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The gas diffusion coefficient (Ds,g) and solute diffusion coefficient (Ds,l) and their dependencies on fluid content (κ) (equal to soil-air content θ for Ds,g and soil-water content ε for Ds,l) are controlling factors for gas and solute transport in variably saturated soils. In this study, we propose unified, predictive models for Ds,g(ε) and Ds,l(θ) based on modifying and extending the classical Maxwell model at fluid saturation with a fluid-induced reduction term including a percolation threshold (εth for Ds,g and θth for Ds,l). Different percolation threshold terms adopted from recent studies for gas (Ds,g) and solute (Ds,l) diffusion were applied. For gas diffusion, εth was a function of bulk density (total porosity), while for solute diffusion θth was best described by volumetric content of finer soil particles (clay and organic matter), FINESvol. The resulting LIquid and GAs diffusivity and tortuosity (LIGA) models were tested against Ds,g and Ds,l data for differently-textured soils and performed well against the measured data across soil types. A sensitivity analysis using the new Maxwell's Law based LIGA models implied that the liquid phase but not the gaseous-phase tortuosity was controlled by soil type. The analyses also suggested very different pathways and fluid-phase connectivity for gas and solute diffusion in unsaturated soil. In conclusion, the commonly applied strategy of using the same, soil-type-independent model for gas and solute diffusivity in analytical and numerical models for chemical transport and fate in variably-saturated soils appears invalid, except for highly sandy soils. The unified LIGA model with differing percolation thresholds for diffusion in the liquid and gaseous phases solves this problem. [ABSTRACT FROM AUTHOR]

Landfill sites have been implicated in greenhouse warming scenarios as a significant source of atmospheric methane. In this study, the effects of extreme compaction on the two main soil-gas transport parameters, the gas diffusion coefficient (Dp) and the intrinsic air permeability (ka), and the cumulative methane oxidation rate in a landfill cover soil were investigated. Extremely compacted landfill cover soil exhibited negligible inactive soil-air contents for both Dp and ka. In addition, greater Dp and ka were observed as compared with normal compacted soils at the same soil-air content ([variant_greek_epsilon]), likely because of reduced water-blockage effects under extreme compaction. These phenomena are not included in existing predictive models for Dp([variant_greek_epsilon]) and ka([variant_greek_epsilon]). On the basis of the measured data, new predictive models for Dp([variant_greek_epsilon]) and ka([variant_greek_epsilon]) were developed with model parameters (representing air-filled pore connectivity and water-blockage effects) expressed as functions of dry density (ρb). The developed Dp([variant_greek_epsilon]) and ka([variant_greek_epsilon]) models together with soil-water retention data for soils at normal and extreme compaction (ρb=1.44 and 1.85 g cm-3) implied that extremely compacted soils will exhibit lower Dp and ka at natural field-water content (-100 cmH2O of soil-water matric potential) because of much lower soil-air content. Numerical simulations of methane gas transport, including a first-order methane oxidation rate, were performed for differently compacted soils by using the new predictive Dp([variant_greek_epsilon]) model. Model results showed that compaction-induced difference in soil-air content at a given soil-water matric potential condition is likely the most important parameter governing methane oxidation rates in extremely compacted landfill cover soil. [ABSTRACT FROM AUTHOR]

The soil gas diffusion coefficient (Dp) and its dependency on soil air content, ϵ, and tortuosity-connectivity of the air-filled pore networks control the transport and fate of gaseous-phase contaminants in variably saturated soil. The bimodality pin ore size distribution of structured soil often yields a variation of Dp with ϵ in the intraagggre ate pore region that is distinctly different from that in the interaggregate region. Data imply a highly nonlinear behavior of soil gas diffusivity, Dp(ϵ)/D0 (where D0 is the gas diffusion coefficient in free air), in the interaggregate region of aggregated soils similar to that of structureless soils with a unimodal pore size distribution, probably due to diffusion-limiting effects by connected water films at low ϵ. In contrast, for the intraaggregate region, we show that the impedance factor F* (= Dp/ϵD0 ) and tortuosity factor T [= (1/F*)1/2] are approximately constant for most soil media. We suggest a typically well-defined separation between the two pore regions at the minimum for the pore connectivity factor X* [= log(Dp/D0)/log(ϵ)], at which point the interaggregate pores are devoid of water while the intraaggregate pore region is water saturated. Based on this, a hierarchical two independent region (TIR) Dp/D0 model was developed by applying a cumulative series of Buckingham-Currie power-law functions, FϵX. A nonlinear, water-content-dependent expression for F best described the measured Dp/D0 in the interaggregare region, while constant F (around 0.5) and X (around 1) generally sufficed for the intraaggregate region. The TIR model better predicted gas diffusivities for both aggregate fractions and highly structured soils across the entire range of moisture conditions with RMSE reduced by two to five times compared with traditional-predictive Dp(ϵ)/D0 models. [ABSTRACT FROM AUTHOR]

The soil gas diffusion coefficient (D p) and air permeability (κ a) and their dependency on soil air content (ε) control gas diffusion and advection in soils. This study investigated the effects of average particle size (D 50) and dry bulk density (ρb) on D p and κ a for six sandy soils under variably saturated conditions. Data showed that particle size markedly affects the effective diameter of the drained pores active in leading gas through the sample at -100 cm H2O of soil water matric potential (calculated from D p and κ a) as well as the average pore diameter at half saturation (calculated from the water retention curve), both exhibiting similar and exponential relationships with D 50 Under variably saturated conditions, higher D p and κ a in coarser sand (larger D 50) were observed due to rapid gas diffusion and advection through the less tortuous large-pore networks. In addition, soil compaction (larger ρb) simultaneously caused reduced water blockage effects and a reduction of large-pore space, resulting in higher D p(ε) but lower κ a(ε). Two recent models for D p(ε) and κ a(ε) were evaluated: the water-induced linear reduction (WLR) model for D p, and the reference-point power law (RPL) model for κ a, with reference point κ a set at -100 cm H2O. The performance of both models for the sandy soils (particle size range 0.02-0.9 mm) was improved if the pore connectivity-tortuosity factor and water blockage factors were assumed to be functions of D 50 and ρb Water blockage factors, N for the WLR D p(ε) model and M for the RPL κ a(ε) model, showed a strong nonlinear relationship (R² = 0.95) that seems promising for predicting D p(ε) from the more easily measureable κ a(ε). [ABSTRACT FROM AUTHOR]

The soil gas diffusion coefficient (Dp) and its variations with soil air content ϵ and soil water matric potential ψ control vadose zone transport and emissions of volatile organic chemicals and greenhouse gases. This study revisits the 1904 Buckingham power-law model where Dp is proportional to ϵX, with X characterizing the tortuosity and connectivity of air-filled pore space. One hundred years later, most models linking Dpϵ to soil water retention and pore size distribution still assume that the pore connectivity factor, X, is a constant for a given soil. We show that X varies strongly with both and matric potential [given as pF = log(-ψ, cm H2O)] for individual soils ranging from undisturbed sand to aggregated volcanic ash soils (Andisols). For Andisols with bimodal pore size distribution, the X-pF function appears symmetrical. The minimum X value is typically around 2 and was observed close to ψ of -1000 cm H2O (pF 3) when interaggregate voids are drained. To link Dp with bimodal pore size distribution, we coupled a two-region van Genuchten soil water retention model with the Buckingham Dpϵ model, assuming X to vary symmetrically around a given pF. The coupled model well described Dp as a function of both and for both repacked and undisturbed Andisols and for other soil types. By merely using average values of the three constants in the proposed symmetrical X-pF expression, predictions of Dp were better than with traditional models. [ABSTRACT FROM AUTHOR]

The soil-air permeability (ka) and its dependency on air-filled porosity (ε) govern convective air and gas transport in soil. For example, accurate prediction of ka(ε) is a prerequisite for optimizing soil vapor extraction systems for cleanup of soils polluted with volatile organic chemicals. In this study, we measured ka at different soil-water matric potentials down to 5.6-m depth, totaling 25 differently textured soil layers. Comparing ka and soil-gas diffusivity (Dp/D0) measurements on the same soil samples suggested an analogy between how the two soil-gas transport parameters depend on ε. The exponent in a power-law model for ka(ε) was typically smaller than for Dp(ε)/D0, however, probably due to the influence of soil structure and large-pore network being more pronounced for ka than for Dp/D0. In analogy to recent gas diffusivity models and in line with capillary tube models for unsaturated hydraulic conductivity, two power-law ka(ε) models were suggested. One ka(ε) model is based on the Campbell pore-size distribution parameter b and the other on the content of larger pores (ε100, corresponding to the air-filled porosity at -100 cm H2O of soil-water matric potential). Both new models require measured ka at -100 cm H2O (ka,100) as a reference point to obtain reasonably accurate predictions. If ka,100 is not known, two expressions for predicting ka,100 from ε100 were proposed but will cause at least one order of magnitude uncertainty in predicted ka. The ka(ε) model based on only ε100 performed well in the model tests and is recommended together with a similar model for gas diffusivity for predicting variations in soil-gas transport parameters in the vadose zone. [ABSTRACT FROM AUTHOR]