Question 7 of 16: Hazards of Methane in Landfill Gas

Question 7: Hazards of Methane in Landfill Gas (3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(1) Explosion/fire hazard — methane is flammable between roughly 5–15% concentration in air; if LFG migrates laterally through soil into an enclosed space (a building basement, utility trench or manhole adjacent to the landfill) it can accumulate to within the explosive range and ignite. (2) Asphyxiation hazard — in a confined space, LFG (largely CH₄ and CO₂) can displace oxygen to levels unsafe for workers or the public, even without ignition. (3) Greenhouse gas contribution — uncaptured methane has a global-warming potential roughly 25–28× that of CO₂ over a 100-year horizon, making uncontrolled LFG emission a disproportionately large contributor to the site's — and the municipality's — carbon footprint. (4) Vegetation and cap damage — LFG migrating through the final cover can displace oxygen in the root zone and, being warm and reduced, stress or kill the vegetation the cap depends on for erosion control (see Q12).

Managing these hazards in practice combines passive and active controls: perimeter gas-migration probes monitor methane concentration in the soil at the property boundary against a typical action level of 25% of the lower explosive limit (LEL, i.e. roughly 1.25% v/v methane in soil gas), triggering investigation or an engineered gas-migration barrier (a vented trench or slurry wall) well before any hazardous concentration could reach an off-site structure. Buildings constructed on or adjacent to a closed landfill are also commonly required to include passive sub-slab venting or an active depressurization system as a standing precaution, since gas generation — and therefore migration risk — can persist for decades after a cell stops accepting waste.