| Due to rapid urbanization, a large number of contaminants, including organic and inorganic matters, were discharged into the urban environment by human activities, which has provoked some serious environmental problems. Among the various environmental problems, heavy metal contamination in urban soils is most serious. Meanwhile, urban dust and atmospheric dry and wet deposition are the main storage medium of heavy metals besides the soil in the urban environment. Excessive inputs of heavy metals in the urban dust and atmospheric dry and wet deposition into the surface environment as a consequence of atmospheric deposition and other channels can impose a long-term burden on biogeochemical cycling in the ecosystem. Moreover, compared with the heavy metals in the soil, the heavy metals absorbed in the urban dust and atmospheric dry and wet deposition pose a much greater threat to human health and the urban environment, because which can hazard on human health through diet, contacting and respiratory more easily. However, the heavy metals contamination in urban dust and atmospheric deposition doesn't get enough concern and attention from people.Studies on urban soil, urban dust and atmospheric deposition have started lately in our country. But the majority of these studies has been taken in southern cities such as Chengdu, Guangzhou, etc., while the relevant studies in the northern cities have seldom been reported, especially in Changchun, the studies on urban soil, urban dust and atmospheric dry and wet deposition have not been carried out systematically.Changchun city is located in the heart of Northeast Plain, China, and is the capital of Jilin Province, so it has an important political, economic and cultural status. In recent years, massive economic growth and urban development in Changchun has led to serious environmental problems. The previous multi-target geochemical surveys showed that the urban soil encountered potential threaten by heavy metals contamination in Changchun City. However, the sampling density of previous studies was too small to fully and accurately reflect the heavy metals contamination in urban soil, and previous studies merely focused on survey, but seldom took into account the in-depth analysis and interpretation of survey data. The spatial distribution features, chemical forms, contamination degree, sources of heavy metals, migration and conversion law of heavy metal elements in the urban soil were not clear, and the studies haven't paid sufficient attention to heavy metal contamination of topsoil which more easily come into contact with human body and do greater damage to the urban environmental. At the same time, the study on heavy metal contamination on atmospheric dry and wet deposition and urban dust (especially, the heavy metal contamination of the near-surface urban dust at the scope of the human body height, which is about 1.5 m, from the earth surface) was limited in Changchun City. Therefore, the present study has theoretical significance and a wide range of practical value.Based on a systematic sampling strategy with a high sampling density, and an accurate sample determination, the aims of this paper are: (1) to find out the concentration characteristic and its influencing factors of heavy metal in the urban topsoil, in near-surface urban dust and in atmospheric dry and wet deposition in Changchun City; (2) to analyze the spatial variability and distribution of heavy metals in the urban topsoil and in near-surface urban dust using GIS method; (3) to assess the pollution degree and environment quality of heavy metal in the urban topsoil and in near-surface urban dust; (4) to study the chemical forms characteristics and its influencing factors of heavy metals in urban topsoil; (5) to study the eco-geochemical characteristics of atmospheric dry and wet deposition; (6) to do predictive and early-warning research on heavy metal contanmiation in urban topsoil; (7) to analyze the effects of atmospheric dry and wet deposition on heavy metal accumulation in urban topsoil; (8) to identify the possible sources of heavy metal pollution in topsoil and urban dust. From the above research, some results are achieved.1.Comparied with Changchun's topsoil (0-20cm), atmospheric dry and wet deposition of As,Cd,Cr,Cu,Hg,Pb and Zn in Changchun (38.9,2.24,92.3,76.17,0.275,115.2,462mg/kg, respectively) have elevated concentration as a whole. It was also found that Cu,Hg,Pb and Zn concentration were generally higher in the non-heating period than that in the heating period which could be caused by the washout effect of the rain season. There was no clear relationship between the season changes and the Cr concentration in the atmospheric deposition. According to the results, atmospheric dry and wet deposition of As,Cd,Cr,Cu,Hg,Pb and Zn in Changchun (4.79,0.25,10.67,8.22,0.030,12.31,48.15 mg/m2/a, respectively) was significantly elevated compared with other regions, e.g. the Great Lakes region in North America and North Sea in European. It was also found that atmospheric dry and wet deposition of As,Cd and Cr was generally higher in the heating periods than that in the non-heating periods. The potential impact analysis showed that the elevated atmospheric deposition of heavy metals could cause potential long-term concern over heavy metal enrichment in surface soils and potential health concerns in Changchun, especially, Zn was the worst case.2. The results of grain size analysis show that the mean median grain size, arithmetic average grain size and the range of mean median grain size of the urban dust are 21.58μm, 24.24μm and 10.75-32.88μm respectively. The frequency distribution curves of grain size of the urban dust are bimodal and non-normal, which may indicate that the multiplicity of the sources of urban dust in Changchun City. A significant spatial variability was not found for the grain size of urban dust in Changchun. Mineral compositions of the urban dust were determined by X-ray diffraction instrument, the results showed that the quartz, albite and calcite are the dominant mineral composition in the urban dust.3. The mean concentrations of As, Cd, Cr, Cu, Hg, Pb and Zn in the urban dust in Changchun (23.26, 0.62, 94.53, 68.41, 0.24, 93.63 and 416.71mg/kg, respectively) were significantly higher than those in the topsoil in the study area, but lower than those in the atmospheric dry and wet deposition. The concentrations of heavy metals in the urban dust in Changchun city were in wide ranges, which were typical for Hg and Cd.4.A significant spatial variability was found for the heavy metals in urban dust in Changchun using a spatial variation analysis based on geological statistics. The structure of spatial variability of the concentrations of heavy metals was obvious. The spatial variabilities of the concentrations of As, Hg, and Cd were well fitted by the line model, while Cr, Cu, Pb and Zn were well fitted by spherical model. As, Cr, Hg, Cd demonstrated the moderate spatial correlation, while Cu, Pb, and Zn has a weak spatial correlation. The ratios of nugget (C0) to sill (Co+C) were all over 50%, which may indicate that the heavy metals concentrations in the urban dust were affected seriously by some random factors (mainly the artificial inputs). High concentrations of heavy metals in urban dust in Changchun City were mainly found in the northeast of the city which were the industrial zone, as well as in the industrial district of the FAW in southwest of the city. In addition, the impact of vehicular emissions on the concentrations of Pb, and Zn was fairly obvious. The spatial distribution characteristics of heavy metals in the urban dust were closely related to the industrial layout and human activities.5. The urban dust in the Changchun City has been contaminated by heavy metals in varying degrees, and shows significant spatial variability. Among them, the degrees of Cr, Hg, Pb contamination in the urban dust were relatively light, at the level of potential contamination, while the degree of Zn contamination was most serious. The integrated environment quality of the urban dust in Changchun City had a relatively low level, about 78% of the urban area was at gradeⅢ. The level of the potential ecological hazard index for urban dust in Changchun city was high, which had reached to a strong ecological risk level, Cd and Hg made the main contribution to the potential ecological risk of urban dust.6. The background values of As, Cd, Cr, Cu, Hg, Pb and Zn in near-surface urban dust of Changchun city (22.77,0.528,91.52,65.11,0.20,89.5 and 378mg/kg, respectively)were determinated using a Mean-Variance method. And then, Standards grading of envirnomental quality grade for urban dust were also established tentatively based on Envirnomental Quality Standard for soil which were published by China, and literature review.7. The pH value of urban topsoil increased in Changchun City because of the human activities. The mean concentrations of As, Cd, Cr, Cu, Hg, Pb and Zn in the urban topsoil in Changchun (12.5, 0.132, 66.0, 29.4, 0.118, 35.4 and 90.0mg/kg, respectively) are significantly higher than the regional topsoil background value in the study area. The concentrations of heavy metals in the urban topsoil in Changchun city were in wide ranges, which were typical for Hg and Cd. Depending on the locations, the heavy metals concentrations differ according to the element types in the urban topsoil in investigated settings. That is, the concentrations of As and Cu in the urban topsoil were comparable from different administrative districts, while the concentrations of Hg, Pb, Zn and Cd of urban topsoil in Nanguan District were the highest, and the concentration of Cr of urban topsoil in Lvyuan District was the highest.8. The Pearson correlation analysis shows that there were strong associations among heavy metal elements in the urban topsoil in Changchun city. In general, As, Hg, Zn, Pb, Cd, and Cu were significantly positively correlated, while Mn and Cr form another group based on their positive correlation. The degree of correlation among elements is closely related to the element origin and its geochemistrical characters. The concentrations of Hg, Zn, Pb, Cd, Cu and As in the urban topsoil were positively correlated to the pH value of urban topsoil. However, no significant correlations between the concentrations of Mn and Cr and the pH value were detected.9. A significant spatial variability was found for the heavy metals in urban topsoil in Changchun using a spatial variation analysis based on geological statistics and Geographical Information System (GIS). The structure of spatial variation of the concentrations of heavy metals was obvious. The spatial variability of the concentrations of As, Hg, Cd and Cr in urban topsoil was well fitted by exponential model, while Cu, Pb and Zn were well fitted by spherical model. As, Hg, Cd, Cu, Pb and Zn demonstrate the moderate spatial correlation, while Cr has a strong spatial correlation. The ratios of nugget (C0) to sill (Co+C) for heavy metals were all less than 50%, which may indicate that the heavy metals concentrations in the urban topsoil were affected by some random factors (mainly the anthropogenic inputs) with a certain degrees, but still controlled by the structural factors (such as climate, the parental material, soil types etc.), and the original spatial structures have not been destructed. The range of spatial correlation of the concentrations of As, Hg, Cu, Pb and Zn in urban topsoils was much bigger, while the range of spatial correlation of Cd and Cr was smaller (4650m and 1230m, respectively). In the whole study area, the concentrations of heavy metals in the urban topsoil gradually declined from the central region to the suburban region and showed no clear directivity. High concentrations of heavy metals were mainly found in the area which pH was larger than 7.5. The properties of the spatial structures and distributions of heavy metals in urban topsoil are closely related to the layout of urban industry and enterprises, the intensity of human activities and the land utilization.10. The percentages of different heavy metals in the same chemical form vary greatly; meanwhile, the percentages of different chemical forms of the same heavy metal also vary greatly in the urban topsoil. Residual phase is the main fractions of the heavy metals in the urban topsoil excepted Cd. Ion exchangable fractions and water soluble fraction take up small proportion excepted Cd. The chemical distribution of Cd is a little special. The chemical association of Cd within the urban topsoil is dominated by the Carbonate fraction and water soluble fraction and a higher percentage with the ion exchangabe fraction and the water soluble fraction. Compared with suburban topsoil, the chemical distribution characteristics of As and Hg are similar in the urban and suburban, but the percentage of residual Pb and Zn decline modestly in urban topsoil, on the contrary, the percentage of residual Cr, Cu, Cd increase in a small degrees. Meanwhile, it is worth noting that, with the exception of Cu, the percentage of water-soluble fraction of heavy metal increases slightly in urban topsoil. Non-residual fraction (active fraction) in the urban topsoil increases compared with those in the suburban soil, but the change is not significant. Residual fraction of heavy metals in urban topsoil shows significant positive correlation with the concentrations of the heavy metal in the urban topsoil, and the degree of correlation between other active fraction and the concentrations of heavy metals is affected by the types of heavy metals. The effects of soil pH value and the content of Fe, Mn on the chemical forms of heavy metals in soil are related with the types of heavy metals. Meanwhile, the degree of correlation between the percentages of different chemical forms of the same heavy metal and the soil pH value and the content of Fe, Mn is depend on the chemical forms of the heavy metal. Though the residual phase is the main fractions of the heavy metals in the urban topsoil in Changchun at present, they may be activated under certain conditions, and would bring disastrous consequences to the city soil environment.11. The urban topsoil in Changchun City has been contaminated by heavy metals in varying degrees. However, the overall quality of soil environment is satisfying, of which 79% of the sample unit meets the Class I national soil environmental quality standards. Hg is the most dangerous pollutant to the urban topsoil and has a high degree of potential ecological harm. The composite pollution level of urban topsoil by heavy metals reaches at mild degree (the average integrated pollution index is 1.22) with moderate and severe pollution locally. The level of the potential ecological hazard index for urban topsoil in Changchun city is medium. The spatial distributions of the extent of soil pollution are related closely with urban functional areas and the intensity of human activities.12. The trend of the development of the urban topsoil heavy metals contamination was forecasted and early-warned using a heavy metal accumulative model. The results show that the concentrations of heavy metals in most of the urban topsoil will not exceed the second kind of the Environmental quality standard for soils recommended by the Enviromental Protect Agency of China in 50yr under the uniform accumulative mode. However, under the accelerated accumulative mode, the urban topsoil quality will deteriorate rapidly in 20-50yr, and the degrees of As,Cd,Cu,Hg and Zn contamination in uran topsoil will be serious.13. The Factor analysis showed that the sources of different heavy metals in urban dust of Changchun City were various. Cr and As mainly originated from the soil dust and natural origin; Cu and Pb mainly originated from traffic pollution, mainly from vehicle exhaust; Zn mainly originated from transport pollution, mainly from the tire wear dust of motor vehicles, but also by the impact of industrial activities to a considerable extent; Hg mainly originated from the coal dust generated by the settlement, including the industrial coal-fired and civilian coal-fired; Cd mainly originated from the released Waste of industrial activities.The Factor analysis also showed distinctly different assosciations among heavy metals in the urban topsoil of Changchun City. Pb, and Zn mainly originated from traffic contamination; Cr was still mainly controlled by natural factors, originated from natural sources; coal-fired was the main source of As and Hg; Cd mainly originated from the industrial and chemical fertilizer. The results of Pb isotope tracer showed that the lead pollution in the urban topsoil of Central district in Changchun mainly come from the residual contamination of automobile exhaust in the past and the coal-fired industrial emissions, generated from the Second Thermoelectric Plant, as a typical case, by the, and was related less to the current auto exhaust emissions. The construction dust had also taken an impact on the urban soil to a certain extent.
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