## 1 Introduction  
There are several cost-effective and green remediation processes (e.g., natural attenuation or bioremediation) for hydrocarbon-contaminated sites in current practice [1, 2, 3]. Although biological treatments are promising methods, they are tedious because the cycle times of bioremediation for total petroleum hydrocarbon (TPH) are relatively long and exhibit incomplete removal of hydrocarbons [4, 5]. Furthermore, the remedial agents may suffer poor contact or poor infiltration in the soil [6], owing to which the degradation potential may be insufficient to meet soil-TPH regulatory limits throughout the soil. The remediation of contaminated soils in the unsaturated zone is challenging because the access of the remedial agents to the contaminants is limited by factors such as depth, water saturation, soil anisotropy and gravity in unsaturated soils [7]. Chemical oxidation is a more powerful means for remediation compared to the bioremediation method. However, its efficiency is often limited by low soil permeability, high reactivity of oxidants with soil components (e.g., natural organic matter), radical scavenging, and the difficulty in controlling the heat emitted from oxidation reactions, which could further affect biological forms in the soil [6; 8; 9; 10; 11]. For faster and enhanced remediation efficiency, current practice is to combine chemical oxidation with bioremediation treatment for the remediation of petroleum-contaminated soil [11; 12; 13; 14; 15; 16; 17]. Although chemical oxidation pretreatment is required for faster remediation of oil contaminants, good contact between contaminants and remedial agents is essential to achieve the remediation goals. The uniform delivery of chemical reactants to the unsaturated zone is a difficult task [18]. The inherent issues related to solution-based delivery to an unsaturated medium (e.g., preferential flow and contaminant mobilization) can be overcome by using foam to deliver the remedial agents [19]. Some methods for foam transport in vadose zone remediation using unsaturated narrow columns (with an internal diameter (i.d.) of 2.4 to 5 cm) have been documented [18; 19; 20]. Surfactant foam can be applied for non-aqueous phase liquid (NAPL) flushing; thermal remediation; and nanoparticle, chemical, and nutrient delivery to enhance remediation of organic contaminants [19; 20; 21; 22]. Furthermore, [PERSON] et al. [22] demonstrated that surfactant foam was effective at carrying iron nanoparticles (NZVI) to enhance TCE volatilization in unsaturated sand-packed columns (16 cm long, 2 cm i.d.). They suggested that there was a relative improvement in the delivery of reagents when pressurized foam was injected into porous media. Indeed, most studies of foam application to porous media have used pressure-driven injection. Recently, surface foam spraying remediation technology using natural infiltration without soil mixing or disturbance has been suggested. As such, serial oxidation-bioaugmentation foam spraying has been tested in small scale reactors containing 5 cm thick unsaturated soil [23; 24]. However, the delivery of remedial agents to unsaturated soil thicker than 5 cm using natural infiltration has been scarcely studied. In this study, we evaluated the natural infiltration of surfactant foam carrying reactive reagents through vertical holes in order to improve poor and uneven distribution of remedial agents throughout the soil. Furthermore, spreading reagents using foam and slowly introducing reagents into the soil are the main ideas of the study. The remediation reagents into the soil can be gradually introduced, as foam dissolves, but the instant release of liquid may result in preferential flows. The first objective of this study is to evaluate the oxidation-bioaugmentation coupled system with unsaturated 30 cm thick soil to check the delivery limitations of reagents to the soil via gravity driven natural infiltration.