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A unified measurement system with Suction control (UMS—SC) was developed for measuring soil water characteristics curves (SWCC) and gas transport parameters under alternating drying and wetting cycles. The new system consists of a diffusion chamber, sample ring, porous plate, tensiometer, moisture sensor, oxygen electrodes, and air pressure gauges. The SWCC and gas transport parameters [gas diffusion coefficient (Dp) and air permeability (ka)] for two different porous materials, Toyoura sand and granulated slag, were simultaneously measured under drying and wetting cycles. The SWCC and gas transport parameters measured by UMSSC were consistent with recent models and independently measured data on exactly the same materials using standard experimental setups from literature. For an applied water suction head of less than 50 cm and corresponding water saturation of around 0.3-0.5, the UMS—SC data documented hysteretic (nonsingular) behavior in both measured Dp and κa under repeated drying and wetting cycles. The hysteretic behavior was insignificant for water and air contents but large for both gas transport parameters when applying suction, and hysteretic effects were larger for air permeability than for gas diffusivity. Additionally, hysteresis in the percolation threshold (soil-air content where gas transport ceases due to interconnected water-filled pores) for both gas diffusion and air permeability was insignificant for both materials. These findings should be taken into account when developing models for diffusive and convective gas transport and their parameters in variably saturated porous media. [ABSTRACT FROM AUTHOR]

Abstract: Landfill sites are emerging in climate change scenarios as a significant source of greenhouse gases. The compacted final soil cover at landfill sites plays a vital role for the emission, fate and transport of landfill gases. This study investigated the effects of dry bulk density, ρb , and particle size fraction on the main soil–gas transport parameters – soil–gas diffusivity (Dp /Do , ratio of gas diffusion coefficients in soil and free air) and air permeability (ka ) – under variably-saturated moisture conditions. Soil samples were prepared by three different compaction methods (Standard and Modified Proctor compaction, and hand compaction) with resulting ρb values ranging from 1.40 to 2.10gcm−3. Results showed that Dp and ka values for the ‘+gravel’ fraction (<35mm) became larger than for the ‘−gravel’ fraction (<2mm) under variably-saturated conditions for a given soil–air content (ε), likely due to enhanced gas diffusion and advection through less tortuous, large-pore networks. The effect of dry bulk density on Dp and ka was most pronounced for the ‘+gravel’ fraction. Normalized ratios were introduced for all soil–gas parameters: (i) for gas diffusivity Dp /Df , the ratio of measured Dp to Dp in total porosity (f), (ii) for air permeability ka /ka ,pF4.1, the ratio of measured ka to ka at 1235kPa matric potential (=pF 4.1), and (iii) for soil–air content, the ratio of soil–air content (ε) to total porosity (f) (air saturation). Based on the normalized parameters, predictive power-law models for Dp (ε/f) and ka (ε/f) models were developed based on a single parameter (water blockage factor M for Dp and P for ka ). The water blockage factors, M and P, were found to be linearly correlated to ρb values, and the effects of dry bulk density on Dp and ka for both ‘+gravel’ and ‘−gravel’ fractions were well accounted for by the new models. [Copyright &y& Elsevier]

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 transport of gaseous compounds in soil takes place by gas diffusion, advection, and dispersion. Gas transport processes arc influenced by the soil-gas diffusion coefficient (DH), air permeability (ka) and soil-gas dispersion coefficient (DH), respectively. Of three gas transport parameters, DH is the least understood, especially how it is correlated to soil type, moisture conditions, and other transport parameters (i.e., Dp and ka). In this study, a unified measurement system (UMS) that enables sequential measurement of Dp, ka, and DH on the same soil core was developed. The experimental sequence is based on a two-chamber measurement of DH and ka, followed by a one-chamber measurement of Dp. Gaseous oxygen concentration and air pressure sensors are located in inlet and outlet cham6ers as well as at multiple points along the soil column. Using different particle-size fractions of non-aggregated (Toyoura sand) and aggregated (Nishi-Tokyo loam) soils, the effects of soil structure, particle (aggregate) size, and column scale (5-cm i.d. and 30-cm or 60-cm length) on the three gas transport parameters were investigated. For both soils, DH linearly increased with increasing pore-air velocity. For Toyoura sand, gas dispersivity (λ = DH/u0) decreased with increasing soil-air content, while for Nishi-Tokyo loam, gas dispersiviry decreased with increasingsoil-air content to a minimum value when inter-aggregate pores were drained and increased again when the pores inside the soil aggregates started to act as tortuous air-filled pathways. In the arterial pore region (corresponding to the total pore volume for Narita sand and the inter-aggregate pore volume for Nishi-Tokyo loam), a linear relation between tortuosity of the air-filled pore network (T, calculated from Dp) and the gas dispersivity λ was observed. [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]