2.1.2. Experimental Device

The experiment designed three plexiglass columns with the following exact specifications: the inner diameter was 20 cm, the height was 110 cm, the top was open, and the opening of the glass column was covered with plastic film; the bottom was connected to a pressure-measuring tube and a Markov flask through a three-way valve. Between the Markov flask and the cylinder, a peristaltic pump was equipped to control the water level to simulate the process of groundwater fluctuation (rising–falling); the organic glass column was equipped with four sampling ports from top to bottom, at 15, 25, 35, and 45 cm away from the bottom of the column; adjacent points were separated by 10 cm. The sampling points were equipped with a Rhizon solution sampler (inner diameter: 1 mm); we covered the top and bottom of each column with 5 cm of quartz sand (diameter 2~3 mm), to ensure that the water level rose and fell uniformly. The experimental device is shown in Figure 2.

**Figure 1.** Location of sampling points.

**Table 1.** Soil particle composition and its parameters.


**Figure 2.** Schematic diagram of the experimental device.
