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. For the thousands of people living in the Alewife floodplain, combined sewer overflows are not an abstract infrastructure problem.
During an October 2021 storm that brought nearly an inch of rain, the Tannery Brook combined sewer outfall—which drains from the Davis Square area—discharged 1.42 million gallons over 45 minutes. Other outfalls contributed roughly another 1.1 million gallons, flushing about 2.5 million gallons of sewage-contaminated water into Alewife Brook.
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?
Somerville has already shown what residents can accomplish with one important piece of that idea.
In October 2023, Somerville High School students, staff and community volunteers planted approximately 410 native trees and shrubs representing more than 30 species in just 1,500 square feet beside the high school. Students continue to measure the forest’s growth and have reported a 93% survival rate.
Created using the principles of the Miyawaki method of dense native planting, the young forest provides habitat for myriad birds and other wildlife, along with bees, butterflies and other pollinators.
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 be combining subsurface wastewater treatment with a densely planted native forest designed to provide habitat and other ecological benefits.
A pilot system could divert a portion of the combined sewer overflow for treatment rather than allowing it to continue down the brook. 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.
As water moved through the soil, plant roots, fungi, bacteria, and other soil organisms could provide additional biological processing. Trees would return some water to the atmosphere through transpiration. Their roots and accumulated organic matter could improve soil structure and infiltration capacity while 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 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.
Somerville, known as the most densely populated community in New England and a city with precious little open space, knows the value of every available acre. Creating more woods ordinarily means finding—and often purchasing—land. Here is an unusual possibility: creating acres of native forest on public land already sitting beside the brook.
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.
Somerville residents already know what it means to create a dense, diverse native forest together. High school students, teachers and volunteers have done it. 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.
Somerville still needs to solve its sewage problem. But perhaps solving it can also give Somerville 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.
