Field-derived phosphorus accumulation rates and solid phase speciation in the filter medium of an urban bioretention...

Bioretention cells are a popular type of green stormwater management infrastructure. In urban catchments, they reduce the 'flashiness' and volume of surface flow resulting from storms by promoting storage, groundwater infiltration, and evapotranspiration. Although they can also remove pollutants from stormwater runoff, their effectiveness in reducing phosphorus (P) loads is highly variable, in part because of the complex biogeochemical cycling of P in bioretention cell systems. Using time series solid-phase P concentrations measured over a 9-year period on the filter medium from a multi-cell bioretention system in Mississauga, Ontario, Canada, we estimated average total P (TP) accumulation rates of 88 mg P kg⁻¹ y⁻¹ in the top 10 cm and 33 mg P kg⁻¹ y⁻¹ at depths of 25-45 cm. These accumulation rates translate into a net sequestration of TP by the filter medium across the entire system of roughly 4 kg per year. The linear TP concentration versus time trends further imply that the filter medium has yet to reach P saturation. The system can, therefore, be expected to continue reducing stormwater runoff P loads in the foreseeable future. On cores collected in 2019 and 2021 (7 and 9 years post-construction), the depth-dependent solid-phase P chemical speciation was analyzed using an established sequential chemical extraction protocol. The more reactive P fractions and organic P compounds were generally enriched in the upper 10 cm of the filter medium. Accumulation of P associated with organic matter contributed most to the net TP accumulation on the filter medium. Non-metric dimensional scaling (NMDS) further indicated that variations in the concentrations of TP and the P fractions among filter media samples were mainly linked to variations in the concentrations of the redox-sensitive metals iron and manganese and that of organic carbon. Overall, our study highlights that the spatial and temporal distributions and chemical speciation of P can help unravel the mechanisms controlling P abatement of urban stormwater by bioretention systems.