CSO Control to Date

Over the past several decades, MWRA, in collaboration with CSO communities, including Cambridge and Somerville, has achieved significant CSO reductions and improvements in water quality in the water bodies of metro Boston.

CSO Reduction

The MWRA’s original CSO control plan - CSO Long-Term Control Plan (LTCP) - was first developed in the late 1990s. MWRA and the CSO communities have:

• implemented 35 CSO projects.
• spent over $1 billion on implementation efforts.
• closed or accomplished LTCP goals for 72 out of the 86 CSO outfalls:
  • 10 of 19 Charles River CSO outfalls have been closed with a CSO volume reduction of 98% since 1992.
  • 9 of 16 outfalls on the Upper Mystic River and Alewife Brook have been closed, with a CSO volume reduction of 69% since 1992.
• accomplished an 88% reduction of CSO volume discharges, from the 1980’s average of 3,200 millions gallons annually to 396 million gallons in the typical year as of the end of 2022. The CSO discharge volume required by the LTCP is 404 million gallons annually.
• treated 95% of the remaining CSO volume discharged. Treatment of the CSO discharges includes screening and disinfection.
• improved pumping capacity at the Deer Island Wastewater Treatment Plant.
• improved the way the pipe and pump system is operated.

Water Quality

Water quality in the Lower Charles River Basin and the Alewife Brook/Upper Mystic River Basin has improved tremendously over the last 30 years, in part due to significant reductions in CSO discharges and stormwater pollution controls implemented by communities along these water bodies. As shown in the figures below, there has been noticeable improvement in the level of the sewerage indicator bacteria (Enterococcus) in the rivers and harbor since MWRA began implementation of the LTCP. Average bacteria counts during heavy rain, when the rivers are affected by contaminated stormwater and CSO discharges, have decreased substantially. While showing substantial improvements since the 1990s, Alewife Brook continues to have higher bacteria levels than the other water bodies. Certainly there is more work to be done and MWRA and CSO community partners are continuing to do our part to further the remarkable achievements that have been made to date.