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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority


CSO and SSO Notifications

Parts of metropolitan Boston has a combined sewer system, where surface runoff enters the sewer, reducing system capacity, particularly in extreme weather events with very heavy rains.  When stormwater and groundwater overwhelm the sewer system, a combined sewer overflow (CSO) or a sanitary sewer overflow (SSO) can result.


MWRA has recently completed the final construction milestone of its Long Term Combined Sewer Overflow (CSO) Control Plan, a 25-year plan to reduce the impact of combined sewer overflows to surface waters in the metropolitan Boston area. The plan comprised 35 wastewater system improvement projects that bring CSO discharges at CSO outfalls in the Boston area into compliance with the Federal Clean Water Act.

Somerville Marginal CSO outfall

What is a CSO?

CSO receiving water quality monitoring

CSO Variances

CSO Annual Discharge Estimates and Rainfall Analyses Reports (2014-present)

CSO Annual Progress Reports (2004-2015)

A Sanitary Sewer Overflow (SSO) is an unintentional discharge of wastewater to the environment prior to reaching a treatment facility. SSOs occur on infrequent occasions when the capacity of the sewer system is exceeded, usually due to extreme conditions such as heavy rainfall, or an obstruction or break in sewer lines.

Manhole cover

More about SSOs and how to help prevent them

Updated February 9, 2021