Kinetics of Methane Oxidation in a Landfill Cover Soil:  Temporal Variations, a Whole-Landfill Oxidation Experiment, and Modeling of Net CH₄Emissions

Rates and controlling variables for methanotrophic oxidation of methane at a northeastern Illinois landfill with pumped gas recovery were examined in a field study from June to December 1995. Cover materials consisted of a simple clay-topsoil sequence without geomembranes. Through use of a static enclosure (closed chamber) technique supplemented by soil gas concentration profiles and field incubations, the study concentrated on proximal (near gas recovery well) and distal (between well) sites established in 1994. A personal computer-based three-dimensional finite−difference model was also developed which includes both gaseous mass transfer (CH 4, CO 2, O 2 ) and microbial CH 4 oxidation. Mass transfer is modeled through a modified chemical potential gradient within a cubic network of nodes; a strict mass balance for each gas is maintained through successive timesteps. Methane-oxidizing conditions with no net CH 4 emissions to the atmosphere persisted into full winter conditions in December, 1995. Rates of CH 4 oxidation (negative fluxes) from closed chamber experiments were similar to rates obtained from in vitro field incubations with initial headspace CH 4 at ambient atmospheric concentrations (1−2 ppmv). Composited data from the chamber tests and field incubations demonstrated that oxida tion rates were able to rapidly increase over 4 orders of magnitude as a direct kinetic response to broad ranges of initial CH 4 concentrations (from ambient to 8.4 vol %). The maximum observed rate was 48 g m - 2 day - 1 . Kinetic plots indicated at least two major trophic groups of methanotrophs: a CH 4 -limited group (low CH 4; ambient O 2 ) and an O 2 -limited group (high CH 4; subambient O 2 ). The whole-landfill CH 4 oxidation experiment was conducted 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 CH 4 concentrations. Although the modeling relies on theoretical considerations for both gaseous flux and development of microbial populations, the Landfill CH 4 Emissions Model requires a limited number of input variables and provides a practical tool for order-of-magnitude prediction of net CH 4 fluxes at field sites.