| In recent years,with the rapid development of China’s economy and the rapid growth of urbanization rate,the problem of non-point source pollution in mountainous cities has become increasingly prominent.In the city,the traffic is developed and there are many roads,which is one of the main components of the urban surface.Because of the large traffic flow,there are more pollutants in the road runoff during the rainfall,which makes a great contribution to the urban non-point source pollution.The main materials of urban roads are cement and asphalt.The new urban areas are mainly asphalt roads,while the old urban areas are mainly cement roads.However,the existing relevant studies on the characteristics of pollutant migration and transformation in the runoff of the two materials are still relatively simple,and the mechanism explanation of pollutant migration and transformation in the runoff is also lacking,which requires more in-depth study.In this paper,six asphalt and cement roads with different slopes in Shapingba District of Chongqing are selected,and the artificial rainfall test is carried out on the spot by using the constructed simulated rainfall device.The water quality of the road runoff produced is sampled and analyzed,and the temporal and spatial changes of p H,SS,TN,TP and nitrogen and phosphorus pollution in the runoff of asphalt and cement roads with different slopes and different rainfall conditions are obtained The composition and characteristics of things.This study also analyzes the correlation between the pollutants and the relationship between the slope,rainfall and the transport and transformation of pollutants in the pavement runoff.In addition,a simulation test was carried out in the laboratory to further explore the effect of nitrogen and phosphorus pollutants in the runoff intercepted by different pavement materials,and the asphalt and cement pavement before and after the test was characterized by means of SEM,EDS,FT-IR,XPS,bet and other means to clarify the interception mechanism of different materials for each pollutant.The main conclusions are as follows:(1)In each simulated rainfall test,the p H value in the runoff of asphalt and cement roads remained relatively stable and weak alkaline,and the concentration of other pollutants decreased with the increase of rainfall duration.In the process of single runoff,in the early stage of rainfall,there are more pollutants accumulated,which gradually increases with the increase of the process.In the later stage of rainfall,the concentration of pollutants basically remains unchanged or slightly decreases.(2)Under the condition of simulated moderate rain,the p H value in the runoff of asphalt and cement pavement is weak alkaline and relatively stable;the concentrations of SS and TN in the runoff of asphalt pavement are much higher than that of cement pavement,and the concentrations of TP are slightly higher than that of cement pavement;with the increase of slope,the concentrations of SS,TN and TP in the runoff of asphalt pavement gradually increase and then decrease;the concentrations of SS and TP in the runoff of cement pavement first increase and then decrease,while the concentrations of TN are relatively high Degree is different from the former two,showing the trend of first decreasing and then rising.TN and TP in the runoff of the two roads are similar.Nitrogen is mainly nitrate nitrogen,followed by organic nitrogen,while nitrite nitrogen and ammonia nitrogen are less;phosphorus is mainly organic phosphorus and colloidal phosphorus,while phosphate content is relatively low.In order to effectively control the urban non-point source pollution,in the process of urban construction,under the feasible circumstances,the pavement construction should try to select asphalt pavement and increase the slope as much as possible.(3)Under the three kinds of rainfall conditions,the p H value of the runoff of asphalt and cement pavement with small slope is also weakly alkaline and keeps relatively stable;the concentrations of SS,TN and TP in the runoff of asphalt pavement are higher than those of the runoff of cement pavement;and with the increase of rainfall,the three pollutants in the runoff of asphalt pavement change differently,the concentration of SS increases gradually,and the concentration of TN decreases first and then increases,and the concentration of TP is shown in the table Now it is a trend of rising first and then falling.SS concentration in cement pavement runoff increases gradually,and TN and TP concentrations change in the same way,which are rising first and then falling.TN and TP in the runoff of asphalt and cement pavement are similar in form and composition.Nitrogen mainly exists in nitrate nitrogen,followed by organic nitrogen,while nitrite nitrogen and ammonia nitrogen are less;phosphorus mainly exists in the form of organic phosphorus and colloidal phosphorus,and the proportion of phosphate is relatively low.Although the concentration of pollutants in the runoff of asphalt pavement is slightly higher than that of cement pavement,due to the advantages of asphalt pavement,such as flat surface,wear resistance and low noise,asphalt pavement is still preferred in the future urban construction process,but some supporting non-point source pollution control projects need to be properly constructed.The initial runoff control volume should be set at 5-10 mm to better protect the urban water environment.(4)Under different rainfall conditions,TN and TP have little correlation in the runoff of asphalt and cement pavement,while in moderate rainfall,some TN and TP exist in the form of particles in the runoff of asphalt pavement.Under the condition of light rain and heavy rain,part of TN in asphalt pavement runoff exists in the form of particles;part of TP in cement pavement runoff exists in the form of particles.Therefore,increasing the frequency of road cleaning and building some non-point source pollution control projects based on sedimentation,filtration and other technological principles can effectively intercept suspended solids and achieve the goal of simultaneous removal of nitrogen and phosphorus pollution in runoff.(5)The significant degree of influence factors of asphalt and cement on nitrogen and phosphorus pollutant retention effect is: TN,asphalt and cement pavement:temperature>rainfall>prevailing time;TP,asphalt pavement: temperature>prevailing time>rainfall,cement pavement: temperature>rainfall>flowing time.In spring and autumn,asphalt road with large slope has the best interception effect on nitrogen and phosphorus pollutants in light rain runoff.In spring and autumn,the moderate slope cement pavement has the best retention effect for nitrogen and phosphorus pollutants in the runoff.This conclusion has certain guiding significance for the design of urban non-point source pollution in the later stage.(6)Both asphalt and cement pavement materials retain nitrogen and phosphorus pollutants in runoff by physical and chemical means.For asphalt pavement,the amount of retained nitrogen and phosphorus pollutants: chemical action > physical action;for cement pavement,the amount of retained nitrogen and phosphorus pollutants: physical action > chemical action.The occurrence of chemical interception needs more stringent conditions.Therefore,in the current process of sponge city construction,we should pay more attention to the construction of supporting urban non-point source pollution control projects. |
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