Study On Characteristics And Influencing Factors Of Heavy Metal Pollution In Urban Soils

Urban soil is the most important storage and decomposition of heavy metal and other pollutants.Heavy metal elements from the industrial "three wastes",motor vehicle exhaust and dust,fossil fuel combustion,living waste and other sources continue to be discharged into the urban soil.The accumulation of heavy metals will be a huge burden on the biogeochemical cycle of urban ecosystems,causing deterioration of soil function,changes in soil properties and other environmental problems,having a huge risk to the health of the inhabitants,especially to children.The study of heavy metal pollution in urban soil has been the hotspot of domestic and foreign research.At present,the study of heavy metal pollution in urban soil is in the ascending period.But due to the high cost of heavy metal sampling and test,the investigation of heavy metal pollution in the whole city needs a lot of manpower and material resources,a comprehensive survey of urban soil heavy metal pollution characteristics and spatial distribution of the study is still rare,the methods of source apportionmen of urban soil are also relatively simple.In addition,there is little research on the coupling analysis of urban heavy metal pollution and its influencing factors combined with the characteristics of urban areas.Therefore,this paper takes the typical urban city of Yangtze River Delta in China as an example to study the urban soil pollution characteristics and spatial distribution pattern in the process of rapid urbanization,and selects the relevant factors to quantitatively characterize the characteristics of heavy metals in urban soils,with a view to providing decision-making basis for urban management and planning.The main conclusions are as follows:(1)Overall,the average contents of Cu,Zn,Pb,Cd,Ni,Hg and As in Nanjing city soil are only slightly higher than Nanjing natural background value,except that the average content of soil Cr in Nanjing is lower than the background value.But the enrichment of heavy metals of some samples is obvious.The heavy metal elements of Cu,Zn,Hg and Pb in Nanjing are disturbed by external factors such as anthropogenic activities,which show the dispersion and enrichment of non-natural sources.There are significant differences in the main urban area,suburbs and rural areas of Nanjing.Pb,Zn,Hg and Cu should be the focus of the heavy metal pollution control in Nanjing area.The distribution of Cr,Cd,Ni and As is relatively uniform,which means that the four elements are mainly affected by the intrinsic factors such as parent material.(2)The average nemerow pollution index of Nanjing is 0.81,the overall view of heavy metal pollution is at the level of alert.The average potential ecological risk index is 92.40.The overall potential ecological risk is low.However,the coefficient of variation of nemerow pollution index and hakanson potential ecological risk index is large,which means nemerow pollution index and hakanson potential ecological risk between the sample points are highly different.There are significant differences in the nemerow pollution index and hakanson potential ecological risk index of heavy metals in the main urban area,suburbs and rural areas of Nanjing.The size of the average Nemero index is the main urban area>suburbs>villages,the size of ecological risk index is the main urban area>villages>suburban,the heavy metal pollution of main urban area is higher than the suburbs and rural areas,indicating that the strong disturbance of human activities in the main urban area makes the heavy metal pollution of the soil heavier.There are some differences in the contents of heavy metals in the four functional areas of commercial area?residential area?public green area and science and education area in Nanjing.However,the differences are not significant.Heavy metal pollution as a whole shows a distribution characteristics of that commercial area>residential area>public green area>science and education area.(3)Kriging interpolation of the geo-accumulation index of heavy metal elements,nemero pollution index and potential ecological risk index in Nanjing are carried out.The geo-accumulation index of Pb,Zn,Cu and H,nemero pollution index and potential ecological risk index show similar spatial distribution,The decreasing distribution of the main urban-suburban-rural gradient shows that the heavy metal pollution in Nanjing is greatly affected by human activities.High-value areas are relatively high associated with the main city and the business district,which means commercial and life activities are the main human factors of Nanjing soil heavy metals.The geo-accumulation index of As,Cd,Cr and Ni are at the safe level,high-value region in general is not highly associated with the main city and commercial and chemical plant feature points.The spatial distribution of the geo-accumulation index of As,Cd,Cr and Ni is different but has certain continuity,indicating that these four heavy metal elements mainly by the parent material,geological and other internal factors.(4)Comprehensive application of principal component analysis(PCA)method and positive definite matrix factor(PMF)analysis method to analyse the sources of heavy metals in urban soil of Nanjing.Taking the principal component analysis method,four potential sources of the generalized elements of urban soil in Nanjing are extracted,On the basis of this,the positive definite matrix factor analysis method is used to quantitatively characterize the source contribution of each source to heavy metal elements.Considering the Q value and the PCA analysis result,four calculation factors are determined.Combined with the contribution spectrum of the analytical component,the spatial distribution of each factor and the results of PCA analysis.Factor 1 is identified as a foreign input,factor 2 is identified as soil physical and chemical properties,the soil physical and chemical properties of factor 2 is related to the elevation,factor 3 is identified as soil parent material source,factor 4 is identified as human activities.In terms of heavy metals,the contribution of human activities to Hg,Pb,Zn and Cu is the highest,which was 38.9%,25.31%,21.83%and 19.65%,respectively.The contribution to Cd,Cr,Ni and As is only 9.37%,7.93%,6.57%and 5.63%,respectively.(5)Among the geochemical elements,P and Ca,which are disturbed by anthropogenic activities,are positively correlated with heavy metal elements,nemetro pollution index and potential ecological risk index,which are also greatly disturbed by human disturbance,indicating its pollution(living pollution,building materials dust)and heavy metal pollution has a certain homology.Among the factors of human activities,each of them has a positive correlation with the heavy metal pollution in the soil,that is,the heavy metal pollution is more serious in the areas with strong human activities.The population density is the most important factor affecting the heavy metal pollution in Nanjing,which is positively correlated with multiple heavy metal elements and nemetro pollution index and potential ecological risk index.The correlation between other factors of human activities and heavy metal is not high and the contribution is not high,which indicates that they are not the main human activity factor affecting heavy metal pollution in Nanjing.(6)Multiple regression and path analysis results of nemetro pollution index and potential ecological risk index show that human activities has a significant effect on the heavy metal pollution in Nanjing,and heavy metal pollution also shows characteristics having a certain homology with Ca and P pollution.The main human activities affecting the Nanjing urban soil heavy metal pollution are commercial and life activities.Therefore,in order to reduce the heavy metal pollution and ecological risk of urban soil,it is necessary to further promote the construction of urban sub-centers in urban planning to guide the outflow of urban population.