Abstract: Climate change is an important factor affecting material transport in the basin. Annual rainfall has increased significantly in recently years in the North China. The environmental impact of material transport should be considered during the construction of sponge city. In this study, river water samples in four important sections of the Haihe River and samples from city roadrunoff were collected during rainy season to explore the effects of heavy rainfall events on the dynamics of nitrogen pollution in Haihe River in Tianjin. The rainfall data were taken from the relevant hydrological stations. The results showed that NO₃^--N concentrations of all the studied sections sharply increased after the heavy rainfall events and recovered to pre-rainfall levels after one week. However, δ¹⁵NNO₃^- and δ¹⁸ONO₃^- first decreased and then increased. Integrated analysis suggested that mixing effects, biogeochemical processes and different responses to rainfall affected the dynamics of nitrogen pollution of the Haihe River in Tianjin. The substantial increases of nitrogen load during the rainfall period indicated that the factitious pollution during rainevents was very serious, which led to elevated nitrogen pollution in urban rivers. In summary, the material transportation in urban river during the rainfall period did not follow the dilution effect. Controlling of urban point source pollution should be given priority in the management of urban water environment quality.

Key words: temporal and spatial variation, soil acidification, Guangdong Province