Green roofs - A BMP for urban stormwater quality?

The focus of this research is the impact of green roofs on urban stormwater quality and builds on a laboratory study that evaluated several potential green roof media to produce a mix that produced the "best" overall reduction in pollutants from simulated rainwater. Because of the limitations of the laboratory testing, this second phase (field test) was performed. A second purpose was to determine if this actual media mix would support plant life with minimal maintenance. The third question raised by the researchers was whether the green roof, at the low concentrations often seen in many pollutants in wet and dry deposition, could remove these pollutants. If not, was the green roof at least "chemically neutral" (e.g., did not remove or add pollutants to the runoff, such as those added to rainwater by passage over a galvanized metal roof?) The field study showed that, during the early life of these roofs, water quality from the green roofs was comparable to the control roofs and better, with the exception of phosphorus and added color, to that of the galvanized roof. The increase is color is believed to be from washout of the organics in the media. Plant development on these roofs has been very slow, probably due to the lack of added fertilizers and minimal organic matter; however, the water quality benefits may substantially outweigh the need for rapid plant growth.