Characteristics Of Particle Size Distribution And Heavy Metal Pollution In Urban Road Runoff

With the increasing urbanization in China,road runoff has been the primary contributor of urban nonpoint source?NPS?pollution.The pollutants containing in road runoff are mainly derived from the wash-off of road-deposited sediments?RDS?and associated organic matter,nutrients and heavy metals?HMs?,et al.In light of the high toxicity,persistency,concealment and irreversibility of HMs,the discharge of road runoff into receiving water body will pose increasing risk to aquatic ecosystem and even human health.In order to control the urban NPS pollution effectively,it is indispensable to investigate the distribution of HMs in RDS,as well as the characteristics of stormwater pollution derived from RDS wash-off during rainfall process.As HMs generally adsorb on the surfaces of particles,HM pollution of road runoff is closely associated with the distribution of HMs in RDS of different sizes,and the mobility of RDS.Therefore,in this study,the size distributions of stormwater particles and associated HMs were studied through laboratory experiments and theoretical analysis to elucidate the process of HM pollution in road runoff.The research outcomes provide valuable information for urban NPS pollution control.The research work is supported by the National Major Science and Technology Program for Water Pollution Control and Treatment.In urban area of Yixing City,representative traffic roads were selected to collect RDS and runoff samples.By laboratory measurements and data analyses,the method to characterize the particle size distributions?PSD?of stormwater runoff was firstly studied.Then,the transport processes of HM pollution in road runoff were analyzed,and relevant influential factors were discussed.The research elucidates the characteristics of HM pollution in road runoff.The main outcomes of this work are as follows:?1?Based on the differences of the scattered light intensity by dispersant with diffrent particle sizes in water column,the ratio of turbidity and total suspended solids?Tur/TSS?of water sample was proposed as a synthetic indicator characterizing the PSD of stormwater runoff.Coarse disperse system presents relatively low Tur/TSS value while fine disperse system presents relatively high Tur/TSS.The PSD analyses and SEM images of RDS and stormwater particles indicated that the proportion of nano-and submicron-scale?<10?m?particles is closely correlated with Tur/TSS value.By Tur/TSS measurements,the PSD of disperse particles in stormwater runoff can be quickly understood.?2?The transferal of HM pollution from RDS to stormwater runoff during transport processes was elucidated by analyzing the differences of PSD between RDS and stormwater particles,the heterogeneity of typical HMs associated with RDS of different sizes.It is found that fine particles were so easy to be transported by stormwater that the associated HMs were the dominated contributor to stormwater pollution.Taken five typical HMs as examples,the average Cu,Pb and Zn content in stormwater particles were 2.2⁶.3 times higher than that in RDS,while the average Cr and Ni content were comparable to that in RDS,suggesting that Cu,Pb and Zn preferred to accumulate in fine particles.The HM evaluation of stormwater particles with geoaccumulation index(I_geo)and potential ecological risk index?RI?also showed higher risk of Cu,Pb and Zn than that of Cr and Ni.Furthermore,the HMs pollution of stormwater particles increased linearly with Tur/TSS.?3?The impacts of traffic condition,road slope and antecedent dry days?ADD?on RDS and associated HMs were investigated.For typical traffic roads,the content of Cu,Pb and Zn preferring to accumulate in fine particles increased with higher vehicle volume.Specifically,Cu and Pb were closely correlated with car volume,and Zn is closely correlated with lorry volume.The influence of road slope demonstrated that higher proportion of fine particles generally deposited on flat roads.Furthermore,higher RDS loads and percentage of fine particles were observed with longer ADD.Therefore,appropriate strategies should be adopted to remove RDS according to RDS characteristics under various conditions,especially those with longer ADD,in order to attenuate the HM pollution of stormwater runoff from source.?4?Taken the relationships between RDS and HM pollution of stormwater runoff into consideration,the research proposed the control strategies of HM pollution in urban stormwater runoff.Firstly,as the source of HM pollution,large amount of RDS will lead to high risk of HM pollution in stormwater runoff,and thus should be eliminated timely by effective measures.Secondly,the research found that the risk of HM pollution increased linearly with higher Tur/TSS of stormwater,highlighting the necessity of controlling stormwater with high Tur/TSS.And the preferential control area can be identified according to traffic condition,topographical feature and rainfall characteristics.Thirdly,given the characteristics of HMs associated with particles,control of the particles in stormwater runoff,especially in the initial runoff,is of great significance in stormwater pollution control.The best management practices should be chosen to enhance the removal of particles.