## 2 Experimental Principles  
### Experimental Setup and Materials  
The experimental setup utilizes a custom-made transparent plexiglass sandbox, measuring 120 \(\times\) 20 \(\times\) 55 cm, which is primarily composed of two sections: the test area and the water level adjustment area. The test area is centrally filled with a medium, while the water level adjustment area is located on both sides of the sandbox. The sidewalls of this area are equipped with water level adjustment holes, with eight holes per side spaced 5 cm apart, connected to Mariotte bottles to maintain a stable water head. The test area and the water level adjustment area are hydraulically connected by a perforated partition with a hole diameter of 0.1 cm. The partition is wrapped with gauze to ensure stable water flow and to prevent soil from entering the water level adjustment area. At the bottom of the sandbox, seven drainage holes are installed, each equipped with a filter to prevent soil loss, and controlled by water-stopping clips for drainage. The medium is placed inside the sandbox, and water is introduced through the water level adjustment area, allowing the water level to rise gradually. Once a water film forms on the surface of the soil, the system is left to stand for 24 h before the bottom drainage is opened. The water is then naturally drained by gravity until completely dry, and this process is repeated three times to achieve a uniform and stable unsaturated state of the medium. The leakage point is strategically located at the center of the test area. The entire leakage process is controlled by a peristalic pump to regulate the amount and rate of pollutant discharge. The test setup is shown in Figure 1. The method of data acquisition used in this test is the intelligent high-density electro photometry system of Chongqing Top Technology Co. Ltd., which consists of the electro photometer mainframe, computer, power supply, intelligent switches, wireless transmission system, cables and copper electrodes. The particle analysis of the test medium gave the media properties as indicated in Table 1.  
In this experiment, No. 0 light diesel oil was used as a representative of LNAPL. With a density of 0.84 g/mL, a kinematic viscosity coefficient of 3.0-8.0 mm\({}^{2}\), a boiling point of  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{\begin{tabular}{c} **Unevenness** \\ **Coefficient** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Curvature** \\ **Factor** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Average** \\ **Particle** \\ **Size/mm** \\ \end{tabular} } & \multicolumn{6}{c}{**Percentage of Particle Size/\%**} \\ \cline{5-10}  & & & & **2-0.5 mm** & **0.5–0.25 mm** & **0.25–0.075 mm** & **0.075–0.005 mm** & **\textless{}0.005 mm** \\ \hline Primary & \multirow{2}{*}{1.89} & \multirow{2}{*}{0.87} & \multirow{2}{*}{0.131} & \multirow{2}{*}{0} & \multirow{2}{*}{0} & \multirow{2}{*}{94.4} & \multirow{2}{*}{3} & \multirow{2}{*}{2.6} \\ Medium & & & & & & & & \\ Weak lens & & & & & & & & \\ group & & & & & & & & \\ Intense lens & & & & & & & & \\ group & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Indicators of media properties.  
Figure 1: Diagram of the experimental setup.  
180-370 \({}^{\circ}\)C, being flammable and non-volatile, insoluble in water, soluble in ethanol and other organic solvents, and with a high degree of safety in operation, No. 0 diesel oil was chosen as a representative pollutant in the study of LNAPL. At the same time, in order to clearly record and photograph the LNAPL migration process, the diesel fuel was dyed red using Sudan Red as it melts in organic matter and does not dissolve in water. The physical properties of the test LNAPL representative media are shown in Table 2.