The identification of the source of methane (CH4) and carbon dioxide (CO2) in gases within unconsolidated deposits outside municipal landfill sites, where there are multiple potential contributing gas inputs, is important for regulation and to inform gas management policy. Stable isotope analysis was applied to ground gas samples from boreholes close to but external to landfill cells, and landfill gas, to differentiate CH4 from biological and abiological geological sources at sites in NW England. Landfill flare gas and landfill without external gas inputs were biogenic references. Ground gases adjacent to one landfill had potential external CH4 inputs from underlying Coal Measures. Mineshaft vent gas and commercially-supplied North Sea gas were used as non-biological, thermogenic, source references. Ground gas samples were subjected to Compound-Specific Isotope Ratio Mass Spectrometry (GC-IRMS) and conventional IRMS examination. values for landfill flare gas and landfill Site 1 were −65.0 and −61.0‰, and values were −319 and −298‰ respectively, consistent with a wholly biogenic origin. values for Site 2 landfill were −55 to −48‰, and values were −173 to +55‰. Mineshaft vent gas and North Sea gas CH4 values were −46.2 and −43.5‰, and values were −214 and −203‰, respectively. Given their similarity to these data, gases in the vicinity of Site 2 landfill are considered to have a thermogenic CH4 contribution, due to 13C and D enrichment in CH4.