HR: 0800h
AN: H21B-1348    [Abstracts]
TI: Relative Abundances of Calcite and Silica in Fracture Coatings as a Possible Indicator of Evaporation in a Thick Unsaturated Zone, Yucca Mountain, Nevada
AU: * Marshall, B D
EM: bdmarsha@usgs.gov
AF: US Geological Survey, Box 25046, MS 963 Denver Federal Center, Denver, CO 80225-0046 United States
AU: Moscati, R J
EM: rmoscati@usgs.gov
AF: US Geological Survey, Box 25046, MS 963 Denver Federal Center, Denver, CO 80225-0046 United States
AB: Yucca Mountain, a ridge of shallowly dipping, Miocene-age volcanic rocks in southwest Nevada, is the proposed site for a nuclear waste repository to be constructed in the 500- to 700-m-thick unsaturated zone (UZ). At the proposed repository, the 300-m-thick Topopah Spring Tuff welded unit (TSw) is overlain by approximately 30 m of nonwelded tuffs (PTn); the Tiva Canyon Tuff welded unit (TCw) overlies the PTn with a range in thickness from 0 to approximately 130 m at the site. The amount of water percolation through the UZ is low and difficult to measure directly, but local seepage into mined tunnels has been observed in the TCw. Past water seepage in the welded tuffs is recorded by widespread, thin (0.3 cm) coatings of calcite and silica on fracture surfaces and within cavities. Abundances of calcite and silica in the coatings were determined by X-ray microfluorescence mapping and subsequent multispectral image analysis of over 200 samples. The images were classified into constituent phases including opal-chalcedony-quartz (secondary silica) and calcite. In the TCw samples, the median calcite/silica ratio is 8; in the TSw samples within 35 m below the PTn, median calcite/silica falls to 2, perhaps reflecting an increase in soluble silica from the presence of glass in the nonwelded tuffs. In the deeper parts of the TSw, median calcite/silica reaches 100 and many samples contain no detectable secondary silica phase. Evaporation and changing pCO₂ control precipitation of calcite from water percolating downward in the UZ, but precipitation of opal requires only evaporation. Calcite/silica ratios, therefore, can constrain the relative importance of evaporation in the UZ. Although calcite/silica values scatter widely within the TSw, reflecting the spatial variability of gas and water flow, average calcite/silica ratios increase with stratigraphic depth, indicating less evaporation at the deeper levels of the UZ. Coupled with the much smaller calcite/silica ratios observed in coatings from the TCw, these data indicate that evaporation decreases with depth in the UZ. Evaporation at the repository horizon and in the overlying units is an important process that reduces the amount of seepage at the repository horizon.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1094 Instruments and techniques
DE: 1838 Infiltration
DE: 1875 Vadose zone
DE: 3694 Instruments and techniques
SC: Hydrology [H]
MN: Fall Meeting 2005