Pre- and post-fire pollutant loads in an urban fringe watershed in Southern California

Abstract

Post-fire runoff has the potential to be a large source of contaminants to downstream areas. However, the magnitude of this effect in urban fringe watersheds adjacent to large sources of airborne contaminants is not well documented. The current study investigates the impacts of wildfire on stormwater contaminant loading from the upper Arroyo Seco watershed, burned in 2009. This watershed is adjacent to the Greater Los Angeles, CA, USA area and has not burned in over 60 years. Consequently, it acts as a sink for regional urban pollutants and presents an opportunity to study the impacts of wildfire. Pre- and post-fire storm samples were collected and analyzed for basic cations, trace metals, and total suspended solids. The loss of vegetation and changes in soil properties from the fire greatly increased the magnitude of storm runoff, resulting in sediment-laden floods carrying high concentrations of particulate-bound constituents. Post-fire concentrations and loads were up to three orders of magnitude greater than pre-fire values for many trace metals, including lead and cadmium. A shift was also observed in the timing of chemical delivery, where maximum suspended sediment, trace metal, and cation concentrations coincided with, rather than preceded, peak discharge in the post-fire runoff, amplifying the fire's impacts on mass loading. The results emphasize the importance of sediment delivery as a primary mechanism for post-fire contaminant transport and suggest that traditional management practices that focus on treating only the early portion of storm runoff may be less effective following wildfire. We also advocate that watersheds impacted by regional urban pollutants have the potential to pose significant risk for downstream communities and ecosystems after fire.

Characterization and evaluation of controls on post-fire streamflow response across western US watersheds

“…In terms of water quality, contaminant levels can be dramatically increased for many years after a wildfire in both soil (Burke et al, 2010) and stream systems (Emelko et al, 2011;Stein et al, 2012;Burke et al, 2013), increasing the workload on source water protection organizations in communities reliant upon burned watersheds for drinking and agricultural water. Furthermore, wildfires are readily attributed as the cause of substantial increases in debris flows (Benavides-Solorio and MacDonald, 2001;Cannon et al, 2001;Meyer et al, 2001).…”

Application of MODIS snow cover products: wildfire impacts on snow and melt in the Sierra Nevada

“…Soil and vegetation change after fires result in increased flooding, mass wasting, increased runoff intensities, long-term changes in energy and water budgets, and increased air pollutants (Swanson, 1981;Kattelmann et al, 1983;Stednick, 1996;Webb et al, 2012). Storm runoff also liberates atmospherically deposited contaminants and mobilizes particulate-bound constituents, degrading post-fire water quality (Stein et al, 2012;Burke et al, 2013). Vegetation recovery significantly controls long-term hydrologic conditions; additionally, elevated discharge has been observed for nearly 10 years post-fire (Kinoshita and Hogue, 2011).…”

Wildfire, water, and society: Toward integrative research in the “Anthropocene”

“…The remobilization of natural and industrial lead, mercury, and other trace metals and contaminants are released during fire and are more readily transported in waterways, causing environmental health concerns (Odigie and Flegal, 2011;Burke et al, 2013;Kristensen et al, 2014;Odigie et al, 2016). Water quality typically deceases after wildfires, which may liberate atmospherically deposited contaminants in soils and vegetation, which are then mobilized through erosion, runoff, and sediment transport processes (Kristensen et al, 2014).…”

“…Nutrient loading in streams are generally elevated after fire (Ranalli, 2004). Nitrates increase up to 40 times (Mast and Clow, 2008;Riggan et al, 1994) along with increases in other metals including lead, cadmium, and mercury (Stein et al, 2012;Burke et al, 2013). Nitrates, metals, and large quantities of sediment itself from burned landscapes contribute substantially to pollutant loads downstream.…”

“…Nitrates, metals, and large quantities of sediment itself from burned landscapes contribute substantially to pollutant loads downstream. These pollutants have significant implications for water management, including the quality of drinking water (Riggan et al, 1994;Stein et al, 2012;Burke et al, 2013).…”