Fate and Transport of Mercury in a Watershed-Reservoir System Burned During the 2012 Hewlett Gulch Fire, Ft. Collins, Colorado

Abstract:

Deposition and accumulation of atmospheric mercury has led to the ubiquitous enrichment of this toxic metal in surficial soils around the planet. Remobilization of mercury into the atmosphere is generally associated with landscape disturbance, in particular wildfire, and is a significant component of annual global atmospheric mercury loading. In addition to atmospheric transport, wildfire may indirectly mobilize mercury into local surface waters through soil destabilization and increased watershed runoff. Transport of mercury into surface water and sulfate reducing environments may result in conversion of ionic mercury into methylmercury; a highly toxic and bioaccumulative form of the metal. Although increases in methylmercury production have been observed following wildfire, there is relatively little information regarding chemical and physical processes responsible for post-fire mercury mobilization and methylation. In this study, soils, fire debris, and reservoir sediments have been collected from a watershed-reservoir system burned in the 2012 Hewlett Gulch Fire, near Ft. Collins, Colorado. Subsequent coring of the reservoir sediment was conducted at multiple times to assess how early diagenesis of fire debris affects the fate of associated mercury. In addition to environmental sampling, laboratory microcosm experiments using fire debris are employed to assess the influence of redox driven processes on the fate of mercury in recently deposited reservoir sediments. Debris collected from the drainage and reservoir bottom are represented by charred and destabilized organic material which have total mercury concentrations of around 50 ng g^-1, similar to native sediment; however, post-wildfire reservoir sediment have dramatically increased methylmercury concentrations. The influence of sulfur speciation, determined by X-ray absorption near-edge structure spectroscopy, on mercury binding is measured using competitive ligand exchange techniques and indicates enhanced mercury binding capacity in aged wildfire debris and sediments.