Migration Capacity and Bioavailability of Metals in Bottom Sediments of Surface Water Bodies (Chemical and Analytical Aspects)

The paper summarizes results of the study of the coexisting forms of metals (Cd, Pb, Mn, Co, Cu, Zn, Fe, Cr, and Al) in bottom sediments of surface water bodies of various types, in particular the reservoirs of the Dnieper cascade and small water bodies of the urbanized territories (for example Lake Verbne) located in the city of Kyiv. On the whole, five fractions - ion exchange, carbonate, oxide, organic/sulfide, and residual, including the metals found in the crystal lattice of minerals, were obtained using the five-stage extraction technique. It has been found that the distribution of metals between the specified fractions significantly differs due to the chemical properties of metals and the type of bottom sediments. The major portion of cadmium is concentrated in the first three fractions of bottom sediments and therefore it is characterized by the highest migration capacity. An increase in water mineralization and a decrease in pH values in the near-bottom layer of water are the most important factors contributing to cadmium migration from bottom sediments. Therefore, this metal should be considered as potentially bioavailable to aquatic organisms. The lowest migration capacity is characteristic of aluminum, copper, chromium, and lead occurring mainly in the composition of organic/sulfide and residual fractions. These metals migrate from the bottom sediments under specified conditions, the probability of the occurrence of which in the natural aquatic environment is rather low. Thus, these metals belong to the category of poorly available for hydrobionts. Iron and manganese, as well as those metals, the share of which is quite high in the composition of Fe(III) and Mn(IV) oxyhydroxides occupy an intermediate position. Zinc and partly cadmium, cobalt, and lead belong to this group of metals. The migration of these metals from bottom sediments increases significantly with long-term oxygen deficiency in the near-bottom layer and the formation of anaerobic conditions. These research results are relevant under conditions of climate changes that occur now and will occur in future, as the risk of secondary pollution of the water environment of surface water bodies by various chemical substances, including metals, due to their migration from bottom sediments increases. Dissolved oxygen deficiency in surface waters, salinization of fresh water, and the increase in the concentration of organic substances and decrease in pH and Eh-potential in the near-bottom layer of lakes and reservoirs contribute to the increase of secondary pollution of the water environment by metal compounds. The urgency of this problem makes it necessary to perform such research at the present time.