# Effect of Water Content on Light Nonaqueous Phase Fluid Migration in Sandy Soil
## 2 Materials and Methods
### Research Methods
#### 2.3.2 Unsaturated Soil Column Experiment  
The saturated weight water content and saturated weight LNAPL content of the media utilized in the study were determined to be 13.45% and 11.3%, respectively, based on the formulas applied. The higher saturated weight moisture content in comparison to the saturated weight LNAPL content can be attributed to the inherent properties of LNAPL. LNAPL, being a hydrophobic liquid, exhibits low surface tension, which leads to diminished intermolecular interactions and suboptimal contact with the pore walls. Therefore, the weight oil contents of the samples were 5%, 6%, 7%, 9%, 10%, 11%, and 13%. The weight water contents of the test soil columns were 4%, 6%, 8%, 10%, 12%, 14%, and 16%. As shown in Table 3, a total of 49 groups of experiments were conducted using the orthogonal experimental method, and each group of experiments was repeated three times to avoid the effect of chance error. In the experiment, the movement of LNAPLs was simulated under different water content conditions. The whole soil column system test group could be divided into three types: an oil-gas two-phase system, an oil-water two-phase system, and an oil-water-gas three-phase system. The variation of water content on the adsorption rate, as well as the migration rate of LNAPLs under unsaturated conditions, was analyzed.  
The experimental procedure is shown in Figure 4. First, we prepared and filled the sand samples. Before filling the sand samples, however, we applied petroleum jelly (Vaseline) to the inner walls of the acrylic glass sand sample columns to avoid the wall effect. The sand columns were filled with different water contents, with the porosity calculated according to the height and mass of the filled sample; these samples were put into the test sand columns, sealed, and placed in a standard curing chamber (temperature (20 \(\pm\) 2) \({}^{\circ}\)C and relative humidity \(\geq\) 95%) for 24 h to facilitate the wetting of sand samples. A gravel layer was also placed in the upper and lower layers of the sand sample to prevent the loss of sand particles. In the third step, the volume of quantitative LNAPL designed by the experimental program was passed through a peristaltic pump. The amount of diesel fuel flowing through the graduated tank was recorded, and the amount of diesel oil flowing out was recorded through the graduated base. When the diesel fuel stopped flowing out of the sand column, we opened the base drain to discharge the pollutants and collected them into a beaker for data comparison.  
The flow of oil-water two-phase fluid in porous media is an immiscible multiphase flow problem. The residual saturation within the unsaturated zone is the LNAPL saturation attained after initially saturated soil is allowed to drain by gravity to equilibrium [26]. Generally, the residual saturation will be lower within the unsaturated zone compared to the saturated zone. Thus, the LNAPL becomes an intermediate wetting fluid, which allows more of the fluid to drain from the soil than the saturated zone, where the gas phase is small, and the LNAPL is the non-wetting fluid. This study considered the water phase as the wetting phase fluid, the oil phase as the non-wetting phase, and the injection method  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Test Number** & **Water Content (\%)** & **Oil Content (\%)** & **Test Number** & **Water Content (\%)** & **Oil Content (\%)** \\ \hline
1\#–7\# & 4, 6, 8, 10, 12, 14, 16 & 5 & 29\#–35\# & 4, 6, 8, 10, 12, 14, 16 & 10 \\
8\#–14\# & 4, 6, 8, 10, 12, 14, 16 & 6 & 36\#–42\# & 4, 6, 8, 10, 12, 14, 16 &  
of replacing the wetting phase with the non-wetting phase was employed. The residual LNAPL content in the soil is an important feature parameter in the oil-water two-phase flow system, defined as:  
\[Q_{r}=(Q-Q_{l})/Q \tag{2}\]  
where \(Q_{r}\) is the residual LNAPL by the soil, \(Q_{i}\) is the outflow volume of \(i\) phase fluid, and \(Q\) is the initial flow rate of i phase fluid.