Are green roofs a source or sink of runoff pollutants?

Abstract

Retention of rainwater runoff is a primary function of green roofs in their contribution as one of the main indicators of “Sponge City”. However, the influence of green roofs on runoff quality and whether they are a source or sink of pollutants is ambiguous. A concept model of pollutant cycling is proposed in this paper, based on a systematic literature review, which includes the direct factors substrate and structure of the green roof system; and indirect factors such as plant species, irrigation, fertilizer, atmospheric deposition, and age of the green roof. In addition, the model identifies each factor as a pollutant source or sink and explores relations among the factors. Examples in the literature were drawn to demonstrate the factors and pollutant materials including metallic elements such as Zn, Pb, Cd, Fe, Mn, Cr, Cu, and Ca; and nutrients such as N, NO3−-N, NH4-N, DON, P, PO43−-P, K, and DOC. The concept of “moderate growth vigor” is proposed; moderate growth is optimal for runoff quality and low and persistent fertility, and a low benchmark to control diseases and insect pests. The concept of “clean sources and pollutant retention” should be adopted, i.e., pollutant sources such as substrate, irrigation water, fertilizer, pesticide, and other local pollution sources should be removed or cleaned, and pollutants should be detained or decreased by plant degradation, slow-release material, filter layer, and actinic and microbial degradation. These results will aid understanding of the mechanism of runoff pollution and improve stormwater runoff quality from source to sink in green roofs.