How well do stormwater green infrastructure respond to changing climatic conditions?

Urban green infrastructure (GI) such as biofiltration systems (also known as bioretention, biofilters, rain gardens) are being increasingly implemented in different parts of the world to enhance urban greening whilst controlling stormwater pollution. Treatment effectiveness has been shown to be reliant upon the surrounding climate, including rainfall patterns (e.g. length of wet and dry weather) and temperature. Plant species play a key role in treatment, yet, the response of different plant species to pollutant removal under different climatic and environmental conditions is still not fully known. This information is pertinent to ensure sustained biofilter performance under a range of conditions. This study investigates the change in biofilter nutrient and E. coli removal performance in the presence of four plant species (Canna indica, Carex appressa, Ginkgo biloba and Miscanthus sinensis) with distinct above and below ground characteristics, when exposed to wet weather, dry weather and cold weather conditions. The extent of the decline in nutrient treatment performance following extended drying and under cold conditions was found to be species-specific. Miscanthus sinensis was the best performer and was also found to be cold-resistant. With the exception of Miscanthus sinensis, all plant species were more impacted by the cold temperature (decrease from 70% to 45% TN removal on average; <1 to 30% decrease across species) compared to 4-weeks drying (decrease from 60% to 54% TN removal on average; <1–16% decrease across species) in systems equipped with a submerged zone. In contrast, the 4-weeks drying (E. coli decrease from 2.60 to 1.08 log removal on average) had a more pronounced effect on E. coli removal performance compared to the cold conditions (E. coli decrease from 1.70 to 1.38 log removal on average). This study shows that plant species relative contribution to pollutant removal is pollutant-specific and varies under different climatic conditions. A mix of plant species may ensure higher level of system resilience under a variable climate. The study also highlights the need to understand more about the microbial ecology of these plant systems to optimise both nutrient and pathogen removal for sustained long term performance.