Observations on the methane oxidation capacity of landfill soils

The objective of this study was to determine the role of CH₄ loading to a landfill cover in the control of CH₄ oxidation rate (g CH₄ m⁻² d⁻¹) and CH₄ oxidation efficiency (% CH₄ oxidation) in a field setting. Specifically, we wanted to assess how much CH₄ a cover soil could handle. To achieve this objective we conducted synoptic measurements of landfill CH₄ emission and CH₄ oxidation in a single season at two Southeastern USA landfills. We hypothesized that percent oxidation would be greatest at sites of low CH₄ emission and would decrease as CH₄ emission rates increased. The trends in the experimental results were then compared to the predictions of two differing numerical models designed to simulate gas transport in landfill covers, one by modeling transport by diffusion only and the second allowing both advection and diffusion. In both field measurements and in modeling, we found that percent oxidation is a decreasing exponential function of the total CH₄ flux rate (CH₄ loading) into the cover. When CH₄ is supplied, a cover’s rate of CH₄ uptake (g CH₄ m⁻² d⁻²) is linear to a point, after which the system becomes saturated. Both field data and modeling results indicate that percent oxidation should not be considered as a constant value. Percent oxidation is a changing quantity and is a function of cover type, climatic conditions and CH₄ loading to the bottom of the cover. The data indicate that an effective way to increase the % oxidation of a landfill cover is to limit the amount of CH₄ delivered to it.