Afield study of green roof hydrologic and water quality performance

Recent regulations intended to minimize the amount of nitrogen and phosphorus in stormwater runoff have resulted in interest in stormwater treatment practices to reduce nutrient loadings. In ultra-urban areas where typical treatment practices are not optimal because of large surface area requirements, green roofs are an option to improve stormwater runoff. The hydrologic and water quality performance of two extensive green roofs in eastern North Carolina's Neuse River basin were investigated. The two green roof designs were a flat 70 m(2) area with an average media depth of 75 mm, and a 3% pitch 27 m(2) surface area roof with an average media depth of 100 mm. Extensive green roofs typically have shallow media depths (less than 150 mm) with vegetation that requires minimal irrigation and maintenance. Each green roof retained a significant (p < 0.05) proportion of the rainfall observed, 64% of the total rainfall measured at each site. Peak outflow of runoff was significantly reduced (p < 0.05) from the green roof (average peak flow reductions of more than 75% were observed from each green roof), and each green roof substantially delayed runoff. On average, the total nitrogen (TN) concentrations in the green roof outflow were 2.7 mg L-1 higher than the rainfall (significant at p < 0.05) and 1.3 mg L-1 higher than the control roof outflow; TN amounts in the green roof outflow were 0.02 g higher than the rainfall and 0.12 g lower than the control roof outflow. On average, total phosphorus (TP) concentrations in the green roof outflow were 1 mg L-1 higher than the rainfall and 0.8 mgL(-1) higher than the control roof oufflow (both significant at p < 0.05); TP amounts in the green roof oufflow were 0.07 g higher than the rainfall (significant at p < 0.05) and 0.05 g higher than the control roof outflow. It was determined that the media, composed of 15% compost, was leaching TN and TP into the green roof outflow. This field study demonstrated the importance of green roof media selection in locations where nutrient removal is a concern. Results from this study serve as a benchmark for the development of an optimal media that contains fewer nutrients initially within the media mix, yet provides adequate plant growth.