Study On The Migration And Transformation Of Heavy Metals In Urban Surface Runoff

Recent years,with the rapid development of urbanization and the continuous expansion of urban area,the traditional roads with good permeability such as farmland and forest land are gradually replaced by the roads with poor permeability such as asphalt,cement and concrete,resulting in the increasingly serious problem of urban runoff pollution,which has become one of the main sources of urban water environment pollution.Therefore,in order to effectively reduce the impact of runoff pollution on the urban water environment,it is imperative to study the migration and transformation characteristics of pollutants in urban surface runoff.There are many kinds of pollutants in urban surface runoff,among which heavy metals,as persistent pollutants,exist in the water environment for a long time and are easy to combine with surface particles and migrate with them.They are typical pollutants that cause runoff pollution,and are widely concerned by researchers.In this study,heavy metals are taken as the main research object.Firstly,through the way of sampling and investigation,the occurrence characteristics of heavy metals in the study area are revealed from the content,form and particle effect of heavy metals in the surface particles,and the corresponding evaluation methods are selected to evaluate the risk of heavy metals from multiple perspectives.Then through the simulated runoff erosion experiment and dynamic column experiment,the migration and transformation of heavy metals in the particles under different rainfall conditions are explored,and the interaction between heavy metals migration and various environmental factors,such as redox potential,dissolved oxygen content,organic matter content,etc.in the biological retention media is clarified.The purpose of this study is to reveal the migration and transformation of heavy metals in the whole process from enrichment to erosion by runoff,and then to rainwater facilities with surface runoff.The main conclusions are as follows:(1)The overall average content of heavy metals from low to high is Cd < as < Pb < Cu < Zn < Cr < Mn.The average content of all heavy metals except as and Cu in the industrial area is higher than that in other functional areas.Most of the heavy metals in the study area are in the form of residue.Cr has the highest proportion of residue,and Mn,Cd and Zn have relatively high proportion of exchangeable and residue.In the area of urban-rural junction,the surface particles are mainly small-sized particles,and the content of heavy metals in particles,except for Pb,has obvious particle effect.The results of heavy metal pollution risk show that the industrial area is the most seriously polluted by heavy metals,the natural area and residential area are almost not polluted by heavy metals,and each area is not polluted by heavy metals.Except for natural areas,there are some potential ecological pollution risks in all urban functional areas.(2)Under the condition of runoff scour,the scour rate of each particle size increases with the increase of rainfall return period,and the scour rate of small particle size is particularly affected by rainfall return period.Under different return periods,with the increase of scour time,the change trend of the content of Zn,Cr,Cd in runoff is relatively stable,and the difference of the change trend among the particle sizes is small,while the change trend of the content of Cu,Pb,Mn in runoff has some fluctuations.With the increase of rainfall return period,the reduction rate of heavy metals in runoff particles increased significantly,and the reduction rate of heavy metals in small particles was more obvious,among which the reduction rate of Cu particles was the highest.Under different rainfall return periods,the variation trend of various dissolved heavy metals in runoff is obviously different.With the increase of the recovery period,the difference of the content of various dissolved heavy metals decreased gradually,and the peak time of the content was earlier.In addition,under different rainfall return periods,the change range of heavy metal content in different forms is relatively low,and there is basically no conversion between different forms.(3)The experimental results of heavy metals migration in the biological retention media show that the leaching of Cu and Zn in the four media is relatively low.With the increase of influent time,DO,pH,ORP and TOC in the effluent of the four media all showed a downward trend,which was positively correlated with the dissolution of Cu and Zn.In the stage of water distribution experiment,the concentration of Cu and Zn in the effluent of the four media decreased obviously,and Cu and Zn in the coarse sand media showed better mobility.The change trend of DO in the media is basically the same as that of Cu and Zn content,indicating that there is a close relationship between do and heavy metal migration characteristics in the media of biological retention media.At the same time,pH and ORP have a great influence on the migration of heavy metals in the medium,while TOC content has a relatively small influence on the migration characteristics of Cu and Zn.