Ground Water at Yucca Mountain How High Can It Rise? (1992) / Chapter Skim
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Water Levels in the Vicinity of the Proposed Repository in the ast 100,000 Years
Water Levels in the Vicinity of the Proposed Repository in the ast 100,000 Years
Pages 15-61
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From page 15... ...
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.
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From page 17... ...
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.
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From page 18... ...
18 GROUND WATER AT YUCCA MOUTON UMIT, BASIN ED 1/ Jo RANGE PROVINCE t~ ~~ ~~, , ~ "~ : ~40°N 36°N 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)
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From page 19... ...
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.
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From page 20... ...
20 z m ~ CO _ Ul <: ~ Y ~ \ Z Z I~ w~ h}14 CD ~ Z .N ~ CO Z Z ~ O cn C) Z ~: CL cn I llJ ~o -1~1 0< Z G LL 1 cZ6 CO CC CD O '1: Z cn - 1,, J Z ~ O CO Z m ~1=, tilil~ I ~1 01 ~ 1 ~ IrllU Ct5 .— a'— ~ Ct ~ Cl ~ ~D _ cn~ ~ ~D C)
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From page 21... ...
21 kilometers from sea leve!
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From page 22... ...
area to the east (Lake Mead extensional fault system) , and another within the shortened sedimentary wedge (Death Valley extensional fault system)
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From page 24... ...
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 experiences!
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From page 25... ...
25 t.;.t ~ l ~ ~ J o ~ cc 3 A at: o , _ UJ ~ ~ :—1 c::, 1 o3 ~ c, ~ m A' =~ o ~ c, 0 my a At, _ := F= s Jo gel a: J at: z o o _ 0=F ~ < off ~ 0 z ~ a J ~ 3 ~ t g ' ~ 031~390VX3 3~1~:'S 1~311~1~/` z ~ ,., It we lo.> 0 ~ ~ w uJ <( _ J o o = w in 0 ,~ u.
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From page 26... ...
See Chapter 5 of this report for a detailed discussion of, and constraints on, active deformation in the Yucca Mountain region. EPISODIC GROUND-WATER DISCHARGE THESIS The thesis that ground water was periodically forced up from depths of hundreds of meters and discharged repeatedly on the surface of the earth near Yucca Mountain has great importance for the longterrn capability of the proposed Yucca Mountain repository to isolate radioactive waste.
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From page 27... ...
However, it depended heavily upon its own observations and evaluation of the evidence available to it, and its own and others' modeling, for its deliberations and findings relative to the origin of the carbonate deposits and the likelihood of ground water upwelling hundreds of meters. The evidence bearing on whether or not the potentiometric (water table)
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From page 28... ...
Secondary carbonate can accumulate in the soil profile by a variety of pedogenic processes, as described by Machette (1985~. These processes include in situ weathering of calcium carbonate from the parent materials and leaching of calcium carbonate from calcareous materials emplaced on the ground surface as airborne dust.
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From page 30... ...
Intermediate-aged surfaces, such as the slopes of Yucca Mountain and Busted Butte, a steep-sided isolated remnant of Tiva Canyon and Topopah Spring luffs, have calcrete horizons from a few centimeters to fractions of a meter in thickness. The youngest surfaces identified, in the localized contemporary drainages throughout the region, have weakly developed soils, characterized by thin vesicular A horizons and incipient, discontinuous thin laminae of calcrete.
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From page 31... ...
and could provide dissolved carbonate in upwelling ground water. In the Yucca Mountain region, however, considerable surface exposure of this limestone is present in the Spring Mountains, and even nearer Yucca Mountain, in Bare Mountain, on the west side of Crater Flat.
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From page 32... ...
Although proponents of the thesis of rising ground water assume a single source, that of upwelling creep ground water, for the secondary carbonates in the Quaternary of the Yucca Mountain region, multiple origins and ages for the carbonates are discernible. Some examples of the different sources and ages of carbonates of the region are: (1)
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From page 33... ...
The latter observation combined with an integrated soil profile encompassing the Bk/K horizons constitutes evidence that the surface-parallel calcite cements formed in the root zone of soil horizons' not on the ground surface. In response to the observation by panel members of increasing carbonate thickness upslope of the fracture, proponents of the upwelling ground water origin for the carbonate deposits suggested that a fault at higher elevation than that observed was probably the source of the upwelling water.
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From page 34... ...
There are many different circumstances that can result in a spring, and the cause of any one spring may not be immediately apparent. Warm water springs may result from dee~seated ground water rising rapidly to the surface; hot water springs often arise from near a deep volcanic source that heated the water; cold water springs suggest shallow ground water.
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From page 35... ...
Yet at Devils Hole where the water table is only 16 m below the surface, and where there have no doubt been earthquakes over the hundreds of thousands of years of the cavern's existence, no emdence of episodic large and protracted upwellings of ground waters is present. There are, indeed, small, short-lived, surging changes in the water level In Devils Hole in response to earthquakes.
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From page 36... ...
The Wahmonie site is a barren outcrop of white to buff gypsum-beanng (CaSO4~2H2O) sediments interpreted by proponents of the upwelling ground water thesis as a young spring deposit 1000 years old or younger.
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From page 37... ...
water flowing through the sulfide mineral deposits is at least as credible as upwelling deep ground water, or more so, as the following discussion of Cane Spring will suggest. Cane Spring Cane Spring, located near the northeast end of Skull Mountain, about 20 km southeast of Yucca Mountain, was cited as a very recent upwelling, having broken through to the surface and wetted that
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From page 38... ...
The fourth factor, as discussed later in this chapter, is the oxygen and hydrogen isotope analysis of Cane Spring waters, which shows them to be similar in isotopic content to local rainwater, although enriched in both the hydrogen isotope deuterium (D) and 78O in relation to ground water by evaporation (see Figure 2.~.
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From page 39... ...
Larger mineral grains are typical of higher temperature deposits. If, as has been suggested, deep ground water has risen repeatedly and has discharged onto the surface in the Yucca Mountain area, then spring mounds should be widespread and common occurrences.
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From page 40... ...
suggest that the very fine grain size of the calcite found in fracture fillings in the Yucca Mountain region is the result of very rapid cleposition of calcite where CO2-rich waters rapidly degas. However, at Mammoth Terrace in Yellowstone National Park, where deposition of travertine occurs at a very rapid rate from highly CO2-charged water, indiviclual carbonate crystals are large enough to be identified with a hand lens in the most fine-grained materials sampled from feeder veins.
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From page 41... ...
The current persistent growth of saltgrass (Distichlis spicafa) in the immediate area suggests that surface water is present, and may imply a "perched" water table, a reservoir of ground water maintained above the local water table because of some characteristic of the rock strata.
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From page 42... ...
EVI DENCE EXHIBITED BY BRECCIAS The widespread occurrence of "mosaic,' breccias at Yucca Mountain and Busted Butte has been viewed as supporting evidence for
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From page 43... ...
The panel observed four types of breccias at Yucca Mountain and Busted Butte distinguishecl on the basis of their structure or structur
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From page 44... ...
"jigsaw" breccias consisting of fragments of luff that have been broken and moved apart up to several millimeters, often with little or no subsequent rotation of the fragments. None of these can be attributed unequivocally to upwelling pressurized ground water; on the contrary, evidence strongly supports a surface process origin for some.
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From page 45... ...
These were generally absent in and above the lowermost welded zone in the Tiva Canyon tuff. The evidence is strong that the ground-water table was relatively high at the start of deposition of the Tiva Canyon tuff or that a lot of rain fell onto the still hot and highly porous basal tuff units during their accumulation between 13 and 10 Ma.
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From page 46... ...
The panel concludes that there is no need for, or good evidence
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From page 47... ...
Calcite would likely be the first phase to precipitate from this water, followed later by precipitation of silica as hotter water moved to a shallower level. Cool ground waters in siliceous, ash-flow luff environments commonly contain 25-50 mg/kg dissolved silica, while well over 120 mg/kg dissolved silica would be required to precipitate amorphous silica.
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From page 48... ...
The panel concludes that me preponderance of geochemical and mineralogical evidence supports the interpretations that the static water level has been close to its present level deep below Yucca Mountain for at least the last 100 ka, and possibly as long as 10 Ma, and that the carbonate-rich fracture fillings exposed in trenches in the region are composed of wind-blown dust cemented by material deposited from evaporating water that had infiltrated unsaturated rock along open fractures ISOTOPIC EVIDENCE Stable Isotopes: ,3C and RIO Advances in geochemical research make it possible to predict the isotopic composition of calcite precipitated from a specified ground water of known isotopic composition. Methods of isotope geochemistry can be applied to the available data on calcite deposits and ground waters of Yucca Mountain to test if these deposits are evidence for episodes of ground-water upwelling and discharge at the Earth's surface.
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From page 49... ...
Isotopic evidence for the origin of calcite deposits at Yucca Mounta~n is in strong contrast to that presented for Devils Hole because the isotopic content of calcites predicted to precipitate from Yucca Mountain ground water and measured isotopic concentrations of calcites at Yucca Mountain do not agree. Table 2.2 shows the results of predicted and measured isotopic compositions for calcites at Yucca Mountain and Busted Butte.
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From page 50... ...
Ground water has the lowest D and ~80 concentration because it originates from rain and snow falling high in the Spring Mountains Table 2.2 Comparison of Predicted vs Measured Isotopic Compositions of Calcite Precipitates at Yucca Mountain and Busted Butte, Nevada Isotopes Predicted Measured Oxygen (~18OVSMOW) +12.0 to +14.4% +18 6 to +22.0% IStrontium (87Sr/86Sr)
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From page 51... ...
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.
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From page 52... ...
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.
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From page 53... ...
GROUND WATER ALK./FURN. 8 o o o 1,,,,,,, ~ _ 0 10 {j 18 0 SMOW °/°° 20 30 Figure 2.9 Plot of ~l3CpDB vs bl80sMow for ground waters of Alkali Flat/ Furnace Creek flow systems, and Yucca Mountain vein calcites and soil calcites.
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From page 54... ...
The calcites with inconsistent 73C content may have foxed as a result of hydrothermal activity that occulted 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)
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From page 55... ...
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.
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From page 56... ...
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.
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From page 57... ...
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.
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From page 58... ...
1989. Stable isotopes in precipitation and ground water in the Yucca Mountain region, Nevada - paleoelimatic interpretation.
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From page 59... ...
Geologic map of the Nevada Test Site, southern Nevada.
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From page 60... ...
In Proceedings of the American Nuclear Society International Meeting on High Level Radioactive Waste Management 2:924-929. Quade, J., T
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From page 61... ...
1990. Reconnaissance 6~3C and 8~80 data from Trench 14, Busted Butte, and drill hole Gal, Yucca Mountain, Nevada Test Site.
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