A communication transmitted from Yi-An Huang, City Manager, relative to combined sewer overflows

City of Cambridge Department of Public Works John F. Nardone, Commissioner 147 Hampshire Street Cambridge, MA 02139 theworks@cambridgema.gov Voice: [phone removed] TDD: [phone removed] To: Yi-An Huang, City Manager, From: John F Nardone Commissioner DPW Date: December 17, 2025 Re: Policy Order 2025 #136: requests that the City Manager work with relevant City Departments and watershed advocacy groups to consider various measures, including: • Costs and benefits to meet the 2050-25 year storm level of control for CSOs. • Updating regulations for stormwater management on private development projects • Implementation of green stormwater infrastructure • Improving meaningful public outreach on sewer and stormwater investments including the creation of a Combined Sewer Overflow Commission This Policy Order follows a public hearing of the Council’s Health and Environment Committee on September 9, 2025, where the City’s on-going work to reduce combined sewer overflows was discussed. As a response to this policy order, the City offers the following update. Background Combined Sewer Overflows (CSOs) are part of a broader set of interconnected challenges within the City’s sewer and stormwater collection systems. CSOs are, generally, the process by which sewage from the City’s combined sewer systems is released into waterways in order to prevent sewage backups into homes, businesses and streets. CSO control is a broad description of policies and infrastructure used to reduce the frequency and volume of CSOs. As the City prepares the Draft CSO Control Plan, it is important to contextualize CSOs and other methods the City has used to address sewer and stormwater quality over decades. The City has consistently invested in infrastructure improvements to provide the level of service expected from modern sewer and stormwater systems. Accordingly, the City’s planning and infrastructure improvements extend beyond CSO control to address sanitary sewer overflows, basement backups, street flooding and the overall quality of stormwater runoff. These issues are inherently linked, and the City has adopted an integrated approach that manages the sewer and stormwater systems holistically - enhancing system capacity, improving water quality, and strengthening climate resilience. Cambridge has been advancing these initiatives since the adoption of the Clean Water Act in 1972, often ahead of regulatory requirements, demonstrating a longstanding commitment to proactive system management. Through initiatives such as the Ten-Year Sewer and Drain Infrastructure Plan, the FloodViewer and Flood Resilience Zoning programs and the implementation of green stormwater infrastructure, the City continues

to adopt a comprehensive system-wide approach that integrates ongoing refinement of modeling tools, technical guidance, and design standards in response to new data and emerging trends. This proactive and adaptive strategy ensures that infrastructure investments achieve multiple environmental and regulatory objectives while enhancing the long-term reliability and resilience of the collection system. The stormwater management measures summarized in this response – regulatory oversight of private development and green stormwater infrastructure – are central to the City’s holistic sewer and stormwater planning. DPW actively reviews the City's stormwater management regulations against the latest information related to the City's flood model, stormwater program, and zoning amendments. Cambridge’s stormwater regulations for private development have always exceeded state and federal requirements, applying rigorous standards citywide to support flood resilience and water quality. DPW routinely evaluates and revises these standards, most recently proposing updates to require smaller developments to comply with the City’s stormwater storage criteria for the 2070 projected storm. DPW actively integrates green stormwater infrastructure, such as bio-basins, tree filters, and porous pavement into roadway, sidewalk, and open space projects. While large-scale flood mitigation through green infrastructure is limited by Cambridge’s dense urban environment, DPW incorporates these systems whenever possible in order to enhance stormwater quality and sustainability. Update on the Draft CSO Plan The City is currently preparing a Draft CSO Control Plan as required under Water Quality Variances for the Charles River and Alewife Brook issued by the Massachusetts Department of Environmental Protection (DEP) and the United States Environmental Protection Agency (EPA). Cambridge is working collaboratively with the Massachusetts Water Resources Authority (MWRA) and the City of Somerville on the Draft Plan. The original submission deadline for the Draft CSO Control Plan was December 31, 2025. However, DEP and EPA have requested additional information and analysis as part of the Draft Plan, so Cambridge, MWRA and Somerville have requested a 120-day extension for submission. With this extension, there will be an additional public meeting in January 2026 to present further information on CSO Control Plan alternatives and financial capability assessments. When the Draft CSO Control Plan is submitted, there will be a five-month public review and comment period. Following that review period and any further adjustments, the Final Updated CSO Control Plan will be submitted in January 2027. 2050 25-Year Storm Level of Control for CSOs Development of the 2050 Typical Year Rainfall One requirement of the EPA’s CSO Control Policy, which directs the Draft CSO Control Plan, is to model a “Typical Year” of rainfall. A Typical Year is a full year of rainfall data that best represents rainfall conditions over 365 days. A “Typical Year” is required in order to assess how planned improvements will perform under those rainfall conditions.

A Typical Year is used throughout the CSO control planning process for the following: • at the onset, to understand current system performance, against which proposed alternatives will be assessed. • during development, to identify and test alternatives; and • during implementation, to set a benchmark to measure and assess progress. Beyond simply establishing a current Typical Year, Cambridge, Somerville, and MWRA have jointly developed a “Future 2050 Typical Year” for the Draft CSO Control Plan. This dataset will be used to evaluate CSO discharge volumes and activation frequencies at locations throughout the combined sewer system in the Alewife Brook/Upper Mystic River and Charles River Basin. This effort is notable because it creates a Typical Year based on projected future climate conditions, including more intense rainstorms than those reflected in the current Typical Year. At present, EPA and MassDEP have not issued guidance on incorporating future precipitation projections into the development of Typical Years for CSO Control Plans. The method used to develop the 2050 Typical Year is the first of its kind. It was vetted and peer reviewed by climate scientists and provides a scientifically supported Future Typical Year for use in the Draft CSO Control Plan. A Future Typical Year is developed by considering both historic observed, and modeled future, precipitation levels to understand how proposed system improvements will perform under the impacts of climate change. It also allows planners to develop alternatives that are sized for this predicted increase in precipitation. While it is being referred to as the 2050 Typical Year, the analysis actually covered a 30-year climate period centered around 2050, using the years 2040–2069 for the analysis. This 30-year window offers a more stable, representative projection of climate change trends rather than the variability that occurs year-to-year. The CSO control program, i.e., the program by which actions in the Final Updated CSO Control Plan take effect, is likely to reach implementation during 2040-2069. Developing alternatives based on future conditions of the 2050 Typical Year helps avoid designing solutions that would become obsolete soon after they are fully operational. In addition to the 2050 Typical Year, the Draft CSO Control Plan process has considered the “2050 25-Year Storm.” This means, in lay terms, planning for the heaviest of storms; a storm with a 4% chance of occurring in a given year. The 2050 25-Year Storm is modeled as 7.8 inches of rain in 24 hours. Such a storm is larger than the current 50-year storm and will cause widespread flooding in Cambridge in areas served by combined sewers as well as areas with separate sewers and storm drains. This flooding will include overland flooding, backups into buildings, and sanitary sewer overflows.

Figure 1. System Performance During the 2050 25-Year Design Storm: Manhole Flooding to the Ground Surface Figure 2. System Performance During the 2050 25-Year Design Storm Design Storm: Inland Over the Ground Flooding

Because of this widespread flooding, even building CSO control infrastructure to the level of a 2050-25 Year Storm may provide limited public health and water quality benefits. Conceptual design of the facilities needed to control overflows for the 2050-25 Year Storm indicates that large storage tanks and tunnels will be needed. The amount of available land/space for such facilities will be a significant constraint, even including any potential use of Department of Conservation and Recreation (DCR) open space. However, the Cambridge, Somerville, and MWRA project team have evaluated alternatives that can achieve the 2050 25-Year Storm level of CSO control and detailed information will be available in the upcoming Draft CSO Control Plan. Updating Stormwater Management Regulations for Private Development The City's Wastewater and Stormwater Drainage System Ordinance (Chapter 13.16 of the Cambridge Municipal Code), enacted in 1998, was significantly revised in 2008 to establish regulatory authority to require stormwater management standards for private development. This Ordinance is implemented through the Land Disturbance Regulations as well as the Wastewater and Drainage System Use Regulations, which specify criteria for projects that require Stormwater Control Permits. Since adopting these regulations, Cambridge has consistently applied more stringent project criteria than any overlapping federal or state requirements. Cambridge has also chosen to apply its stormwater management criteria to development projects citywide, in both separated and combined sewer areas. Projects that meet one or more of the following criteria require a Stormwater Control Permit from DPW: • Disturbance of one or more acres of land, • Construction exceeding 50,000 square feet of Gross Floor Area, • Project parcel(s) size equal to or greater than one acre, • Inclusion of outdoor parking for 10 or more cars, • Requirement of a Special Permit from the Planning Board, or • Determination by the City Engineer that the project may result in an adverse impact on the municipal Sewer, Combined Sewer, Stormwater Drainage Systems or Water Resources. DPW has issued approximately 200 Stormwater Control Permits since the Land Disturbance Regulations were established. Cambridge’s Wastewater and Stormwater Management Guidance Document (2008) establishes the City’s technical standards for stormwater control. The current standard for the management of stormwater volume requires a permit holder to demonstrate that their property will be able to store the difference between the 2-year storm (3.6 inches) pre- construction, and the 25-year storm (8.2 inches) post construction, using the City’s projected rainfall data for the 2070 storm. A storage requirement of this magnitude (equivalent to approximately 4.6 inches of rainfall on-site) and the use of projected storm events incorporating climate change are not mandated by any federal or state regulations. Cambridge adopted this rigorous standard to directly support citywide stormwater management improvements.

Due to the density of redevelopment in Cambridge, on-site storage of the 2070 100-year storm (11.7 inches) is not feasible due to the space required to store this volume. Despite this, by enforcing the 2-year to 25-year 2070 storage requirement across a wide range of projects, Cambridge has achieved a substantial level of on-site stormwater management and will continue to require developers to incorporate significant stormwater storage into their projects. Cambridge also extends stormwater management to even smaller projects requiring a Building Permit, but not a Stormwater Control Permit, including any project with: • A building addition greater than 150 square feet, or • Any proposed increase in impervious area. For these projects, DPW requires stormwater controls to the maximum extent practicable, including evaluation of storage for the 2070 2-year to 25-year storm difference. DPW reviews an average of 320 building permit applications annually. Of those, an average of 60 reviews each year include a review of civil site plans and stormwater management. The goal of these reviews is to identify opportunities for smaller projects to make improvements that contribute to the goals of the City’s stormwater management program. DPW continually evaluates existing regulations and project thresholds to align with changes in development and City zoning. In the summer of 2025, DPW initiated a detailed review of project criteria for Stormwater Control Permits, evaluating recent changes to zoning and recent development projects (parcel size, project criteria triggering a Stormwater Control Permit, etc.). As a result of this review, DPW plans to revise its criteria for projects that require a Stormwater Control Permit, in order to maintain the effectiveness and impact of this regulatory program. In parallel, DPW is revising the Wastewater and Stormwater Management Guidance Document to clarify requirements for smaller projects. Proposed updates include requiring all projects undergoing DPW site plan review for a Building Permit to store the difference between the 2-year (pre-construction) and 25-year (post-construction) 2070 storm events to the maximum extent practicable, with a minimum on-site storage requirement this is still being evaluated. Cambridge’s stormwater regulations already exceed all federal and state requirements and set a high standard for stormwater storage in an urban setting. The proposed revisions to project criteria will allow the City to incorporate additional private development projects into the City's program and support stormwater improvements citywide.

Implementation of Green Stormwater Infrastructure As a standard practice, DPW evaluates roadway and capital improvement projects for opportunities to incorporate green stormwater infrastructure where feasible. The City’s Green Streets Guidance document outlines the City’s approach to integrating green infrastructure and stormwater management into the design of public rights-of-way while balancing other public infrastructure needs. The City integrates green infrastructure features into roadway, sidewalk, and open space improvements whenever practicable. Examples of green stormwater infrastructure implemented across Cambridge include the Alewife stormwater wetland which provides stormwater quantity and quality control implemented during previous CSO control work in West Cambridge. The City has also constructed porous bike paths, porous pedestrian walkways, bio-basins, rain gardens, subsurface infiltration systems, tree filters, infiltrating catch basins, and drywells. Subsurface infiltration system at Longfellow Park.

Bio-basin on Lakeview Ave. Porous pavement bike path along Galileo Galilei Way. In addition to City projects, green stormwater infrastructure is also constructed as part of private redevelopment projects, following City stormwater requirements. In Cambridge, there are limited opportunities to construct green infrastructure systems on City property

or within the public right-of-way that would be large enough to store the volume necessary for CSO control or flood mitigation. While green stormwater infrastructure has multiple benefits, including local stormwater management, the effectiveness of green infrastructure for CSO control is limited in dense, urban environments such as Cambridge, where available space for infiltration and storage is constrained. As such, green infrastructure in City capital projects is primarily used for the improvement of stormwater quality. Green stormwater infrastructure remains a key component of the City’s overall stormwater management strategy and the City is committed to continuing to install new green infrastructure as part of City projects where feasible. Improve Meaningful Public Outreach Including the Creation of a Combined Sewer Overflow Commission. Robust public outreach for the Draft CSO Control Plan The extensive scope and complexity of the alternatives being developed to reduce CSOs as part of the Draft CSO Control Plan will impact both the communities abutting these water bodies as well as MWRA communities. From the beginning of the plan development process, Cambridge, Somerville and MWRA have been committed to engaging impacted parties and clearly sharing critical information. Below is an overview of the substantial efforts to not only inform stakeholders of CSO Planning, but to facilitate public input on CSO control alternatives. Over the last three years, Cambridge, Somerville and MWRA have focused on organizing public meetings providing up to date information and materials, hosting technical meetings with watershed organizations, facilitating community feedback, and conducting outreach to environmental justice groups. To assist with the Draft Plan public involvement efforts, Cambridge, Somerville and MWRA partnered with a consultant, the Consensus Building Institute (CBI), who has played an integral role in public presentations and the wider engagement approach. Need for Public Input A key goal of engagement efforts has been informing and inviting active participation from as many stakeholders as possible. The communities around these CSO locations feel the impact of overflows today and will be subject to the disruption caused by the construction of CSO control projects. It is important to keep them informed on Plan development and let them share their perspective. Cambridge, Somerville and MWRA have been, and continue to be, receptive and responsive to public inquiries and feedback related to developing the Draft CSO Control Plan. In 2026, staff expect to hear significant input from the public on the Draft Plan and integrate that input into the planning process.

Public Engagement Considerations Early in the public engagement process, Cambridge, and Somerville and MWRA committed to three main goals: conducting expansive outreach so that as many interested stakeholders as possible could participate, providing up to date information so that participation could be knowledgeable, and ensuring the participation of members of Environmental Justice (EJ) populations. Sixty percent of the residents of MWRA’s service area live in EJ communities and nearly all of Cambridge and Somerville neighborhoods include EJ populations according to the State’s Executive Office of Energy and Environmental Affairs’ criteria. Boston, Medford, Everett, and Arlington, which border the Charles River, Alewife Brook and Mystic River, also have significant EJ populations. Outreach to these stakeholders, including translation and interpretation services, is a critical component of the public engagement. Additionally, stakeholders have a broad range of familiarity with CSOs and the efforts to reduce them. Some have been involved for a while and have benefited from past meetings or published information while others are new to the process. This presents a challenge in designing the information to meet everyone’s needs. Methods of Engagement To engage with as many stakeholders as possible, Cambridge, Somerville and MWRA utilized a number of methods. Project Website One of the first steps in the outreach was the creation of a joint project website as a public resource for everything associated with updating the CSO Control Plan. The website has a running timeline of all meetings and events, the presentation slides and recordings of all previous public meetings as well as any associated materials, an FAQ on CSOs and the development of plans and translated flyers for any upcoming meetings. The public can also subscribe to receive emails on the project and see the contact information of project leaders for each entity. This website provides a critical platform for both past project materials and future engagement. Public Meetings Cambridge, Somerville and MWRA hosted five virtual public meetings to date. These meetings were designed to inform the public over the course of the project by (a) sharing information about the process and draft concepts and content as it was developed, and (b) getting timely input and feedback from the public about the topics the project team was working on. General topics included: • what CSO discharges are, why they occur, and what has been done already to reduce their frequency and volumes. • an overview of the planning process. • an overview of potential tools to help reduce or eliminate CSO discharges.

• potential control alternatives under consideration and the range of costs; and • results of alternative analysis. In addition to the public meetings, there was a listening session in April 2025 to provide a forum for anyone interested in CSOs and the Draft CSO Control Plan to share their priorities, concerns, and questions on the planning process. To offer the public multiple perspectives, the listening session also included brief presentations by three watershed groups – Charles River Watershed Association, Mystic River Watershed Association, and Save the Alewife Brook – as well as the MWRA Advisory Board. See the table below for the topics, dates, and the number of remote participants for each public meeting. Public Meeting Topic Date Participants 1 CSO Overview & Intro to CSO Plan June 29, 2022 226 2 Plan Priorities & New Typical Year Development December 15, 2022 177 3 CSO Tools & Alternative Development November 15, 2023 243 4 Alternatives Screening & Affordability Analysis January 22, 2025 355 Listening Session April 3, 2025 173 5 Results of Alternative Analysis September 25, 2025 170+ 6 Present Draft Plans Spring 2026 Planned 7 Present Final Plans Winter 2027 Planned Given the technical complexity of the project, the effort to design the content of these meetings so the plan could be understood by members of the public with vastly different levels of knowledge about CSOs was significant. Meetings were two to three hours long and involved both presentations and time for questions to be answered in writing and/or verbally. During these meetings staff answered over 450 questions to inform and enable continued public participation in real time. Additionally, surveys associated with the second and third public meetings were made available and open for several weeks to gather input from people who were not able to attend the meeting live. All public meetings were recorded and posted online, and all meeting materials were posted online so the public could access them at any time. Outreach for Public Meetings Cambridge, Somerville and MWRA conduct broad outreach in anticipation of the public meetings. Prior to each one, notifications are posted on the joint website as well as each entity’s website. Notification flyers with QR codes for registration are developed and translated into eight languages prevalent in the surrounding communities – Spanish, Portuguese, Mandarin, Haitian Creole, Nepali, Amharic, Arabic, and Bengali in adherence to Massachusetts Environmental Policy Act Office’s (MEPA) Public Involvement Protocol for Environmental Justice Populations. These flyers are posted to

the joint website and distributed in EJ communities, at community events, through the watershed associations, and through MWRA’s water fountain program across the metro area. Notifications of public meetings and the listening session were widely distributed. Flyers were emailed to nearly 200 contacts, including the MEPA Environmental Justice distribution list and specific EJ organizations identified by Cambridge and Somerville. In addition to outreach, accessibility was prioritized at the public meetings. All have been conducted over Zoom to maximize participation, which proved to be successful, with at least 170 people attending all the meetings to date. Interpretation has also been a focus. The presentation for the first meeting was prerecorded in English and live interpreted into four languages – Portuguese, Spanish, Amharic, and Bengali – followed by a question-and-answer period. The second meeting was entirely live interpreted in eight languages – Spanish, Portuguese, Mandarin, Haitian Creole, Nepali, Amharic, Arabic, and Bengali. Participants had the option to join distinct language channels where an interpreter would be repeating the original speaker in the given languages. The third meeting was live interpreted in Spanish and Portuguese as potential participants did not request additional languages during outreach. Since attendance on the translation channels cannot be seen in Zoom, interpretation was provided on request for the fourth and fifth public meetings. Flyers continue to be translated into eight languages with instructions on how to request interpretation for upcoming meetings. Surveys Another strategy implemented for encouraging and obtaining feedback is to provide opportunities for people to submit input via asynchronous written online surveys. These were conducted both during public meetings through live polling and, several times, through surveys shared with the broader public that were open for several weeks. See below for a list of each survey conducted to date. Survey Date Format Respondents Public Meeting # 2 December 2022 - January 2023 Broad Survey 86 Public Meeting # 3 November 15, 2023 Live Poll 81 Public Meeting # 3 December 2023 - January 2024 Broad Survey 1,312 Public Meeting # 4 January 22, 2025 Live Poll 167 The aim of these surveys was to learn who was participating at the public meetings (we asked where people lived and worked), in some cases to hear their primary reason for

attending, and to provide a chance for people who might not want to speak in front of nearly 200 people a chance to share their concerns about the current CSO situation, suggestions for the plan development process, and thoughts on what is most important for the Plan to include. Over the past several years, responses to these surveys, as well as discussions with the watershed groups, have yielded several themes: • Cambridge, Somerville, and MWRA need to act with urgency to develop and implement both short-term and long-term CSO reduction alternatives. • Green infrastructure and nature-based solutions should be a key part of mitigation. • Input from the public and watershed groups should be taken seriously and their feedback should be integrated into the Draft CSO Control Plan (and Final Updated CSO Control Plan). • The projected impacts of climate change should be considered in evaluating alternatives. Engagement with Watershed Organizations In the fall of 2023, Cambridge, Somerville and MWRA also began meeting separately with watershed groups, specifically the Mystic River Watershed Association (MyRWA), the Charles River Watershed Association (CRWA), and Save the Alewife Brook (StAB). While communication with these organizations has been frequent, the watershed groups expressed a desire to be more deeply involved in the development of the Draft CSO Control Plan. To date, four separate meetings have been held with these groups. The purpose of these meetings has been (a) to share technical information and answer questions, (b) to hear feedback, and (c) to get advice on the public meeting designs. Since the watershed groups are much more familiar than the general public with CSOs in the relevant water bodies and the types of potential mitigation measures, these meetings have allowed for in-depth dialogue on specific topics and more technical discussions. Watershed Group Meeting Date Format Number of Participants 1 October 24, 2023 In-person 21 2 June 24, 2024 In-person 32 3 December 17, 2024 Virtual 35 4 September 4, 2025 In-person 35 In addition to these group sessions, leaders from the watershed organizations were in touch with project staff informally via email and phone during the process to request or share updates or provide recommendations. The watershed groups have helped the project significantly through outreach to interested constituents in the area, and their help

was key to achieving high levels of attendance at public meetings to date. In addition to these group sessions, leaders from these organizations were in touch with project staff informally via email and phone during the process to request or share updates or provide recommendations. Commitment to Community Feedback Cambridge, Somerville and MWRA heard and directly responded to many of the concerns raised through engagement with the public and watershed groups. The 2050-25 Year Storm model was developed specifically to project the impacts of climate change. Many near-term improvements have been investigated and implemented. Notification, including additional signage on bike paths, in advance of large rainstorms has been implemented and two evaluation reports have been completed: 1) evaluating the effectiveness of current floatables control at outfalls to the Alewife Brook and the Upper Mystic River and recommending improvements; and 2) identifying potential best management practices for reducing odors near CSO structures in the three water bodies. Lastly, in response to comments from both the public and the watershed groups, green stormwater infrastructure will be integrated into many alternatives where feasible. Next Steps Cambridge, Somerville, and MRWA will continue to have monthly meetings with MassDEP and the EPA as the Draft CSO Control Plan is finalized and will also continue to answer questions from the public and the watershed organizations. The Draft CSO Control Plan submittal will be followed by a public meeting and hearing on the Draft Plan within a five-month DEP/EPA and public review and comment period. Public outreach throughout impacted communities and to various stakeholders will continue through this comment period to encourage additional engagement. Cambridge, Somerville and MWRA envision robust public involvement during these few months, to inform further adjustment to the Draft Plan prior to submitting the Final Updated CSO Control Plan in January 2027 for MEPA review. The Final Updated CSO Control Plan will provide a conceptual level overview of potential alternatives and their impacts rather than design-level details on specific projects. The Plan itself will not require Environmental Impact Report review at that time. Instead, project-specific reviews will be conducted and those that exceed MEPA thresholds will comply fully with applicable MEPA regulations, including requirements relating to public outreach and involvement of EJ populations and an analysis of impacts on these residents. Although Cambridge, Somerville and MWRA are close to achieving a significant milestone by finalizing recommendations for reducing CSOs as part of the Draft Plan,

there are still many opportunities for the public to engage and provide input over the next year plus as the development of a final plan continues. City of Cambridge CSO Control and Stormwater Management Advisory Committee DPW has evaluated the creation of an advisory committee to provide feedback to City staff on the development and implementation of the Draft CSO Control Plan (and Final Updated CSO Control Plan) as well as the City’s broader Stormwater Management Program. There are currently several advisory committees that provide input on City’s Capital Investment Plan. Having an advisory committee for capital improvements specific to the City’s wastewater and stormwater systems will be beneficial. An advisory committee comprised of members of watershed advisory groups and technical experts in the design, construction and financing of large capital projects can provide valuable input to City staff implementing these plans. DPW will begin a process early in 2026 to advertise this advisory committee and recommend members to the City Manager.