Long-term dynamics of total mercury in surficial bottom sediments of the Volga River's reservoir in central Russia

The long-term dynamics of mercury in bottom sediments of Uglich Reservoir were examined. Samples of the 0-5-cm surface layer of bottom sediment collected during research expeditions in 1958, 1977, 1991, and 2012 were analyzed. An increase in the average mercury concentration from 1958 to 1977 (0.440 +/- 0.039 and 0.668 +/- 0.065 mg/kg d.w., respectively) was followed by a decrease in 2012 (0.260 +/- 0.021 mg/kg d.w.). During all years except 1991, a significant correlation was found between mercury content and the amount of organic matter in the sediments (r = 0.54-0.87, p < 0.01), flow velocity (r = 0.38-0.53, p < 0.01), and the rate of sedimentation in the reservoir (r = -0.51 to -0.69, p < 0.01). Calculations based on these results and previously published data from the study region on the density of atmospheric mercury precipitation showed that the watershed as a whole received 2.4-3.0 t on an annual basis, and in the same period, the water surface received 9.7-12.8 kg of mercury. The bottom sediment contained 4.31 (2012)-7.40 (1977) tons of mercury. Thus, most of the mercury in the Uglich Reservoir is assumed to originate from runoff and terrigenous sources within the watershed.