Formation of Water Chemistry of Small Rivers of the Volga Basin in the Tver Region during the Summer Low-Water Season

Studies of natural processes of river water chemistry formation and the effect of anthropogenic factors on it remain a topical task of hydrochemistry. In July 2021, water quality was assessed based on hydrochemical indicators of the upper reaches of 29 small rivers in the Volga River basin within the Tver region. The location of most of the gages was selected, if possible, in homogeneous landscape conditions of catchments, minimally affected by economic activity. The obtained characteristics can be considered as close as possible to natural background indicators for the summer low-water period. The analysis of spatial differentiation of natural factors was carried out on the basis of landscape maps, geochemical and hydrogeological cartographic materials of different years of publication. The set of hydrochemical characteristics was processed using the method of hierarchical cluster analysis. Significant spatial differentiation of the chemical composition of waters was revealed, and landscape diversity was not found to play a clear determining role. The chemical composition of water in small rivers is mainly influenced by two factors. The high degree of swampiness of the catchment area leads to an increase in the concentrations of Fe, organic matter by COD and PO, phenols and color index in surface waters. The presence of channel lakes levels out this factor to some extent, which is most noticeably manifested in the association of river waters of the Volga and Selizharovka. The second factor is the recharge by deep groundwaters, which increase the TDS of river waters, the content of the main ions, F, Sr, and P, and the leaching of silicates from bedrock. The combined effect of both factors apparently causes the high concentrations of Mn characteristic of the region.