Future sewage discharges would be worse in storms that cause Alewife Brook flooding
Cambridge, Somerville, and the MWRA have proposed a new plan1 to address combined sewer overflow (CSO) discharges into Alewife Brook. The plan is required by law2 and projects at Alewife Brook will cost $340 million. It is designed to address discharges in typical annual storms3.
Because the draft plan is not engineered to be resilient to Climate Change4, more untreated sewage will be discharged in future storms - when the Brook is most likely to overtop its banks.
Even after the proposed projects are built, Massachusetts Water Resources Authority’s own projections5 show a worsening of conditions in larger, climate-driven storms: substantially more sewage would be discharged into the brook. Today, during a 5-year storm, the sewer system discharges about 10.5 million gallons of CSO into Alewife Brook. Under the proposed plan, a 5-year storm event in 2050 would discharge 15.6 million gallons.
That is about 50% more sewage during storms most likely to flood our parks, paths, and homes. A 5-year storm has a 20% chance of occurring in any given year.
The Storms That Flood Alewife Brook
Alewife Brook has flooded over its banks in storms smaller than the 5-year event, yet the plan is designed to allow greater sewage discharges during storms most likely to cause flooding.
In 2023, the Brook flooded five times6, sending untreated sewage into DCR parkland and the bike path. After these storms, we documented children riding bikes through untreated sewage-contaminated floodwater, joggers running through it, and parents pushing baby strollers through it. This is a popular path that folks from Somerville, Cambridge, Medford, and Arlington use to get to the Alewife T.
Why this matters: Untreated Sewage is a Health Hazard
According to MWRA, when CSOs are discharging at Alewife Brook, E. coli levels are 950 times higher than what EPA considers safe7. Untreated sewage can also carry other bacteria, parasites, viruses - including norovirus, and intestinal worms. Area residents have testified to getting sick from exposure to CSO sewage.
Emerging science from researchers at the BU School of Public Health8 shows that living near CSOs9 is associated with health impacts severe enough to lead to Emergency Room visits. Infants, older adults, cancer patients, and people with weakened immune systems are especially vulnerable.
Alewife Brook runs through Environmental Justice neighborhoods10, past subsidized housing in Somerville, Arlington, and Cambridge. About 5,000 people live in the Alewife's 100-year floodplain.
The plan does not look at flooding along Alewife Brook, does not address public health risks, and does not keep up with climate change.
The Solution:
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Virtual Elimination
Engineer a plan to eliminate CSO discharges to our rivers.
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Sewer separation
Keep stormwater out of the sewer system. Sewer separation is a climate-resilient strategy that increases local and regional sewer system capacity and reduces CSOs, SSOs, basement backups, and “blended events” at Deer Island Wastewater Treatment Plant.
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Large Green Stormwater Infrastructure
Build stormwater wetlands that capture, clean, and slow stormwater while reducing flooding.
What you can do to help
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A
Submit a public comment to MassDEP and EPA.
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B
Monday Attend the Cambridge City Council meeting on Monday, October 5th, 2026 at 5:30 pm and speak out.
Ask the City Council to:
- Reject the proposed plan
- Demand Virtual Elimination of CSOs in our rivers
- Demand a Climate Resilient Strategy of Sewer Separation with Green Stormwater Infrastructure, which scales with increase in rainfall.
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C
Attend the October 28 hearing, 6 to 8 PM Hybrid
At Somerville City Hall and on Zoom.
Download Fact Sheets here:
- PDFCSO Plan does not keep up with Climate Change at Alewife Brook
- PDFAlewife Brook Flooding in 2023
- PDFLiving near CSOs is a public health risk
Footnotes
- Draft Updated Combined Sewer Overflow Control Plan for Alewife Brook, Upper Mystic River, and Charles River ↩
- Massachusetts Department of Environmental Protection, Final Determination to Adopt a Water Quality Standards Variance for CSO Overflow Discharges to the Alewife Brook/Upper Mystic River Basin (Aug. 30, 2024), at conditions requiring MWRA and the Cities of Cambridge and Somerville to implement their approved scopes for Updated CSO Control Plans. The Variance is effective September 1, 2024, through August 31, 2029, and requires the Updated CSO Control Plans to conform to EPA’s CSO Control Policy and MassDEP’s 1997 Guidance for Abatement of Pollution from CSO Discharges. ↩
- The selected Alewife Brook plan, Alternative 3.AB Hybrid 2, is designed to achieve “0 CSOs in 2050 Typical Year” conditions. Note: that does not mean zero CSOs! A “Typical Year” is the plan’s modeled annual rainfall condition, not a large design storm such as the projected 2050 five-year, 24-hour storm. Under the plan’s modeled 2050 five-year storm, the alternative would still discharge approximately 15.6 million gallons of CSO to Alewife Brook. Massachusetts Water Resources Authority, City of Cambridge, and City of Somerville, Draft Updated Combined Sewer Overflow Control Plan (Apr. 30, 2026), ch. 9, § 9.5.3.1, Table 9-25, printed pp. 9-51–9-52; Table 10-21, p. 10-25. ↩
- The draft plan is 99.9% storage and only 0.1% sewer separation. Storage can temporarily hold combined sewage and stormwater, but once the storage conduit is full, additional flow is discharged to Alewife Brook. Storage does not scale with increase of stormwater from Climate Change. ↩
- Massachusetts Water Resources Authority, Cambridge, and Somerville, Draft Updated Combined Sewer Overflow Control Plan (Apr. 30, 2026), Table 10-21, p. 10-25 (projected 2050 five-year-storm discharge of approximately 15.6 million gallons to Alewife Brook); compare MWRA, Alewife Brook and Lower Charles River System Optimization Evaluations Report (Dec. 28, 2022), Table 1-2, p. 3 (five-year-storm discharge of 10.53 million gallons). The proposed-plan projection is approximately 48% higher. ↩
- Alewife Brook flooded over its banks with sewage in the floodwater at least 5 times in 2023. Not one of these events reached even a 2-year storm, and four of the five were well below a 1-year storm. These are rainfall levels that occur multiple times a year. ↩
- MWRA tests show that during storms, when the CSO outfalls discharge raw sewage, E. coli levels in Alewife Brook are roughly 950 times the standard that EPA considers safe. E. coli data from the MWRA Board of Directors meeting presentation, page 18: https://www.mwra.com/media/file/2025-10-29-mwra-board-directors-meeting-presentations ↩
- Association between Combined Sewer Overflow Events and Gastrointestinal Illness in Massachusetts Municipalities with and without River-Sourced Drinking Water, published May 2024
A Review of Combined Sewer Overflows and Gastrointestinal Illness in a Changing Climate, published 09/11/2025 ↩ - Conservation Law Foundation’s affidavit of Bethany M. Haley, Court docket no.: 1981-2 Date: July 17, 2026 Dr. Bethany M Haley is a researcher whose peer-reviewed work links CSO events to gastrointestinal illness in Massachusetts communities. This is the scientific backbone of CLF’s public-health argument. ↩
“There are three commonly accepted possible exposure pathways to pathogens from CSO discharges. The most well-supported of these is incidental ingestion while recreating. Some studies have also found a link to AGI from drinking water drawn from CSO waters, and there is emerging research on the potential for aerosolized contaminated water to cause illness when inhaled.”
- Massachusetts Environmental Justice Viewer ↩
2 Responses
The area of greatest pollution is within the state of Massachusett’s “Environmental Justice” zones, indicating a bias that Cambridge claims not to tolerate. The area must be cleaned up and flooding attenuated if Cambridge continues to taut its globally recognized educational institutions as having any environmental, social or economic beneficial effect on its policies and its expanding populations.
Working hard on the Alewife Reservation floodway in the DCR permitted Alewife garden over the years, we have seen a rise in Arlington/Cambridge Reservation ground water and have monitored it. Thus, with previous design plan by “Biodiversity of Salem Inc.” retention ponds already there are a perfect remedy for attenuation and for flood waters from culverts in Arlington under route 2 that reach Little River. A well developed basin will attenuation more in order to answer some of the flooding probs. and protect the Research Park, once the contamination prob. has been properly addressed. Green infrastructure is the way to go, and the 3 acres of natural biodiverse flora for that is waiting in the wings. Use the previous plan to save money in the early MDC Master Plan for this area, and in the Alewife storm water wetland designs, already completed.
Ellen Mass, Founder of Friends of Alewife Reservation