In 2008-2010, monitoring work in the national park «Smolensk Lakeland» showed an excess of mercury in the water of Lake Loshamye located in the protected area of the natural reserve. In 2014-2019, together with the staff of the national park, we studied the ecological and geochemical features of the catchment area and the water area of Lake Loshamye. We studied the patterns of distribution of several heavy metals: Hg, Cr, Mn, Fe, Co, Cu, Zn, As, Cd, Pb, Sr. We did not find an anthropogenic influence on the catchment area; this article presents the results of the study of the bottom sediments of the lake. As a result of the work, we determined that all heavy metals have a natural distribution in this lake, except mercury. High values of mercury in bottom sediments (up to 1000 ppb) are not typical for such natural object. The median of the sample is 152 ppb, and it is four times higher than expected mercury content considering its content in the soils of the catchment area and 2-5 times higher than the values obtained for similar objects by other researchers. The analysis of the patterns of mercury distribution in bottom sediments indicates a one-time, local anthropogenic pollution of the lake.