Two different systems are under stress for different reasons—and Cohasset’s own records show that infiltration and inflow have long challenged the sewer system.

July 11, 2026

Cohasset residents are currently being asked to conserve drinking water because of drought conditions. At the same time, the town’s wastewater treatment facility has been operating under a High Flow Plan because incoming wastewater is exceeding the amount the facility can fully process under normal operating conditions.

At first glance, those two warnings may seem contradictory: How can Cohasset have too little water and too much wastewater at the same time?

The answer is that the town’s drinking-water and wastewater systems are separate systems facing different problems.

The drinking-water problem: drought and reduced supply

The Cohasset Water Department’s June 30 drought-warning notice says abnormally dry conditions are affecting the town’s surface-water supplies. The current restrictions prohibit the use of irrigation systems. Hand-held watering of ornamental plants and vegetable gardens is permitted between 5 p.m. and 9 a.m., except on Sundays, when watering is not allowed.

The town says its drought-warning criteria include three consecutive months of precipitation below 40 percent of normal, stream-flow targets not being maintained for 21 consecutive days, and storage falling below 300 days. Violations can result in a $50 penalty for a first offense and $100 for later violations, with each day treated as a separate offense.

North Cohasset is subject to a different and stricter restriction through the Weir River Water System, which serves Hingham, Hull and North Cohasset. That system has imposed a total outdoor water ban because of low storage-tank reserves attributed to overconsumption and drought. The notice specifically connects conservation to maintaining drinking-water supply and firefighting capability.

The wastewater problem: incoming flow is exceeding treatment capacity

In a June 12 notice, Cohasset’s Sewer Department said the wastewater treatment facility was unable to produce enough fully treated water to keep pace with incoming flow and had therefore been implementing its state-approved High Flow Plan on a daily basis.

The town said the facility was maximizing flow diversion to Hull, proactively cleaning treatment membranes to increase production capacity, and replacing related equipment as needed.

The most significant disclosure was that incoming wastewater was receiving primary treatment and chemical disinfection before discharge to the harbor. The town said state and federal regulators, as well as the Massachusetts Department of Marine Fisheries, had been notified.

How can both conditions exist at once?

A drinking-water drought concerns the amount of raw water available in reservoirs and other supply sources. A wastewater high-flow condition concerns the volume entering sewer pipes and arriving at the treatment plant.

Not all of the water entering a sanitary sewer system necessarily comes from sinks, toilets, showers and businesses. Groundwater and stormwater can enter through what engineers call infiltration and inflow.

Infiltration occurs when groundwater enters sewer pipes through cracks, leaking pipe joints or deteriorated manholes. Inflow occurs when stormwater or other surface water enters through roof-drain connections, openings in manhole covers, sump pumps or foundation drains connected to the sewer system.

That unwanted water still has to be pumped, managed and treated. During major rain events or periods of high groundwater, it can dramatically increase the amount of flow reaching a treatment plant even when the community’s drinking-water supplies remain depleted.

Cohasset’s records show this is a longstanding concern

A 2018 town request for proposals for operation of the wastewater treatment plant said Cohasset had an active program to reduce infiltration and inflow. It also said that during wet seasons or unusually heavy rain, peak flows could exceed the treatment plant’s hydraulic capacity and require implementation of a High Flow procedure.

More recent records underscore the size of the issue. Minutes from a January 2026 Cohasset Harbor Committee meeting report that Public Works Director Brian Joyce discussed continuing traces of infiltration and inflow. The minutes state that the plant’s capacity is approximately 450,000 gallons per day, but that flow during a major rain event can exceed one million gallons per day. The same discussion described infiltration and inflow reduction as an ongoing effort beyond the normal operating budget that would require grant funding.

A town Harbor Climate Resilience presentation also explains that infiltration can enter through cracked pipes and deteriorated manholes, while inflow can come from downspouts, manhole openings, sump pumps and foundation drains improperly connected to the sanitary sewer.

What the town has not yet publicly explained

The town’s current High Flow Plan notice does not identify the precise cause of the present high-flow condition. It does not say whether the immediate trigger is groundwater infiltration, stormwater inflow, pipe defects, improper private connections, equipment limitations, membrane performance, or some combination of those factors.

It would therefore be premature to state as fact that infiltration and inflow caused the current event. What can be said is that Cohasset’s own records identify infiltration and inflow as a longstanding sewer-system issue capable of pushing flows beyond the plant’s hydraulic capacity.

Questions residents deserve to have answered

  • What is the primary cause of the current high-flow condition?
  • How much wastewater is being diverted to Hull each day?
  • How often has the plant relied on primary treatment and chemical disinfection before harbor discharge?
  • What monitoring is being performed in Cohasset Harbor and nearby shellfish areas?
  • Which sewer lines, manholes or neighborhoods contribute most heavily to infiltration and inflow?
  • What repairs, equipment replacements or capital projects are planned?
  • What will those projects cost, and will they be funded through sewer rates, taxes, borrowing or grants?

Two problems, one infrastructure challenge

Cohasset is not literally suffering from both “too little water” and “too much water” in the same place. It is dealing with reduced drinking-water supply on one side and excessive flow entering the wastewater system on the other.

The immediate message for residents is to follow the water restrictions that apply to their particular water system. The larger civic issue is whether Cohasset’s wastewater infrastructure is resilient enough to handle infiltration, extreme rainfall, groundwater and future coastal conditions without relying repeatedly on emergency high-flow procedures.

Official sources