Changes in the water chemical composition in the tailings pond and the Kenti lake-river system under the impact of the Kostomuksha mining and processing plant were determined using data of long-term hydrochemical observations (1970–2015). It was found that releases from the tailings pond influenced the Kenti river system the most, and led to a significant increase in water salinity, concentrations of K+, SO42-, NO3-, Ni and Li, and to a decrease in organic matter content in comparison with the natural state. Pollution of the tailings pond waters and Kenti river system was estimated using high-priority pollution indices.