Field investigations of lateral gas transport and subsequent emissions in soil adjacent to an old landfill in Denmark have been conducted during a one-year period. A significant seasonal variation in the emissions with high carbon dioxide and low methane fluxes in the summer (May to October) was observed. This was attributed to methane oxidation. Diumal measurements during a drop in barometric pressure showed that the fluxes of landfill gas changed dramatically within a very short time. The concentrations and the soil moisture content in the upper part of the soil profile had significant influence on the fluxes, as did the distance from the landfill border, temperature, barometric pressure and the pressure gradient. Statistical analyses proved that soil moisture described the largest part of the variation. No methane at all emitted during the summer. Calculations and isotope analyses showed that very high fractions of the laterally migrating methane were oxidised.

Soil exposed to elevated methane concentrations can develop a high capacity for methane oxidation. Methane oxidation at high and low methane concentrations is performed by different types of methanotrops and therefore oxidation rates found at low temperatures at the atmospheric methane content cannot be extrapolated to soils exposed to high methane concentrations. Four sandy soils with different organic matter content (1–9% w/w) from two landfills in Denmark were investigated in batch experiments in the laboratory to determine the response of methane oxidation at low temperatures and different soil moisture regimes. At 2°C the methane oxidation rates were 0.005 to 0.17 µmol g−1 h−1, and calculations showed that it was possible to oxidize all the produced methane at older landfills, even during the winter. Therefore, methane oxidation in top covers of landfills is an alternative to gas recovery at smaller and older landfills in northern Europe. Equations have been developed that describe the dependency of temperature and soil moisture content for each soil. The oxidation rates depended significantly on the soils (and thereby organic matter content), temperature, and soil moisture content. Soil moisture was the most important factor. However, high Q10 values indicate that temperature also was important. The four soils tested had optimum soil moisture content between 11 and 32%. At increasing organic matter content, both the optimal soil moisture content and the maximum oxidation rate increased.

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The Danish counties have performed numerous investigations of old landfills. These investigations have been presented in several reports, but no comprehensive summary of the findings has been carried out. The objective of this study was to evaluate the typical composition of leachates from old smaller landfills by a comprehensive review of the investigations carried out by the counties. In total 106 landfills were selected by criteria avoiding dilution effects. A database was constructed using a standard program. Statistical evaluations showed that the leachate concentrations in general decreased with the age of the landfill, and that the leachate concentrations were lower than found in other similar studies.