The fate of heavy metals in highway stormwater runoff: The characterization of a bioretention basin in the Midwest

The usual wear of automobiles and road surfaces deposits numerous environmental pollutants on roadways and parking lots, including heavy metals such as copper, zinc, lead, nickel and cadmium. During rainfall and snow events, these metals are washed from the paved surface in stormwater runoff or plowed onto shoulders where they are trapped in snow and ice. The metal contaminants carried in the stormwater runoff and eventual snowmelt often enter nearby surface waters and contribute to their degradation as well as threaten the survival of aquatic organisms at all levels of the food chain. This pathway of heavy metal contamination has become a significant concern due to increased urbanization and roadway construction.;Bioretention is a best management practice used to remove heavy metals and other pollutants from stormwater runoff and snowmelt that utilizes the inherent physical and chemical properties of soil compounds. As runoff passes through a bioretention basin, heavy metals are deposited in mulch and soil layers through processes including sedimentation and filtration as well as adsorption and complexation to soil compounds such as organic matter, clay and metal oxides. Numerous factors affect the retention of these metals, including the use of deicing salts. Deicing salts in highway runoff are thought to hinder heavy metal removal by interfering in the chemical attraction between metals and soil compounds.;Investigations, including a pilot column study and field experiments at an existing bioretention basin in Johnson County, Kansas, documented the effectiveness of bioretention in removing elevated levels of heavy metals, particularly copper, from highway stormwater runoff in the Great Plains region. Total suspended solids (TSS) and dissolved and total metal concentrations were analyzed in influent and effluent runoff samples from the field bioretention basin. The column study included tracer and retention tests that investigated water and dissolved copper retention, respectively, in two columns.;Removal rates of particulate-associated metals (based on concentration) from runoff samples were 90 percent or greater for lead, nickel, copper, cadmium and zinc. From March to November 2007, the bioretention basin removed an average of 33 percent of total copper and 47 percent of dissolved copper from runoff samples. The TSS removal rate was 80 percent. Short-term dissolved copper retention was achieved in the laboratory column study. The two columns retained 91 and 94 percent of dissolved copper from the influent solution during the final tests.