Whole-watershed Mercury Balance in a Sierra Nevada Ecosystem

Xavier Fan1, Daniel Obrist1, Ashley Pierce1, Cornelia Barth1, Mae S. Gustin2 and Douglas P. Boyle1

Little data is available on mercury (Hg) dynamics at high-elevation mountain sites. A wholewatershed approach was used to quantify major fluxes and pools of Hg in Sagehen basin, a closed basin in the Sierra Nevada Mountains in California (NADP site CA50). Over a period spanning 9 months (January-September 2009), we estimated wet deposition inputs to the watershed at 3.8 g m-2. Dry deposition added additional Hg in the range of 0.30-2.45 g m-2 during this time period, and was the dominant deposition process during summer time. Seasonal snowpack accounted for only half of the Hg deposited by wet deposition. We suggest that photo-induced reduction of Hg(II) in snow and subsequent volatilization was responsible for this loss. Thus, snowpacks in the Sierra Nevada Mountains likely reduce the effective atmospheric mercury flux via wet deposition due to significant re-emission fluxes prior to snowmelt. As such, wet Hg deposition could be of lesser importance as a Hg source in snow-dominated systems. Finally, stream runoff collected at the outlet of the watershed could account for only 6% of total Hg wet deposition suggesting that a large fraction of mercury deposition was sequestered in the ecosystems, specifically in the soils.

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1 Desert Research Institute, Reno NV, USA
2 University of Nevada, Reno NV, USA