Combined Sewer Overflow (CSO) Control

  • CSO Activation Notifications

  • Annual CSO Progress Update

  • Learn About CSOs

  • Annual Reports

  • CSO Long Term Control Plan

Combined Sewer Overflow (CSO) Event on October 30, 2021 at 6:15 PM at SOM-001A on the Alewife Brook

Public Health Warning:  Public health officials recommend avoiding contact with water bodies during rainstorms and for 48 hours afterwards, as there may be increased health risks due to bacteria or other pollutants associated with urban stormwater runoff and CSO discharges.

A CSO outfall location north of Massachusetts Avenue is the subject of this CSO event

In accordance with the National Pollutant Discharge Elimination System Combines Sewer Overflow (NPDES CSO) Permit No. MA0101982, which authorized the City of Somerville to discharge flows form two outfalls into the Alewife Brook (SOM-001A) and Mystic River (SOM-007A), and the conditions of the Department of Environmental Protection's (DEP) Alewife Brook/Upper Mystic River Variance, we are hereby providing public notice that a CSO occurred at SOM-001A and discharged into the Alewife Brook.

For additional information, you may contact the Stormwater Program Manager at

To receive CSO activation notifications for CSO outfall SOM007A/MWR205A, which is a CSO outfall shared between the City of Somerville and the Massachusetts Water Resources Authority (MWRA), please follow the instructions provided on the MWRA’s website.

Disclaimer: The information is based upon remotely operated equipment and may be subject to change upon actual observed conditions.


Current Year CSO Activations

  • 10/30/2021 6:15 P.M.
  • 10/26/2021 6:10 A.M.
  • 9/2/2021 12:00 A.M.
  • 8/22/2021 9:05 A.M.
  • 8/19/2021 11:45 A.M.
  • 7/27/2021 7:40 P.M.
  • 7/9/2021 11:30 A.M.
  • 7/2/2021 2:00 A.M.
  • 12/1/2020 12:45 A.M.
  • 8/23/2020 4:05 P.M.
  • 7/23/2020 4:00 P.M.
  • 8/29/2019 12:55 A.M.
  • 8/7/2019 11:25 P.M.
  • 7/17/2019 5:55 P.M.
  • 7/6/2019 5:25 P.M.
  • 6/30/2019, 1:55 P.M.
  • 4/15/2019, 6:10 A.M.
  • 11/25/2018 7:10 A.M.
  • 11/3/2018 6:40 A.M.
  • 10/29/2018 9:35 A.M.
  • 9/25/2018 11:25 P.M.
  • 9/18/2018 11:10 A.M.
  • 8/14/2018 2:05 P.M.
  • 8/11/2018 12:25 P.M.
  • 8/9/2018 3:40 P.M.
  • 7/26/2018 4:50 P.M.
  • 7/17/2018 4:50 P.M.
  • 7/6/2018 12:40 P.M.
  • 6/28/2018 9:00 P.M.
  • 6/25/2018 10:00 P.M.
  • 6/24/2018 7:00 P.M.
  • 5/15/2018 4:30 P.M.
  • 4/25/2018 5:40 P.M.
  • 4/16/2018 3:20 P.M.
  • 1/13/2018 at 5:50 A.M.
  • 10/30/2017 at 12:35 A.M.
  • 9/30/2017 at 7:50 A.M.
  • 9/15/2017 at 6:05 P.M.
  • 9/14/2017 at 7:50 P.M.
  • 8/23/2017 at 2:50 A.M.
  • 7/19/2017 at 7:20 P.M.
  • 7/12/2017 at 7:10 P.M.
  • 6/27/2017 at 7:05 P.M.
  • 6/16/2017 at 6:15 P.M.
  • 6/12/2017 at 5:30 P.M.
  • 5/26/2017 at 12:50 A.M.
  • 5/5/2017 at 4:50 P.M.
  • 4/25/2017 at 2:50 P.M.
  • 4/6/2017 at 7:50 P.M.
  • 12/29/2016 at 7:50 P.M.
  • 10/21/16 at 9:55 P.M.
  • 6/5/16 at 10:20 P.M.
  • 5/30/16 at 6:45 A.M.
  • 1/10/16 at 1:20 P.M.
  • 11/20/15 at 4:10 A.M.
  • 10/29/15 at 3:55 A.M.
  • 9/30/15 at 7:49 A.M.


CSO Discharge Table

The CSO discharge table provides additional details for current CSO activations and their estimated discharge duration and volume. Data for CSOs is preliminary and subject to change.

CSO Number Potentially Affected Area Start Time Stop Time Duration Volume (million gallons) Event Rainfall (inches)
SOM-001A Alewife Brook 10/30/2021
6:15 P.M.
6:35 P.M.
25 min 0.19 0.89
SOM-001A Alewife Brook 10/26/2021
6:10 A.M.
6:55 A.M.
45 min 1.42 0.92
SOM-001A Alewife Brook  9/2/2021
12:00 A.M.
275 min 6.07 4.24
SOM-001A Alewife Brook 8/22/2021
9:05 A.M.
9:30 A.M.
25 min 0.27 0.65
SOM-001A Alewife Brook 8/19/2021
11:45 A.M.
1:20 P.M.
95 min 2.89 1.97
SOM-001A Alewife Brook 7/27/2021
7:40 P.M.
8:20 P.M.
40 min 1.06 1.02
SOM-001A Alewife Brook 7/9/2021
11:30 A.M.
2:15 P.M.
125 min 3.06 2.5
SOM-001A Alewife Brook 7/2/2021
2:00 A.M.
3:05 A.M.
65 min 3.02 3.14

In August 2019, the Massachusetts Department of Environmental Protection (DEP) issued, and the U.S. Environmental Protection Agency (EPA) subsequently approved, a Final Determination to Adopt a Variance for Combined Sewer Overflow Discharges to the Alewife Brook/Upper Mystic River Basin. The Variance is a short-term modification of the Massachusetts Water Quality Standards, which allows for limited combined sewer overflow (CSO) discharges from the outfalls along the Alewife Brook/Upper Mystic River Basin permitted to the Massachusetts Water Resources Authority (MWRA) and the Cities of Cambridge and Somerville, subject to specific conditions. One of the conditions in the Variance requires the MWRA and the Cities of Cambridge and Somerville to annually issue a press release, which includes: (1) general information on CSOs; (2) the location of outfalls in the Alewife Brook/Upper Mystic River watershed; and (3) potential health risks posed by exposure to CSO events. The MWRA and the Cities of Cambridge and Somerville also provide herein an update on the progress of CSO control measures to improve the water quality of the Alewife Brook/Upper Mystic River Basin.

I. General Information

The water quality in the Alewife Brook/Upper Mystic River Basin is often impaired due to bacterial and other pollutants from a number of sources, including CSOs, cross connections between sanitary sewers and storm drains, and stormwater runoff. Water quality in the Alewife Brook during both wet and dry weather generally fails to meet state bacteria standards for fishing and swimming. State bacterial standards for the Upper Mystic River Basin are typically only not met during and after large storm events.

Contaminant sources that contribute to water quality impairment include CSO, stormwater and other discharges from communities along the Alewife Brook, including Belmont, Arlington, Cambridge, and Somerville. All of these communities are undertaking programs to identify and control sources of pollution to the Alewife Brook. Although similar contaminant sources contribute to water quality impairment of the Upper Mystic River, as well contributions from Alewife Brook, only one treated CSO discharge remains from MWRA’s Somerville Marginal CSO facility (MWR205A/SOM007A).  This outfall is located approximately a half mile upstream of the Emilia Earhart Dam, just below the Rout 28 Bridge over the Mystic River. Portions of Cambridge and Somerville are served by combined stormwater and sanitary sewer systems, which is common in older cities. Six remaining CSO outfalls permitted to the MWRA and the Cities of Cambridge and Somerville along the Alewife Brook (see the map in Section II. below) are designed to release CSO (a mixture of wastewater and stormwater) during heavy rainfall to provide critical relief to the sewer system when flows exceed system capacity. While these discharges can contribute to the impairment of water quality, they are necessary to prevent sewer backups into homes, businesses, and streets.

II. Location of Outfalls

There are 6 existing―and two closed―CSO outfalls along the Alewife Brook

III. Potential Human Health Risks

CSOs and contaminants that can get into separate stormwater pipes can include microbial pathogens (i.e. bacteria, viruses, and parasites) and toxins (i.e. metals and synthetic organic chemicals). Some of the health risks associated with contact and ingestion of these discharges include: gastrointestinal illnesses, acute diarrhea, and skin irritation. Because of increased health risks associated with these contaminated discharges, public health officials recommend avoiding contact with the Alewife Brook/Upper Mystic River Basin during rain events and for a period of 48 hours following rainfall. Contact with floodwaters should also be avoided, as they may contain similar contaminants and pose associated health risks. Proper precautions are necessary to minimize these risks during flooding events. DEP has developed guidance for homeowners for responding to flooding onto property from large storms or sewer backups, which can be found at:

MWRA and the Cities of Cambridge and Somerville have implemented a system for rapid reporting of CSO discharges to the Alewife Brook/Upper Mystic River Basin. These subscriber-based systems provide timely information regarding a CSO activation including the start time and location of the discharge. To subscribe, sign up at these links:

IV. MWRA and the Cities of Cambridge and Somerville Actions to Address Alewife Brook CSOs

As part of the 36-year-old Federal District Court Order in the Boston Harbor Case (D. Mass. C.A. No. 85-0489-RGS), MWRA is required to undertake certain corrective actions to reduce or eliminate CSO discharges to Boston Harbor, the Mystic, Charles, and Neponset Rivers and Alewife Brook. MWRA and its communities with permitted CSO outfalls, including Boston, Cambridge, Chelsea and Somerville, have been reducing CSO discharges since the 1980s. Major improvements to the regional wastewater collection and treatment system by MWRA, together with local system improvements by the communities, including the separation of combined sewers with construction of new storm drain systems, have contributed to the closing of many CSO outfalls and an 87% reduction in the total annual volume of CSO discharge region-wide during an average/typical year.

To date, 35 projects in the Long-Term CSO Control Plan (LTCP) approved by the U.S. Environmental Protection Agency (EPA) and DEP, and mandated in the federal court order, have been completed. Seven of the 35 projects have contributed to the control of CSO discharges to the Alewife Brook. In December 2021 MWRA submitted the Final Post Construction Monitoring and Performance Assessment report, which summarize the results of CSO inspections, overflow metering, hydraulic modeling, system performance assessments and water quality assessments. Together, these seven projects have closed seven CSO outfalls (more than half of the outfalls that were active in the late 1980s) and contributed to reducing average annual CSO volume to the Alewife Brook in the Typical Year, to a volume less than the total required under the CSO LTCP. However, one outfall (SOM001A) remains that did not meet the activation and volume goals established. Notwithstanding this shortfall, recent water quality modeling documented in MWRA’s report has shown that at current levels of control, if only CSOs were discharging to the Alewife Brook, the water quality standards are met 99.6% of the time in an average/typical year. However, the water quality is greatly reduced when including all other pollutant sources contributing to Alewife Brook (predominantly stormwater contributions), resulting in the water quality standard being met only 45% of the time in an average/typical year. The work to meet the goals at SOM001A continue under a 3-year extension to the Federal District Court order, which will focus on further CSO improvements at SOM001A and 15 other CSOs not meeting LTCP goals system wide. Annual reports will be provided on MWRA’s progress and a supplement to the December 2021 Final Performance Assessment report will be submitted by December 2024 on the 16 CSOs not currently meeting LTCP goals.

Pursuant to the terms of the Variance, MWRA completed and reported on an optimization evaluation of the newly rehabilitated Alewife Brook Pump Station to ensure its pumping strategies are appropriately configured, and are currently performing an optimization evaluation of all CSO systems tributary to the Variance waters, to evaluate if further adjustments to overflow weirs and connections can be made that would further reduce CSOs without contributing to upstream flooding. In addition, MWRA, Cambridge and Somerville are each proceeding with developing separate CSO Control Plans for CSO outfalls owned by each entity that discharges to Variance waterbodies of the Alewife Brook/Upper Mystic and Charles River Basin. These CSO Control Plans will further analyze the costs, performance, and the impacts to water quality associated with additional CSO control alternatives, up to and including elimination of CSOs. Additional consideration will be given to the use of Green Infrastructure technologies and the impacts on Environmental Justice communities. MWRA, Cambridge and Somerville will continue to partner in this effort.

For more information on CSOs and the CSO control program, visit the MWRA, Cambridge, and Somerville websites at:


Ria Convery
Special Assistant
Massachusetts Water Resources Authority
(617) 788-1105

Katherine F. Watkins, PE
Assistant Commissioner for Engineering
Cambridge Department of Public Works
(617) 349-4751

Brian Postlewaite, PE
Director of Engineering
City of Somerville
(617) 625-6600, ext. 5400


What is a CSO?

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.

CSOs in Somerville

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.