Proceedings Sardinia 2009, Twelfth International Waste Management and Landfill Symposium S. Margherita di Pula, Cagliari, Italy; 5 - 9 October 2009 © 2009 by CISA Publisher, Italy ROLE OF SOIL GAS DIFFUSIVITY FOR THE MICROBIAL OXIDATION OF METHANE IN LANDFILL COVERS J. GEBERT AND A. GROENGROEFT University of Hamburg, Institute of Soil Science, Allende-Platz 2, 20146 Hamburg, Germany SUMMARY: The microbial oxidation of methane in cover soils bears great potential for the reduction of methane emissions from landfills. High methane degradation rates can only be accomplished if the supply of atmospheric oxygen to the methanotrophic community is adequate. Thus, system performance is strictly governed by the share of pores available for gas transport. Diffusion tests as well as column studies were conducted to investigate the effect of air-filled porosity and compaction on diffusivity and methane oxidation efficiency. Results show that the effective diffusion coefficient governing oxygen transport through soil is exponentially related to the air-filled pore volume and can be significantly decreased by compaction. Soil gas composition and methane oxidation rates in the column study correlated with both the degree of compaction and magnitude of advective bottom flux. Low aeration and correspondingly low methane oxidation rates prevailed at high compaction rates and/or high bottom fluxes whereas high rates could be maintained at lower fluxes and/or low compaction rates. At low compaction (75 % of the Proctor density), fluxes of 5.3 l CH 4 m -2 h -1 could be fully oxidized at all times by a sandy loam; the capacity limit was not reached during the experiment. From the measured relationship between air-filled pore volume and effective diffusivity it is recommended that soils intended for use as methane-oxidizing biocovers should maintain an air-filled pore volume of at least 17 vol.%. This is provided by sands, loamy sands, sandy loams and some of the coarsely textured loams. 1. INTRODUCTION At a global warming potential of 25 (IPCC, 2007), methane is a large potential contributor to climate change. Subsequently, understanding the environmental controls of methane cycling has received increased attention. Making up 22 % of the total anthropogenic methane emissions, landfills constitute the second largest anthropogenic source of methane in Europe (EEA, 2006) with an estimated annual release of 3.3 Gg. The potential of methanotrophy to mitigate landfill methane emissions at the interface between the landfill body and the atmosphere, using biofilters, biowindows or engineered cover soils (biocovers), has been affirmed in a number of laboratory and field scale studies (Streese and Stegmann, 2003; Barlaz et al., 2004; Scheutz et al., 2004; Gebert and Gröngröft, 2006; Haubrichs & Widmann, 2006; Powelson et al., 2006; Zeiss, 2006); and recently, the IPCC Working Group III assessment report (Bogner et al., 2007) has listed biocovers and biofilters as key mitigation technologies and practices to mitigate landfill