Analysis and modeling of vadose zone gas transport : modeling methane emissions from landfills
Spokas, Kurt A.
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The movement of gases between the soil and the atmosphere can occur by both diffusion and convection. This thesis examines the potential driving and controlling forces of gas movement through the vadose zone. A three-dimensional gas diffusional model was developed and tested with field data. Field instruments for the continuous monitoring of subsurface pressures and temperatures were developed and surface methane emission values were obtained from three landfill sites in California, Illinois, and Alabama. These emission values were compared to the developed gas diffusional model (LMEM [Landfill Methane Emission Model]) for the quantification of methane emission from the surface of landfills. The developed LMEM model differs from previous gas transport models in that it deals with the soil separately from the gas diffusion algorithm. The soil is handled as an obstruction in the flow path versus modifying the gas diffusional coefficient to account for the soil matrix. In this fashion, different soil characteristics can be more accurately modeled. The model’s dependency on soil moisture, porosity, and depth of cover were also examined. The collected field pressure and temperature data indicate a strong correlation between the air temperature and shallow subsurface pressures (<100 cm). This correlation disappeared at depths that exceeded 100 cm. This indicates that the primary variable affecting shallow vadose zone gas transport are air temperature fluctuations. No observable correlation exists between the measured subsurface pressures and the barometric pressure. The relationship between air temperature and subsurface differential pressure was further substantiated with the gas flow data collected from an open soil gas extraction well. The flow rate from the extraction well was in-phase with the air temperature fluctuations. The influence of air temperature could be explained partially by the correlation between water vapor pressure and temperature. The developed LMEM model was found to be a useful tool in predicting the magnitude of the methane flux from the surface of landfills in the range of 10^-2 to 10^3 g CH[sub 4]/m^2/day. The model did however have difficulties at higher soil moistures (>15 %). The model predicted the surface methane emissions better at the semi-arid site (California) than at the humid sites (Illinois and Alabama). However, even though the model overestimated the absolute magnitude of the methane flux at the humid sites, the direction of the flux still was accurately determined. This discrepancy at higher moisture contents exists because of the oversimplification in the moisture algorithm of the model. The moisture interactions in the model were all assumed to follow Henry’s law. The model was found to be highly sensitive to moisture and porosity conditions in the shallow (<25 cm) soil. As the thickness of the cover increased the sensitivity of the model to these variables decreased.