Heavy Metal Pollution In Different Soil Particle Size Of The Xi'an City Park

City soil is an important component of city ecological environment system. In the process of rapid development of urbanization, heavy metal contamination in the city soil is quite serious, which is impacted by urban transportation, commerce, construction and industry. Park is the primary venue for leisure in which people can be in touch with nature. The soil in the park has also been impacted by urbanization and urban activities. Especially the heavy metals' accumulation in the soil, not only does it affect and change the ecological functions of urban soils, but also endanger human health directly or by contacting with dust. Therefore, soil's physical and chemical properties and heavy metal content in different sizes soil, the study of which have the vital practical significance.In this paper, the surface soil (0-20cm) in the Xi'an 14 main parks has been carried on the synchronized open country investigation, sample gathering and the analysis test. Soil's physical and chemical properties and heavy metal content in different sizes soil have been studied by a large number of measured data and comprehensive analysis. The correlation analysis, cluster analysis and principal component analysis are used to discuss the relationships between the soil physics and chemistry nature and the heavy metal content as well as the relationship between heavy metals. From the different angle, heavy metal pollution of park soil in different sizes is assessed by means of single-factor pollution index, the synthetic pollution index, Muller's Geo-accumulation Pollution Index and Lars Hokinson's Potentially Ecological Risk Index. The following results and cognitions are attained.(1) The LOI in different sizes soil are different in the city parks of Xi'an. In the 0.45-0.9mm mm and 0.3-0.45mm, the LOI in Chang Le Park is the smallest. In the 0.15-0.3mm and 0.097-0.15mm, the LOI in Urban Sports Park is the smallest. In the 0.097-0.15mm and<0.05 mm, the LOI in Xing Qing Park is the biggest. Soil in most parks is strong alkaline, in some areas shows alkaline. In the 0.45-0.9mm, soils in Lao Dong Park are alkaline. In other different sizes soil, Yanta Custom Garden is alkaline. Conductivity (EC) values in different sizes soil are from 0 to 0.5. There is no problem of Stalinization in city parks of Xi'an. In the 0.05-0.097mm, the average low-frequency magnetic susceptibility of every park is 138.69×10-8m3/mg and the average high-frequency magnetic susceptibility is 127.54×10-8m3/mg.The LF Sus and HF Sus of Children's park are the maximum and Urban Sports Park's are the minimum. In other different sizes soil, the average low-frequency magnetic susceptibility in every park are from 124.78×10-8 to 159.8×10-8m3/mg and the average high-frequency magnetic susceptibility are from 114.345×10-8 to 148.51×10-8m3/mg.The LF Sus and HF Sus of Huan Nan park are the maximum and Urban Sports Park's are the minimum. The average frequency dependent susceptibility in every park are from 7.19%to 8.38%.Calcium content in different sizes soil are in the range of 9.04-10.37 g/kg. Maximum content of calcium carbonate is in Ge Ming Park and the minimum in Small Wild Goose Pagoda.(2) The results of Screening Experiment showed that quality distribution of soil in 0.45-0.9mm,0.3-0.45mm,0.15-0.3,0.097-0.15,0.05-0.097 and<0.05mm are 21.0%,24.0%, 14.0%,10.9%,16.8% and 13.2%. Heavy metals concentrate in the finer particle size of 0.097-0.15, 0.05-0.097 and 0.15-0.3 mm. Heavy metals in the remaining particle size are relatively small.(3) In soil physical-chemical indicators associated with the majority of heavy metal elements analysis, the correlation among pH, EC, loss on ignition, CaCO3 contents, the common elements(CaO,MgO,Fe2O3,SiO2,Na2O,K2O,Al2O3 and TiO2 and heavy metal contents is better, reflecting the above-mentioned physical-chemical indicators are the major impact factors of most heavy metal enrichment. A significant correlation among Pb, Zn and Cu indicate that their origins have the big similarity. Pb, Zn, Cu are generally pro-Cu elements, which have a strong pro-sulfur features. There are sulfide or sulfur mineral salts. A significant correlation among Ni and Cr, Co, As indicate that they have similar pollution sources and methods. Cluster analysis and principal component analysis indicate that according to its own physical and chemical properties, heavy metals in the soil have showed similar geochemical characteristics.(4) The results of synthetic pollution index method showed that the city parks in different sizes soil of Xi'an are medium pollution. Potential ecological index method shows that ecological risk of heavy metals is at the middle level. The results of Muller's Geo-accumulation Pollution Index show that the order of every element in different size soil is different. In the<0.05 mm, 0.097-0.15 mm and 0.05-0.097 mm, Pb>Co>Cu>Zn>Cr>Ni>As. In the 0.15-0.3 mm and 0.3-0.45mm, Pb>Cu>Zn>Co>Cr>Ni>As. In the0.45-0.9mm, Pb>Zn>Cu>Co>Cr>Ni>As.