Characteristics of Heavy Metals Pollution of Farmland and the Leaching Effect of Rainfall in Tianjin

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The leaching effect of the rainfall process for removing heavy metals from farmland soils is a critical physical process. To study the leaching effect of the rainfall process on heavy metals in soil, the pollution characteristics of seven heavy metals in farmland soil have been investigated in the suburban Tianjin region by combining the current data with previously reported runoff data in the same region. There is a large difference for these tested heavy metals in soils, with Zn having the highest concentration at 106.61±56.24 mg·kg-1 and Cd having the lowest concentration at 0.31±0.31 mg·kg-1. Compared with the background value in Tianjin, only the content of Cd is at higher levels (a four-fold increase). The results from a single factor pollution index and comprehensive pollution index analysis indicate that the comprehensive pollution index ranking is Cd > Cu > Ni > Zn > As ≈ Cr > Pb. Cd, Cu, Ni, and Zn are in the moderate pollution, mild pollution, and alert level categories, respectively. Except for As, the results from a correlation analysis, the principal component analysis (PCA), and the cluster analysis indicate the possible similar origins of Cr, Ni, Cu, Zn, Cd, and Pb. The multivariate statistical analysis indicates that Cd is mainly derived from human activities, Zn, Cu, Cr, Ni, and Pb are affected by anthropogenic and natural sources, and As is mainly influenced by natural conditions. By comparing the ratios of individual heavy metals to Cr in soils and runoffs, the leaching behaviors have been studied, with the leaching capacity ranked as Cd > As > Cu ≈ Pb > Ni > Cr ≈ Zn. In summary, Cd pollution in soil is more prominent than pollution from other metals and should receive attention.