October 2025 CSO Draft Plan — File 26-0650

Draft Combined Sewer Overflow Updated Long Term Control Plan File ID 25-1451 Officer’s Communication to City Council Infrastructure & Asset Management 23 October 2025

Presenter's Preamble • This extended presentation deck, including supplemental slides (26 through 89), is provided to the Council as a reference document and to facilitate follow up discussions • Many slides provide links to other presentations, videos, and resources for further information • Much like the May 2025 Officer’s Communication to the Council, even the presented slides (3 through 23) could be topics of prolonged discussion • It is the presenter’s intention to move very quickly through the slides and only provide the highest-level overview on each topic • The presenter is happy to follow up on any of these topics in person or at subsequent meetings 2

Formal regulatory process 3 Continuation of CSO control efforts System Wide CSO Reduction Since the 1980s *Annual discharge volume based on the prior Typical Year Variance Water Study Area *Volumes consider climate change

Formal regulatory process parties 4 Drafting & executing plan City of Cambridge (Stantec, W&S) Massachusetts Water Resources Authority (MWRA) (AECOM) City of Somerville (Dewberry, CBI) Reviewing & approving plan Massachusetts Department of Environmental Protection US Environmental Protection Agency Input & comments • Charles River Watershed Association • Mystic River Watershed Association • Save the Alewife Brook • MWRA Advisory Board • Neighboring cities & towns • Various State agencies • General public

Formal Variance Water Schedule 5

What it means for Somerville • Legal (and moral) obligation to mitigate CSOs • Federal/State Administrative Order will bind Sewer CIP for a generation 6 • Will constrain future Capital Investments & operating budgets Photos by David Stoff & Ann McDonald / Save the Alewife Brook

Council action on this item • Council approval not required for submission of plan to DEP/EPA • However, plan will drive sewer CIP and rates for decades • Therefore, plan will influence future Council actions on rates • Infrastructure & Asset Management would like a sense of the Council’s position on the plan 7

High level summary of process & conclusions Supplemental Slides (24 through 87) provide support for Q/A • Planning process • Community outreach & engagement • Tools for predicting CSO mitigation • Vocabulary: “Typical Year” and “Level of Control” • Alternatives, analysis & evaluation • Eliminated alternative: Sewer separation • Upstream contributions & downstream constraints • Difficulties with & costs of sewer separation • Eliminated alternative: 25-Year level of control • Large storms causes multiple system failures resulting in sanitary pollution • Other sources of pollution and Charles/Alewife/Mystic water quality • Scale & cost of CSO control • Limitations of the Variance Water plan compared to success stories nationally • Local efforts outside of the Variance Water plan to improve water quality • Eliminated alternative: Lower level of control 8

Draft recommended plan Cambridge, Somerville & MWRA technical team’s Goldilocks alternative 9

Proposed construction 10

Alewife Brook Pump Station Dilbo y Field MWR003 Fresh Pond Not to Scale Little Pond Spy Pond CAM401 B MWR003 1.5 MG Storage Tank CAM401A 1.5 MG Storage Tank CAM401A Increased Conveyance CAM00 2 CAM401A Legend: Combined Sewer System Facility CSO Outfall CSO Storage Tank Sewer Separation CSO Storage Conduit Near-surface Conveyance N SOM001 A CAM00 1 CAM401B and SOM001A 2.3 MG Microtunnel Storage Conduit 9 ft diameter CAM001 8 Acres of Sewer Separation 3.AB Hybrid 2 Level of Control: 0 CSOs in 2050TY Storage: - Tanks: 2 - Tunnel: 0 - Microtunnel: 1.0 miles long Key Features Conveyance: 0.75 miles long Sewer Separation: 8 acres GSI: with separation/ other street excavation Land Acquisition: Yes Time to Complete: 13-18 years Preliminary Cost: $340 Million

95 Acres of Sewer Separation Mystic River Legend: Combined Sewer System Facility CSO Outfall CSO Storage Sewer Separation Storm Drain Mystic River Somerville Marginal CSO Facility DeLauri Pump Station MWR20 5 SOM007A / MWR205A SOM007A/MWR205 A 2.7 MG Storage Tank Stormwater Trunk Line to Mystic River Remaining Activation Frequency and Treated Discharge Volume in the 2050 TY Total Volume (MG) Activation Frequency 6.77 2 SOM007A/ MWR205A 6a.MR Hybrid 1 Level of Control: Limited CSOs in 2050TY Storage: - Tanks: 1 - Tunnel: 0 - Microtunnel: 0 Key Features Conveyance: 0 Sewer Separation: 95 acres GSI: with separation/ other street excavation Land Acquisition: Yes Time to Complete: 5-10 years Preliminary Cost: $170 Million Not to Scale N

CAM00 5 CAM00 7 MWR01 0 MWR20 1 CAM017 MWR20 3 RE046- 19 RE046- 30 RE046- 381 RE046- 100 RE046- 105 RE046-54 RE046-55-1 Not to Scale N Legend: Combined Sewer System Facility CSO Outfall Tributary to Charles River CSO Outfall Tributary to Boston Harbor RE046 CSO Regulator Stormwater Storage CSO Storage Sewer Separation RE046- 62A Bosto n Harbor Ward Street Headworks Facility Prison Point CSO Facility CAM017 80 Acres of Partial Sewer Separation CAM005 2.5 MG Storage Tank RE046-100 0.06 MG Storage Conduit MWR02 3 188 Acres of Partial Sewer Separation MWR01 8 MWR01 9 MWR02 0 Not to Scale N Columbus Park Headworks Facility 8.CR Hybrid 1 Level of Control: Limited CSOs in 2050TY Storage: - Tanks: 1 - Tunnel: 0 - Microtunnel: 0 - Storage Conduits: 1 Key Features Sewer Separation: 268 acres GSI: with separation/ other street excavation Land Acquisition: Yes Time to Complete: 23-28 years Preliminary Cost: $360 Million Remaining Activation Frequency and Volume in the 2050 TY Total Volume (MG) Activation Frequenc y 0.0 0 CAM005 0.0 0 CAM007 0.0 0 CAM017 0.88 – 0.94 2 MWR018-020 0.11 2 MWR023 0.0 0 MWR010 26.8* 4 MWR201 *treated discharge

Projected CSO elimination 14

Plan performance – 2050 Typical Year 15 Hydraulic Model Predictions 2050 TY Level of Control Receiving Water CSO Discharge Volume (MG) Activation Frequency Recommended Plan Current Conditions Recommended Plan Current Conditions 6.7 Treated 29.3 2 8 1 – Limited CSOs in 2050 Typical Year Upper Mystic 0 20.9 0 13 2 – Zero CSOs in 2050 Typical Year Alewife Brook 1.2 Untreated 26.8 Treated 38.4 4 6 1 – Limited CSOs in 2050 Typical Year Charles River

Plan performance – Historical record Alewife Brook Recorded/reported events at all 7 Alewife outfalls 1 Jan 2015 thru 31 Dec 2024 versus model results for recommended alternative for that rainfall record 16 92% reduction in activation frequency (109 to 9 for 10-year period) 79% reduction in total volume (170 to 35 MG for 10-year period) 0 5 10 15 20 25 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Number of Activations Existing Conditions Preferred Alternative 0 10 20 30 40 50 60 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Volume of CSO (MG) Existing Conditions Preferred Alternative

Plan performance – Historical record Mystic River upstream of dam Recorded/reported events at SOM007A outfall 1 Jan 2015 thru 31 Dec 2024 versus model results for recommended alternative for that rainfall record 17 94% reduction in activation frequency (102 to 6 for 10-year period) 96% reduction in total volume (239 to 9 MG for 10-year period) 0 5 10 15 20 25 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Number of Activations Existing Conditions Preferred Alternative 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Volume of CSO (MG) Existing Conditions Preferred Alternative

Projected financial impact & Financial Capability Assessment (FCA) 18

Provisional FCA results • Alt 1 Residential Indicator • Cost per Household = 1.34% MHI – Mid-Range Burden • Alt 2 Residential Indicator • Mid-Range Burden FY2033 through FY2040 • Low Burden before and after • Alt 2 Lowest Quintile Residential Indicator • Low Burden FY2027 through FY2029 • Mid-Range Burden FY2030 through end of projection • EPA methodology indicates Somerville has the financial capacity to undertake this plan on this schedule 19 Note: At the time of this presentation, costs and allocations are being QA/QC’ed. FCA will be revised in November.

Provisional bill impacts: 208% increase from FY26 to FY55 FY 2036 FY 2035 FY 2034 FY 2033 FY 2032 FY 2031 FY 2030 FY 2029 FY 2028 FY 2027 FY 2026 Dashboard 2.0% 2.0% 2.0% 5.0% 5.0% 10.0% 15.0% 15.0% 15.0% 20.0% 12.0% Usage Rate Increase $ 6,039,416 $ 4,796,173 $ 656,941 $ 5,212,712 $ 9,680,613 $ 14,456,825 $ 24,211,545 $ 36,706,503 $ 31,759,034 $ 24,433,561 $ 16,293,191 Balance Excess/Deficiency 1.65 1.67 1.56 1.60 1.62 1.66 1.59 1.55 1.70 2.12 2.11 Debt Service Coverage $1,838.62 $1,804.92 $1,771.88 $1,739.49 $1,662.37 $1,588.93 $1,455.39 $1,281.21 $1,129.75 $998.04 $851.70 Annual Bill SFR 38.60% 38.46% 39.09% 37.56% 36.48% 35.55% 35.54% 33.45% 27.12% 19.86% 15.42% Debt Service/Revenue $ 10,000,000 $ 10,000,000 $ 15,000,000 $ 15,000,000 $ 15,000,000 $ 20,000,000 $ 20,000,000 $ - $ - $ - Cash Funded CIP $ 1,447,815 $ 1,114,383 $ 4,927,781 $ 4,347,360 $ 3,783,845 $ 4,785,746 $ 4,063,831 $ 16,039,966 $ 15,572,782 $ 15,119,206 Debt Funded Project Costs 20 Note: At the time of this presentation, costs and allocations are being QA/QC’ed. MWRA rates will then be calculated, leading to refined Somerville rates. Rate projections will be revised in November.

Next steps 21

Variance Waters CSO LTCP Update 22 2025 Activities • Presentations to MWRA Boards • Finalize FCA • Produce report • Submit draft recommended plan to EPA and DEP by December 31, 2025 2026 Activities • Public mtg #6 on draft recommended plan • Public hearing and public comment period • Additional outreach in affected communities • Team reviews comments and modifies plan 2027 and Beyond • Final plan submitted January 2027 • EPA and DEP review and set the plan for further CSO control • Design of projects • Construction!

FY2027 Water & Sewer Rates • Long-term Capital Investment Plan requirements • DEP/EPA approval of January 2027 Final Plan will establish legally- enforceable CIP for next 20+ years • Somerville ust also consider critical pipeline rehabilitation and flood mitigation components of CIP • Future revenue requirements and enterprise fund health • Critical to maintain financial metrics (debt service coverage, debt service to revenue ratio, total debt, revenue vs. expenses) to maintain bond rating • Raise rates early to cash-fund future projects • Prepare for annual rate increases of 15% to 20% for next 4 to 5 years 23

Questions / Supplemental Slide Index 24 • Planning process Slide 27 • Community outreach & engagement Slide 28 • Tools for predicting CSO mitigation Slide 33 • Vocabulary: “Typical Year” and “Level of Control” Slide 38 • Alternatives, analysis & evaluation Slide 44, 48 • Eliminated alternative: Sewer separation Slide 53 • Upstream contributions & downstream constraints Slide 54 • Difficulties with & costs of sewer separation Slide 58 • Eliminated alternative: 25-Year level of control Slide 64 • Large storms causes multiple system failures resulting in sanitary pollution Slide 65 • Other sources of pollution and Charles/Alewife/Mystic water quality Slide 69 • Scale & cost of CSO control Slide 72 • Limitations of the Variance Water plan compared to success stories nationally Slide 77 • Local efforts outside of the Variance Water plan to improve water quality Slide 84 • Eliminated alternative: Lower level of control Slide 88

Council action on File ID 25-1451 • Straw poll • “Does the Council support the recommended plan as presented, including the proposed level of control and projected rate impacts?” • Discussion • Statements in support or opposed • Item • Place on file, or • Keep open for further discussion 25

Supplemental Slides

Planning process Mechanics of drafting a recommended plan 27

Community outreach and engagement Included in scope approved by DEP/EPA 28

CSO Plan Public Meetings 29 1. CSO Control Plan Update - Public Mtg #1 June 29, 2022 2. CSO Public Mtg #2 - 12-15-22 3. Updated CSO Control Plan Public Meeting #3 Cambridge, Somerville, MWRA 11-15-23 4. Combined Sewer Overflow (CSO) Control Planning Update – January 2025 5. Updated Combined Sewer Overflow Control Plan Public Meeting 5 - September 25, 2025 – YouTube 6. Listening Session (https://www.youtube.com/watch?v=k3jLXiO Dafg)

CSO Plan Additional Engagement • Monthly updates with DEP/EPA • Workshops with CRWA, MyRWA & StAB • October 2023 in person – Tools, typical year, process • June 2024 in person – Alternatives development, mapping exercise • December 2024 virtual – Alternatives development, cost assumptions • September 2025 in person – Alternatives overview, evaluation criteria • Asynchronous input through website and Menti polls • Tabling at Civic Days and other events 30

City Council Presentations City Council 22 May 2025 ID# 25-0966 – CSOs, Updated LTCP, Examples & Stormwater Billing Plan City Council 13 March 2025 ID# 25-0510 – Impacts of CSOs on Alewife Brook 31

Additional Resources • Combined and Sanitary Sewer Overflow Control | City of Somerville • Combined Sewer Overflow Control Planning Program - Joint efforts by Cambridge, MWRA, and Somerville | SomerVoice • Save the Alewife Brook – environmental health is community health • It's Time to Cut the Crap — Charles River Watershed Association • Mystic River Watershed Association | MyRWA | Boston 32

Tools for predicting CSO mitigation Predicting CSO frequency and volumes for different size storms for current and future conditions 33

Hydraulic model of sewer system 34

CAM007 CAM005 Cottage Farm MWR201 MWR010 MWR020 MWR019 MWR018 CAM017 MWR023 Water quality model of receiving waters

Typical Year, design storms, and climate change 36 The presentation from public meeting # 2 includes more details on climate change and precipitation simulations. The presentation from public meetings #3 and #4 includes details on resulting CSO frequency and volumes. The presentation and recording is on the project webpage: https://voice.somervillema.gov/joint-cso-planning Future Baseline Condition Model Results Receiving Water CSO Discharge Volume (MG) Activation Frequency 2050 25-year Storm (7.8 inches) 2050 5-year Storm (5.3 inches) 2050 Largest Storm in the Typical Year (3.3 inches) 2050 Typical Year Prior Typical Year 2050 Typical Year Prior Typical Year 120.6 65.5 16.6 38.4 7.9 6 3 Charles River 40.1 20.9 4.84 20.9 9.9 13 8 Alewife Brook 27.2 17.4 10.5 29.3 1.3 8 2 Mystic River

Looking into the future to select a ‘Preferred Alternative’ – the 2050 Typical Year and 2050 Design Storms 37 24-hr Precipitation Depths (in) Cornell 20502,3 NOAA Atlas 14 Baseline1 Return Period 4.0 3.3 2-yr 5.3 4.3 5-yr 6.4 5.2 10-yr 7.8 6.3 25-yr Total Depth: 10.3 in over 59 hours 6.2 (24-hr depth around the peak) March 2010 Total Depth: 5.2 in over 17 hours 5.2 Ida (Sept, 2021) REFERENCES 1. Perica, S., Pavlovic, S., StLaurent, M., Trypaluk, C., Unruh, D., Martin, D. 2015. NOAA Atlas 14: Precipitation-Frequency Atlas of the United States; Volume 10 Version 3.0: Northeastern States. https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html 2. Steinschneider, S., & Najibi, N. (2022). Observed and Projected Scaling of Daily Extreme Precipitation with Dew Point Temperature at Annual and Seasonal Scales across the Northeastern United States, Journal of Hydrometeorology, 23(3), 403-419. https://journals.ametsoc.org/view/journals/hydr/23/3/JHM-D-21-0183.1.xml 3. Climate Resilience Design Standards & Guidelines. Climate Resilience Design Standards Tool. https://resilientma.org/rmat_home/designstandards/ • Selected the 2050 planning horizon for the CSO Control Plan (first in the nation!) • Developed the 2050 Typical Year (TY) as a rainfall series of 90 storms that range in intensity and duration • Identified 2050 design storms

“Level of Control” Term used to describe how big a storm the system will handle 38

39 Four Levels of Control Being Evaluated Hydraulic Model Predictions Receiving Water CSO Discharge Volume (MG) Activation Frequency 2050 25-year Storm 2050 5- year Storm 2050 Largest Storm in TY 2050 TY 2050 TY 27.2 17.4 10.5 29.3 8 Upper Mystic 40.1 20.9 4.84 20.9 13 Alewife Brook 120.6 65.5 16.6 38.4 6 Charles River

40 Four Levels of Control Being Evaluated Hydraulic Model Predictions Receiving Water CSO Discharge Volume (MG) Activation Frequency 2050 25-year Storm 2050 5- year Storm 2050 Largest Storm in TY 2050 TY 2050 TY 27.2 17.4 10.5 29.3 8 Upper Mystic 40.1 20.9 4.84 20.9 13 Alewife Brook 120.6 65.5 16.6 38.4 6 Charles River Level 1: Significantly reducing CSO discharges from those predicted to occur in a 2050 Typical Year (“Limited CSO in 2050 Typical Year”)

41 Four Levels of Control Being Evaluated Hydraulic Model Predictions Receiving Water CSO Discharge Volume (MG) Activation Frequency 2050 25-year Storm 2050 5- year Storm 2050 Largest Storm in TY 2050 TY 2050 TY 27.2 17.4 10.5 29.3 8 Upper Mystic 40.1 20.9 4.84 20.9 13 Alewife Brook 120.6 65.5 16.6 38.4 6 Charles River Level 2: No CSO in a 2050 Typical Year (“0 CSOs in 2050 Typical Year”) Level 1: Significantly reducing CSO discharges from those predicted to occur in a 2050 Typical Year (“Limited CSO in 2050 Typical Year”)

42 Four Levels of Control Being Evaluated Hydraulic Model Predictions Receiving Water CSO Discharge Volume (MG) Activation Frequency 2050 25-year Storm 2050 5- year Storm 2050 Largest Storm in TY 2050 TY 2050 TY 27.2 17.4 10.5 29.3 8 Upper Mystic 40.1 20.9 4.84 20.9 13 Alewife Brook 120.6 65.5 16.6 38.4 6 Charles River Level 2: No CSO in a 2050 Typical Year (“0 CSOs in 2050 Typical Year”) Level 1: Significantly reducing CSO discharges from those predicted to occur in a 2050 Typical Year (“Limited CSO in 2050 Typical Year”) Level 3: No CSO in a 2050 5-year, 24-hour design storm (“0 CSOs in 2050 5-year storm”)

43 Four Levels of Control Being Evaluated Hydraulic Model Predictions Receiving Water CSO Discharge Volume (MG) Activation Frequency 2050 25-year Storm 2050 5- year Storm 2050 Largest Storm in TY 2050 TY 2050 TY 27.2 17.4 10.5 29.3 8 Upper Mystic 40.1 20.9 4.84 20.9 13 Alewife Brook 120.6 65.5 16.6 38.4 6 Charles River Level 2: No CSO in a 2050 Typical Year (“0 CSOs in 2050 Typical Year”) Level 1: Significantly reducing CSO discharges from those predicted to occur in a 2050 Typical Year (“Limited CSO in 2050 Typical Year”) Level 3: No CSO in a 2050 5-year, 24-hour design storm (“0 CSOs in 2050 5-year storm”) Level 4: No CSO in a 2050 25-year, 24- hour design storm (“0 CSOs in 2050 25- year storm”)

Alternatives Envisioning ways to reconstruct the sewer systems 44

CSO Reduction and Elimination Tools 45 The presentation from public meeting # 3 includes more details on each tool. The presentation and recording is on the project webpage: https://voice.somervillema.gov/joint-cso-planning Sewer Separation Green Stormwater Infrastructure Storage Conveyance Regional Tunnel

Developing CSO Control Alternatives 46 The presentation from public meeting # 4 includes more details on building alternatives. The presentation and recording is on the project webpage: https://voice.somervillema.gov/joint-cso-planning

39 Alternatives Under Consideration 47 Number of Alternatives Under Consideration Level of CSO Control Charles River (Lower): 9 CSO Outfalls Mystic River (Upper): 1 CSO Outfall Alewife Brook: 6 CSO Outfalls 2 3 2 Limited CSOs in the 2050 TY 7 4 6 0 CSOs in the 2050 TY 2 3 2 0 CSOs in the 2050 5-Year Storm 2 3 2 0 CSOs in the 2050 25-Year The presentation from public meeting # 5 describes 37 alternatives. The presentation and recording is on the project webpage: https://voice.somervillema.gov/joint-cso-planning

Analysis & Evaluation Examining the relative pro’s and con’s of alternatives 48

• Level of CSO control • Permitting uncertainty • Site acquisition risks • Capital Cost and Life Cycle Cost • Timeline to implementation/CSO benefits • Impact on priority, vulnerable, and environmental justice populations • Benefits criteria Preferred Alternative Selection Considerations 49 The presentation from public meeting # 5 provides details on the selection considerations. The presentation and recording is on the project webpage: https://voice.somervillema.gov/joint-cso-planning

50 Benefits Criteria Evaluation Criterion Criteria Category Water quality impact; improve/reduce phosphorus loads CSO Performance Schedule: minimize duration to CSO reduction benefit Minimize construction impacts Construction • Impacts to public uses during construction • Neighborhood impacts during construction Minimize construction complexity/risk • Depth to excavation • Construction complexity Operation and maintenance/safety considerations Operations, Maintenance & Resiliency Resiliency and adaptability Opportunity to upgrade existing infrastructure Flooding: reduce sewer/stormwater flood risk Community & Ancillary Benefits Community co-benefits and long-term site impacts • Community co-benefits • Permanent impacts to public uses Impacts to non-variance CSOs

Stakeholder Input 51 • Reduce / eliminate CSOs and sanitary sewer overflows • Improve ecosystem health • Include near-term improvement to the Alewife • Maximize nature-based solutions when possible • Provide meaningful benefits to EJ communities and abutters • Have a short construction timeline • Reduce flooding and flood risk • Are site-able • Allow for increased recreation • Rehabilitate sewer infrastructure • Are coordinated with other municipal plans and goals • Minimize / Do not minimize neighborhood disruption during construction • Prioritize / Do not prioritize cost and affordability to rate payers The presentation from public meeting # 5 provides details on the selection considerations. The presentation and recording is on the project webpage: https://voice.somervillema.gov/joint-cso-planning

Scoring example 52

Eliminated alternative: Sewer separation Full sewer separation seems like an obvious fix, why is that not the recommendation? 53

Sewer separation does not fully eliminate CSOs Upstream contributions & downstream constraints create CSOs in any storm larger than the Typical Year 54

Infiltration and Inflow (I/I) 55 *Source: 2025 MWRA I/I Report page 107. 0 5 10 15 20 25 Lexington Belmont Arlington Medford Cambridge Somerville Annual Average (mgd) Estimated Area Flow Components Sanitary Infiltration Inflow

Infiltration and Inflow (I/I) 56 *Source: 2025 MWRA I/I Report page 107. 0 5 10 15 20 25 Lexington Belmont Arlington Medford Cambridge Somerville Annual Average (mgd) Estimated Area Flow Components Sanitary Infiltration Inflow Sewer separation only removes combined inflow (and some infiltration) Sewer separation ignores separate I/I

Regional system constraints 57

Sewer separation is very difficult to achieve The region grew around pipes from the 19th Century, the replacement systems need to accommodate climate change, and outfalls must mitigate stormwater pollution. 58

Multiple new outfall pipes, stormwater storage tanks, pumping & treatment 59 Information on preliminary design Mystic River Outfall and Sewer Separation project: https://voice.somervillema.gov/mystic-river-outfall Virtual community meeting 29 October 2025: https://us06web.zoom.us/webinar/register/WN_N2eQE99FRECRyJa6srkrqg#/registration

60 *Preliminary cost estimates are under review and subject to change. 6.AB Sewer Separation 5.MR Sewer Separation 7.CR Sewer Separation Level of Control: 0 CSOs in 2050TY Storage: - Tanks: 0 - Tunnel: 0 - Microtunnel: 0 Key Features Conveyance: 0 AB Sewer Separation: 900 acres MR Sewer Separation: 690 acres CRSewer Separation: 4,400 acres GSI: with separation Land Acquisition: Yes Time to Complete: 50+ years Preliminary Cost: $6,840 Million • Any storm greater than the largest storm in the 2050 TY storm produces CSOs!

S O M E R V I L L E F C A A N A L Y S I S 2024 FCA Guidance – Measure of Financial Burden Alternative 1: The financial impact of current and proposed CSO controls on residential users “Snapshot in time” of current & projected system costs Alternative 2: Long-term financial plan and rate model that evaluates the annual impacts of current and proposed CSO controls. Annual utility rate increases necessary to fund the plan Resulting annual residential customer bills 61 EPA has published guidance on how to complete an FCA and offers two alternative approaches

Provisional FCA results • Alt 1 Residential Indicator • Cost per Household = 4.52% MHI – High Burden • Alt 2 Residential Indicator • Mid-Range Burden FY2029 through FY2033 • High Burden starting in FY2034 • Alt 2 Lowest Quintile Residential Indicator • High Burden starting in FY2031 • EPA methodology indicates Somerville does not have the financial capacity to undertake this plan on this schedule and would need more than 50 years to complete it 62 Note: At the time of this presentation, costs and allocations are being QA/QC’ed. FCA will be revised in November.

Bill impacts: 673% increase from FY26 to FY55 63 Note: At the time of this presentation, costs and allocations are being QA/QC’ed. Rate projections will be revised in November. FY 2036 FY 2035 FY 2034 FY 2033 FY 2032 FY 2031 FY 2030 FY 2029 FY 2028 FY 2027 FY 2026 Dashboard 3.0% 5.0% 10.0% 10.0% 15.0% 19.0% 29.0% 29.0% 30.0% 30.0% 12.0% Usage Rate Increase $ 6,733,895 $ 5,262,677 $ 3,726,979 $ 11,452,378 $ 31,270,018 $ 60,397,348 $ 72,556,918 $ 47,697,330 $ 34,243,369 $ 26,656,046 $ 16,293,191 Balance Excess/Deficiency 2.64 2.66 2.53 2.37 2.25 2.02 1.79 1.63 1.59 1.75 2.11 Debt Service Coverage $3,805.16 $3,697.82 $3,527.45 $3,217.68 $2,936.07 $2,568.76 $2,177.78 $1,715.18 $1,356.57 $1,071.21 $851.70 Annual Bill SFR 31.20% 30.97% 31.76% 33.13% 34.31% 37.26% 39.92% 39.71% 35.08% 26.52% 15.42% Debt Service/Revenue $ 80,000,000 $ 80,000,000 $ 80,000,000 $ 80,000,000 $ 80,000,000 $ 50,000,000 $ - $ - $ - $ - Cash Funded CIP $ 26,746,055 $ 23,636,947 $ 20,618,395 $ 17,687,762 $ 14,842,488 $ 42,080,085 $ 89,398,141 $ 86,794,312 $ 84,266,322 $ 81,811,963 Debt Funded Project Costs

Eliminated alternative: 25-Year level of control We’re planning for the future, why not aim higher? 64

7.8 inches of rain in 24 hours breaks the system Even with CSO mitigation to the 25-year level of control, sanitary flows contaminate neighborhoods and waterways 65

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Other sources of polution 69

These pollutants have negative impacts on water quality, environmental health, and public health . Eliminating CSOs alone does not result in swimmable and fishable waterbodies. CSOs Stormwater Dry weather • Pathogens (bacteria, viruses) • Oil and grease • Wipes • Nutrients (Phosphorus, Nitrogen) • Pharmaceuticals • Industrial waste • Others • Pathogens (bacteria, viruses) • Oil and grease • Nutrients (Phosphorus, Nitrogen) • Trash • Others • Illicit connections • Leaky sewer pipes • Wildlife and dog excrement • Decomposing leaves Example sources of pollution in our waterbodies

Water Quality model results 71 Percentage Time Entire Modeled River is in Compliance with Previous Single Sample Maximum Criterion Enterococcus (61#/100mL) E. coli (235#/100mL) Model Run Stormwater Only CSO Only Non-CSO Sources All Sources Stormwater Only CSO Only Non-CSO Sources All Sources Alewife Brook 38% 99% 35% 35% 38% 99% 36% 36% 2050 Typical Year Mystic River 46% 95% 30% 30% 46% 96% 45% 45% 2050 Typical Year

CSO Projects for 25-Year level of control 72

Alewife Brook Pump Station MWR003 Fresh Pond Little Pond Spy Pond CAM401 B Alewife 41.6 MG CSO Storage Tunnel 32-ft. diameter CAM401A CSO Conveyance to Storage Tunnel CAM401A SOM001 A CAM00 1 Dilbo y Field TBM Mining Shaft and Dewatering Pump Station TBM Retrieval Shaft and Odor Control Facility CAM00 2 Not to Scale N Legend: Combined Sewer System Facility CSO Outfall CSO Storage Tunnel Drop Shaft Storage Tunnel Near-surface Conveyance 11.AB Tunnel Level of Control: 0 CSOs in 2050, 25-Year Storm Storage: - Tanks: 0 - Tunnel: 1.5 miles long, 32 ft dia. - Microtunnel: 0 Key Features Conveyance: 0.75 miles long Sewer Separation: 0 GSI: with street excavation Land Acquisition: Yes Time to Complete: 12-15 years Preliminary Cost: $1,700 Million

95 Acres of Sewer Separation Mystic River Legend: Combined Sewer System Facility CSO Outfall CSO Storage Sewer Separation Mystic River Somerville Marginal CSO Facility DeLauri Pump Station MWR20 5 SOM007A / MWR205A SOM007A/MWR205A 14.2 MG Storage Tank Stormwater Trunk Line to Mystic River 12.MR Hybrid 1 Level of Control: 0 CSOs in 2050, 25-Year Storm Storage: - Tanks: 1 - Tunnel: 0 - Microtunnel: 0 Key Features Conveyance: 0 Sewer Separation: 95 acres GSI: with separation/ other street excavation Land Acquisition: Yes Time to Complete: 5-10 years Preliminary Cost: $340 Million Not to Scale N

CAM00 5 CAM00 7 MWR01 0 MWR20 1 CAM017 MWR20 3 RE046- 19 RE046- 30 RE046- 381 RE046- 100 RE046- 105 RE046-54 RE046-55-1 MWR02 3 Not to Scale N Legend: Combined Sewer System Facility CSO Outfall Tributary to Charles River CSO Outfall Tributary to Boston Harbor RE046 CSO Regulator CSO Storage Tunnel Drop Shaft CSO Storage CSO Storage Tunnel CSO Storage Conduit RE046- 62A Bosto n Harbor Ward Street Headworks Facility Prison Point CSO Facility MWR01 8 MWR02 0 TBM Mining Shaft and Dewatering Pump Station MWR01 9 132 MG CSO Storage Tunnel 32 ft. diameter 0.05 MG Storage Conduit 2.22 MG Microtunnel Storage Not to Scale N 12.CR Tunnel Level of Control: 0 CSOs in 2050, 25-Year Storm Storage: - Tanks: 0 - Tunnel: 4.5 miles long - Microtunnel: 1 mile long, 9 ft dia. - Conduit: 1 Key Features Sewer Separation: 0 GSI: with street excavation Land Acquisition: Yes Time to Complete: 15-20 years Preliminary Cost: $2,600 Million

Bill impacts: 347% increase from FY26 to FY55 76 Note: At the time of this presentation, costs and allocations are being QA/QC’ed. Rate projections will be revised in November. FY 2036 FY 2035 FY 2034 FY 2033 FY 2032 FY 2031 FY 2030 FY 2029 FY 2028 FY 2027 FY 2026 FY 2025 Dashboard 2.0% 2.5% 3.0% 8.0% 13.5% 16.0% 22.0% 22.5% 23.0% 23.0% 12.0% N/A Usage Rate Increase $ 869,548 $ 3,386,967 $ 2,755,596 $ 6,070,403 $ 10,409,732 $ 4,071,708 $ 2,001,801 $ 392,989 $ 253,001 $ 1,124,577 $ 16,293,191 $ 10,686,098 Balance Excess/Deficiency 2.04 2.11 2.05 2.10 2.06 1.94 1.80 1.70 1.76 2.08 2.11 1.90 Debt Service Coverage $2,653.25 $2,603.58 $2,543.00 $2,472.43 $2,298.18 $2,039.10 $1,774.39 $1,476.06 $1,226.99 $1,019.99 $851.70 $817.59 Annual Bill SFR 36.70% 35.80% 35.62% 34.36% 34.27% 35.01% 35.68% 34.11% 28.57% 20.72% 15.42% 14.97% Debt Service/Revenue $ 40,000,000 $ 40,000,000 $ 40,000,000 $ 40,000,000 $ 25,000,000 $ 22,000,000 $ 15,000,000 $ 10,000,000 $ 10,000,000 $ 24,000,000 Cash Funded CIP $ 24,414,285 $ 22,538,141 $ 20,716,642 $ 18,948,196 $ 32,231,258 $ 33,564,328 $ 38,945,950 $ 42,374,708 $ 40,849,231 $ 25,368,186 Debt Funded Project Costs Still Mid-Range Burden for Somerville. Other MWRA communities TBD

Comparison to plans in other regions If other cities can do it, why can’t we? 77

Variance water combined sewer area • 9 square miles 78

Philadelphia GSI Solutions • 143 square miles • 143 square miles 79

New York City DEP GSI Solutions • 299 square miles 80

Washington DC Stormwater Solutions • 982 square miles 81

Milwaukee Metro Sewer District Stormwater Solutions & Flood Control • 350 square miles 82

Responsibilities & opportunities CSOs, stormwater & overall water quality 83

Efforts outside the Variance Waters CSO LTCP Update 84

Somerville Stormwater Fee & Credits • Program is designed in incentivize on-site stormwater management 85 See City Council 22 May 2025 meeting ID# 25-0966 for details. IAM-Engineering is working on public outreach materials. Please stay tuned.

Cambridge City Council Policy Order 2025 #136 • Consider costs & benefits of meeting 2050 25-year level of control • Update stormwater regulations for private development • Public outreach on sewer/stormwater investments • Create Combined Sewer Overflow Commission 86

Pending legislation • In Joint Committee on Environment and Natural Resources • S.608/H.1031 - An Act Relative to Combined Sewer Overflows • H.1046 – An Act to Eliminate Combined Sewer Overflows in Massachusetts Waterways • S.658/H.3938 – An Act Pertaining to Regional Resilience and Flood Protection Entities • In Joint Committee on Housing • S.1014 – An Act to Provide Sewer & Water Rate Relief 87

Eliminated alternative: Lower level of control Why not submit a modest mitigation plan that minimizes rate impacts? 88

Marginal cost savings do not justify discharge 89 $800 $1,800 $2,800 $3,800 $4,800 $5,800 $6,800 FY 2026 FY 2028 FY 2030 FY 2032 FY 2034 FY 2036 FY 2038 FY 2040 FY 2042 FY 2044 FY 2046 FY 2048 FY 2050 FY 2052 FY 2054 Annual SFR Sewer Bill Recommended plan = 3.1x FY26 rates 25-Year control = 4.5x FY26 rates Sewer Separation, TY control = 7.7x FY26 rates Lower control = 2.8x FY26 rates

Draft Combined Sewer Overflow Updated Long Term Control Plan File ID 25-1451 Officer’s Communication to City Council Infrastructure & Asset Management 23 October 2025