## 3 Results and Discussion
### Vertical TPH Removal at Three Depths in Soil Column  
Soil samples were taken at three depths (5, 15, and 25 cm) to verify the vertical TPH remediation efficiency of chemical oxidation and biodegradation. Figure 4 shows residual TPH concentrations in soil columns vertically at the three tested depths. TPH concentrations at 25 cm depth always showed higher values than those at the shallower depths. The results also demonstrated a difference in TPH removal efficiency between with holes and without holes. In the soil column with holes, the residual TPH concentrations decreased to 3125 (5 cm depth), 3143 (15 cm depth), and 3742 mg kg\({}^{-1}\) (25 cm depth) by using only 0.2 PV of Na\({}_{2}\)S\({}_{2}\)O\({}_{8}\) (1.64 L) in the first 2 days of the oxidation period with corresponding TPH removal efficiencies of 43.8, 52.8, and 53.1% at the respective depths of 5, 15, and 25 cm. In the soil column without holes, soil-TPH was reduced to 4296, 4785, and 4807 mg kg\({}^{-1}\) from the respective depths of 5, 15, and 25 cm with corresponding efficiencies of 24.9, 25.2, and 32.9%. In the control, there was no distinct breakthrough at 2 days with the residual TPH in the range 6427-6545 mg kg\({}^{-1}\). Figure 3: Effect of vertical soil holes on TPH removal by surfactant foam spraying. “Control (water)” indicates that water was poured onto the soil surface. “(PS + Bio) w/o holes” indicates persulfate (PS) oxidation (2 days) followed by bioaugmenation (Bio) (30 days) in the soil column without holes. “(PS + Bio) w/holes” indicates those with holes. The TPH was extracted from composite samples by mixing soils from three depths. The different letters (**a–e**) above the bars indicate significant differences (\(p<0.05\), Tukey’s test). After 30 days, the TPH concentrations in the soil column with holes fell to 628, 653, and 825 mg kg\({}^{-1}\) at the respective depths of 5, 15, and 25 cm with corresponding TPH removal efficiencies of 90.6, 90.2, and 87.6%. The additional 37-44% removal of TPH from the 2 day results was attributed to the biodegradation process. The residual soil-TPH values in the column without holes were 658, 1750, and 1800 mg kg\({}^{-1}\), with removal efficiencies of 89.7, 72.7, and 71.9% from the corresponding depths of 5, 15, and 25 cm. Although the TPH degradation for 5 cm region was similar in both cases, the difference in TPH degradation between soil regions at depths of 15 and 25 cm was higher. These results were as expected by the VWC measurements at 25 cm, as shown in Figure 2. The VWC in the column with holes was twice as high as without holes, implying that the aqueous phase remediation agents were better delivered at the 25 cm depth. Biodegradation generally relies on the ability of aqueous solutions containing amendments to infiltrate into contaminated areas. Therefore, the success of aqueous-supplied amendments is limited on the infiltration capacity and persistency since the electron acceptor or nutrient is often metabolized before it reaches the contaminated area [39]. The surfactant AOS used in the remedial agents not only enhanced the infiltration of remedial agents but also accelerated the dissolution of oil from soil which assisted biodegradation of soil-TPH [28; 40]. However, [PERSON] et al. [28] reported that surfactant AOS did not result in any significant reduction in TPH concentration during 33 days of foam spraying, although there was a small variation during the initial 3-6 days of the observation period. Moreover, no significant contaminant movement occurred by surfactant foam flow; therefore, contaminants stay in foam-occupied regions and react with reactants [18; 20]. The result of the vertical soil-TPH change during experiment implies that it would be difficult to remove TPH present in a region deeper than those tested using remediation methods depending on natural-gravity-driven infiltration.