The capacity of greening roof to reduce stormwater runoff and pollution

To examine the stormwater retention capacity and runoff water quality of a green roof in Chongqing, China, 19 rainfall event samples of runoff and dry and wet deposition were measured. The results showed that the green roof effectively retained stormwater runoff, with retention ranging from 35.5% to 100%, with an average retention of 77.2%. The annual runoff retention of the green roof showed that the retention volume and rate reached 758.7 mm and 68.0%, respectively. When we compared the stromwater quality among the green roof, asphalt (control) roof, dry and wet deposition and rainfall samples, found that the green roof reduced the concentration of TSS, and could neutralize the pH of rainfall; however, it increased the concentrations of TN, NH4+-N, NO3−-N, TOC, COD, BOD5, F−, Cl−, SO42−, K+, Ca2+, Si4+, DPb, DAl, DMn, and DFe. When we compared the pollutant loads from the green roof and rainfall samples, found that the green roof was a sink for NH4+-N, but was sources of NO3−-N, K+, Si4+, Ca2+, TOC and DAl. Overall, the green roof was effective in reducing stormwater runoff, neutralizing acid deposition and abating the loading of most pollutants; however, the water quality of runoff was seriously affected by the soil substrate. Therefore, we suggest that green roofs need to be constructed of suitable materials in order to avoid deterioration of runoff water quality.