Cohasset outlines TTHM response, possible treatment plant study

Cohasset’s response to elevated total trihalomethanes, or TTHMs, is moving on two tracks: near-term operating changes are underway, while larger treatment options remain under review. The Cohasset Select Board scheduled a public discussion of TTHM water quality for 8:15 p.m. Tuesday, July 28.

Town materials say Cohasset’s drinking water remains safe for drinking, cooking, bathing and other ordinary household uses. The town’s current guidance says residents do not need to stop using the water or switch to bottled water and filters.

What TTHM means for Cohasset residents

TTHMs are disinfection byproducts that can form when chlorine reacts with naturally occurring organic material in source water. Cohasset uses Lily Pond as a source, and the town says the issue is measured through a locational running annual average, or LRAA, rather than a single short-term test.

The town’s second-quarter 2026 drinking-water notice said the federal Environmental Protection Agency maximum contaminant level for TTHMs is 80 micrograms per liter, equivalent to 80 parts per billion. Based on routine quarterly monitoring conducted May 19, the LRAA was 97 parts per billion at 208 Sohier Street and 92 parts per billion at 2 Whitehead Road.

The notice said the standard is based on potential health risks from long-term exposure over many years, not short-term or occasional use. It also said Massachusetts and federal health agencies advise that some people who drink water containing TTHMs above the standard over many years may face risks involving the liver, kidneys or central nervous system, as well as an increased risk of cancer. The notice said young children, pregnant people and those who may become pregnant may be more susceptible to risks from exposure to chemicals such as TTHMs.

The town describes the situation as not an emergency and says the water remains safe for everyday use. Residents with specific health questions can consult a health care provider or the state agencies identified in the notice.

Work already underway

The Water Department says treatment operations are being optimized to improve the removal of organic material before disinfection. The town is also continuing targeted hydrant flushing, expanded sampling and monitoring, and coordination with the Massachusetts Department of Environmental Protection and engineering consultants.

A water-system engineering analysis is complete. According to the town, the water-age study found that further reducing water age in distribution pipes is not expected to significantly reduce TTHMs, helping officials focus on other measures.

Source-water work is also underway. The town began updating a Lily Pond limnology study on July 16 to evaluate opportunities to improve source-water quality. Improvements to the sludge lagoon are in design and permitting and are intended to improve the plant’s ability to remove organic material before disinfection.

Possible treatment changes are not final decisions

Cohasset is evaluating alternative chemical treatment, including a possible conversion of filter media to granular activated carbon at the water-treatment plant. The town is also preparing an application for state infrastructure funding for water-storage-tank mixers.

A possible new treatment plant remains under review. The Water Department has received a proposal for a pilot study and preliminary design work, but the progress tracker states that no decisions have been made. The proposal should not be read as an approved, funded or scheduled construction project.

Rates and the next public step

The town’s TTHM information page says improving and maintaining Cohasset’s drinking-water system will require significant investment and that a future water-rate increase will be necessary. It does not establish a specific increase, project cost or construction timeline. Any proposed rate changes would go through public town processes.

Tuesday’s Select Board discussion is the next scheduled public point for residents seeking more detail about treatment alternatives, costs, timing and the status of the possible plant study. Any confirmed decisions should come through later meeting records or official town documents.

Sources

• Cohasset Water Department TTHM Solutions Progress Tracker