Spatial Variation in Mercury Concentrations and Fluxes in the Upper Big Thompson Watershed, Colorado

Mast, M; Campbell, D; Krabbenhoft, D

Mercury (Hg) concentrations and fluxes were compared for three streams at different elevations in the upper Big Thompson Watershed in Colorado during 2003-2005: Andrews Creek (elev. 3,215 m), The Loch (elev. 3,109 m), and Big Thompson River above Lake Estes (elev. 2,283). This study investigates how Hg behavior in snow-melt dominated streams changes moving from the alpine headwaters downstream to montane forests in the lower part of the study area. A better understanding of the processes that control Hg transport in high-elevation streams is important for assessing the potential for Hg contamination in mountain ecosystems. Streamwater concentrations of total Hg (THg) in unfiltered samples ranged from 0.5 to 6.5 ng/L and showed a similar seasonal pattern among sites with the highest concentrations just prior to peak snowmelt in June. At all sites, THg concentrations were positively correlated with organic carbon, suggesting organically complexed Hg was flushed into streams from near-surface soil horizons. During snowmelt, THg concentrations increased with decreasing elevation (2.4 ng/L at Andrews Creek, 3.7 ng/L at The Loch, and 6.5 ng/L at the Big Thompson River), which may reflect differences in soil coverage and organic carbon pools. Annual export of THg during 2005 was 1.38 mg/m2 at Andrews Creek, 1.68 mg/m2 at The Loch, and 1.00 at mg/m2 at the Big Thompson. The lower yield at the Big Thompson primarily results from lower runoff (34 cm), which was less than half that of the upper elevation sites (79 cm at The Loch and 101 cm at Andrews Creek). Annual THg deposition measured near the high-elevation sites during 2005 was 7.5 mg/m2, indicating that less than 20% of atmospherically deposited Hg was exported by these streams.

Spatial Variation in Mercury Concentrations and Fluxes in the Upper Big Thompson Watershed, Colorado

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