*3.4. Migration Characteristics of Pb in Soils with a Fluctuating Water Table*

Figure 8 shows how the contents of Pb adsorbed by the sand sample varied at different heights. The maximum Pb content occurred at the height of 35 cm in three columns. Trace amounts of Pb were detected at the height range of 25–45 cm. The analyses in Figures 5 and 8 show that the maximum Pb content occurred where the minimum pH was detected. Therefore, the presence of heavy metal Pb decreases pH as a consequence of acidic soil. Zhai L et al. determined that precipitates and complexes may solidify the soil, causing grievous pollution to soils and crops [35].

**Figure 8.** The variation of Pb concentration in the medium at different heights after experiments.

As the water table fluctuated, Pb2+ migrated upward or downward in the medium, but the migration distance of Pb2+ was more positively affected by the adsorption. The migration distance of Pb2+ in coarse sand was further than that in the medium sand or fine sand. The water table varied in coarse sand, the upward migration distance was only 5 cm high, and the downward migration distance was <10 cm high. In medium and fine sand, the upward and downward migration distance height was <5 cm.
