CHARACTERIZATION AND POLLUTANT LOADING ESTIMATION FOR HIGHWAY RUNOFF

Three highway segments typical of urban, semiurban, and rural settings in the Piedmont region of North Carolina were monitored to characteri ze the respective runoff constituent concentrations and pollutant disc harge 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 nitrog en and phosphorus loadings are similar to agricultural runoff Site II included a pervious roadside shoulder with traffic volume equal to tha t of Site I. Site III was a nonurban highway having lower traffic coun ts and imperviousness due to the presence of a roadside median. The ex isting roadside shoulder and median appeared to attain at lease 10-20% hydrologic attenuation of peak runoff discharges, more than 60% reduc tion of event mean concentration of TSSs, and attenuation of the first -flush concentrations for most pollutant constituents. Bulk precipitat ion data collected at the bridge deck site indicated that 20% of TSS l oadings, 70-90% of nitrogen loadings, and 10-50% of other constituent exports from the roadway corridors might have originated from atmosphe ric deposition during dry and wet weather conditions. The long-term hi ghway pollutant loadings have been derived to provide a basis for comp aring highway runoff with other categories of nonpoint sources (NPSs).