A Modified Method for the In Situ Measurement of Soil Gas Diffusivity

Abstract

Gas diffusion coefficients are of importance for the calculation of gas fluxes in soils. Since many aspects of soil gas fluxes are affected by scale variability of soil properties, diffusion coefficients should be measured in situ and preferably for a large volume. In the present study, we modified the method of McIntyre and Philip. Gas diffusion through a laterally confined soil volume from a gas reservoir at the bottom to an accumulating chamber on the top was measured directly in the field. Thus, a large and well-defined soil volume was controlled. Apparent gas diffusion coefficients were calculated from the time course of the gas concentrations by inverse modeling. The method was tested on a sand soil and a loam soil. Since retardation due to soil water was considered, measurements were not restricted to inert gases like sulfurhexafluoride (SF6) but were also possible for water soluble gases like N2O. The results were in good agreement with comparison measurements with the McIntyre and Philip Method and a commonly used laboratory method (soil cores attached to a diffusion chamber). By controlling the lower boundary of the measured soil volume, the presented measurement setup overcomes the temporal and spatial restrictions inherent in the McIntyre and Philip Method and enables the measurement of a larger soil volume.