Road salt induced mobilization and accumulation of heavy metals in roadside bioretention in the field: the roles of season, plant uptake, media, and catchment properties

Bioretention systems can effectively remove heavy metals from urban stormwater runoff. However, seasonal dynamics of metal accumulation and the role of road salt in metal mobilization from bioretention media under field conditions remain poorly understood. Moreover, limited research has examined metal accumulation in plants in bioretention systems in field. This study investigated seasonal variation in metal (Cadmium, Chromium, Copper, Nickel, Lead, and Zinc) and major cation (Sodium, Magnesium, Calcium, and Potassium) contents and metal accumulation in leaf tissue of 14 plant species across 19 field-scale bioretention sites in Toronto, Canada. Media and catchment characteristics influencing metal retention in bioretention systems were examined. All metals were positively correlated with electrical conductivity and exchangeable sodium percentage; Copper, Lead, and Zinc showed positive correlations with chloride, indicating mobilization by ion exchange and chloride complexation. Metals and major cation contents (dry weight basis) were 2-26% lower in summer compared with winter, except for Zinc and Calcium, indicating mobilization with potential risks to freshwater degradation. Media with higher organic matter and fine sediments retained more metals. Sites receiving runoff from high-traffic roads had elevated Chromium and Nickel contents, while those with large directly connected impervious areas had higher Copper, Lead, and Zinc. Older systems (∼10 operational years) retained higher metals, with Copper, Zinc, and Chromium exceeding environmental toxicity thresholds, particularly in the inlet and middle zones at some sites. Therefore, bioretention systems older than 10 years and those draining large impervious areas, high-traffic roads will likely require more frequent maintenance, which can be done through periodic removal of the top 2-5 cm of accumulated surface sediment. Plant species from the Asteraceae family accumulated a wide range of metals at potentially phytotoxic levels. These species may aid phytoremediation in systems receiving high metal loads, though seasonal pruning may be required to prevent release back into media.