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Combined sewer overflows

Updated October 29, 2026

What's happening

Written comment on the region's $1.3 billion CSO plan closes October 30, and the Somerville City Council is now formally against it. On October 22 the Council voted 9-1, with one member absent, to oppose the draft plan and call for a regional solution aimed at the "virtual elimination" of combined sewer overflows (Cambridge Day, Oct 28). The resolution came from Ward 7 Councilor Emily Hardt and At-large Councilors Jonathan Link and Will Mbah. It asks the Legislature and MWRA for solutions that are "regional, diversified and address stormwater," with costs shared so the people least able to pay aren't overburdened (Hardt newsletter, Oct 26). "This is a technical plan to address what I see as a political problem," Hardt said. "We don't want to have a halfway solution that just kicks the can down the road to future generations." She cited studies tying big overflow events to spikes in emergency-room visits for gastrointestinal illness.

At-large Councilor Ben Wheeler cast the only no, and he did it on cost: "The current plan's projected annual increases of 15 percent or more year after year for every Somerville ratepayer is already a colossal burden." By the cities' own projections, a typical Somerville household's sewer bill goes from about $850 a year now to $2,600 by 2055. In Cambridge a single-family bill goes from about $1,200 in fiscal 2026 to $3,300 over 30 years. Those figures already assume the cost is spread across MWRA's 43 sewer communities.

Somerville went further than its neighbors did. On October 5 the Cambridge City Council voted unanimously to ask City Manager Yi-An Huang to push for stronger controls and virtual elimination by 2050, without opposing the plan. "While the city stands behind this, we, the City Council, really want more," said Councillor Patty Nolan. Boston's Council formally opposed the draft in a nonbinding August 26 resolution from President Liz Breadon. It asks for protection up to a 25-year storm under 2050 conditions, which is short of elimination.

Cambridge's fight is mostly about the tank under Magazine Beach. On October 20 its Council unanimously urged the MWRA to move or redesign the 10.1-million-gallon tank, which as designed would fence off 2.3 acres of the park permanently and take more than eight acres out of use for six to eight years (Cambridge Day, Oct 21). The order lists alternatives: Goose Meadow, a lot by the Cottage Farm facility at the BU Bridge, buying or sharing private property, or a design that reopens the park on top. Cambridge Day reported October 28 that at a contentious Cambridgeport meeting, MWRA officials said they would consider other locations for the tank, or restoring public access above it once construction ends.

Nobody has negotiated who pays. "We have started to get comments from other municipalities in the MWRA sewer system that are very worried about the cost of this plan as presented," Cambridge DPW project manager Lucica Hiller told Cambridge Day (Oct 5). Asked whether those towns would get any tangible benefit, she said: "No."

The overflows keep coming, even in a dry year. The September 25–27 nor'easter caused three discharges into the Mystic from MWRA's Somerville facility, more than a million gallons (Cambridge Somerville Independent, Sept 28). A much smaller rain on October 10–11 did it again, with the longest discharge running from 8 p.m. to 2:36 a.m. (Independent, Oct 13). Infrastructure Director Rich Raiche says the Somerville site overflows so often because MWRA throttles flows at its Everett and Chelsea pumping facilities "so that we don't wash out Deer Island." Alewife Brook flooded nearby recreation areas five times in 2023.

The story

What overflows, and where

Most of Somerville has combined sewers: one pipe carries both sewage and stormwater to MWRA's Deer Island plant. In a big rain the pipe fills, and relief structures dump the mix into a river so it doesn't back up into basements. Somerville owns two outfalls (city CSO page). SOM-001A sends untreated combined sewage into the Alewife Brook. SOM-007A/MWR205A, permitted jointly with MWRA, sends screened and disinfected flow into the Mystic. MWRA's Somerville Marginal facility sent 8.38 million gallons into the Upper Mystic across five events in 2025 (MWRA 2025 CSO annual report). A 2022 state law requires public notice of any discharge longer than two hours, and the advice is to stay out of the water for 48 hours afterward.

The plan and its price

The region has spent more than $1 billion on 35 CSO projects since the late 1990s, but SOM-001A is one of 16 outfalls that never met their targets. MassDEP's 2019 variances sent Cambridge, Somerville and MWRA back to study control "up to and including elimination." A first set of alternatives drew backlash in October 2025 and was shelved. The replacement, developed over four years, promises "zero CSOs in a typical year" of projected 2050 rainfall. Raiche calls that a global first, and it roughly doubled the price. Five-year and 25-year storms would still overwhelm the system. If regulators accept it, the work runs into the 2050s.

The draft was submitted April 30 in three packages:

  • Alewife Brook, $340 million: two tanks totaling three million gallons, eight acres of separation and a one-mile micro tunnel, built over 13–18 years.
  • Mystic River, $260 million: a 7.4-million-gallon tank and 95 acres of separation, built over 5–7 years.
  • Charles River, $690 million, over 28–33 years.

Raiche told the Council on April 23 that the plan had grown to about $1.69 billion. The packages add up to about $1.29 billion, and our sources don't reconcile the two numbers. The Council already approved 15 percent water and sewer increases on June 11 (rates).

Two pieces land hard in Somerville. The Mystic tank is nearly double the size of the 4-million-gallon Poplar Street tank and about 70 feet deep, and it would likely go under the former Christmas Tree Shop parking lot at Assembly Square. The Alewife micro tunnel would need Dilboy Field's parking lot for staging for two to three years.

Magazine Beach: the first siting revolt

At an MWRA presentation at Cambridge's Morse School on October 5, officials told residents that the tank's footprint would stay fenced off permanently. Huang wrote October 9 that the city, after five years of joint work, "had not been notified of these plans prior to the meeting," and called it "not a viable proposal" (Cambridge Day, Oct 11). The park belongs to the state Department of Conservation and Recreation, and giving up protected parkland under Article 97 takes a two-thirds vote of both legislative chambers. State Rep. Mike Connolly, whose 26th Middlesex district takes in East Somerville as well as Magazine Beach, said the plan would be "dead on arrival" at the Legislature. Cambridge Councillor Cathie Zusy says the tank could stay if the park doesn't permanently lose ground: "Maybe it could be 5 million gallons."

Emily Norton, executive director of the Charles River Watershed Association, wrote that public engagement has been "consistently inadequate" (Independent op-ed, Oct 14). She pointed to a "25-year Tunnel" option, a deep tunnel from Boston to Cambridge, that would virtually eliminate Charles River CSOs and shrink the Magazine Beach footprint. The lesson for Somerville is that the plan's tank sites, including Assembly Square, can still be negotiated. Every relocation or tunnel also changes a price nobody has agreed to split.

The critics

The Mystic River Watershed Association goes after the standard itself: "Zero CSOs in a typical year does not mean CSO elimination" (Feb 5 release). Because a "typical year" is modeled on average 2050 rainfall, about half of real years will be wetter. On October 24 Norton and Patrick Herron, the Mystic association's executive director, called the draft "another temporary, incomplete solution." They urged residents to ask regulators to "reject the current draft" and require a plan with no sewage in the rivers: "we will not pay for a plan that continues to treat our rivers as sewers" (Cambridge Day op-ed, Oct 24).

Save the Alewife Brook brought a counter-plan to the Council's Sustainability and Infrastructure Committee on June 29. Its Community CSO Elimination Plan replaces storage with 126 acres of sewer separation plus green infrastructure, for about $229 million. It argues that MWRA frees up about $2.1 billion in bonding capacity as the Boston Harbor bonds retire, and that full elimination would cost about $99 per household per year across the service area.

The Council's leverage

Sustainability and Infrastructure Chair Jesse Clingan, the Ward 4 councilor, was frank about it: MWRA is a state agency, and the state delegation is probably the better channel. The October 22 resolution follows that advice by aiming at the Legislature. Ward 2 Councilor JT Scott noted that Raiche also chairs the MWRA Advisory Board, which advocates call a potential conflict. Mayor Jake Wilson wants the region to "equitably share the financial costs of progress." Raiche's answer on the mandate is blunt: "Not to be too cavalier about it, but it is the law." Two big sewer contracts go out right after comment closes: a $50 million Morrison Avenue storage procurement and a $65 million Mystic River Outfall and Foss-area separation procurement (Flooding and stormwater).

What's next

  • October 28, 2026 (passed): final public hearing on the draft plan at Somerville City Hall and on Zoom (outcome not in our sources yet).
  • October 30, 2026: written comment deadline, including on the Magazine Beach tank (portal).
  • Fall 2026: Clingan and Scott intend to bring Raiche and possibly MWRA officials before Sustainability and Infrastructure. Save the Alewife Brook is drafting a policy order backing its 126-acre alternative.
  • Undated: MWRA's formal answer on Magazine Beach and the tunnel option; the cost-sharing negotiation among Cambridge, Somerville, MWRA and the other member communities.
  • December 2026: the two big sewer procurements go out (The capital plan: fix it first, borrow last).
  • May 31, 2027: the final plan is due to MEPA, and then goes to EPA and MassDEP.
  • August 31, 2029: MassDEP and EPA's review window closes, and the current variances expire.