| With the rapid development of regional socio-economic, the accumulation and pollution of heavy metals in urban soils has become increasingly prominent, directly related to soil quality and human quality of life. The study of distribution and sources of soil heavy metals can provide a theoretical basis to protect the ecological environment and construct suitable living environment in some sense. Soil heavy metals concentrations along the typical urban-transect in Shanghai were analyzed to indicate the effect of urbanization and industrialization on soil environment quality. Spatial variation structure and distributions of 5 heavy metals (Cu, Cr, Mn, Pb and Zn) in the top soil in the urban-transect were analyzed both in scale effects and gradient analysis. The soil heavy metals pollution was evaluated, obtaining main results as follows:(1) The special analysis results of five soil heavy metal elements in the study transect showed that Soil Cu, Cr, Mn, Pb, Zn were all moderate variability, and coefficient of variation of Pb was the maximum. Five heavy metals existed spatial correlation in a certain range, containing significantly spatial autocorrelation of Mn, moderate spatial correlation of Zn, and weak spatial correlation for all Cu, Cr and Pb. By a large influence and interference of human activity, the variation of Cu, Cr and Pb were mainly random. Distribution of soil Cu was banded, decreasing from downtown to the outskirts. Soil Cr, Mn was island-shape distributed, radiating decrease from the center to around. The spatial distribution of Soil Pb, Zn showed the characteristics which combined of banded and island-shape distribution.(2) Analysis of heavy metal pollution in soil showed that the pollution of soil Cr, Zn, Pb is relatively serious. The exceeded rate of soil Cr, Zn, Pb and Mn reached 45% or even more, while the exceeded rate of Cr was 86.2%. Highly significant correlation was existed between soil Cr, Zn, Pb, Mn and Cu, showing out the heavy metal compound pollution or homologous. Heavy metal pollution of urban soil was evaluated by using of single factor index and Nemerow Evaluation index. In the transect, the pollution of Cu, Pb, Zn were reducing from the city to the suburbs, and ascending order according to the degree of pollution illustrates Xuhui District,Minhang District, and Fengxian District. The composite pollution index was in the range of 0.78～5.08,including 75.7% of the sample which had composite pollution index between 1～2 is light pollution, and the integrated pollution index of 5% of all greater than 2, pertaining moderate pollution. The heavy metal pollution level reflects an overall gradient difference from urban,suburban to rural areas, also reflecting the affect of heavy metal distribution in industrial areas, urban transport, waste emissions.(3) The demographic characteristics value and the variation coefficient of the heavy metals in urban soils were significantly different in different scales. Soil Mn, Cr was normally distributed and content of Cu, Pb, and Zn followed Lognormal Distribution in transect scale. Five heavy metals are all in line with normal distribution in both campus and district-level scale. The spatial autocorrelation of soil Mn was significant, and soil Cr, Zn, Pb, Cu were weakly spatial correlated in transect scale. Soil Cr, Zn, Cu had strong spatial correlations; spatial autocorrelation of soil Pb was also significant while spatial correlation of soil Mn was weak in district-level scale. Soil Pb had a strong spatial correlation, and soil Zn had a significant spatial autocorrelation while the spatial correlations of soil Cr, Cu, Mn were weak.(4) In the scale study, spatial variability structures of heavy metal reflect different variation. Heavy metals exist in a better spatial structure in campus and district-level scale, the spatial structure of three heavy metals (Cu, Pb and Zn) was pure nugget form, reflecting the randomness and independence without spatial continuity between samples. The structure of spatial variability of these four heavy metals can not well be revealed in transect. The distribution of soil Cu, Pb, Zn were generally similar, with the characteristics of combining island and banded, and soil Cr, Mn were island-shape distributed in transect scale. The spatial distributions of soil Cu, Cr, Zn were island-shape, and the spatial distributions of soil Pb, Mn were general in banded structure in campus scale. Industrial production, waste emissions and the development of transportation were the important factors which affected soil heavy metal pollution.(5) Comparative analysis results of soil heavy metals in the gradient urban-transect showed as follow: Soil Cu, Cr, Zn had strong spatial correlations and the spatial autocorrelation of soil Pb was significant, while the spatial correlation of soil Mn was weak in Xuhui district. Soil Mn had an obvious spatial autocorrelation and variation of soil Cr, Cu, Pb, and Zn were mainly random with very a weak spatial correlation in Minhang District. Soil Cu, Pb, and Zn had strong spatial correlations, and soil Mn had a significant spatial autocorrelation while soil Cr showed a moderate spatial correlation in Fengxian District. The variation of soil Cu, Pb, and Zn were in the weakening and the structural factors of the spatial distribution were in the increasing by the urbanization level reducing and the human disturbance weakening from the city center to the outer suburbs. The interference of random factors, enhanced human activities in particular, produced a strong spatial structure variation of soil heavy metal pollution with the rapid advance of urbanization and industrialization in the city outskirts.(6) Semi-variance analysis showed that the soil Cu, Pb, and Zn agreed spherical model, and soil Cr agreed exponential model and soil Mn satisfied the linear model in Xuhui District. Soil Mn agreed exponential model, and soil Cr satisfied the linear model while the semi-variance of soil Cu, Pb, and Zn were fitting linear model, showing pure nugget form in Minhang District. Soil Mn was consistent with the spherical model, and soil Cr and Pb lined with the linear model and soil Cu was consistent with the Gaussian model, in the meanwhile soil Zn agreed exponential model in Fengxian district. Xuhui District as a fully urbanized area and Fengxian District with agriculture land-based, soil Cu, Cr, Mn, Pb, Zn generally showed obvious spatial correlation. The Minhang District, as city outskirts, soil Cu, Cr, Pb, and Zn had shown strong randomness and independence and the weak spatial correlations by the strong impact of human activities and disturbance. Soil Cu, Cr, and Zn were island-shape distributed and the spatial distribution of soil Pb, Mn were in general banded structure in Xuhui district; soil Cr, Mn, Pb, and Zn showed the spatial distribution of multi-island and soil Cu was the combination island and banded, decreasing from northwest to southeast in Minhang District; soil Cu and Zn were the combination distribution of the banded and island while soil Cr, Mn, and Pb showed a banded distribution, decreasing from north to south in Fengxian district. The urbanization, distribution of industrial zone, corporate waste emissions and traffic pollution were all important factors that affected the formation of soil heavy metal distribution.(7) Descending order of mean soil magnetic susceptibility within the study area were 69.31×10-8 m3/kg of Xuhui District,50.90×10-8m3/kg of Minhang District,and 37.48×10-8 m3/kg of Fengxian District. The low-frequency magnetic susceptibility of soil declined from the suburbs downtown in Xuhui, Minhang and and Fengxian. The soil magnetic susceptibility and the content of Cu, Zn, Pb, and Cr were significant correlated in both Minhang and transect. Comprehensive pollution index of soil heavy metals and soil magnetic susceptibility was significantly positively correlated, Showing that soil magnetic susceptibility of the urban, can be used as quantitative indicators, was closely related to heavy metal pollution. |
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