Urban environments are dominated by impervious surfaces like roads, parking lots, sidewalks and rooftops. These urban surfaces replace non-impervious natural surfaces that can absorb water like grasslands or forest floors. More impervious surface means larger amounts of water run-off in urban area where less absorption can take place.
Natural areas absorb water:
Urban areas have less absorbing capacity and much greater run-off:
Large rain storms in cities can lead to flooding and erosion during flash events. In New York City a Combined Sewer system can discharge sewage during high-storm events leading to pollution of waterways.
Green roofs act as sponges on rooftops. They absorb large quantities of water into the soil substrate. There, plants can extract water to photosynthesize, fixing CO2 a greenhouse gas from the air. Green roof plants can also filter out other air pollutants like ozone. Water may also evapotranspirate from the plants through stomata. Any water then left in substrate will evaporate. It becomes clear that green roofs can mitigate stormwater quantity, and could reduce the occurrence of overflow if widely implemented. Although this is an excellent attribute, green roof substrate contains nutrients that allow plants to grow which, may be deleterious to the environment if they make their way into water bodies.
Nutrient loading from fossil fuel combustion, fertilizers and other sources in water bodies can lead to eutrophication. Eutrophication can create algal blooms or red tide, which can wreck havoc to marine organisms threatening seafood production, while simultaneously preventing human swimmers from safely enjoying the water during 95 degree July days. Plants require nutrients to grow, as do you. Nitrogen and Phosphorous are critical elements for all living organisms including yourself. Nitrogen is needed to make every protein in your body, and Phosphorus is a key building block of DNA. Therefore, these nutrients must be in growing substrate on rooftops for plants to grow; however, these nutrients may also leech out of the soil into stormwater systems. Currently, there is conflicting scientific evidence demonstrating that green roofs can both decrease and increase nutrient (N, C, P) content in stormwater run-off. These data sets suggest leeching is dependent on growing substrate attributes, fertilizer use and plant types.
FUSE is currently investigating both the quantity and the quality of stormwater run-off from an agricultural rooftop. Our study investigates the three different substrate types and four different species of plants. Here you can see FUSE team member Julia diligently preparing water samples to be analyzed.