# Investigating Effects of Landfill Soil Gases on Landfill Elevated Subsurface Temperature
## 2 Background  
Volumetrically, landfill soil gases usually comprises 40 to 60% carbon dioxide (CO\({}_{2}\)), 45 to 60% methane (CH\({}_{4}\)), 2 to 5% nitrogen (N\({}_{2}\)), and 0.1 to 1% oxygen (O\({}_{2}\)) along with low fractions of ammonia, CO, hydrogen, sulfides, and nonmethane organic compounds (NMOCs) such as benzene, trichloroethylene, and vinyl chloride [35]. Landfills produce gases in three processes: most gases are produced by aerobic and anaerobic bacterial decomposition; solid or liquid wastes convert into vapor via volatilization, e.g., nonmethane organic compounds (NMOCs) from disposal chemicals; chemical reactions between the remaining chemicals in waste [35]. Over a period of decades, landfill waste goes through bacterial decomposition in five phases:  
Phase I: Refuse matters are converted into water and CO\({}_{2}\) in the aerobic decomposition process by aerobic bacteria that live in an oxygen enriched environment. Phase II: In the absence of oxygen, anaerobic bacteria initiate anaerobic decomposition. Phase III: Some anaerobic bacteria produced in Phase II consume the organic acids in this decomposition phase. Organic acids formed via a microbial hydrolysis process convert to soluble acids and further break down to nitrous oxide, CO\({}_{2}\), CH\({}_{4}\) and a small amount of VOCs [36; 37]. Phase IV: This phase is relatively steady with the production rates and composition of the landfill gases. These gases are produced at a steady rate for 50 or more years after the waste pile is placed in the landfill [38]. Phase V: Some organic matters are available to be decomposed and oxygen is introduced in the environment. Figure 1 shows a flow chart of the chemical decomposition processes and the by-products generated during the different phases of waste decomposition. Factors such as waste characteristics (age and composition of the refuse), disposed chemicals in landfills, and some environmental factors (moisture content, temperature, the presence of oxygen) influence the volume and rate of generated landfill gases (e.g., CO\({}_{2}\), CH\({}_{4}\), N\({}_{2}\), and hydrogen sulfide) [39]. Bacterial activity increases as the landfill's temperature rises resulting in increased gas production. Changes in gas compositions have been observed to occur in advance of increases in wellhead temperatures, which can be an indicator of an approaching SET event [34]. MSW landfills produce CO\({}_{2}\), water, and heat in the process of aerobic decomposition ([PERSON] et al., 2017a). In the absence of oxygen, anerobic decomposition initiates after aerobic decomposition with the resultant production of CO\({}_{2}\), CH\({}_{4}\), and heat. Equations (1) and (2) can be used to express the aerobic and anaerobic transformation of organic waste, respectively [34]. A comparison between the enthalpies of both reactions indicates that the heat generated in aerobic decomposition is approximately 19 times higher than the heat produced from an anaerobic reaction [34]. As a result, the temperature typically ranges between 60 and 80 \({}^{\circ}\)C for a waste pile in aerobic conditions [34; 40], whereas the temperature in the anaerobic landfills ranges from approximately 25 to 45 \({}^{\circ}\)C [31; 33]. Heat accumulation by some exothermic reactions with the intrusion of oxygen or aerobic degradation creates a suitable condition to initiate and continue the smoldering combustion of MSWs [20; 39]. The tetrahedron of combustion theory describes the four conditions required for a combustion to happen [39; 41]: (1) an oxidizing agent (e.g., oxygen via air intrusion); (2) a source of fuel in MSWs; (3) a source of energy (e.g., heat generated from any exothermic reaction or aerobic decomposition); (4) chain reaction of combustion that is self-sustaining (e.g., charred waste). In MSW landfills, it is essential to limit any air intrusion that can be readily controlled. Subsurface combustion in landfills usually spreads through smolderingcombustion directly occurring on the solid fuel surface [10].