Evaluation of a Field Method for Determining the Gas Diffusion Coefficient in Soils

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Abstract
An in situ method, based on spherical diffusion of gas from an injection point, was evaluated for determining the diffusion coefficient of N2O in field soils. It was found that the analytical solution for spherical diffusion used previously with this method assumes unrealistic initial conditions to calculate concentration of gas at the injection point with time. A finite‐difference model was developed that incorporated initial conditions based on experimental data. Concentrations predicted by the model matched measured concentrations of N2O more closely than concentrations predicted by the analytical solution as determined by the mean absolute difference between predicted and measured concentrations. Mean absolute differences using the numerical model avg 2.4% as compared to 5.2% using the analytical model. Diffusion coefficients determined with the method were reproducible and compared well with those determined by a common laboratory core technique using sand and undisturbed soil cores. The in situ method is a rapid, nondistructive procedure for the determination of gas diffusion coefficients. The method gives reproducible results and is especially useful when many data are needed to determine the spatial or temporal variability of diffusion coefficients in the field.