Behaviour and distribution of heavy metals at the boundary of “water-bottom sediments” in mouth zones of rivers

The aquatic elementary landscapes of the Don and Northern Dvina Rivers mouth zones as arenas of mass transfer of heavy metals (HM) are studied. The results of long-term comprehensive studies of the authors, which included sampling of water and the upper layer of bottom sediments in order to determine the content of mercury, copper, methane, hydrogen sulfide, as well as pH and Eh values, are presented. According to the type of prevailing geochemical conditions and the level of anthropogenic impact, natural, man-made and natural-anthropogenic aquatic elementary landscapes are distinguished. The bottom water layers in both rivers were characterized by positive Eh values, but it should be noted that they were generally relatively low for surface watercourses. According to the values of the redox potential and the hydrogen index, the methane content and the total hydrogen sulfide, the presence of the following conditions is proved – oxygen, gley, hydrogen sulfide, oxygen-gley and gley-hydrogen sulfide. At the water - sediment interface, in most cases, there was a decrease in pH values by the first tenths of the values, while Eh usually decreased quite sharply, often to negative values. In terms of methane content, the bottom sediments of the aquatic elementary landscapes of the Don and Northern Dvina rivers were similar, while the concentration of total hydrogen sulfide was higher in the Don. It is shown that in the mouth zone of the Don and Northern Dvina rivers, in areas subject to constant organic pollution and eutrophication, there is an increase in the total content of mercury and copper in the bottom sediments. Higher concentrations of gross mercury will generally be found in natural and anthropogenic landscapes dominated by hydrogen sulfide and gley-hydrogen sulfide environments. The important role of microparticles of the polydisperse medium of bottom sediments in the deposition of copper and mercury is revealed