Humic substances migration in the surface water of Ukraine

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.

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 50–65% 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 “soil catchment complex - river water – suspended matter - bottom sediments” are established.

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.

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.

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.

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 13–15% of free fulvate ions capable of binding heavy metals, which is an important characteristic for assessing the buffer capacity of water.