FATE OF HEAVY METALS FROM HIGHWAY RUNOFF IN STORMWATER MANAGEMENT SYSTEMS (GROUNDWATER, SPECIATION, RETENTION, SWALES, SEDIMENTS)

The movement and fate of heavy metal inputs (Cd, Zn, Mn, Cu, Al, Fe, Pb, Ni and Cr) from highway runoff were investigated in a three-year study on a 1.3 hectare retention facility near the Maitland Interchange on Interstate 4, north of Orlando, Florida. Stormwater samples were collected over a one-year period, representing a wide range of intensities and antecedent dry periods. A total of 138 core samples were collected in the pond over a three-year period to investigate the horizontal and vertical migrations of heavy metals within the pond. Sediment core samples were also carried through a series of sequential extraction procedures to examine the type of chemical associations and stability of each metal in the sediments. An apparatus was built which allowed sediments to be incubated under various conditions of redox potential and pH to investigate the effects of changes in sediment conditions on the stability of metal-sediment associations. Five groundwater monitoring wells were installed to monitor metal movement and accumulations under stormwater management systems.; Upon entering the retention pond, most metal species settled into the sediments within 60-90 m of the inlet. Removal efficiencies for metals averaged 70-90 percent for particulate species and about 50 percent for dissolved species. Sediment concentrations of heavy metals were highest near the surface, with rapidly decreasing concentrations with increasing depth. Metal-sediment associations appear to be very strong for most metals, with the vast majority of metal inputs into the pond over the eight-year life still remaining in the top 10 cm.; Concentrations of all heavy metals measured were higher in groundwaters beneath the pond than in the pond water; but for most metals, the increases only extended to depths of 1-3 m beneath the pond. In general, metal concentrations beneath swale areas were significantly higher than concentrations beneath the retention pond. Due to slow groundwater movement in the area, the effects of increased metal concentrations were very localized. Evidence was presented to suggest that the mobilization of metals into groundwaters could substantially increase with time if maintenance procedures are not conducted.