Chemistry of stormwater runoff from a large green roof in Syracuse, NY

A green roof can provide a wide variety of ecological benefits in urban environments such as reducing the amount of stormwater runoff. In addition, the growth medium may filter contaminated rainwater, providing runoff that is cleaner than the incoming rain. However, the growth medium might contribute other contaminants, thereby increasing the concentration relative to the rainwater. Thus, field experiments are needed to determine which contaminants are filtered out and which are leached from the growth medium during large rainstorms. It is also possible that some contaminants deposited from the atmosphere are picked up by the runoff. To investigate these issues, we determined the chemistry of stormwater runoff from two adjacent rooftops: A 1.5 acre extensive green roof on the Onondaga County Convention Center and a similarly sized gray roof on the war memorial sports arena, both located in downtown Syracuse, New York. We assumed that the same atmospheric contaminants depositing on the green roof are also depositing on the gray roof. This allowed us to compare concentrations of chemical species in the rain, green roof runoff, and gray roof runoff to determine which contaminants are contributed by the rain, by leaching from the growth medium, and by atmospheric deposition. Results from storms sampled in 2014, 2015, and 2016 suggest that sulfate and copper are leached from the growth medium while lead is filtered. Additional experiments will be conducted to better understand the leaching potential of the green roof.