Understanding Relationships Between Mercury Concentrations and Certain Water Quality
Characteristics: Biogeochemical Cycling of Mercury and Methyl Mercury in
Great Smoky Mountains National Park

Suzanne Fisher
Tennessee Valley Authority
400 W. Summit Hill Dr., Knoxville, TN 37902

Weekly sampling of total mercury in wet deposition has been underway at Clingmans Dome in Great Smoky Mountains National Park since 2002 as part of the Mercury Deposition Network (MDN, TN-12). While concentrations of mercury from the Clingmans Dome site have seasonally (May-October) averaged 9.10 ng/L in precipitation, yearly rainfall amounts for this high-elevation ecosystem are over 86 inches. Monitoring results indicate that mercury concentrations in Great Smoky Mountains National Park are similar to most MDN sites throughout the U.S., and mercury deposition is similar to other monitoring sites located in the southeastern U.S., albeit higher than Northeastern and Midwestern states. Wet deposition alone, however, does not represent the total amount of mercury deposited to the ecosystem. Both dry deposition (particles or gases that are deposited onto foliage and the landscape) and cloud deposition (cloud or fog droplets containing pollutants enshrouding vegetation or other surfaces) of mercury also contribute to the total atmospheric deposition the ecosystem receives. These two forms of deposition are not currently being measured in Great Smoky Mountains constituting a gap in data relevant to the atmospheric deposition of mercury.

Once atmospheric mercury is deposited onto the land and surface waters, complex changes and cycling throughout the ecosystem occur. The fate and mobility of atmospherically deposited mercury to terrestrial environments is poorly understood. Preliminary results from the METAALICUS study suggest that terrestrial systems impose a time lag for the delivery of atmospheric deposition to lakes via runoff. The presence of organic matter in soils influences watershed mercury cycling, as mercury has been shown to form strong complexes with organic matter. Several studies have demonstrated foliar exchange of gaseous elemental mercury through leaf stomata and subsequent loading to the forest floor via litterfall. Studies have also shown upland soils to be a sink for methyl mercury and that in streams draining high-elevation forest ecosystems, particularly with high dissolved organic carbon content, methyl mercury production could occur.

I will discuss the mid-season results of measurements collected from a high-elevation catchment taken at two elevations (2,025 and 1,800 m) in Great Smoky Mountains National Park. Measurements are being taken to determine 1) the relative input of total and methyl mercury concentrations in throughfall, leaf litter, and soils in conjunction with current wet deposition sampled by the Mercury Deposition Network, 2) concentrations of total and methyl mercury and the proportion of mercury in dissolved versus particulate phases of stream water, 3) measurements of watershed components such as sulfate, dissolved organic carbon, and redox potential that influence mercury distribution and cycling in terrestrial-aquatic interface, and 4) the impact of seasonal snow melt and storm events on discharge of total and methyl mercury in the watershed.