Alewife Brook Combined Sewer Overflow (CSO) Control

  • CSO Activation Notifications

  • CSO Control Progress Update

  • Annual CSO Report

Combined Sewer Overflow (CSO) Event on 13 January 2018
at 5:50 AM at SOM001A on the Alewife Brook

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


In accordance with the conditions of the Department of Environmental Protection's (DEP) Alewife Brook/Upper Mystic River Variance we are hereby providing public notice that a Combined Sewer Overflow (CSO) occurred at SOM001A and discharged into the Alewife Brook. The Variance approved workplan requires the Somerville Department of Public Works (DPW) to notify local health agents, the DEP, the EPA and the MRWA within 24 hours of a CSO event. The notice is intended to confirm to users of the resource that untreated sewage discharges to the Brook/River has occurred.

The water quality in Alewife Brook is often impaired due to bacterial and other pollutants from a number of sources, including stormwater runoff, CSOs and cross connections between sanitary sewers and stormwater drains. Water quality in the Brook during both wet and dry weather generally fails to meet state bacteria standards for fishing and swimming. Contaminant sources originate in the watershed communities of Belmont, Arlington, Cambridge and Somerville, all of which are undertaking programs to identify and control the sources of pollution to the brook.

Portions of Somerville are served by combined stormwater and sanitary sewer systems, common in older cities. The CSO outfalls on Alewife Brook (see the attached map for location) which discharge untreated CSO (a mixture of wastewater and stormwater) during moderate and heavy rainfall relieve the system and prevent sewer backups into homes, businesses, and streets. In addition, bordering communities also have separate drainage pipes that collect stormwater runoff and carry it to the brook. Discharges from CSOs and from separate stormwater pipes include bacteria and other pathogens, oxygen-demanding pollutants, solids and other contaminants. Public health officials recommend avoiding contact with the Brook during storm events and for a period of 48 hours following rain storms, as there may be increased health risks during these periods. Contact with floodwaters should also be avoided as they may contain similar contaminants and pose associated health risks. Clean up information following a flood is available on the MA Department of Environmental Protection website at For real-time water data at Fresh Pond Reservoir, you can view the United States Geological Survey National Water System website at:

For additional information on the Somerville system, you may contact the Engineering Department at 617.625.6600 ext.5400.

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

Past CSO Activations

  • 10/30/2017 at 12:35 A.M. (SOM001A)
  • 9/30/2017 at 7:50 A.M. (SOM001A)
  • 9/15/2017 at 6:05 P.M. (SOM001A)
  • 9/14/2017 at 7:50 P.M. (SOM001A)
  • 8/23/2017 at 2:50 A.M. (SOM001A)
  • 7/19/2017 at 7:20 P.M. (SOM001A)
  • 7/12/2017 at 7:10 P.M. (SOM001A)
  • 6/27/2017 at 7:05 P.M. (SOM001A)
  • 6/16/2017 at 6:15 P.M. (SOM001A)
  • 6/12/2017 at 5:30 P.M. (SOM001A)
  • 5/26/2017 at 12:50 A.M. (SOM001A)
  • 5/5/2017 at 4:50 P.M. (SOM001A)
  • 4/25/2017 at 2:50 P.M. (SOM001A)
  • 4/6/2017 at 7:50 P.M. (SOM001A)
  • 12/29/2016 at 7:50 P.M. (SOM001A)
  • 10/21/16 at 9:55 P.M. (SOM001A)
  • 6/5/16 at 10:20 P.M. (SOM001A)
  • 5/30/16 at 6:45 A.M. (SOM001A)
  • 1/10/16 at 1:20 P.M. (SOM001)
  • 11/20/15 at 4:10 A.M. (SOM001)
  • 10/29/15 at 3:55 A.M. (SOM001)
  • 9/30/15 at 7:49 A.M. (SOM001)

This notice is required by the Massachusetts Department of Environmental Protection (DEP) as an annual update on the progress of Combined Sewer Overflow (CSO) control measures to improve the water quality of Alewife Brook. You are receiving this notice because your property lies within the extended 100-year floodplain of Alewife Brook as established and currently in effect by the Federal Emergency Management Agency (FEMA).

The water quality of Alewife Brook is often impaired due to bacteria and other pollutants from a number of sources, including cross connections between sanitary sewers and storm drains, urban stormwater runoff, and CSOs. During both wet and dry weather, the water quality of Alewife Brook can be impaired and fail to meet state bacteria standards for fishing and swimming.

Portions of Cambridge and Somerville are served by combined stormwater and sanitary sewer systems common in older cities. Six CSO outfalls along Alewife Brook can release an untreated mix of stormwater and sanitary flow during large storms when stormwater can overwhelm the capacity of the combined sewers. While these discharges can impair water quality, they provide critical relief to the sewer system when flows exceed capacity, preventing sewage backups into homes, businesses and streets.

As part of the Federal District Court Order in the Boston Harbor Case (D. Mass. C.A. No. 85-0489-RGS), the Massachusetts Water Resources Authority (MWRA) is required to undertake certain corrective actions to reduce or eliminate CSO discharges along 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 continuously since the 1980s. Major improvements to the regional wastewater collection and treatment system by MWRA and 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 88% reduction in the total volume of CSO discharge.

In December 2015, MWRA, in partnership with the CSO communities, completed the last of the 35 projects in the Long-Term CSO Control Plan approved by the U.S. Environmental Protection Agency (EPA) and DEP and mandated in the federal court order. Since the program began, MWRA has effectively met more than 180 federal court schedule milestones related to CSO control.

Seven of the 35 projects have contributed to the control of CSO discharges to the Alewife Brook. Together, these completed projects have closed six CSO outfalls (half of the outfalls that were active in the late 1980’s) and are predicted to reduce average annual CSO volume to Alewife Brook by 85% and reduce the frequency of CSO discharges among the six remaining outfalls from 63 activations a year to seven activations a year. Remaining CSO discharges are predicted to comply with Class B “fishable/swimmable” water quality standards 98% of the time.

For more information about these and the other projects in the Long-Term CSO Control Plan, their CSO control benefits, and water quality conditions and improvement, see MWRA’s final CSO Annual Progress Report, filed with the Federal Court in March 2016, at

MWRA continues to meet its remaining federal court obligations and state and federal regulatory requirements related to CSO control. The federal court schedule includes two remaining milestones: commencement of a 3-year post-construction monitoring program and CSO performance assessment by January 2018 and submission of a related report to EPA and DEP by December 2020 intended to verify that CSO discharges meet the levels of control in the approved Long-Term Control Plan.

On August 31, 2016, DEP issued a determination extending the water quality standards variance for CSO discharges to the Alewife Brook and Upper Mystic River by another three years, to August 31, 2019. The variance extension and a supporting fact sheet can be found at: The variance extensions since 2000 have included conditions requiring MWRA to conduct certain water quality sampling, CSO reporting and proper sewer system operation and maintenance to minimize CSO discharges, as well as implement the Long-Term Control Plan in compliance with the federal court schedule. The current variance extension requires MWRA to submit a modified water quality monitoring program by December 2016 (completed) and a scope of work for the 3-year post-construction monitoring program and CSO performance assessment by May 1, 2017, which DEP intends to issue for public review. 

DEP plans to reissue the variance through 2020, after which it intends to make long-term determinations on water quality standards and related level of CSO control for Alewife Brook/Upper Mystic River with information gained in part from the water quality sampling and the court-mandated CSO performance assessment.

During large storms, CSO's are released to Alewife Brook. Because of increased health risks associated with CSO discharges, public health officials recommend avoiding contact with waterbodies during rainstorms and for at least 48 hours following a CSO release, as there may be increased health risks due to bacteria or other pollutants. It is important to understand that floodwaters from extreme storms, in all cases, can present health risks, and proper precautions are necessary to minimize these risks during flooding events. DEP has developed guidance for homeowners for responding to flooding onto your property from large storms or sewer backups, which can be found at:

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

There are 7 CSO outfalls along the length of Alewife Brook

Map note: Four additional CSO outfalls along Alewife Brook (not shown) were permanently closed by the City of Somerville.


Ria Convery
Special Assistant
Massachusetts Water Resources Authority
[email protected]
(617) 788-1105

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

Richard E. Raiche, PE, PMP
Director of Engineering
City of Somerville
[email protected]
(617) 625-6600, ext. 5410