Abstract: In this paper, the mobility of heavy metals Pb, Cu, Cr and Zn in urban soil in Tongchuan City and the leaching characteristics with ascorbic acid were investigated with once balance method. The pollution degree was assessed with the ecological risks index. The results showed that the contents of heavy metals Pb, Cu, Cr and Zn in urban topsoil were significantly different, and the contents of Pb and Cu were high. The holistic pollution degree of heavy metals was serious. The speciations of these 4 heavy metals were dominated by the residual forms, and Zn and Cu was mobile in soil environment. However, Pb was not mobile due to its existence forms of hydroxide, carbonate or phosphatic in the environment. The total potential ecological risk degree was high. It was found that Pb was a main element of potential ecological risk, Cr, however, was easy to be leached and transferred in soil organism, which could be paid more attention to in the future.

Key words: urban soil, heavy metal, speciation and transfer, leaching, ascorbic acid, Tongchuan city, Shaanxi province