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

The stable isotopic signatures (delta(13)C, delta D) of CH4 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 delta(13)C-delta D isotopic signatures [delta(13)C = (-59.0 +/- 2.2)parts per thousand VPDB (n = 104); delta D = (-304 +/- 10)parts per thousand VSMOW (n = 46)]. In contrast, emission samples taken with static chambers on soil-covered landfill areas exhibit a considerable delta(13)C-delta D variability, mainly due to the influence of aerobic bacterial CH4 oxidation, which occurs when the biogas CH4 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 CH4 oxidation were determined to be alpha(delta(13)C) = 1.008 +/- 0.004 and alpha(delta D) = 1.039 +/- 0.026. On average, about 80% (70 - 97%) of CH4 is oxidized during the transport through cover soils, while no significant CH4 oxidation was found in uncovered areas consisting of freshly dumped waste. Area-integrated delta(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 (delta(13)C = (-55.4 +/- 1.4)parts per thousand VPDB (n = 13; four different landfills)). CH4 budgets were established for two landfill sites, indicating that projected CH4 surface emissions from uncovered and covered areas are significantly lower compared to total CH4 production (for a landfill without gas collection) or compared to the difference between CH4 production and recovery (for a landfill with a gas collection system). For these two landfill sites the overall fraction of CH4 oxidation is estimated to be 46 and 39% (53%) of total CH4 production (minus recovery). Furthermore, the delta(13)C balance (comparing the delta(13)C values of the different emission pathways with the area-integrated delta(13)C results) implies that direct CH4 emissions via cracks or leakages constituted the major transport pathway (similar to 70%) into the atmosphere in both landfills.