Zhou, B., Shafii, M., Parsons, C., Passeport, E., Rezanezhad, F., Van Cappellen, P. (2022) Assessing the efficacy of bioretention cells for phosphorus and nitrogen load reduction in stormwater runoff: Interrogating the International Stormwater Best Management Practices Database. In Fall Meeting 2022. AGU. https://agu.confex.com/agu/fm22/meetingapp.cgi/Paper/1152243

Bioretention cells are a Low Impact Development (LID) technology widely promoted as a green solution to attenuate loads of phosphorus (P) and nitrogen (N) exported with urban stormwater runoff and, hence, mitigate downstream eutrophication. Despite their increasing implementation worldwide, reported removal of P and N by bioretention cells is highly variable. To evaluate P and N concentration and loading reduction performance, we conducted a comprehensive analysis, including calculation and analysis of seasonal and long-term trend of event-based reduction efficiency, of total P (TP), soluble reactive P (SRP), total N (TN), and dissolved inorganic N (DIN) concentration data, in concert with hydrologic records for 26 bioretention facilities in the International Stormwater Best Management Practice Database. We applied machine-learning methods, including random forest and decision tree modeling, to identify the strongest factors controlling nutrient removal in bioretention cells on event basis. These factors comprise watershed properties, climate conditions, hydrologic event characteristics, and design parameters. Results demonstrate that while SRP and DIN concentrations in surface outflow from bioretention cells often exceed inflow concentrations, their outflow loads are typically lower due to infiltration that reduces the surface outflow ostensibly by increasing groundwater infiltration. Moreover, because SRP tend to be removed less efficiently than DIN, bioretention cells on average decrease the DIN:SRP ratios of the outflow. The efficacy of bioretention cells for nutrient removal was found to be lower under lower age, lower inflow concentration and higher precipitation intensity. Overall, our findings demonstrate that, although bioretention cells typically decrease N and P loadings in surface runoff, their effectiveness for mitigating eutrophication in downstream waterbodies is less straightforward. We suggest that groundwater P and N contamination, local nutrient control criteria, limiting nutrient patterns in downstream surface water bodies, and regional climate (change) conditions should be considered when planning the implementation of bioretention cells for eutrophication control.

https://agu.confex.com/agu/fm22/meetingapp.cgi/Paper/1152243