Runoff Quality from Paved Low-Volume Roads

Vehicular traffic contributes a large fraction of the pollutant load in stormwater runoff from roadways. Although 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<2,000) in the US state of Minnesota was monitored at 10 locations during 172 rainfall events. The initial concentrations of total suspended solids (TSS), total phosphorus (TP), nitrite+nitrate, and metals in the runoff, along with 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 significantly higher mean TP and zinc concentrations and lower nitrite+nitrate concentrations than wooded sites, which can also be related to the type of soil that would get transported onto the roadways. Only TSS showed a statistically significant relationship with total rainfall depth. TP and total copper showed weak but statistically significant correlation with antecedent dry period (ADP). ADT of the monitored sites, however, was not a significant explanatory variable for the measured initial concentrations. When compared with 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 nitrite+nitrate. In rural settings, local sediment sources, agricultural activities, and other nontraffic sources can impact runoff constituent concentrations.