Combined Sewer Overflow Control Planning Program - Joint efforts by Cambridge, MWRA, and Somerville

Share Combined Sewer Overflow Control Planning Program - Joint efforts by Cambridge, MWRA, and Somerville on Facebook Share Combined Sewer Overflow Control Planning Program - Joint efforts by Cambridge, MWRA, and Somerville on Twitter Share Combined Sewer Overflow Control Planning Program - Joint efforts by Cambridge, MWRA, and Somerville on Linkedin Email Combined Sewer Overflow Control Planning Program - Joint efforts by Cambridge, MWRA, and Somerville link

About the project

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

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.

Cambridge, MWRA, and Somerville (the team) are developing updated control plans that will reduce their respective CSOs and protect our waterways, as part of the Variances for CSO Discharges to the Lower Charles River/Charles Basin and to the Alewife Brook/Upper Mystic River Basin issued by MassDEP in 2019, .

We want to hear from you!

Public participation is a critical element in the successful development and implementation of the CSO Control Plans. The team wants to understand how the Charles River, Alewife Brook and the Mystic River affect your life and what impact potential improvements to the stormwater and sewage systems may have on your communities.

By participating in this planning process, you can weigh in on and influence key considerations, such as:
  • What tools should be used to reduce CSOs? And in what areas should we pursue these tools?
  • How should we weigh potential tradeoffs, such as the pace of CSO reduction and its relation to increases in sewer rates?

While CSO reduction and stormwater management can be highly technical, there are decisions to be made that affect your communities and neighbors. We want to hear from you whatever your level of experience is with these issues. Stay informed, attend project meetings, and keep checking this website and the team’s CSO pages at:


About the project

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

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.

Cambridge, MWRA, and Somerville (the team) are developing updated control plans that will reduce their respective CSOs and protect our waterways, as part of the Variances for CSO Discharges to the Lower Charles River/Charles Basin and to the Alewife Brook/Upper Mystic River Basin issued by MassDEP in 2019, .

We want to hear from you!

Public participation is a critical element in the successful development and implementation of the CSO Control Plans. The team wants to understand how the Charles River, Alewife Brook and the Mystic River affect your life and what impact potential improvements to the stormwater and sewage systems may have on your communities.

By participating in this planning process, you can weigh in on and influence key considerations, such as:
  • What tools should be used to reduce CSOs? And in what areas should we pursue these tools?
  • How should we weigh potential tradeoffs, such as the pace of CSO reduction and its relation to increases in sewer rates?

While CSO reduction and stormwater management can be highly technical, there are decisions to be made that affect your communities and neighbors. We want to hear from you whatever your level of experience is with these issues. Stay informed, attend project meetings, and keep checking this website and the team’s CSO pages at:


  • Public Meeting #4 - January 22, 2024

    Share Public Meeting #4 - January 22, 2024 on Facebook Share Public Meeting #4 - January 22, 2024 on Twitter Share Public Meeting #4 - January 22, 2024 on Linkedin Email Public Meeting #4 - January 22, 2024 link


    You are invited to a virtual public meeting to discuss improving water quality in our rivers through the development of an updated CSO Control Plan. This meeting is a collaboration between the City of Somerville, the City of Cambridge, and the Massachusetts Water Resources Authority (MWRA). Join us for this joint public meeting to learn about the alternatives developed so far to control combined sewer overflows and how the affordability will be evaluated for each entity.


    The meeting will take place virtually on Zoom.

    Wednesday, January 22, 2025 at 6 p.m. 

    Register here: https://tinyurl.com/CSOmeeting4


    For Access Help

    The City of Somerville can provide you with an interpreter in your language for free. To request an interpreter, please contact us at somerviva@somervillema.gov or call 311 (617-666-3311) at least 7 days in advance of this event.

    La ciudad de Somerville puede proporcionarle un intérprete en su idioma de forma gratuita. Para solicitar un intérprete, comuníquese con nosotros a través de somerviva@somervillema.gov o llame al 311 (617-666-3311) al menos con 7 días de anticipación al evento.

    A Prefeitura de Somerville pode fornecer um intérprete em seu idioma gratuitamente. Para solicitar um intérprete, entre em contato conosco por somerviva@somervillema.gov ou ligue para 311 (617-666-3311) com pelo menos 7 dias de antecedência ao evento.

    Vil Somerville ka ba ou yon entèprèt nan lang ou gratis. Pou mande yon entèprèt, tanpri kontakte nou nan somerviva@somervillema.gov oswa rele 311 (617-666-3311) omwen 7 jou davans evènman sa a.

    समरभिल सिटिले तपाँईलाई तपाँईको भाषामा नि:शुल्क अनुवादक उपलब्ध गराउन सक्छ। अनुवादक अनुरोध गर्न कृपया हामीलाई कार्यक्रमको कम्तिमा ७ दिन अगाडि somerviva@somervillema.gov मा इमेल मार्फत वा ३११ (६१७-६६६-३३११) मा फोन मार्फत सम्पर्क गर्नुहोस्।

    薩默維爾市政府可以免費提供您所說的語言的口譯。如需要口譯服務，請於活動前最少7日發送電郵至somerviva@somervillema.gov 或撥打 311 (617-666-3311) 提出申請。

    萨默维尔市政府可以免费提供您所说的语言的口译。如需要口译服务，请于活动前最少7日发送电邮至somerviva@somervillema.gov 或拨打 311 (617-666-3311) 提出申请。

    Somerville সিটি আপনাকে বিনামূল্যে আপনার ভাষার একজন দোভাষী প্রদান করতে পারে। দোভাষীর জন্য অনুরোধ করতে, অনুগ্রহ করে somerviva@somervillema.gov--এ আমাদের সাথে যোগাযোগ করুন বা এই ইভেন্টের অন্তত 7 দিন আগে 311 (617-666-3311) নম্বরে কল করুন।

    የSomerville ከተማ አስተዳደር እርስዎ ባሹት ቋንቋ፣ ያለ ምንም ክፍያ አስተርጓሚ ሊያቀርብልዎ ይችላል። አስተርጓሚ እንዲቀርብልዎ ለመጠየቅ፣ እባክዎ ከስብሰባው ቢያንስ 7 ቀን አስቀድመው ወደ somerviva@somervillema.gov ኢሜይል ይጻፉ ወይም ወደ 311 (617-666-3311) ይደውሉ።

    ADA Notice

    Persons with disabilities who need auxiliary aids or reasonable modifications should please contact Adrienne Pomeroy in advance at 617-625-6600 x 2059 or ADA@somervillema.gov.

    Las personas con discapacidades que necesiten ayudas auxiliares o modificaciones razonables deben comunicarse con anticipación con Adrienne Pomeroy al 617-625-6600 x 2059 o ADA@somervillema.gov.

    Pessoas com deficiências que necessitem de suporte adicional ou acomodações razoáveis devem entrar em contato com Adrienne Pomeroy antecipadamente pelo telefone 617-625-6600, ramal 2059, ou pelo e-mail ADA@somervillema.gov.

    Moun ki gen andikap ki bezwen èd oksilyè oswa modifikasyon rezonab dwe kontakte Adrienne Pomeroy davans nan 617-625-6600 x 2059 oswa ADA@somervillema.gov.

    अपाङ्गता भएका व्यक्तिहरू जसलाई सहायताहरू वा उचित परिमार्जनहरू चाहिन्छ कृपया एड्रियन पमोरोईलाई (६१७)६२५-६६०० X२०५९ / (617)625-6600 X2059 वा ADA@somervillema.gov मा अग्रिम सम्पर्क गर्नुपर्छ।

    身障人士如需要任何協助溝通的輔助工具和服務，請致電 617-625-6600，分機號碼 2059 或發送電郵至ADA@somervillema.gov聯繫 Adrienne Pomeroy。

    残疾人士如需要任何协助沟通的辅助工具和服务，请致电 617-625-6600，分机号码 2059 或发送电邮至ADA@somervillema.gov联系 Adrienne Pomeroy。

    প্রতিবন্ধী ব্যক্তি যাদের কার্যকরভাবে যোগাযোগের জন্য সাহায্য বা যুক্তিসঙ্গত পরিবর্তন প্রয়োজন, তারা অনুগ্রহ করে অগ্রিম Adrienne Pomeroy-এর সাথে 617-625-6600 x 2059 নম্বরে অথবা ADA@somervillema.gov এই ঠিকানায় যোগাযোগ করুন।

    አካል ጉዳተኛ የሆናችሁ ተጨማሪ እገዛ ወይም ምክንያታዊ ማሻሻያዎች እንዲደረግላችሁ የምትፈልጉ ሰዎች እባካችሁ አስቀድማችሁ Adrienne Pomeroy በ 617-625-6600 x 2059 ወይም ADA@somervillema.gov ላይ ማነጋገር አለባችሁ።

  • CSO Control to Date

    Share CSO Control to Date on Facebook Share CSO Control to Date on Twitter Share CSO Control to Date on Linkedin Email CSO Control to Date link

    Over the past several decades, MWRA, in collaboration with CSO communities, including Cambridge and Somerville, has achieved significant CSO reductions and improvements in water quality in the water bodies of metro Boston.

    CSO Reduction

    The MWRA’s original CSO control plan - CSO Long-Term Control Plan (LTCP) - was first developed in the late 1990s. MWRA and the CSO communities have:

    • implemented 35 CSO projects.
    • spent over $1 billion on implementation efforts.
    • closed or accomplished LTCP goals for 72 out of the 86 CSO outfalls:
      • 10 of 19 Charles River CSO outfalls have been closed with a CSO volume reduction of 98% since 1992.
      • 9 of 16 outfalls on the Upper Mystic River and Alewife Brook have been closed, with a CSO volume reduction of 69% since 1992.
    • accomplished an 88% reduction of CSO volume discharges, from the 1980’s average of 3,200 millions gallons annually to 396 million gallons in the typical year as of the end of 2022. The CSO discharge volume required by the LTCP is 404 million gallons annually.
    • treated 95% of the remaining CSO volume discharged. Treatment of the CSO discharges includes screening and disinfection.
    • improved pumping capacity at the Deer Island Wastewater Treatment Plant.
    • improved the way the pipe and pump system is operated.


    Water Quality

    Water quality in the Lower Charles River Basin and the Alewife Brook/Upper Mystic River Basin has improved tremendously over the last 30 years, in part due to significant reductions in CSO discharges and stormwater pollution controls implemented by communities along these water bodies. As shown in the figures below, there has been noticeable improvement in the level of the sewerage indicator bacteria (Enterococcus) in the rivers and harbor since MWRA began implementation of the LTCP. Average bacteria counts during heavy rain, when the rivers are affected by contaminated stormwater and CSO discharges, have decreased substantially. While showing substantial improvements since the 1990s, Alewife Brook continues to have higher bacteria levels than the other water bodies. Certainly there is more work to be done and MWRA and CSO community partners are continuing to do our part to further the remarkable achievements that have been made to date.

Page last updated: 19 Dec 2024, 01:38 PM