Characterization and Pollutant Loading Estimation for Highway Runoff

Abstract

Three highway segments typical of urban, semiurban, and rural settings in the Piedmont region of North Carolina were monitored to characterize the respective runoff constituent concentrations and pollutant discharge or export loadings. Runoff from the impervious bridge deck (Site I) carried total suspended solids (TSSs) concentrations and loadings that are relatively higher than typical urban highways, whereas nitrogen and phosphorus loadings are similar to agricultural runoff. Site II included a pervious roadside shoulder with traffic volume equal to that of Site I. Site III was a nonurban highway having lower traffic counts and imperviousness due to the presence of a roadside median. The existing roadside shoulder and median appeared to attain at least 10–20% hydrologic attenuation of peak runoff discharges, more than 60% reduction of event mean concentration of TSSs, and attenuation of the first-flush concentrations for most pollutant constituents. Bulk precipitation data collected at the bridge deck site indicated that 20% of TSS loadings, 70–90% of nitrogen loadings, and 10–50% of other constituent exports from the roadway corridors might have originated from atmospheric deposition during dry and wet weather conditions. The long-term highway pollutant loadings have been derived to provide a basis for comparing highway runoff with other categories of nonpoint sources (NPSs).