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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
  • Verification of the predicted water quality impacts of remaining CSOs through analyses of extensive water quality data collected during the performance assessment period

MWRA plans to issue a total of five (5) 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 (PDF)
(2.5 months)
Nov 2018
2 July 1 to December 31, 2018
(6 months)
Apr 2019
3 January 1 to June 30, 2019
(6 months)
Sep 2019
4 July 1 to December 31, 2019
(6 months)
Apr 2020
5 January 1 to June 30, 2020
(6 months)
Sep 2020


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Updated December 11, 2018

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