Distribution Characteristics of Heavy Metals and Pb Isotope in Profile Soils from a Mining and Smelting Area in Northwestern China

The distribution characteristics of typical heavy metals (HMs) and Pb isotopic compositions in profile soils from different areas were investigated in Baiyin district, northwest China. The soil samples from the ore district and the sewage irrigation area showed an obvious enrichment of HMs in different depths, and the difference in the longitudinal migration depth of Pb, Cu, Zn, and Cd was insignificant, which might be due to the migration behavior of HMs mainly controlled by carbonate in alkaline soil. With the sewage irrigation activities, the content of soil organic matter was no longer the main controlling factor for the migration behavior of HMs. In ore district and sewage irrigation area, the HMs contents of nonresidual fractions in topsoil were much higher than that in the corresponding deep soil, which indicated that HMs activities in topsoil were significantly high. The Pb isotopic compositions of profile soils indicated that the long-term mining and smelting activities remained the main source of vertical enrichment of HMs in Baiyin district. Moreover, the Pb isotope ratios Pb-206/Pb-207 of nonresidual fractions (1.1359-1.1916) were all lower than that in the corresponding residual fractions (1.1641-1.2010), showing the characteristics that HMs input from anthropogenic source were in different degrees. The Pb isotopic compositions of residual fractions in the topsoil of ore district and sewage irrigation area (1.1641 and 1.1703) were between the two end-members composed of background soil and local mineral samples, which suggested that some anthropogenic HMs might enter into the residual components in the soil that was greatly affected by the input of HMs.