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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority

MWRA Sanitary Sewer Overflow (SSO) Notifications

A sanitary sewer is a system of underground pipes that carries sewage from homes and businesses to the wastewater treatment plant. If this sewer accidentally discharges wastewater to the environment before it's treated, that's a sanitary sewer overflow (SSO). They occur infrequently, and are often due to a break in the sewer line or heavy rain. Excess flow may be discharged to local water body without treatment, as a last resort to reduce the risk of sewage backups.

Public health officials recommend avoiding contact with water bodies during rainstorms and for 48 hours afterwards, as there may be increased health risks due to bacteria or other pollutants associated with urban stormwater runoff and SSO discharges. Because SSOs contain untreated sewage and waste, the Massachusetts Department of Environmental Protection requires MWRA to notify regulators and the public when an SSO occurs. More information about SSOs and how to help prevent them.

Subscribe to MWRA SSO discharge notifications

Select SSO/bypass events to display on map:
* *

Events active within the past two days appear as red markers    ;

Older events are yellow     and bypass events are green     .

To refresh the web page for the latest CSO event information, press Ctrl +F5 in Windows or Shift + ⌘ + R in Mac. For mobile devices, clear the browser cache first and then refresh.

Mouse over individual markers on the map to view more detail. Past events include links to detailed SSO reports. Additional information will appear in the popup window if multiple events occurred during the selected period. If there are no SSO events during the selected period, the map will display no markers.

SSOs displayed on the map have been reported to the Department of Environmental Protection by MWRA, this map does not include SSOs that may occur outside of the MWRA system.

*Information is preliminary and is subject to change.
**SSO events that do not require notification.


The SSO discharge table below provides additional details. Discharge duration and volume are estimates. Data are preliminary and subject to change.

**SSO events that do not require notification.

The map and table above display SSOs occurring only within the MWRA system and does not include SSOs for community systems.  For additional SSO information, see:

Subscribe to MWRA SSO discharge notifications
Mass. Division of Marine Fisheries emergency shellfish closure and opening notices