SUMMARY
The purpose of this study is to analyse the hydrochemical regime and surface water quality of the upper reaches of the Tisza River from its source to Tiachiv, which also includes the transboundary section of the river. The data of the Borys Sreznevskyi Central Geophysical Observatory of the State Emergency Service of Ukraine for the period 1984-2018, as well as the results of route surveys during 2015-2018 served as materials for the study. The hydrological parameters of the river, in particular the level and flow of water, the distribution of the flow and the change of power sources throughout the year were analysed. Based on of the collected materials, a detailed analysis of the hydrochemical indicators of the water quality of the Tisza, Black Tisza, and White Tisza rivers was carried out. During the research, the natural conditions of the territory and economic activity were studied and their influence on the values of physic-chemical indicators, main ions and mineralization of water, biogenic substances, heavy metals and specific pollutants was analysed. The ratio of seasonal, average annual, average multi-year and maximum permissible concentra-tions (MPC) of hydro-chemical indicators was considered to determine their seasonal variability and multi-year dynamics. The role of natural factors in the formation of the chemical composition of river waters is determined, and the main sources of anthropogenic influence are also indicated. Seasonal variability of the indicated groups of indicators was also analysed considering different phases of river water content. It was determined that the waters of the headwaters of the Tisza River are calcium carbonate, moderately fresh with average mineralization (192-245 mg·dm-3) and clear seasonal variability. Cases of increased concentrations of nitrogen-containing compounds in the waters of the upper reaches of the Tisza River due to the intensive decomposition of organic substances from house- hold wastewater were revealed. High concentrations of iron in river waters were recorded, which is related to the peculiarities of the geological structure of the territory. The seasonal variability of the content of synthetic surfactants was established. A decrease in the content of specific pollutants during the study period was observed.