Isotopic characterisation and mobile detection of methane emissions in a heterogeneous UK landscape

Characterising methane (CH4) sources and their stable isotope values at the regional level is important for taking effective mitigation actions as well as more accurately constraining global atmospheric CH4 budgets. We performed dual stable isotope (13C, 2H) analysis of CH4 emission sources as well as mobile 13C measurements in North-West England, in a region with a mix of natural and anthropogenic emission sources as well as potentially exploitable shale gas deposits. Dual isotope analysis was performed for enteric fermentation, animal waste, landfill gas, wetlands, and natural gas from the regional distribution network. Microbial emission sources’ δ13C values ranged from −72.1 ± 0.31‰ to −53.1 ± 1.17‰ with agricultural sources and landfills showing partially overlapping values (−65.3 ± 0.41‰ to −72.1 ± 0.31‰ and −59.2 ± 0.26‰ to −70.4‰, respectively). However, the use of a dual isotope approach with δ2H provided additional separation between agricultural (−340 ± 0.8‰ to −322 ± 19.5‰) and landfill (−312 ± 0.3‰ to −282‰) CH4. All microbial sources were clearly distinct from natural gas with mean values of −39.5 ± 1.38‰ and −184 ± 4.9‰ for δ13C and δ2H, respectively. Mobile measurements conducted over a distance of 557 km detected emissions from two out of four surveyed managed landfills in the region. Multiple gas leaks were detected, which may confound emissions from other thermogenic sources. When separating the surveyed area by land-use into agricultural and urban, we found that background levels of CH4 were more depleted by around 1‰ in areas with agricultural land use compared to urban areas, but emissions from gas leaks and landfills are present in both categories. Our findings highlight the complexity of isoscapes in regions with multiple types of emission sources and the value of dual-isotope measurements in source attribution.