ROLE OF SOIL BIOGEOCHEMISTRY IN SEQUESTRATION OF HG AND PB FROM ATMOSPHERIC DEPOSITION IN AN ALPINE WATERSHED, ROCKY MOUNTAIN NATIONAL PARK, CO

Atmospheric deposition of trace metals from human sources is documented for several national parks in the Western United States (Landers et al., 2008). The effects of atmospheric deposition of Hg originating from coal-fired power plants and Pb originating from leaded gas combustion have been documented in surface water in Rocky Mountain National Park, which includes the Loch Vale watershed. In a watershed mass-balance calculation, Mast et al. (2005) found that only 20% of the Hg from atmospheric deposition to the Loch Vale watershed could be accounted for in stream and lake water concentrations.

Our study tests the hypothesis that alpine soils act as a sink for Hg and Pb from atmospheric deposition, accounting for the Hg missing from Mast’s watershed mass-balance. Mercury accumulates in soils through sorption to iron oxides and complexation with the organo-sulfide component soil organic matter. Lead sorption is associated with soil organic matter and clay minerals. Our data from Loch Vale mineral soils collected in forest and meadow settings showed elevated concentrations of both Pb (48-96 mg/kg) and Hg (0.12-0.22 mg/kg) relative to the continental mean soil concentrations (27 mg/kg Pb; 0.06 mg/kg Hg; Smith et al., 2006) with concentrations decreasing with depth. Wetland soils, with 28-37% organic carbon, had consistently elevated Hg concentrations (0.15-0.18 mg/kg Hg) throughout the profile. These data suggest that the cycling of Hg and Pb in surface and ground water in this alpine ecosystem is regulated through sequestration by soil organic matter. Projections for climate change suggest that the Rocky Mountain region will see warmer, drier winters. Reduced spring runoff and subsequent wetland desiccation could result in the release of Hg through the breakdown of associated soil organic matter. Thus, climate-driven shifts in runoff from basins in the southern Rocky Mountains may have deleterious effects on downstream aquatic ecosystems and on the quality of municipal water supplies drawn from these watersheds.

Landers, D.H., et al. 2008. The Fate, Transport, and Ecological Impacts of Airborne

Contaminants in Western National Parks (USA). EPA/600/R-07/138, 2008. Corvallis, Oregon.

Mast, M.A. et al., Water, Air, Soil Poll. 2005. 164, pp. 21–42.

Smith, D. B. et al. 2005. U.S. Geol. Survey Open-File Report 2005-1253.