Distribution of heavy metals in water and bottom sediments in reservoirs of the Dnipro cascade (Ukraine).

The Dnipro river is the main catchment in Ukraine. Within its territory the river flow is regulated and  represents a cascade of six reservoirs in direction from upper to down: Kyiv,  Kanevsky, Kremenchug, Kamensky, Dniprovsky, and Kakhovsky.  Reservoirs have a total water mirror area of 6950 km² and a total water volume of 44 km³. These figures make 95% and 91% of the total number of all large reservoirs in Ukraine, respectively. Their location in various natural and climatic zones, the difference in morphometric and hydrodynamic characteristics, hydrobiological, physicochemical, and sedimentation processes, as well as levels of anthropogenic load make them a unique object for studying intra-water processes, including processes of transformation, interphase exchange and accumulation of heavy metals in bottom sediments.

The study presents long-term data on the dynamics of the content and patterns of distribution of heavy metals (Fe, Mn, Cu, Zn, Pb, Ni, Co, Cd) in water, as well as in suspended solids, and bottom sediments of these reservoirs.

The influence of hydrological factors, physicochemical and hydrobiological processes on the nature of the transformation and interphase distribution of the heavy metals in the system “water – suspended substances – bottom sediments” has been shown. The contribution of mineral particles, Fe hydroxides and Mn oxides, and high molecular weight fractions of humic substances (humic and fulvic acids) on sorption processes and the removal of heavy metals from the solution phase were studied.

It has been found that the shift of the equilibrium of the calcium-carbonate system towards the formation of the CaCO₃ solid phase, which is associated with intensive hydrobiological processes in the Kremenchug and Kakhovsky reservoirs, leads to the additional removal of heavy metals from the solution due to deposition.

The study reveals the special aspects of the interphase distribution of heavy metals between the liquid and solid phases, due to the physicochemical conditions of the aquatic environment, as well as the morphometric characteristics of the reservoirs, flow rate, water temperature, duration of the ice cover during winter, and the content of organic substances.

The results of the distribution of heavy metals among various types of bottom sediments and their different particle size fractions are presented.

 It has been shown that a specific feature of the first in the cascade Kyiv Reservoir is the high content of dissolved humic substances, which play an important role in the migration of heavy metals.

The co-existing forms of metals in the solution phase are investigated with the thermodynamic modeling approach. This method was also used to evaluate the flow of metals from pore solutions of bottom sediments into the aquatic environment. Differences in the rate of molecular diffusion of metals from bottom sediments have been shown. They are caused by both the water chemistry and pore solutions, and the concentrations of metals and binding ligands.

The levels of anthropogenic load with heavy metals on the reservoir ecosystems have been evaluated over the past 30 years.