Combined Sewer Overflows

Public Health Warning:  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.

What is a CSO?

Combined sewers are sewers that are designed to collect both wastewater and stormwater runoff in the same pipe.


Approximately 70% of Chelsea is serviced by combined sewers, and the remaining 30% is serviced by separate sanitary sewers and storm sewers. Most of the time, Chelsea’s combined sewers transport all of the wastewater and stormwater to the Massachusetts Water Resources Authority’s (MWRA’s) Deer Island Treatment Plant, where it is treated and then discharged to the Atlantic Ocean. However, during a rain storm, runoff over hard surfaces rushes quickly into sewers and can cause a dramatic increase of water flowing into and through the combined sewers. When this happens, the total amount of this combined wastewater and stormwater flow can exceed the capacity of the sewer pipes.

For this reason, combined sewer systems such as Chelsea’s are designed with special overflow structures that can release the excess wet-weather flow directly to a nearby water body, to prevent it from backing up into buildings or out of manholes. These overflows, called Combined Sewer Overflows (CSOs), contain not only stormwater but also untreated wastewater and debris so are a public health and water pollution concern for Chelsea, as well as the approximately 770 other U.S. cities with combined sewer systems. Learn more about CSOs (PDF)

Long Term Control Plan

The City of Chelsea has developed a master plan to separate the remaining sewer and stormwater drainage systems throughout the City that will, in time, nearly eliminate CSO discharges. Recent sewer separation project (in conjunction with roadway improvements), include:

  • Upper Broadway (Chelsea City Hall to Revere line), 2020 - current
  • Central Ave (Highland Ave to Eastern Ave), 2022 - current
  • Downtown Broadway (Chelsea City Hall to Williams St), planned 2024

In addition, Chelsea’s CSOs are included in the MWRA’s long term control plan. Several projects have already been undertaken by the MWRA to increase the flow of sewerage and stormwater to the Deer Island Treatment Facility and therefore reduce the likelihood of an overflow event.

In 2001, the MWRA completed construction of the Chelsea Branch Sewer Relief Project and outfall repairs and floatables control at CHE008. The Chelsea Branch Sewer Relief Project included the installation of 42-inch and 66-inch pipes to relieve the Chelsea Branch Sewer and the Revere Extension Sewer located along Eastern Avenue in Chelsea. The work at CHE 008 included the relining and replacement of part of the 42-inch outfall pipe, the replacement of the headwall, the placement of new shore protection and the installation of an underflow baffle at the sole regulator to provide floatables control. These projects are intended to minimize CSO discharges to Chelsea Creek and reduce surcharging in the wastewater transport system.

In 2023, the MWRA completed replacement of thirty feet of piping between the Chelsea’s CHE008 regulator structure and the MWRA’s Chelsea Branch Sewer and Relief Sewer and installed new structures to maintain the sewerage flow at the intersection of Willoughby Street and Eastern Avenue.