<p>A comprehensive study of humic substances (HS) in the surface water of Ukraine have been carried out. It reveals the nature of the basic laws of humic substances intake, the spatial-temporal distribution, and the physicochemical processes of the intra-water transformation. The spatial differentiation of the studied parameters largely depends on the zonal-genetic conditions of humus formation. The HS content in the azonal system of cascade of the Dnipro reservoirs, decreases from the upper - the Kyiv - to the lower - the Kakhovka - reservoirs. Based on the coefficient of the HS water migration, which is characterizes their ratio in the water and catchment soils, the zoning of the Ukrainian territory is carried out.</p><p>The basic properties of the surface water HS - polydispersity, solubility, and the effect on the water blooming formation - are studied. It is shown that humic acids (HA) dissolved in water are more highly-molecular and polydisperse in comparison with fulvic acids (FA). Their most fine particle fractions, which content is 50&#8211;65% for HA and about 50% for FA, dominate in the surface water HS. The quantitative characteristics and features of the interphase distribution of HA and FA and the change in their molecular weight in the system &#8220;soil catchment complex - river water &#8211; suspended matter - bottom sediments&#8221; are established.</p><p>It is shown that sorption by suspended substances play a major role in the processes of self-purification of water from humus, while the oxides and hydroxides of Fe, Al, and Mn have the maximum sorption capacity for the HS. Conducting the laboratory experiments, sorption isotherms of the HS are obtained by Fe hydroxides and clay minerals of suspended substances of the Dnipro reservoirs. Also, the study states that among the various natural water factors, the conformational changes of HS are predominantly due to a change in pH.</p><p>Moreover, the quantitative indicators of the HS wash-off from the Prypiat catchment surface, the marshiest river basin in Ukraine, are calculated. The role of different pathways of water flow in the formation of the HS runoff is shown. Based on the data gathered, the material balance of HS within the cascade of the Dnipro reservoirs is calculated, and the role of the Prypiat river as the main source of HS is explained.</p><p>The results of the study indicate that the cascade of the Dnipro reservoirs is a powerful biogeochemical barrier that facilitates the transition of HS and associated with them pollutants from solution phase into bottom sediments, and in the reservoirs with delayed water exchange, FA are characterized not only by the transporting but also accumulating function.</p><p>Finally, the thermodynamic calculations of coexisting forms of HS in water were conducted. It is concluded that in the physicochemical conditions typical for the surface water of Ukraine, water contains 13&#8211;15% of free fulvate ions capable of binding heavy metals, which is an important characteristic for assessing the buffer capacity of water.</p>
<p>The Kyiv Reservoir and the Desna river are the main sources of drinking water supply for Kyiv, the capital of Ukraine. The impact of surface air temperature on the change of the aquatic system chemistry mentioned above water objects was studied based on long-term regular observations (1995&#8211;2018).</p><p>The findings are based on the analysis of daily air and water temperature, water pH, oxygen and carbon dioxide concentrations, water color index, dissolved organic substances (COD<sub>Mn</sub>), Fe, Mn, and phytoplankton abundance (without identifying their species composition).</p><p><strong>The winter period. </strong>Despite different hydraulic conditions in the Kyiv Reservoir and in the Desna river, the lack of ice cover due to an increase in winter air temperatures (December-February) led to significant improvement of the water oxygen regime in recent years. This fact, as well as the subsequent chain of changes in the water chemistry, contributed to the cheaper drinking water supply.</p><p>The ratio of the oxygen content change to the duration of the ice cover, determined by air temperature, was obtained. This allowed us to reconstruct years with observed hypoxia phenomena since 1850. Changes in the water chemistry, triggered by oxygen deficit, were described.</p><p><strong>The summer period. </strong>The increase in summer air temperature led to a decrease in oxygen concentrations. It had the effect of slowing down the process of N-NH<sub>4</sub><sup>+</sup> nitrification. Because of the high content of natural organic matters in these water bodies, dangerous hypoxia phenomena were reported in summertime. Such conditions contributed to the restoration of manganese to a mobile Mn<sup>2+</sup>.</p><p>All these features created additional difficulties at water treatment plants for supplying drinking water to residents of Kyiv.</p><p>In shallow waters, occupying almost half of the Kyiv Reservoir, an increase in water temperature during the summer period led to the intensification of the hydrobiological processes and &#8220;water blooming.&#8221; This is compounded by the high income of nitrogen and phosphorus from the point and diffuse sources.</p><p>The observed decrease in the Dnieper water flow does not allow the active use of releases from the Kyiv Reservoir. The only way to minimize negative consequences in the future is to prevent pollution and eliminate shallow water zones.</p><p>This is complicated by the fact that a significant amount of nitrogen compounds enters the Kyiv Reservoir with the water of its main tributary, Pripyat, which basin is highly marshy.</p><p>Dredging in the shallow areas will require further research because of the bedding of radionuclides of Chernobyl in the bottom sediments.</p>
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