| With the accelerated urbanization process and effective control of point source pollution, urban runoff, as the second pollution source for urban water environment besides domestic sewage and industrial wastewater, has become the main reason for deterioration of water quality. Road runoff, as the strongest polluted part of urban runoff, affects the quality of receiving waterbodies with special severity. In recent years, research about road runoff has developed into an independent field and become multidisciplinary scientific field of environmental science, hydrology, road ecology, physical geography and other disciplines.Based on analysis of road runoff research status, existing problems and necessity of pollution control, a sampling station, which was equiped with a self-made volume proportional collecting device, was built at south second ring road in Xi’an city. Road runoff caused by rainfall events were sampled from Mar.2009to Nov.2010. Pollution characteristics, pollution load, first flush effect and pollution control technology of urban road runoff in Xi’an city were studied in the dissertation.Study on pollution characteristics of urban road runoff in Xi’an city showed that:(1) EMC of main runoff pollutants varied greatly and the median EMC of SS, COD, dissolved COD, NH3-N, Pb, Zn and dissolved Zn were1601mg/L,613mg/L,143mg/L,1.9mg/L,0.042mg/L,0.223mg/L and0.111mg/L respectively. The coefficient of variation ranged from0.47to0.8. EMCs of SS and COD were far greater than the values of Integrated Wastewater Discharge Standard Grade III, so SS and COD were the main pollutants of road runoff.(2) The occurrence of Pb was mainly granular state and46.2%of Zn,26.4%of organic pollutants was dissolved.(3) The correlation of COD, Pb and Zn with SS were significant at0.01level and the correlation coefficients were0.797,0.825and0.893respectively, which showed that particulate matters were carrier for many other pollutants.(4) The descending order for influence weights of rainfall characteristics on pollutants’concentration was as follows: rainfall volume> maximum rainfall intensity> rainfall duration. Antecedent dry period (ADP) had no correlation with EMC of many pollutants except dissolved Zn and dissolved COD.(5) Result of one factor analysis of variance showed that EMC of SS, COD and NH3-N varied with seasons significantly, while the variation tendency of other pollutants, such as dissolved COD, Pb and Zn was not obvious.Study on pollution loads of urban road runoff in Xi’an city showed that:(1) The pollution load of SS, COD, dissolved COD, NH3-N, Pb, Zn and dissolved Zn were4.56-778.39kg/hm2,2.22-308.7kg/hm2,0.76-52.33kg/hm2,0.01-1.39kg/hm2,0.05-33.09g/hm2,1.38-115.82g/hm2and0.27-86.12g/hm2respectively. The amount of pollutants entering into the receiving waters carried by road runoff in different runoff events were quite different and caused shock effect on receiving water bodies.(2) Rainfall had most significant impact on pollution load positively and the correlation coefficients were0.734-0.943at the significant level of0.01. Maximum rain intensity and rain duration were also positively correlated with pollution loads, while ADP had few relationship with it.(3) The established multiple regression models of runoff pollution loads with rainfall, maximum rain intensity, rain duration and cumulative seasonal rainfall passed the model significant test unanimously and can be used for forecast of road runoff pollution load.Annual pollution loads of urban road runoff in Xi’an city were calculated. In addition, impact of sampling size on estimation accuracy of annual pollution load was investigated. The results were as follows:(1) The annual pollution load of SS, COD, dissolved COD, NH3-N, Pb, Zn and dissolved Zn were11090.29kg/(hm2.a),3462.5kg/(hm2.a),812.48kg/(hm2.a),13.49kg/(hm2.a),0.28kg/(hm2.a),1.35kg/(hm2.a) and0.59kg/(hm2.a) respectively, which indicated that urban road runoff in Xi’an city had a rather high polution potential.(2) The pollution loads of urban road runoff in Xi’an city were quite different between various rainfall events as EMC of road runoff varied widely. The mean EMC can not represent the statistical characteristics of road runoff pollution accurately when the sampling size is not big enough. The error of calculated annual pollution load would be quite big if SMC is represented by the mean EMC of the small sample.(3) Results of random sampling showed that when sampling size is more than16, the maximum relative error between SMC with mean EMC is below30%. That is to say, if at least16runoff events are sampled and monitored, the mean EMC of those runoff events can represent SMC with rather high accuracy and the maximum relative error of calculated annual pollution load is below30%.Study on first flush effect of urban road runoff in Xi’an city showed that:(1) The rule of runoff concentration variation was related with the mode of occurrence of pollutants. The concentrations of granular state pollutants, such as SS, COD and Pb, were mainly influenced by rainfall intensity during runoff process, while concentrations of soluble pollutants, such as NH3-N, dissolved COD, dissolved Zn, were gradually reduced during runoff process and were not influenced by rainfall intensity seriously.(2) Urban road runoff in Xi’an city was seriously polluted and the final part runoff after hours’rainfall still had a high pollution level. SS remained as high as358-422mg/L, which was close to or above the value of Integrated Wastewater Discharge Standard Grade Ⅲ. COD remained as high as179-367mg/L, which exceeded the value of Integrated Wastewater Discharge Standard Grade II.(3) First flush phenomena of road runoff were not common, which was influenced by the mode of occurrence of pollutants and rainfall characteristics. Dissolved contaminants in runoff were prone to first flush phenomena, while first flush effect of granular state pollutants were seriously influenced by rainfall characteristics.(4) The load ratios of SS, COD, dissolved COD, NH3-N, Pb, Zn, and dissolved Zn of initial30%runoff volume to that of total runoff volume were21.8%-50.0%,25.5%-49.3%,36.3%-52.6%,52.6%-66.7%,26.8%-45.0%,27.2%-63.4%and36.2%-62.6%, respectively, which indicated that only initial runoff treatment will not successful pollution control of urban road runoff in Xi’an city be finally realized.Study on pollution control technology of urban road runoff in Xi’an city showed that:(1) Road runoff pollution in Xi’an city should be controled successfully through the combination of non-engineering measures and engineering measures. The minimization of contamination was suggested from the full process of discharges of contaminants in the runoff.(2) Sedimentation was an effective method to remove particles in runoff. The average removal efficiency of SS was60.2%of60minutes’settling and72.5%of2hours. The surface loads achieving same removal efficiency of different runoff events varied greatly. In order to achieving removal efficiency of45%、60%and80%, the surface loads of sedimentation tank were1.1-9.3cm/min,0.6-5.9cm/min and0.1-2.7cm/min respectively. The surface load decreased1.3-3times as the required particle settling removal efficiency increased10%.(3) The particle size distribution of road runoff varied greatly with regions and rainfall events. The dio, dso and d90of road runoffs in Xi’an city were3-23μm,17-56μm and40-65μm respectively. It’s obvious that most of the particles in runoff were fine particles and we should pay more attention to the pollution intensity and moving and changing rules of fine particles with the diameter of3-65μm.(4) The effective filter height of interception bag made by geotextile of130g/m2，150g/m2and300g/m2was at least75mm,94mm and175mm respectively, and the overflow will not occur under the condition of5-year recurrence interval runoff flow. In addition, removal rates of SS of interception bag made by geotextile of130g/m2,150g/m2and300g/m2was56.0%,69.4%and76.5%respectively. |
PDF Full Text Request