Study On Heavy Metal Pollution Of Vegetable Soils In Suburbs Of Xi'an

As the development of economy, the transportation service, vehicle ownership and traffic mileage are growing rapidly. The surrounding environment of highway were contaminated, including noise pollution, air pollution, soil pollution and water pollution, and that seriously threat to the health and safety of ecological environment of surrounding highway. The five vegetable fields in the south suburb of Xi’an, including two vegetable fields (S1, S2) nearby107provincial highway, two vegetable fields (XI, X2) nearby university avenue and one vegetable field (Z) nearby meridian avenue were selected and the soil samples were collected. The contents of heavy metals and the variations of contents, chemical speciation and contamination levels were analyzed and evaluated. It was of great significance for providing scientific basis for road planning, the layout of agriculture production along highway road and pollution prevention of heavy metals in vegetable soil along the highway.The physical and chemical properties of soil and the content, spatial distribution features, chemical speciation and bioavailability of heavy metals in soil were analyzed by a research process which including data collection, site inspection, sample collection, pretreatment, experimental analysis and data analysis. The following conclusions were obtained by studying.(1) The soil of X1, X2and Z is alkaline, and S2is neutral soil, whereas S1is acidic soil. Although the pH of soil in vegetable field X2increases between10and20meters, it decreases linearly after20meters, and the pH value of soil in other vegetable fields decrease with the increasing distance from the highway. The soil organic matter decreased in the order of plot X1> Z> S1> X2> S2, and the CaCO3content decreased in the order of plot Z> X1> X2> S2> S1.(2) The content of nine heavy metals in vegetable fields X1, X2, S2and Z is all excess the background values of surface soil in Shaanxi province. The content of As, Ni and Zn in vegetable field S1and As in S2is below the background values while the other heavy metals are beyond. Because the excess levels of Co is more serious, and the coefficient of variation is higher, so the Co is greatly influenced by human activities. According to the correlation analysis, the content of nine heavy metals is negative correlation with distance, showing that the content of heavy metals in five vegetable fields decreases with increasing distance in the overall trend, but each element has different significance levels. The content of heavy metals is constant fluctuating with the distance increasing, but the volatility of each heavy metal is different in the different segments of the distance.(3) The result of BCR extraction method shows that the unsteady fractionation of Mn, Cu, Co and Cd is higher than other elements. Above all, the unsteady fractionation of Mn in X2, S1, S2and Z is nearly90%, and in X1nearly80%. Furthermore, the unsteady fractionation of eight heavy metals is nearly30%. This shows that the transfer ability of all heavy metals is strong, and they are more dangerous to the ecological environment.(4) The result of bioavailability shows that the content of Cd and Pb in five vegetable fields is relatively high, and the content of Cr in S1and S2is also. So the eight heavy metals are more easily absorbed by plants, and have high risk. By comparing the two extract methods, the result can be found that the content of unsteady fractionation of BCR is higher than bioavailability, especially for Mn, Ni and Zn.(5) The result of single pollution index of Co in the five vegetable fields is the highest, and the polluted level is serious. The comprehensive pollution index is in order of X2> Z> S2> X1> S1. The result of enrichment factor evaluation shows that the nine heavy metals are moderately polluted in Z; most of heavy metals are intense polluted in X1and X2; the Co is moderately polluted level and other heavy metals are light polluted level in S1; the nine heavy metals are light polluted level in S2. The result of single pollution index and enrichment factor evaluation indicates that the polluted level of Co and Cr is relatively high. The pollution load index evaluation result displays that the spatial variation of five vegetable fields is different:X1and S2constantly reduce with the increasing of distance, X2increases, while S1and Z have small change. The pollution load index of five vegetable fields decreased in the order of X2> Z> S2> X1>...