Stable isotopic signatures (δ¹³C, δD) of methane from European landfill sites

The stable isotopic signatures (δ 13 C, δD) of CH 4 from four German and Dutch landfill sites have been characterized using different techniques for isotope analysis (tunable diode laser absorption spectroscopy and isotope ratio mass spectrometry). Samples taken directly from the gas collection systems show fairly uniform, biogenic δ 13 C‐δD isotopic signatures [δ 13 C = (−59.0±2.2)‰ VPDB ( n = 104); δD = (−304±10)‰ VSMOW ( n = 46)]. In contrast, emission samples taken with static chambers on soil‐covered landfill areas exhibit a considerable δ 13 C‐δD variability, mainly due to the influence of aerobic bacterial CH 4 oxidation, which occurs when the biogas CH 4 encounters atmospheric oxygen available in the uppermost region of the cover soil. Soil gas samples from the landfill covers clearly show the progressive isotopic enrichment within the aerobic regions of the soil. Isotope fractionation factors due to CH 4 oxidation were determined to be α(δ 13 C) = 1.008±0.004 and α(δD) = 1.039±0.026. On average, about 80% (70–97%) of CH 4 is oxidized during the transport through cover soils, while no significant CH 4 oxidation was found in uncovered areas consisting of freshly dumped waste. Area‐integrated δ 13 C values of total emissions were derived from upwind‐downwind measurements around the landfill and show very little temporal and site‐to‐site variation (δ 13 C = (−55.4±1.4)‰ VPDB ( n = 13; four different landfills)). CH 4 budgets were established for two landfill sites, indicating that projected CH 4 surface emissions from uncovered and covered areas are significantly lower compared to total CH 4 production (for a landfill without gas collection) or compared to the difference between CH 4 production and recovery (for a landfill with a gas collection system). For these two landfill sites the overall fraction of CH 4 oxidation is estimated to be 46 and 39% (53%) of total CH 4 production (minus recovery). Furthermore, the δ 13 C balance (comparing the δ 13 C values of the different emission pathways with the area‐integrated δ 13 C results) implies that direct CH 4 emissions via cracks or leakages constituted the major transport pathway (∼70%) into the atmosphere in both landfills.