Wednesday, November 4, 2009
Convention Center, Exhibit Hall BC, Second Floor
Accurate prediction of gas diffusivity (Dp/Do) and air permeability (ka) and their variations with air-filled porosity (ε) in soil are critical for simulating the migration and fate of gaseous phase compounds, for example in relation to climate gas emissions or uptake in soil and indoor and outdoor air pollution at contaminated sites. Soil type/texture and soil compaction are two important parameters controlling gas movement in soils. This study investigates the effect of soil type and compaction on Dp/Do and ka and introduces two new compaction-corrected predictive models. We used Dp/Do and ka measurements from Danish soil profile data (total of 150 undisturbed soil samples) representing different soil types/textures and compaction levels (total porosities). The measurements were within a given range of matric potentials (-10 to -500 cm H2O) typically representing natural field conditions in subsurface soil. Data were regrouped into 4 categories based on soil compaction (total porosity (Φ) <0.4 and Φ >0.4 m3 m-3) and soil texture (volume-based percentage of clay, silt and organic matter (CSOvol) >15% and CSOvol <15%).Results revealed that both gas diffusivity and air permeability significantly depend on soil compaction. We developed a new Dp(ε)/Do model that is generalized from a previous macroporosity (ε100, air-filled porosity at -100 cm H2O)-dependant model developed specifically for Dp/Do at -100 cm H2O of matric potential. The model was further modified for compaction effects and performed well across different soil types, moisture conditions (pF 1-3) and compaction levels when compared to existing models. A similar compaction-corrected model was introduced to estimate reference-point air permeability (ka at -100 cm H2O), and combined with a power law model with an exponent linked to the exponent of gas diffusivity. A test against independent Dp/Do and ka data was promising and suggests that we have taken a step towards a unified compaction-corrected model concept for gas diffusivity and air permeability in undisturbed, variably saturated soils.