| Soil was one of the important natural resources. It was not only the foundation of agricultural production, but also the important component of the environment. It could affect human health directly or indirectly through various channels.Heavy metals such as Pb, Cd, Cr, As and Hg, are the important parts of persistent toxic substances, which attached more and more attention by scholars. Chongming Island, located in the middle of Yangtze, is one of the most potential developing areas of Shanghai. Agriculture is the leading industry of Chongming Island, and farmland in the study area predominantly consists of paddy fields and vegetable plots. The population census in 2004 reported that more than 75% of employed people are engaged in agriculture for both subsistence and commercial purpose. However, the high population density and modern agricultural development have put great pressure on the natural and economic resources. A number of substantial environmental pressures and symptoms of environmental degradation have been identified in recent years.With the support of the National Natural Science Foundation (No.40701164), the bidding projects of Shanghai Environmental Protection Bureau (No.07-15), the Public Project of Environmental Protection Ministry of China (No. WFLY-2009-1-SK-06-03)and the National Key Water Program during the 11th Five-Year Plan Period (No.2009ZX07317-006),the concentrations, the spatial and temporal distribution characteristics, the distribution and accumulation in vegetable soil, atmospheric deposition fluxes of five heavy metals (Pb, Cd, Cr, As and Hg)in agricultural soils of Chongming Island were studied. Besides, the environmental quality of heavy metals in soils of three ecological agriculture demonstration areas were also studied in order to provide basis data and scientific support for further exploring heavy metals in the agricultural soil in the study area.The main conclusions can be summed up as follows.1.The Average concentrations of Pb, Cd, Cr, As and Hg in agricultural soils in the study area were 21.60 mg·kg-1,0.176 mg·kg-1,69.40 mg·kg-1,9.209 mg·kg-1 and 0.128 mg·kg-1, respectively. Except for Pb and Cr, the average contents of Cd, As and Hg in soils were 33.0%,1.2% and 26.3% higher than the background values of soil heavy metals in soils of Shanghai. Of the three land use types, vegetable plot was polluted most seriously by heavy metals, which was probably related to the over-application of pesticides and fertile.2. Most of the agricultural soils quality is still good overall. The percentages of ecological soil, good soil, qualified soil and unqualified soil were 1.26%,97.1%, 1.47% and 0.12%, respectively. The ecological soil shows a small area of block distribution. The superior soil, accounting for 72.11% of the total area, distributed widely in the island.The satisfactory soil mainly distributed in Chenjia Town, Gangyan Town, Shuxin Town, Sanxing Town and some areas of Nongchang. The unqualified soil mainly distributed in Gangyan Town. Compared to 2003, the soil quality decreased to some extent in the survey. The soil area of ecological grade, superior grade reduced to some extent and the other three grades has existed.3.The contents of Pb, Cd, Cr, As and Hg in vegetable soil averaged 20.51 mg·kg-1,0.22 mg·kg-1,72.15 mg·kg-1,12.299 mg·kg-1 and 0.1894 mg·kg-1, respectively. The contents of Pb, Cd, Cr, As and Hg in the edible portions of vegetables averaged 0.0081 mg·kg-1,0.0078 mg·kg-1,0.0100 mg·kg-1,0.0186 mg·kg-1 and 0.0049 mg·kg-1, respectively. In all of the vegetable samples, the contents of Pb,Cd and Cr were less than the tolerance limit of the maximum levels of contaminants in foods (GB 2762-2005).While for As and Hg,9.52%(eggplant and water spinach) and 14% (water spinach and cabbage) vegetable samples were beyond the tolerance limit of the maximum levels of contaminants in foods, respectively. For Pb, Cd, Cr and As, there were significantly positive correlations between the contents and the Bioconcentration factor (BCF) in vegetables. As to Hg, there were significantly positive correlations between the Hg contents in vegetables and the contents in vegetable soils.4.Vegetables of lower bioconcentration factors could be grown in soil, which was moderately polluted by Pb, Cd, Cr or As, not Hg. The average annual health risk value of child was higher than that of adult affect by Pb, Cd, Cr, As and Hg in the edible portions of vegetables in the study area. For carcinogens (Cd, Cr and As), The average annual risk values of adult and child were 2.53E-06 a- and 5.37E-06 a-1, respectively. The risk values were higher than the maximum allowance levels recommended by some organizations and researchers, but lower than the maximum allowance levels recommended by US EPA and ICRP.For non-carcinogens (Pb and Hg), The average annual risk values of adult and child were 5.81E-07 a-1 and 1.23E-06 a-1, respectively. The average annual risk values of adult were lower than the maximum allowance levels recommended by some organizations and researchers。The average annual risk values of child were higher than the maximum allowance levels recommended by some organizations and researchers, but lower than the maximum allowance levels recommended by US EPA and ICRP.5.The annual deposition fluxes of Pb, Cd, Cr, As and Hg in the study area were 7735μg·m-2·a-1,208μg·m-2·a-1,4620μg·m-2·a-1,2238μg·m-2·a-1 and 52.78μg·m-2·a-1, respectively. The annual deposition fluxes were in the order of Pb>Cr>As>Cd> Hg. The deposition fluxes of particulate Pb and As were higher than the fluxes of soluble Pb and As, but Cd and Hg was on the contrary. The atmospheric deposition fluxes of heavy metals were higher in December, January, February and March, highest in December and decreased gradually from December to March. Additionally, the deposition fluxes were also higher in May, June, August and September, highest in June. Compared with other regions, the atmospheric deposition fluxes of heavy metals were relatively lower in the study area, which was probably related to the location, less industrial pollution and the construction planning of eco-island. The annual contribution rates of As,Hg in the agricultural topsoil (0～20cm) of the study area were 16% and 2%, respectively.6.According to Grading Standards of Environment Quality in Vegetable Greenhouses(GB/T 18407.1-2001),soils of three asparagus greenhouses in different age were suitable for planting non-environmental pollution vegetable. Concretely speaking, the soil environmental quality in LS04 and LS07 greenhouse was grade one, and that in LS98 was grade two.The correlation analysis showed that there were positive correlations between five heavy metals concentrations and the greenhouse planting years. Especially for Cd, the Cd concentration significantly correlated to the greenhouse planting year (P<0.05).The reaping work had a significant effect on the heavy metals concentrations in the greenhouses. Frequent fertilization and irrigation during the period lead to the increase of some heavy metals concentration. The soil environmental quality in cauliflower and paddy-wheat rotation demonstration area was grade two, namely superior grade, conformed to Environmental Standard of Safe, Healthy, High Quality Agricultural Products (Shanghai Local Standard, DB31/T391-2000). Several heavy metals presented certain accumulation in the topsoil.
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