Closing the methane mass balance for an old closed Danish landfill

In this study, a methane (CH₄) mass balance was established for Hedeland landfill. CH₄ generation rates were modelled using a multiphase first-order decay model (The Afvalzorg model) and determined at between 57 and 79 kg h⁻¹. The CH₄ emission rate was quantified at between 2 and 14 kg h⁻¹, using the tracer gas dispersion method and the CH₄ gas recovery efficiency was between 8 and 21%. At three places along the perimeter of the landfill, gas remediation systems have been installed to protect the residential houses from any risk of migrating landfill gas. About 0.76 kg h⁻¹ of CH₄ was extracted from these three remediation systems. Using a carbon mass balance for the lateral migrating landfill gas showed a fractional oxidation of about 78%, which corresponded to a CH₄ flux of 3.5 kg h⁻¹ from the three remediation systems, including the oxidised CH₄. The total lateral CH₄ flux (un-oxidised) from the total landfill perimeter was estimated at between 6.9 and 10.4 kg h⁻¹. CH₄ oxidation efficiency in the landfill cover soil, determined from stable carbon isotope analyses, was found to be between 12% and 92%. This resulted in an average CH₄ oxidation rate of 32 kg h⁻¹, using an average CH₄ emission rate of 8 kg h⁻¹. CH₄ surface screenings and surface flux measurements supported the hypothesis that oxidation efficiency was in the higher range and that oxidation could close the CH₄ mass balance.