Final Report: Assessing Mercury Contamination and Bioavailability in Great Smoky Mountains National Park Aquatic Habitats

Buchwalter, David B; Xie, Lingtian; Arai, Yuji

This study represents an initial exploration of Hg contamination in sediments, soils and aquatic macroinvertebrates collected from different locations in the Great Smoky Mountains National Park. Mercury was readily measurable in all soil, sediment and macroinvertebrate samples, though the magnitude of Hg concentrations varied widely across locations. The range of all sediment/soil total Hg measurements ranged from 14-378 ng/g on a dry weight basis, with mean and median values of 101.6 and 93.3 ng/g, respectively. A limited number of sediment samples (5) analyzed for methyl Hg had concentrations ranging from 0.02 – 3.83 ng/g on a dry weight basis. These values represent a range of contribution (0.2 to 8.2 %) to the total Hg pool present at the sites. Macroinvertebrate samples were taken from each of three locations – Kerr Branch, Chilogatee Creek and Mossy Branch. These sites differed considerable in their physical habitats, sediment geochemistry, and Hg loading. For example, Mossy Branch (47 ng/g), Kerr Branch (78 ng/g) and Chilogatee Creek (117.6 ng/g) differed almost 3-fold in total Hg concentrations, yet the methyl Hg profiles did not reflect the total Hg profiles. Mossy Branch, despite having the lower total Hg among the 3 sites, had methyl Hg concentrations (3.8 ng/g) that were comparable to Kerr Branch (3.7 ng/g). Chilogatee Creek had the lower methyl Hg concentration (1.6 ng/g) of the three sites despite its higher total Hg concentrations. The texture of soils and sediments are sandy loam and loamy sand, respectively. Quartz, biotite, kaolinite, and potassium feldspar are major mineral components. Most of sediments and soils are moderately acidic (4.2) to near neutral pH (6.8). While soils exhibit cation exchange capacity (CEC) of 13-23 cmol/kg, CEC of sediments are CEC 7-12 cmol/kg, suggesting that lack of cation exchangeability in sediments. Mercury desorption in sediments with high C (e.g., Kerr Branch) was much lower than that from low C sediments (Laffete and Mossy Branch sediments). These results indirectly suggest that organic carbon sources might strongly retain the labile Hg fractions in sediments (1-7%). Comparisons of invertebrate tissue Hg concentrations among sites are complicated by the fact that the same taxa did not occur at each site. Nevertheless there are some interesting observations. First, Hg (total and methyl) concentrations in sediment are likely not good predictors of food web Hg incorporation. For example, Hg burdens in the caddis fly Diplectrona modesta were more than two fold higher in Mossy Branch than in Kerr Branch, despite the fact that Kerr Branch had higher sediment total Hg and comparable methyl Hg. Second, trophic position did not appear to be a major driver of Hg body burdens among species at a given site. However, based on limited data, it appears that the proportion of methyl to total Hg increases with trophic status.

Final Report: Assessing Mercury Contamination and Bioavailability in Great Smoky Mountains National Park Aquatic Habitats

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