The article describes the changes in the chemical composition of surface waters under the impact of the exploited Korpangskoye iron ore deposit of the Kostomuksha mining and processing plant. The rate of contamination of the water bodies within its impact zone was found to be higher than at the Kostomukshskoye iron ore deposit. There has been a growing trend in mineralization and the content of K⁺, SO₄^2-, NO₃^-, Li and Ni in the surface waters. Contamination of the water bodies was assessed against national and regional maximum permissible concentrations.