2

Water Levels in the Vicinity of the Proposed Repository in the Last 100,000 Years

A brief summary of the geologic history and setting will serve to put into context the panel's field observations and evaluation of the geologic and isotopic evidence used to support the hypothesis of cyclic upwelling of ground water in the Yucca Mountain region. Moreover, models assessing future behavior of the earth systems in the area, which are considered later in this report, require realistic assumptions if the results are to be credible. Therefore, consideration of the geologic setting is key to evaluating such models.

GEOLOGIC SETTING OF YUCCA MOUNTAIN AND ENVIRONS

Yucca Mountain is situated in the southwestern part of the Great Basin physiographic province. The Great Basin is coincident with the northern part of the Basin and Range physiographic province ( Figure 2.1), which is characterized by numerous north-trending mountain ranges and intervening broad, flat valleys, or basins, spaced about 20-30 km apart. For the area of interest for this report, from west to east, Bare Mountain, Crater Flat, Yucca Mountain, and Jackass Flats constitute two such pairs of basins and ranges (Figure 2.2).

Yucca Mountain is located geologically within the Cordilleran mountain belt, an elongate region of active deformation on the western margin of the North American tectonic plate. While this tectonic belt is currently active, it has a long and complex geologic history (Figure 2.3). The modern surface appearance, or physiography, of the Cordilleran belt is mainly the result of geologic activity over approximately the last 10 million years, but the rocks record a geologic history over the last 2 billion years. Crustal deformation that defines the Cordilleran



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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? 2 Water Levels in the Vicinity of the Proposed Repository in the Last 100,000 Years A brief summary of the geologic history and setting will serve to put into context the panel's field observations and evaluation of the geologic and isotopic evidence used to support the hypothesis of cyclic upwelling of ground water in the Yucca Mountain region. Moreover, models assessing future behavior of the earth systems in the area, which are considered later in this report, require realistic assumptions if the results are to be credible. Therefore, consideration of the geologic setting is key to evaluating such models. GEOLOGIC SETTING OF YUCCA MOUNTAIN AND ENVIRONS Yucca Mountain is situated in the southwestern part of the Great Basin physiographic province. The Great Basin is coincident with the northern part of the Basin and Range physiographic province ( Figure 2.1), which is characterized by numerous north-trending mountain ranges and intervening broad, flat valleys, or basins, spaced about 20-30 km apart. For the area of interest for this report, from west to east, Bare Mountain, Crater Flat, Yucca Mountain, and Jackass Flats constitute two such pairs of basins and ranges (Figure 2.2). Yucca Mountain is located geologically within the Cordilleran mountain belt, an elongate region of active deformation on the western margin of the North American tectonic plate. While this tectonic belt is currently active, it has a long and complex geologic history (Figure 2.3). The modern surface appearance, or physiography, of the Cordilleran belt is mainly the result of geologic activity over approximately the last 10 million years, but the rocks record a geologic history over the last 2 billion years. Crustal deformation that defines the Cordilleran

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 2.1 Digital shaded relief map of the Cordilleran mountain belt at the latitude of California, showing positions of Yucca Mountain, the Great Basin, and the Basin and Range Province. The northern Great Basin portion of the province drains internally, while the southern portion (south of dashed line) drains into the Gulf of California.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 2.2 Map showing major geologic features of the Yucca Mountain area. Unpatterned areas are alluvium in modern valleys. Major active faults include the Northern Death Valley Fault zone (NDFZ) and Bare Mountain fault (BMF). Detachment faults at Tucki Mountain, in the Funeral-Grapevine Mountains, and Bullfrog Hills, and at Bare Mountain accommodated rapid, largemagnitude crustal extension over the last 15 million years, but are now mainly inactive. The Timber Mountain-Oasis Valley Caldera Complex (outlined by T's) represents the volcanic source region for tuffs that comprise most of Yucca Mountain. Furnace Creek and Ash Meadows are major hydrologic discharge areas in the southern Great Basin. NTS and associated dashed lines refer to the Nevada Test Site boundary. belt proper dates back approximately 600 million years. The geologic evolution of the region that includes Yucca Mountain is divisible into three main episodes, each corresponding to the three major geologic eras of the past 600 million years, the Paleozoic (570-

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 2.3 Map showing distribution of major tectonic features of the Cordilleran mountain belt. Yucca Mountain lies between an area of thin Paleozoic sediment and minor Mesozoic deformation to the east (craton) and a zone of intense igneous intrusion to the west (Mesozoic batholith belt). The intervening area (Cordilleran (PLZ) miogeocline) is characterized by a thick wedge or prism of Paleozoic sediments deposited mainly in a shallow marine sea (Figure 2.4) that was strongly shortened in Mesozoic time (Figure 2.5). The east limit of strong shortening of the miogeocline (Sevier front) coincides with the edge of thick Paleozoic sediment. Dashed contours show thicknesses of the lower, predominantly sandstone and mudstone, portion of the miogeocline. After Wernicke et al. (1988).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? 245 million years ago (Ma)), Mesozoic (245-65 Ma), and Cenozoic (65 Ma to the present) eras. Paleozoic The Paleozoic was a time of major crustal down-warping along the continental shelf and margin, which formed the miogeocline (Figure 2.4), and of sediment deposition on top of Precambrian (prePaleozoic) crystalline crust (formed ca. 1.7 billion years ago). In southern Nevada, shallow-marine deposits of the miogeocline, originally of sand, clay mud, and calcium carbonate (lime) precipitates, thicken northwestward from about 1 km thick in the Las Vegas area to more than 10 km thick in the Yucca Mountain region (Figure 2.4; Wright et al., 1981). The sand and clay muds of this wedge-shaped sedimentary deposit, called a clastic wedge or prism, are now predominantly quartzite, slate, and other impermeable rocks in its lower part. These rocks now constitute an aquitard, a rock unit that does not readily permit water to pass through. The carbonate, now limestone, that lies above it is an aquifer through which ground water flows easily in fractures and solution channels. Strong Mesozoic and Cenozoic tectonic events deformed these rocks and destroyed the simple wedge-shaped geometry and continuity of the original deposits. The thick Paleozoic limestone that once covered the region as a continuous layer is now a widespread but discontinuous deep regional aquifer through which large volumes of water flow in the southern Great Basin. Mesozoic Beginning approximately 250 Ma, the sedimentary prism was horizontally compressed into a major system of folds and west-dipping thrust faults (Figure 2.5). The total contraction of the prism was probably more than 100 km between the Las Vegas and Sierra Nevada regions (Wernicke et al., 1988). Yucca Mountain is situated in the central part of the zone of Mesozoic thrusting and crustal shortening. In the Las Vegas region, major compression persisted until approximately 90 Ma. In the westernmost part of the sedimentary wedge, molten rock of high silica, or granitic, composition was forced up into the crust, crystallizing several granitic bodies into a larger granite mass called the Sierra Nevada batholith (Figure 2.3). The Yucca Mountain area lies mainly to the east of this area of Mesozoic igneous activity. Both the shortening and igneous activity are believed to be related to subduction, or downthrusting, of the ancient Pacific plate beneath western North America (Hamilton, 1969).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 2.4 Cross-section of the miogeoclinal prism as it existed prior to Mesozoic thrust faulting. The lower sandy and muddy sediments (clastics) thicken dramatically westward. Vertical exaggeration is approximately 3:1. After Wernicke et al. (1988).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 2.5 Cross-section of southern Great Basin region as it existed following Mesozoic shortening but before Cenozoic extension, showing geologic details of miogeoclinal sediments with respect to major Mesozoic thrust faults. YM shows approximate position of Paleozoic strata in the Yucca Mountain area. After Wernicke et al. (1988).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Cenozoic A major quiet period lasting 60 million years occurred from late Mesozoic to middle Cenozoic time, from 90-30 Ma. There is no record of sedimentary deposition, or of igneous or tectonic activity during this time. Only relatively minor erosion of the thickened crust occurred. Following this hiatus, a major deformational and igneous event affected the region, and continues to the present. Although subduction of Pacific ocean crust continued into Cenozoic time and was active at the onset of renewed deformation and volcanism following the hiatus in tectonic activity, the deformation beginning about 30 Ma involved extension, or pulling apart the crust, instead of contraction. The molten rock produced by the renewed igneous activity emerged onto the earth's surface mainly as volcanoes and lava flows. This type of activity was in contrast to the deep granitic bodies that intruded the crust during the Mesozoic. Millions of years after they formed in the crust, uplift and erosion of the rocks that covered them exposed the massive granite bodies at the earth's surface, which are now called the Sierra Nevada Mountains. Extension Extension of the crust occurred mainly along west-dipping normal faults (roughly parallel to the older thrust faults) and associated northeast-and northwest-trending strike-slip faults (Figure 2.6). Two major north-trending belts of extension and normal faulting developed, one largely in the Las Vegas/Lake Mead area to the east (Lake Mead extensional fault system), and another within the shortened sedimentary wedge (Death Valley extensional fault system), which includes the Yucca Mountain area. The entire Basin and Range at the latitude of Las Vegas has extended about 200-300 km across the province in two phases (Wernicke et al., 1988). Although evidence of extensional activity and related development of basins and ranges dates back to approximately 30 Ma in the southern Great Basin (Axen et al., 1991), the majority of extensional strain and volcanism in the Yucca Mountain area appears to have occurred in the last 10-20 million years (Scott, 1990; Wernicke et al., 1988). Extension appears to have been most rapid from 15-5 Ma, when the rate is thought to have been 10-30 mm per year. Extension has resulted in local severe thinning of the sedimentary crust, completely removing it in some areas. As a result the thickness of the Paleozoic limestone beneath Yucca Mountain is uncertain, but was likely very much thinned or pulled apart by extension in mid-Miocene time (15-10 Ma). Extension in the southern

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 2.6 Map showing major Cenozoic strike-slip and normal faults in the Basin and Range province at the latitude of Yucca Mountain. OVF, Owens Valley fault; NFZ, SDF, northern and southern branches of the Death Valley fault zone; BMF, Bare Mountain fault zone; LVVSZ, Las Vegas Valley shear zone; LMFS, Lake Mead fault system. Two strongly extended areas of the Basin and Range, the Lake Mead and Las Vegas normal fault systems, lie east and west respectively of a medial unextended block that includes the Spring Mountains. Active strong extension is restricted to region west of Death Valley. After Wernicke et al. (1989).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Great Basin region (Figure 2.1) slowed down after 5 Ma to its present rate of approximately 5-10 mm per year. The waning of major extensional activity migrated westward across the Death Valley region, beginning 10-15 Ma in the eastern part of the Basin and Range, but extension persists to the present day in the western part (Wright et al., 1984; Hamilton, 1988). Volcanic Activity Between 14 and 11.3 Ma, the Yucca Mountain area experienced large-volume, explosive volcanism resulting in extensive fallout of volcanic ash. The accumulation of massive layers of this ash resulted in the formation of rocks known as ash-flow tuffs (Frizzel and Shulters, 1990). Yucca Mountain itself is composed mainly of such tuffs, including the Crater Flat, Paintbrush, and Timber Mountain tuffs. In some places where there were thick accumulations of ash composed mainly of volcanic glass shards (fragments), the shards became welded together under the combined action of the heat retained by the particles, the weight of overlying material, and hot gases. Welded zones thus formed within ash-flow tuffs are very dense and, for the most part, impermeable, but they are also brittle and have responded to stresses by forming many fractures through which water may flow. The proposed repository at Yucca Mountain is designed to be excavated in a welded tuff, the Topopah Spring Member of the Paintbrush Tuff (Figure 2.7). The sources of the tuffs are volcanic centers of silica-rich rocks distributed throughout the region immediately to the north of Yucca Mountain (Figure 2.2). Nearby volcanic eruptions associated with these centers in the later stages of silicic activity extruded low silica/high iron lavas which cooled to form basaltic rocks (e.g., on Skull Mountain south of Jackass Flats). It is important to note here that the high silica volcanic activity ended about 11.3 Ma, and that the low silica basalt volcanism occurred in two phases, about 10 Ma and between 3.7 and 0.13 Ma. Further discussion of the volcanic history of the Yucca Mountain vicinity appears in Chapter 4 of this report. The age relationship between major extension and volcanism in the region is complex. While broadly synchronous, major extension largely preceded volcanic activity in some parts of the southern Great Basin and followed it in others. At Yucca Mountain, an episode of rapid extension likely occurred prior to the accumulation of the Paintbrush and Timber Mountain Tuffs, extending the previously shortened Paleozoic sedimentary wedge tenfold or more. Subsequently, the tuffs were mildly extended, resulting in a series of gently tilted domino-like

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 2.7 East-west cross-section through Yucca Mountain showing the tuff units and location of the proposed MGDS relative to the water table (dotted line). (From DOE Site Characterization Plan Overview, 1988).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 2.8 Plot of hydrogen vs. oxygen isotopic concentrations of Yucca Mountain ground waters, springs, and precipitation on the Nevada Test Site. The global meteoric water line is labeled “MWL”; the local meteoric water line is “LMWL”. The ground-water samples are from the Ash Meadows and regional aquifers of the Alkali Flat/Furnace Creek subdivision of the Death Valley ground water system in which Yucca Mountain is located. Cane Spring flows from a perched water table beneath the east end of Skull Mountain, 30 km east of Yucca Mountain. Whiterock Spring is 45 km northeast of Yucca Mountain. Data and additional discussion in Claasen (1985, 1986); Benson and Klieforth (1989); Benson and McKinley (1985); Benson et al. (1983); Ingraham et al. (1990); and Lyles et al. (1990). Analytical uncertainty is ±0.2‰ for δ18O and ±2.5‰ for δD. SMOW is Standard Mean Ocean Water, the standard by which oxygen and hydrogen isotopic concentrations are measured. and Pahute Mesa, among the recharge areas for the Ash Meadows– Alkali Flat/Furnace Creek part of the Death Valley ground-water system, in which Yucca Mountain is located. The low temperatures in the high mountains influence the isotopic content of the ground water. The isotopic composition of the waters from Whiterock Spring, located 45 km NE of Yucca Mountain, indicates a lower elevation than the source of ground water but a higher elevation than Cane Spring. Waters from Cane Spring show a shift in δ18OVSMOW towards

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? higher concentrations, indicating local evaporation. Samples with the highest D and 18O abundances are local precipitation (especially summer samples). They reflect the low elevation and a higher average temperature than the waters of the other sources. A significant graphical plot that brings together several categories of data (Figure 2.9) illustrates the relationship between carbon and oxygen isotopic abundances among calcite of local soils of Yucca Mountain, calcite of Trench 14 and Busted Butte, ground waters at Yucca Mountain, and calcite that would precipitate in isotopic equilibrium from these ground waters. The darker triangles represent the 13C and 18O abundances of the ground water in the Tertiary/ Quaternary tuff aquifer of the Alkali Flat/Furnace Creek subsystem. Note that the 18O abundance shows little variation, but the 13C abundances vary over a range of 10‰. This is because 18O content depends mainly on elevation/temperatures of precipitation in the recharge area while the amount of 13C of ground water depends on the extent of exchange with atmospheric CO2, types of vegetation, soil pH, oxygen pressure, and other localized effects. In Figure 2.9, lighter triangles represent calcites that would precipitate in carbon and oxygen isotopic equilibrium from Yucca Mountain ground water (darker triangles) assuming 25°C. Next, the measured 13C and 18O abundances of the calcites of Trench 14 and Busted Butte are plotted (gray circles). They clearly do not plot as calcites that precipitated from the illustrated ground water. Finally, the isotopic composition of calcite found in the local soils, plotted as diamonds, is distinct from the ground water and predicted calcite composition, but strongly overlaps the Trench 14 and Busted Butte calcite compositions. Note that if a more realistic temperature for ground water is chosen (e.g., 33°C), the δ18OVSMOW content predicted for calcite shifts approximately 1.0‰ to lower abundances, thus increasing the discrepancy between measured and predicted calcite compositions. The panel concludes, therefore, that the calcites of Trench 14 and Busted Butte formed from the same waters and by the same surface processes as the soil carbonates, and therefore are pedogenic in origin. Tracer Isotopes: Sr, U, and Th Analyses of the abundances of the radiogenic isotopes of strontium, uranium, and thorium of the carbonates of Trench 14 and other Yucca Mountain localities (see Appendix A) provide similar results, i.e. the ratios or amounts of these isotopes in the calcites indicate that

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? the calcites could not have precipitated from ascending ground waters of the Yucca Mountain region. Moreover, they show pronounced similarities to the Sr, U, and Th abundances in soil-carbonate compositions. Subsequent to the panel's detailed analyses of stable and radiogenic isotopes, the panel received some new isotope analyses from the project scientists. The data consisted of analyzed calcites at various depths from drill cores of the area. The elevations of the samples ranged from 600 m above the water table to 1,200 m below. The samples analyzed were secondary calcite from cavities and cements in the tuffs and from vein fillings in fractures. Figure 2.9 Plot of δ13CPDB vs δ18OSMOW for ground waters of Alkali Flat/ Furnace Creek flow systems, and Yucca Mountain vein calcites and soil calcites. Yucca Mountain calcite data from Whelan and Stuckless (1990), Quade and Cerling (1990), and Quade et al. (1989). The values of calcite in isotopic equilibrium with analyzed ground-waters (equilib. cc) were calculated for 25 °C with 1000 ℓn α (18O/16O) for CaCO3 − H2O = 28.5 and (13C/12C) for HCO3- − CaCO3 = −2.2 (Friedman and O'Neil, 1977). Ground-water analyses from Benson and McKinley (1985). PDB refers to the standard by which the carbon isotopic content of carbonates and water is measured.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? The results showed a consistent relationship between the depth of the calcite and the content of 18O and 13C, with δ18O decreasing and δ13C increasing with greater depth. Preliminary analyses of strontium isotopic ratios showed a clear distinction in strontium isotopic composition between calcites below the water table and those above. The calcites above the water table showed a narrow range of 87Sr/ 86Sr composition (0.7106 to 0.7128) that overlaps the range of soil calcite and the calcite veins of Trench 14. Samples from 600 m to 1,200 m below the water table had Sr values similar to the low end of the range for waters from the Tertiary/Quaternary aquifers of the Alkali Flat/Furnace Creek ground-water subsystem. These results lead the panel to conclude that the water table remained relatively stable over the period of time during which and since the secondary calcite was deposited. It was reported that some inconsistent results were obtained in the 13C abundances from core samples above the water table; they were in the range usually found at depths below the water table. The significance of these “outliers” is not presently clear, as nearby core samples from above the water table do not indicate a rise in the water table. The small amount of dissolved carbon in ground water and the possibility of isotopic exchange between dissolved carbon and local wall rock make variations subject to a wide range of possible causes. Moreover, the ages of the calcites have not been determined. The calcites with inconsistent 13C content may have formed as a result of hydrothermal activity that occurred millions of years ago, soon after deposition of the volcanic rock as was seen at Harper Valley discussed earlier in this chapter. CONCLUSIONS Taking into account the expert testimony at its meetings, published information, and what was observed on the field trips and analyzed independently by panel members, the panel found no compelling evidence for the widespread discharge of deep ground water in the vicinity of Yucca Mountain that would have resulted if the regional water table had been elevated to a height sufficient to cause flow of water along the fault exposed in Trench 14 at an elevation of about 1,150 m AMSL (above mean sea level). For comparison, the estimated elevation of the present water table beneath Trench 14 is about 730m, and the design elevation of the floor of the proposed MGDS ranges from about 950 to 1,150 m (Dudley, pers. comm., 1991). Indeed, in the panel's opinion, the morphologic, textural, mineralogic, and isotopic evidence is strong that the fault-filling

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? materials formed by evaporation of descending meteoric water that only partly filled near-surface open fractures in the unsaturated zone. Surficial calcite deposits consistently show a correlation between thicker surface-parallel calcretes occurring in better developed soil horizons and progressively older ground surfaces. The widespread occurrence of root casts within the calcretes associated with Bk or K soil horizons confirms that the calcium-rich waters concentrated in the soil zone rather than on the surface. This is characteristic of infiltrating rain water, not of ascending pressurized thermal waters. These observations lead the panel to conclude that surface-parallel calcretes in soils originated from meteoric water by surficial soil-forming process rather than from upwelling ground water. In the panel's view, those breccias that appear to have formed in response to hydrothermal processes in the vicinity of Yucca Mountain and Busted Butte originated at the time of emplacement of the ash-flow tuff sequence from 13-10 Ma. They formed as a result of rapid heating of locally derived ground and rain waters. The source of energy appears to have been the heat initially contained in the tuffs, rather than the thermal and mechanical energy in the postulated hot water from a source many kilometers deep. Younger breccias formed by a variety of processes through time, many of which show evidence of surficial water and progressive carbonate accumulation. A variety of evidence, outlined earlier in this chapter, indicates that the water table in Devils Hole (presently 16 m below the surface) has varied less than 9 m in the past 45 ka and probably has not risen to the land surface in the past several hundred thousand years. Considering that Devils Hole is located in the same active tectonic region, and is extending at two to three times the rate of the Yucca Mountain area, the fact that earthquakes have not resulted in even a 15 m rise in the Devils Hole water table inspires serious doubt that the seismic pumping mechanism can cause a greater than 100 m rise in the water table in the Yucca Mountain area. The currently active spring at Site 199 may rise from a perched water table. The isotopic composition of Cane Spring water is significantly different from that of ground water in the Alkali Flat/ Furnace Creek and Ash Meadows aquifer, but similar to that of local rain water, modified by evaporation. The gypsum deposits at Wahmonie are ancient and probably formed well before the present erosion cycle. The panel concludes that none of the springs or spring deposits in the Yucca Mountain area provide evidence of origin from ascending deep-seated ground water. Isotopic evidence shows that none of the surficial calcite depos-

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? its analyzed to date could have precipitated from known ground waters. The analyzed deposits, including those at Trench 14 and Busted Butte that inspired the upwelling hypothesis, show isotopic affinities with local soils and pedogenic carbonates. The panel concludes that to date the preponderance of evidence supports the view that the calcretes and other secondary carbonates in veins of the area formed from meteoric water and surface processes. The evidence cited here has convinced the panel that the ground-water level at the proposed Yucca Mountain repository site has not reached or exceeded the level of the proposed repository at any time during the last 100 ka. RECOMMENDATIONS The panel has identified some studies that may be useful. Because only a small number of the site characterization study plans are available, the panel is not aware of all the studies that are planned. If, therefore, some recommendations include studies that are already planned, these recommendations may be viewed as endorsements of the project plans. The panel recommends that further efforts in the study of secondary calcite deposits be refocused. In the panel's opinion, it is well established that surface calcite deposits at Yucca Mountain, such as those at Trench 14 and Busted Butte, did not precipitate from ground waters sampled in deep wells. Evidence for the isotopic concentrations of ancient ground waters is incomplete, however. It is recommended that analyses of calcite veins intersected in drill cores be carried out for δ18OVSMOW and δ13CPDB, as well as of fluid inclusions. Age dating with U and Th isotopes would be an essential part of this study to reconstruct the ground-water history of the Yucca Mountain area. Additional Characterization of Carbonate Veins in Core The panel strongly endorses the efforts that are under way to characterize the ages and isotopic compositions of calcites in core obtained from both above and below the present water table. Along with the isotopic and age results, information should be obtained on grain sizes, chemical variations found in the carbonates (particularly Ca-Mg-Mn-Sr), and whether or not fluid inclusions are present.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Fluid Inclusion Studies Fluid inclusions provide samples of ancient waters that were present at the time of entrapment. Information on the temperature and chemistry of the water at the time of mineral crystallization may be obtained from analysis of these inclusions. A search should be made for suitable fluid inclusions in vein materials found in core to extract for hydrogen isotope analyses. The isotopic composition of the corresponding oxygen in the inclusion water can be calculated from the oxygen isotopic composition of the host mineral. The panel is aware that vein materials that contain mixtures of primary and secondary fluid inclusions that formed at different times (or that contain more than one generation of secondary fluid inclusions) are not suitable for this type of analysis unless the volume percent of one type of inclusion is much greater than that of the other. Isotopic Composition of Windblown Dust Windblown particles that have been collected over a long period of time in dust traps in the vicinity of Yucca Mountain should be analyzed to determine if the average isotopic composition of this dust is essentially the same or significantly different from that of pedogenic carbonates and rock units exposed in the region. This study also would provide information about the possible extent to which windblown dust might influence the isotopic composition of meteoric water percolating downward through the vadose zone. Additional Studies at Site 199 Detailed studies should be carried out to describe and document the geology, hydrology, and geochemistry of the apparent spring deposit at Site 199. Trenching and drilling of the tufa mound at this locality should be a high priority to determine if a perched water table is present, and if carbonate is currently depositing or has deposited in veins underground. This information will be valuable as an aid in further understanding the paleohydrology of the region. The hydrologic history of the region is essential, as understanding the past will bear on predicting future changes in the hydrologic system.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? REFERENCES Allen, E. T., and A. L. Day. 1935. Hot springs of the Yellowstone National Park. Carnegie Institution of Washington Publication 466. 525 pp. Archambeau, C. B., and Price, N. J. 1991. An Assessment of J. S. Szymanski's Conceptual Hydrotectonic Model, Minority Report of the Special DOE Review Panel. September. 144 pp. plus appendix. Arnorsson, S. 1978. Precipitation of calcite from flashed geothermal waters in Iceland Contributions to Mineralogy and Petrology 66: 21–28. Axen, G. J., W. J. Taylor, and J. M. Bartley. 1991 . Space-time patterns of the onset of extension and magmatism, southern Great Basin, Nevada, Utah, and California. Geological Society of America Abstracts with Programs. 26: (6) A190. Bargar, K. E. 1978. Geology and thermal history of Mammoth Hot Springs, Yellowstone National Park, Wyoming. U.S. Geological Survey Bulletin 1444. 55 pp. Bargar, K. E., and M. H. Beeson. 1984. Hydrothermal alteration in research drill hole Y-6, Upper Firehole River, Yellowstone National Park, Wyoming. U.S. Geological Survey Professional Paper 1054-B, pp. B1-B24. Beatley, J. C. 1976. Vascular Plants of the Nevada Test Site, and Central-Southern Nevada National Technical Information Service Report TID-26881. 308 pp. Benson, L. V., and H. Klieforth. 1989. Stable isotopes in precipitation and ground water in the Yucca Mountain region, Nevada - paleoclimatic interpretation In Aspects of Climate Variability in the Pacific and W. Americas. D. H. Peterson, ed. American Geophysical Union, Monograph 55. pp. 41-49. Benson, L. V., and P. W. McKinley. 1985. Chemical composition of ground water in the Yucca Mountain area, Nevada, 1971-84. U. S. Geological Survey Open-File Report 85-484. 10 pp. Benson, L. V., J. H. Robinson, R. K. Blankennagel, and A. E. Ogard. 1983. Chemical composition of ground-water and the locations of permeable zones in the Yucca Mountain Area, Nevada U. S. Geological Survey Open-File Report 83-484. 19 pp. Burk, C. A. 1952. The Big Horn hot springs at Thermopolis, Wyoming. In Wyoming Geological Association Guidebook Seventh Annual Field Conference, pp. 93–95. Chafetz, H. S., and R. L. Folk. 1984. Travertines: Depositional morphology and bacterially constructed constituents. Jour. Sed. Petrology 54: 289-316. Clayton, R. N., J. R. Goldsmith, K. J. Johnson, and R. C. Newton. 1972. Pressure effect on stable isotope fractionation. Abstracts of the American Geophysical Union 53: 555. Claasen, H. C. 1985. Sources and mechanisms of recharge for ground-water in the west central Amargosa Desert, Nevada: A geochemical interpretation U. S. Geological Survey, Professional Paper 712-F, pp. F1-F31. Claasen, H. C. 1986. Late-Wisconsin paleohydrology of the west-central Amargosa Desert, Nevada, USA. Chem. Geol. 58: 311-323. Dettinger, M. D. 1989. Distribution of Carbonate-Rock Aquifers in Southern Nevada and the Potential for their Development. Summary of Findings, 1985-88. U.S. Geological Survey, and Desert Research Institute, Univ. of Nevada, Program for the Study and Testing of Carbonate-Rock Aquifers in Eastern and Southern Nevada. Summary Report No. 1 Dudley, W. W. 1991. Personal Communication. U.S. Geological Survey. Ellis, A. J., and W. A. J. Mahon. 1977. Chemistry and Geothermal Systems. New York, Academic Press. 392 pp. Engebretson, D. C., A. Cox, and R. G. Gorden. 1985. Relative motions between oceanic and continental plates in the Pacific basin. Geological Society of America. Special Paper 206. 59 pp.

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