Characterization of Metals and Solids in Urban Highway Winter Snow and Spring Rainfall-Runoff

Abstract

Urban highways are a nonpoint source of metal elements and solids. These constituents are generated from traffic activities, pavement degradation, roadway maintenance, and atmospheric deposition. Pavement degradation and abrasion generates solids ranging in size from submicron particles to gravel-size aggregates. These constituents accumulate on the highway until they are transported from the pavement by traffic-induced turbulence or precipitation-runoff processes. Total metal elements and solids concentration are higher in snow washoff from urban highways than rainfall runoff. One reason is the snow washoff volume is less than the volume generated during a rainfall-runoff event. Another reason is that the residence time of a highway snowbank can range from hours to months depending on weather conditions. In addition, porous snow banks act as repositories that trap metals and solids. Metal elements and solids in urban highway winter snow and spring runoff at one highway site located along I-75 in Cincinnati, Ohio, are described and compared. Both snow and rainfallrunoff samples were fractionated into particulate-bound and dissolved metals. Solids characterization included dissolved, inorganic, and organic fractions. Results from snow samples taken at daily intervals from late January through mid-February 1995 indicate a significant increase in metal elements and solids accumulation for several days after two snow events. Results from runoff events in April 1995 indicate that the particulate-bound metal element washoff response was a function of rainfall intensity. In contrast, the dissolved response exhibited a dependence on the solubility of the metal element. Metal elements in rainfall-runoff were predominately dissolved as compared with more particulate-bound metals in snowfall.