# Investigating Effects of Landfill Soil Gases on Landfill Elevated Subsurface Temperature
## 2 Background
CO, CO\({}_{2}\), water vapor, and heat are yielded from the partial smoldering combustion of cellulose [42], as shown in Equation (3). \[\text{C}_{6}\text{H}_{12}\,\text{O}_{6}+\,6\text{O}_{2}\,\rightarrow\,6\text{ CO}_{2}\,+\,6\text{H}_{2}\,\text{O}\,;\,\Delta\text{H}=\,-2815\text{ kJ}/\text{mol} \tag{1}\]  
\[\text{C}_{6}\text{H}_{12}\,\text{O}_{6}\,\rightarrow\,3\text{CO}_{2}\,+\,3 \text{CH}_{4}\,\,;\,\Delta\text{H}=\,-145\text{ kJ}/\text{mol} \tag{2}\]  
\[\text{C}_{6}\text{H}_{10}\,\text{O}_{5}\,\left(\text{s}\right)+\,5.7\text{O}_{2 }\,\left(\text{g}\right)\rightarrow\,5.4\text{CO}_{2}\,\left(\text{g}\right)+ \,0.6\text{CO}\,\left(\text{g}\right)+\,5\text{H}_{2}\,\text{O}\,;\,\Delta\text{H }=-2440\text{ kJ}/\text{mol} \tag{3}\]  
Smoldering ignition does not proceed to completion due to the limited O\({}_{2}\) available but can propagate at a low level of oxygen, e.g., less than 3% volume-to-volume ratio (_v/v_) [43; 44; 45]. Smoldering combustion within an MSW landfill has been documented to persist at temperatures between 100 and 120 \({}^{\circ}\)C [46]. In some cases, the temperature during smoldering combustion has been detected in the range of 200-300 \({}^{\circ}\)C and even as high as 700 \({}^{\circ}\)C in MSW landfills [28; 23]. [PERSON] and [PERSON] [47] measured a deep subsurface fire with a range of 80-230 \({}^{\circ}\)C. Research has shown that the integrity and service life of landfill leachate control systems, gas-extraction systems, materials and cover of the composite liner systems can be impacted by a sustained temperature as low as 85 \({}^{\circ}\)C [48]. During a  
Figure 1: Chemical processes and byproducts generated in 5 phases of bacterial decomposition. SET, the landfill gas composition rapidly changes from CO\({}_{2}\) (40-55% _v/v_) and CH\({}_{4}\) (50-60% _v/v_) to a gas composition of CO\({}_{2}\) (60-80% _v/v_), hydrogen (10-35% _v/v_), CO (>1500 ppmv) along with the ratio of CH\({}_{4}\) to CO\({}_{2}\) falling below 1 [34]. [PERSON] et al. [49] observed the subsurface temperature rising from 55 to 75 \({}^{\circ}\)C in their case study, when CH\({}_{4}\) to CO\({}_{2}\) ratio reduced from 1.2 to 0.3 in a course of 50 days [49]. The technical procedures for detecting, evaluating, and mitigating elevated temperatures or landfill fires vary in the literature. The elevated temperature considered in this study refers to an increase in gas-well temperature beyond a certain threshold. The United States Environmental Protection Agency (US EPA) recommends a normal level of O\({}_{2}\), CH\({}_{4}\), and temperature ranges for landfills to be, respectively, \(<\)5%, 45% to 60%, and less than \(<\)55 \({}^{\circ}\)C, and a CO level of 2000 ppm is regarded as an action level (the concentration level in excess requires regulatory or remedial action) [50]. The Solid Waste Association of North America (SWANA) suggests temperature \(<\)52 \({}^{\circ}\)C, O\({}_{2}\) levels \(<\)1%, and CH\({}_{4}\) levels from 45 to 58% as normal ranges, and traces of CO in excess of 25 ppm calls for an action level to take precautionary steps against SETs [39; 51]. The difference in the ranges for O\({}_{2}\), CH\({}_{4}\), and temperature recommended by the SWANA from those of the US EPA are minimal. The level for CO suggested by the US EPA is more tolerant, up to 2000 ppm, whereas the SWANA has a strict interpretation of the action level with the presence of only 25 ppm CO. Unlike the US EPA, the SWANA considers CO and residual nitrogen (RN\({}_{2}\)) to be possible indicators of smoldering events. Table 1 displays the ranges of subsurface RN\({}_{2}\) level along with their indications as described by [PERSON] [52]. The SWANA emphasizes the monitoring of CO to assess landfill fires. Table 2 simplifies the details regarding landfill operations and fire prevention.