Review of Field Methods for the Determination of the Tortuosity and Effective Gas‐Phase Diffusivity in the Vadose Zone (original) (raw)
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Gas-phase Diffusive Tracer Test for the In-Situ Measurement of Tortuosity in the Vadose Zone
Water Air and Soil Pollution, 2007
Robust measurements of porous-medium tortuosity are one of the many components needed for accurate characterization and prediction of fluid flow and contaminant transport in the subsurface. A gas-phase diffusive tracer-test method is evaluated for the in-situ measurement of tortuosity in the vadose zone. This technique presents an alternative to employing widely-used correlations to estimate tortuosity. A small-scale field study was conducted using a single well and a non-reactive gas-phase tracer (sulfur hexafluoride; SF6). Gas samples were collected from the injection point periodically after tracer injection into the soil matrix. An effective radius of influence of 50 cm was determined for the tests. An analytical solution was calibrated to the measured temporal concentration distribution, producing a fitted value for tortuosity. The value obtained from the tracer tests was compared to values estimated with several widely-used correlations. The value obtained from the tracer tests was generally larger than the values estimated with the correlations, which spanned a relatively wide range. The tracer-test method provides a means by which to determine in-situ measurements of tortuosity, allowing for better characterization of contaminant transport in the vadose zone.
In Situ Method To Measure Effective and Sorption-Affected Gas-Phase Diffusion Coefficients in Soils
Environmental Science & Technology, 2003
Transport modeling, risk assessment, and the evaluation of remediation strategies at contaminated sites require the knowledge of gas diffusivities in soil. A field method is presented, which determines the tortuosity factor and the mass fraction in the air phase of a volatile compound in situ. The compound is injected into the unsaturated zone together with a conservative gaseous tracer to form a point source. Concentrations are monitored at the injection point during 8 h and evaluated with an analytical equation for reactive transport. The air-filled porosity is determined independently. From these data, both the effective and the sorption-affected diffusion coefficients are obtained. Results are reported for volatile organic pollutants in both a lysimeter and a sandy soil. The measurements show good reproducibility. Batch experiments suggest that tracers were not truly conservative at subsurface temperatures. This may lead to a systematic underestimation of the effective diffusion coefficient by less than 10%, but the sorptionaffected diffusion coefficients were probably overestimated by 15-20%. Nevertheless, the in situ method can avoid considerable uncertainties associated with choosing appropriate empirical relationships for the tortuosity factor or deviations from natural soil conditions in laboratory experiments.
Measurement of gas diffusion through soils: comparison of laboratory methods
Journal of Environmental Monitoring, 2008
Gas movement through soils is important for ecosystems and engineering in many ways such as for microbial and plant respiration, passive methane oxidation in landfill covers and oxidation of mine residues. Diffusion is one of the most important gas movement processes and the determination of the diffusion coefficient is a crucial step in any study. Five laboratory methods used for measuring the relative gas diffusion coefficient (D s /D o ) were compared using a loamy sand, a porous media commonly found in agricultural fields and in several engineered structures, such as in landfill final covers. In the absence of macropores, all methods gave rather similar values of D s /D o . Methods allowing the study of microscale variability indicated that the presence of macropores highly influenced gas movement, thus the value of D s /D o , which, near a macropore may be one order of magnitude higher than in regions without macropores. Repacked columns do not allow the study of heterogeneity in D s /D o . Natural spatial variability in D s /D o due to water distribution and preferential pathways can only be studied in large systems, but these systems are difficult to handle. Advantages and disadvantages of each method are discussed.
Gas Transport Parameters in the Vadose Zone: Gas Diffusivity in Field and Lysimeter Soil Profiles
Vadose Zone Journal, 2006
The main soil-gas transport parameters, gas diffusivity and air permeability, and their variations with soil type and air-filled porosity play a key role in soil-gas emission problems including volatilization of toxic chemicals at polluted sites and the production and emission of greenhouse gases. Only limited information on soil-gas transport parameters across the vadose zone is available, especially for soil layers below the root zone. In a series of studies, we developed new data for the soil-gas transport parameters in different soil profiles and tested existing and new predictive models. In this first study, we measured gas diffusivity at different soil-water matric potentials on undisturbed soil samples for three lysimeter soil profiles down to 1.4-m depth and for two field soil profiles down to 5.6-m depth, representing a total of 22 different soil layers with soil texture ranging from sand to sandy clay loam. Five commonly used predictive gas diffusivity models were tested. The three-porosity model (TPM) performed best for both shallow and deep soil layers. The tortuosity-connectivity parameter X in the TPM varied with soil texture and pore size distribution, and the TPM predicted well the depth distributions of measured soil-gas diffusivities. The TPM also requires less detailed information on the soil-water characteristic curve than other well-performing predictive models, and is therefore recommended for predicting variations in soil-gas diffusivity within the vadose zone.
Laboratory Measurements and Predictive Equations for Gas Diffusion Coefficient of Unsaturated Soils
2002
Molecular diffusion is an important mechanism for gas transport through soil covers placed over municipal dumps (from which bio-gas must not escape) and above acid generating mine tailings (where oxygen availability must be controlled). Gas flux through cover systems depends on the effective diffusion coefficient De of the cover materials. In this paper, the authors describe a laboratory procedure for oxygen diffusion tests, and present results obtained on different types of porous materials at various degrees of saturation. The determination of De from experimental measurements is based on analytical and numerical solutions to the diffusion equations. The measured values of De are compared to values calculated from two predictive models. A new expression is finally proposed as a simple means of estimating De. RÉSUMÉ La diffusion moléculaire est un mécanisme important de transport de gaz à travers les couvertures en sols mises en place sur les décharges municipales (d'où le bio...
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