Abstract
The control and treatment of highway-pavement storm water at the edge of the highway shoulder pose unique challenges due to the unsteady nature of processes, including rainfall runoff, mobilization and partitioning of heavy metals, variations in storm-water chemistry, residence time on the pavement, and delivery of particulate mass. Presented are heavy-metal partitioning results as influenced by pavement-runoff chemistry and hydrologic parameters from a series of eight rainfall-runoff events over a 2-year period. Water-quality characteristics such as low alkalinity, low hardness, and short pavement residence times cause a majority of the heavy-metal mass to remain in solution at the edge of the pavement, with partitioning coefficients approaching equilibrium conditions only toward the end of the event, as heavy metals partition to entrained solids. There are two primary implications when considering the application of typical best management practices (BMPs) for highway runoff within the right-of-way. The first implication is to use a BMP such as a detention basin or roadside swale to detain runoff and produce sufficient residence time so that partitioning to the entrained solids occurs. The second implication is to use a BMP such as an engineered infiltration trench to provide surface complexation for dissolved metals and filtration mechanisms for the particulate-bound metals. Although no simple solutions exist for the removal of a heavy metal or particle once it is released in the highway environment, knowledge of the dynamic processes in highway runoff can provide insights for the proper selection of BMPs, depending on the conditions at the highway site. A design should be based on the concept that BMPs, to be effective, are essentially garbage cans for heavy metals and solids and as such must be emptied and maintained.
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