2.3.2. Experimental Methods for Water Table Fluctuations

The columns were filled with sand collected and processed from the field, including coarse, medium, and fine sand. The column was filled with gritstone around the 5 cm length, unpolluted medium around the 30 cm length, polluted medium around the 10 cm length, unpolluted medium around the 40 cm length, and gritstone around the 10 cm length, respectively, from bottom to top. A Pb(NO3)2 solution was mixed with the sample sand to produce 2000 mg·kg−<sup>1</sup> contaminated mixture. After loading, the deionized water at pH 7 flowed up from the base of the column, whose pressure tube was used to measure the water table. The peristaltic pump controlled the water table variation, rising or falling. The initial water table was set in the column at the beginning of the experiment at about 20 cm high. Then, the water table was adjusted to increase 10 cm per day until it had continuously risen to 60 cm by adjusting the peristaltic pump. The same method was applied to the water table decrease, which decreased 10 cm per day until it reached a height of 20 cm. This process represented the completion of one water table fluctuation cycle. In the column for water table fluctuations, two cycles were continuously conducted. Figure 2 shows how the water table varies with time. In order to further understand the migration process of Pb in the medium under water table fluctuations, the Pb2+ concentration adsorbed in the medium was measured by digestion treatment after the experiment. The sampling medium was obtained from the 20–60 cm height at an interval of 5 cm.
