What public testing shows about Cohasset's water

The system

The Cohasset Water Department (Massachusetts PWS ID MA4065000), overseen by an elected Board of Water Commissioners, provides public drinking water to about 7,550 residents through roughly 2,700 connections — about 90% of the town's population. This is a distinct system from the Weir River Water System that serves the North Cohasset section of town (see the note below).

Supply comes from two surface water sources — Lily Pond (about 100 million gallons capacity, 52 acres) and the Aaron River Reservoir (up to 550 million gallons, 136 acres, used to supplement Lily Pond during high demand) — plus one groundwater source, the Ellms Meadow well-field, approved for up to 141,000 gallons per day. The combined watershed for the two surface sources covers 5,892 acres, only 40% of which lies within Cohasset itself.

Before reaching taps, water is drawn from Lily Pond and passed through screening, then dosed with ferric chloride, sodium hydroxide, sodium permanganate, and polyaluminum chloride to coagulate dissolved organic matter, filtered through coal and sand media, then treated with corrosion-control chemicals, chlorine for disinfection, and fluoride (adjusted to an optimal 0.7 ppm since May 2015).

MassDEP's 2004 Source Water Assessment rated this system's susceptibility to contamination as high, and specifically flags the Route 3A transportation corridor running through the watershed as a moderate spill/accident hazard — the town has since installed an oil/water separator and rain gardens in response.

A more complicated water map than it looks

Cohasset's water situation isn't a single clean system serving the whole town. Three separate things are true at once:

• The Cohasset Water Department (this page) serves about 90% of the town from Lily Pond, the Aaron River Reservoir, and Ellms Meadow — entirely within town control, not shared as a primary source with any neighboring town.
• North Cohasset — the section of town closest to Hingham — is served instead by the Weir River Water System, operated by Aquarion Water Company jointly with Hingham and Hull. That's almost certainly most of the remaining 10% of Cohasset residents on the Aquarion side. We don't duplicate that system's data here; see Hingham Water Watch's North Cohasset page for it.
• Emergency interconnections only: the Cohasset Water Department's own distribution system is physically connected to both the Hingham system and the Scituate system, to be opened only during a water emergency to move supply between systems. Separately, a paid interconnection (funded entirely by the customer, not the town) supplies water to the Linden Ponds retirement complex through the Hingham system — the town's own CCR is explicit that "no water is sold or given to Hingham" the other direction. Neither of these is a shared source arrangement like Weir River; they're one-way emergency and commercial connections.

Violation history

The Cohasset Water Department's own 2023 Consumer Confidence Report — the most recent one published as of this writing — states plainly: "There were no other violations to report during Calendar Year 2023." Every regulated contaminant table in that report, including the PFAS6 table, is marked "No" under Violation.

We were not able to independently pull a multi-year violation history directly from EPA's ECHO/SDWIS online tools, which run as JavaScript dashboards not accessible to our research process. We're reporting what the town's own official CCR states for 2023, rather than repeating an unverified "no violations since [year]" claim we couldn't confirm ourselves.

PFAS: a real disagreement worth explaining, not picking a side on

This is the one place our research turned up two sources that don't agree, and we think Cohasset residents deserve to see both rather than have us pick the more (or less) alarming number.

Source	What it shows	MA PFAS6 standard
Cohasset Water Department, 2023 CCR (quarterly testing)	Range 5.5–16.1 ppt; quarterly average 12.08 ppt	Below 20 ppt — compliant
Sierra Club Massachusetts PFAS tracker (MassDEP data, since 2018, as of June 2022)	Peak of 25.8 ppt recorded for Cohasset's system	Above 20 ppt

The Sierra Club's compiled figure explicitly notes it "combines historical raw and finished water" and "does not represent current finished drinking water quality coming out of your tap" — meaning the 25.8 ppt figure may reflect a raw (pre-treatment) source-water sample rather than treated tap water, and it's not clear from that source exactly when between 2018 and mid-2022 it was recorded. We could not find a specific MassDEP exceedance notice for Cohasset from that period in our research. What we can say plainly: the town's own most recent published testing (2023) shows compliance with the state standard, and a credible third-party compilation of state data shows this system exceeded that same standard at some point in the several years before that. Both facts are real; neither should be hidden.

PFAS6 is the sum of six specific compounds under the Massachusetts standard: PFOS, PFOA, PFHxS, PFNA, PFHpA, and PFDA. The Cohasset Water Department's public CCR reports only this combined sum, not a breakdown by individual compound — so unlike some neighboring towns' EPA UCMR5 results, we don't have a sourced, town-specific number to compare against the separate federal 4 ppt individual limits for PFOA and PFOS. We're flagging that gap rather than guessing at a number.

Other PFAS compounds monitored (2020)

Outside the regulated PFAS6 sum, the town's CCR also lists two additional PFAS compounds monitored in 2020, under the "unregulated contaminants" heading (no enforceable limit applies to either individually):

Compound	Range detected	Average detected	Individual federal limit
PFBS (perfluorobutane sulfonic acid)	ND – 2.82 ppt	1.51 ppt	No individual MCL (part of 2024 federal Hazard Index mixture)
PFHxA (perfluorohexanoic acid)	ND – 2.89 ppt	1.46 ppt	No individual MCL

ND = not detected above the lab's detection limit. Both compounds were found at low, single-digit ppt levels. Source: Cohasset Water Department 2023 Consumer Confidence Report.

Other things the 2023 report shows

A few other results worth knowing, none of them violations:

• Disinfection byproducts (TTHMs): the highest running quarterly average in 2023 was 79 ppb, against an MCL of 80 ppb — compliant, but close enough to the limit to be worth watching in future reports.
• Lead and copper: testing of 33 homes in mid-2022 found a 90th-percentile lead level of 2 ppb (action level 15 ppb) and copper of 0.162 ppm (action level 1.3 ppm) — well within limits, with zero of the 33 sites sampled above the action level for either.
• Sodium: detected in a range of 41–61 ppm. There's no enforceable limit, but it's well above the state's 20 ppm health-advisory guideline for people on sodium-restricted diets — worth knowing if that applies to your household, even though it's not a violation.

Regulatory timeline

How the rules around PFAS in drinking water have actually changed over the past several years — and where they stand right now.

Sources: Mass.gov — Massachusetts PFAS Drinking Water Standard (MCL); Federal Register — PFAS National Primary Drinking Water Regulation (April 2024); EPA — Proposed PFOA and PFOS Compliance Extension Rule; EPA — Proposed PFAS Rescission Rule.