# An Experimental Study on the Migration of Pb in the Groundwater Table Fluctuation Zone
## 3 Results and Discussion
### Migration Law of Pb Due to Water Table Fluctuations  
Due to the strong adsorption capacity of fine sand, the migration ability of Pb in fine sand is weak. The Pb of each sample solution in the columns did not reach the instrument's detection limit (the instrument's detection limit (TAS-990A) is 0.01 mg/L). Therefore, we only analyzed the migration law of Pb in coarse and medium sand in this article. During groundwater table fluctuations, the variation in Pb concentration at different sample solutions is shown in Figure 7(a) (coarse sand) and Figure 7(b) (medium sand). The results of how Pb concentration is altered with water table fluctuations are shown in Table 3 and can be used to study the migration of Pb when the water table rises and falls in the experiment. Our analysis is as follows. (1) In the sample solution, when the height was 30 cm high in coarse sand, the water table height increased from 20 to 40 cm, and Pb\({}^{2+}\) concentration increased in the range of 8.29-42.42% in the early-stage relative to the initial concentration on the first day. Then, with the water table fluctuations, Pb\({}^{2+}\) concentration decreased in the range of 37.12-94.34%. This concentration declined by 9.54% on average within 8 days. (2) At the height of 35 cm for sample solutions in coarse sand, Pb\({}^{2+}\) concentration decreased in the range of 4.01-97.47% with water table fluctuations. This concentration decreased by 6.68% on average within 14 days. (3) At the height of 40 cm for the sample solution in coarse sand, the concentration declined by 5.54% on average within 12 days. (4) At the height of 45 cm for the sample solution, Pb\({}^{2+}\) was absorbed by coarse sand after 4 days. (5) At the heights of 30 and 40 cm for the sample solution in medium sand, Pb\({}^{2+}\) was only detected on the first day. Afterward, Pb\({}^{2+}\) was completely absorbed by the medium. (6) At the 30 and 40 cm heights for the sample solution in medium sand, Pb\({}^{2+}\) concentration decreased in the range of 24.26-100%. This concentration decreased by 7.57% on average until the water table fluctuated at between 50 and 60 cm. Figure 4: Effects of pH on adsorption (**a**) and desorption (**b**) on different media, including coarse, medium, and fine sand. Figure 5: pH variation at different heights of sampling locations in the first and second cycles: (**a**) pH variation of coarse sand at sampling positions with heights of 20, 30, and 35 cm; (**b**) pH variation of coarse sand at sampling positions with heights of 35, 40, 45, 50, 55, and 60 cm; (**c**) pH variation of medium sand at sampling positions with heights of 20, 30, and 35 cm; (**d**) pH variation of medium sand at sampling positions with heights of 35, 40, 45, 50, 55, and 60 cm; (**e**) pH variation of fine sand at sampling positions with heights of 20, 30, and 35 cm; (**f**) pH variation of fine sand at sampling positions with heights of 35, 40, 45, 50, 55, and 60 cm. Figure 6: Average pH at different heights of sampling locations. Figure 7: The Pb\({}^{2+}\) concentrations vary in water table fluctuations. **(a)** The Pb\({}^{2+}\) concentrations vary in columns filled with coarse sand; **(b)** the Pb\({}^{2+}\) concentrations vary in columns filled with medium sand. Therefore, at the height of 35 cm for the sample solution, Pb\({}^{2+}\) concentration reached its maximum compared with the others. When the sample solutions were further away, the maximum concentration of Pb\({}^{2+}\) was smaller. In a column of coarse sand, when the water table rose to 30 cm, the desorption quantity of Pb\({}^{2+}\) reached its maximum, and the Pb\({}^{2+}\) concentration in the solution reached its maximum. Throughout the whole experiment, the maximum concentration of Pb\({}^{2+}\) at the 20, 30, 35, 40, and 45 cm sample solutions were 0.072, 36.061, 38.973, 16.941, and 0.042 mg\(\cdot\)L\({}^{-1}\), respectively.