PubMed · 2026-05-15
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.
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.