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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority

MWRA CSO Reporting

Combined Sewer Overflows (CSOs)Somerville Marginal CSO outfall

Sewer systems owned and operated by MWRA, Boston Water and Sewer Commission and the cities of Cambridge, Chelsea and Somerville include portions with older "combined" sewers that carry both rainwater and sewer flows. During most rainfall events, these combined flows are delivered to the Deer Island wastewater treatment plant. But during heavy rains, the large volumes of stormwater can overwhelm the capacity of the sewer system. Built-in relief points (called combined sewer overflows or "CSOs") release excess flows into the nearest body of water to prevent backups into neighborhoods.

MWRA CSO Treatment Facilities
Annual CSO flows have been reduced by 84% since 1987. More than 90% of the remaining CSO flows discharged in the MWRA service area receives treatment at MWRA-owned CSO treatment facilities, that provide screening and removal of floatable materials, and disinfection and dechlorination prior to discharge. Treated discharges from these facilities occur more frequently than the CSO discharges at untreated outfalls to the same water bodies.

CSO Variance Reporting
MWRA has been granted variances by the state Department of Environmental Protection from having to meet water quality standards in the Lower Charles River Basin and in the Alewife Brook/Upper Mystic River; the variances are expected to be extended until MWRA fully implements its plan to reduce the impacts of combined sewer overflows (CSOs) in the harbor and rivers.


More about CSOs

CSO Annual Progress Reports

CSO receiving water quality monitoring in Boston Harbor