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.
Khaniya B, Szota C, Fletcher TD, Drake J
Phytoremediation
That rain garden strip between the parking lot and the street is quietly accumulating lead, copper, and zinc from passing cars — and winter road salt is pushing those metals back out into nearby waterways every cold season.
Scientists studied 19 real-world rain gardens in Toronto and found that the salt spread on roads in winter shakes heavy metals loose from the soil in these gardens, sending them potentially into streams and lakes. Gardens older than about 10 years and those next to busy roads had the most metals, sometimes at levels that could harm plants or wildlife. Some plants in the daisy family were actually soaking up those metals — which could be useful for cleaning contaminated sites, but gardeners would need to trim them regularly so the metals don't cycle back into the soil when the plants die back.
Key Findings
Metal concentrations in bioretention media were 2–26% lower in summer than winter, pointing to road salt as a seasonal driver that flushes heavy metals out of the soil and toward waterways.
Systems in operation for roughly 10 years had copper, zinc, and chromium levels exceeding environmental toxicity thresholds, especially near inlet zones — suggesting periodic removal of the top 2–5 cm of surface sediment as a maintenance practice.
Plant species from the Asteraceae (daisy) family accumulated metals at potentially phytotoxic levels across 14 species studied, identifying them as candidates for phytoremediation in high-metal urban rain gardens.
chevron_right Technical Summary
Road salt used for winter de-icing flushes heavy metals out of bioretention gardens and rain gardens in cities, raising contamination risks — especially in older systems and those near busy roads. Plants in the daisy family (Asteraceae) absorb a wide range of these metals and could help clean the soil, but need seasonal pruning so they don't release metals back when they decompose.
Abstract Preview
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 bioreten...
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