Combined Sewer Overflow (CSO) Control Plan

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About the project

The City of Somerville, the City of Cambridge, and the Massachusetts Water Resources Authority are collaborating to improve water quality in the Alewife Brook and Mystic River through the development of an updated combined sewer overflow (CSO) control plan.

A combined sewer overflow (CSO) occurs when a large rainstorm overwhelms a combined sewer system, causing stormwater to mix with wastewater (or sewage) and discharge to a nearby river or lake.

As part of the Final Determination to Adopt a Variance for Combined Sewer Overflow Discharges to Alewife Brook/Upper Mystic River Basin of 2019

About the project

The City of Somerville, the City of Cambridge, and the Massachusetts Water Resources Authority are collaborating to improve water quality in the Alewife Brook and Mystic River through the development of an updated combined sewer overflow (CSO) control plan.

A combined sewer overflow (CSO) occurs when a large rainstorm overwhelms a combined sewer system, causing stormwater to mix with wastewater (or sewage) and discharge to a nearby river or lake.

As part of the Final Determination to Adopt a Variance for Combined Sewer Overflow Discharges to Alewife Brook/Upper Mystic River Basin of 2019 issued by MassDEP (the Variance), the City is, along with regional partners, developing an updated plan to reduce such occurrences and protect our waterways.

This updated CSO control plan is one of the many ways that we are working to create a more sustainable and climate-resilient Somerville.

We want to hear from you

Public participation is a critical element in the successful implementation of the Somerville CSO Control Plan. The City wants to understand how local rivers affect the lives of our residents and what impact potential improvements to our stormwater management systems may have on our communities.

By participating in this planning process, you can weigh in and influence key considerations such as

  • What type of interventions are prioritized as part of efforts to reduce CSOs? And in what areas should we pursue them?

  • How should we weigh potential tradeoffs such as that between the pace of CSO reduction and increases in sewer rates?

  • To what extent do we factor in future climate conditions when formulating this plan?

While CSO reduction and stormwater management can be highly technical, there are decisions to be made that effect our community and we want to hear from your regardless of your level of experience with these issues.

  • CSO Control Planning Meeting #2, Dec 15, 2022

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    The City of Somerville, the City of Cambridge, and the Massachusetts Water Resources Authority (MWRA) invite you to a virtual public meeting to discuss improving water quality in our rivers through the development of an updated Combined Sewer Overflow (CSO) Control Plan.

    Join us for this joint public meeting to learn more, including how climate change predictions are being used to prepare our infrastructure for future storms and what this means for the future of our sewer and stormwater systems.

    The meeting will take place virtually on Thursday, December 15, at 6 p.m.

    Register here: tinyurl.com/CSOmeeting2

    Interpretation will be available into Arabic, Amharic, Bengali, Nepalese, Mandarin, Spanish, Portuguese, and Haitian Creole. The presentation materials and the recording will be made available following the meeting.

    Persons with disabilities who need auxiliary aids and services for effective communication (i.e., CART, ASL), written materials in alternative formats, or reasonable modifications in policies and procedures in order to access the programs, activities, and meetings of the City of Somerville should please contact Adrienne Pomeroy at 617-625-6600 x 2059 or apomeroy@somervillema.gov.


  • What is a Combined Sewer Overflow (CSO)?

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

    Figure 1. Dry weather conditions in the combined sewer system versus Wet weather conditions in the combined sewer system - high flows can cause a combined sewer overflow (CSO)




  • CSOs in Somerville

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    The majority of Somerville is served by a combined sewer system, which is common in older cities. 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 Atlantic Ocean. 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, 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 special overflow structures that can release the excess directly to a nearby water body and prevent it from backing up into buildings or out of manholes. These combined sewer overflows (CSOs) contain not only stormwater but also untreated wastewater and debris and are a public health and water pollution concern for Somerville, as well as the approximately 770 other U.S. cities with combined sewer systems. Somerville owns two CSO outfalls that discharge to the Alewife Brook and the Mystic River. The City of Cambridge and MWRA own additional CSO outfalls that discharge into these two rivers.

    Separated systems are designed and constructed to convey only stormwater to the rivers and only sanitary waste to a treatment plant. In the recent years, Somerville has increased its sewer separation and stormwater management efforts because of stricter environmental compliance regulations and a desire to provide a better quality to residents' daily lives. To see what Somerville is doing to address pollutants from stormwater runoff please visit our stormwater management website and learn how you can also help reduce stormwater pollution.

  • How do CSOs and stormwater discharges impact you?

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    During large storms, CSOs discharge to the Alewife Brook, the Charles and Mystic River.

    CSOs are mostly stormwater but do contain sanitary wastewater, and discharge bacteria, nutrients, and other pollutants into waterways.

    This can impact water quality and recreational uses during and immediately following those heavy rain events.

    Stormwater from separate systems also impacts water quality and causes flooding. CSOs do not cause flooding but do contribute additional pollutants to stormwater contamination.

    CSOs and stormwater discharges can impact your communities during and after storm events by:

    • Making rivers unsafe for swimming or fishing

    • Impacting appearance - debris, oil slicks, odor

  • What is a CSO Control Plan?

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    supporting image
    • Establishes water quality and CSO discharge requirements.
    • A plan to meet water quality and CSO discharge requirements.
    • Evaluates how well proposed improvements perform under typical (average) rainfall conditions.


Page last updated: 10 Jan 2023, 10:19 AM