Anthropogenic constituents in highway runoff include metal elements and suspended solids which result from traffic activities, atmospheric deposition, roadway degradation and highway maintenance. A best management practice (BMP) for immobilizing heavy metals and suspended solids is a partial exfiltration trench (PET). A PET is designed to exfiltrate a percentage of infiltrated runoff to subsoil with the balance discharged through an underdrain. The PET functions through adsorptive-filtration, where dissolved metals are immobilized through sorption and metals associated with suspended solids are immobilized through filtration. An important PET design consideration is potential association of heavy metals with suspended solids. This paper investigates the correlation between heavy metals and suspended solids in highway runoff. Results indicate a strong positive correlation between heavy metal and suspended solid concentrations for snow washoff events and a poor correlation for rainfall-runoff events. Similar results are observed for correlations between metals and suspended particle sizes. From these findings, it is argued that a PET holds promise as a device for immobilizing metals associated with suspended solids transported during snow washoff and long duration rainfall events.

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