Distribution and Transport of Residual Lead and Copper Along Soil Profiles in a Mining Region of North China

Residual heavy metals are commonly considered to be immobile in soils, leading to an underestimation of their environmental risk. This study investigated the distribution and transport of residual heavy metals along soil profiles, using the Xiaoqinling gold mining region in North China as a case study. Soil samples were collected at three depths from three locations near the tailing heap. The speciation of copper (Cu) and lead (Pb) (exchangeable, carbonate-bound, Fe-Mn oxide-bound, organic matter-bound, and residual fractions) was determined using a sequential extraction procedure. The residual fraction's morphology was observed using scanning electron microscopy (SEM). Results showed that metal fraction distributions along the soil profiles were influenced by each fraction's mobility. Residual fraction with high chemical stability can not be transformed from or into other fractions. This led to the conclusion that the high concentration of residual metals in soils mainly resulted from residual fraction transport. The SEM analysis showed that fine particles (submicrons) were mainly attached to large particles and were likely released and transported by water flow. The more sorptive fractions (non-residual fractions) were mainly retained in the top soil, and the more mobile fractions (residual fraction) were mainly leached to the deep soil. Cu and Pb concentrations in the residual fraction decreased slightly and those in the non-residual fractions decreased significantly with soil depth. These suggest a relatively higher residual metal mobility along the soil profiles. Therefore, residual metals can be transported in soils and their environmental risk can not be ignored in assessing soil contamination.