Evaluation of Residence Time for Rock Gas-Atmospheric Exchange for a Fractured Tuff at Yucca Mountain, Nevada
Abstract
Rock gas in the TCw (Tiva Canyon member of the Paintbrush Tuff), the uppermost fractured tuff unit at Yucca Mountain, is subject to substantial advective transport, arising from a combination of barometric pumping, thermosyphon effects, and wind pumping. The exchange of rock gas with the atmosphere induced by these mechanisms is of interest for its potential to remove water vapor from the unit and thus reduce infiltration to underlying rocks, which are proposed as a nuclear waste repository horizon. Measurements of gas-phase CFC (chlorofluorocarbon) concentrations and 14CO2 activities provide a means for evaluating such exchange. Box models may be invoked to determine the residence time for each gas species, based on the assumptions of complete gas-phase mixing and of instantaneous equilibration of the somewhat soluble gases with residual rock moisture. Residence times for selected gas species pairs may be used with their known or estimated Kw's (gas-liquid partitioning coefficients) to determine both the residence time for air and for the effective water saturation of the fractured porous medium. For the analyses, residence times for the CFCs CCl3F, CCl2F2, and C2Cl3F3 and for 14CO2 were determined using box models and historical concentrations in the atmosphere. Kw's for the CFCs were computed from published equations, and Kw for 14CO2 was estimated from an assumed dissolved inorganic content of the pore water. Computed residence times, which include effects of differing Kw's for the different species, were 2.6 to 4 years for the CFCs and about 70 years for 14CO2. Based on analyses for various species pairs, the residence time of air in the TCw ranged from 1.6 to 3.0 years, with a mean value of 2.5 years. This residence time allows for a vapor loss of only about 0.02 mm/yr, which would have little effect on infiltration through the unit. Estimates of the effective liquid saturation ranged from 0.3 to 0.8, with a mean value of 0.55, which is only somewhat less than the water saturation (typically about 0.8, but ranging from 0.3 to 0.9) determined for the porous rock matrix from cores. These analyses indicate that a substantial fraction of water in the rock matrix, as well as that in the fractures, participates in gas-water exchange, even during relatively rapid advective gas transport.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.H52D..10W
- Keywords:
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- 1875 Unsaturated zone;
- 1899 General or miscellaneous