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 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 claytopsoil 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 finitedifference model was also developed which includes both gaseous mass transfer (CH4, CO2, O2) and microbial CH4 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 CH4 emissions to the atmosphere persisted into full winter conditions in December, 1995. Rates of CH4 oxidation (negative fluxes) from closed chamber experiments were similar to rates obtained from in vitro field incubations with initial headspace CH4 at ambient atmospheric concentrations (1-2 ppmv). Composited data from the chamber tests and field incubations demonstrated that oxidation rates were able to rapidly increase over 4 orders of magnitude as a direct kinetic response to broad ranges of initial CH4 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 CH4-limited group (low CH4; ambient O2) and an O2limited group (high CH4; subambient O2). The wholelandfill CH4 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 CH4 concentrations. Although the modeling relies on theoretical considerations for both gaseous flux and development of microbial populations, the Landfill CH4 Emissions Model requires a limited number of input variables and provides a practical tool for order-of-magnitude prediction of net CH4 fluxes at field sites. Background