LAUSR

The total mercury content of the surface layer (0–5 cm) of different types of bottom sediment in Gorky and Cheboksary reservoirs was determined. The content of mercury in bottom sediments varied within a wide range, 0.001–0.234 mg/kg. In spite of large industrial enterprises near the watersheds, which are potential sources of mercury in the environment, the content is comparable to background values. Depending on the sediment type, the metal concentration rises within a sequence: sand and loamy sand > sandy loams > clay and silt. The content of mercury positively correlates with the amount of organic matter in bottom sediments (r = 0.49–0.86, p ≤ 0.001) and the proportion of clay silt particles (r = 0.55–0.76, p ≤ 0.001). This relationship indicates the presence of two forms of mercury: sorbed on fine particle surfaces and as part of complex compounds. A negative correlation between Hg concentration and the flow velocity in the reservoir was revealed (r = − 0.40 to − 0.58, p ≤ 0.05). This relationship is due to the effect of pulsating discharge and wind currents on sedimentation conditions, and thus on the accumulation of mercury. The rate of accumulation was calculated as 15 kg/year in Gorky reservoir and 9 kg/year in Cheboksary, with average concentrations of 0.057 and 0.046 mg/kg, respectively, in the surface layer of bottom sediments.