September 2025 Board Briefs: Crunch Time for CSO Planning

At the September 17th MWRA Board of Directors meeting, staff presented the joint scoring criteria that MWRA, Cambridge, and Somerville will use to evaluate potential projects for the Updated CSO Long-Term Control Plan. With the draft plan due to MassDEP and EPA by December 31, 2025, the process is entering its final, decisive stretch.

The calendar is now packed: on September 25th, MWRA and its partners held the fifth public meeting to review alternatives and take community input. In October, MWRA staff will present the recommended suite of projects to the MWRA Board, followed by a full Advisory Board presentation the very next day. November will bring a final round of discussion before the MWRA Board of Directors’ anticipated vote, leading to submission of the Draft Plan in December. A five-month public comment and regulatory review period will follow.

With cost estimates reaching $5.6 billion, the projects chosen will have long-lasting implications for communities and ratepayers across the MWRA service area. Advisory Board members’ questions, perspectives, and feedback are vital as we move through this critical phase. Visit our new CSO Resource Page to explore the scoring criteria, learn about upcoming meetings, and find ways to make your community’s voice heard.

MWRA, Cambridge, and Somerville are weighing four potential levels of Combined Sewer Overflow (CSO) control, ranging from significant reductions under “Breakpoint” conditions to full elimination during major storm events projected for 2050. Options include localized storage tanks, microtunnels, and full sewer separation, as well as regional tunnel systems that would capture all discharges to the Charles River and Alewife Brook. Each path carries very different costs, timelines, and neighborhood impacts.

Preliminary capital costs for the alternatives span from $400 million to $5.6 billion, with wide variation in construction complexity and risk. Notably, while the different approaches offer different levels of reliability and resiliency, MWRA’s water quality modeling shows only limited differences in annual water quality benefits. For example, under 2050 rainfall projections, CSO discharges account for just a handful of exceedance days per year, while stormwater and upstream flows continue to drive hundreds of exceedances annually.

Stakeholder groups, from watershed associations to city councils, are pushing for stronger elimination targets and more green infrastructure investment. At the same time, MWRA, Cambridge, and Somerville are working on an equitable cost allocation framework for whichever control plan is chosen. Staff will return to the MWRA Board in October with recommended approaches for each of the three variance waters (Charles, Alewife, and Mystic), and Advisory Board members will play a central role in ensuring that community perspectives and ratepayer impacts are part of the conversation.