Urban activities cause a number of hydrogeological environmental problems. The aim of this study was to undertake hydrochemical evaluation of the urban aquifer system in Seoul, Korea. Well monitoring, hydrochemical analysis, identification of contamination sources, and an assessment of the mechanisms of hydrogeological impact were undertaken. The aquifer quality is affected by several processes of groundwater contamination. Hydraulic conductivity in the Seoul aquifer, geologically consisting of alluvium, granite, and metamorphic rocks, varies from 2.3×10−9 to 1.5×10−3 m/sec. Interpretation of the hydrochemical data indicates that pollution causes a general decline of baseline quality downgradient. Sliding stiff diagrams illustrate the mixing effect between polluted water and background fresh groundwater. Groundwater of the Seoul area is affected by diffuse pollution from industrial complexes, seepage from oil storage tanks, infiltration of sewage, and leachate percolation from abandoned landfills.