Advisory Board blog post analyzing the updated CSO Control Plan—reviewing past gains, highlighting cost-per-gallon tradeoffs, and urging strategic, equitable, flexible investments to maximize water-quality benefits.
Updated Long-Term Control Plan:
Resources Hub
On February 25, 2026, the MWRA Board of Directors voted to authorize submission of the Draft Updated Combined Sewer Overflow (CSO) Long-Term Control Plan to the Massachusetts Department of Environmental Protection (MassDEP) and the U.S. Environmental Protection Agency (EPA) by the extended deadline of April 30, 2026.
The Plan was originally expected to be filed on December 31, 2025, but the deadline was extended to allow additional technical review and coordination prior to submission.
Once submitted, the Draft Plan will formally enter regulatory review, including public hearings and a formal public comment period administered through DEP and EPA. As emphasized during the Board discussion, submission of the draft marks the beginning of the regulatory process, not a final decision. Final requirements will ultimately be shaped through agency review and public input.
The Draft Plan outlines how MWRA, Cambridge, and Somerville propose to further reduce CSO discharges, including achieving zero CSOs in a 2050 Typical Year rainfall scenario, while balancing environmental progress, affordability, and long-term system flexibility.
This page serves as a central hub for Advisory Board members and the public. It includes key MWRA documents, Advisory Board analyses and recommendations, and selected background resources explaining the history of the CSO program, the progress achieved to date, and the policy and financial considerations surrounding the Updated Plan.
Key Dates & Timelines
September 2025
- September 17 — MWRA Board of Directors: Presentation on joint scoring criteria
- September 25 — Public Meeting #5 (virtual, 6:00 PM)
October 2025
- Oct. 29 — MWRA Board of Directors: Presentation of recommended suite of projects
- Oct. 30 — Advisory Board: MWRA presentation to full Board for discussion and feedback
November 2025
Nov. 19 — MWRA Board of Directors: Final presentation and vote on draft planNov. 20 —Advisory Board / WAC / WSCAC: MWRA presentation on final draft plan
December 2025
Dec. 31 — Draft Updated CSO Control Plan submitted to MassDEP/EPA(120-day extension granted by MassDEP)
January 2026
February 2026
- Feb. 4 – MWRA Board of Directors Meeting – Draft Updated CSO Control Plan Alternatives
- Feb. 25 – MWRA Board of Directors Meeting: Final Presentation & vote on draft plan
April 2026
- April 2026 – Draft Updated CSO Control Plan submitted to MassDEP/EPA)
2026 and Beyond
- May–September 2026 — Five-month DEP/EPA & public review period
- Jan. 2027 — Final Updated CSO Long-Term Control Plan submitted for MEPA review
Background Resources
- February 25, 2026 – Submission of Draft Updated CSO Control Plan
- February 4, 2026 – Draft Updated CSO Control Plan Alternatives Recommendation
- Editorial Board. “Is making the Charles swimmable worth the cost”, Boston Globe, December 11, 2025, https://app.clickup.com/9013485918/v/dc/8ckxray-2373
- January 15, 2026 – Updated CSO Control Plan: Additional Information on Alternatives
A detailed presentation by the CSO partners on the scope of the study, results of the first-in-the-nation forward looking planning horizon (2050 typical year) and the development and selection of potential solutions (alternatives). - October 29, 2025 – MWRA Board of Directors Draft Updated CSO Control Plan Alternatives Recommendation
A presentation and staff summary outlining the evaluation and selection of recommended Combined Sewer Overflow (CSO) control alternatives for the Alewife Brook, Upper Mystic River, and Charles River basins. The discussion detailed climate-adjusted modeling for 2050 rainfall conditions, assessment of control levels (from limited CSOs to full elimination), and recommended alternatives balancing water quality benefits, feasibility, cost, and community impacts. The materials also summarized stakeholder engagement, financial capability analysis, and next steps toward submitting the Draft Updated CSO Control Plan to MassDEP and EPA by December 2025. - September 17, 2025 – Progress on Development of Updated Combined Sewer Overflow Control Plan
An update on the joint MWRA, Cambridge, and Somerville effort to develop an updated CSO Control Plan, detailing progress on evaluating control alternatives, cost and water-quality analyses, public engagement, and next steps toward submission to MassDEP and EPA - October 22, 2025 – Public Engagement and Involvement for Development of the Draft Updated CSO Control Plan
A summary of the extensive public outreach and stakeholder involvement undertaken by MWRA, Cambridge, and Somerville during the CSO planning process, including multilingual public meetings, surveys, engagement with watershed groups, and environmental justice community participation - Combined Sewer Overflow Control Planning Program – Joint efforts by Cambridge, MWRA, and Somerville A joint project website from Cambridge, Somerville, and MWRA providing information, updates, and public engagement opportunities on plans to reduce combined sewer overflows and improve water quality in local rivers.
- Progress on Development of Updated Combined Sewer Overflow Control Plan – September 17, 2025 MWRA BOD Staff Summary MWRA staff brief the Board on progress toward the updated CSO Control Plan—covering regulatory requirements, evaluated alternatives, preliminary costs, public engagement, and next steps toward a draft submittal and recommendations.
- Advisory Board Blog: What Are We Paying for Clean Water?
- Just the FAQs video (CSOs in the MWRA System)
A “Just the FAQs” video from the MWRA Advisory Board explaining the history, progress, and ongoing management of Combined Sewer Overflows (CSOs) in the MWRA system.
- MassDEP: Sanitary Sewer Systems & Combined Sewer Overflows
This state-government page explains how Massachusetts regulates sanitary sewer and combined sewer overflow systems — covering infrastructure, overflow causes, public notification rules, and CSO control requirements under state and federal law.
- MWRA: Combined Sewer Overflows
A public-education page by MWRA explaining what Combined Sewer Overflows (CSOs) are, how their system handles them, historical and ongoing control efforts, health and environmental impacts, and MWRA’s monitoring and planning framework.
- City of Cambridge: Combined Sewer Overflows
A City of Cambridge page explaining how combined sewer overflows (CSOs) work, their impacts on the Charles River and Alewife Brook, and the city’s ongoing projects, regulations, and data reporting to reduce CSOs and improve water quality.
- City of Somerville: Combined and Sanitary Sewer Overflow Control
A City of Somerville page providing real-time CSO/SSO activation notifications, annual reports, public health guidance, and details on ongoing efforts with MWRA and Cambridge to reduce sewer overflows and improve water quality.
Advisory Board Materials
- The Advisory Board’s FY26 Comments & Recommendations on the CSO Long-Term Control Plan, urging MWRA to pursue only cost-effective projects with measurable water quality and equity benefits while broadening focus to rising risks like SSOs, stormwater, and infiltration/inflow.
- The Advisory Board’s FY24 Comments & Recommendations on the CSO Long-Term Control Plan, reviewing MWRA’s 2022 performance, legislative proposals, and advocacy group pressures, while opposing full CSO elimination as financially unsustainable despite environmental benefits
