## 3 Results and Discussion  
### Volumetric Soil Water Content (VWC) during Soil Column Experiment  
This study monitored volumetric soil water content (VWC, m\({}^{3}\)/m\({}^{3}\)) over a one-month period (2 day oxidation and 28 day bioaugmentation). The liquid uptake in the sediment is an indicator of the amount of remedial agent delivered to the contaminated area [18]. Figure 2a depicts the change in soil water content at 25 cm soil depth during the oxidation period of 2 days (48 h). During Na\({}_{2}\)S\({}_{2}\)O\({}_{8}\) foam spraying, the soil water content sharply increased up to maximum value (for the first five hours) which remained constant for 48 h of oxidation period. Na\({}_{2}\)S\({}_{2}\)O\({}_{8}\) can be transported to the bottom region twice as fast using vertical soil holes. The VWC in soil reached 0.171 m\({}^{3}\)/m\({}^{3}\) when Na\({}_{2}\)S\({}_{2}\)O\({}_{8}\) was applied to the soil surface with holes, whereas without holes, the VWC only reached 0.094 m\({}^{3}\)/m\({}^{3}\). Moreover, a greater value of VWC at 25 cm soil depth was recorded rather than at 15 cm soil depth (Supplementary Figure S2), implying a faster movement of remedial agents through holes. Initially, a slight increase in VWC from 0.027 to 0.032 m\({}^{3}\)/m\({}^{3}\) was observed in the control soil column, though the change was negligible. The results show that the combination of surfactant foam use and holes plays an important role in the movement of remedial agents into the bottom region of unsaturated soil. Figure 2b shows the change in soil water content during the bioaugmentation 28 day-period. In this period, a 2-3-fold increase in VWC oxidation period was observed. The VWC in the soil column with vertical holes, Na\({}_{2}\)S\({}_{2}\)O\({}_{8}\) foam + bio foam (with holes), had a value  
Figure 2: Change in volumetric water content (VWC) in soil columns with or without using vertical holes at 25 cm depth during (**a**) persulfate oxidation (PS) for two days, and (**b**) oxidation followed by bioaugmentation (PS + bio) for 28 days (4 weeks). Note that foam was sprayed onto the soil surface. Control indicates the treatment of water in soil columns with holes. of 0.322 m\({}^{3}\)/m\({}^{3}\) at soil depth of 25 cm; however, the VWC without holes, Na\({}_{2}\)S\({}_{2}\)O\({}_{8}\) foam + bio foam (w/o holes), reached only 0.151 m\({}^{3}\)/m\({}^{3}\) holes. The results show that small vertical holes improved the distribution of remedial agents in soil, demonstrating a 2-3-fold increase in VWC. Enhanced delivery of remedial agents occurred in the bottom area of the soil column when using a combination of surface surfactant foam spray and vertical small holes. In the control test using holes, \"Water for control (with holes)\" in Figure 2, the VWC reached a maximum value of 0.076 m\({}^{3}\)/m\({}^{3}\), even though the same volume of water as the volume of surfactant solution (for foam) was poured onto the soil. The results imply that water may infiltrate only through preferential flow paths in the unsaturated soil, showing a low VWC at 25 cm soil depth [37,38]. Leachate collected from the bottom of the soil column (see Figure S3) clearly showed that the volumes thereof in \"water for control (with holes)\" were always higher than that in other conditions. These results imply that water infiltrated only through preferential flow paths in the unsaturated soil. After being broken, surfactant foam may slowly change into a solution, allowing its gradual infiltration into the soil. This suggestion was supported by the visualization experiment shown in Figure S4. During this experiment, foam was poured into a plastic box of 22 cm long, 13 cm wide, and 7.5 cm tall; it had a 2.5 cm hole at the bottom that was connected to the neck (2.5 cm i.d., similar to the soil holes used in this study) of a 1 L volumetric flask (see Figure S4). No foam movement through the neck was immediately observed after the foam was poured into the box.