Using stable isotopes as an indicator for the remaining gas generation potential in decommissioned municipal waste landfills – a concept study

Stable isotope measurements have been used as a tool for understanding landfill processes for over two decades. The stable isotope natural abundance signatures of CH₄ and CO₂ give insight into the extent and duration of processes forming and consuming landfill gas, based on known kinetic fractionation factors for carbon turnover, carbon decomposition, methanogenesis and methane oxidation. Variations in isotopic ratios of carbon in CH₄ isotopocules have been documented for many landfills and can be interpreted in terms of methanogenesis, gaseous transport (both diffusive and by mass-flow) and oxidation. The aim of this contribution is to test that δ13C signatures of inorganic carbon in leachate and in CO₂/CH₄ as well as the DH signature of CH₄ and leachate can also be used to estimate the biodegradability of remaining organic matter in a closed landfill based on principles of Rayleigh fractionation. Our strategy is to perform laboratory experiments with excavated landfill waste from three different landfills in Norway with contrasting waste quality. Apparent fractionation coefficients will be compared with independently measured biodegradation potentials and physicochemical properties of the waste. Laboratory results will be integrated with field measurements of the isotopic composition of seeped and collected landfill gas and integrated into a landfill isotopic model. The landfill isotopic model will be used to estimate the remaining CH₄ emission potential of decommissioned and covered municipal landfills. The results from this study are relevant for landfill owners and operators as a tool to estimate the duration and volume of gas emissions at a particular site and to define the landfill management strategy appropriately.