Somerville staff outline multi‑hundred‑million dollar CSO plan; rate increases likely

Richard Raich, director of infrastructure and asset management, presented the Somerville portion of the draft long‑term control plan for combined sewer overflows (CSOs) at the Oct. 23 council meeting. The plan covers the Alewife, Mystic and Charles watersheds and was developed in coordination with Cambridge and the Massachusetts Water Resources Authority (MWRA).

Raich said the technical team evaluated 39 project alternatives and concluded the most practicable approach blends targeted sewer separation, storage tanks and a microtunnel rather than full, citywide sewer separation. Staff explained that full separation is impractical in Somerville’s dense, nineteenth‑century street grid, that downstream constraints at MWRA facilities (including Deer Island and several pump stations) limit capacity gains, and that separating stormwater alone would not eliminate untreated discharges during the largest storms.

For the Alewife watershed the plan includes limited sewer separation, two storage tanks and a roughly 9‑foot diameter microtunnel expected to be constructed over a multi‑year schedule; the Alewife package was estimated at about $340 million shared among Cambridge, Somerville and MWRA. The Mystic package would add targeted separation in Winter Hill and a storage tank in Assembly Square tied to the MRAS outfall project, with an estimated cost near $170 million to be split with MWRA. The Charles watershed work focuses on targeted separation and storage with longer timelines and substantial cost.

Using the draft plan, model runs show a substantial reduction in the number of spill events and total discharge volumes in the Alewife and Mystic watersheds for a 2050 climate‑adjusted “typical year,” though very large storms (25‑year events) would still generate sanitary overflows in street systems that can route back to receiving waters.

City staff said the plan will materially affect Somerville’s capital improvement program, borrowing and operating budgets. Preliminary financial modeling followed EPA guidance on financial capability and indicates Somerville can undertake the plan on schedule, but that preserving bond rating metrics will likely require front‑loaded rate increases. Raich said city staff are currently modeling a potential near‑term rate path that could require annual water/sewer increases in the range of roughly 15–20 percent in the initial years to build cash reserves and preserve debt capacity; the numbers are provisional and will be refined with MWRA inputs.

Council members pressed staff on MWRA cost‑sharing, the choice of targeted separation sites (including MRAS work in Winter Hill), and equity of fee design. Raich and staff recommended additional regional conversation and possible legislative approaches to address the stormwater component at scale. Councilors voted to place the draft plan on file and referred it to the Finance Committee for more detailed review before final decisions on rates and CIP timing are taken.

What’s next: MWRA is scheduled to vote on its portion of the draft; Somerville will submit the draft to state regulators by year end and enter a public review phase in 2026. The council’s Finance Committee will take a deeper briefing on modeling, cost‑sharing and rate options.