Ground Water at Yucca Mountain How High Can It Rise? (1992) / Chapter Skim
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Appendix A: A Review of the Isotopic Geochemistry of the Yucca Mountain, Nevada, Proposed Nuclear Waste Repository Site
Appendix A: A Review of the Isotopic Geochemistry of the Yucca Mountain, Nevada, Proposed Nuclear Waste Repository Site
Pages 145-171
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From page 145... ...
Appendixes
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Tests of the hypothesis carried out with isotope geochemistry show that the calcites did not precipitate from presentday ground water but, on the contrary, from surface waters. Changes in the isotopic compositions of ancient ground waters to bring them to parental compatibility with the calcites are larger than those inferred from available data.
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I NTROD UCTION Szymanski (1989) has proposed that calcite veins in the vicinity of the Yucca Mountain, Nevada, Nuclear Waste Repository Site were deposited from upwelling ground water.
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In each section) the efficacy of isotopic methods to fingerprint the origin of calcite is verified by comparing data on Devils Hole calcite veins with that on ground waters of the Ash Meadows flow system.
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In the discussion that follows it grill be demonstrated that known surface calcite deposits at Yucca Mountain did not precipitate from analyzed presently ground waters. Whether or not the calcites could have precipitated from ancient ground waters cannot be proven because
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Assuming that the temperature of ancient ground waters was 33.7°C, equal to the present value, the range of 618OVSMOW of paleo-ground waters was-13.7 to -11.2%o at Devils Hole over the past 566 ka. These changes are not large enough to justify the ground waters as parental to Yucca Mountain surficial calcite deposits.
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The similar values of the Alkali Flat/ Furnace Creek ground-water system suggest a similar control on ~sotopic content. Note that precipitation collected at stations on the Nevada Test Site (north and east of Yucca Mountain)
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The ground water samples are from the Ash Meadows and Alkali Flat/Furnace Creek regional aquifers. 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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Ground waters and surface waters of the Salton Sea Geothermal Field located in S California 384 km south of Yucca Mountain show the effects of both evaporation and isotopic exchange between waters and wall rocks under conditions of geothermal heating (Figure 3~.
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. I will first consider the test case of Devils Hole calcite veins and the Ash Meadows flow system to verify that it is possible to accurately predict the 6~3C and 678O values of calcites precipitated from ground waters of known isotopic composition.
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~ -2 ~ CALCITE DEVILS HOLE EQUILIB. CC GROUND WATER ASH MEADOWS DEVILS HOLE WATER ASH MEADOWS CALCITE & WATER I I I_ ~ ~ _ _ ~ ~ I t 0 10 618 O SHOW °/oo 20 30 Figure 4 Plot of 6~3C vs TRIO for ground water of Ash Meadows flow system (references given for Figure 1 )
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Values of calcites in equilibrium with analyzed ground waters calculated as for Figure 4. Wells drilled at Yucca Mountain give ground-water values of 6~80 from -14 to -12.8% and 6~3C from -12.7 to - .9~.
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ecl calcites expected to precipitate from analyzed ground waters and the measured calcite veins of Yucca Mountain. There is, however, a significant overlap between Yucca Mountain calcites and analyzed soil carbonates collected from localities surrounding the Repository Site in the southern Great Basin.
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Analyses of drill hole calcites are shown in Figure 7 together with data on Devils Hole, Yucca Mountain, and soil calcites. The values of drill hole calcites overlap those of Yucca Mountain soil calcites but also extend towards PESO values as low as 15.4%~.
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These data directly contradict the hypothesis that Yucca Mountain calcites were deposited from upwelling ground water. On the contrary, there is a strong implication that calcite isotopic compositions are dominated by surface waters to depths as close as 186m.
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U AND TH ISOTOPES The activity ratios of U and Th isotopes in ground waters and calcites of the Yucca Mountain area are shown in Figure 9. Values of 234U/238U for ground waters from both the Ash Meadows and the Alkali Flat/Furnace Creek flow systems are plotted in a stacked histogram on the left-hand side of the figure.
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The symbol GYMS denotes ground waters sampled near Yucca Mountain. Histogram is rotated 90° to facilitate comparison with calcite data.
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of ground waters sampled by drilling at Yucca Mountain. There are two interesting discrepancies between GO data, and U-Th data measured for drill hole calcites.
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I conclude that the U-Th data are consistent with stable isotope data that shows Yucca Mountain calcite veins were not deposited from analyzed ground waters. SR ISOTOPES Strontium isotopes (87Sr and 865r)
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it,, ,, ~ ASH MEADOWS ~ l t13 6 ~ DEVILS HOLE CC 4 LO o i2 ~ 8 m 4 By , 1 ALK./FURN. TRENCH 14 & BE, CC SOIL CC O ~ ,~ ~~/~/~/~ ~ 0.708 0.709 0.71 0 0.711 0.71 2 0.71 3 0.714 0.71 5 87Sr/86Sr Figure 10 Upper panel shows stacked histogram of 87Srp6Sr values for ground waters of Ash Meadows flow system and Devils Hole calcites.
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Many of the samples from the saturated zone appear to be in equilibrium with analyzed ground waters. Temperatures measured at depths of -500 to -1200 m below static water level range from 40°C to 60°C.
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The majority of presently available data for drill hole calcites suggests a stable elevation for the static water level, over the period of time during which secondary calcite was deposited. CONCLUSIONS i: Analytical data on both stable isotopes and radiogenic isotopes agree that the calcite vein deposits of Yucca Mountain did not precipitate from analyzed ground waters.
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From page 168... ...
Analysis of wind-blown dust would help to quantify its role in the origin of surficial calcite deposits. New data on drill hole calcites are needed to explore the possibility that some of them may have precipitated from mixtures of ground waters and surface waters The increased use of mass spectrometry methods (Ludwig, et al., 1990;
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Nevertheless, in a project of such importance to the public as the Yucca Mountain Repository Site Evaluation it would seem prudent to leave nothing to chance including the verification of analytical data by independent laboratories. ACKNOWLEDGEMENTS The task of gathering data for this review was greatly aided by the work of J
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1990. Uranium-series dating of secondary carbonates near Yucca Mountain, Nevada: applications to tectonic, paleoclimatic, and hydrologic problems.
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1991. Isotopic discontinuities in ground water beneath Yucca Mountain, Nevada.
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