Estimation of Gas Permeabilities for the Maricopa Site, Arizona

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1998

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Upward and downward migration of gases from waste-disposal facilities is a critical issue for low-level radioactive waste disposal. Gaseous radionuclides in low-level waste include H-3, C-14, and Rn-222. Upward migration of gases to the surface can be important, particularly during operation of the facility (Kozak and Olague, 1994). High tritium values (for example 1,100 TU at 24 m depth, 162 TU at 109 m depth) have been found adjacent to the Beatty site, Nevada, that cannot readily be explained by liquid or combined liquid and vapor transport (Prudic and Striegl, 1995; Striegl et al., 1996). Because disposal practices at Beatty varied in the past and included disposal of as much as 2,000 m3 of liquid waste, further research in tritium movement at Beatty is warranted. Transport mechanisms for gases include not only diffusion but also advection. Analysis of gas transport is important at many low-level waste disposal facilities as shown by the intensive program to monitor concentrations and concentration gradients of gaseous radionuclides proposed for the California low-level radioactive waste disposal facility (Harding Lawson & Assoc., 1991). Performance assessment calculations require information on parameters related to gas transport to predict long-term migration of gases in the subsurface. The purpose of this study is to evaluate different techniques of estimating gas transport parameters and monitoring subsurface gas migration.

The objective of this study is to examine different techniques for evaluating gas permeability. Pneumatic pressure tests will be conducted to estimate vertical and horizontal air permeabilities at different levels. In addition, permeabilities will be calculated from atmospheric breathing data that will include evaluation of subsurface response to barometric pressure fluctuations. Computer simulations suggest that air from the surface can move several meters into the ground during typical barometric pressure cycles (Massmann and Farrier, 1992). Gas ports will be installed at different depths in two boreholes to evaluate atmospheric pumping. The results of this study will provide valuable information on subsurface gas transport processes and the various techniques to obtain data on parameters required for simulation of such processes. These data will be required for performance assessment calculations.

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