Combined Sewer Overflows

Public Works

The City of Cambridge's sewer and drain collection systems consists of separated and combined stormwater and sewer systems. Combined systems are common in older cities like Cambridge. During significant rain storms combined sewer systems may fill up beyond their capacity with a mixture of sanitary waste and rainwater. Combined Sewer Overflows (CSO) act like a relief valve allowing sewerage to discharge into waterways preventing sewerage backups into homes, businesses and streets. Cambridge’s combined sewer system ties into the Massachusetts Water Resources Authority's (MWRA) wastewater collection system which is treated and discharges at Deer Island. During periods of heavy rain both MWRA and Cambridge’s wastewater systems can become overwhelmed by rainwater releasing CSOs into both the Charles River and the Alewife Brook.

Combined Sewer Overflow Diagram
To view a map of all the outfalls and contributing catchment areas in Cambridge, click here

What is being done about CSO's?

As part of the Federal District Court Order in the Boston Harbor Case (D. Mass. C.A. No. 85-0489-RGS), the 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 developed a CSO Control Plan in 1994 which was implemented based on design and construction schedules mandated by the Federal Court as part of the Boston Harbor Court Case.

MWRA and its communities with permitted CSO outfalls, including Boston, Cambridge, Chelsea and Somerville, have been reducing CSO discharges continuously since the 1980s. MWRA, Boston Water and Sewer Commission and the City of Cambridge have completed a number of projects to reduce CSOs to the Charles River. These projects have reduced average annual CSO discharge volume to the Charles River basin by 98% since 1988.

Seven of the MWRA Long-Term CSO Control Program projects have contributed to the control of CSO discharges to the Alewife Brook. These completed projects have closed six CSO outfalls (half of the outfalls that were active in the late 1980s) 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 sixty-three 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.

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 overall. The CSO work is in part also guided by the CSO Variances issued by the Massachusetts Department of Environmental Protection for the Lower Charles River/Charles Basin and Alewife Brook/Upper Mystic River.

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 and MWRA CSO Control Plan Goals and Costs by Receiving Water (PDF, 2016). 

Six CSO outfalls along Alewife Brook remain following the completion of the MWRA’s Alewife Brook Long-Term CSO Control Plan (1 owned by MWRA, 4 owned by the City of Cambridge and 1 owned by the City of Somerville). 

CSOs in the Alewife Brook  

CSOs in the Charles River

There are 15 CSOs in the lower Charles River. Seven of these CSOs are within Cambridge, including 2 MWRA CSO treatment facilities and 5 City of Cambridge CSO outfalls. Currently 2 of the 5 Cambridge CSO outfalls are temporarily closed pending further study.

Important Health Information

During large storms CSO's are released to the Charles River and Alewife Brook. Because of increased health risks associated with CSO discharges, public health officials recommend avoiding contact with bodies of water during rainstorms and for at least 48 hours following a CSO release, and wearing protective clothing such as gloves and boots if contact is unavoidable. Contact with floodwaters should also be avoided as they may contain similar contaminants.

Page was posted on 7/24/2019 1:32 PM
Page was last modified on 10/10/2019 8:36 AM
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