Submitted to: Waste Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/2010
Publication Date: 3/16/2011
Citation: Chanton, J., Abichou, T., Langford, C., Spokas, K.A., Hater, G., Goldsmith, D., Barlaz, M. 2011. Observations on the methane oxidation capacity of landfill soils. Waste Management. 31(5):914-925. Interpretive Summary: The only known biological sink for atmospheric methane are methanotrophs (bacteria) that are universally present in all soils. This microbial process is pivotal at landfill sites, since metanotrophs reduce the emission of methane from the landfill surface. Determining the magnitude of this methane oxidation is one of the major uncertainties in estimating national or global landfill methane emissions. Landfill gas that is not collected passes through landfill cover soils on the way to being released to the atmosphere. As the gas migrates through the cover soils the gas can be oxidized by methanotrophs. However, there are uncertainties in the magnitude of this methane sink. This manuscript clearly shows, that this microbial process is dynamic (fluctuating in time) and is difficult to assess as a static or fixed quantity. This was accomplished both through field measurements as well as mathematical modeling. This research is of importance in the study of methane oxidation activity in soils, and not just limited to landfill environments. This research will contribute to improving annual greenhouse gas inventory guidelines where the soil processes need to be included in the assessment, such as soil methane oxidation. This research will lead to improved estimations of soil methane oxidation activity.
Technical Abstract: Field data and two independent models indicate that landfill cover methane (CH4) oxidation should not be considered as a constant 10% or any other single value. Percent oxidation is a decreasing exponential function of the total methane flux rate into the cover and is also dependent on climate and cover type. Landfill cover soil oxidation is conducted by a biological system and it behaves as such. Once a soil oxidation limit is reached, increasing the delivery of methane to the soil does not continue to increase the rate of oxidation, which stays constant. When methane is supplied, a cover’s rate of methane uptake is linear to a point, and then the system becomes saturated. Our results indicate that the best way to increase the % oxidation of a landfill cover is to limit the amount of CH4 delivered to it. In the early life of a landfill, this is best accomplished by an efficient gas collection system. The presence of a gas collection system reduces the concentration and pressure of methane at the base of the cover and thereby reduces the source strength of methane entering the cover system. However, the cover may be relied upon to consume a portion of the methane so that the extraction strength of gas collections systems can be adjusted downward to obtain landfill gas with an elevated CH4 composition to better power energy generating systems Geospatial mean landfill methane emissions ranged from 0.3 to 236 g m^-2d^-1 as a function of cover type. The best maintained final covers only allowed the emission of small quantities of CH4 while covers in different stages of development or disrepair released more CH4. Five landfill flat tops were compared with slopes in this study, and in 3 of 5 cases, the slopes had greater emissions by a factor of 3, factor of 7, and a factor of 30. In two cases the slopes emitted less methane than the tops did. Thus while slopes have the potential to contribute higher emissions relative to flat tops, this is clearly not always the case. As previously reported, CH4 emissions from the landfill surface were dominated by “hotspots”. We observed persistence of these features indicating that with increased maintenance such hot spots could be identified, repaired and the cover emissions attenuated by coverage with biocells.