cohasset actually drinks.
About 90% of Cohasset gets its water from the town's own Water Department, drawing from Lily Pond, the Aaron River Reservoir, and the Ellms Meadow well-field, overseen by an elected Board of Water Commissioners. That's the system this site covers, and we think it's been under-covered relative to how much of the town it actually serves.
Our first deep dive turned up the most genuinely newsworthy thing we found: a real disagreement between the town's current 2023 testing (12.08 ppt average PFAS6, within the 20 ppt state standard) and a third-party compilation of state data showing this same system at 25.8 ppt at some point since 2018 — above that standard. We're publishing both numbers, not just the reassuring one. Full breakdown on the Water data page.
System-wide data only tells part of the story — service lines, home plumbing, and private wells can all change what actually comes out of your tap.
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