Fate and transport of polycyclic aromatic hydrocarbons in an urban runoff diversion system

Urban storm water discharge is a recognized source of pollution to water bodies throughout the world. In Southern California, ongoing population growth and its associated urban sprawl suggest the problem of storm runoff pollution will become more important in the years to come. Understanding how pollutants are transported during runoff events is essential to devise control strategies in order to protect the receiving waters. This study characterized transport pathways of polycyclic aromatic hydrocarbons (PAHs) from three drainage basins into an urban runoff diversion system (URDS) designed to protect a drinking water reservoir in Southern California. Five storms were monitored at the exit of forebay structures, three storms were also monitored at the entrance.; PAHs were found associated to the suspended solids transported by runoff. Fluoranthene, pyrene, benzo(b)fluoranthene, and benzo(g,h,i)perylene were the dominant PAHs. Compositional profiles varied between storms and locations. Sources of PAHs to the URDS include a mix of petroleum products, burned fossil fuels, and wood, grass and coal combustion. Higher PAH concentrations occurred in the first half of the sampling period at forebay exits. A wide range in suspended solids (TSS) and PAH concentrations were observed within a storm. Both parameters were correlated only at the forebay exits. Concentrations and pollutant loads varied between storms and locations but were consistently lower at the exit of one of the forebays. Forebays attenuated pollutant variations during storm events, but inconsistently trapped TSS and PAHs. Enrichment of PAHs in the smaller particle size fractions, not retained by the forebays, could be responsible. Spatial patterns of PAH contaminated sediments in the forebays were studied. Runoff travels through the forebays in well defined longitudinal paths, with little spread occurring in the lateral direction. The amount of sediments deposited decreased along each path. PAHs are associated to organic carbon in the sediments.; This research showed runoff pollution can pose a water quality management problem at the Sweetwater reservoir in terms of PAHs. As presently operated, forebay structures in the URDS do not appear effective to control these pollutants. Further studies are recommended to understand settling of PAH containing suspended solids in the forebays in order to optimize their removal from runoff.