First flush effect of urban trunk road runoff in Xi＇an
西安市城市主干道路面径流初期冲刷效应

Urban trunk road runoff caused by three rainfall events were sampled by labor with equal interval of time during the rainfall process at south second ring road in Xi’an from Sep.to Nov.2010.Concentration variations of SS,COD,dissolved COD,NH3-N,Pb,dissolved Pb,Zn and dissolved Zn were tested during runoff processes.The first flush effect and the influencing factors of road runoff were investigated.The result shows that urban trunk road runoff of Xi’an is seriously polluted and the final part runoff still has a high pollution level.The rule of runoff concentration variation is related with the mode of occurrence of pollutants.The concentrations of granular state pollutants,such as SS,COD and Pb,are mainly influenced by rainfall intensity during runoff process,while concentrations of soluble pollutants,such as NH3-N,dissolved COD,dissolved Zn,are gradually reduced during runoff process and are not influenced by rainfall intensity seriously.First flush phenomena of road runoff are not common,which is influenced by the mode of occurrence of pollutants and rainfall characteristics.Dissolved contaminants in runoff are prone to first flush phenomena,while first flush effect of granular state pollutants are seriously influenced by rainfall characteristics.The load ratios of SS,COD,dissolved COD,NH3-N,Pb,Zn,and dissolved Zn of initial 30% runoff volume to that of total runoff volume are 21.8%~50.0%,25.5%~49.3%,36.3%~52.6%,52.6%~66.7%,26.8%~45.0%,27.2%~63.4% and 36.2%~62.6%,respectively.Thereby,it can be concluded that only initial runoff treatment will not realize the pollution control of urban road runoff successfully in Xi’an.