# Investigating Effects of Landfill Soil Gases on Landfill Elevated Subsurface Temperature
## 1 Introduction  
To date, landfills have been the most commonly used solid-waste disposal option. Solid-waste landfills have been designed to contain municipal solid waste in a large scale containment system to prevent and control the harmful effects on the environment and surrounding communities. A totalof 262 billion kilograms of municipal solid waste were assessed to be produced each year in the United States (US), and approximately 52.7% of this waste is buried within landfills [1]. In recent decades, the rate of municipal solid waste (MSW) production has increased substantially in the US, from 211.64 billion kilograms in 1990 to 254.42 billion kilograms in 2011 [2], whereas the number of landfills has substantially declined, from about 8000 in 1988 [3] to 1908 in 2011 [2]. This decline in landfill numbers is mostly because of the strict guidelines enforced by the Environmental Protection Agency (EPA), which has led to accommodating the increasing amount of MSW along with large waste piles within the few remaining landfills and resulting in the smaller surface-to-volume ratios. When the self-heat produced in the natural biodegradation processes exceeds the rate of heat dissipation through the landfill surface, temperatures may increase such that the landfill can ignite spontaneously [4; 5]. This article mainly focuses on subsurface fire events of the MSW landfills. Landfills are prone to subsurface fires because of their unique compositions and construction. The National Fire Incident Reporting System (NFIRS), reported in [6], found that more than 25% of these incidents occurred repeatedly at the same landfills [7]. Landfill fires can be classified as surface and subsurface fires. Surface fires that ignite over noncompacted freshly covered solid waste can occur due to different reasons such as discarding of the smoldering materials, spontaneous ignition and improper control systems of landfill-gases. Any combustion happening under the ground level within the waste mass is defined as a subsurface combustion which is not visible at the ground surface. These fires may continue undetected for many years; hence it becomes difficult to determine the extent of landfill damage. These events can cause tremendous internal and structural damage by consuming large amounts of waste and may result in the collapse of landfill sections while personnel are actively engaged in an attempt to contain the fire [8]. A smoldering event is initiated when temperatures are high enough, which can emit air pollutants including but not limited to volatile organic compounds (VOCs), semivolatile organic compounds (SVOCs), carbon monoxide (CO), polycyclic aromatic hydrocarbons (PAHs), and pose serious environmental and health hazards [9; 10; 11; 12; 13]. Subsurface combustion in landfills may cause severe damage to the leachate collection and liner system. Most subsurface fires burn slowly without showing visible flame, making it more difficult to detect compared to the surface landfill fires. Detecting subsurface fires is generally very difficult unless smoke or areas of settlement are observed, or a continuous measurement system is in place that records gas levels. A recent and existing example of such events is a dumpsite underground fire in the Forestdale area of Alabama, which has continued to burn since May 2020, damaging the ground stability and covering this residential area with hazardous smoke [14]. In such cases, the levels of CO (1000 ppm or more) are higher, well temperatures exceed 170 \({}^{\circ}\)F (67.67 \({}^{\circ}\)C) or the gas temperature within the extraction system is over 140 \({}^{\circ}\)F (60 \({}^{\circ}\)C). Elevated temperatures in landfills can occur by events characterized by high temperatures near the surface or in the subsurface at depths of greater than 20 m [15; 16].