Field Evaluation of Storm-Water Control Measures for Highway Runoff Treatment

The thousands of kilometers of highways in North Carolina have the potential to generate large amounts of storm-water runoff. Thus, investigation of storm-water control measures (SCMs) for these somewhat unique linear catchments, where space is limited for SCM implementation, was needed. This study examined the quantity and quality of highway runoff at four sites over a 48-km stretch of Interstate 40 in the coastal plain of North Carolina. The highway had a 4-cm overlay of permeable asphalt, known as permeable friction course (PFC), which influenced the export of sediment-bound pollutants and produced median effluent concentrations of total suspended solids (TSS) of 8 mg/L , 8 mg/L , 9 mg/L , and 17 mg/L at the four sites, well below concentrations observed from standard asphalt highway runoff. Two vegetative filter strips (VFSs), two traditional dry swales, and two wetland swales were also tested for pollutant removal efficacy at the four highway research sites. The filter strips generally produced higher pollutant concentrations than the edge of pavement due to a lack of vegetative cover, substantial soil compaction, and high slopes. Total nitrogen (TN) median effluent concentrations were significantly lower for wetland swales ( 1.02 mg/L and 1.03 mg/L ) than for dry swales ( 1.50 mg/L and 1.63 mg/L ). TN load export appeared lower from wetland swales when compared with dry swales, potentially due to the greater number of nutrient removal mechanisms that are present in wetland swales. Maintenance of highway SCMs was shown to be extremely important, as one of the dry swales produced an effluent TSS concentration of 70 mg/L due to a head cut in the thalweg of the channel. As noted in other scientific literature, vegetative SCMs did not further reduce pollutant concentrations released by the PFC because TSS and sediment-bound pollutant concentrations seemingly were at or near irreducible concentrations. This study showed that incorporating wetland elements in the design of swales and the use of PFC should be considered by roadway designers.