A New Underground System Protecting Casco Bay

Portland’s Hidden Pollution Solution

If you have visited Back Cove recently, you might have noticed the new athletic field and obstacle course. But beneath the grass lies something much bigger: Portland’s largest pollution-preventing project yet, the Back Cove South Storage Facility.

This underground system consists of massive concrete conduits capable of holding 3.5 million gallons of stormwater and sewage during heavy rains. Together with earlier projects under Payson Park and Baxter Boulevard, it forms a “crown jewel” of trenches, pipes, and tanks around Back Cove that can store a total of 8 million gallons of polluted water until it can be sent to the wastewater treatment plant.

Combined Sewer Overflows

Portland, like many older cities, has some combined sewer systems where stormwater and wastewater share the same pipes. On dry days and during small storms, everything flows to the treatment plant. But when big rains hit, the pipes can’t handle all that extra water. Untreated stormwater and wastewater then flow through gates that open directly into Casco Bay. These events are called Combined Sewer Overflows (CSOs). While these CSO events prevent sewage from backing up into homes and streets, they have a direct, negative impact on the Bay. For example, sometimes CSOs deliver tens of millions of gallons of untreated water into Back Cove.

Storage with a Mission

“There are really only two ways to reduce CSOs,” explained Bill Boornazian, Portland’s Water Resources Manager. “You can separate the pipes, or you can build storage tanks. These tanks buy the system time by holding water until it can be treated.” Portland is also working to separate sewage pipes from stormwater pipes in other areas of the city.

Back Cove South is expected to cut Portland’s CSO discharges by half. Engineers say that if the system had been online earlier this year, 14 out of 15 overflow events would have been fully captured and treated.

Back Cove South Captures Stormwater Pollution

Stormwater itself isn’t always bad. “Casco Bay is an estuary, where the land meets the sea and freshwater flows into the ocean,” says Casco BAYKEEPER® Ivy Frignoca. “In its natural state, freshwater nourishes the Bay, creating a rich habitat that supports many species.”

But in a built city like Portland, stormwater picks up pollution, like fertilizers, pesticides, road runoff, and litter, and carries it to the Bay. That’s why Ivy supports projects like Back Cove South. “We love these storage tanks. They’re designed to capture what first washes off the landscape, the most polluted waters.”

Building for Resilience

The project does more than store polluted water. The entire field above the tank was raised three feet to help guard against sea level rise and flooding. “We’ve already seen the benefit,” said Brad Roland, senior project engineer with Portland’s Department of Public Works. “Even during the biggest storms last winter, this area didn’t flood.”

For Ivy, projects like this are part of the Bay’s future. They show what’s possible when the community invests in solutions that protect clean water and coastal resilience.

Want to see this hidden infrastructure in action?

Watch our new 8-minute video, Running Around Back Cove: Portland’s Hidden Pollution Solution. Communications Coordinator Meghan Vigeant laces up her sneakers to show where Portland stores millions of gallons of polluted water until it can be treated. You’ll also hear from Casco BAYKEEPER® Ivy Frignoca and City Engineer Brad Roland on why these underground tanks are such a big win for Casco Bay.