, Professor Emeritus, Civil, Environmental and Geo-Engineering
Vehicular traffic contributes a large fraction of the pollutant load in stormwater runoff from roadways. While runoff concentrations have historically been characterized for urban roads with high average daily traffic (ADT), the runoff quality from paved rural roads that have relatively low ADT is largely unknown. In this study, runoff from low-volume roads (ADT < 1500) in Minnesota was monitored at 10 locations during 174 rainfall events in 2018 and 2019. The initial concentrations of total suspended solids (TSS), total phosphorus (TP), nitrate+nitrite, and heavy metals in the runoff, and the relationship between measured concentrations and site-specific conditions were analyzed. Concentrations were strongly influenced by the surrounding land use and soil type. Sites with agricultural lands had higher mean TSS, TP, and zinc concentrations, and lower nitrite+nitrate concentrations than wooded sites, which can be related to the type of soil that would get transported onto the roadways. When compared to existing urban runoff quality data, the estimated event mean concentrations (EMCs) in rural road runoff were substantially lower for copper and zinc and marginally lower for TSS, TP, and nitrate+nitrite. Based on detailed cost-benefit analysis of various roadside treatment options, roadside drainage ditches/swales are recommended for cost-effective treatment of runoff from low-volume roads over ponds, sand filters, and infiltration basins. Example road-widening projects were also modeled to determine how stormwater management requirements can be achieved using drainage ditches/swales.