Table 111 Field comparison of three techniques for landfill CH4 emissions Deviation between measured CH4 release rate and controlled CH4 release during blind field test Wisconsin October 2008

Total CH4 release rates (range for trials 1-4; each trial with 1-3 release points)

Deviation of total measured CH4 flux versus total rate of controlled CH4 release (range for trials 1-4)

FLUXSENSE (mobile plume/FTIR)

WMX/Arcadis (VRPM)

NPL (DIAL)

1.01—3.28g CH4 sec-1 released from 1-3 points on 40 X 40m area in 300 X 500m evaluation area (approx. 59-177g CH4 m-2 d-1 for 40 X 40m release area only, or 0.06-0.19g CH4 m-2 d-1 for 300 X 500m evaluation area)

SD for technique 4-9%

-3 to -21% for closest vertical plane -42 to 48% for farthest vertical plane

SD for technique 18-33%

SD for technique 24-31%

Note: Wind speed ranged from 3.8 to 4.4m s-1. SD = standard deviation. Source: Babillotte et al (2009)

near-surface processes (soil respiration, organic matter oxidation, activity of soil invertebrates, photosynthesis) (Kuzyakov, 2006). In fact, from laboratory incubations, no significant correlation was observed between CH4 oxidation and the corresponding CO2 production in peat soils (Moore and Dalva, 1997). A further complication is that CO2 is more soluble than CH4 and thereby more readily partitioned to soil moisture (Stumm and Morgan, 1987; Zabowski and Sletten, 1991). In non-landfill soils, elevated CO2 soil gas concentrations have also been observed to suppress microbial respiration (Koizumi et al, 1991).

During the last few years, the development of robust engineering designs for landfill 'biocovers' has indicated that the limits of natural CH4 oxidation in landfill settings could be expanded with engineered solutions (Barlaz et al, 2004; Huber-Humer, 2004; Abichou et al, 2006b; Stern et al, 2007; Bogner et al, 2010) In general, the common elements of landfill biocovers have included a gas dispersion layer of coarse materials above the waste (to reduce the variability in CH4 fluxes to the base of the cover) overlain by compost or other organic substrates which have been shown to have high CH4-oxidizing capacity. Issues requiring further investigation include the possibility of in situ methanogenesis under saturated conditions in organic cover materials, as well as N2O emissions from N-enriched organic materials such as sewage sludge compost. Incidentally, N2O emissions from landfills are considered an insignificant source globally (Bogner et al, 1999; Rinne et al, 2005). However,

N2O emissions may be locally important where there is abundant N, high moisture, and restricted aeration; this includes cover soils amended with sewage sludge (Borjesson and Svensson, 1997) or where aerobic or semi-aerobic landfilling practices are implemented (Tsujimoto et al, 1994).

At larger scales, there have been few research studies that have attempted to quantity the contribution of landfill CH4 to regional air quality. In large part this is due to the dispersed nature of landfill sources and the complexity of multiple CH4 sources in most areas, including wide ranges in emission rates and overlapping isotopic signatures with other sources (such as wetlands). A 'top-down' study for sources of atmospheric CH4 in London used stable carbon isotopes to model the relative contribution of several potential sources, including landfill CH4 (Lowry et al, 2001). Zhao et al (2009) estimated the landfill contribution to atmospheric CH4 for a region in central California. However, directly quantifying the contribution of landfill CH4 to regional atmospheric CH4 remains a significant challenge for the future.

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