Elevated mercury levels measured in lake sediments collected from several northeast Minnesota lakes have been attributed to atmospheric deposition of mercury originating from anthropogenic sources. The concentration of mercury in the lake sediment cannot be entirely accounted for by direct precipitation to the lake surface. The surrounding watershed appears to influence the concentration of mercury in the lake sediment and provide a mechanism for the loading atmospherically deposited mercury to the lake system. Four watersheds (Dunnigan, Meander, Kjostad, and Thrush) located in northeast Minnesota were studied to determine their interaction with atmospherically deposited mercury and the relationship between watershed characteristics and lake sediment mercury concentration. Results from bedrock mercury analyses show that bedrock contains substantially lower concentrations of mercury than the associated lake sediments. This indicates that contributions of mercury from bedrock sources are minor. Analysis of surficial material that contained greater than 30 percent of clay and organic matter generally contained higher concentrations of mercury. A positive correlation also exists between the composition of the surficial material in the watershed and lake sediment mercury concentration. The watersheds containing surface material with the highest percentages of clay and organic matter also contained the highest concentration of mercury in its lake sediment. Other watershed characteristic that correlate with lake sediment mercury concentration are the ratio of immediate watershed/lake surface area and percent forest cover. Results from watershed mercury loading calculations indicate that approximately 26-56% of the mercury measured in the associated lake sediments of the four watersheds is contributed from the surrounding watershed. This suggests that the watershed functions as a secondary contributor of mercury, primarily originating from atmospheric deposition, to the lake sediment.
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Eric Allen Smith in partial fulfillment of the requirements for the degree of Master of Science, December 1990.
Smith, Eric Allen.
Mercury Geochemistry of Four Watersheds in Northeast Minnesota.
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