No abstract

This paper discusses the results of the monitoring of anthropogenic mercury contamination of the upper part of Bratsk reservoir. The paper shows that in the long-term, (1998-2018) there is an ongoing decrease of Hg concentration in the surface layer of bottom sediments. At the same time the study demonstrates that 20 years after the cessation of mercury electrolysis at the Usoliehimprom chemical plant, Hg concentration in the bottom sediments exceeds background levels by 5 to 54 times. Significant part of the Hg in the bottom sediments is stored in organic form, which is potentially dangerous since it may cause secondary contamination of aquatic environment and hydrobionts. The high amplitude of water-level fluctuations in the reservoir is one of the main factors influencing the distribution of mercury in the surface layer of bottom sediments and preventing the sealing of contaminated bottom sediments by terrigenous material.

This study aimed to identify the factors responsible for the major ion composition of pore water from the bottom sediments of the Bratsk water reservoir, which is part of the largest freshwater Baikal-Angara water system. In the Bratsk reservoir, the overlying water was characterized as HCO3–Ca–Mg type with the mineralization ranging between 101.2 and 127.7 mg L−1 and pore water was characterized as HCO3–SO4–Ca, SO4–Cl–Ca–Mg and mixed water types, which had mineralization varying from 165.9 to 4608.1 mg L−1. The ionic composition of pore waters varied both along the sediment depth profile and across the water area. In pore water, the difference between the highest and lowest values was remarkably large: 5.1 times for K+, 13 times for Mg2+, 16 times for HCO3−, 20 times for Ca2+, 23 times for Na+, 80 times for SO42−, 105 times for Cl−. Such variability at different sites of the reservoir was due to the interrelation between major ion concentrations in the pore water and environmental parameters. The major factor responsible for pore water chemistry was the dissolution of sediment-forming material coming from various geochemical provinces. In the south part of the reservoir, Cl−, Na+ and SO42− concentrations may significantly increase in pore water due to the effect of subaqueous flow of highly mineralized groundwater.

This study was aimed at identifying the processes responsible for the major ion composition of pore water from the bottom sediments of the Bratsk water reservoir, which is a part of the largest freshwater Baikal-Angara water system. The pore water ionic composition varies both along the sediment depth profile and across the water area. In pore water, the difference between the highest and lowest values is remarkably large: 5.1 times for K+, 13 times for Mg2+, 16 times for HCO3-, 20 times for Ca2+, 23 times for Na+, 80 times for SO42-, 105 times for Cl-. Such a variability suggests that the dominant factors, influencing pore water chemistry, depend on the location. At the first stages of the sedimentation process, the chemistry of the pore water in the Bratsk reservoir is dependent on HCO3-Ca of the overlying water. Later on, due to the interaction with the sedimentary terrigenous material, they changed to SO4, SO4-HCO3, HCO3-SO4, HCO3-Cl-SO4-water types with exchangeable cations, mainly Ca. Some of pore waters may have a complex genesis associated with subaqueous groundwater discharge. The change in the redox potential observed in the pore water is the indicator of early-diagenetic transformations taking place in bottom sediments.