Vapor Phase Transport at a Hillside Landfill

摘要

This paper reports the results from a series oftwo-dimensional, time-dependent simulations of heat and masstransfer through a partially saturated mesa-top landfill innorthern New Mexico. Simulations use the finite elementapproach to solve the governing equations for a problem,wherein air mass, water mass, and energy are conserved. Weuse a computational grid that maintains complex topographicand stratigraphic boundaries. The 30 m of topographic relief atthe site allows atmospheric communication with the subsurfaceair and water vapor within the mesa. Time-dependent heat andgas generated in the landfill through the decomposition oforganic waste provide the main driving forces for vapor phasemigration. We show that the magnitude of vapor phasemigration is primarily controlled by gas generation sourcestrength. Increased temperature has a secondary effect on vaporphase flux. Flow paths change considerably from pre-landfillto post-landfill conditions. Pre-landfill upflow of air throughthe mesa with maximum flux of 2 cm/yr is driven by ambientdensity gradients. Post-landfill gas input reverses the directionof flow beneath the landfill, forcing gas into dry, permeablepathways that lead into the mesa. Vapor advection along highpermeability zones beneath the landfill may explainobservations of landfill gas found at depth. Post-landfill vaporflux most likely peaked with a maximum flux on the order of30 m/yr, within the first 20 years since closure. Advectivetransport of gas below the landfill is shown to dominate duringthe high productivity phase of gas generation. Transport oflandfill gas is shown to be dominated by diffusion when thevapor phase flux falls below 1 to 3 m/yr. Model results suggestthat capping the landfill with a low permeability layer couldcause the vapor flux to be diverted into the surrounding mesa via dry pathways.