Metal Element and Particulate Relationships at the Upper End of an Urban Watershed: Investigations of Disproportionate Delivery and Control

While there is little debate regarding the generation of anthropogenic metal elements and particulates in urban rainfall-runoff, there is significant debate regarding the delivery and solid-phase distribution of metal elements. Debates regarding delivery concepts such as mass or concentration first-flush and regarding the distribution of metal elements mass and concentration across the particle gradation continue. Beyond these debates are the larger issues of non-structural vs. structural controls vs. source controls, the fate and management of such constituents and emerging concepts such mass trading and offsetting. This paper reviews concepts regarding the delivery of metal element and particulate mass; and distribution of both, since these issues are fundamental to control, fate, management and trading/offsetting.Urban storm water mobilizes a wide gradation of entrained anthropogenic solids that are capable mediating the equilibrium partitioning of metal elements resulting in a distribution of metal element mass across the gradation. For residence times, flow characteristics, infrastructure interactions, particle granulometry and rainfall-runoff chemistry that result in partitioning equilibration (1 - 12 hours), metals have the potential to partition across the particle gradation. In general results indicate that particle specific surface area (SSA) increased (by measurement and definition) with decreasing particle size for all gradations. In contrast, total particle surface area (SA) decreased with decreasing particle size. Particulate-bound concentrations (μg/g) of Zn, Cu, Cd and Pb increased (by measurement and definition) with decreasing particle size. Although the gradation of particles ranged from less than 1 μm to greater than 10,000 μm, most of the metal mass is associated with the sand-size fraction of particles. Results are useful inputs when considering structural treatment design as a function of design particle size, non-structural control, fate, TMDL development, stormwater management and trading/offsetting.With respect to delivery, this study investigates the phase-specific temporal delivery of Cd, Cu, Pb and Zn at a paved transportation land use site in Baton Rouge, LA as a function of hydrology. Events were characterized as either mass or flow-limited. Results indicate that mass-limited events generally exhibited similar but temporally disproportionate mass deliveries for Cd, Cu, Pb and Zn and an initial rapidly declining concentration profile. In contrast, flow-limited events produced mass deliveries that were strongly proportional to the hydrograph. In contrast to other sites of similar land use and, despite a site mean rainfall pH of 4.3, partitioning of Cd, Cu, Pb and Zn in runoff was predominately to the particulate fraction. Examination of rainfall chemistry indicated that wet atmospheric deposition accounted for less than 10% of Cd, Cu, Pb and Zn in runoff and metal concentrations in rainfall loadings were in general two orders of magnitude lower than in the corresponding runoff responses from the sites. Delivery results suggest that since metal mass is driven by the hydrograph for flow-limited events, and to a lesser extent by mass-limited events, in-situ capture and control should also be driven by hydrology. Partitioning and distribution results indicate that structural controls or non-structural practices should be based on particle control as a first (but not exclusive) consideration when considering control, fate, management and trading/offsetting at the upper end of the urban watershed.