In situ measurement of gas diffusion characteristics in unsaturated porous media by means of tracer experiments
A gas-diffusion tracer experiment was conducted at the Chem-Nuclear, Inc., nuclear waste burial site near Barnwell, South Carolina, on June 1-10, 1981, testing a new methodology to measure the in situ gaseous diffusion characteristics of unsaturated porous media for the purpose of estimating the diffusive flux of volatile contaminants from the burial ground. The tracers used were CClBrF/sub 2/ and SF/sub 6/. They were released in the subsurface from permeation devices that closely approximate an ideal point-diffusion source. The permeation devices contain the tracer in liquid form and allow the tracer to escape at a constant rate by diffusion through a Teflon membrane. The release rates for CClBrF/sub 2/ and SF6 during the test were 105 and 3.3 nanograms/second, respectively. These compounds were selected on the basis of their compatibility with the permeation-release device, their absence in the subsurface, and detectability in the part-per-trillion range in soil gas. The diffusion sources were placed in the unsaturated soil at depths of 2 meters and 13 meters below land surface. Diffusive movements of tracer were monitored for a period of 7 days and tracer breakthrough was observed at points up to 3.5 meters away. Diffusion was modeled using a three-dimensional, continuous point source, transient-state, analytical model which allowed estimation of the effective diffusion coefficient of the porous media, and an independent assessment of the media's sorptive effects on the tracer gas. Field values obtained for effective diffusion coefficient ranged from 0.026 to 0.037 cm/sup 2//sec. The average tortuosity factor observed for test site was 0.705.
- OSTI ID:
- 6715343
- Resource Relation:
- Other Information: Thesis (Ph. D.)
- Country of Publication:
- United States
- Language:
- English
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