New combined sewer overflow proposal is a slight improvement over plans from last fall, but still falls short of what the public and our rivers deserve
Arlington, MA, February 5, 2026 — After public backlash in October around a proposed plan to address combined sewer overflows – intentional discharges of raw and partially treated sewage into local rivers and streams – project teams from the Massachusetts Water Resources Authority (MWRA) and the cities of Cambridge and Somerville shelved their initial “preferred alternatives,” which would have short-changed our rivers and guaranteed the ongoing release of sewage.
After being granted a 120-day extension to submit a new plan, the project teams presented their new alternatives at an MWRA board meeting on February 4th. While these new plans are an incremental step in the right direction, promising “Zero CSOs in a typical year” in the Mystic and Charles Rivers and Alewife Brook, they don’t go far enough. Zero CSOs in a typical year does not mean CSO elimination, and with only a modest cost increase, virtual CSO elimination is possible in Greater Boston.
“The question all of us should be asking – board members, regulators, residents of the Commonwealth – is how can we spend less than the limit of what we can afford to address this problem?” said Patrick Herron, Executive Director for the Mystic River Watershed Association. “The current plans on the table guarantee continued raw sewage – completely untreated sewage – going into Alewife Brook, for instance, essentially permanently. Why are we not asking what is the maximum we can achieve, given affordability calculations, rather than what is the minimum we are obliged to do?”
It is important to understand what a “typical year” is (and is not), as used in this context. A “typical year” is a model of precipitation patterns that MWRA thinks will represent average conditions in 2050. Project teams have designed their solutions up to but not exceeding this threshold. But by definition, half the years covered by the model will be above average in frequency and intensity and volume of rain – this is how averages work. MWRA has told us they have not modeled what this would mean, say, over a ten-year period in terms of actual CSO discharges.
What we do know is that MWRA’s draft plan would ensure the release of CSOs to all three waterbodies, including the Alewife Brook, in some, if not most, years, but MWRA fails to provide details about how much and how often these future CSOs will occur. They should be required to do so.
MWRA, Cambridge, and Somerville have presented possible alternatives that go further, and these plans are not budget-breaking. In the data they presented, even the most expensive plans would amount to only about a 10% increase above average base MWRA rates – the cost of an extra lunch out every month. And this rate change would be rolled out slowly, over decades.
Rising costs are a top concern for everyone who lives in and around Boston, but the scale of the impact of virtual CSO elimination, once distributed across the hundreds of thousands of households in MWRA’s service area and over the course of many years, will be worth it, especially if additional state support can reduce that impact and help ensure it is distributed equitably. In the Mystic specifically, the cost difference between the least ambitious plan and the most ambitious one is quite modest.
Large infrastructure projects are expensive, but Greater Boston is no stranger to big investments toward the greater good. CSO control will provide regional benefits that extend far beyond Cambridge and Somerville. Costs do not need to be borne exclusively by ratepayers. There is room for the state to supplement the cost of CSO elimination.
