Somerville outlines CSO plan, proposes stormwater enterprise and parcel-based fees

Richard Raich, Somerville's director of infrastructure and asset management, told the City Council on May 20 that the city is carrying out a comprehensive update to its long-term control plan for combined sewer overflows (CSOs) and is proposing a new stormwater enterprise and parcel-based billing system to help pay for capital work.

The update, Raich said, is built on unified hydraulic modeling with the Massachusetts Water Resources Authority (MWRA) and Cambridge and is the first regionwide plan the presenters know of to explicitly incorporate climate-change projections into planning. "We are the first in the nation to incorporate climate change into that regulatory framework," Raich said as he described revised typical-year metrics and conceptual alternatives for separation, storage, and tunnels.

Why it matters: CSOs discharge untreated or partially treated sewage and stormwater to local waterways during heavy rain. Somerville officials said the city still discharges treated and untreated flows to the Mystic River and Alewife Brook in larger storms and that climate change increases both the frequency and volume of discharges. The scale of infrastructure needed under the updated scenario makes clear the city must coordinate regionally and change how it funds stormwater management.

Key facts from the presentation

- Regional framing: the long-term control plan update unifies Somerville, Cambridge and MWRA hydraulic models and evaluates separation, storage (including deep tunnels), green stormwater infrastructure (GSI), and inflow/infiltration reduction as alternatives. The MWRA and state regulatory process require a draft by December 2025 and a final plan (subject to review) afterward.

- Costs and scale: Raich presented a planning-level model that forecasts roughly $50 million in incremental revenue needed by the start of construction (~2030) to support programs identified in the plan; that estimate gives a per-household planning number near $1,200 over the planning horizon and places Somerville in a "low impact" category under current EPA financial-capability indicators because of relatively high median household income.

- Engineering examples: full sewer separation would require new outfalls and neighborhood storage (a conceptual 5,500,000-gallon storage tank near Gilman was used as an example); alternatives include partial separation plus storage and MWRA-scale storage tunnels that temporarily hold combined flow for later treatment at Deer Island.

- Flooding vs. CSOs: councilors and staff stressed that flooding and CSO pollution are related but distinct problems. Raich emphasized that flooding is driven by watershed runoff and downstream hydraulic constraints as well as local pipe capacity; many sources of runoff sit outside Somerville's combined-sewer boundary.

Stormwater enterprise and billing proposal

Raich proposed creating a stormwater enterprise to bill parcels based on measured impervious area rather than relying solely on water use to fund stormwater and sewer capital. The proposal uses high-resolution aerial imagery and GIS to measure impervious area per parcel, assigns most parcels to tiered billing bands and bills very large commercial parcels on exact impervious area. Credits would be available for on-site retention and green stormwater infrastructure.

- Equity and revenue mechanics: the city would re-split the combined water/sewer/stormwater revenue requirement so that sanitary sewer and stormwater are funded from separate enterprise billing: roughly a two-thirds (sanitary) / one-third (stormwater) split was shown in the planning numbers. That shift would likely lower bills for many residential customers while increasing bills for large commercial and high-impervious-area properties.

- Expected effects: model runs in the presentation showed that if 25% of property owners implemented on-site retention at modest levels the Mystic outfall volume could shrink materially, and complete adoption of simple retention measures citywide would sharply reduce CSO volumes in the modeled typical year.

Council questions and context

Councilors asked about how regional partners would pay (Councilor Burley), the limits of permeable pavement in New England (Director Raich), the legal ability to target low-income rate relief (Raich and staff said EPA financial-capability guidance and Massachusetts law constrain direct low-income exceptions), and how developers and large property owners could be required to pay or to contribute infrastructure (Raich said existing infiltration and inflow fees are being recalculated and would increase). Several councilors expressed concern about affordability and the potential for rate increases to affect residents on fixed incomes.

Next steps and scheduling

Raich said the city is finalizing cost estimates for alternatives, will hold neighborhood meetings on impacts and design, will bring proposed FY26 water/sewer/stormwater rates to the council for public hearing, and expects to submit a draft long-term control plan to regulators in late 2025. He said authorizations from Council will be required to create the stormwater enterprise and related ordinance changes.

Ending

Councilors and staff framed the work as a multiyear, regionally coordinated program that will require neighborhood outreach, ordinance changes, and Council action on rates and billing structure; staff committed to follow-up briefings and neighborhood-level public meetings in the fall.