Chemical Fractions and Mobility of Cd, Cu, Pb and Zn in Soil Profile of a Sewage Irrigation Area, Northwest China

The Chinese government recently planned to supply financial support for comprehensive prevention and control of heavy metal pollution in 30 cities nationwide, among which Baiyin city took the first place. The present study aimed to investigate the chemical fractions and mobility of Cd, Pb, Cu and Zn in the soil profile (0–60 cm depth) of cornfields in the Dongdagou region, Baiyin city, Northwest China. The modified BCR (now the Standards, Measurement and Testing Programme of the European Community) method was employed to analyse the chemical fractions of heavy metals, and mobility index used to assess heavy metal mobility. Besides, Pearson correlation matrix was adopted to study the effects of soil properties (depth, total organic matter and pH) on heavy metal mobility. Results show that the soils are seriously polluted with heavy metals, especially by Cd. Cadmium is particularly presented in the acid soluble fraction, while Pb is mainly in the reducible fraction. As for Cu and Zn, they are likely to be retained in the residual fraction. With few exceptions, heavy metal concentrations in non-residual fractions generally decrease with the deepening of soil depth. Heavy metal pollution in soils has reached at least 60 cm deep, and the main pollution is found in the soils of 0–30 cm depth. In soil profile, heavy metal mobility is in the order of Cd > Cu > Pb > Zn; meanwhile, total organic matter has a significant role in retaining heavy metals in soils. Immediate measures should be taken to prevent the potential risk of heavy metals for local agro-ecosystem and the public.