Ground Water at Yucca Mountain How High Can It Rise? (1992) / Chapter Skim
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Summary of Conclusions and Recommendations
Summary of Conclusions and Recommendations
Pages 128-144
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From page 128... ...
dealt with the potential for ground water to rise more than 200 meters to the level of the proposed repository below Yucca Mountain: "Has it happened? and Can it happen?
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From page 129... ...
The panel took two approaches in its examination of the potential for flooding of the proposed repository at Yucca Mountain by a rise in the grouncl-water level for a prolonged period. These were (1)
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From page 130... ...
discountecl hydrothermal systems as a potential mechanism for raising the water table level in the Yucca Mountain area. HAS IT HAPPENED2 Field Evidence The panel spent several days in the fielc} on separate occasions with proponents of the hypothesis that upwelling ground water caused the surface-parallel soil carbonates and the calcite-silica vein deposits in Trench 14 and elsewhere in the Yucca Mountain area, and with others claiming a surface soil-clev~elopment, or pedogenic, origin for those deposits.
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From page 131... ...
The hypothesized cyclic upwelling of ground water along faults demonstrably does not account for their presence at Busted Butte. Nlodern and Paleo-Springs The abundant carbonate veins in the Yucca Mountain area, if they are the conclu~ts of ascending subsurface waters cluring specific events of the past, should be capped by abundant tufa, or travertine, mouncls.
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From page 132... ...
In contrast, the veins in Trench 14 and other exposures in and around Yucca Mountain show no mouncls, are composed of extremely fine-grainecl calcite (less than 6 microns, or one thousandth the size of the Travertine Point calcites) , have no symmetry, and contain thin interlayered bands of low-temperature, amorphous silica, and other materials, such as clay and volcanic ash, commonly found in desert soils.
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From page 133... ...
ground water of the isotopic composition measured for the Tertiary/Quaternary aquifer beDeath Yucca Mountain. Available data suggest that the isotopic content of ground waters has not changed greatly over the past 300 ka.
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From page 134... ...
Recommendations Further efforts should refocus away from the descendingJascending water controversy. Studies should concentrate on improving the knowledge of the ground water history of the Yucca Mountain area, to ascertain Me validity of the widely held view that me isotopic composition of ground water of an area does not change much with time and ~at, therefore, differences in isotopic composition of present day ground water and carbonates at the surface are not the result of differences in the isotopic compositions of present and past ground waters.
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From page 135... ...
CAN IT HAPPENS Water Table Response to an Increase in Rainfall In considering the various mechanisms that could conceivably cause a significant rise in the water table in the Yucca Mountain area, the pane} identified a change from an arid to a pluvial (wet) climate and the consequent increased recharge of the saturated zone as a possible scenario.
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From page 136... ...
Concurrent and subsequent basaltic volcanism, related to the change to extensional tectonics that producecl the Basin and Range structure, has experienced a progressive decline in volume, as expressed in the low-volume volcanic eruptions of Crater Flat which bounds Yucca Mountain on the west, and the latest and lowest in volume, Lathrop Wells cone, a short distance to the south. Thus the geologic record of waning basaltic volcanism indicates that the only likely style of intrusion into the Yucca Mountain area during the lifetime of the repository is a low-volume basaltic dike.
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From page 137... ...
Water Table Response to Earthquakes To evaluate the effects of earthquake-induced changes in crustal stresses on the water table, the panel looked at some of the better known, more recent historic earthquake records and at some model calculations, its own and those of others. The panel was unsuccessful in its attempts to obtain information on water level responses to measured earth strains due to underground nuclear explosions on
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From page 138... ...
, and the low probability of occurrence of a large ear~qualce close to Yucca Mountain, the panel finds no reason that site characterization of the area should not proceed as planned. The panel supports continued site characterization efforts to obtain the critical information necessary for more definitive assessments of the future behavior of the natural systems in the Yucca Mountain region.
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From page 139... ...
In addition, knowledge of the Paleozoic carbonate properties is essential for the general characterization of the flow regime needled to assess effects on increased recharge as described earlier. ADDITIONAL ISSUES OF CONCERN Steep Hydrologic Gradient Few data are available to constrain the complex hydrologic system acting in the vicinity of the proposed repository in the unsaturated zone.
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From page 140... ...
The panel regards the general approach to acquiring the data needled for characterization of the Yucca Mountain regional flow system as given in the Study Plan 8.3.1.2.1.3 to be sound. Continued review of available data, coupled with the judicious use of preliminary modeling results, provides a useful framework for guiding
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From page 141... ...
Adequate site characterization for Yucca Mountain will demand an un~lerstanding of vertical, as well as lateral, fluxes of ground water and so will require new modeling delineating the flow system in three dimensions, considering the carbonate aquifer, the volcanic aquifers and the unsaturated zone. To address the hydrologic information needs, the panel recommends new and additional drill hole data.
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From page 142... ...
Moreover, an integratecl program guided by a strong scientific coordinator would probably have identified the steep hydrologic gradient early on as a major project-wide concern and would have approached it from a multidisciplinary point of view e in the panelrs view, the anticipated higher-order systems integration efforts would be more effective if the complex solid earth and hydrological sciences studies for site characterization were coordinatec] and integrated first.
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From page 143... ...
The detailed study plans apparently leave little room for possible changes in direction of a study. Such an inflexible approach inhibits scientific progress in achieving the objectives of the studies.
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