Impacts of Runoff Water Quality from Extensive Green Roofs

Green roofs are well-documented for mitigating volume and flow rate of stormwater runoff. However, impacts associated with nutrients in green roof runoff are less well-known. This study compares water quality of green roof runoff with precipitation and runoff from a traditional shingle membrane roof. A demonstration-scale extensive green roof reduced overall runoff volume and served as a sink for nitrate and ammonium but was a source for phosphate, sulfate, dissolved organic carbon, and organic nitrogen, particularly notable during large storms (>0.5 inches rainfall). Zinc and lead data suggested higher mass loads in traditional roof runoff than in precipitation and green roof runoff, although results were not statistically significant. Life Cycle Assessment software was used to map pollutant mass loads to environmental impacts, taking into account runoff volume reduction by the green roof. As a source of phosphorous and a sink for nitrogen, LCA results show negligible or reduced impacts from the green roof in all categories except freshwater eutrophication. Therefore, impacts of fertilization should be considered when installing and maintaining green roofs, especially near inland freshwaters where phosphorus is the limiting nutrient.