Diffusion Barrier Transport Properties of Unsaturated Paintbrush Tuff Rubble Backfill

by James L. Conca, Pacific Northwest Lab, United States,

Document Type: Proceeding Paper

Part of: High Level Radioactive Waste Management 1990


Diffusion coefficients (D) were experimentally determined in unsaturated tuff gravel to evaluated the effectiveness of tuff gravel and rubble as a diffusion barrier to ionic transport in the vadose zone. Gravel surface volumetric water contents and the D values ranged from 1.5% to 7% and 10-9 to 10-7 cm2/sec. respectively, and depended upon gravel grain size. Internal water does not contribute to the diffusion coefficients measured in the tuff gravel which are a function only of the surface water contents. Humidity chamber experiments with both initially wet and initially dry tuff gravel show that the volumetric water content in the backfill in equilibrium with the near-field environment pore/fracture atmosphere of 100% humidity will be between 2% and 3%, but the surface will be air-dry and the diffusion coefficient will be below 10-11 cm2/sec.

Subject Headings: Surface water | Water content | Diffusion | Riprap | Backfills | Humidity | Material properties | Waste sites

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