KINETICS OF METHANE OXIDATION IN A LANDFILL COVER SOIL - TEMPORAL VARIATIONS, A WHOLE LANDFILL OXIDATION EXPERIMENT, AND MODELING OF NET CH4 EMISSIONS

Rates and controlling variables for methanotrophic oxidation of methan e at a northeastern Illinois landfill with pumped gas recovery were ex amined in a field study from June to December 1995. Cover materials co nsisted of a simple clay-topsoil sequence without geomembranes. Throug h use of a static enclosure (closed chamber) technique supplemented by soil gas concentration profiles and field incubations, the study conc entrated on proximal (near gas recovery well) and distal (between well ) sites established in 1994. A personal computer-based three-dimension al finite-difference model was also developed which includes both gase ous mass transfer (CH4, CO2, O-2) and microbial CH4 oxidation. Mass tr ansfer is modeled through a modified chemical potential gradient withi n a cubic network of nodes; a strict mass balance for each gas is main tained through successive timesteps. Methane-oxidizing conditions with no net CH4 emissions to the atmosphere persisted into full winter con ditions in December, 1995. Rates of CH4 oxidation (negative fluxes) fr om closed chamber experiments were similar to rates obtained from in v itro field incubations with initial headspace CH4 at ambient atmospher ic concentrations (1-2 ppmv). Composited data from the chamber tests a nd field incubations demonstrated that oxidation rates were able to ra pidly increase over 4 orders of magnitude as a direct kinetic response to broad ranges of initial CH4 concentrations (from ambient to 8.4 vo l %). The maximum observed rate was 48 g m(-2) day(-1). Kinetic plots indicated at least two major trophic groups of methanotrophs: a CH4-li mited group (low CH4; ambient O-2) and an O-2-limited group (high CH4; subambient O-2) The whole-landfill CH4 oxidation experiment was condu cted over a 2 day period when the pumped gas recovery system was shut down and restarted; oxidation rates increased and then decreased more than 2 orders of magnitude in response to changing CH4 concentrations. Although the modeling relies on theoretical considerations for both g aseous flux and development of microbial populations, the Landfill CH4 Emissions Model requires a limited number of input variables and prov ides a practical tool for order-of-magnitude prediction of net CH4 flu xes at field sites.