Somerville Combined Sewer Overflow Plan

Somerville’s efforts to reduce combined sewer overflows (CSOs) are built on more than 30 years of work to improve water quality across the region, including in the Alewife Brook, Mystic River, and Charles River watersheds. Over that time, the MWRA and cities in the greater Boston urban core, including Somerville, have eliminated 88 percent of CSO discharges in the region. Today, the system is facing new challenges.

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Stronger storms and changing weather patterns are placing increasing pressure on infrastructure that, in many cases, was built more than a century ago. This requires a forward-looking approach. 

The Updated Combined Sewer Overflow Control Plan reflects that shift away from using historical rainfall data. It is the first in the nation to incorporate climate change projections into long-term CSO planning, using advanced modeling to evaluate how the system will perform under future conditions through 2050. The recommendations are designed to achieve zero CSOs in a 2050 typical year. This sets a new standard for infrastructure planning.

The planning process has also made clear that CSOs are only one part of a larger challenge. Other stormwater pollutants, aging infrastructure, and system capacity limitations across the region all affect water quality and system performance. Addressing these issues will require coordinated investments not just in Somerville, but across the broader watersheds and state.

The plan is now entering a formal public review process, including a public comment period and ongoing coordination with state and federal regulators. Residents will have opportunities to learn more about the Plan and submit public comments. Public input will help shape the final plan and the investments that follow. 

Typical in older cities, combined sewers collect both wastewater and stormwater runoff in the same pipe. A combined sewer overflow (CSO) occurs when a large rainstorm overwhelms the combined sewer system causing the mix of stormwater and wastewater (or sewage) to discharge to a nearby river or lake. This relief measure prevents sewage backups into homes, businesses, and streets. 

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The majority of Somerville is served by a combined sewer system, which is not just common in older cities, but was considered state of the art when it was built in the late 1800s and early 1900s. Most of the time, Somerville’s combined sewer system transports all the wastewater and stormwater to the Massachusetts Water Resources Authority’s (MWRA’s) Deer Island Treatment Plant, where it is treated and then discharged to the Massachusetts Bay. However, during a rainstorm, stormwater runoff rushes quickly into sewers and can cause a dramatic increase of water flowing into and through the combined sewer pipes. When this happens during large storms, the total amount of this combined wastewater and stormwater can exceed the capacity of the sewer pipes.

For this reason, combined sewer systems such as Somerville’s are designed with overflow structures that can release the excess directly to a nearby water body and prevent it from backing up into buildings or out of storm drains on our streets. These combined sewer overflows (CSOs) contain not only stormwater but also wastewater and debris. They are a public health and water pollution concern for Somerville and the hundreds of other U.S. cities and towns with combined sewer systems. Somerville has one permitted CSO outfall that discharges to the Alewife Brook and is co-permitted for a second outfall in the Mystic River. The City of Cambridge and MWRA own additional CSO outfalls that discharge into these two rivers.

Somerville, Cambridge, and the MWRA have worked together, with input from the U.S. Environmental Protection Agency (EPA), Massachusetts Department of Environmental Protection (MassDEP) and local watershed organizations, to develop a Draft Updated CSO Control Plan (Draft Plan) based on several years of technical analysis, modeling, and public engagement.

The Draft Plan, which was completed and submitted to regulators on April 30, 2026, is open to public comment until September 30, 2026. The project team will make revisions based on feedback from the public and regulators and submit the final Updated CSO Control Plan in 2027. For more information on how to provide a public comment, see the “How to Provide Public Comment” below.

How was the plan was developed? 

To develop the plan, Somerville, Cambridge, and MWRA:

• worked with experts to develop a model of rainfall conditions for the year 2050 that accounts for increases in rainfall volumes and intensity due to climate change.  
• analyzed the expected performance of our infrastructure under those future conditions.
• developed a series of proposed project alternatives to increase system capacity, mitigate CSOs, and improve the health of our local waterways. 

How were various options assessed? 

The planning process produced dozens of potential project alternatives resulting in different levels of CSO control/reduction. These options were analyzed and refined across a range of criteria including:  

• water quality improvements in local waterways
• cost and affordability for our communities
• construction impacts of the proposed work
• impacts on the performance of the city's sewer system

Public input played a critical role in evaluating the proposed alternatives throughout. For more information on the public process that helped informed this role visit voice.somervillema.gov/joint-cso-planning.

What are the final recommendations? 

The recommended path forward eliminates CSOs in the typical year 2050 while balancing construction impacts, preservation of public open space, and impacts to water and sewer rates across Cambridge, Somerville, and the MWRA communities. 

The recommendations include a range of infrastructure improvements including targeted sewer separation, underground storage, conveyance improvements, and other system optimization. Together the recommended construction projects amount to a $1.3 billion investment from Cambridge, Somerville, and the MWRA over several decades.

The recommendations are grouped by waterbody in the plan: 

Alewife Brook: 

The recommended plan for the Alewife Brook includes $340 million worth of projects both in Somerville and Cambridge. In Somerville, a microtunnel is proposed adjacent to the Alewife Brook to store and convey large volumes of combined sewer and stormwater. Additional storage, conveyance, and sewer separation are recommended in Cambridge. Together, projects in the Alewife are expected to take up to 18 years of construction to complete.  

Mystic River: 

Recommendations for the Mystic River include a 7.4 million gallon storage tank for combined sewer and stormwater in Assembly Square and sewer separation in the Ten Hills and Winter Hills neighborhoods. Somerville has begun detailed design of the Mystic River Outfall and Sewer Separation (MROSS) project. More information on that project can be found here: somervillema.gov/mysticoutfall. Together, work in the Mystic is estimated to cost $260 Million and take up to 7 years of construction to complete.  

Charles River: 

Recommendations for the Charles River are limited to sewer separation, storage tanks, and storage conduits in Cambridge and Boston. This work is expected to cost $690 million and up to 33 years of construction to complete.

Cleaner Water and Fewer Overflows

The recommended plan is designed to eliminate combined sewer overflows during a 2050 typical year. The projects recommended by the plan will improve the health of local waterbodies, mitigate flooding, and support long-term environmental and public health goals. 

Costs and Impact to Ratepayers

Project costs are shared among Somerville, Cambridge, and the MWRA based on regulatory responsibility for each outfall and system component.

Somerville is both a direct project partner and a member of the MWRA system. As a result, the City contributes to local infrastructure improvements while also paying a share of regional MWRA investments.

For more info on water and sewer rates in Somerville visit the Water and Sewer Department's webpage. 

Construction Impacts

The large scale construction projects recommended by the plan will have significant impacts to residents in the surrounding neighborhoods including the need for trucking materials in and out of the worksites, traffic detours, elevated noise levels, and more. 

How to Provide Public Comment 

Until September 30, 2026, community members, advocacy groups, and organizations are invited to review the Draft Updated CSO Control Plan and share feedback with the Cities of Cambridge and Somerville, along with the MWRA.  

The public can provide feedback via:

Online comment portal - Comments can be submitted to the project team electronically using an online comment portal hosted by Plan Engage at: https://us.planengage.com/updatedcsocontrolplan/page/home 

Email - Comments can be emailed to the project team at UpdatedCSOControlPlan@aecom.com 

Mail - Electronic comments are preferred, however written comments addressed to “Draft Updated CSO Control Plan Partners” can be mailed to the project team at the following address:   

MWRA   
Attn: Draft Updated CSO Control Plan   
2 Griffin Way   
Chelsea, MA 02150   

Public hearing - Two virtual public meetings to be held in September. Once more information about these meetings is available. It will be shared broadly through City channels and meeting details will be posted on voice.somervillema.gov/joint-cso-planning.

In-Person Office Hours - The project team will be hosting in person and virtual office hours to answer your questions about the Draft Plan. Find us at one of the following venues this summer.

Combined Sewer Overflow Control Planning Program - Joint efforts by Cambridge, MWRA, and Somerville. This page includes information and public meeting presentations from throughout the plan's development. 

Somerville CSO Activations - Information on CSO activations at SOM-001A (Alewife Brook). Sign up here to receive discharge notifications. 

MWRA CSO - Information on CSOs across the MWRA system.