ABSTRACT
Bioretention systems are popular low impact development stormwater management features designed to remove pollutants, including phosphorus (P), from urban stormwater runoff. While the performance of bioretention systems in retaining P has been well studied, seasonal variability of P retention in field-scale systems installed in cold climates, including the influence of high road de-icing salt (sodium chloride) inputs, remains unclear. Two large field-scale bioretention systems installed in London, Ontario, Canada were monitored over their initial operational period to evaluate the seasonal trends in the retention of different forms of P in bioretention systems and the impact of high salt loading. Over the 12-month monitoring period, a net retention of total P and dissolved organic P, and a net release of soluble reactive P and total dissolved P mass were observed. Reduced hydrological performance and increased effluent P concentrations resulted in high P release from the bioretention systems in early to mid-spring (March and April), with most release occurring during a few individual large precipitation events. Laboratory-scale column experiments were performed using the engineered soil media installed in the field-scale bioretention systems to isolate the effect of high salt loading on P release. Column experiments combined with field data indicate that prolonged high salt loads through winter and spring may have contributed to elevated spring P release, mostly in the form of soluble reactive P, from the field-scale bioretention systems. Findings from this study are needed to better understand the performance of bioretention systems with respect to P retention as required to improve urban stormwater management in cold climates. Results have implications for further investigations of the impact of road salt on P mobility in bioretention systems and more broadly in roadside soils and groundwater systems.
