When heavy rain hits Alewife Brook, combined sewer overflows (CSOs) can send sewage-contaminated water into the brook, onto public trails, and into surrounding habitat. Combined sewer overflows are not an abstract infrastructure problem for the thousands of people living in the Alewife floodplain,
Tannery Brook from Davis Square (SOM01A) discharged 1.42 million gallons over 45 minutes during an October 2021 storm that brought nearly an inch of rain, while the other outfalls contributed roughly 1.1 million gallons—burdening Alewife Brook with a flush of about 2.5 million gallons.
The conventional engineering response is to capture and store that water until it can be treated. A proposed and much-needed 3-million-gallon storage pipe is now part of the public discussion.
But living systems invite another question: Can we also slow some of this water, clean it, and return it to the landscape?
Cambridge has already shown what residents can accomplish with one important piece of that idea.
Many volunteers helped plant approximately 600 native trees and shrubs representing many species in just 4,000 square feet in Danehy Park in September 2021. This was the first Miyawaki forest planted in the Northeast. The following fall, volunteers planted a second dense and highly diverse native forest at Greene-Rose Heritage Park.
Created using the principles of the Miyawaki method of dense native planting, the young forests provide a home for myriad birds and other wildlife, along with bees, butterflies and other pollinators. A native forest now grows where a building once stood at Greene-Rose Heritage Park in East Cambridge, restoring wildlife habitat in a densely developed neighborhood.
Now imagine applying that community experience to a larger environmental challenge just downstream.
Caption: The open areas between Alewife Brook Parkway and the wooded brook corridor total roughly 60,000 square feet.
Roughly 60,000 square feet of mowed grass lies between Alewife Brook Parkway and the brook from Route 2 to Massachusetts Avenue. The Miyawaki method densely plants young trees and shrubs, often about 18 inches apart. At that density, this corridor could potentially support about 25,000 native trees and shrubs representing a rich diversity of species, many valuable to pollinators.
But this would be more than a forest. It would be an experiment in living infrastructure.
The underlying concept is familiar: a forested leach field. The innovation would combine subsurface wastewater treatment with a densely planted native forest designed to provide habitat and other ecological benefits.
Instead of sending all combined sewer overflow down the brook, a pilot system could divert a portion of it for treatment. After screening to remove sediment and floating debris, the water could be pumped to the upland, where it could pass through engineered biological filters—potentially including wood chips colonized by fungi and beneficial microbes, along with biochar designed to adsorb contaminants.
The water could then be distributed underground through a carefully engineered infiltration system beneath the forest. Much as in a conventional leach field, the soil itself would provide further treatment—but here the treatment zone would also support thousands of native trees and shrubs.
Plant roots, fungi, bacteria, and other soil organisms could provide additional biological processing as water moved through the soil. Trees would return some water to the atmosphere through transpiration while their roots and accumulated organic matter could improve soil structure and infiltration capacity, thus creating more biomass above and below ground.
This pilot project is not intended to replace modern wastewater treatment or the conventional infrastructure required to manage combined sewer overflows. It proposes investigating as to whether living systems can complement and enhance conventional infrastructure.
If successful, the benefits could extend well beyond water quality. A native forest could provide habitat for birds and pollinators, cool its surroundings, capture carbon, filter air pollution, soften traffic noise from Alewife Brook Parkway, and make the Alewife Greenway more beautiful and welcoming.
Cities across the country are exploring nature-based approaches to managing stormwater in a changing climate. Alewife Brook offers an extraordinary place to test what happens when ecological restoration and conventional engineering are designed together rather than treated as separate worlds.
And Cambridge has already helped plant the sapling of a mighty oak.
How You Can Help
The current public-comment process is an opportunity to broaden the conversation about Alewife Brook beyond sewage and grey infrastructure to include native forests and living infrastructure.
Cambridge citizens already know what it means to create a dense, diverse native forest together. Now Greenway walkers, tree lovers, birders, gardeners, botanists, mushroom foragers, students, educators, ratepayers and others can bring their own perspectives to the discussion.
What could a living forest mean for birds? Pollinators? Fungi and mycorrhizal networks? Salamanders and small mammals? Summer heat? Air pollution? Traffic noise? The experience of walking the Alewife Greenway? Or simply getting more lasting public benefit from the enormous investment required to clean up Alewife Brook?
You don’t need to design the infrastructure. That’s the work of experts. Tell them why the possibility of a living forest matters to you.
Cambridge still needs to solve its sewage problem and solving it can also give Cambridge something to crow about.
Perhaps the next forest can do something even bigger: help keep sewage-contaminated water out of Alewife Brook while making its Greenway cleaner, healthier and more resilient. Planting thousands of native saplings would be an investment not just in today’s infrastructure, but in a forest that could grow and mature for generations.
