Evaluation of an in situ method for measurement of gas diffusivity in surface soils

Diffusion of gases across the soil/atmosphere interface is a major mechanism in water evaporation, soil aeration, and volatilization of N species, pesticides, and volatile organic compounds. Rapid methods to directly measure the soil-gas diffusivity of surface soils in the field are needed. An in situ method for measuring the soil-gas diffusivity was evaluated both theoretically and experimentally in order to provide guidelines for using the method with minimal experimental error. The method involves insertion of a cylinder into the surface of field soil and supplying a tracer gas to the confined soil core from a well-stirred reservoir of finite volume placed over the inserted cylinder. An analytic solution for unsteady diffusion of a gas in one dimension in semiinfinite porous media is used to determine the soil-gas diffusivity. In order to establish an estimate of the amount of time for which a diffusion experiment may be conducted without resulting in error due to finite soil core length, calculations of gas reservoir concentration were made for two extreme cases of the lower soil boundary condition. These calculations indicate that tests using a 10-cm soil cylinder can be conducted for > 50 min for soils with soil-air contents (epsilon) < 0.3 cm3 cm-3. For dry soil (epsilon > 0.3), soil cylinder depths of 15 or 20 cm should be used to extend the sampling time. Diffusivities determined from the in situ method are in reasonable agreement with cores removed from the field and analyzed in the lab using an independent method. The method should be used for times greater than approximately 15 min in order to minimize errors associated with experimental measurements and perturbations associated with initiating the initial gas reservoir concentrations. The method offers a relatively rapid approach for measuring the diffusivity of surface soils in situ as long as the initial concentration of the tracer gas is accurately estimated and the assumption of uniform soil-air content within the soil depth of interest is not greatly violated.