# Investigating Effects of Landfill Soil Gases on Landfill Elevated Subsurface Temperature
## 6 Discussions  
Our main goal of this study was to investigate the effects of soil gases on the SET over a certain threshold. There are several regulations recommended by the regulatory agencies regarding acceptable ranges of these landfill soil gases and subsurface temperatures. A temperature threshold of 80 \({}^{\circ}\)C was selected in this study since temperatures up to 80 \({}^{\circ}\)C have been observed during normal biological decomposition processes. We selected the thresholds for some of the gas parameters according to those reported in the 40 CFR SS60.753 operational standards--Thalhamer [39] and Estabrooks [52]. Our observations from the statistical analysis conducted above, can be drawn:  
* Unsafe temperature events have occurred more in the unsafe range for CH4, the CH4-to-CO2 ratio, and RN2, but not for O2. The combination associated with the highest SET did not include an unsafe O2 range, a finding which contradicts the suggested safe O2 range. * The conditional inference tree algorithm shows that the CH4-to-CO2 ratio of the four parameters has the strongest association with temperature. Ratios less than 1, O2 concentrations less than 5% and CH4 <45% and >60% resulted in the highest probability of events ranging from 80 to 149 \({}^{\circ}\)C. However, the safe O2 range was not associated with a safe temperature range, which is also consistent with the observation above. * Some combinations of gas parameters, i.e., (CH4 = 1, CH4-CO2 = 1, O2 = 0, RN2 = 0), (CH4 = 1, CH4-CO2 = 1, O2 = 0, RN2 = 1), and (CH4 = 1, CH4-CO2 = 1, O2 = 1, RN2 = 0) have an insignificant effect on temperature. This implies that there are other confounding variables affecting these three combinations. * Only the (CH4 = 1, CH4-CO2 = 1, O2 = 0, RN2 = 1) combination shows a high occurrence in the 93-149 \({}^{\circ}\)C range based on the naive Bayes conditional probability, whereas a gradual upward trend is observed for the 80-93 \({}^{\circ}\)C range in the four combinations of (CH4 = 1, CH4-CO2 = 1, O2 = 1, RN2 = 0), (CH4 = 1, CH4-CO2 = 1, O2 = 1, RN2 = 1), and (CH4 = 1, CH4-CO2 = 1, O2 = 0, RN2 = 1). Using the conditional probability results, the gas combinations have been summarized according to their potential risk levels (low, moderate, or high) with respect to SETs. * The unsafe ranges of the gas parameters are not always associated with high temperature ranges, but rather different combinations of parameters. Figure 9: Naive Bayes conditional probability [54] of four temperature ranges corresponding with different gas combinations. * A three-step procedure can be followed to evaluate the risk of a landfill subsurface fire. The diagram in Figure 10 provides a general idea of this step-by-step procedure for preventing subsurface fire incidents. Step 1 is to check the temperature to determine if it falls within the unsafe range, which would require that other parameters be controlled to bring the temperatures into the safe range. Otherwise, landfill personnel should proceed to step 2 which involves checking the gas combinations. Other preventive measures should be considered if one of the four combinations of (CH4 = 1, CH4:CO2 = 1, O2 = 1, RN2 = 0), (CH4 = 1, CH4:CO2 = 1, O2 = 0, RN2 = 0), (CH4 = 1, CH4:CO2 = 1, O2 = 1, RN2 = 1) and (CH4 = 1, CH4:CO2 = 1, O2 = 0, RN2 = 1) occurs. Step 3 proposes to check gas-wells showing similar combinations of (CH4 = 1, CH4:CO2 = 0, O2 = 0, RN2 = 1) and (CH4 = 1, CH4:CO2 = 0, O2 = 1, RN2 = 1) that correspond to a medium risk, and then monitor these gas-well locations more carefully or more often to ensure that they do not become one of the riskiest combinations. Landfill authorities can repeat this three-step action plan as part of their regular monitoring practices of the gas collection system. The dataset used for this research does not contain CO data, which may be a significant parameter affecting landfill subsurface fire incidents. [PERSON] et al.