During rainfall/snowfall runoff events, constituents are conveyed from highway surfaces, which may degrade receiving water quality. Utilization of a natural wetland to mitigate storm water constituents from a portion of a 30,000-average-daily-traffic highway was studied. The runoff area consisted of two innermost lanes of a four-lane highway draining to a median drain via a vegetated median; a conduit conveyed this runoff into the wetland. Distributed runoff from the outermost lane along the wetland flowed across a vegetated highway embankment into the length of the wetland. During 57 rainfall-runoff events, in situ data collection included rainfall volume and frequency, conduit flow rate, and temperature, conductivity, pH, and dissolved oxygen. During each event, sequential samples were obtained and analyzed for metals, total suspended solids, and chemical oxygen demand from the wetland inlet and outlet. Flow-proportioned composite samples of sheetflow from the highway pavement edge were analyzed along with rainfall samples. Sediment samples were analyzed for metal constituents and organic material. Heavy metal concentrations in the storm water runoff consisted primarily of iron, zinc, lead, nickel, and copper. Concentration of metals was reduced via flow through the grassy median and then the wetland system. Over a 1-year study, average removals of heavy metals were 45% to 67%. First-order reaction rate constants were estimated to be 2.8, 3.7, and 6 per day for iron, nickel, and zinc, respectively. Metals in bed sediments were representative of the hierarchy of concentrations analyzed in the runoff, and concentrations tended to increase with sediment organic fraction.
Abstract