Overview of trace metals in the urban soil of 31 metropolises in China

This overview provides an up-to-date assessment of the trace metal contamination (As, Cd, Cr, Cu. Hg, Ni, Pb, Sb, Se, and Zn) in urban soils of 31 metropolises in China. This systematic soil geochemical survey summarizes the characteristics of trace metals in Chinese urban soils, including concentration, accumulation, spatial distribution, and major sources. Mercury was ranked first followed by Cd and Se in geo-accumulation among all of the contaminant metals in urban soils in China; this finding is likely due to the Hg and Se emissions from fossil fuels. However, the lack of studies on Se contamination in urban soils, not only in China but also in the rest of the world, implies that Se contamination may have been unobserved for a long time. Shanghai, Kunming, Shenyang, and Changsha may be some of the most heavily contaminated Chinese cities based on the concentrations, spatial dimensions, and associations among the contaminant metals. Numerous hotspots with high concentrations of metals were found in Changsha, Shanghai, and Shenyang, clearly indicating a significant contribution from both the metallurgical industry and smelt mining to the contamination of urban soils. Conversely, the levels of Sb, Cu, and Cd in Kunming originated from their naturally high geochemical background in soils. Heavy Se contamination was found in Guiyang and Taiyuan. The natural source of Se may be important in defining the pattern of pollution in Guiyang, whereas anthropogenic sources are likely more accurate than is the natural background in Taiyuan city. We review the existing limits and types of pollutants in the current soil guidelines and find that an international agreement on the range of the limits and the types of pollutants contained in the soil guidelines is urgently needed.