Fire pollutant loading to urban fringe watershed - 61 AbstrAct This study investigates the impacts of wildfre on storm runoff chemistry in an urban fringe watershed that is highly impacted by regional atmospheric pollutants. Pre- and post-fre storm runoff was collected and analyzed for basic cations and anions, trace metals, and total suspended solids. The loss of vegetation and changes in soil properties due to fre reduced lag time and greatly increased the magnitude of storm runoff, resulting in sediment laden foods carrying high concentrations of particulate-bound constituents. Post-fre concentrations and loads were up to three orders of magnitude greater than pre-fre values for many trace metals, including lead and cadmium. A shift was also observed in the timing of chemical delivery, where maximum sediment, trace metal, and cation concentrations coincided with peak discharge in the post-fre runoff. Our results highlight the need for treatment of sediment-bound chemicals, in order to mitigate potentially detrimental impacts on downstream water quality. IntroductIon Wildfres can contribute to contaminant loading to downstream areas. The responsible mechanisms included increased runoff associated with soil hydrophobicity, increased mobility of contaminants from soil and vegetation, and higher erosion rates (Bitner et al. 2001, Moody and Martin 2009, Jung et al. 2009, Smith et al. 2011). Increased nutrient concentrations, especially nitrate and phosphate, have been previously reported in post-fre runoff from Southern California’s San Gabriel Mountains (Riggan et al. 1985, Ranali 2004, Meixner et al. 2006). Total suspended solid (TSS) concentrations in stormwater often increase by orders of magnitude in post-fre systems, accelerating the potential for mobilization and transport of particulate bound contaminants (Malmon et al. 2007, Moody and Martin 2009, Smith et al. 2011). Signifcantly higher concentrations of several metals, including As, Al, Ba, Cd, Cr, Fe, Mn, Pb, and Zn, were reported in post-fre runoff following New Mexico’s 2000 Cerro Grande Fire and Australia’s 2003 bushfres in Alpine National Park, (Bitner et al. 2001, Gallaher et al. 2002, Leak et al. 2003, Smith et al. 2011). However, few studies have focused on trace metal delivery following fre, particular in the urban fringe areas of southern California (Gallaher et al. 2002, Smith et al. 2011). The recent Station fre provides an opportunity to investigate the effects of fre on stormwater loadings of metals in an urban fringe watershed. The Station fre burned for six weeks (Aug-Oct, 2009), consum- ing 660 km 2 of primarily Angeles National Forest in the San Gabriel Mountain Range. The burned region is immediately adjacent to dense urban areas on the east side of the Los Angeles basin. Watersheds in the front range of the San Gabriel Mountains supply Dynamics of pre- and post- fre pollutant loads in an urban fringe watershed Megan P. Burke 1 , Terri S. Hogue 1 , Janet Barco 1 *, Christopher Wessel 1 **, Alicia Y. Kinoshita 1 and Eric D. Stein 1 University of California, Department of Civil and Environmental Engineering, Los Angeles, CA *Current address: Sandia National Laboratories, Albuquerque, NM **Current address: Geosyntec Consultants, Los Angeles, CA