Green roofs could be the answer to clean D.C.’s rivers

WASHINGTON — Fifteen feet from a busy bus stop in Northwest D.C., a small patch of plants on one of American University’s academic buildings is doing more than providing students with something to look at while they grab a smoke or take a phone call between classes.

This patch could be the answer to cleaning up urban waterways.

Stephen MacAvoy, assistant professor of environmental sciences at American University, is studying green roofs’ ability to keep common pollutants out of the Anacostia and Potomac rivers.

“The [pollutants] that get into the waterways end up killing oysters and fish. Those animals can’t breathe if they’re in really dirty water, as you can imagine,” MacAvoy says.

But green roofs may be able to change that.


Some of the green roofs at American University are open for students to enjoy. (WTOP/Rachel Nania)

The most common rainwater pollutants in urban environments are nitrogen, a by- product of car exhaust; and suspended solids, a combination of dust and un-burnt hydrocarbons that collect on urban surfaces.

When it rains in the city, these solids run off the roads, sidewalks and concrete buildings, and into the water.

“Anything living on the bottom [of the water] gets buried, so you end up with degraded water,” MacAvoy says.

Green roofs hold on to storm water and prevent it from picking up suspended solids. The nitrogen in the rainwater permeates the green roof and is used by the roof’s plants.

“What the green roofs try to do is clean the water of nitrogen before it gets into the waterways,” MacAvoy says.

MacAvoy, who has been studying green roofs’ impact on water pollution for the past two years, says they clean up to 60 percent of the suspended solids and 80 percent of the nitrogen that would otherwise get into the rivers.

This spring he’s begun collecting data from various roofs at American University. His previous work was in partnership with some of D.C.’s fire stations, which were equipped with green roofs in 2011 after receiving a $300,000 grant.

Green roofs have long been studied for their ability to save energy. Since installing the green roofs a few years ago, D.C.’s Engine 12 station reduced its energy costs by about 5 percent annually.

But in the past few years, there’s been more of a focus on nutrient retention.


Green roofs, such as this one at American University, could help clean up urban waterways. (WTOP/Rachel Nania)

“It’s still very much an area of discovery,” MacAvoy says. “We’re not confirming someone else’s results, at all.”

To quantify the impact green roofs have on urban water systems, MacAvoy and his research students will collect rainwater directly from the sky and rainwater that filters through pans on two different types of green roofs — an engineered-soil one and a foam-based one.

“We’re also collecting water that runs off of untreated roofs to get an idea of how dirty it is,” he says.

The researchers will compare levels of nitrate, ammonium, phosphorous and suspended solids in the collected samples. All of the economic and environmental findings will be reported to the National Fish and Wildlife Foundation.

MacAvoy says if the data shows that green roofs can keep pollutants out of D.C.’s rivers and streams, he anticipates more of them will be built in the District and other cities.

“The green roofs save energy; they retain storm water; they possibly reduce nutrient pollution; they possibly reduce sediment pollution; there are a bunch of positives. So I think they will be increasingly used,” he says. “D.C., in particular, wants to make its signature river — what’s going to become a signature river, the Anacostia — fishable and swimmable by 2032. So that gives us 18 years.”

While businesses and government buildings may move toward green roofs, he predicts homeowners won’t jump on the idea unless there is some sort of tax advantage or incentive.

The U.S Environmental Protection Agency says the cost of installing a green roof ranges from $10 to $25 per square foot. D.C. has started on an inventive program, though. A 2013-2014 rebate program from The District Department of the Environment offers D.C. residents and businesses base funding of $7 per square foot for installing a green roof.

“The value of property goes up when it’s environmentally sound, no doubt about it. So if you can make the environment prettier, more functional, less polluting — all of that’s a bonus,” MacAvoy says. “It’s not just the right thing to do; it’s the smart thing to do.”

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