Migration and transformation of Pb and Cd in the unsaturated zone induced by water table fluctuations

Heavy metal migration poses serious threats to soil and groundwater environments, especially under fluctuating water table conditions, where the transport and transformation behaviors of heavy metals are more complex and difficult to predict. However, there are still limited studies on the effects of groundwater level fluctuations on the migration and transformation behavior of Pb and Cd in an unsaturated zone. This study conducted column experiments to investigate the migration, transformation, and interaction of Pb and Cd in an unsaturated zone. The results showed that Pb and Cd generally migrate to the upper sand with increasing water levels owing to their higher activity under low pH conditions. After many water table fluctuations, Pb and Cd accumulated in the sand and the Cd content in the lower layer was reduced compared to that in the upper layer. This may be due to the coexistence with Pb, leading to the conversion of some of the Cd in the sand solids to unstable (Cd(OH)₂), which is susceptible to migration. The mechanism of the migration and transformation of Pb and Cd may be based on the synergistic effect of single or multiple processes, such as surface adsorption, surface precipitation, homocrystalline substitution, and interlayer cation exchange. The results show that hydrodynamic conditions affect the migration and transformation of Pb and Cd by changing the soil-groundwater hydrochemical environment, and this finding not only deepens the understanding of the heavy metal migration mechanism, but also provides an important scientific basis and technical support for the remediation of contaminated sites in areas of frequent fluctuations of the groundwater table.