MWRA outlines 2050-focused CSO control plan; Arlington share estimated at about $39 million

MWRA interim executive director Steve Estes Marjosse and Chief Operating Officer Kathy Murtaugh presented a draft updated Combined Sewer Overflow (CSO) control plan to the Arlington Select Board on July 27, 2026. The joint plan for the Alewife, Mystic and Charles basins is intended to eliminate combined stormwater and sanitary overflows in a 2050 “typical year” by combining storage tanks, conveyance piping and targeted sewer separation while adding green stormwater infrastructure and climate-change projections to the design.

"This plan is the 1st in the nation that we know of to estimate future rainfall in developing CSO control measures," Kathy Murtaugh said during the presentation, describing the project as forward-looking and designed for a 3.3‑inch, 24‑hour design storm in 2050. MWRA staff said the recommended package for all three water bodies has a planning‑level cost shown in the presentation as about $1.29 billion in current dollars; MWRA's share of the recommended plan was presented as about $763 million. Escalation and debt service in the modelling increase the long‑term repayments shown on the dashboard, with MWRA saying the green‑line plan corresponds to roughly $2.0 billion in long‑term payments across the system. The presenters noted that the MWRA share would be spread across member communities and highlighted an Arlington share estimate of about $39 million under the mid‑range scenario.

Board members pressed MWRA staff on what the plan does and does not promise. "If you believe that we should spend the $6,000,000,000 to eliminate them, that's an appropriate comment to provide to DEP and EPA," Steve Estes Marjosse said, urging residents to use the public comment period to tell regulators their preferences. MWRA emphasized it evaluated 39 alternatives and chose a package it says balances water‑quality benefits, open‑space impacts and construction disruptions. The draft plan is out for a five‑month comment period that ends in September; the presenters said the final plan will be submitted to state and federal regulators in the spring and that DEP and EPA must sign off.

Why it matters: MWRA members — including Arlington ratepayers — will ultimately bear increased sewer rates to pay debt service on large infrastructure projects. The plan seeks large reductions in typical‑year overflows but acknowledges some overflows will still occur in very large storms. The Select Board and resident advocates urged MWRA and regulators to consider more sewer separation and additional public‑health analyses for overland sewage flooding.

What’s next: MWRA will collect comments through the public comment period, review them with DEP and EPA, and return with a final plan next spring. Arlington board members said they will consider whether to send formal comments or a town position to DEP, EPA and the federal court that continues to supervise aspects of the Boston Harbor/CSO process.