FATE AND TRANSPORT OF MERCURY IN A WATERSHED-RESERVOIR SYSTEM BURNED DURING THE 2012 HEWLETT GULCH FIRE, FT. COLLINS, COLORADO

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 were collected from a watershed-reservoir system burned in the 2012 Hewlett Gulch Fire, near Ft. Collins, Colorado. Debris collected from the drainage and reservoir bottom represented charred and destabilized organic material and were found to have total mercury concentrations of around 50 ng g^-1. Subsequent coring of the reservoir sediment was conducted over a one year period 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 were conducted to assess the influence of redox driven processes on the fate of mercury in recently deposited reservoir sediments.