LANDFILLS AS ATMOSPHERIC METHANE SOURCES AND SINKS

Sanitary landfills are recognized as globally significant sources of atmospheric methane, but field measurements are rare. Existing country-specific landfill emissions have been estimated from solid waste statistics and a series of assumptions regarding methane generation and emission rates. There has been no attempt to reconcile the national and global estimates with limited field data on landfill methane emissions which range over six orders of magnitude (Bogner and Scott, 1995). This paper addresses controlled field measurements of methane emissions at sites in Illinois and California (USA) using a closed chamber technique. Overall, observed rates from various controlled monitoring experiments during 1988-1994 ranged from 0.003 io more than 1000 g CH4 m(-2) d(-1). Rates were related to the presence or absence of gas recovery wells, physical properties of cover soils (texture, moisture, and temperature) relating to their aeration status for diffusional flux, and rates of methane oxidation by indigenous methanotrophs. Surprisingly, at the Illinois site during spring, 1994, the landfill surface was consuming atmospheric methane rather than emitting landfill methane. This was attributed to high capacities for methane oxidation in well-aerated soils which had reduced landfill methane compared to 1993, the result of an effective pumped gas recovery system. Three independent methods confirmed that the landfill cover soils were functioning as a methane sink: (a) static closed chamber measurements yielding negative flux rates (uptake of atmospheric methane); (b) rates of methane oxidation similar to chamber results from in vitro field incubation studies using ambient methane; and (c) a reversal in the soil gas methane concentration gradient at the 25 cm depth. Field verification of landfill cover soils functioning as methane sinks has profound implications for revision of landfill contributions to global methane budgets; furthermore, it should be feasible to develop mitigation stategies incorporating a combination of engineered and natural methanotrophic controls.