Spatial Variability Of Heavy Metals In Cultivated Soil Of Rapid-Urbanizing Regions

The rapid urbanization in China, which is characterized by remarkable growth of economic, industrial development and large-scale nature exploitation, has intensified critical ecological and environmental problems, such as heavy metal pollution in soil. The heavy metal pollution of agricultural products has become increasingly prominent. Analysis on characteristics of heavy metal pollution, environmental assessments and pollution sources provides important theoretical basis for the prevention and treatment of heavy metals in soil, which is particularly necessary in developed areas. Xishan District of Wuxi, which is one of the rapid urbanization regions in South Jiangsu, is taken as the study area in this paper. The characters of spatial distribution, source identification and affecting factors, evaluations of environmental pollution and the potential ecological risk of heavy metals in cultivated soil in Xishan district were carried out in this study following the theories of traditional statistics analysis, geo-statistics and spatial heterogeneity using SPSS17.0and Arcgis9.3. The main conclusions are as follows:(1) Carrying on the statistical analysis on the contents of six heavy metals such as As, Cd, Cu, Hg, Pb, Zn in cultivated soil, we found that the concentration of heavy metals in Xishan was obviously enriched and the variation degree was Hg>Cd> Pb>Zn>Cu>As. Hg had the greatest variation with coefficient of variation of84.20%and some fractions belong to severe pollution of Hg in Xishan. Compared with the soil environmental background valuesin Wuxi, as well as other cities and regions in China, the contaminations of heavy metals in Xishan was significantly higher than other areas, the prevention of soil heavy metal pollution is particularly necessary. It is particularly necessary to control and remediate the heavy metal contamination in study area. (2) The characteristics of spatial variability of heavy metals in cultivated soil in Xishan were analyze through the semi-variogram and geo-statistics method. It was showed that the academic model of As and Hg is exponential model, the academic model of Cd, Cu and Zn is Gaussian model. Nugget coefficient of each heavy metal was in the order of Hg>Cd>Zn>Cu>As. Among them, the nugget coefficient of As and Cu is between25%and75%, which showed that As and Cu had a medium spatial autocorrelation, and the spatial variability is effected by structural factors and random factors. However, the nugget coefficient of Cd, Hg and Zn was bigger than75%, from which we found that these metals had a weak spatial autocorrelation and were more strongly influenced by the randomness factors. The enrichment of soil heavy metals in economical developed area was relative higher due to the long-term effects of human activities; therefore the impact of human factors on the soil can’t be ignored.The interpolation results showed that the spatial distribution of heavy metals in cultivated soil in Xishan District was patchy. The concentrations of As was bigger in the most parts of Ehu town and the junction of Donggang town, Anzhen Town and Yangjian town. The contents of Cd were dispersed concentrated, and the high values of Cd were mainly located in the Dongbeitang Town, Ehu town and the junction of Anzhen town and Yangjian town. The distribution of Cu was very high in the south of Ehu town and Anzhen town. The content of Hg was high overall, and was gradually increased from north to south. The highest values of Hg located in Ehu town, and it was also higher in Dongbeitang Town and Dongting streets with the dense population. The spatial variability of Pb distribution was great in the study area, higher in the south of the Ehu town, and lower in Xibei town and Donggang town. The distribution trend of Zn was out obvious, high and low values of Zn was staggered, and are less harmful to the ecological environment of this area.(3) The evaluation analysis such as Nemerow Index and potential ecological risk were used to assess the pollution of heavy metals in cultivated soil in Xishan. We can find that the single pollution indexes of As, Cu and Zn in the study area were at safe-light pollution level, the pollution of Cd was at clean-moderate pollution level, the pollution of Pb and Hg were at all levels. The heavy metal pollution was spatially distributed in patchy and the pollution levels in different regions were not identical. The fragmentation of single pollution indexes of Hg was the biggest in this area and Ehu town which located in southeast was at the heavily pollution level. The integrated pollution index in Wuxi was in order of light-pollution> alert-level pollution> severe-pollution> moderate-pollution> security-level, and was increasing gradually from north to south, just similarly to the distribution characteristics of Hg pollution.The single potential ecological risk indexes of As, Cu, Pb and Zn were at a slight grade, in which potential ecological risk of Hg ranking the most. The integrated potential ecological risk of Xishan was relatively high, which was at a medium level. In the space domain, the potential ecological risk were at a minor level in the most areas in Xishan, and the higher risk scattered in Yangjian Town, Chang’an Town, and Ehu Town. There was a strong ecological hazard area in the south of the Ehu Town which is close to Tai Lake. The potential hazards that soil environment quality had impacted on ecosystem of the study area is more serious. Therefore, we need to take immediate and effective measures for monitoring and control the heavy metals in this area, in order to prevent the adverse effects of the ecological environment deterioration on human life.(4) By the methods of correlation analysis, principal component analysis and cluster analysis, we can infer the pollution sources of heave metals in Xishan, combing the main sources of heavy metals in Xishan. It was founded that heavy metal pollution was affected by the combined effects of natural factors such as soil properties, soil type, soil parent material, etc. and anthropogenic factors such as rural township, transportation, agricultural production activities, land use, etc. The impact of natural factors on soil heavy metal enrichment in Xishan was smaller, and it was emphasized that the anthropogenic activities among the influence factors was the most important and critical in this regions with rapid industrialization and urbanization. Hg, Pb and Cu in the cultivated soil of this study area were related significantly, and might originate from the same sources of pollution, such as publishing and printing industries, transportation systems, agricultural production activies and living waste emissions. As and Zn existed a certain correlation, and their contents in study area were low and were mainly affected by the influence of soil parent material and other natural factors. It surely offers a scientific basis to control and prevent the pollution of heavy metals in agricultural soil.