Emerging investigator series: the role of vegetation in bioretention for stormwater treatment in the built environment: pollutant removal, hydrologic function, and ancillary benefits†
Vegetation influences both the hydrologic and pollutant-removal performance of bioretention cells for green infrastructure stormwater management in the built environment. Vegetation can intercept rainfall, lessen erosive sheetflow, ameliorate bioretention soil media clogging to maintain infiltration capacity, and decrease total stormwater volume through transpiration. Plants influence multiple pollutant removal processes, including phytoextraction, in planta phytotransformation, and alteration of the rhizosphere and associated microbial community. We present the current state of knowledge of vegetative influence on pollutant-removal performance and mechanisms, including for total suspended solids, nitrogen, phosphorus, toxic metals, hydrocarbons, pathogens, and emerging contaminants in urban stormwater. Additional benefits and opportunities for vegetation in bioretention include improved aesthetics of stormwater infrastructure, lessened irrigation/fertilizer demand, provision of urban micro-habitats, thermal attenuation, public education, increased resilience for climate change adaptation, and the potential for air quality improvement as well as biomass and/or food production. We describe plant traits and species that improve pollutant removal and hydrologic function, such as plant biomass and growth rate. We identify key areas of future research need, including a focus on transferrable findings/mechanistic studies, a better understanding of root system/rhizosphere impacts, quantification of the impact of plant shoot harvesting, and further study of emerging organic contaminants and metals. We conclude that vegetation in bioretention systems produces measurable water quality and hydrologic performance benefits, but that plant processes could be substantially further researched and developed to improve stormwater systems.