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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority


Combined Sewer Overflow (CSO) Variances

A Variance allows MWRA to temporarily exceed water quality standards in a CSO location while working to improve conditions.

Massachusetts Water Resources Authority and the cities of Cambridge and Somerville must comply with requirements of variances granted by the state Department of Environmental Protection that allow them to temporarily discharge combined sewage to the Charles River Basin and to the Alewife Brook/Upper Mystic River. The variances require receiving water quality monitoring, public notifications of CSO discharges, and special studies to see if particular CSOs can be reduced still further. MWRA's water quality monitoring report tracks changes in water quality as MWRA implements CSO remediation. 

The variances currently extend through August 2024, allowing time for MWRA and the CSO communities to evaluate further CSO controls to reduce the CSO impacts to the Charles River Basin and the Alewife Brook/Upper Mystic River.

Combined Sewer Overflow
Combined sewers can overflow during large storms, and discharge a mix of sewage and stormwater into the Charles and Mystic Rivers, Alewife Brook and Boston Harbor, temporarily degrading water quality.

MWRA is also implementing these nine system controls intended to limit the duration and impact of combined sewer discharges:

  • Operate and maintain CSO facilities properly
  • Maximize storage of combined sewage
  • Minimize overflows of combined sewage
  • Maximize flows to treatment plants
  • Prohibit dry weather CSOs, unrelated to rain
  • Control solids and floatables
  • Institute pollution prevention programs
  • Notify the public of impacts
  • Observe monitoring and reporting requirements


Updated CSO Control Plans

MWRA, Somerville and Cambridge are updating their CSO Control Plans for the variance waters. Information about this project can be found on the City of Cambridge website.


MWRA CSO Reports
CSO Variances Mitigation Projects
Summary of CSO Receiving Water Quality Monitoring in Upper Mystic River/Alewife Brook and Charles River (2021)
CSO post-construction monitoring and final reports (semiannual, 2018-2021)
Annual Public Briefing on CSO post-construction assessment
Annual CSO discharge estimates
CSO Control & Performance Assessment (7/18/19 presentation)

MWRA CSO web pages
What is a CSO?
MWRA CSO Notifications
MWRA CSO Receiving Water Monitoring
CSO discharges at the Cottage Farm CSO Treatment Facility (2010-2019)

Updated January 3, 2023