Removal,by Vegetated Biofilter,of Medium and Low Concentrations of Pollutants from Simulated Highway Runoff

The removal of pollutants by a prototype of a vegetated biofilter was investigated at medium and low concentrations. The biofilter was 4 ft (1.2 m) wide by 14 ft (4.3 m) long and was tilted at slopes of 8:1, 4:1, and 2:1. Artificial runoff, formulated with metals, native soil, and oil at medium concentration, was delivered at a rate equivalent to a simulated 2-year storm event (medium flow) at each slope; an additional experiment using a 10-year storm event (high flow) was conducted at the 2:1 slope. The flow rates at the low concentration represented 10-year storm events. During each simulated storm event, samples were obtained from the inlet, surface runoff, and underdrain and analyzed for total and dissolved metals, total suspended solids (TSS), and oil and grease. Before and after all tests, specimens were extracted from the bed and analyzed for metal content in soil, roots, and grass. Results indicated that all constituents (seven total metals and TSS) were removed at levels above 75% (event mean concentration) for medium-concentration influent. Metals above background levels were found primarily in the first half [7 ft (2.1 m)] of the bed. Oil removal was also achieved. Removals at low concentration were lower and erratic, and residual metal concentrations in the bed were near background levels. Soil particles in the influent flow, tagged with lanthanum, were neither resuspended nor measured at any significant concentration in the outlet surface flow.