Somerville's $1.3 Billion Combined Sewer Overflow Plan

A summary of the regional sewer plan headed to state and federal regulators on April 30, and its projected impact on Somerville households.

The plan in brief

On April 30, the cities of Somerville and Cambridge and the Massachusetts Water Resources Authority will submit a draft updated Combined Sewer Overflow (CSO) control plan to MassDEP and the U.S. EPA. The plan addresses overflows of mixed sewage and stormwater into the Alewife Brook, Upper Mystic River, and Charles River — events that occur when heavy rain exceeds the capacity of 19th-century combined pipes.

Submission opens a five-month public comment period. A final plan is expected in May 2027.

The plan recommends infrastructure that would eliminate CSO discharges in a projected 2050 “typical year,” at a total cost of about $1.29 billion shared among the three partners. According to Director of Infrastructure and Asset Management Rich Raiche, who presented the plan to the City Council on April 23, it is the first CSO plan in the country to design around climate change projections rather than historical rainfall data.

What changed since October

When Raiche presented an earlier version of the plan last October, it carried an estimated cost of $870 million and allowed a limited number of discharges in a typical year. During subsequent public meetings and consultations with MassDEP, stakeholders and regulators indicated that even a single discharge in the typical year would not meet the standard of “elimination.” The partner agencies revised the plan to target zero discharges in the 2050 typical year, increasing the cost to $1.29 billion. The MWRA Board of Directors approved submission of the revised plan in February.

Projected impact on Somerville sewer bills

The Financial Capability Assessment prepared for the plan projects the following annual sewer bills for a typical Somerville single-family home (assumed at 54 CCF of annual usage), currently $852:

• Baseline, no new CSO work — $2,094 by FY 2055, a 146% increase

• Limited CSOs in the 2050 typical year — $2,367, a 178% increase

• Recommended plan, zero CSOs in the 2050 typical year — $2,600, a 205% increase

• Zero CSOs in the 2050 5-year storm — $2,976, a 249% increase

• Zero CSOs in the 2050 25-year storm — $3,109, a 265% increase

• Full sewer separation — $7,409, a 770% increase

The baseline figure reflects projected costs of maintaining the existing system, including debt service on the Poplar Street Pump Station and anticipated MWRA reinvestment at the Deer Island treatment plant, which is approaching 30 years of operation. About 60% of each Somerville sewer dollar passes through to the MWRA assessment. Put another way: of the projected $1,748 increase between today’s bill and the FY 2055 bill under the recommended plan, about $1,242, or roughly 71%, would happen even without any new CSO work. The marginal cost of the recommended plan, compared to doing nothing new on CSOs, is closer to $506 per year by FY 2055.

Projected rate increases

To fund the recommended plan, the city’s draft rate model projects the following annual usage rate increases:

• FY 2027: 20%

• FY 2028: 17%

• FY 2029: 17%

• FY 2030: 17%

• FY 2031: 10%

• FY 2032: 9%

• FY 2033: 5%

• FY 2034: 3%

• FY 2035–2044: 2% per year, declining to 1% per year from FY 2045 onward

Somerville sewer bills have two components: a fixed base charge tied to the size of the property’s water meter, and a volumetric charge based on the amount of water a household uses. The proposed increases apply only to the volumetric portion. The base charge is not projected to change, following a restructuring last year that adjusted it to align with industry norms. The city plans to bring formal rate proposals to the Finance Committee next month.

Debt service on the recommended plan would extend to approximately 2072. A separately planned stormwater infiltration fee, which would shift a portion of stormwater costs onto properties with large impermeable surfaces, has been delayed by data issues in the city’s billing database and is not expected to roll out until mid–FY 2027.

Equity and cost-sharing concerns raised by councilors

Several councilors questioned the cost allocation among the three permittees. Under the regulatory framework dating to the 1970s, CSO outfalls were assigned to political entities, and the outfalls along the Boston side of the Charles River are permitted to MWRA rather than to the City of Boston directly. As a result, Boston’s contribution to the plan is indirect, through its share of MWRA assessments.

Raiche, who also chairs the MWRA Advisory Board, said the current cost distribution places a significant burden on sewer ratepayers across Cambridge, Somerville, and the MWRA’s 43 sewer member communities, and he encouraged the council and constituents to engage their state delegation on funding alternatives. The council passed a resolution last fall asking the state delegation to advocate for policy changes that would give MWRA a role in stormwater management. Raiche also told the council that EPA’s Financial Capability Assessment methodology (which determines whether a community qualifies for an extended schedule based on the ratio of sewer costs to median household income) is poorly suited to high-cost, high-income Massachusetts communities, and that the city has no avenue under the federal framework to negotiate relief.

Councilor Kristen Strezo raised concerns about impacts on lower-income residents, including tenants of the Somerville Housing Authority, whose water bills are paid by the authority. Raiche agreed that housing authority impacts should be part of the upcoming rate discussion. Raiche said Massachusetts law prohibits utilities from offering income-based subsidies through sewer rates, but Strezo and others have asked for a broader conversation about ratepayer assistance.

Construction impacts in Somerville

The recommended plan includes three major project areas:

• Alewife Brook ($340 million): Two storage tanks and a 9-foot-diameter microtunnel running roughly from Mass Ave in Cambridge to Dilboy Field, which would also hold roughly 2.3 million gallons of combined sewage and stormwater during storms. Construction would require a mining shaft in the Dilboy parking lot for two to three years, with roughly 20 truck trips per day traveling along Alewife Brook Parkway to haul excavated soil away from the site. A small permanent footprint would remain after construction.

• Mystic River ($260 million): 95 acres of sewer separation in Somerville and a 7.4-million-gallon storage tank in Assembly Square, nearly double the size in the previous plan version. Site negotiations with Federal Realty would be required.

• Charles River ($690 million): Storage tanks and approximately 446 acres of sewer separation in Cambridge and Boston’s Back Bay. The Back Bay separation work is expected to require approximately 25 years of phased construction.

Why not full sewer separation?

The recommended plan’s headline outcome — zero CSO discharges — applies only against a projected 2050 “typical year,” a planning benchmark representing typical rainfall conditions, not the largest storms. Storms above that benchmark would still cause discharges. Tested against the past decade’s actual rainfall, for example, the Alewife system under the recommended plan would have produced no discharges in most years but would have spilled during 2021’s Hurricane Ida and a 2023 storm that dropped nearly two inches of rain in an hour.

In response to questions from Councilors Clingan and Wheeler, Raiche explained why even full sewer separation (at an estimated $2.64 billion in additional capital cost on top of Somerville’s baseline) would not improve on those outcomes. The downstream MWRA interceptors built in the 1800s have limited capacity and were designed with hydraulic relief points. Even if Somerville and Cambridge fully separated their systems, stormwater inflow from upstream communities and the inability to expand downstream conveyance would still produce overflows during large storms. Walling off the existing relief points, Raiche said, would back sewage into homes — an outcome the partners consider worse than river discharges.

The water quality modeling also indicates that stormwater, not CSO discharges, is the largest source of bacterial pollution in the Alewife, Mystic, and Charles rivers. Eliminating CSOs alone would not make the rivers fully fishable or swimmable.

What happens next

• April 30, 2026: Draft plan submitted; five-month public comment period begins

• June 2, 2026: Public meeting on the draft recommendation

• September 2026: Public hearings

• Late October 2026: EPA and DEP issue comments and direction

• May 31, 2027: Final updated CSO plan due

• Next month: Somerville Finance Committee considers the FY 2027 rate proposal

The full draft plan, presentation slides, and Financial Capability Assessment are available through the city’s joint CSO planning portal at voice.somervillema.gov/joint-cso-planning. Public comments will be received by MassDEP and EPA throughout the formal review period.