The spatial variations in the chemistry of first- and second-order stream waters (N = 65) were investigated in the easterly bound of Seoul in order to assess the effects of urban air pollution on surface water chemistry. The sulfate (SO4(2-)) was high (range 3.9-17.8 mg l(-1), mean 11.8 mg l(-1)) within 30 km away from the center of Seoul, compared to the levels (range 1.1-7.7 mg l(-1), mean 4.3 mg l(-1)) observed in remote areas (30-70 km away). Both graphical examination and statistical evaluation (variogram) of sulfate concentration data consistently showed the decrease of sulfate concentration with increasing distance. The results of mass balance modeling also indicate that the concentrations of SO4(2-), Cl- and Na+ may be affected mainly by dry/wet deposition. However, the spatial variations of major cations such as Ca2+ and Na+ are well explained by the reaction of rainwater with diverse rocks in the watercourse. The water type was found to change from Ca(-Na)-SO4) type to Ca(-a)-HCO3 type with the increasing distance. It is thus inferred that the pollutants like SO2 emitted from strong man-made source areas of Seoul are transported to the considerable distance (at least 30 km away) by westerlies and that such mechanism may lead to the changes of the anion composition in surface water. In the remote area (> 30 km away from Seoul), the stream water chemistry appears to be influenced more effectively by the weathering of rock-forming minerals.