Chemical fractions and potential mobility of lead in soils irrigated by sewage in Pearl River Delta, South China

The chemical fractions, i.e., water soluble (WS), exchangeable (Ex), carbonate (Car), weakly organic (WO), Fe-Mn oxide (FMO), strongly organic (SO), residual (Res) fraction, of Pb in irrigated soils in South China were investigated by a modified Tessier sequential extraction technique. The results show that the chemical fraction of Pb in soil is mainly the Res fraction and followed by FMO fraction, and the WS, WO, FMO, and SO fractions in topsoils (0-10 cm) are higher than those in subsoils (30-40 cm). The sum of contents of WS and Ex fractions (S (WE)) in topsoils is significantly positively related with that in subsoils, indicating the strong mobility of Pb in WS and Ex fractions in soils, and the S (WE) in soils is higher than the German trigger value for the transfer path soil-plant, indicating the high bioavailability of Pb in soils of this area. Fortunately, S (WE) and the ratio of WS and Ex fractions (R (WE)) to the sum of all fractions generally decrease with the soil depth in soil profile and the R (WE) in soil profile is lower than 0.5%, indicating the low pollution risk for Pb in groundwater. In addition, soil particles, pH and Fe2O3 play an important role in the impact of mobility and chemical fractions of Pb in soils.