Methane mass balance of a Spanish landfill

Landfills represent a great opportunity to reduce anthropogenic methane (CH4) emissions in the near future. Although they accounted for 19% of global anthropogenic CH4 emissions in 2020 (Saunois et al., 2024), emissions mitigation strategies, such as landfill gas capture or microbial methane oxidation systems, have been well documented and successfully implemented in the past. However, accurate monitoring methods are required to assess the efficiency of such mitigation efforts. Currently, the IPCC recommends the use of first-order decay (FOD) models to estimate landfill CH4 generation potentials (IPCC, 2006). Such models remain highly uncertain due to large uncertainties in the model inputs (e.g., waste amounts or waste compositions) and/or model parameters (e.g., waste carbon contents, decay constants, biochemical CH4 potential, oxidation potential of landfill cover) (Rasouli et al., 2025, Wang et al., 2024, Mou et al., 2015). Therefore, estimating the CH4 generation potential of a landfill using direct in-situ measurements is preferred. Combining measurements of CH4 emissions, landfill gas collection and CH4 oxidation allows for more accurate estimates of the landfill CH4 generation potential and CH4 recovery efficiency. Although many studies have measured CH4 emissions using a wide range of in-situ methods (e.g., static flux chamber, atmospheric inversion modelling or tracer dispersion), few have also quantified the oxidation capacity of the landfill cover soils (Agdham et al., 2018; Abichou et al., 2006; Chanton et al., 2009). In this study, we performed a CH4 mass balance to estimate the CH4 generation potential at a landfill in Madrid, Spain receiving more than one million tons of household waste yearly. We collected 76 stable isotope samples and over 200 tracer dispersion measurements over the course of two campaigns in May and October 2025. Combining our measurements with landfill gas collection data, we estimated the landfill CH4 generation potential and CH4 recovery efficiency. Additionally, we compared our estimate to the modelled CH4 generation potential using different FOD models (e.g., IPCC FOD, Afvalzorg, LandGEM, GasSim). Leveraging the strength of direct in-situ measurements, this study provides valuable insights into CH4production at landfill sites and how to further enhance CH4 mitigation.