Chongqing Highway Pavement Runoff Pollution Characteristics Study

As the rapid development of highway, the environmental issues around it come into being continuously, especially the non-point source pollution caused by rainfall runoff. Due to the geographical factor, the highways in Chongqing were mostly constructed in mountainous region and the slope protection soil close to the roadsides was mainly the purple soil with low degree of weathering, along with the wet weather, large and concentrated rainfall, which could easily cause the soil erosion. The other factors may increase the pollution of highway surface runoff in Chongqing that included low pH value of acid rain, the traffic emissions, the leakage of fuel and lubricants and the tires friction. The real situation of the pollution of highway surface runoff in Chongqing was still unknown.The research objects of this thesis were gathered at Lanhai highway (Yuwu section) G75 and Circle highway in Chongqing G5001,and the ordinary road and suspension bridge were studied. During the period of January to September in 2009,10 batches including 55 highway surface runoff samples were collected, furthermore, duration sampling was applied in three rainfall incidents happened on April 30,2009 (spring),June 9,2009 (summer) and August 29,2009 (autumn) respectively,at the same time, dust samples and rainfall samples had been collected by means of dust dissolving simulation experiments. On these foundations, pollution emission characteristics and the influencing factors of highway surface runoff were studied. The related strategies and measures of pollution control in highway surface runoff were put forward to provide scientific bases for executive decisions. The main conclusions were listed as follows:The results of samples monitoring in Chongqing highway surface runoff showed that pH value and COD met the normal distribution and the arithmetic mean was 7.63mg/L and 293.16mg/L respectively. The indexes of SS, BOD5, TN, TP, Cu, Zn, Pb and oil were better in line with lognormal distribution and their geometric mean concentrations were 174.39,2.11,3.17,0.10,0.08, 0.59,0.34 and1.75mg/L respectively. Based on the standard of GB8978-1996, the comprehensive evaluation by Nemerrow index indicated that the pollution degree of Yuwu highway and Circle Highway reached moderate pollution and slight pollution respectively and the main pollutants in turn were COD, SS, Pb and oil. By comparison with other domestic research results, SS concentrations in Chongqing highway surface runoff were generally lower than those in other cities, the biodegradability was poorer with higher COD concentrations and lower BOD₅ concentrations.Correlation analysis of the pollutants in Chongqing highway surface runoff indicated that there existed significant or extremely significant correlation between indexes except the correlation between Cu and other indexes. BOD₅/COD of the runoff was between 0.002 and 0.069, which is far less than that of the domestic sewage and was also lower than that in other domestic studies. The result reflected the poor biodegradability. Moreover, with the increase of the concentration of SS, the ratio of BOD₅ and COD declined. This indirectly showed that the amount of road dust determined the control measures of the runoff sewage.Analysis of main components of pollutants in Chongqing highway surface runoff showed that the first component reflected road surface dust, the second, traffic exhaust and the third, rain. Variance analysis exhibited that all concentrations of runoff pollutants didn't reach the significant differences between the ordinary road surface and the suspension bridge in the Chongqing highway. The concentrations of COD, BOD5, Oil and other organic pollutants reached the significant differences between these two highways, it reflected that the impact of the traffic flow on runoff perhaps related to the settlement of organic exhaust emitted from the vehicles. The comparison between the four seasons indicated that all indexes were relatively higher in winter except the index of SS, and the degree of pollution in four seasons was spring>winter>summer>autumn. There had no obvious correlation between water quality of runoff and that of rain.The smaller the particle diameter of surface dust on Chongqing highway was, the higher the pollutants concentration being loaded was, and more serious the pollution was. Load coefficient and cumulative load coefficient showed that more than 80% pollutants accumulated in dust particle whose diameter was smaller than 2.0 mm.The simulation experiments of acid rain leaching exhibited that the leaching ratio of the pollutants was between 0.76% and 15.94%. The particulate pollutants produced along with the rain had higher leaching ratio.Through establishment of the simplified and quantitative model of first flush effects of the highway surface runoff, the first flush effects of rainfall incidents happened on the April 30,2009 and August 29,2009were determined as 68.11% and 50.48% respectively and the duration of the first flush effects was 15min and 25min respectively. The rainfall incident happened at June 9,2009 indicated no obvious first flush effects.69.37% to 92.21% runoff pollution loading could be removed by intercepting 30% initial runoff flow.On account of the limited condition, the sampling frequencies and the sampling sites within the study were not too much, so the final results need further confirmation. Quantifying the contribution of the main pollution sources effect size and the direction and mode of action of pollutant factors in highway surface runoff and etc need to be studied in future.