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The soil gas diffusion coefficient (DP) and its dependency on air‐filled porosity (ε) govern most gas diffusion‐reaction processes in soil. Accurate DP(ε) prediction models for undisturbed soils are needed in vadose zone transport and fate models. The objective of this paper was to develop a DP(ε) model with lower input parameter requirement and similar prediction accuracy as recent soil‐type dependent models. Combining three gas diffusivity models: (i) a general power‐law DP(ε) model, (ii) the classical Buckingham (1904) model for DP at air saturation, and (iii) a recent macroporosity dependent model for DP at −100 cm H2O of soil–water matric potential (ψ), yielded a single equation to predict DP as a function of the actual ε, the total porosity (Φ), and the macroporosity (ε100; defined as the air‐filled porosity at ψ = −100 cm H2O). The new model, termed the three‐porosity model (TPM), requires only one point (at −100 cm H2O) on the soil–water characteristic curve (SWC), compared with recent DP(ε) models that require knowledge of the entire SWC. The DP(ε) was measured at different ψ on undisturbed soil samples from dark‐red Latosols (Brazil) and Yellow soils (Japan), representing different tillage intensities. The TPM and five other DP(ε) models were tested against the new data (17 soils) and data from the literature for additional 43 undisturbed soils. The new TPM performed equally well (root mean square error [RMSE] in relative gas diffusivity <0.027) as recent SWC‐dependent DP(ε) models and better than typically used soil type independent models.

The soil gas diffusion coefficient (D(P)) and its dependency on air-filled porosity (epsilon) govern most gas diffusion-reaction processes in soil. Accurate D(P)(epsilon) prediction models for undisturbed soils are needed in vadose zone transport and fate models. The objective of this paper was to develop a D(P)(epsilon) model with lower input parameter requirement and similar prediction accuracy as recent soil-type dependent models. Combining three gas diffusivity models: (i) a general power-law D(P)(epsilon) model, (ii) the classical Buckingham (1904) model for D(P) at air saturation, and (iii) a recent macroporosity dependent model for D(P) at -100 cm H2O of soil-water matric potential (psi), yielded a single equation to predict D(P) as a function of the actual epsilon, the total porosity (phi), and the macroporosity (epsilon100; defined as the air-filled porosity at psi = -100 cm H2O). The new model, termed the three-porosity model (TPM), requires only one point (at -100 cm H2O) on the soil-water characteristic curve (SWC), compared with recent D(P)(epsilon) models that require knowledge of the entire SWC. The D(P)(epsilon) was measured at different psi on undisturbed soil samples from dark-red Latosols (Brazil) and Yellow soils (Japan), representing different tillage intensities. The TPM and five other D(P)(epsilon) models were tested against the new data (17 soils) and data from the literature for additional 43 undisturbed soils. The new TPM performed equally well (root mean square error [RMSE] in relative gas diffusivity <0.027) as recent SWC-dependent D(P)(epsilon) models and better than typically used soil type independent models.

Soil‐water‐characteristic‐dependent (SWC‐dependent) models to predict the gas diffusion coefficient, D P , in undisturbed soil have only been tested within limited ranges of pore‐size distribution and total porosity. Andisols (volcanic ash soils) exhibit unusually high porosities and water retention properties. The Campbell SWC model and two Campbell SWC‐based models for predicting D P in undisturbed soil were tested against SWC and D P data for 18 Andisols and four Gray‐lowland (paddy field) soils from Japan. The Campbell model accurately described SWC data for all 22 soils within the matric potential range from ≈ −10 to −15000 cm H 2 O. The SWC‐dependent Buckingham‐Burdine‐Campbell (BBC) gas diffusivity model predicted D P data well within the same matric potential range for the 18 Andisols. The BBC model showed a minor but systematic underprediction of D P for three out of the four Gray‐lowland soils, likely due to a blocky soil structure with internal fissures. A recent D P model that also takes into account macroporosity performed nearly as well as the BBC model. However, D P in the macropore region (air‐filled pores >30 μm) was consistently underpredicted, likely due to high continuity of the macropore system in both Andisols and Gray‐lowland soils. In agreement with previous model tests for 21 European soils (representing lower porosities and water retention properties), both SWC‐dependent D P models gave better predictions for the 22 Japanese soils than soil‐type independent models. Combining D P and SWC data, a so‐called gas diffusion fingerprint (GDF) plot to describe soil aeration potential is proposed.

Soil air permeability (k a ) governs convective air and gas transport in soil. The increased use of soil venting systems during vadose zone remediation at polluted soil sites has created a renewed interest in k a and its dependency on soil type and soil air‐filled porosity (ε). Predictive k a (ε) models have only been tested within limited ranges of pore‐size distribution and total porosity. Andisols (volcanic ash soils) exhibit unusually high porosities and water retention properties. In this study, measurements of k a (ε) on 16 undisturbed Andisols from three locations in Japan were carried out in the soil matric potential interval from −10 cm H 2 O (near water saturation) to −15000 cm H 2 O (wilting point). Two simple power‐function k a (ε) models, both with measured k a at −100 cm H 2 O as a reference point, gave similar and good predictions of k a (ε) between −10 and −1000 cm H 2 O. For one location comprising finely textured and humic Andisols, both models largely underpredicted k a (ε) in dry soil (<−3000 cm H 2 O), suggesting a sudden occurrence of highly connected air‐filled pore networks during drainage. For the two other locations, the models satisfactorily predicted k a also in dry soil. Using recently published data for gas diffusivity and soil‐water retention together with the k a data in the Millington and Quirk (1964) fluid flow model, a plot of equivalent pore diameter as a function of soil matric potential was made for each soil. This plot, labeled a soil structure fingerprint (SSF), proved useful for illustrating effects of soil cultivation and high organic matter content on soil structure.

Diffusion of hydrophobic organic compounds (HOC) is a key process controlling transport of contaminants in soils. However, the separate effects of sorption and diffusion on net (effective) HOC diffusion are not fully understood. In this study, effective diffusion of naphthalene in five unsaturated soils was evaluated by: (i) naphthalene adsorption-desorption experiments (batch method), (ii) naphthalene effective diffusion experiments (half-cell method), and (iii) trace-gas diffusivity experiments (chamber method). There was no soil type effect on gas diffusivity in repacked unsaturated soil, but a pronounced soil type effect on naphthalene sorption behavior. Varying degree of adsorption nonlinearity (Freundlich n'^sub a^) and apparent adsorption-desorption nonsingularity ([omega]) and [omega] increased with decreasing n'^sub a^ were observed. In the half-cell experiments, gas diffusion was the governing naphthalene transport mechanism. Three effective diffusion coefficients were calculated from the half-cell experiments, based on concentration profile from either the whole (D^sub eff^) cell, the source (desorption) half-cell (D^sub eff,D^), or the recipient (adsorption) half-cell (D^sub eff,A^). Generally, the observed D^sub eff^ decreased with naphthalene-soil contact time, because of aging effects. The D^sub eff^, D^sub eff,A^ and D^sub eff,D^ values (half-cell method) could only to some extend be estimated from Freundlich isotherm parameters (batch method). A suggested index of effective diffusion nonsingularity, H = D^sub eff,D^/D^sub eff,A^ showed that H was correlated with [omega] and inversely correlated with n'^sub a^. Thus, the sorption nonlinearity (n'^sub a^) was found to provide good indications of degree of nonsingularity in both HOC adsorption-desorption and effective diffusion. The combination of batch and half-cell experiments generally gave useful insight towards understanding and predicting the influence of sorption nonlinearity and nonequilibrium on HOC diffusion.

Diffusion of hydrophobic organic compounds (HOCs) may be exploited to control spreading of these contaminants in the vadose zone. Effective (net) diffusion of naphthalene was studied in five unsaturated soil types using several experimental approaches on repacked soil columns. Soil type did not appear to affect gas diffusion, but it did affect naphthalene sorption behavior, with nonlinear and nonsingularity effects being observed. Gas diffusion was the primary naphthalene transport mechanism in half-cell experiments; effective diffusion coefficients were calculated from various concentration profiles. These and other findings suggest that the sorption nonlinearity was a good indicator of the degree of nonsingularity in both HOC adsorption-desorption and effective diffusion. Batch and half-cell experiments provided useful information for predicting HOC diffusive transport.

Diffusion of hydrophobic organic compounds (HOC) is a key process controlling transport of contaminants in soils. However, the separate effects of sorption and diffusion on net (effective) HOC diffusion are not fully understood. In this study, effective diffusion of naphthalene in five unsaturated soils was evaluated by: (i) naphthalene adsorption‐desorption experiments (batch method), (ii) naphthalene effective diffusion experiments (half‐cell method), and (iii) trace‐gas diffusivity experiments (chamber method). There was no soil type effect on gas diffusivity in repacked unsaturated soil, but a pronounced soil type effect on naphthalene sorption behavior. Varying degree of adsorption nonlinearity (Freundlich n′a) and apparent adsorption‐desorption nonsingularity (ω) and ω increased with decreasing n′a were observed. In the half‐cell experiments, gas diffusion was the governing naphthalene transport mechanism. Three effective diffusion coefficients were calculated from the half‐cell experiments, based on concentration profile from either the whole (Deff) cell, the source (desorption) half‐cell (Deff,D), or the recipient (adsorption) half‐cell (Deff,A). Generally, the observed Deff decreased with naphthalene‐soil contact time, because of aging effects. The Deff, Deff,A, and Deff,D values (half‐cell method) could only to some extend be estimated from Freundlich isotherm parameters (batch method). A suggested index of effective diffusion nonsingularity, H = Deff,D/Deff,A, showed that H was correlated with ω and inversely correlated with n′a Thus, the sorption nonlinearity (n′a) was found to provide good indications of degree of nonsingularity in both HOC adsorption‐desorption and effective diffusion. The combination of batch and half‐cell experiments generally gave useful insight towards understanding and predicting the influence of sorption nonlinearity and nonequilibrium on HOC diffusion.

Soil‐water‐characteristic‐dependent (SWC‐dependent) models to predict the gas diffusion coefficient, DP, in undisturbed soil have only been tested within limited ranges of pore‐size distribution and total porosity. Andisols (volcanic ash soils) exhibit unusually high porosities and water retention properties. The Campbell SWC model and two Campbell SWC‐based models for predicting DP in undisturbed soil were tested against SWC and DP data for 18 Andisols and four Gray‐lowland (paddy field) soils from Japan. The Campbell model accurately described SWC data for all 22 soils within the matric potential range from ≈ −10 to −15000 cm H2O. The SWC‐dependent Buckingham‐Burdine‐Campbell (BBC) gas diffusivity model predicted DP data well within the same matric potential range for the 18 Andisols. The BBC model showed a minor but systematic underprediction of DP for three out of the four Gray‐lowland soils, likely due to a blocky soil structure with internal fissures. A recent DP model that also takes into account macroporosity performed nearly as well as the BBC model. However, DP in the macropore region (air‐filled pores >30 μm) was consistently underpredicted, likely due to high continuity of the macropore system in both Andisols and Gray‐lowland soils. In agreement with previous model tests for 21 European soils (representing lower porosities and water retention properties), both SWC‐dependent DP models gave better predictions for the 22 Japanese soils than soil‐type independent models. Combining DP and SWC data, a so‐called gas diffusion fingerprint (GDF) plot to describe soil aeration potential is proposed.

Soil air permeability (ka) governs convective air and gas transport in soil. The increased use of soil venting systems during vadose zone remediation at polluted soil sites has created a renewed interest in ka and its dependency on soil type and soil air‐filled porosity (ε). Predictive ka(ε) models have only been tested within limited ranges of pore‐size distribution and total porosity. Andisols (volcanic ash soils) exhibit unusually high porosities and water retention properties. In this study, measurements of ka(ε) on 16 undisturbed Andisols from three locations in Japan were carried out in the soil matric potential interval from −10 cm H2O (near water saturation) to −15000 cm H2O (wilting point). Two simple power‐function ka(ε) models, both with measured ka at −100 cm H2O as a reference point, gave similar and good predictions of ka(ε) between −10 and −1000 cm H2O. For one location comprising finely textured and humic Andisols, both models largely underpredicted ka(ε) in dry soil (<−3000 cm H2O), suggesting a sudden occurrence of highly connected air‐filled pore networks during drainage. For the two other locations, the models satisfactorily predicted ka also in dry soil. Using recently published data for gas diffusivity and soil‐water retention together with the ka data in the Millington and Quirk (1964) fluid flow model, a plot of equivalent pore diameter as a function of soil matric potential was made for each soil. This plot, labeled a soil structure fingerprint (SSF), proved useful for illustrating effects of soil cultivation and high organic matter content on soil structure.