Using a Wetland to Mitigate Highway Storm Water Runoff

Highway traffic, maintenance, natural background sources, and deposition of air pollutants contribute to buildup of contaminants on highway surfaces. Consequently, during a rainfall-runoff event highways can produce an exorbitant per area loading of contaminants, which can subsequently degrade receiving water quality. The capacity of a wetland to mitigate pollutants in highway runoff, particularly metals, was investigated. Storm water runoff from a portion of a 30,000 average daily traffic highway was analyzed during 57 rainfall-runoff events. The runoff area consisted of two inner-most lanes of a four-lane highway composed of 1189 m² of asphalt pavement, draining to 279 m² of vegetated median, and then collected by median drain and discharged into a 557 m² wetland. In addition, a distributed input from 673 m² of pavement flowed across a vegetated highway embankment of 184 m² along the length of the wetland. In-situ data collection included rainfall volume and frequency, conduit flow rate, and temperature, conductivity, pH and dissolved oxygen in the runoff. During each rainfall-runoff event sequential samples were obtained and analyzed for metals, total suspended solids, and chemical oxygen demand from the point source inlet and wetland outlet. Flow-proportioned composite samples of sheetflow from the highway pavement edge were collected and analyzed along with rainfall samples. Sediment samples were obtained longitudinally from the wetland inlet to the outlet and analyzed for metal constituents.