Distribution, Source And Quality Assessment Of Heavy Metals In Urban Street Dust Of Xigu Region

Urban street dust is an important research object in urban environics.Compared with urban soil and atmospheric particles,urban street dust is an environmental medium with complicated composition and multi-sources.Influenced by the human activities including traffic transportation,industrial production and urban construction,etc.,urban street dust itself accumulates a component of pollutants such as heavy metals,contains important indicator information for environment pollution,and becomes a significant source of urban environment pollution.As a valley and a heavy industrial city,Xigu region of Lanzhou city has more than a half century's history of industrial development.Heavy metals accumulated in the street dust of this city may be not only a non-point pollution source of ecological environment,such as urban soil and rivers,but also a potential threat to urban human health.And so,it significantly has realistic and scientific meaning to study on heavy metal pollution in urban street dust at the study region.The particle size composition,organic matter content and concentrations of six heavy metals(Cu,Zn,Pb,Ni,Cr,Cd)in urban street dust of Xigu region were determinated.On this basis,this dissertation studies content levels,accumulation characteristics,source, pollution status and potential ecological risk of heavy metals.The main conclusions of this study can be summed up as follows:(1)Rough sand particle(1-0.25mm)content(w/w)accounted for only 7.79 percent, fine sand particle(0.25-0.05mm)content(w/w)47.05 percent,and powder clay particle (＜0.05mm)content(w/w)45.16 percent.The contribution ratio of natural dustfall and human activities are 37.67 and 62.33 percent,respectively.The average organic matter content is 7.10 percent,higher than those of topsoil and gray calcium soil of Lanzhou city.(2)The results of statistical analysis indicate that:①The average contents of Cu,Zn, Pb,Ni,Cr,Cd in urban street dust of study region are 75.70mg/kg,350.03mg/kg, 110.77mg/kg,167.38mg/kg,144.24mg/kg and 2.87mg/kg respectively,which are 2.99, 3.38,2.38,3.70,1.37,1.74 times as high as those of heavy metals in suburb street dust of control points.②The contents of Ni and Cr have higher levels in the world,and Cu is at a lower level.However,the contents of Zn,Pb,Cd are at medium levels in domestic and lower levels in abroad street dust.③The contents of six heavy metals are higher than crustal abundance and natural background in alkaline soil of China.Moreover,Cd,Zn,Ni are significantly cumulative and may threat to human health and roadside plant.(3)The distributions of heavy metals in street dust:①The impact of anthropogenic activities' intensity on the accumulation of heavy metal contents is positive,and the content distribution from high to low is following northern industrial area,central buffer zone,Southern residential area and suburb control point.What is,from north to south,the concentrations of heavy metals gradually reduce,especially for Zn,Pb,Ni and Cr.②The distributions of Cu and Cd are less influenced by land use types.Industrial activities play a more important role in the contents of Zn,Pb and Ni in street dust of Xigu region,but not the same to the content of Cr.③The difference of three dust particle sizes have no obvious impact on the contents of Cu and Zn.However,Pb and Cr are more easily accumulated in street dust particles of 0.25-0.074mm,Ni those of 0.074-0.050mm,and Cd those of＜0.050mm(powder clay).(4)The results of pearson correlation analysis(CA)and principal component analysis (PCA)showed that there are significantly positive correlations among the contents of Cu, Zn,Pb and Cd,which may mostly come from traffic transporation and industrial production emissions.Furthermore,the contents of Zn,Pb and Ni also have significantly positive correlations,and the source maybe fretted steel products.However,there is no significantly correlation between Cr and any other heavy metal or organic matter content, which may indicate that Cr mainly comes from weathering soil.Totally speaking,the source of industrial and traffic activities occupies 61.54 percent when the nature source occupy 18.57 percent and the other source 19.89 percent.(5)The queue of geoaccumulation indexes of six heavy metals in street dust is:Cd＞Ni＞Pb＞Zn＞Cu＞Cr,and that of potential ecological risk indexes is:Cd＞Ni＞Pb＞Cu＞Zn＞Cr.The potential ecological risk of Cd is the highest element whose content is LevelⅤ,and all the other five element contents are LevelⅠ.However,total potential ecological risk index of six heavy metals is 698.29,LevelⅣ.