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.

Concentration and composition of hydrocarbons in the river Ural mouth zone in the period of high water

High concentrations of hydrocarbons were detected in the surface water suspended solids of the near-mouth zone of the river Ural during periods of high water level. On average, 198 and 270 μg/l suspended solids were detected in 2016 and 2017, respectively. Weathered oil alkanes were prevalent in the composition of hydrocarbons. The hydrocarbon content was low in sands with an average of 7.8–14.6 μg/g. Terrigenous high-molecular mostly stable alkanes predominated in their composition. The interannual variability of the content of organic compounds is connected with the river runoff and the supply of oil contaminants is connected with the flood water. The Ural River’s runoff during high-water periods is expected to significantly affect the ecological conditions of the North Caspian Sea.