Combined sewer overflows

Updated September 21, 2026

What's happening

The public now has until October 30 to comment on a roughly $1.29 billion plan that would stop Cambridge, Somerville and the MWRA from dumping sewage into the Alewife Brook, the Mystic and the Charles — in an average year. The three governments pushed the deadline back a month from September 30 "in response to requests from the public" and added a third hearing, the only one in Somerville (comment portal and notice, updated August 24). The first formal hearing was September 17 at Cambridge City Hall (outcome not in our sources yet). Two remain: September 24, 1–3 p.m., virtual only; and October 28, 6–8 p.m., in person at Somerville City Hall Council Chambers, 93 Highland Ave., or by Zoom.

Somerville's piece of the plan is getting concrete. The city's Construction News newsletter for September 2026 describes the proposed Assembly Square storage tank as 7.4 million gallons, nearly double the roughly 4-million-gallon tank recently built at the Poplar Street pump station, with an early concept footprint of about 205 by 120 feet, about 70 feet deep, and roughly 100,000 cubic yards of excavation. Ward 7 Councilor Emily Hardt told constituents in her September 10 newsletter that she shares the Mystic River Watershed Association's and Save the Alewife Brook's "serious concerns" about the draft.

The three governments submitted the Draft Updated CSO Control Plan to EPA and MassDEP on April 30, after screening 39 approaches. It breaks into three packages: Alewife Brook, $340 million (two storage tanks totaling three million gallons, eight acres of sewer separation, a 0.75-mile conveyance pipe and a one-mile micro tunnel, 13–18 years); Mystic River, $260 million (one 7.4-million-gallon tank and 95 acres of sewer separation, 5–7 years); and Charles River, $690 million (two tanks totaling 12.6 million gallons, 446 acres of separation and two storage conduits, 28–33 years).

Somerville ratepayers are already paying for it. The Council approved 15 percent increases in both water and sewer rates on June 11, and Infrastructure Director Rich Raiche has told councilors the CSO program alone could otherwise demand 15 to 20 percent a year for four or five years. See water and sewer rates.

The story

Most of Somerville sits on a combined sewer — one pipe carrying both sewage and stormwater to MWRA's Deer Island plant. In a big rain the pipe fills, and relief structures dump the mixture into a river rather than into people's basements. Somerville owns two of these outfalls: SOM-001A, which sends untreated combined sewage into the Alewife Brook, and SOM-007A/MWR205A, jointly permitted with MWRA, which sends screened and disinfected flow into the Mystic below the Route 28 bridge (city CSO page).

The numbers are small in a dry year and ugly in a wet one. SOM-001A activated three times in 2025 — August 29 (0.039 million gallons), September 6 (0.338 MG) and October 8 (0.729 MG) — and twice so far in 2026, on July 4 and July 22. MWRA's Somerville Marginal facility discharged five times in 2025 for 8.38 million gallons into the Upper Mystic, while its Alewife outfall MWR003 did not activate at all (MWRA 2025 CSO annual report, Table 2-2). Compare 2023, when Save the Alewife Brook counts 29 million gallons of untreated sewage into the brook and five floods.

How the plan got bigger

The region has spent more than $1 billion on 35 CSO projects since the late 1990s and closed or hit goals at 72 of 86 outfalls, cutting regional volume 88 percent. But SOM-001A is one of 16 outfalls that never met its Long-Term Control Plan targets, and the 2019 MassDEP variances required Cambridge, Somerville and MWRA to go back and study control "up to and including elimination."

A first set of preferred alternatives drew public backlash in October 2025 and was shelved. After a 120-day extension, the teams presented new alternatives to the MWRA board on February 4, 2026, promising "zero CSOs in a typical year" — and the price roughly doubled. Raiche told the Council on April 23 the plan had grown from $870 million to about $1.69 billion, called it "the most sophisticated CSO plan in the country," and previewed water/sewer increases of about 20 percent, then three years of 17 percent, leveling near 2 percent about a decade out. A typical single-family sewer bill would go from $852 to roughly $2,600 a year, with debt service running to 2070. (Raiche's $1.69 billion and the $1.29 billion implied by the city's own three waterbody figures are not reconciled in the sources.)

Two pieces land hard in Somerville: the Mystic River Outfall project — 95 acres of sewer separation plus the 7.4-million-gallon tank, likely under the former Christmas Tree Shop parking lot at Assembly Square — and the Alewife micro tunnel, which would need a two-to-three-year construction staging area in Dilboy Field's parking lot.

The critics

The Mystic River Watershed Association attacked the standard itself. "Zero CSOs in a typical year does not mean CSO elimination," it argued in a February 5, 2026 press release: a "typical year" is a model of average 2050 precipitation, so by definition half of real years will be wetter, and MWRA has not modeled what a decade of actual weather would produce. "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?" said Executive Director Patrick Herron. On affordability: even the most ambitious alternatives came to about a 10 percent increase over average base MWRA rates — "the cost of an extra lunch out every month."

Save the Alewife Brook brought a counter-plan. Kristen Anderson and Michael Lenotto presented it to the Council's Sustainability and Infrastructure Committee on June 29, timed so residents could watch and comment during the open window. Their Community CSO Elimination Plan swaps storage for 126 acres of sewer separation plus green stormwater infrastructure at about $229 million, eliminates the Tannery Brook regulator behind SOM-001A, and demands full MWRA funding. (Their website prices a fuller version, including river restoration, at $450 million over 15 years.)

Their charges are specific: the official Alewife tanks and tunnel can't hold a two-year storm, a single five-year storm would still push an estimated 15 million gallons into the brook, the package is 99.9 percent storage with zero acres of separation planned in Somerville, and tanks and tunnels can't be enlarged later. They cite Cambridge-commissioned projections of a 25 percent increase in two-year storm rainfall by 2050 and MWRA modeling showing average-year Alewife discharges nearly tripling. More than 5,000 people live in the brook's 100-year floodplain, in environmental-justice neighborhoods.

On money, they argue MWRA will free up roughly $2.1 billion in bonding capacity over the next decade as the original Boston Harbor bonds retire, that the harbor case's second stipulation makes MWRA — not Somerville — responsible for bringing the outfall into compliance, and that the gap between the minimum plan and full elimination across all three waterbodies works out to about $99 per household per year across MWRA's million-household service area.

The Council's problem

Committee chair Jesse Clingan, the Ward 4 councilor, pressed on the $99 figure and was frank about leverage: this is a state agency, ratepayers are already facing double-digit increases, and the state delegation is probably the more effective channel. He warned against the Council contradicting its own engineers. Ward 7 Councilor Emily Hardt asked whether the advocates had heard back from Raiche since the submission — they said they'd asked for months with no response — and pushed on his earlier claim that Somerville lacks underground room for separation, which Anderson answered by pointing at the 95 acres planned in Winter Hill. Ward 2 Councilor JT Scott noted that Raiche also chairs the MWRA Advisory Board, whose job is holding costs down for member cities; Anderson called that a potential conflict. Scott said he left with more questions than he came in with.

The asks on the table: Council backing for the 126-acre plan, a policy order and letters to the mayor, Raiche, MWRA and Governor Maura Healey demanding full MWRA funding, and an appearance by MWRA's interim director or CFO.

Mayor Jake Wilson is not defending the price tag so much as the bill. In the April 30 announcement he hoped the region would "rally together as a region to equitably share the financial costs of progress," and Clingan noted on June 29 that he had written a Boston Globe op-ed about the unfair financial burden on Somerville. Hardt was blunter in her August 21 ward newsletter: "I believe that the state and Mass. Water Resources Authority (MWRA) should be shouldering more of the costs," and she pointed constituents to both advocacy groups' comment tools.

The comment window has already moved once. Somerville, Cambridge and MWRA extended it from September 30 to October 30 at the public's request and scheduled an October 28 hearing at Somerville City Hall, 6–8 p.m. — the first hearing held in the city whose ratepayers are being asked to carry the Alewife share; the other two are in Cambridge and online.

Two construction contracts sit just past the comment period: the city's spring 2026 buying plan schedules a $50 million Morrison Avenue linear-storage procurement and a $65 million Mystic River Outfall and Foss area separation procurement for December 2026 — see flooding and stormwater.

What's next

September 17, 2026 — first public hearing, Cambridge City Hall (outcome not in our sources yet).
September 24, 2026 — virtual public hearing, 1–3 p.m.
October 28, 2026 — public hearing, Somerville City Hall Council Chambers, 93 Highland Ave., and virtual, 6–8 p.m.
October 30, 2026 — written comment deadline, extended from September 30 (portal, email to UpdatedCSOControlPlan@aecom.com, or mail to MWRA in Chelsea). More hearings and office hours may still be added.
Fall 2026 — Clingan and Scott intend to file a committee item bringing Raiche and possibly MWRA officials before Sustainability and Infrastructure; Save the Alewife Brook is drafting a policy order for the full Council. Whether the Council formally endorses the 126-acre alternative is the live question.
December 2026 — the two big construction procurements go out.
May 31, 2027 — Final Updated CSO Control Plan due to MEPA; a public meeting follows in winter 2027.
August 31, 2029 — MassDEP and EPA must act on the final plan; the current variances expire.