| Xi’an city, located in the Guanzhong Plain, is the core city of western China. With the urbanization of Xi’an city, especially the construction of the international metropolis, the emissions of pollutants from human activities into the urban ecological environment caused some urban environment pollution, and urban soil was one of bearing medium. In this paper, the physical and chemical properties, concentrations and chemical speciation of heavy metals were measured with a variety of experimental methods. The contamination characteristics and environmental ecological risks of heavy metals in Xi’an topsoil were evaluated based on the pollution load index and potential ecological hazard index. The Kriging interpolation methods with GIS were used to studying the characteristics of the spatial distribution of heavy metals and integrated pollution level in Xi’an urban topsoil. Combined statistical and mathematical methods, the main source of heavy metal accumulation in the surface soil from Xi’an city were sorted out. And a new research method had been done to probe the sources of heavy metals in topsoil. The conclusions would provide some reference for the soil environment management of Xi’an city. Through the above research work, the following conclusions as follows:1) Some spatial differences with related to greenland types and the level of urbanization, were found in the physical and chemical properties of the topsoil from Xi’an city. The average concentrations of As, Ba, Co, Cr, Cs, Cu, Mn, Ni, Pb, Sn, Sr, V and Zn in Xi’an topsoil were0.99,1.20,1.46,1.35,1.08,1.67,1.05,1.10,2.12,2.28,1.48,1.00and1.71times soil background values of cinnamon soil, respectively. The average concentrations of As, Cu, Pb, Zn, Cr, Ni were below the three national soil environmental quality standard (GB15618-1995). Some significant difference were found in the concentrations of Pb, Zn, Cu, Sn, Cr in those soil from different types greenland, but that of As, Co, Cs, V, Ni, Mn, Sr and Ba were little difference. The concentrations of Mn, Ni, V were slightly higher than the background values of cinnamon soil.2) The results of chemical speciation analysis showed that the migration of heavy metals was in the order of Pb (60.79%)> Mn (60.12%)> Zn (46.64%)> Cr (44.55%)> Co (41.59%)> Ni (32.75%)> Cu (31.43%). And Pb and Mn should become a priority objects to be prevented and controlled, of which more than60%of the concentrations of Pb, Mn could be migrated and cause some harm for ecological environment.3) The study results of spatial distribution showed that the accumulation of heavy metals in different spheres were different due to varying intensive degrees of human activities and different sources of heavy metals. It should be paid attention that Cr, Cu and Co were relatively stable and Pb, Mn and Zn were converse and easy migration in those areas within the city walls and from the city wall to the Second Ringroad. It was more stable for Cu, Ni, Zn, but lower concentrations of stable states for Mn and Pb in the area from the Second Ringroad to the Third Ringroad. The different features of the different administrative area of Xi’an city resulted in the greater difference in the concentrations and chemical speciation of heavy metals. The concentrations of As, Ba, Co, Cr, Cs, Cu, Mn, Ni, Pb, Sn, Sr, V and Zn in topsoil inside Third Ringroad in Xi’an city showed the non-linear relationship with the distance from the center of the city.The characteristics of spatial distribution of every element were different based on the spatial analysis of GIS and geostatistical model. The spatial distribution characteristics of As, Mn, Ni and V content are similar, and the highest and lowest values of Mn, Ni and V appears to the same area. The spatial distribution characteristics of Cu and Pb in the topsoil in Xi’an are similar. Their high-value areas were distributed in those area of the city center and traditional industries intensive western region with heavily traffic and a large number of industrial enterprises, and low value areas were mainly distributed in Qujiang Resort District in southeast of city and Chanba ecological District in northeast of city with the low traffic, low population density and less distribution of industrial enterprises. The spatial distribution characteristic of the concentrations of Sn was not obvious, and the overall level was much higher than the background value of cinnamon soil. For Ba, the concentration was higher in the western region and northeast region, but the lower in middle region. The high value area of Zn had the characteristics of the high traffic density (mixing distribution of urban road and rail) and the intensive industrial enterprises. And the concentration of Zn in topsoil from Xi’an traditional industrial zone was also higher, but that from Chanba ecological District in the northeast and the Qujiang vacation area in the southeast of the city were lower. The spatial distribution characteristics of Co in topsoil manifested as higher in the western and northeastern regions and the lower in southeast of the city. Those areas of the concentrations twice the background value of cinnamon soil were mainly located in the northeast region and western car-city nearby. The low concentration areas of Cr was distributed Chanba Ecological District in the northeast of Xi’an city, and high-value areas were located in the western industrial district of Xi’an, surrounding the railway line distribution. The concentrations of Sr in topsoil within the Third Ringroad in Xi’an city decreased from northwest of the city to southeast of the city, and that was lower in topsoil from Qujiang tourist Resort in the southeast of city. The concentrations of Cs were lower than the background value of cinnamon soil in most parts of city, but slightly higher than the background value of cinnamon soil in the local area of the north and southwest of the city. And the characteristic of spatial distribution of different speciation of the same element were different.4) The pollution evaluation results showed that PLI was1.27, indicating moderately polluted level in Xi’an topsoil. The contaminations of Mn and V were affected to a lesser extent by human activities. There were large regional differences in the contaminations of As, Ba, Co, Cr, Cs, Cu, Ni, Pb, Sn, Sr, Zn in the surface soil of Xi’an city, and the extent of the accumulation of these elements were different. Some sever pollution area for Cu, Pb, Sn, which related to larger cumulative impact of human activities were found. The results of ecological risk analysis showed that there was moderate potential ecological harm in Xi’an topsoil generally. And there were moderate, slight degree, higher degree, high potential ecological risk in95.58%、1.84%、1.84%、0.74%of272samples, respectively. The average individual potential ecological risk index of heavy metals in the surface soil of Xi’an reduced in the following order:As (9.90)>Cu (8.38)>Pb (7.37)>Co (7.29)>V (5.68)>Ni (5.51)>Zn (4.11)>Cr (2.71)>Mn(1.05). And there was slight potential ecological harm for all elements.5) All elements were grouped into five categories using multivariate statistical analysis. There were the same classification results based on minimum spanning tree classification model, which were As-Mn-Ni-V, Ba, Zn, Co-Cr-Sr, Cu-Pb-Sn, Cs. Therefore the classification model based minimum spanning tree was a valuable research method for the field of soil source classification. The GIS interpolation maps with five factors from factors analysis, overlaying traffic maps and enterprises maps were used for analyzing the sources of heavy metals in Xi’an urban topsoil. The first set of elements (As-Mn-Ni-V) was natural factors, which were mainly affected by the soil parent material. The accumulation of second set of elements (Cu, Pb, Sn) in urban soils were mainly related with urban traffic, which was traffic factor. The third group of elements (Ba, Zn), as the result of the combined affects of the industrial production activities, traffic emission and the activities of citizens, was comprehensive factor. The fourth set of elements (Co, Cr, and Sr) in urban soils was mainly affected by the impact of the industrial production activities, which was industrial factor. The fifth group element, Cs, which was affected by natural factors and human activities, was the blending factor. |
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