HR: 14:55h
AN: B32C-05    [PDF]
TI: Forest Fire Effects on Mercury and Other Trace Metal Concentrations in a Rocky Mountain Forest Ecosystem
AU: * Biswas, A
EM: biswasa@umich.edu
AF: Department of Geological Sciences, University of Michigan, 425 E. University Ave., Ann Arbor, MI 48109 United States
AU: Blum, J D
EM: jdblum@umich.edu
AF: Department of Geological Sciences, University of Michigan, 425 E. University Ave., Ann Arbor, MI 48109 United States
AU: Keeler, G J
EM: jkeeler@umich.edu
AF: Air Quality Laboratory, University of Michigan, 109 South Observatory, Ann Arbor, MI 48109 United States
AB: The impacts of forest fires on pools of major elements including carbon, calcium, and sulfur, have been extensively studied while their effects on potentially toxic trace metals are not as well understood. We examined the effect of the summer 2001, 4470 acre Green Knoll Fire (GKF) in northeastern Wyoming on mercury (Hg) and other trace metal concentrations in forest ecosystem pools. A paired watershed study was conducted using a burned and unburned watershed of similar stand age, climate, vegetation, and geology to investigate wildfire effects on the evolution and dispersal of pools of Hg and trace metals in forests. Mercury and other trace metal concentrations were determined through a 15 cm soil profile as well as in vegetation. Atmospheric sampling suggests that possibly due to geothermal inputs, ambient atmospheric and soil mercury concentrations are elevated in this region compared to other rural areas in the US, with typical concentrations of vapor phase mercury $>$5 ng/m$^{3}$. The burned watershed soil profile had much lower mercury concentrations than that of the unburned watershed, suggesting Hg volatilization by wildfires. Previous studies have suggested that leaf litter releases 97-100%\ of its mercury content, while this study suggests that the Hg release from soil organic matter may not be as complete. Mercury concentrations in the unburned soil column decreased from an average of 158 ppb at the surface to 38 ppb at 10-15 cm. In contrast, average concentrations in the burned soil were near 30 ppb from 0-15 cm. This result indicates the GKF released only $\sim$ 85%\ of the mercury present in the organic soil horizon, which may be attributed to relatively low fire intensity. Extrapolations from our results indicate that this relatively small fire released $\sim$ 0.1 Mg of Hg from the soil. On average, 4.2 million acres are burned yearly in the US, suggesting that soil burning releases an approximate 100 Mg annual Hg flux into the atmosphere, 70%\ of the estimated US anthropogenic flux of 144 Mg. Past fire suppression practices in the US have caused a buildup of flammable materials, which combined with the legacy of pollution in the 20$^{th}$ century, suggests that modern fires may release more mercury than in the past. This study reinforces the importance of wildfires to the global mercury cycle and indicates that forests in the Rocky Mountain region may contain large reservoirs of Hg that can be released during fires and add to the global budget of mercury emissions.
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 1065 Trace elements (3670)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting