Concentrations and chemical forms of heavy metals in urban soils of Shanghai, China

The influence of previous irrigation of sewage water from industry and daily life on the concentrations and chemical forms of heavy metals in soils was examined in Xijia Village, Pudong District, Shanghai, China. Soil samples were taken from three regions in the upland fields that varied in distance from the abandoned open canal. To understand the background levels of heavy metals in the agricultural soils in Pudong District, soils under different land use (upland fields, greenhouses and orchards) were sampled as references from four localities with no history of sewage water irrigation. Analyses of total amounts of Cr, Mn, Ni, Cu, Zn, As, Pb, Cd and Hg indicated that the amounts in the soils from the sewage irrigated field were markedly higher than the background levels. In particular, the total amounts of Cd and Zn in the soil at distances of 2 in and 10 in from the canal exceeded the permissible value set by the Chinese Environmental Quality Standard (GB 15618-1995). Although the contamination showed a trend of decreasing with increasing distance from the canal, the total amount of Cd was still higher than the permissible value 20 m from the canal. The chemical forms of Zn and Cd in the soils taken from Xijia Village and the reference upland soils were then evaluated using a sequential extraction method. In the case of the soils 2 m and 10 m from the canal, the highest amounts of Zn and Cd were found to indicate an occlusion in Fe oxides and retention by the clay exchange sites of the soils. Based on these results, it was recommended not to use these sewage irrigated fields in Xijia Village for agricultural purposes unless the necessary remediation measures were taken to lower the heavy metal concentrations to safe levels.