Partitioning of metals in urban drainage from paved source area catchments

Representation of metal partitioning in urban drainage is required when determining loadings, treatment, maintenance and regulatory compliance. This is particularly the case for paved urban source areas where, if a first-flush exists, the design volumetric capture and treatment will be based on such regulatory requirements. However, such treatment for metals requires partitioning information if metals are to be effectively separated from flows from paved source areas. This study examined partitioning and transport of metals from paved surfaces of four differing land uses: highway, airside and landside within aviation site, commercial/tourism terminals within port area. Equilibrium concentrations of metals and particulate matter (PM) using a non-parametric analysis for a series of six paved surface area catchments were compared. In particular two American highway sites, and four Italian sites located in the Liguria Region (two port terminal sites and two aviation sites) were examined and compared with event based concentrations collected from catchments with similar land uses. The role of hydrology on mass transport and partitioning of aqueous and particulate-bound metal fractions was evaluated. Finally transport of metal phases on an event basis, thus relating hydrology and PM mass delivery impacting metal partitioning were examined. Partitioning, indexed through a partitioning coefficient, Kd (L/kg) accounts for hydrologic transport, PM transport and interactions with and between metals. Kd can vary by orders of magnitude across a runoff event for the source area catchments of this study. Highway sites were more heavily loaded with PM and metals than airport sites. The metals results from the six sites of this study were compared to similar sites around the world. Based on the variability of partitioning, whether intra- or inter-event, results indicate that paved source area treatment requires a combination of sedimentation, filtration and adsorption mechanisms for in-situ unit operations and processes (UOP).