## 4 Results
### Temperature Trends
During the years of the study, the maximum temperature was recorded in the lower region 81% of the time, and in the upper region only 12% of the time. Taking the upper region as a control, Figure 5 depicts the spatial temperature distribution in the landfill. Figure 5b shows that the lower region was measured to have the highest temperature, followed by the middle region. However, there were times where the upper region recorded the highest temperature. Figure 5a,c is analogous to Figure 5b with respect to the minimum and the mean temperature values. Thus far, we have shown that by examining the landfill spatially, we were able to detect temperature anomalies (Figure 3) from satellite images, after which we detected regions with a higher temperature and revealed the historical health status of the landfill (Figures 4 and 5). Next, we delve deeper to show changes at the pixel level and how the movement of the hot spots within the landfill can be traced during the years of the study. Figure 6 shows multitemporal maps (thermal maps) of the Bridgeton landfill in normalized scale (\(-8\) to \(8\)), in which the hotspots are easy to discern based on the LST. Figure 6a-l shows twelve snapshots of the LST hotspots from 2000 to 2011, respectively. These hotspots are consistently evident in the south and/or southeastern part of the South Quarry. Intermittent hotspots appear in the Neck area. Figure 6m-u shows nine snapshots of the LST hotspots from 2013 to 2016. These hotspots expanded to cover the entire southern portion of the landfill over the time period spanned by Figure 6.  
Figure 5: Taking the upper region as a “control” and comparing it with the lower/middle regions. Figure 6: _Cont._  
Figure 7 shows locations that reached the highest (or near-highest) temperature values most frequently (frequency of maxima). The bar on the right side of the figure indicates the number of highest temperature occurrences at that spot. This information can be used to predict where internal fires are likely to occur. By continuously monitoring the frequency of maxima, we can identify new hotspots and track the beginning and end of SSEs. The red squares in Figure 7 indicate hotspots  
Figure 6: Thermal map of Bridgeton landfill displaying LST in degree Celsius (\({}^{\circ}\)C) at various times and dates from 2000–2011: 7 September 2000 (**A**); 9 October 2000 (**B**); 8 July 2001 (**C**); 14 July 2003 (**D**); 15 August 2003 (**E**); 27 April 2004 (**F**); 1 June 2005 (**G**); 6 July 2006 (**H**); 27 September 2007 (**I**); 3 September 2010 (**J**); 5 October 2010 (**K**); 6 September 2011 (**L**); 25 July 2013 (**M**); 26 August 2013 (**N**); 11 September 2013 (**O**); 30 November 2013 (**P**); 13 August 2014 (**Q**); 29 August 2014 (**R**); 26 April 2015 (**S**); 12 May 2015 (**T**); 30 May 2016 (**U**). reported in February 2014 [23]. Some of the yellow squares correspond to other reported SSEs, such as that reported in 2012 [31]. The analysis of temperature trends shown above are based on the historical configuration of the Bridgeton Sanitary Landfill (North Quarry, Neck, and South Quarry) by the names used by the landfill operator. The purpose of this analysis was to monitor the movement of the hot spots in the landfill and the subsurface heat elevation to prevent them from reaching the North Quarry, which is adjacent to nuclear waste. The approach of an elevated heat area to the Neck serves as a warning and the need for action to be taken to prevent heat from spreading northward. However, for any landfill, the historical data collected and processed from satellite images can be grouped to identify regions for further study and to cluster them into three or more regions according to their temperature profiles. We used a simple method based on the areas of hottest, moderate, and cooler temperature to cluster the Bridgeton landfill into three regions. These newly define clusters (Figure 8) yield very similar results to those obtained above, and details of our analysis can be found in Appendix A.