Smalii O. Regularities of the Hydrological and Hydrochemical Regime and Surface Water Quality Formation of the Siverskyi Donets River in the Early 21st Century

The thesis is devoted to identification of the patterns of fluctuations in the hydrometeorological characteristics, the chemical composition and quality of water in the early 21st century under conditions of the regional climate change and the significant anthropogenic load. The meteorological and hydrological data from the beginning of observations up to 2015 and the hydrochemical data for the period from 1990 to 2015 were used in the paper. The main trends for the fluctuations in climatic factors of the annual runoff (precipitation and air temperatures) were identified. A statistically significant trend has been revealed in fluctuations of average long-term air temperatures since 1989. The basic research methods are the methods of linear regression with paired data, rank criteria and difference integral curves. Directed changes in annual precipitation fluctuations were not revealed. It is shown that fluctuations in the annual runoff at various gauges within the basin are in-phase, but not synchronous owing to the anthropogenic impacts (the runoff diversion from the Dnipro River and along the length of the river; the discharges of industrial, mine and municipal water into surface watercourses; long-term runoff regulation by reservoirs etc.). The anthropogenic influence retards the runoff response to the changes in climatic factors. An assessment of the environmental status of the river according to the degree of water resource use is «very unsatisfactory», «unstable» and with a low level of environmental reliability. The hydrochemical parameters were also considered. It was found that the main pollutants with the greatest excess of maximum permissible concentrations are chromium 6+, petroleum products, zinc and iron. Pollutants such as nitrites and ammonium are emitted at the tributaries with significant discharges of municipal wastewater. The oxygen regime of rivers, the relationship between the chronological course of air and water temperatures, and the BOD5 regime were studied in the paper. It was determined that the oxygen regime of the rivers is satisfactory and the average concentration of dissolved oxygen varies from 8.0 to 10.0 mg O2/dm3. A decrease in its content up to the range of 4.0 – 6.0 mg O2/dm3 was observed on the Udy and Bakhmutka rivers. Statistically significant trends of water temperature fluctuations were not revealed in the period of 1990 – 2015. It was found that wastewater can have a significant effect on the water temperature regime. The temperature rise can reach 5 – 7 ℃. An assessment of the qualitative status of water was based on BOD5. The river waters are classified as «moderately polluted» and the environmental status is «a threshold». The environmental status at the observation points within the city of Lysychansk was recognized as «irreversibly altered» and the water as «polluted» during the entire observation period. According to application of the «vulnerability criterion», no zones affected by pollution with nitrogen compounds have been identified. The highest values of the criterion were found for the Udy River (9 km downstream from Kharkiv) and the Bakhmutka River at the city of Bakhmut (0.2 km downstream from the city). An integrated water quality assessment, based on hydrochemical observation data and maximum permissible concentrations, was performed. An assessment of the river status by hydrochemical indices (a generalized environmental index by the national methodology, and a generalized water status index by the methods of OSENU) was given. It is shown that the river waters at the Siverskyi Donets Basin are characterized mainly as «polluted» and «very polluted». The dependences of the river pollution degree on the water content were identified by means of an analysis of the relationships between the block indices and the annual runoff module. The research results can be used in the development of programmes for the environmental enhancement of the Siverskyi Donets Basin, introduction of the latest monitoring projects, development of international cooperation to reduce transboundary impacts on the basin ecosystem, the public engagement and raising environmental awareness.