Removal Efficiencies of a Bioretention System for Trace Metals, PCBs, PAHs, and Dioxins in a Semiarid Environment

The negative effects of urbanization on water quantity and quality and the performance of bioretention for treating runoff and reducing concentrations and loads of sediment, nutrients, pathogens, and selected trace metals have been well documented. This study evaluated the efficiency of a bioretention system installed in semiarid Daly City, California, for a large suite of pollutants, including some that have been rarely evaluated in such systems. Stormwater flow and pollutant concentrations were measured before and after the construction of a bioretention system consisting of rain gardens and a bioswale. After installation, concentrations of most pollutants were reduced and met water quality guidelines. Water quality improvements (concentration or load reductions) were demonstrated for total Hg, polychlorinated biphenols (PCBs) and dioxins ranging between 18 and 100%, and a suite of more conventional pollutants [SSC, Cd, Cu, Ni, Pb, Zn, and polycyclic aromatic hydrocarbons (PAHs)] ranging between 20 and 90%. Results were less favorable for methylmercury. The balance of evidence suggests that such systems, when designed and functioning correctly, can be highly effective in treating pollutant concentrations and loads in stormwater runoff in semiarid environments.