A combined sewer overflow (CSO) occurs when a large storm overwhelms the sewage system causing rainwater to mix with wastewater and discharge to a nearby water body.
This relief measure prevents sewage backups into homes and businesses. MWRA owns combined sewer outfalls that discharge a mixture of stormwater and sewage to the Charles and Mystic Rivers, Alewife Brook, and Boston Harbor.
When a CSO occurs at an MWRA-owned outfall, there is a notification displayed on the map below.
The cities of Boston, Cambridge, Chelsea and Somerville manage their own combined sewer outfalls, including CSO reporting. These communities must be contacted directly for information regarding their CSO discharges:
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 CSO discharges. Discharges consist of partially treated or untreated sewage and waste. More Information on Combined Sewer Overflows and how to prevent them.
Interactive discharge map
Click on individual markers on the map to view more detail. Additional information will appear in the popup window.
Data are preliminary and are subject to change. MWRA publishes final data in an Annual Report.
CSO Map
Treated discharge (last 2 days)
No Recent Treated discharge
Untreated discharge (last 2 days)
No Recent Untreated discharge
Boston Water & Sewer Commission
Cambridge
Chelsea
Somerville
Additional discharge details
The CSO discharge table provides additional details. Discharge duration and volume are estimates. Data for CSOs are preliminary and subject to change.
Potentially Affected Area: Everett/Somerville-Lower Mystic River (marine)
Volume (million gallons): 1.97 Treated
Volume 3-Year Average (million gallons): 1.22 Treated
Event Rainfall (inches): 1.42
* Review of metering data indicates that no activation occurred.
Event rainfall is from the National Weather Service gauge at Logan Airport, which may vary substantially from local rainfall conditions.
Volumes for MWR205 include all flow treated by the Somerville Marginal treatment facility, some of which may also be included in the volume reported for SOM007A/MWR205A if it discharged.
