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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Technical Papers

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE This page in the original is blank.

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE 2 Development of the Conceptual Model of Unsaturated Zone Hydrology at Yucca Mountain, Nevada Alan L. Flint,1 Lorraine E. Flint,1 Gudmundur S. Bodvarsson,2 Edward M. Kwicklis,3 and June Fabryka-Martin3 ABSTRACT Yucca Mountain is an arid site proposed for consideration as the nation's first underground high-level radioactive waste repository. Low rainfall and a thick unsaturated zone are important physical attributes of the site because the quantity of water likely to reach the waste and the paths and rates of movement of the water to the saturated zone under likely future climates will be major factors in estimating the concentrations and times of arrival of radionuclides at the surrounding accessible environment. The framework for understanding the hydrologic processes that occur at this site and that control how quickly water will penetrate through the unsaturated zone to the water table has evolved during the past 15 years. 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. This required water to flow vertically in the high-permeability fractures of the potential repository host rock. 1   U.S. Geological Survey, Sacramento, California 2   Lawrence Berkeley National Laboratory, Berkeley, California 3   Los Alamos National Laboratory, Los Alamos, New Mexico

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE The development of numerical modeling methods evolved concurrently with the conceptual model in order to account for the observations made at the site, particularly fracture flow deep in the unsaturated zone. This paper presents the history of the evolution of conceptual models of hydrology and numerical models of unsaturated zone flow at Yucca Mountain, Nevada. INTRODUCTION On-land geologic disposal of high-level nuclear waste has been an issue in the United States for nearly half a century. In 1958, the National Academy of Sciences recommended considering geologic disposal of high-level nuclear waste (HLW). In 1959, concerns about the thermal effects of nuclear-waste disposal were added to the recommendation. In the early 1970s, Winograd (1972, 1974) proposed storing nuclear waste in the unsaturated zone, although it was not until the early 1980s that such a design was seriously considered. In 1976, the Director of the U.S. Geological Survey (USGS) suggested to the U.S. Energy Research and Development Administration [ERDA, the predecessor to the U.S. Department of Energy (DOE)] that a nuclear test site in Nevada [Nevada Test Site (NTS)] be examined for potential sites for HLW disposal. The major attributes of the NTS as a potential site for disposal are that it is in a remote location, it is a large contiguous block of land under federal ownership, there is much information on the unsaturated zone based on studies of the underground nuclear testing and the associated presence of radionuclides in the subsurface, rainfall is low, and there is a thick unsaturated zone with a variety of rock types (Winograd, 1971). Initially, however, use of the saturated zone as a nuclear waste repository was the prevailing choice (Roseboom, 1983; Hanks et al., 1999). By 1978, the first boreholes were being drilled on Yucca Mountain to explore the character of the saturated zone for disposal of nuclear waste. The high fracture transmissivity and elevated groundwater temperature of the saturated zone below Yucca Mountain made this zone undesirable as a repository site. In 1982, USGS scientists suggested to DOE that the unsaturated zone at Yucca Mountain be considered instead. Because Nuclear Regulatory Commission (NRC) draft regulations 10 CFR 60 “Disposal of High Level Waste in Geologic Repositories,” published in 1981, covered only repositories in the saturated zone, the USGS also suggested to the NRC that the regulations be modified to include the unsaturated zone, and Roseboom (1983) pointed out how such a repository would differ from one in the saturated zone. 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. For this discussion, a conceptual model is simply a relevant set of concepts that describe, in a qualitative way, the behavior of a natural system. Numerical models of the same system, which also will be discussed in this paper, are based on

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE the same set of concepts but describe the behavior of the system in a quantitative manner. It is important to note that the history of the characterization of Yucca Mountain or, in particular, the evolution of a conceptual model of groundwater flow at the Yucca Mountain site, cannot be accurately reconstructed solely on the basis of citable literature. To fully understand this history requires reference to unpublished or draft reports, memoranda, and rough notes. In addition, many of the concepts for the model were developed during discussions between DOE and such entities as the Nuclear Waste Technical Review Board (NWTRB), the Advisory Committee on Nuclear Waste (ACNW), or NRC technical interchanges. In many cases, ideas were developed and worked out during informal get-togethers and, as such, many important ideas and information used in the development are not readily available or directly citable. YUCCA MOUNTAIN SITE DESCRIPTION Yucca Mountain is located in southern Nevada about 145 km northwest of Las Vegas (Figure 2-1). The study area covers approximately 45 km2, of which approximately 5 km2 covers the potential repository site. Beneath the crest of Yucca Mountain, the water table ranges from approximately 350-750 m below land surface, with an average of 500 m. 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). The climate in the Yucca Mountain area is arid to semiarid. Weather patterns vary seasonally. Summer precipitation comes primarily from the south and southeast. Winter winds bring moisture from the west, and hence the climate is subject to a regional rain shadow east of the Sierra Nevada and has been for the entire geologic history of Yucca Mountain, more than 13 million years. Topographic effects cause substantial variability in average annual precipitation in the Yucca Mountain area. Precipitation averages from less than 130 mm for lower elevation locations in the south to more than 280 mm for higher elevation locations in the north, with an estimate of 170 mm directly over the potential repository location (Hevesi and Flint, 1996). Regional Hydrogeology Yucca Mountain is located within the Basin and Range physiographic province (Grayson, 1993). The linear mountains and valleys of this area that have a

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE FIGURE 2-1 Yucca Mountain regional (on left) and site-scale study areas (expanded on right). Major block bounding faults (as represented by the project lithostratigraphic model), the Exploratory Studies Facility, and the potential repository boundary are marked.

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE distinct north to northwest trend define the Basin and Range physiography. Within the Basin and Range physiographic province, there are several topographic regions. Yucca Mountain is in the Death Valley region, which has the largest and most prominent desert basin in the Basin and Range physiographic province. The Death Valley region is primarily in the northern Mojave Desert; the region extends northward into the Great Basin Desert and lies in the rain shadow of the Sierra Nevada. Death Valley itself is the ground-water discharge area for a large part of the Death Valley region. The Death Valley region is composed largely of closed topographic basins that apparently coincide with several closed shallow groundwater flow systems (Winograd and Thordarson, 1975). Recharge in these systems is sparse, and is derived mostly from the higher altitudes and comes as infiltration of precipitation or the infiltration of ephemeral runoff. Discharge occurs primarily by spring flow and by evaporation and transpiration of shallow ground water from playas. The deepest part of the saturated flow system consists of extensive Paleozoic carbonate aquifers that connect the closed shallow groundwater systems at depth. Discharge from the system occurs in several intermediate areas that are geomorphically, stratigraphically, and structurally controlled; but ultimately, most groundwater flow discharges to Death Valley. The predominant direction of drainage for surface-water and groundwater flow in the Death Valley region is generally from north to south because of a decrease in the average altitude from north to south in the southern Basin and Range area. Site Geology Yucca Mountain consists of a 1-3-km-thick sequence of ash flow and ash fall tuffs erupted from Timber Mountain, a source caldera complex located directly to the north. The unsaturated zone at Yucca Mountain is about 500-750 m thick (Snyder and Carr 1982; Buesch et al., 1996), characterized by pyroclastic flows that consist of separate formations. From youngest to oldest, the formations are the Rainier Mesa Tuff (11.6 million years) of the Timber Mountain Group; the Tiva Canyon, Yucca Mountain, Pah Canyon, and Topopah Spring Tuffs of the Paintbrush Group (12.7 million years); the Calico Hills Formation (12.9 million years); and the Prow Pass, Bullfrog, and Tram Tuffs of the Crater Flat Group (13.5 million years) (Carr et al., 1986; Sawyer et al., 1994) (Figure 2-2). Interstratified with these formations are bedded tuffs that consist primarily of fallout tephra deposits and small amounts of pyroclastic flow deposits and reworked material (Moyer and Geslin, 1995; Buesch et al., 1996). The bottom and top of the Tiva Canyon and Topopah Spring Tuffs contain vitric, nonwelded to densely welded tuff; the interiors of the tuffs are thick, crystallized, and moderately to densely welded and fractured. 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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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE FIGURE 2-2 Hydrogeologic units and lithostratigraphy currently used at Yucca Mountain, and as used in earlier publications. both nonwelded and welded intervals. The welded intervals of these units, however, thin southward starting near Drill Hole Wash, and, therefore, only thin welded intervals occur in the center of the potential repository location. These welded intervals are absent altogether from the southern half of the repository location (Moyer et al., 1996). The nonwelded tuffs of the Paintbrush Group, including the nonwelded intervals of the Yucca Mountain and Pah Canyon Tuffs, the interstratified bedded tuffs, 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). The Calico Hills Formation (CHn) is composed of nonwelded pyroclastic flow and fallout deposits (Moyer and Geslin, 1995). Tuffaceous rocks have been zeolitized (CHz) at the north end of Yucca Mountain, yet parts of the formation remain largely vitric

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE (CHv) towards the south end of the mountain. The Prow Pass Tuff is a compound cooling unit and consists of nonwelded to partially welded tuff at the top and bottom with intervals of welded tuff. The vitric parts of this unit are typically zeolitized in the north, but only in the southwestern part of Yucca Mountain does a significant part of this unit remain vitric, with partially to moderately welded, crystallized tuff in the interior of the unit. Site Geomorphology The hydrology of Yucca Mountain has largely been influenced by interrelationships between tectonic and geomorphic processes. Faults and fault scarps, and erosional processes on the eastern sloping ridge, have defined the topography of the mountain, and have created a series of washes (Figure 2-1) that are downcut to varying degrees into different bedrock layers. The topography generally is controlled by high-angle faults that tilt the resistant volcanic strata eastward. Locally, slopes are steep on the west-facing escarpments of the Solitario Canyon Fault and in some of the valleys that cut into the more gentle eastward-facing dip slopes. Narrow valleys and ravines have been cut into the bedrock. Floors of wider valleys consist of alluvial deposits that have formed terraces into which intermittent streams have cut channels. Locally, small sandy fans flank the lower slopes and spread out on the valley floors. East of the crest of Yucca Mountain, drainage is into Fortymile Wash; west of the crest of the mountain, streams flow southwestward down fault-controlled canyons and discharge in Crater Flat. The study site area can be divided into two parts north and south of Drill Hole Wash. The washes in the southern area trend eastward, are relatively short (less than 2 km), and are defined by erosional channels that produce gently sloping sideslopes. The washes north of Drill Hole Wash are controlled by faults, are northwest trending, and are approximately 3-4 km long with steep sideslopes. Alluvial deposits in the valley floors and washes include fluvial sediments and debris-flow deposits. Soil development and thickness of the alluvial deposits are variable, and the soils are gravelly in texture. The deposits range from 100 m thick in the valleys to less than 30 m thick in the mouths of the washes. Midway up the washes, most alluvial fill is less than 15 m deep in the center of the wash. Many of these deposits have developed cemented calcium carbonate layers (Flint and Flint, 1995). 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. It describes the physical framework within which the processes can be understood and numerical relations can be developed.

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Conceptual Model Issues at Yucca Mountain Many current and historical issues are relevant to the discussion of the conceptual model of unsaturated zone hydrology at Yucca Mountain. We discuss the most significant issues and how the associated components of the conceptual model developed or were modified as the conceptual model changed. A simplified schematic that highlights these issues is presented in Figure 2-3. In general, the major components of the conceptual model include the following processes and features: (1) surface infiltration rates and their distributions in space and time, (2) lateral flow in the nonwelded PTn, (3) lateral flow at the vitric-zeolitic interface in the matrix of the deep nonwelded tuffs (CHv and CHz), (4) the role of faults as conduits or barriers to flow, (5) the occurrence and stability of perched water, (6) the distribution and significance of fast pathways, and (7) 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. Initial Data Collection Initial data collection at Yucca Mountain consisted of mapping the bedrock surface and drilling boreholes to describe the geology and water table depths at the site. By 1986, more than 100 boreholes had been drilled at or near Yucca FIGURE 2-3 Generalized conceptual model of the hydrology for Yucca Mountain, Nevada. Arrows denote direction of flow; numbers denote the major components described in the text; and abbreviations for lithostratigraphy are described in Figure 2-2.

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Mountain. Many of the early holes drilled during the late 1970s extended to the water table, yielding important data such as potentiometric surfaces, groundwater chemistry, detailed lithostratigraphy, and matrix properties (Anderson, 1981; Rush et al., 1983; Weeks and Wilson, 1984; Whitfield et al., 1984; Flint and Flint, 1990). These were followed in 1984 by an extensive series of shallow boreholes that were drilled to investigate shallow infiltration processes (Hammermeister et al., 1985). Studies of the surface geology at Yucca Mountain had already been ongoing for more than 10 years (Byers et al., 1976; Scott et al., 1983; Scott and Bonk, 1984) prior to the time that investigations of infiltration and percolation processes began in earnest. A stop-work order for most site characterization activities was issued by the DOE in early 1986 because of concerns related to the quality assurance of data collection; however, selected surface and laboratory investigations (those considered to be collecting irretrievable data) were allowed to continue in order to characterize the geology, faults, and matrix and fracture properties of Yucca Mountain (Klavetter and Peters, 1987; Istok et al., 1994; Flint et al., 1996b). After the stop-work order was lifted in late 1991, a series of shallow, cored neutron boreholes were drilled to study infiltration processes (Flint and Flint, 1995). On completion of the neutron boreholes, deep boreholes were drilled for long-term monitoring and geotechnical boreholes were drilled along the surface projection of the underground Exploratory Studies Facility (ESF) prior to its construction (Rousseau et al., 1998) to provide design information for the construction of the ESF. The results of core analysis and borehole instrumentation and geophysics, which measured subsurface conditions, have aided in the development of the conceptual model used to help understand infiltration and percolation rates and processes at Yucca Mountain. 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. (1983) (Figure 2-4). Their conceptual model of hydrology at Yucca Mountain is a component of a larger geologic/hydrologic framework model that is presented in a very straightforward manner. First, they identified the problem and stated that groundwater was one of the most critical parameters for nuclear waste isolation. Second, they described the stratigraphic, structural, and hydrologic framework that is the basis of their geologic/hydrologic framework model by presenting the detailed geologic setting. Third, they identified the relevant hydrologic processes needed to describe the hydrology for their geologic/hydrologic framework model. Finally, using these processes and applying them to their model, they described the hydrologic consequences of groundwater flow.

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE is probably about 1 to 2 mm/yr (Wolfsberg et al., 1999). Third, the residence time of water in the soil cover must be less than 50 years, i.e., the soil thickness must be less than 3 m so that the bomb pulse is not retained within the soil profile. The ESF studies also support the theory that the capacity of the unfaulted PTn to buffer and redistribute infiltration appears to decrease in the south of the repository where the unit thins to 25 m (Wolfsberg et al., 1999). Current Conceptual Model of Flow Through the Unsaturated Zone at Yucca Mountain The current (2000) 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. The existing sitescale numerical flow models effectively encompass and integrate hydrologic processes that operate at multiple scales at Yucca Mountain. The ability of these numerical models to provide simulations that are consistent with a broad suite of characterization data and observations collected independently is substantial corroboration of the viability of the conceptual model on which the numerical models are based. The numerical gridding has become more detailed for the area of the potential repository (Figure 2-16) and currently contains approximately 10,000 surface grid nodes that extend vertically through approximately 28 hydrogeologic unit layers from the land surface to the water table (Bodvarsson et al., 1998). The four most important features of the current conceptual model are (1) the existence of relatively high spatially and temporally variable infiltration rates that virtually eliminate (2) large-scale lateral diversion of water above and within the PTn and that force (3) the pervasive flow of water through fractures in densely welded tuff units, despite nonequilibrium water potentials between fractures and the adjacent matrix, and (4) vertical flow in the CHv and extensive lateral flow and perching of water at the zeolitic boundary abutting faults. This simplified version of the conceptual model is very similar to the earliest conceptual models for Yucca Mountain, particularly that of Scott et al. (1983). These four features basically control most of the remaining details of the hydrologic processes represented schematically in Figure 2-17. The infiltration processes are governed primarily by the distribution and timing of precipitation, the properties of the surface soils and bedrock, and the components controlling evapotranspiration. Average annual rates of infiltration range from 0 to more than 80 mm/yr and average approximately 5-10 mm/yr across the repository block area, or 3 to 6 percent of the average annual precipitation of 170 mm/yr. Most water that becomes net infiltration infiltrates from the ridgetops and sideslopes where fractures are present and soils are shallow to depths below the effects of evapotranspiration. Net infiltration is negligible in

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE FIGURE 2-16 Plan view of the current (1999) site-scale unsaturated zone numerical model grid. From Bodvarsson et al. (1998). deep soils because of the large storage component and evapotranspiration, except in the deep soils of channels fed by large volumes of runoff following extreme periods of precipitation. Most of the infiltrating water passes quickly through the fractures of the TCw to be slowed during transition to matrix flow in the PTn except where

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE FIGURE 2-17 Current (2000) conceptual model of flow in the unsaturated zone at Yucca Mountain. From Flint et al. (in press, Figure 4b). faults or broken zones disrupt the PTn, providing fast pathways for a small component of the flow. A small percentage of water in the TCw (less than 0.5 mm/yr) is lost to the atmosphere by way of upward vapor flow (barometric pumping; vapor diffusion; and convective, buoyancy-driven gas flow). Most flow is vertical and slow through the PTn matrix with possible local-scale lateral diversion just above the altered, nonwelded base of the Tiva Canyon Tuff, at linear contacts, or above low-permeability layers. Zones at the TCw-PTn and PTn-TSw contacts are nearly saturated but do not constitute perched layers. Water enters the TSw through faults or through localized broken zones at a low-conductivity vitrophyre at the top of the TSw and, to a lesser extent, through microfractures within the vitrophyre. The transition from highly porous tuffs to densely welded rocks occurs across a very short vertical distance, resulting in high saturations in this part of the flow system. Relatively pervasive broken-up areas that probably formed as the vitrophyre cooled provide ready access for entry of water into the underlying vapor-phase corroded nonlithophysal rocks of the TSw. Once through the PTn, the bulk of the percolating water transitions back to vertical flow through the fractures and the matrix in the upper TSw and flows dominantly through fractures in the middle of the TSw. Fracture flow apparently occurs primarily under conditions of disequilibrium with the surrounding matrix

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE when averaged for relatively large matrix blocks and may occur by channeling or focused flow. In many locations, particularly in the northern part of the site, the densely welded basal vitrophyre of the TSw coincides with the vitric-zeolitic boundary and therefore serves as a permeability barrier to vertical flow, which results in perched water at this stratigraphic location. Where the vitric-zeolitic boundary does not extend upward as far as the base of the vitrophyre, perching occurs locally at the vitric-zeolitic contact in the CHn. It is surmised that the sloping alteration boundary promotes lateral flow within the perched layers in which transport velocities may be quite high. This mechanism functions similarly in the vitric-zeolitic contacts within the Prow Pass Tuff. The role of faults in the deep unsaturated zone is not yet fully understood. In the shallow unsaturated zone, faults are highly permeable to air flow and may be major conduits for rapid water flow in the PTn. Fast flow in the TSw may be more dispersed because of its high fracture permeability. Fast-flow pathways persist through the PTn where faults disrupt the generally matrix-flow-dominated nonwelded tuffs, but water moving along such paths probably is only a small fraction of the total flux because of matrix imbibition. Fault zones may have the capacity to conduct substantial volumes of water through the entire unsaturated zone to the water table; but without a supply from lateral flow, major faults may conduct little water. Faults may be locally impermeable to lateral flow, resulting in perching, where the fault has offset a permeable bed opposite a less permeable bed. In general, faults are vertical conduits for both air and water flow. 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. The unsaturated zone is 500 m thick with the potential repository located 300 m below land surface. Precipitation ranges from less than 130 mm/yr at the lower elevations in the south to more than 280 mm/yr at the higher elevations in the north and averages 170 mm/yr across the potential repository. Yucca Mountain is in the Basin and Range physiographic province and consists of a Tertiary volcanic sequence that varies between 1 and 3 km in thickness with an unsaturated zone that varies from 500 m in thickness near the potential repository to more than 750 m thick in the north part of the study area. The sequence consists of alternating layers of welded and nonwelded tuffs ranging in age from approximately 11.6 million years to 13.5 million years. The layers have been downcut in fault-controlled washes and in erosion-controlled washes forming on uplifted fault blocks. During the last 15 years, several iterations of conceptual models for Yucca Mountain have been developed. Most of the models have the same general nature but differ significantly in detail. The most persistent concepts assumed negligible infiltration rates, extensive lateral flow in the PTn, and virtually no fracture flow

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE through the TSw (the potential repository horizon), resulting in extremely low vertical percolation rates and long (105 yr) travel times. Most of the conceptual models allowed some lateral flow in the underlying CHn for various reasons. The current conceptual model has relatively high infiltration rates (as much as 80 mm/yr in some locations), fracture-dominated flow in the TCw, vertical matrix-dominated flow in the PTn (little lateral flow), fracture-dominated flow in the TSw, vertical matrix-dominated flow in the vitric rocks of the Calico Hills and Prow Pass, and extensive lateral flow above the zeolitic boundary in those units, all of which lead to much shorter (<104 yr) travel times through the system. The faults are assumed to be permeable to water and air and to transmit bomb-pulse isotopes (fast-flow paths) through continuous, connected, fractured paths through the PTn, in which flow is otherwise slow because it is matrix-dominated. Once through the PTn, the water can continue in the fault zone or move into the fracture-dominated flow field of the TSw. Perched water bodies form when lateral flow along a zeolitic contact reaches a less permeable rock unit that has been offset by faulting. 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. During the next several years as underground testing continues and more data are collected, analysis of and insights into the mechanisms of unsaturated flow likely will lead to a better understanding of the unsaturated zone at Yucca Mountain. REFERENCES Altman, S. J., B. W. Arnold, R. W. Barnard, G. E. Barr, C. K. Ho, S. A. McKenna, and R. R. Eaton, 1996. Flow Calculations for Yucca Mountain Groundwater Travel Time (GWTT-95) . Albuquerque, N. Mex.: Sandia National Laboratories. SAND96-0819. Anderson, L. A., 1981. Rock Property Analysis of Core Samples from the Calico Hills UE25a-3 Borehole, Nevada Test Site, Nevada. Denver, Colo.: U.S. Geological Survey Open-File Report 81-1337. 30 p. Barnard, R. W., M. L. Wilson, H. A. Dockery, J. H. Ganthier, P. G. Kaplan, R. R. Eaton, F. W. Bingham, and T. H. Robey, 1992. TSPA 1991: An Initial Total-System Performance Assessment for Yucca Mountain. Albuquerque, N. Mex.: Sandia National Laboratories. SAND91-2795. Bodvarsson, G. S., and T. M. Bandurraga, eds., 1996. Development and Calibration of the Three-Dimensional Site-Scale Unsaturated-Zone Model of Yucca Mountain, Nevada-Yucca Mountain Project Milestone OBO2. Berkeley, Calif.: Lawrence Berkeley National Laboratory Report LBNL-39315. Bodvarsson, G. S., E. Sonnenthal, and Y-S. Wu., eds., 1998. Unsaturated Zone Flow and Transport Modeling of Yucca Mountain, FY98 . Berkeley, Calif.: Lawrence Berkeley National Laboratory Milestone Report SP3CKJM4. Botkin, D. B., R. A. Nisbet, S. Bicknell, C. Woodhouse, B. Bentley, and W. Ferren, 1991. Global climate change and California's natural ecosystems. In: Knox, J. B., and A. F. Scheuring, eds. Global Climate Change and California, Potential Impacts and Responses : Berkeley, Calif., University of California Press. p. 123-146.

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Buesch, D. C., R. W. Spengler, T. C. Moyer, and J. K. Geslin, 1996. Proposed Stratigraphic Nomenclature and Macroscopic Identification of Lithostratigraphic Units of the Paintbrush Group Exposed at Yucca Mountain. Nevada: U.S. Geological Survey Open-File Report 94-469. 47 p. Burger, P. A., and K. M. Scofield, 1994. Perched water occurrences at Yucca Mountain and their implications on the exploratory studies facility. Unpublished memorandum to the USGS Technical Project Officer. Byers, F. M., Jr., W. J. Carr, P. P. Orkild, W. D. Quinlivan, and K. A. Sargent, 1976. Volcanic Suites and Related Cauldrons of the Timber Mountain-Oasis Valley Caldera Complex, Southern Nevada: U.S. Geological Survey Professional Paper 919. 70 p. Carr, W. J., F. M. Byers, Jr., and P. P. Orkild, 1986. Stratigraphic and Volcano-Tectonic Relations of the Crater Flat Tuff and Some Older Volcanic Units, Nye County, Nevada: U.S. Geological Survey Professional Paper 1323. 28 p. Fabryka-Martin, J. T., A. L. Flint, D. S. Sweetkind, A. V. Wolfsberg, S. S. Levy, G. J. C. Roemer, J. L. Roach, L. E. Wolfsberg, and M. C. Duff, 1997a. Evaluation of Flow and Transport Models of Yucca Mountain, Based on Chlorine-36 and Chloride Studies for FY97. Los Alamos, N. Mex.: Los Alamos National Laboratory, Yucca Mountain Project Milestone Report SP2224M3. Fabryka-Martin, J. T., and B. Liu, 1995. Distribution of Chlorine-36 in UZ-14, UZ-16, Perched Water, and the ESF North Ramp, Yucca Mountain, Nevada. Los Alamos, N.Mex.: Los Alamos National Laboratory Milestone Report 3431. Fabryka-Martin, J. T., S. J. Wightman, W. J. Murphy, M. P. Wickham, M. W. Caffee, G. J. Nimz, J. R. Southon, and P. Sharma, 1993. Distribution of chlorine-36 in the unsaturated zone at Yucca Mountain: an indicator of fast transport paths. In: FOCUS ‘93, Site Characterization and Model Validation, Las Vegas, Nev., Sept. 26-29, 1993. Proceedings. La Grange Park, Ill.: American Nuclear Society, pp. 58-68. Fabryka-Martin, J. T., S. J. Wightman, B. A. Robinson, and E. W. Vestal, 1994. Infiltration Processes at Yucca Mountain Inferred from Chloride and Chlorine-36 Distributions. Los Alamos, N. Mex.: Los Alamos National Laboratory. Milestone Report 3417. Fabryka-Martin, J. T., A. V. Wolfsberg, P. R. Dixon, S. Levy, J. Musgrave, and H. J. Turin, 1997b. Summary report of Chlorine-36 Studies: Sampling for Chlorine-36 in the Exploratory Studies Facility. Los Alamos, N. Mex.: Los Alamos National Laboratory. Report LA-13552-MS. Fabryka-Martin, J. T., A. V. Wolfsberg, S. S. Levy, K. Campbell, P. Tseng, J. L. Roach, and L. E. Wolfsberg, 1998. Evaluation of Flow and Transport Models of Yucca Mountain, Based on Chlorine-36 and Chloride Studies for FY98. Los Alamos, N. Mex.: Los Alamos National Laboratory . Yucca Mountain Project Milestone Report SP32D5M3. Flint, A. L., and L. E. Flint, 1994. Spatial Distribution of Potential Near Surface Moisture Flux at Yucca Mountain, Nevada. In: Proceedings of the International High-Level Radioactive Waste Conference, Las Vegas, Nev., May 22-26, 1994. La Grange Park, Ill.: American Nuclear Society , pp. 2352-2358. Flint, A. L., J. A. Hevesi, and L. E. Flint, 1996a. Conceptual and Numerical Model of Infiltration for the Yucca Mountain Area, Nevada. Las Vegas, Nev.: U.S. Department of Energy Milestone Report 3GUI623M, September 1996, 210 p. Flint, L. E., 1998. Characterization of Hydrogeologic Units Using Matrix Properties. U.S. Geological Survey Water-Resources Investigation Report 96-4342, 61 p. Flint, L. E., and A. L. Flint, 1990. Preliminary Permeability and Moisture Retention of Nonwelded and Bedded Tuffs at Yucca Mountain, Nevada. U.S. Geological Survey Open-File Report 90-569, 57 p. Flint, L. E., and A. L. Flint, 1995. Shallow Infiltration Processes at Yucca Mountain, Nevada. Neutron Logging Data 1984-93. U.S. Geological Survey Water-Resources Investigations Report 95-4035, 46 p.

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Flint, L. E., A. L. Flint, and J. A. Hevesi, 1993. Shallow infiltration processes in arid watersheds at Yucca Mountain, Nevada,. In: Fifth International High Level Radioactive Waste Management Conference, Las Vegas, Nev., May 22-26. Proceedings. La Grange Park, Ill.: American Nuclear Society , pp. 2315-2322. Flint, L. E., A. L. Flint, C. A. Rautman, and J. D. Istok, 1996b. Physical and Hydrologic Properties of Rock Outcrop Samples at Yucca Mountain, Nevada. U.S. Geological Survey Open-File Report 95-280, 70 p. Flint, A. L., L. E. Flint, G. S. Bodvarsson, E. M. Kwicklis, and J. T. Fabryka-Martin, in press. The hydrology of Yucca Mountain. Reviews of Geophysics. Ganthier, J. H., M. L. Wilson, and F. C. Lauffer, 1992. Estimating the consequences of significant fracture flow at Yucca Mountain. In: Third International Conference on High-Level Radioactive Waste Management, Las Vegas, Nev. Proceedings. La Grange Park, Ill.: American Nuclear Society , pp. 727-731. Glass, R. J., 1993. Modeling gravity-driven fingering in rough-walled fractures using modified percolation theory. In: High Level Radioactive Waste Management Conference, Las Vegas, Nev. Proceedings. La Grange, Ill.: American Nuclear Society, v. 2, pp. 2042-2049. Glass, R. J., A. L. Flint, V. C. Tidwell, W. Peplinski, Y. Castro, 1994. Fracture-matrix interaction in Topopah Spring tuff: Experiment and numerical simulation. In: International High-Level Radioactive Waste Conference, Las Vegas, Nev., May 22-26. Proceedings. La Grange Park, Ill.: American Nuclear Society, 9 p. Grayson, D.K., 1993. The Desert's Past: A Natural Prehistory of the Great Basin. Washington, D.C.: Smithsonian Institution Press, 356 p. Hammermeister, D. P., D. O. Blout, and J. C. McDaniel, 1985. Drilling and coring methods that minimize the disturbance of cuttings, core and rock formation in the unsaturated zone, Yucca Mountain, Nevada, in National Water Well Association Conference on Characterization and Monitoring of the Vadose (Unsaturated) Zone. Proceedings. Denver, Colo.: National Water Well Association , p. 507-541. Hanks, T. C., I. C. Winograd, R. E. Anderson, T. E. Reilly, and E. P. Weeks, 1999. Yucca Mountain as a radioactive-waste repository at Yucca Mountain . A report to the Director: U.S. Geological Survey Circular 1184, 19 p. Hevesi, J. A., and A. L. Flint, 1996. Geostatistical Model for Estimating Precipitation and Recharge in the Yucca Mountain Region, Nevada-California. U.S. Geological Survey Water-Resources Investigations Report 96-4123. Hevesi, J. A., A. L. Flint, and J. D. Istok, 1992. Precipitation estimation in mountainous terrain using multivariate geostatistics. II. Isohyetal maps. Journal of Applied Meteorology 31(7): 677-688. Ho, C. K., 1995. Assessing alternative conceptual models of fracture flow. In: TOUGH Workshop '95, K. Pruess, ed. Proceedings. Berkeley, Calif.: Lawrence Berkeley National Laboratory, Lawrence Berkeley Laboratory Report LBL-37200. Ho, C. K., 1997. Models of fracture-matrix interactions during multiphase heat and mass flow in unsaturated fractured porous media. In: Sixth Symposium on Multiphase Transport in Porous Media. Dallas, Tex. 1997 ASME International Mechanical Engineering Congress and Exposition . Ho, C. K., S. J. Altman, and B. W. Arnold, 1995. Alternative Conceptual Models and Codes for Unsaturated Flow in Fractured Tuff: Preliminary Assessments for GWTT-95, Yucca Mountain Site Characterization Project Report. Albuquerque, N. Mex.: Sandia National Laboratories, SAND95-1456. Hudson, D. B., and A. L. Flint, 1995. Estimation of Shallow Infiltration and Presence of Fast Pathways for Shallow Infiltration in the Yucca Mountain Area, Nevada. Las Vegas, Nev.: U.S. Department of Energy Milestone Report 3GUI611M. Istok, J. D., C. A. Rautman, L. E. Flint, and A. L. Flint, 1994. Spatial variability in hydrologic properties of a volcanic tuff. Groundwater 32: 751-760.

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Klavetter, E. A., and R. R. Peters, 1986. Estimation of Hydrologic Properties of an Unsaturated Fractured Rock Mass. Albuquerque, N. Mex.: Sandia National Laboratories, SAND84-2642. Klavetter, E. A., and R. R. Peters, 1987. An Evaluation of the Use of Mercury Porosimetry in Calculating Hydrologic Properties of Tuffs from Yucca Mountain, Nevada. Albuquerque, N. Mex.: Sandia National Laboratories, SAND86-0286, 74 p. Kwicklis, E. M., and J. P. Rousseau, 1999. Analysis of percolation flux based on heat flow estimated in boreholes . In: Rousseau, J. P., E. M. Kwicklis, and D. C. Gillies, eds. Hydrogeology of the Unsaturated Zone, North Ramp Area of the Exploratory Studies Facility. Yucca Mountain, Nevada: U.S. Geological Survey Water-Resources Investigations Report 98-4050, 244 p. LeCain, G. D., 1997. Air-Injection Testing in Vertical Boreholes in Welded and Nonwelded Tuff. Yucca Mountain, Nevada: U.S. Geological Survey Water-Resources Investigations Report 96-4262, 33 p. Maxey, G. B., and T. E. Eakin, 1950. Ground Water in White River Valley, White Pine, Nye, and Lincoln Counties. Nevada. Nevada State Engineer, Water Resources Bulletin (8):59 p. Montazer, P., and W. E. Wilson, 1984. Conceptual Hydrologic Model of Flow in the Unsaturated Zone, Yucca Mountain, Nevada. U.S. Geological Survey Water-Resources Investigation Report 84-4345, 55 p. Moyer, T. C., and J. K. Geslin, 1995. Lithostratigraphy of the Calico Hills Formation and Prow Pass Tuff (Crater Flat Group) at Yucca Mountain, Nevada. U.S. Geological Survey Open-File Report 94-460, 59 p. Moyer, T. C., J. K. Geslin, and L. E. Flint, 1996. Stratigraphic Relations and Hydrologic Properties of the Paintbrush Tuff Nonwelded (PTn) Hydrologic Unit, Yucca Mountain, Nevada. U.S. Geological Survey Open-File Report 95-397, 151 p. Paces, J. B., L. A. Neymark, B. D. Marshall, J. F. Whelan, and Z. E. Peterman, 1996. Ages and Origins of Subsurface Secondary Minerals in the Exploratory Studies Facility (ESF). U.S. Geological Survey Milestone Report 3GQH450M. Patterson, G. L., 1999. Occurrences of perched water in the vicinity of the Exploratory Studies Facility North Ramp. In: Rousseau, J. P., E. M. Kwicklis, and D. C. Gillies, eds. Hydrogeology of the unsaturated zone, North Ramp area of the Exploratory Studies Facility, Yucca Mountain, Nevada. U.S. Geological Survey Water-Resources Investigations Report 98-4050, Denver, Colo., 244 p. Peters, R. R., and E. A. Klavetter, 1988. A continuum model for water movement in an unsaturated fractured rock mass. Water Resources Research 24(3):416-430. Peters, R. R., E. A. Klavetter, I. J. Hall, S. C. Blair, P. R. Heller, and G. W. Gee, 1984. Fracture and Matrix Hydrogeologic Characteristics of Tuffaceous Materials from Yucca Mountain, Nye County, Nevada. Albuquerque, N. Mex.: Sandia National Laboratories, SAND84-1471, 108 p. Pruess, K., 1998. On water seepage and fast preferential flow in heterogeneous, unsaturated rock fractures. Journal of Contaminant Hydrology 30(3-4):333-362. Pruess, K., and Y. W. Tsang, 1990. On two-phase relative permeability and capillary pressure of rough-walled rock fractures. Water Resources Research 26(9):1915-1926. Robinson, B. A., A. V. Wolfsberg, G. A. Zyvoloski, and C. W. Gable, 1995. An Unsaturated Zone Flow and Transport Model of Yucca Mountain. Los Alamos National Laboratory Yucca Mountain Project Milestone 3468. Rockhold, M. L., B. Sagar, and M. P. Connelly, 1990. Multi-dimensional modeling of unsaturated flow in the vicinity of exploratory shafts and fault zones at Yucca Mountain, Nevada. In: First International High Level Radioactive Waste Management Conference, Las Vegas, Nev. Proceedings. La Grange Park, Ill.: American Nuclear Society, p. 153-162. Roseboom, E. H., Jr., 1983. Disposal of high-level nuclear waste above the water table in arid regions. Alexandria, Va.: Geological Survey Circular 903, 21 p. Ross, B., 1990. The diversion capacity of capillary barriers. Water Resources Research 26(10):2625-2629.

OCR for page 45
CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Rousseau, J. P., E. M. Kwicklis, and D. C. Gillies, eds., 1998. Hydrogeology of the Unsaturated Zone, North Ramp Area of the Exploratory Studies Facility, Yucca Mountain, Nevada. U.S. Geological Water-Resources Investigations Report 98-4050, 244 p. Rulon, J. J., G. S. Bodvarsson, and P. Montazer, 1986. Preliminary Numerical Simulations of Groundwater Flow in the Unsaturated Zone, Yucca Mountain, Nevada. Berkeley, Calif.: Lawrence Berkeley Laboratory , LBL 20553, 91 p. Rush, F. E., W. Thordarson, and L. Bruckheimer, 1983. Geohydrologic and Drill-Hole Data for Test Well USW H-1, Adjacent to the Nevada Test Site, Nye County, Nevada. U.S. Geological Survey Open-File Report 83-141, 68 p. Sass, J. H., and A. H. Lachenbruch, 1982. Preliminary Interpretation of Thermal Data from the Nevada Test Site . U.S. Geological Survey Open-File Report 82-973, 30 p. Sawyer, D. A., R. J. Fleck, M. A. Lanphere, R. G. Warren, and D. E. Broxton, 1994. Episodic volcanism in the Miocene southwest Nevada volcanic field: Stratigraphic revisions, 40Ar/39Ar geochronologic framework, and implications for magmatic evolution . Geological Society of America Bulletin 106(10): 1304-1318. Scott, R. B., and J. Bonk, 1984. Preliminary Geologic Map of Yucca Mountain with Geologic Sections, Nye County, Nevada. U.S. Geological Survey Open-File Report 84-494. Scott, R. B., R. W. Spengler, S. Diehl, A. R. Lappin, and M. P. Chornack, 1983. Geologic character of tuffs in the unsaturated zone at Yucca Mountain, southern Nevada. In: Mercer, J. W., P. S. C. Rao, and I. W. Marine, eds. Role of the Unsaturated Zone in Radioactive and Hazardous Waste Disposal . Ann Arbor, Mich.: Ann Arbor Science, p. 289-335. Sinnock, S., Y. T. Lin, and J. P. Brannen, 1984. Preliminary Bounds on the Expected Post-Closure Performance of the Yucca Mountain Repository Site, Southern Nevada. Albuquerque, N. Mex.: Sandia National Laboratories, SAND84-3918, 83 p. Sinnock, S., Y. T. Lin, and J. P. Brannen, 1987. Preliminary bounds on the expected post-closure performance of the Yucca Mountain repository site, southern Nevada. Journal of Geophysical Research 92(B8): 7820-7842. Snyder, D. B., and W. J. Carr, 1982. Preliminary Results of Gravity Investigations at Yucca Mountain and Vicinity, Southern Nye County, Nevada. U.S. Geological Survey Open-File Report 82-701, 36 p. Striffler, P., G. O'Brien, T. Oliver, and P. A. Burger, 1996. Perched water characteristics and occurrences, Yucca Mountain, Nevada . Unpublished memorandum to the U.S. Geological Survey Yucca Mountain Project Technical Project Officer. Sweetkind, D. S., J. T. Fabryka-Martin, A. L. Flint, C. J. Potter, and S. S. Levy, 1997. Evaluation of the Structural Significance of Bomb Pulse 36Cl at Sample Locations in the Exploratory Studies Facility, Yucca Mountain, Nevada. Las Vegas, Nev.: U.S. Department of Energy Milestone Report SPG33M4. Tucci, P., and D. J. Burkhardt, 1995. Potentiometric-Surface Map, 1993, Yucca Mountain and Vicinity, Nevada . U.S. Geological Survey Water-Resources Investigations Report 95-4149. U.S. Department of Energy, 1984. Draft Environmental Assessment: Yucca Mountain Site, Nevada Research and Development Area, Nevada. Washington, D.C.: U.S. Department of Energy. U.S. Department of Energy, 1992. Report of Early Site Suitability Evaluation of the Potential Repository Site at Yucca Mountain, Nevada. Washington, D.C.: U.S. Department of Energy. Waddell, R. K., J. H. Robison, and R. K. Blankennagel, 1984. Hydrology of Yucca Mountain and Vicinity, Nevada-California: Investigative Results Through Mid-1983. U.S. Geological Survey Water-Resources Investigations Report 84-4267, 72 p. Wang, J. S. Y., and T. N. Narasimhan, 1985. Hydrologic mechanisms governing fluid flow in a partially saturated, fractured, porous medium. Water Resources Research 21(12): 1861-1874. Weeks, E. P., and W. E. Wilson, 1984. Preliminary Evaluation of Hydrologic Properties of Cores of Unsaturated Tuff, Test Well USW H-1, Yucca Mountain, Nevada. U.S. Geological Survey Water-Resources Investigations Report 84-4193, 30 p.

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Whitfield, M. S., Jr., C. M. Cope, and C. L. Loskot, 1984. Borehole and Geohydrologic Data for Test Hole USW UZ-6, Yucca Mountain Area, Nye County, Nevada. U.S. Geological Survey Open-File Report 92-28, 41 p. Wilson, M. L., 1996. Lateral diversion in the PTn unit: Capillary-barrier analysis. In: High Level Radioactive Waste Management Seventh Annual International Conference, Las Vegas, Nev., April 29-May 3. Proceedings. La Grange Park, Ill.: American Nuclear Society, SAND95-2186C, p. 111-113. Winograd, I. J., 1971. Hydrogeology of ash-flow tuff: A preliminary statement. Water Resources Research 7(4): 994-1006. Winograd, I. J., 1972. Near-surface storage of solidified high-level radioactive waste in thick (400-2,000 foot) unsaturated zones in the southwest. Geological Society of America, Abstracts with Programs 4: 708. Winograd, I. J., 1974. Radioactive waste storage in the arid zone. EOS, Transactions of the American Geophysical Union 55: 884-894. Winograd, I. J., 1981. Radioactive waste disposal in thick unsaturated zones. Science 212(4502): 1457-1464. Winograd, I. J., and W. Thordarson, 1975. Hydrogeologic and Hydrochemical Framework, South-Central Great Basin, Nevada-California, With Special Reference to the Nevada Test Site . U.S. Geological Survey Professional Paper 712-C, 126 p. Wittwer, C. S., G. S. Bodvarsson, M. P. Chornack, A. L. Flint, L. E. Flint, B. D. Lewis, R. W. Spengler, and C. A. Rautman, 1992. 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. Proceedings. La Grange Park, Ill.: American Nuclear Society , p. 263-271. Wittwer, C. S., G. Chen, G. S. Bodvarsson, M. P. Chornack, A. L. Flint, L. E. Flint, E. M. Kwicklis, and R. W. Spengler, 1995. Preliminary Development of the LBL/USGS Three-Dimensional Site-Scale Model of Yucca Mountain, Nevada. Berkeley, Calif.: Lawrence Berkeley National Laboratory , LBL-37356, UC0814. Wolfsberg, A. V., J. T. Fabryka-Martin, K. S. Campbell, S. S. Levy, and P. H. Tseng, 1999. Use of chlorine-36 and chloride data to evaluate fracture flow and transport models at Yucca Mountain. In: International Symposium: Dynamics of Fluids in Fractured Rocks: Concepts and Recent Advances, in Honor of Paul A. Witherspoon, February 10-12, 1999. Proceedings. Berkeley, Calif.: Lawrence Berkeley National Laboratory. Wu, Y. S., S. Finsterle, and K. Pruess, 1996. Computer models and their development for the unsaturated zone model at Yucca Mountain, Chapter 4 in Development and Calibration of the Three-Dimensional Site-Scale Unsaturated-Zone Model of Yucca Mountain, Nevada. G. S. Bodvarsson and T. M. Bandurraga, eds. Berkeley, Calif.: Lawrence Berkeley National Laboratory Milestone OBO2, MOY-970317-04. Yang, I. C., G. W. Rattray, and P. Yu, 1996. Interpretations of Chemical and Isotopic Data from Boreholes in the Unsaturated-Zone at Yucca Mountain, Nevada. U.S. Geological Survey Water-Resources Investigations Report 96-4058. Yang, I. C., P. Yu, G. W. Rattray, and D. C. Thorstenson, 1998. Hydrogeochemical Investigations and Geochemical Modeling in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada. U.S. Geological Survey Water-Resources Investigation Report 98-4132.

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