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Green Roofs Keep Pollutants out of Urban Waterways

American University environmental science professor’s research shows green roofs that use a special foam instead of soil capture pollution while reducing energy use.

American University environmental science professor’s research shows green roofs that use a special foam instead of soil capture pollution while reducing energy use.

Rooftop gardens, or green roofs, are known to reduce energy use in buildings and catch stormwater runoff, but new research from American University shows that green roofs also absorb pollutants. The research, which takes on an area that previously has not been explored widely by scientists, has implications for how cities can improve the health of their rivers, streams and estuaries.  

“The potential is that institutions and businesses could reduce their pollution footprint,” said Stephen MacAvoy, assistant professor of environmental science at AU. “If large numbers of green roof systems were installed throughout Washington, D.C., they would contribute greatly to keeping harmful nutrients and suspended solids found in runoff from entering the city's waterways.”

MacAvoy works with D.C. Fire & Emergency Medical Services, which received a $300,000 grant for installation and monitoring of green roofs at its fire stations, financial analysis of energy savings and incentives, and research on the ecological impacts.

Significant Pollutant Capture

On the rooftop at D.C.’s Engine 12, green roof panels cover approximately 2,200 square feet. Instead of soil, a new kind of foam technology by the company Aqualok holds plants. MacAvoy and his students conduct chemical analyses, and after five storms over seven months, their results showed significant capture of pollutants, specifically suspended solids of 78 percent and nitrates by up to 82 percent. In fact, the green roof systems became more effective as time passed. After seven months, they were absorbing more pollutants than at the beginning.

Fewer pollutants entering the Anacostia River, a key waterway in the Mid-Atlantic region, is significant. D.C. has declared it wants the Anacostia "fishable and swimmable" by 2032, and reducing sediments and nutrients will help achieve this goal. “Bacteria and algae rob the river of oxygen, and low oxygen kills most invertebrates. Sediments clog fish gills and bury bottom-dwelling life forms and fish eggs,” said MacAvoy.

Fire Stations = Ideal Subjects

Keen on going green, D.C. Fire & EMS conducts sustainable projects such as collecting rainwater and rooftop gardens. Fire stations provide ideal subjects since green roofs can cool the floor directly below the roof. Each station is a two-story building with a non-air conditioned ground floor garage and a heavily air-conditioned second-floor dormitory. Results comparing utility data on Engine 12 from 2013 and 2012 showed a 5 percent annual reduction in electric consumption after installation of the green roof.

The setup is fairly simple: Pans collect untreated runoff and flow through green surfaces of planted roof panels, unplanted roof panels and a ground-level "bioswale," which looks like a large square rain barrel stuffed with plants (grasses and ferns). Plant varieties growing in a three-to-four inch thick foam matrix capture and absorb nitrogen from rainwater. MacAvoy’s analysis shows fewer suspended solids and nitrates are leaving the bioswale as well. This year, the research will be expanded to include three more green roofs of both foam and soil.

A Model of Best Practices

“At D.C. Fire & EMS, we play an active role in practicing sustainability. Through our efforts with green roofs, we hope to determine a kind of ‘best practices’ approach that can benefit residents, businesses and our colleagues in the municipality, for which kind of green roofs work best,” said D.C. Deputy Fire Chief David Foust. “Green roofs must yield economic benefits and ecological performance to be truly effective and desirable for property owners and municipalities.”

With help from Glenn Williamson, owner of Amber Real Estate LLC, and civil engineers, an economic assessment on energy savings and potential stormwater credits will be done when the project concludes next year. The assessment will provide an estimate of stormwater credits that could be received through public incentive programs and how water fees could be reduced.

“The idea is for businesses or institutions to know clearly what must be invested up front and what kind of actual returns can be generated to repay or offset that investment,” Williamson said.

All of the economic and environmental findings will be reported back to the grantor, the National Fish and Wildlife Foundation. MacAvoy and Williamson also have been invited to present the findings at the annual Cities Alive sustainable building conference in Nashville later this year.

“Fewer chemicals entering our waterways is crucial to ecosystem health,” MacAvoy said. “People want to live in nice, clean cities, and greening our cities must be a priority for all of us.”