Variability in the function of green infrastructure technologies: Lessons from science and practice

Background/Question/Methods: Green infrastructure is an emerging engineering approach used to address stormwater challenges in urban areas using distributed cost-effective technologies that can also provide sustainability, social, and economic benefits. Green infrastructure can decrease water inputs to sewers, wastewater facilities, and surface waters by increasing the available storage and loss of water across the urban landscape.

Results/Conclusions: We found from available literature and data that there is considerable variability in the retention of stormwater quantity and quality across green infrastructure technologies and across storm events. We developed a ternary tool to illustrate the pathways of water loss for different green infrastructure technologies. In general, bioretention cells (mean 90% retention) and green roofs (mean 73% retention), and to a lesser extent porous pavement (mean 57% retention) promote stormwater loss through a combination of enhanced evapotranspiration or infiltration. In contrast, other technologies are less effective at removing water (swales with mean 27% retention; detention ponds with mean 11% retention; media filters with mean 8% removal; retention ponds with mean -2.4% retention; wetlands with mean -11% retention), but accomplish stormwater management largely by allowing for additional storage to accommodate stormwater events. Yet, average retention performance differs more by site than by event within a given green infrastructure technology. We will also address how the performance of green infrastructure varies with the temporal scale of measurement and the magnitude of events. The effectiveness of green infrastructure in removing contaminants in stormwater depends on the contaminant of interest and the technology used. Virtually all green stormwater technologies are effective at removal of suspended solids. Bioretention cells, media filters, detention ponds and retention ponds retain modest quantities of total nitrogen and total phosphorus. In contrast, swales, porous pavement, wetlands and green roofs retain minimal quantities of these nutrients, with some systems releasing quantities to downstream water, likely due to the application of fertilizer to biological systems. We found little evidence of retention of chloride in stormwater structures, but generally trace metals, lead and cadmium, were effectively retained.