Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion (1995) / Chapter Skim
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3 PRECLOSURE HYDROLOGY
3 PRECLOSURE HYDROLOGY
Pages 66-92
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From page 66... ...
3. Is the ground water supply at Yucca Mountain sufficient to provide for the needs of the proposed repository during construction and operation?
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From page 67... ...
to protect against surface flooding. The TBR presents estimates of the water surface elevations associated with the probable maximum flood (PMF)
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From page 68... ...
The maximum predicted depths of flow at these cross sections were compared directly to the elevations of the portals, pads, and shaft to determine their flooding potential. The procedures used to estimate the PMP and peak discharge rates are relatively straightforward engineering calculations.
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From page 69... ...
are a hydraulic resistance coefficient, called Manning's n, and the Froude number, a physically significant dimensioniess ratio that determines the depth and velocity characteristics of the flow.4 A third important input parameter to this model, the "bulking factor," is used to represent the effects of entrained sediment, air, and debris carried by the flowing water on the flow depth. Adequacy of Data Collection and Analysis The application of probable maximum flood procedures in the TBR to estimate flood events is consistent with practices used to design civil structures such as bridges and dams.
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From page 70... ...
The assumption of critical flow velocity is conservative for this assumed value of Manning's n (as noted in the last section, critical flow depths were assumed in cases where the Manning equation predicted supercritical flow velocities)
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From page 71... ...
This could affect the interpretation of flooding potential at critical surface facilities at Yucca Mountain. 6 Exploratory Studies Facility.
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From page 72... ...
More work should be done to assess the sensitivity of Me estimated water surface elevations to the assumed values of Manning's n and the bulking factor, and the assumption of critical flow depth at all locations in the stream channels. This testing will
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From page 73... ...
Because the TBR deals only with preclosure issues, potential adverse effects are to be evaluated within the projected 50- to 100-year life of repository operation prior to closure.7 Adequacy of Data Collection and Analysis The TBR does not explicitly address whether rising water tables or deep percolation of infiltrating surface water could cause flooding of the repository during the preclosure phase. Although neither of these two sources appears to the committee to pose a problem for construction or operation, the report should have demonstrated this.
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From page 74... ...
Modeling of the steady-state position of the ground water table for a doubling of mean-annual precipitation showed that the repository remained in the unsaturated zone, about 30 m (approximately 100 feet) above the final position of the water table (Czarnecki, 19851.
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From page 75... ...
, is grossly inadequate for evaluating the potential relationships between perched water zones and stratigraphic or structural features. An accurately scaled cross section that shows elevations of the land surface and locations of boreholes, the proposed repository, and the water table should have been provided in the TBR to facilitate understanding.
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From page 76... ...
above the water table at that location and ~ Note that pages 3-5 and 3-6 in the TBR are miscollated.
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From page 77... ...
9 The water table in UZ-14 is located at about 780 m (2,558 feet) above mean sea level according to information received by the committee during one of its information-gathering sessions.
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From page 78... ...
First, the extremely high water table levels in wells G-2 and WT#6 to the north of Yucca Mountain are never mentioned in the perched water discussion, even though one alternative interpretation for their existence is that they are in fact perched water, and not part of the regional water table (e.g., Ervin et al., 19941. The proposed location for the repository is at least 200 m (approximately 650 feet)
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From page 79... ...
Interestingly, the water table elevations observed in wells G-2 and WT#6 are not too different from the elevation of perched water (965 m [3, 165 feet] above mean sea level; TBR, p.
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From page 80... ...
. Summary and Conclusions Of the three potential sources of subsurface water that could flood the proposed repository (deep percolation, rising water table, and perched water)
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From page 81... ...
, two questions must be addressed to evaluate water resource potential: I Is adequate water for repository development available from surface or ground water sources?
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From page 82... ...
It is not clear if "available water" is assumed to include all water in the aquifer or if available water is limited to that which could be produced without significant mining. At least three aquifers could be tapped for water supply at Yucca Mountain: a shallow alluvial aquifer in Fortymile Wash, the luff aquifer (shown on the cross sections in Figures I.3 and I.4)
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From page 83... ...
Water levels measured in these wells can provide important constraints on conceptually and numerical models of the ground water flow system. The TBR cites, without any supporting data, an estimate by Young (1972)
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From page 84... ...
The summary of annual production rates and periodic water level measurements from La Camera and Westenburg (1994) , which are shown in Figures 3.3-l through 3.3-3 of the TBR, provides the most quantitative data on water availability in the vicinity of Yucca Mountain.
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From page 85... ...
~ of this report summarizes estimates of water requirements derived from values cited in other documents including the final environmental assessment (DOE, 1986) , site characterization plan (DOE, 1988)
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From page 86... ...
(1992) refer to this same value as "water consumption for lifetime repository construction and operation."
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From page 87... ...
This interpretation is consistent with reported recoveries of water levels during periods of reduced pumping and with the limited drawdowns observed during historic pumping. In discussing the water level monitoring data summarized in Figures 3.3-l through 3.3-3, the TBR notes that these data suggest that no permanent drawdowns have been caused by ground water withdrawals to date in Jackass Flats.
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From page 88... ...
The TBR does not discuss if and how acceptance of any of the alternate conceptual models might affect predictions of drawdowns and sustainability of ground water supplies during repository construction and operation.
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From page 89... ...
Extensive pumping in adjacent basins could have significant effects on water levels in the vicinity of Yucca Mountain, particularly if these rates represent a sign~ficant portion of the regional discharge. For example, La Camera and Westenburg (1994)
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From page 90... ...
Additional data that would be useful to an assessment of competing future demands for ground water would be production histories and projected future pumping rates of wells in nearby areas such as the Amargosa Desert. Alternatively, if bounding estimates of drawdown are all that are required for regulatory purposes, it may be acceptable to perform a sensitivity study by constructing numerical models consistent with the alternative hypotheses for the high-gradient zone and comparing drawdowns predicted by these models to those of Czarnecki (19911.
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From page 91... ...
If DOE chooses to attempt to quantify the absolute availability of ground water in the Jackass Flats area, testing and comparison of alternative models would be required. The lack of data and constraints for ground water flow models throughout the basin, but particularly in the area of apparent high gradient in the water table north of Yucca Mountain, would be significant limitations to answering this technical question.
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From page 92... ...
The possible changes in predicted water level declines that would result from incorporating alternative conceptual models into the simulations would have to be assessed in this case. Especially important to this analysis would be a comparison of results generated by numerical models incorporating the alternative conceptual models that have been proposed to account for the high-gradient area north of the proposed repository.
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