The environmental assessment of the surface water quality of the Western Bug River has been made using the system of classification quality of land surface water of Ukraine in accordance with the approved methodology, which allows comparing water quality of separate areas of water objects of different regions. The calculation of the environmental as-sessment of water quality has been carried according to three blocks: block of salt composition, block of trophic and sapro-bic (ecological and sanitary) indicators and block of indicators of content of specific toxic substances. The results are pre-sented in the form of a combined environmental assessment, based on the final conclusions of the three blocks and consists in calculating the integral ecological index. Comprehensive studies of changes in the water quality of the Western Bug Riv-er have been conducted within the territory of Ukraine for a long-term period. The water quality of the river on the final values of the integral indicators of the ecological condition corresponded mainly to 4nd category of the 3rd class – the wa-ter is “satisfactory” by condition and “little polluted” by degree of purity (except for points of observation that located within the Volyn region, where the water quality corresponded to 3rd category and the 2nd class. It is “good” by condition and “fairly clean” by the degree of purity). Visualization and part of the analysis are performed using GIS technologies in the software of the ArcGIS 10.3.

Monitoring of surface waters within the transboundary section of the Western Bug River showed, that during 2014–2018, a significant excess of the maximum permissible concentration (MPC) was observed for some substances for fish ponds. As a result of this, the water in the river for these substances was rated as “dirty” in terms of purity and correspond-ed to water quality class IV, namely: phosphorus was observed to exceed the MPC at the observation point Ambukіv vil-lage in 2015 (9.7 times), for manganese – an excess of the MPC at the observation point Ambukіv village in 2018 (9.7 times) and in point Zabuzhzhia village in 2014 (7.9 times), 2015 (8.0 times), 2017 (7.1 times), 2018 (8.3 times); for the total iron – the exceeding of MPC at the observation point Ambukіv village in 2016 (5.95 times) and 2017 (6.13 times); at the observation point Ustilug town in 2016 (5.23 times); in the observation point Zabuzhzhia village in 2016 (9.44 times) and 2017 (5.27 times). The assessment of the surface waters based on the determination of the pollution factor showed that during the study period their quality did not deteriorate but did not meet the norms. In general, surface waters of the river correspond to the second class of quality and are characterized as “poorly polluted” waters by the level of pollution.