By ASHLEY D’SOUZA and LUCIE McCORMICK, MIT Graduate Program in Science Writing
CAMBRIDGE, Mass. (AP) — When Ann McDonald went kayaking several weeks after a 2024 rainstorm, she thought the water in the Alewife Brook just outside Boston was safe. Even though sewage discharges into the brook during heavy storms, public health officials recommended waiting just 48 hours after rain for bacteria levels to drop — so when McDonald’s kayak got stuck in debris, she wasn’t concerned about dipping her arms into the water to free herself.
Later that afternoon, she fell sick with diarrhea. She attributes it to sewage in the water.
“I got stuck in some really stinky water,” she said. “I’m naive, and I thought I was above all this.”
McDonald’s experience is emblematic of why environmental organizations are urging local water officials to keep sewage out of waterways by committing to a more modern system with separate pipes for waste and storm runoff. But in February, the Massachusetts Water Resources Authority approved a more conservative approach to updating the existing sewer system in several communities, citing higher costs. The plan is expected to allow continued sewage discharges during heavy storms.
The Alewife Brook runs through parts of Cambridge, Arlington and Somerville, Massachusetts, and is popular with wildlife-watchers, runners and dog walkers. It also receives millions of gallons of untreated sewage pollution each year from a wastewater network known as a combined sewer system.
In a combined system, storm runoff and sewage typically share the same pipes. When the total amount of wastewater exceeds the system’s capacity — like during a rainstorm — untreated sewage is released into nearby water bodies in an intentional event called a combined sewer overflow, or CSO.
A separate sewer system, on the other hand, uses one set of pipes to carry wastewater to treatment plants and another set to transport rainwater to waterways. Most U.S. communities currently use the more modern systems, according to the federal government.
Some parts of the Boston area have separate sewer systems, while others have combined systems. Local advocates want full sewer separation and elimination of sewage releases through outfall pipes along the waterways.
Approximately 700 communities in the U.S. that use combined sewer systems face similar public health concerns as untreated wastewater flows into their water bodies. This is being exacerbated by climate change, which causes more frequent and intense storms and threatens access to freshwater resources.
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A wetland in Alewife Brook Reservation is used to treat stormwater runoff before it enters the brook in Cambridge, Mass., Friday, March 13, 2026. (Lucie McCormick/MIT Graduate Program in Science Writing via AP)
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A wetland in Alewife Brook Reservation is used to treat stormwater runoff before it enters the brook in Cambridge, Mass., Friday, March 13, 2026. (Lucie McCormick/MIT Graduate Program in Science Writing via AP)
The proposed sewer plan
MWRA’s proposed plan would use storage tanks, partial sewer separation and infrastructure improvements to reduce discharges into waterways through 16 outfall pipes owned by the authority, Cambridge and Somerville.
To prepare the proposal, MWRA used precipitation projections for the year 2050 and considered three control levels. One level was based on a typical year of rainfall in 2050, while the other two were modeled on eliminating sewer overflows during more intense storms.
The authority, which state lawmakers established in 1984 to manage water and sewer services in eastern and central Massachusetts, ultimately decided to aim for eliminating CSOs for a typical year of rainfall without the intense storms.
“This plan represents a responsible investment of ratepayer dollars that balances real environmental benefits with actual water quality improvements,” the agency said in a statement in response to questions.
The plan has a total price tag of $1.28 billion, which would be split between the authority, Cambridge and Somerville and ultimately passed on to ratepayers, according to MWRA planning documents.
David Stoff, who’s on the steering committee for the volunteer group Save the Alewife Brook, lives just across a walking path from the waterway. On multiple occasions in past years, he had to stop his children from playing in the yard due to sewage flooding. He’s disappointed in MWRA’s proposal.
“The plan is to keep the waters as open sewers instead of saying, ‘We’re going to rebuild the combined sewer so that sewage goes to the wastewater treatment plant and rainwater goes to the river,’” he said. “It’s not rocket science.”
In their February vote, the water authority’s board considered but decided against full sewer separation, stating that separation “is relatively expensive and can disrupt traffic and other community activities during construction.”
To fully separate it, crews would need to dig up roads and parking lots in communities, causing temporary closures.
The water authority estimated that eliminating CSOs during those less frequent but more intense storms would cost households $82 more per year in 2050 — $46 in today’s dollars.
Emily Norton, executive director of the Charles River Watershed Association, called the vote “abominable and embarrassing.”
“Anyone who lives in Greater Boston knows we live with construction disruptions to improve the infrastructure we rely on to live our lives,” Norton said.
MWRA will submit an updated plan to state and federal environmental regulators at the end of April, followed by a five-month public comment period. A final plan is expected in January 2027.
Calling for full sewer separation
Save the Alewife Brook was formed in 2020 by Arlington resident Kristin Anderson, whose home was flooded with raw sewage multiple times following rainstorms. The group lobbies local and state leaders for full sewer separation and the elimination of sewage discharges during storms.
Another local environmental organization, the Mystic River Watershed Association, enlists volunteers to gather water samples from the brook for lab testing.
While the MWRA says there has been an 88% reduction in CSOs across its system since the late 1980s, there were 23 discharges into the Alewife Brook alone in 2025.
The authority is also the defendant in two court cases dating back to the 1980s regarding sewage pollution in the Boston Harbor, the Charles River and the Alewife Brook. Those cases require the agency to file biannual compliance reports in federal court, and MWRA is court-mandated to control combined sewer overflow.
“(The sewers) were designed for yesterday’s storms, not tomorrow’s,” Anderson said. “If it happens to you, you don’t want it to happen to anybody else.”
‘Don’t tell us any more excuses. Just fix the problem’
Gregory Pierce, director of the Water Resources Group at UCLA, said he believes the current plan will deteriorate quickly and that the water authority should plan for more intense storms.
“Any projection now of extreme weather events tends to be underestimated, so from how we keep breaking weather extreme records, it makes sense to build out the more expensive and comprehensive solution,” he said.
Pierce has seen similar debates play out across the nation as cities move away from combined sewer systems. Beginning in the 1990s, Portland, Oregon, underwent a 20-year, $1.4 billion project to build massive underground tunnels to relieve combined sewers and prevent sewage discharges into waterways. The city reduced its annual CSOs from over 50 to fewer than 10.
Despite its challenges, the Alewife Brook is still a green space that brings people joy, said McDonald, who is also active in the Save the Alewife Brook group. She and fellow group members expect a long fight ahead and are trying to increase awareness through newsletters, tours of outflow pipes and cleanup events.
“I’m hopeful when I see the blue herons land here; I’m hopeful that nature will somehow inch its way along. But I think we’re far from learning how to live with the water,” she said. “People see this as their living room. You got sewage in my living room? Get it out of here. Don’t tell us any more excuses. Just fix the problem.”
EDITOR’S NOTE: This story is a collaboration between the MIT Graduate Program in Science Writing and The Associated Press.
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