The impacts of substrate and vegetation on stormwater runoff quality from extensive green roofs

Green roofs are emerging as practical strategies to retain stormwater runoff and improve water environment quality of urban cities. However, the impacts of green roofs on stormwater runoff quality remains a topic of concern to urban planners and mangers. In this study, we designed the scale based runoff plots of extensive green roofs with different substrate and vegetation types, and investigate whether the studied green roofs act as a sink or source of nutrients and suspended solids in their runoff compared to a conventional roof. The influences of substrate and vegetation on the water quality of green roof outflows were also assessed. The results showed that the average TSS (total suspended solids), TN (total nitrogen), and TP (total phosphorus) concentrations of the green roof runoff were all significantly higher than that of the conventional roof runoff. Except substrate S3 (the composition ratio of rural soil:peat soil:perlite:vermiculite was 2.5:5:2:0.5) and S1 (the composition ratio of rural soil: peat soil was 1:1) in TP, the substrate S2 (the composition ratio of rural soil:peat soil:pine needle:perlite was 1:1:1:1) displayed significantly higher TN and TP concentrations than either of the runoff from the substrate S1 and S3. The TN and TP concentrations of 5 cm substrate depth were significantly lower than that in the 15 cm substrate depth treatments. Significant differences were observed in TN concentrations between vegetation Sedum Spectabile and Radix Ophiopogonis. However, no significant differences were observed between any of the vegetation types with regard to TP concentrations. Therefore, we suggest that green roofs need to be constructed of suitable substrate materials and plants based on their function on runoff water quality in order to mitigate the pollution risk of green roof runoff on urban water environment.