Spatiotemporal complexity of soluble reactive phosphorus behaviour within cold climate field-scale bioretention systems

Bioretention systems are widely implemented to improve urban runoff quality, yet the fate of phosphorus (P) within these systems remains poorly understood, particularly under field conditions in cold climates. This study investigated the spatiotemporal behaviour of soluble reactive phosphorus (SRP) in two near-replicate field-scale bioretention systems, including seasonal dynamics under cold-climate conditions. Influent, effluent, and porewater samples collected across multiple depths and locations within the bioretention media were analysed for SRP and key geochemical parameters influencing SRP retention and release. For both systems, effluent and porewater SRP concentrations significantly exceeded influent concentrations, with sediment P data indicating the media may be an internal SRP source. Despite similar patterns in SRP concentrations at the system-scale, the two systems exhibited distinct internal porewater SRP distributions, with non-monotonic vertical patterns revealing spatially discrete zones of SRP retention and release. Associations between dissolved metals (Fe, Mn, Al) and SRP varied across locations with data showing that metal-associated controls alone do not explain the observed heterogeneity. Seasonal effects on porewater and effluent SRP were pronounced despite no significant seasonal change in influent SRP, with the highest concentrations over the monitoring period observed in spring. These elevated SRP concentrations coincided with elevated Cl, suggesting de-icing salts may have been a contributing factor to SRP mobilization. Overall, the results highlight the need to move beyond black-box influent-effluent assessment to identify internal SRP release zones. This process-based understanding can inform improved bioretention design and operation, particularly where design modifications or real-time control are used to manage hydrologic and geochemical conditions for enhanced pollutant removal.