The impact of green roofs on urban runoff quality: A review

Green roofs are building-integrated greenery systems that provide many ecological and economic benefits to urban environments. Green roofs are an important component of sustainable drainage systems that can potentially improve the quality of urban runoff. Previous research on the environmental impact of leachate from green roofs on receiving waters have generated unfavourable views on the use of these systems. The sink effect of P, N and heavy metals is ambiguous. The benchmarks used to study source-sink effect are particularly disparate, with leachate from green roofs being compared by some authors to leachate from conventional soils and by others to rainwater. Finally, the use or not of fertilizers during the studies varies the result. This dispersion leads to misinterpretations of the results and conclusions, compromising the future use of green roofs. The articles reviewed and detailed has been ordered according to the possible impacts, comparing results and conclusions. The review confirms the source effect for P from 0.03 to 3.86 mg/l and the source effect for N from 0.4 to 8.3 mg/ l. A mixed source-sink effect for P and N can be concluded when fertilizers are not applied and using additional materials, with reductions from 17,4 to 8,4 mg/L in P concentrations and reductions from 14% to 87% in N concentrations. Nutrient leaching can be controlled by appropriate mitigation measures, better system design and management. The source-sink effect for heavy metals, is determined with compared with the rainwater and runoff conventional roofs. The proportions of heavy metals from the green roofs runoff mostly depend on the substrate type. The heavy metals most referenced from the green roof runoff are Pb, Cd, Zn, Fe, Cu and Cr. Finally, a clear consensus is observed on the fact that green roofs mitigate the damage caused by acid rain by increasing pH usually around 8.0.