Abstract

This paper reports the results from a series of two-dimensional, time-dependent simulations of heat and mass transfer through a partially saturated mesa-top landfill in northern New Mexico. Simulations use the finite element approach to solve the governing equations for a problem, wherein air mass, water mass, and energy are conserved. We use a computational grid that maintains complex topographic and stratigraphic boundaries. The 30 m of topographic relief at the site allows atmospheric communication with the subsurface air and water vapor within the mesa. Time-dependent heat and gas generated in the landfill through the decomposition of organic waste provide the main driving forces for vapor phase migration. We show that the magnitude of vapor phase migration is primarily controlled by gas generation source strength. Increased temperature has a secondary effect on vapor phase flux. Flow paths change considerably from pre-landfill to post-landfill conditions. Pre-landfill upflow of air through the mesa with maximum flux of 2 cm/yr is driven by ambient density gradients. Post-landfill gas input reverses the direction of flow beneath the landfill, forcing gas into dry, permeable pathways that lead into the mesa. Vapor advection along high permeability zones beneath the landfill may explain observations of landfill gas found at depth. Post-landfill vapor flux most likely peaked with a maximum flux on the order of 30 m/yr, within the first 20 years since closure. Advective transport of gas below the landfill is shown to dominate during the high productivity phase of gas generation. Transport of landfill gas is shown to be dominated by diffusion when the vapor phase flux falls below 1 to 3 m/yr. Model results suggest that capping the landfill with a low permeability layer could cause the vapor flux to be diverted into the surrounding mesa via dry pathways.

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