Study On The Discharge Characteristics Of Road Runoff Pollution Of Xi’an City

With the accelerated urbanization process, the increase of the vehicles and pavement in the city. As an important part of the second physical geography of urban impervious surface,traffic road pavement is not only an important source region of runoff pollution, but also provids a broad collection space of rainwater harvesting and utilization. As part of the urban road runoff most polluted, road runoff was widely attention. Western countries have adopted the second generation of low-impact development techniques on runoff disposal, in order to reduce runoff flow, control runoff pollution and improve the ecological environment and other purposes. Our country has begun to set points for "sponge city", trying to achieve maximum accumulation of rainwater infiltration of rainwater infiltration and purification within the cities limits, and promoting the use of rainwater resources, control of runoff pollution and protection of environmental.Sampling station of road runoff at south 2nd ring road in Xi,an city, and artificial equal time interval sampling method was adopted in this study. Road runoff caused by 13 rainfall events were sampled from July. 2014 to Aug. 2015. Pollution emission characteristic,first-flush effect of urban road runoff in Xi’an city of atmospheric dust, serious shortage of water resources of western China city were studied in the dissertation. Pollution emission characteristic, first-flush effect of the process of SS, COD, dissolved COD, heavy metal and dissolved heavy metal Cu, Pb, zinc, Cr, Ni, Cd of urban road runoff,and the instantaneous sample number influence on the accuracy of the session EMC estimate results were studied in the dissertation. The main conclusions can be summed up as follows.1.Urban road runoff pollution emission characteristics(1) Road runoff pollution emission characteristics is influenced by the rainfall characteristics and the mode of occurrence pollutants. Regardless of the type of rainfall runoff events, dissolved pollution can achieve maximum concentration at the beginning of the runoff,and it is not affected by the capability flush. And particle state pollutant maximum concentration were slightly behind the time of maximum rainfall intensity occured. For less rainfall intensity and runoff in small rainfall events,runoff scour surface,carrying grain state pollutant ability is limited. Because the whole runoff pollutants concentration fluctuation issmall, even in the late runoff, pollutants concentration also maintain a higher level. And rainfall intensity changes varies, and occurring heavy rainfall, rainfall intensity peak appeared strong flushing action and it has resulted in a particle state runoff pollutants concentration significantly increased, and then quickly reduced.(2) Road runoff pollution varied greatly, not only in the process of single field runoff pollutants concentration fluctuations,but also significantly different session runoff pollutants concentration is a wide variation. Each runoff events of SS, COD maximum concentration range were far greater than the values of Integrated Wastewater Discharge Standard Grade Ⅲ,the highest than standard 13.15, 3.58 times respectively. And most event of SS, COD minimum concentration is greater than Integrated Wastewater Discharge Standard Grade Ⅱ,which indicated that SS, COD were the main pollutants and need to focus on. While the heavy metal pollution is also indispensable, all events of Pb, Cr maximum concentration is higher than Surface Water Environment Quality Standard Ⅲ, the highest than standard 4.9, 3.96 times respectively. And part of the events of Zn, Cd maximum concentration is higher than Surface Water Environment Quality Standard Ⅲ. If discharged directly without treatment,will increase heavy metal pollution levels and more serious effects on the water environment quality of receiving waters.2. First flush effect of urban road runoff1) This study tests the 13 runoff show that in different rainfall events, the difference of proportion of early runoff pollution load carried was significant. If we take effective measures for road runoff in Xi’an, only dealing with the initial runoff will not be able to get the satisfactory results.2) The pollutants of all runoff were not appear strong first flush effect, while part are moderate and weak, and only a small does not appear in all runoff events.3)The presence or absence and the extent of the first flush effect is influenced by the rainfall characteristics and the type of pollutants. The extent of the first flush effect of typeⅠ,Ⅱ rainfall events is more obvious than type Ⅲ of the "multimodal" rainfall events. and strong early rain,total rainfall event Rainfall Ⅱ smaller rainfall,rainfall intensity Ⅰ type small first flush runoff event effect is more obvious. SS, COD, dissolved COD of the initial degree of erosion obviously slightly stronger than the other pollutants. Heavy metals slightly strongerinitial total extent of erosion in its early dissolution of heavy metals and erosion degree and dissolved Pb, Cr, Cd initial degree of erosion was significantly weaker than the rest of heavy metals.3. the number of transient road runoff samples screenings EMC estimates affect the accuracy of the results1) When the number of samples were collected less, EMC of sample concentration combined with runoff volume to calculated was greater dispersion of the coefficient of variation is relatively large. The number of samples with transient increase, EMC was gradually reduced and stabilized and the coefficient of variation was decreased.2) When instantaneous sampling size is more than 13, the maximum relative error between the pollution index calculated value of EMC with EMC true value(approximate value) is below 30%. That is to say, if at least 13 are sampled and monitored, the maximum relative error between calculated value of EMC with EMC true value is below 30%.