The Boston Water and Sewer Commission has 37 active combined sewer overflows (CSOs) that are permitted to discharge combined sewage during wet weather events under a National Pollutant Discharge Elimination System (NPDES) Permit. These CSO outfall pipes are located along Boston Harbor, Dorchester Bay and the Charles River in the Lower Charles River Basin. Whether these pipes discharge during rain events depends upon a number of factors.

As part of the Commission’s Public Notification Program, maps showing CSO outfall pipes were developed and sent to various groups with interests and concerns about the water quality in these receiving waters. The maps, when applicable, also show outfalls owned by the Massachusetts Water Resources Authority and the City of Cambridge.

The CSO outfall pipes shown on the following maps are necessary to provide relief to a piping system that carries stormwater flows in addition to sanitary flows.  The relief occurs only when the capacity of these pipes has been exceeded.  Generally, the combined flows are conveyed to the MWRA’s treatment plant on Deer Island and overflows don’t occur. According to studies conducted by the MWRA, the potential for a CSO discharge increases when the amount of rainfall exceeds 0.5 inches.  Discharges of combined flows depend upon the rainfall amount, its intensity and duration.   When CSO discharges occur the Massachusetts Water Quality Standards for swimming and contact recreation can be violated.  

September 2007

Lower Charles River Basin CSO Outfall Map

This map shows the locations of CSOs owned by the Boston Water and Sewer Commission (Commission), the Massachusetts Water Resources Authority (MWRA) and the City of Cambridge. The Commission’s CSO outfalls are permitted by both the Massachusetts Department of Environmental Protection and the U.S. Environmental Protection Agency to discharge a combination of wastewater and stormwater during wet weather.  The Commission inspects critical structures upstream of these outfalls each month.

The status of the Charles River is monitored by the Charles River Watershed Association.  Conditions in the river can be viewed online at: http://www.crwa.org/index.html?wavestop.html&0

Dorchester Bay CSO Outfall Map

This map shows the locations of CSOs owned by the Boston Water and Sewer Commission (Commission). The Commission’s CSO outfalls are permitted by both the Massachusetts Department of Environmental Protection and the U.S. Environmental Protection Agency to discharge a combination of wastewater and stormwater during wet weather.  The Commission inspects critical structures upstream of these outfalls each month.

The status of the most of the beaches shown on this map can be viewed online at:http://mass.digitalhealthdepartment.com/public_21/index.cfm. 

 

Inner Harbor CSO Outfall Map

This map shows the locations of CSOs owned by the Boston Water and Sewer Commission (Commission), the Massachusetts Water Resources Authority (MWRA) and the City of Chelsea. The Commission’s CSO outfalls are permitted by both the Massachusetts Department of Environmental Protection and the U.S. Environmental Protection Agency to discharge a combination of wastewater and stormwater during wet weather.  The Commission inspects critical structures upstream of these outfalls each month.

The status of the Constitution Beach in East Boston which is not shown on this map can be viewed at:http://mass.digitalhealthdepartment.com/public_21/index.cfm.