The aim of this work is to determine the elemental composition and types of humic substances extracted from the sediment samples from 6 lakes of the foreststeppe zone of the Tyumen region. The fractionation of humic substances is carried out by the pyrophosphate. To determine carbon, hydrogen, nitrogen and oxygen, the method of elemental analysis is used. The average values of the mass fraction of C, H, O, N for humic acids are 52.9, 4.31, 36.9, 5.88 wt. %; for fulvic acids they are 43.4, 4.96, 46.0, 5.61 wt. % respectively. The atomic ratio of H:C, 0:C and C:N are calculated. The average value (H:C)CORR of humic acids is equal to 1.68 (varying from 1.61 to 1.75), which can be interpreted as a combination of aromatic and aliphatic structures with a predominance of aliphatic chains. The average value for fulvic acids (H:C)CORR is the equal to 2.4 (varying from 2.2 to 2.7), which shows the predominance of the saturated fragments in this structure. The predominant type of humus in the sediments is the humate one. It indicates that the process of the organic matter humification of sediment is dominant if compared to mineralization.

The paper studies the regularities of formation of the trace-element composition of benthal deposits of the reference lakes in West siberia by statistical methods. For the statistical analysis, the experimental data of 29 sediment samples are used. 60 trace elements from these samples of benthal deposits are identified. According to the results of the factor analysis, a group of trace elements with similar mechanics of accumulation in benthal deposits of the lakes in West siberia is distinguished. For the Cr, Cu, Ni, Sr, Ti, V, Zn, Zr, Ag trace elements, the close interrelationship with the aluminum oxide content in the benthal deposits is determined. It allows us to make the conclusion of the dominating accumulation of these trace elements with aluminosilicates in the benthal deposits. For Cr, Cu, Sr, V, Zn trace elements, the mathematical expression to describe sorption processes in the water bottom sediments system is obtained with the method of regression analysis. The obtained dependences can be used to evaluate the participation of mineral components in the formation of the trace-element composition of benthal deposits. Besides, they can be used to determine the extent of anthropogenic influence, since they are obtained for the reference lakes of West siberia.

The paper studies the chemical composition of the waters of small lakes in the northern territories of West Siberia in order to evaluate their resistance to the processes of acidification. The study includes the lakes located far from the sources of anthropogenic impact and the ones located in close proximity to them. According to the results of this research, it is concluded that about 15% of conventionally-clear lakes are vulnerable to the loss of acid-forming substances and have a low self-regenerating capacity. These primarily include the low-mineralized water bodies in the northern and middle taiga zones. In addition, there are water bodies in the zone of anthropogenic impact, which at present have the critically low values of acid-neutralizing capacity. However, most of the baseline lakes are referred to the hydrocarbonate class and have a good buffer stock. Chlorides predominate in the anionic composition of the lakes located in the zone of industrial impact. The calculation of acid-neutralizing capacity of these lakes demonstrates that most of them (about 80%) are sufficiently resistant to fallout of acid-forming substances (ANC > 100).

Factor, discriminatory, and correlation analyses of the chemical composition of 130 natural water samples taken form the small lakes in West Siberia are carried out. Over 50 indicators describing general and trace element composition of the waters are used for the analyses. Based on the results of the factor analysis, the main factors and processes are identified according to their impact on the chemical composition of waters: 1) geographical zoning, 2) regional features (marine influence, humification, salinity), and 3) local factors, both natural (waterlogging, eutrophication) and anthropogenic (man-induced acidification). According to the results of the discriminatory analysis, it is concluded that the water composition in the tundra and taiga zones slightly varies, and a sharp difference in the water composition of the forest-steppe lakes is registered. The correlations between macroand microcomponent contents of natural waters are determined.

The article describes the problems of water acidification. Criteria for process estimation and calculation method of the critical load of acids precipitation, based on the indexes of water chemical composition, are given. The described method is used for the lake of Arantur in the Tyumen area. The tendency of buffer lake capacity renewal is shown over the last years. This tendency conforms with the processes observed in the European lakes on the whole.