Investigation of Coupled Hydrologic and Geochemical Impacts of Wildfire on Southern California Watersheds

Southern California's fire regime is characterized by periodic large-scale wildfires that occur in late summer and early fall, leaving little time for the landscape to recover before the winter rainy season. Recent trends suggest that fire frequency and intensity is increasing due to factors such as urban encroachment into wildlands and a changing climate. While there is an awareness of the hydrologic consequences of these fires, wildfire's potential effects on water quality in Southern California are less understood, especially at the urban-fringe. This study investigates the impacts of wildfire on regional soils with respect to mercury (Hg) storage, accumulation, and transport potential; on storm runoff chemistry in an urban fringe watershed highly impacted by regional atmospheric pollutants; and on procedures required to model post fire hydrologic response and predict contaminant loading. The regional soil work showed reduced influence of grain size on Hg storage and accelerated accumulation of Hg in burned soil surfaces over time, as well as sediment-facilitated Hg transport in post-fire storms. Analysis of post-fire runoff from an urban fringe watershed showed up to three orders of magnitude increases from pre-fire levels in the concentrations and loads of many trace metals, including cadmium (Cd) and lead (Pb). These results also highlighted a shift in the timing of chemical delivery in post-fire storms to coincide with, rather than precede, peak discharge, amplifying the fire's impacts on mass loading. The investigation into modeling these impacts allowed for initial estimates to be made of seasonal loading, the development of a model to simulate pre-fire hydrology, and an identification of steps required to develop a model application capable of simulating post-fire sediment and metal concentrations. The resultant body of work provides an increased understanding of wildfire's effects on Southern California's soils and water and on what is required to predict these impacts