Measurement of in Situ Gas-Phase Diffusion Coefficients

ISSN： 0959-3330

Source： Environmental Technology, Vol.21, Iss.6, 2000-06, pp. : 631-640

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Abstract

Vadose zone in situ diffusion coefficients were measured at a former petrochemical plant (Chatterton research site) using a push-pull test and helium tracer. The test is relatively simple to perform, utilizes relatively inexpensive materials and field equipment, and requires only a few hours to complete each series of tests. In situ diffusion coefficients are derived using an analytical solution for spherical diffusion from a point source, assuming instantaneous injection of the tracer. The results of push-pull tests conducted at the Chatterton research site indicate that measured effective diffusion coefficients in sand fill with moisture content between 5 and 12% (by weight) ranged from about 0.01 to 0.07 cm²sec⁻¹. A good comparison was obtained between the measured gas-phase tortuosity and that predicted using a common empirical relationship, with measured tortuosity factors consistently being about twice the predicted values. While further comparisons need to be conducted for various moisture contents and different soil types, the results of this study suggest that the push-pull test is an effective tool for estimating diffusion coefficients and can be used to validate empirical relationships for diffusion coefficient.