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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority

MWRA CSO Discharge Reporting

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.

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 (see locations of CSO outfalls in MWRA's service area that are closed or remaining active).

Notices of discharges from Cottage Farm CSO treatment facility in Cambridge since 2010


Map of MWRA treated CSOs

The map below displays CSO discharges from MWRA's CSO treatment facilities. Click on individual markers to view more detail, including start and stop times for each recent CSO activation. MWRA reports these CSOs to the Department of Environmental Protection.


CSO facilities active within the past two days appear green     and inactive facilities are blue    

*Information is preliminary and is subject to change.

Where CSOs are located: Boston, Cambridge, Chelsea and Somerville have combined sewer systems that connect to MWRA's system. Below are links to their website: