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CSO Post-Construction Monitoring and Performance Assessment

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 require 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 2020 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.

MWRA plans to issue a total of seven (7) semiannual reports on the progress of its post-construction monitoring and performance assessment, as scheduled in the table below.

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


Annual Public Briefings

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

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Updated November 2, 2020

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