Concentrations of main inorganic ions were determined in waters of city streams affected by domestic and industrial wastewaters – the Okhta and the Murinsky creek in St. Petersburg. It was shown that pollution of streams led to increasing of molar fraction for sodium ion and chlorides, decreasing of molar fraction of magnesium, calcium, sulfates and hydrocarbonates. These changes took place while electric conductivity of waters was increasing. Positive correlation coefficients (from 0.35 to 0.63) were found between molar fraction of sodium and parameters connected with wastewater pollutants: total nitrogen (TN), ratio of TN to TOC (total organic carbon), ratio of protein-like fluorescence (excitation at 230 or 270 nm, emission at 350 nm) to humic-like fluorescence (excitation at 230 or 270 nm, emission at 420 nm). The obtained results show that electric conductivity can be applied as an express-method for environmental monitoring in the studied streams. Additional use of ions molar fractions for such purpose is also possible.