Conceptual Models of Flow and Transport in the Fractured Vadose Zone (2001) / Chapter Skim
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Development of the Conceptual Model of Unsaturated Zone Hydrology at Yucca Mountain, Nevada
Development of the Conceptual Model of Unsaturated Zone Hydrology at Yucca Mountain, Nevada
Pages 45-86
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From page 45... ...
Technical Papers 45
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Early conceptual models assumed that very small volumes of water infiltrated into the bedrock, that much of the infiltrated water flowed laterally within the upper nonwelded units because of capillary barrier effects, and that the remaining water flowed down faults with a small amount flowing through the matrix of the lower welded, fractured rocks. When evidence accumulated indicating that infiltration rates were higher than initially estimated, and that mechanisms supporting lateral diversion did not apply at these higher fluxes, the flux calculated in the lower welded unit exceeded the conductivity of the matrix.
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After extensive public comment and review, the final version of 10 CFR 60 that included the unsaturated zone was released in 1985. The purpose of this paper is to describe and trace the evolution of the conceptual model of groundwater flow in the unsaturated zone at Yucca Mountain.
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The potential repository host rock is the Topopah Spring Tuff of the Paintbrush Group, a densely welded and fractured tuff located in the unsaturated zone at an average depth of 300 m below land surface (Hanks et al., 1999~. Climate and Precipitation An understanding of the response of the hydrologic system to current climatic conditions is a prerequisite for predicting the response of the system to potential future climatic conditions (Botkin et al., 1991~.
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Most of the lithostratigraphic units in the Tiva Canyon and Topopah Spring Tuffs are laterally continuous and stratiform (Scott and Bonk,1984~. The Yucca Mountain and Pah Canyon Tuffs are relatively thick to the north of the potential repository location near Yucca Wash and contain
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The nonwelded luffs of the Paintbrush Group, including the nonwelded intervals of the Yucca Mountain and Pah Canyon Tuffs, the inters/ratified bedded luffs, the nonwelded base of the Tiva Canyon Tuff, and the nonwelded top of the Topopah Spring Tuff collectively are commonly referred to as the Paintbrush nonwelded hydrologic unit (PTn)
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DEVELOPMENT OF INITIAL CONCEPTUAL AND NUMERICAL MODELS (1983-1990) A conceptual model describes the physical processes that are part of an environment, how they relate to each other, and which processes dominate the system.
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the flux between fractures and matrix in unsaturated rock. Most conceptual models for Yucca Mountain include these components, but advances in our scientific understanding of these processes and features have greatly influenced the way the conceptual and numerical models have developed over the years.
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The measurements and analyses have provided detailed data sets needed for the development and testing of the site-scale numerical flow model. Early Conceptual Models of Hydrology at Yucca Mountain The earliest detailed conceptual model of the unsaturated zone at Yucca Mountain was published by Scott et al.
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56 sit Cal Ct U)
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Rapid drainage of water through fractures would minimize contact time between water and the waste canisters and therefore repositories in the unsaturated zone would be preferable to repositories in the saturated zone if the host rock had high fracture permeability. In 1984, Montazer and Wilson (1984)
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s9 cd o .
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, who estimated a downward flux of between 0.003 and 0.2 mm/yr in the matrix of the TSw, as support for the matrix flow component of their conceptual model. Montazer and Wilson suggested that lateral flow would lead to perched water on the upgradient side of faults, but that the water would ultimately move into the fault and recharge the saturated zone.
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DEVELOPMENT OF CONCEPTUAL MODELS AT YUCCA MOUNTAIN 1 0 -13 o-l4 -15 ~0 A: CO V' CE: 0 it do A: 43 10 [g 0 -20 10 _ 1 0 m A: LO 1 o 22 :L 10 lll -24 I,L 10 U] 1 0-25 1 0-26 1 1 ' -- 1''~ I~ _ -_ Fracture ~ t~ Critical saturation ~: _ ~ \ - ~ _ ~.~` 1 and 2 _,1:3 .
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They suggested that there is virtually no downward flow in fractures beneath the PTn and that flow between adjacent matrix blocks would occur across fractures only at asperities (Figure 2-8~. They proposed that fracture flow occurs only when the water potential of the fracture is in equilibrium with the water potential of the matrix, which, in the unsaturated zone, occurs only during periods of surface flooding when there is a transition between saturated and unsaturated fracture conditions.
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The first numerical two-dimensional flow model of the unsaturated zone at Yucca Mountain was presented by Rulon et al.
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64 Cal ·_.
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DEVELOPMENT OF CURRENT CONCEPTUAL AND NUMERICAL MODELS (1991-2000) Three-Dimensional Site-Scale Numerical Model The basis for the current site-scale model of the unsaturated zone at Yucca Mountain was established during meetings in 1991 between the USGS and Lawrence Berkeley National Laboratory (LBNL)
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66 CONCEPTUAL MODELS OF FLOWAND TRANSPORT in. ~ to FIGURE 2-10 Alternative conceptual models and their corresponding characteristic curves for relative permeability for flow through fractured rock.
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Development of Conceptual Model of Spatially Distributed Infiltration Hevesi et al.
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Recognizing that there is a large variability in the spatial and temporal distribution of precipitation in the study area, it was noted that spatial and temporal distributions of infiltration could be the most important factors influencing the distribution and rate of percolation in the unsaturated zone (U.S. Department of Energy, 1992~.
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DEVELOPMENT OF CONCEPTUAL MODELS AT YUCCA MOUNTAIN 69 FIGURE 2-12 Infiltration distributed statistically. From Hudson and Flint (1995, Figure 8)
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Modeling with High Infiltration Rates: the Conversion to Fracture Flow Lateral Diversion in the PTn By 1995, most project hydrologists had accepted that the conceptual model of unsaturated zone flow must include fracture flow in the TSw and that numerical models must incorporate approaches other than the equivalent continuum model if they were to accommodate high fluxes through the TCw. However, some hydrologists still believed that lateral diversion in and above the PTn may reduce the volume of water that would penetrate the TSw.
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The results of several studies that had evaluated how water flows through fractures contradicted the traditional, but perhaps outdated, conceptual model of sheet flow in fractures (Rulon et al., 1986) and supported fast fracture flow by demonstrating channeling or fingering (Pruess and Tsang, 1990; Glass, 1993; Glass et al., 1994; Pruess, 1998~.
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To account for the perched water, the site-scale model currently treats faults as permeability barriers to lateral flow, an assumption substantiated by pneumatic evidence (LeCain, 1997) and by the sharp drop in the potentiometric surface across the Solitario Canyon Fault (Tucci and Burkhardt, 1995~.
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Development of Conceptual Model for Fast Pathways Isotopic studies at the site began to support a conceptual model in which pervasive fracture flow and fast pathways through the unsaturated zone altered
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From Bodvarsson and Bandurraga (1996, Figure 1.1.1~. the long-held conceptual model of flow in an arid environment (Winograd, 1981; Roseboom, 1983; Montazer and Wilson, 1984~.
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The concept of deep, rapid infiltration into fractured luffs exposed at the surface or buried under shallow soils is supported by the presence of bomb-pulse 36C1 and tritium at depth, as well as by low concentrations of chloride, which reflect fairly high infiltration rates (Fabryka-Martin et al., 1998~. Isotopic studies of the porewater salts in the underground ESF support a conceptual model for fast pathways in which faulting in the overlying PTn is the dominant factor controlling the spatial distribution of fast pathways to the repository horizon (Fabryka-Martin et al., 1997~.
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conceptual model of flow through the unsaturated zone at Yucca Mountain (Bodvarsson et al.,1998; Flint et al., in press) as presented here reflects those processes invoked and supported by the majority of research participants on the Yucca Mountain Project, as well as those concepts that are most consistent with most of the measured data and observations.
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site-scale unsaturated zone numerical model grid. From Bodvarsson et al.
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conceptual model of flow in the unsaturated zone at Yucca Mountain. From Flint et al.
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SUMMARY This paper has described and traced the evolution of the conceptual model of hydrology for the unsaturated zone at Yucca Mountain. The study area is 145 km northwest of Las Vegas, Nevada, and covers approximately 45 km2.
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The fault will act as a conduit to slowly drain the perched water body in equilibrium with the large area inflow. This paper has presented the most recent conceptual model of the unsaturated zone at Yucca Mountain but is by no means the final word.
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Sharma, 1993. Distribution of chlorine-36 in the unsaturated zone at Yucca Mountain: an indicator of fast transport paths.
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K., 1995. Assessing alternative conceptual models of fracture flow.
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Hydrogeology of the unsaturated zone, North Ramp area of the Exploratory Studies Facility, Yucca Mountain, Nevada.
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Hydrogeology of the Unsaturated Zone, North Ramp Area of the Exploratory Studies Facility, Yucca Mountain, Nevada.
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Design of a three-dimensional site-scale model for the unsaturated zone at Yucca Mountain, Nevada. In: International High-Level Radioactive Waste Conference, Las Vegas, Nev., April 12-16.
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