Cambridge breaks new ground with living infrastructure

When heavy rain hits Alewife Brook, combined sewer overflows can send sewage-contaminated water into the brook, onto public trails, and into surrounding habitat. Thousands of people living in the Alewife floodplain are exposed to flooding with contaminated water.

Consider what happened during an otherwise unremarkable October 2021 storm that brought nearly an inch of rain. Tannery Brook from Davis Square discharged 1.42 million gallons in 45 minutes, while 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 overflow until it can move through the wastewater system for treatment. The draft plan proposes two storage tanks with a combined capacity of 3 million gallons, along with sewer separation, new conveyance systems, and a mile-long microtunnel. The estimated design and construction cost is about $340 million, with completion expected in 13 to 18 years.

The storage is essential. But storage does not make the water disappear. Water captured by the sewer system must eventually move through the wastewater system for treatment.

Could we also slow some of this water, clean it, and return it to the landscape? We at the Ocean River Institute have proposed an Alewife Living Forest pilot to investigate whether living infrastructure could complement the conventional system. Think of the storage tanks and the forested leach field as two different kinds of relief valves.

Now is the time to comment on the draft updated Combined Sewer Overflow Control Plan. The Massachusetts Water Resources Authority and the cities of Cambridge and Somerville are accepting written public comments through Oct. 30, with a final public hearing scheduled for Oct. 28.

Cambridge has already shown that one part of this vision is achievable: Residents can build dense, native forests. In 2021, volunteers helped plant the Northeast’s first Miyawaki forest, approximately 600 native trees and shrubs across 4,000 square feet at Danehy Park. The following fall, volunteers planted a second dense, highly diverse native forest at Greene-Rose Heritage Park. The young forests provide homes for myriad birds and wildlife, as well as bees, butterflies, and other pollinators. Now imagine applying that community experience to a larger environmental challenge just downstream.

Roughly 60,000 square feet of mowed grass lies between Alewife Brook Parkway and the brook from Route 2 to Massachusetts Avenue. This corridor could support as many as 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 innovation would combine subsurface wastewater treatment with a densely planted native forest designed to provide habitat and other ecological benefits. The underlying concept is familiar: a forested leach field.

Instead of sending all combined sewer overflow down the brook, a pilot system could divert a portion for treatment. A pumping station could pretreat and lift the water to the Alewife Living Forest, regulating the rate at which water enters the forest rather than subjecting it to the full pulse of a combined sewer overflow. Much as in a conventional leach field, the water could then be distributed underground, where soil would provide further treatment while supporting thousands of native trees and shrubs. A pilot would determine how much combined sewer overflow water the system could safely receive, how it should be pretreated and distributed, and how treatment performance varies through the seasons.

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.

Cambridge residents 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, salamanders and small mammals, summer heat, and people who walk the Alewife Greenway? Or simply getting more lasting public benefit from the enormous investment required to clean up Alewife Brook?

Cambridge still needs to solve its sewage problem, and doing so could also give Cambridge something to crow about. Perhaps the next forest can help keep sewage-contaminated water out of Alewife Brook while making its Greenway cleaner, healthier, and more resilient, an investment in a forest that will grow for generations.

Moir is the president and executive director of the Ocean River Institute, a nonprofit based in Cambridge.

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