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CSO Post-Construction Monitoring and Performance Assessments

Massachusetts Water Resources Authority

In November 2017, the Massachusetts Water Resources Authority (“MWRA”) commenced a three-year study to measure the performance of its $910 million long-term combined sewer overflow (“CSO”) control plan (the “LTCP”).

The performance assessment is intended to comply with the last two schedule milestones in the now 33-year-old Federal District Court Order in the Boston Harbor Case (U.S. v. M.D.C., et al, No. 85-0489 MA).  From 1987 through 2015, MWRA addressed 182 CSO-related court schedule milestones, including completion of construction of the 35 wastewater system projects that comprise the LTCP by December 2015.

The last two milestones required MWRA to commence by January 2018 a three-year performance assessment including post-construction monitoring (which the MWRA met in November 2017), and to submit by December 2021 the results of its performance assessment to the U.S. Environmental Protection Agency and the Massachusetts Department of Environmental Protection demonstrating that it has achieved the levels of CSO control, “including as to frequency of activation and as to volume of discharge specified in its Long-Term CSO Control Plan,” pursuant to the March 15, 2006, Second Stipulation of the United States and the Massachusetts Water Resources Authority on Responsibility and Legal Liability for Combined Sewer Overflow Control, as amended on April 30, 2018.

MWRA’s CSO performance assessment includes the following key scope elements:

  • Extensive survey and overflow data collection at remaining active CSO regulators
  • Upgrades to and recalibration of MWRA’s hydraulic model of the wastewater system
  • Assessments of system performance for CSO control and the consideration of performance improvements
  • Assessment of the water quality impacts of remaining CSOs and compliance with Massachusetts Water Quality Standards, including receiving water modeling of the Lower Charles River/Charles Basin and the Alewife Brook/Upper Mystic Basin.

    Final CSO Post Construction Monitoring Program and Performance Assessment Report 12/30/2021

MWRA issued a total of seven (7) semiannual reports on the progress of its post-construction monitoring and performance assessment.

Report # Data Collection Period
1 - PDF April 15 to June 30, 2018
(2.5 months)
2 - PDF July 1 to December 31, 2018
(6 months)
3 - PDF January 1 to June 30, 2019
(6 months)
4 - PDF July 1 to December 31, 2019
(6 months)
5 - PDF January 1 to June 30, 2020
(6 months)
6 - PDF July 1, 2020 through December 31, 2020 (6 months)
7-PDF January 1, 2021 through June 30, 2021 (6 months)

CSO Annual Report: CSO Discharge Estimates and Rainfall Analyses for Calendar Year 2023 04/30/2024
CSO Annual Report: CSO Discharge Estimates and Rainfall Analyses for Calendar Year 2022 04/28/2023
CSO Annual Report: CSO Discharge Estimates and Rainfall Analyses for Calendar Year 2021 04/29/2022

Annual Public Briefings

2021 Public Briefing May 21, 2021
2020 Public Briefing May 28, 2020
2019 Public Briefing May 31, 2019

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Updated April 30, 2024

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