Evaluation on Environmental Quality of Heavy Metals in Agricultural Soils of Chongming Island,Shanghai

In order to analyze the contents of heavy metals (Pb, Cd, Cr, As and Hg) in agricultural surface soils, a large scale survey was conducted in Chongming Island. 28 soil samples were collected from vegetable plot, 65 samples were collected from paddy fields and 9 samples were collected from watermelon fields. Different assessment standards of heavy metals were also used for comparison. Results showed that the content of Pb, Cd, Cr, As and Hg averaged 21.597 mg·kg-1, 0.176 mg·kg-1, 69.395 mg·kg-1, 9.209 mg·kg-1 and 0.128 mg· kg-1, respectively. In comparison with the background value of soil in Shanghai, mean concentrations of heavy metals in soils all exceeded their corresponding natural background values, except for Pb and Cr. The contents of Cd, As and Hg were 33.0%, 1.2% and 26.3% higher than the background value of Shanghai, respectively. In addition, inverse distance interpolation (IDW) method was also applied to study the spatial variability of heavy metals based on GIS. The results indicated that most of agricultural soils were good, and the ratio of ecology, good soil, certified soil and disqualified soil were 1.26%, 97.14%, 1.47% and 0.12%, respectively. Taking the background value as a reference, the ratios of all the soil samples above the limit were Pb 10.1%, Cd 85.7%, Cr 27.0%, As 55.4% and Hg 55.2%, respectively. Of the three land use types, vegetable plot was polluted most seriously by heavy metals, which is probably related to the over-application of pesticides. The annual deposition fluxes of Pb, Cd, As and Hg were 7735.470 μg·m-2·a-1, 208.432 μg·m-2·a-1, 2237.738 μg·m-2·a-1 and 52.783 μg·m-2·a-1, respectively.