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April 15, 2003


MWRA, Cambridge Department of Public Works, Somerville Public Works

Combined Sewer Overflow Control Project for Alewife Brook
Area Moves Forward

Relief for the Alewife Brook from decades of combined sewer overflows is on the way. The Massachusetts Water Resources Authority (MWRA) and the Cities of Cambridge and Somerville are finalizing plans for a $75 million series of projects to significantly reduce combined sewer overflows (CSOs) into the brook. The impacts and benefits of the project are currently in a state environmental review process with the Massachusetts Environmental Policy Act Unit (MEPA). MWRA and Cambridge expect to submit a Response to Comments document for MEPA review and public comment by mid-May.

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 storm 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 Cambridge and Somerville are served by combined stormwater and sanitary sewer systems, common in older cities. There are eight CSO outfalls on Alewife Brook (map), which discharge untreated CSO (a mixture of wastewater and stormwater) during moderate and heavy rainfall to 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 and for 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.

As part of a federal court order, MWRA together with the Cities of Cambridge and Somerville are implementing an ambitious CSO abatement program to minimize the impacts of these CSO discharges and reduce discharges by an estimated 84% yearly. Earlier sewer separation work and other system improvements by Cambridge and Somerville, done in the 1990’s, has already allowed closure of several CSO outfalls and significantly reduced the CSO discharges to the Alewife Brook area.

The $75 million project consists primarily of "sewer separation." By removing stormwater from the combined system, sewer separation creates more room in the sewer pipes during storms, thereby reducing the amount that overflows. The newly separated stormwater is then redirected through new pipes to the Alewife Brook. Cambridge’s sewer separation work will involve state-of-the-art measures to improve the quality of the stormwater flows as well as to attenuate, or dampen, the discharges to avoid any effect on Alewife Brook water levels. Flooding along Alewife Brook has long been a problem, so Cambridge has taken great care to ensure that the new stormwater outfall project will not worsen the existing flooding situation. In addition to the sewer separation work, relieving constrictions at key connection points ("hydraulic relief"), will allow more flow from the local system to get into the regional pipes, thereby reducing the overflow amounts.

The main elements of the current project are:

Separating stormwater from the combined sewer system in neighborhoods east of Fresh Pond to eliminate CSO discharges from the Cambridge004 outfall (Early phases of this work were completed in 2002.) This work will ultimately include constructing a new stormwater outfall and an innovative 3.4 acre stormwater wetland to attenuate peak flows to the Little River/Alewife Brook and improve stormwater quality.

Separating common manholes in the CAM400 area (bounded by Alewife Brook Parkway, Whittemore and Massachusetts Avenues) to reduce CSO discharges.

Increasing the size of connections between local sewers and the MWRA interceptor to reduce CSO discharges.

Increasing the size and capacity of the Rindge Avenue siphon to reduce CSO discharges.

Installing mechanisms at all remaining Alewife Brook CSO outfalls to control floatable materials.

Under the current state variance, the MWRA, Cambridge, and Somerville are also in the process of completing their CSO planning in the Alewife/Mystic Watershed to determine if further CSO controls are affordable and cost-effective. A CSO Reassessment Plan will be completed by July 1 for public review. The Massachusetts Department of Environmental Protection (DEP) and the U.S. EPA are expected to make a final decision on the appropriate level of CSO control and water quality standards for the Alewife Brook after a public review process. MWRA’s CSO program is also overseen by a federal judge as part of the Boston Harbor court case. MWRA, along with the CSO communities, is implementing 25 different CSO control projects along Boston Harbor and the Mystic, Charles and Neponset Rivers.

Final design of the Alewife project is slated to begin in 2004 and construction will take several years to complete. While the CSO project will reduce bacteria levels in the brook after rainstorms, there will continue to be significant sources of pollution in the watershed during both dry and wet weather conditions. Various community and watershed organizations are working to identify and reduce these contaminant sources, and to deal with the causes of local flooding conditions.

More information on on CSOs and the CSO Abatement Program is available on the MWRA website. contact MWRA at 617-788-1170 or website at or visit the Cities of Cambridge and Somerville websites at and

Updated information on water quality in the Alewife Brook watershed can be found at the Mystic River Watershed Association website, and at a "real time" site co-sponsored by the City of Somerville at