Chemical leaching behaviour of a full-scale green roof in a cold and semi-arid climate

Abstract

Despite the benefits of green roofs in managing urban stormwater quantity and quality, a number of studies have demonstrated that green roofs can pose negative impacts on the urban environment due to chemical leaching, in particular in their early age. Besides design variables such as growing media composition and depth, the roof age and hydro-meteorological variables are also expected to affect or govern the temporal evolution of chemical leaching. To characterize the chemical leaching behavior of green roofs and explore possible modeling approaches, a full-scale extensive green roof and a reference roof, which are located in a cold and semi-arid climate region, were monitored in both rain and snowmelt events in 2015–2018. The temporal evolution of chemical leaching was examined at both intra- and inter-annual time scales. The roles of hydro-meteorological variables including the growing media temperature (GMT), moisture condition (antecedent moisture condition (AMC) for rain events, and moisture condition (MC) for snowmelt), event rainfall amount (Ra), and cumulative inflow/precipitation amount on chemical leaching were investigated. The field observations demonstrated the leaching of nutrients (i.e., both nitrogen (N) and phosphorous (P)) and conductivity from the green roof, whereas the roof acted as a sink for metals (Zn, Cu, and Pb). The leaching of N appeared to cease, whereas P leaching was still ongoing at the end of the study period. Although the degree of nutrient leaching was not significantly different between the rain and snowmelt events, the nutrient leaching in rain events appeared to be relatively higher in the spring than in the summer and fall. Furthermore, the leaching of nutrients was found to decline annually, but at different rates for different nutrients. Among the investigated hydro-meteorological variables, the cumulative inflow was identified influencing the temporal evolution of chemical leaching considerably. In addition, the results from two modeling approaches, namely multiple linear regression modeling and semi-physically based modeling adapted from the pollutant wash-off concept for urban stormwater runoff, confirmed the critical role of the cumulative inflow on chemical leaching. Besides the cumulative inflow, the GMT was the other important explanatory variable for P leaching. The results suggested that chemical leaching from green roofs in semi-arid regions, where precipitation and moisture level are low, could persist longer but at a lower degree compared to that in mild and temperate climate regions.