Stormwater Runoff Following Wildfire in Southern California
Southern California Water Research Project, 2012
Abstract
Contaminant loading associated with stormwater runoff from recently burned areas is poorly understood, despite the fact that it has the potential to affect downstream water quality. The goal of the present study is to assess regional patterns of runoff and contaminant loading from wildfires in urban fringe areas of southern California. Postfire stormwater runoff was sampled from five wildfires that each burned between 115 and 658 km2 of natural open space between 2003 and 2009. Between two and five storm events were sampled per site over the first one to two years following the fires for basic constituents, metals, nutrients, total suspended solids, and polycyclic aromatic hydrocarbons (PAHs). Results were compared to data from 16 unburned natural areas and six developed sites. Mean copper, lead, and zinc flux (kg/km2) were between 112- and 736-fold higher from burned catchments and total phosphorus was up to 921-fold higher compared to unburned natural areas. Polycyclic aromatic hydrocarbon flux was four times greater from burned areas than from adjacent urban areas. Ash fallout on nearby unburned watersheds also resulted in a threefold increase in metals and PAHs.Attenuation of elevated concentration and flux values appears to be driven mainly by rainfall magnitude. Contaminant loading from burned landscapes has the potential to be a substantial contribution to the total annual load to downstream areas in the first several years following fires. Environ. Toxicol. Chem. 2012;31:2625–2638. # 2012 SETAC
INTRODUCTION
Periodic wildfires are a natural component of forest and scrubland ecosystems in arid and semiarid environments such as those found in the southwestern United States, western South Africa, Chile, northern Spain and Portugal, and southwestern Australia. However, the frequency and intensity of wildfires has increased because of human activities in and near natural forest and foothill areas [1] and is expected to continue increasing as climate patterns change [2]. In addition to habitat destruction, wildfires have been shown to have the ability to mobilize contaminants. For example, combustion of plants and natural materials has been reported to release metals that are subsequently mobilized [3–8], polycyclic aromatic hydrocarbons (PAHs) [9], dioxins [10–13], and nitrogen compounds [14–18]. Runoff from burned natural habitats often flows to adjacent or downstream urban areas, where the contaminants may comingle with similar urban-derived pollutants and have the potential to exacerbate existing water quality issues. In southern California, catchments affected by fire often drain to water bodies that support sensitive riparian or estuarine resources that may already be stressed or impaired by urban contaminants.
Despite the potential effects on downstream water quality, routine monitoring and assessment of postfire runoff seldom occurs. Consequently, the contribution of metals, nutrients, and organic contaminants from postfire runoff to receiving waters is poorly understood, in terms of both the magnitude of potential effect and the persistence of the influence. Because of this, the relative contribution of contaminant loading from postfire runoff compared to other sources, such as urban runoff or non postfire runoff is also poorly understood. In addition to the direct effects of runoff from burned landscapes, the materials left behind at the burned location can be carried away from the fire in the form of smoke and ash. Subsequent atmospheric deposition can markedly increase the quantity of various constituents available to storm flows downwind of fires [19]. For example, Sabin et al. [20] report that during the severe 2003 southern California forest fire season, atmospheric deposition rates of copper, lead, and zinc, went up by factors of four, eight, and six, respectively, at an unburned site in the San Fernando Valley that was approximately 30 miles from the southeastern border of the Piru/Simi fires.
The goal of the present study was to assess regional patterns of runoff and loading of a broad suite of contaminants from wildfires in urban fringe areas of southern California. Information from five large wildfires over the six-year period of 2003 to 2009 was used to answer the following three questions: (1) What is the magnitude of contaminant loading from burned areas relative to natural and urban sources? (2) How long does the fire-related contaminant loading persist? (3) What are some of the factors that may influence patterns of contaminant loading? We also provide some preliminary findings on the indirect effects on contaminant concentration in unburned urban areas that receive aerial deposition of wildfire ash.
