Summary of Groundwater Flow Models of the 

Death Valley Regional Aquifer System and 

Yucca Mountain Area 

Prepared for 

U.S. Nuclear Regulatory Commission 

Contract NRC–02–07–006 

Prepared by 

J. Winterle 

L. Gergen 

Center for Nuclear Waste Regulatory Analyses 

San Antonio, Texas 

September 2011 
ii 

ABSTRACT 

This knowledge capture report provides a brief summary of numerical groundwater models of 

the Death Valley Regional Groundwater Flow System (DVRFS) and the Yucca Mountain area. 

The main focus of the report is on the Death Valley Regional Groundwater Flow Model 

(DVRGFM), developed by the U.S. Geological Survey in cooperation with the U.S. Department 

of Energy (DOE) to support the DOE characterization of Yucca Mountain as a proposed 

repository for high-level radioactive waste. The DVRGFM is important in that it describes the 

regional hydrogeologic setting in which the Yucca Mountain area resides. The second focus of 

this report is to provide a short bibliography of smaller scale groundwater models and modeling 

studies in and near the Yucca Mountain area. This bibliography is not intended to be 

exhaustive and judgment was used in deciding which studies are most relevant. The intent is 

that the included bibliographic summaries would provide a qualified hydrogeologist with a useful 

starting point for understanding the current state of knowledge about the Yucca Mountain 

groundwater flow system. 
iii 

CONTENTS 

Section Page 

ABSTRACT ................................................................................................................................... ii 

FIGURES ..................................................................................................................................... iv 

ACKNOWLEDGMENTS ............................................................................................................... v 

1 INTRODUCTION ............................................................................................................ 1-1

2 THE DEATH VALLEY REGIONAL GROUNDWATER FLOW MODEL .......................... 2-1 

 2.1 Physiography and Geology of the Death Valley Region..................................... 2-1 

 2.2 Model History ..................................................................................................... 2-2 

 2.3 Insights on Groundwater Flow Patterns and Directions in the 

 Yucca Mountain Area Gained From Results of the Death Valley 

 Regional Groundwater Flow Model .................................................................... 2-8 

 2.4 Inputs to the DOE Site-Scale Saturated Zone Flow Model for Yucca Mountain 

 Derived From the Death Valley Regional Groundwater Flow Model ................ 2-14 

3 MODELS OF THE YUCCA MOUNTAIN AREA ............................................................. 3-1 

4 REFERENCES ............................................................................................................... 4-1 
iv 

FIGURES 

Figure Page 

2-1 Schematic Block Diagram of Death Valley and Other Basins Illustrating the 

 Structural Relations Between Mountain Blocks, Valleys, and Groundwater Flow .......... 2-1 

2-2 Recharge Simulated in the Death Valley Regional Flow System Model ........................ 2-5 

2-3 Groundwater Discharge Areas in the Death Valley Regional Flow System ................... 2-6 

2-4 Pumping Locations in the Belcher (2004) Death Valley Regional Groundwater 

 Flow Model ..................................................................................................................... 2-9 

2-5 Interpreted Subregions, Groundwater Basins, and Associated Flow Paths of the 

 Death Valley Regional Groundwater Flow System ...................................................... 2-10 

2-6 Central Death Valley Subregion of the Death Valley Regional Groundwater Flow 

 System Showing Groundwater Basins, Sections, and Flow Directions ........................ 2-11 

2-7 Geographic and Prominent Topographic Features of the Death Valley Regional 

 Groundwater Flow System Region, Nevada and California ......................................... 2-13 

2-8 Map Showing the Boundaries of Regional and Site-Scale Models .............................. 2-15 
v 

ACKNOWLEDGMENTS 

This report was prepared to document work performed by the Center for Nuclear Waste 

Regulatory Analyses (CNWRA®) for the U.S. Nuclear Regulatory Commission (USNRC) under 

Contract No. NRC–02–07–006. The activities reported here were performed on behalf of the 

USNRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste 

Repository Safety. The report is an independent product of the CNWRA and does not 

necessarily reflect the views or regulatory position of the USNRC. 

The authors would like to thank Gordon Wittmeyer for technical review and E. Pearcy for 

programmatic and editorial reviews. The authors also appreciate L. Selvey for providing word 

processing support in preparation of this document. 

QUALITY OF DATA, ANALYSES, AND CODE DEVELOPMENT 

DATA: No original data were generated for this report. 

ANALYSES AND CODES: No technical analyses or codes are presented or used in this report. 
1-1 

1 INTRODUCTION 

This report provides a brief summary of numerical groundwater models of the Death Valley 

Regional Groundwater Flow System (DVRFS) model since 1995. A previous summary of 

models within the DVRFS was completed by Wittmeyer and Turner (1995). Since that time, 

however, a three-dimensional regional model of the entire DVRFS has been developed and has 

gone through several updates and improvements. Additionally, site characterization in support 

of the Yucca Mountain Project has spurred the generation of several smaller scale groundwater 

models within the DVRFS. 

The purpose of this knowledge capture report is to consolidate in one place a summary of the 

groundwater models created since 1995 that are most relevant to Yucca Mountain. The main 

focus of the report is on the Death Valley Regional Groundwater Flow Model (DVRGFM), 

developed by the U.S. Geological Survey (USGS) in cooperation with the U.S. Department of 

Energy (DOE) to support the DOE characterization of Yucca Mountain as a proposed repository 

for high-level radioactive waste. The DVRGFM is important in that it describes the regional 

hydrogeologic setting in which the Yucca Mountain area resides. Accordingly section 2 of this 

report provides a summary of the development DVRGFM and how it was used to support the 

DOE characterization of the Yucca Mountain site. 

A second focus of this report is to provide, in Section 3, a short bibliography of smaller scale 

groundwater models and modeling studies within the DVRFS that provide useful information 

and context for understanding groundwater flow in and around the Yucca Mountain area. This 

bibliography is not intended to be exhaustive and judgment was used in deciding which studies 

are most relevant. The intent is that the included bibliographic summaries will supplement the 

report by Wittmeyer and Turner (1995) to provide a useful starting point for understanding the 

Yucca Mountain groundwater flow system. 
2-1 

2 THE DEATH VALLEY REGIONAL GROUNDWATER FLOW MODEL 

2.1 Physiography and Geology of the Death Valley Region 

The Great Basin, 500,000 km2

 [200,000 mi2] and part of the Basin and Range physiographic 

region, is characterized by arid climates and internal drainage of surface water and ground 

water (Mifflin, 1988). The Death Valley regional flow system lies within the carbonate-rock 

province of the southern Great Basin. A deep regional aquifer formed of thick sequences of 

highly permeable Paleozoic carbonate rocks, fosters interbasinal groundwater flow between 

topographically closed structural basins (DOE, 2008a; Wittmeyer and Turner, 1995). As 

illustrated schematically in Figure 2-1, regional groundwater flow in the DVRFS through the 

Paleozoic carbonate rock sequence is affected by complex geologic structures from regional 

faulting and fracturing that can enhance or impede flow (DOE, 2008a). Permeable Cenozoic 

basin fill throughout the DVRFS, and fractured Cenozoic volcanic rocks near the Nevada Test 

Site (NTS) form locally important aquifers that interact with regional flow through the underlying 

Paleozoic carbonate rocks (Sweetkind, et al., 2004; Winograd and Thordarson, 1975). Primary 

regional confining units include Proterozoic to Lower Cambrian metamorphic and siliciclastic 

rocks and Paleozoic siliciclastic rocks. Local confining units are composed of fine-grained 

Cenozoic basin fill, and zeolitically altered and nonwelded Cenozoic tuffs (Winograd and 

Thordarson, 1975). 

Figure 2-1. Schematic Block Diagram of Death Valley and Other Basins Illustrating the 

Structural Relations Between Mountain Blocks, Valleys, and Groundwater Flow 

(Faunt, et al., 2004a, Figure D–1) 
2-2 

2.2 Model History 

This section provides a summary of the evolution of the DVRGFM and how the different 

model versions and model analyses may have been used in the Yucca Mountain site 

characterization effort. 

D’Agnese, et al. (1997)

USGS developed the DVRGFM in cooperation with DOE to support the DOE evaluation 

of Yucca Mountain as a proposed repository for high-level radioactive waste (D’Agnese, 

et al., 1997). The model covers much of the approximately 100,000 km2

 [39,000 mi2] area 

of the DVRFS in Nevada and California. The initial three-dimensional steady-state model 

was developed by D'Agnese, et al. (1997) using a computational grid with three vertical 

layers and a lateral grid composed of 1,500-m [4,900 ft] square cells. The model was 

constructed and calibrated using the numerical flow model MODFLOWP (Hill, 1992) to 

simulate present-day conditions. 

Calibration included adjusting (i) the location and type of flow system boundary conditions, 

(ii) the extent of recharge areas, and (iii) the arrangement of hydrogeologic framework features. 

Spring flow and water level data were used as calibration targets. During model calibration, the 

original single recharge parameter was reassigned into four zones with percentages of 

infiltrating average annual precipitation designated. Additionally, although the initial model 

included only four hydraulic conductivity zones, refinement was necessary to better represent 

geologic structures and lithology, resulting in assignment of nine hydraulic conductivity zones. 

This 1997 version of the model provided a framework for understanding regional flow patterns 

within the DVRFS. Calculated flow rates from the DVRFS model provided estimated flow rates 

that were used to aid calibration of the smaller-scale DOE site-scale saturated-zone flow model 

of the Yucca Mountain region (BSC, 2004a). 

D’Agnese, et al. (1999)

The 1997 steady-state model provided a starting point for D’Agnese, et al. (1999) to assess the 

effects of climate change on the DVRFS. The purpose of this model was to understand how 

potential future climate conditions might influence groundwater flow rates that, in turn, may 

affect radionuclide transport from the proposed repository to the accessible environment. Past 

(full-glacial) and future (global-warming) climate scenarios were evaluated. Simulated results of 

past climatic conditions 21,000 years before present were compared to observations of 

paleodischarge sites. Past (full-glacial) climate conditions were wetter and cooler than today, 

with lakes, perennial streams, and large wetlands common. In general, the present-day model 

parameters and boundaries (D’Agnese, et al., 1997) were used for modeling the past climate 

conditions with the following changes: 

1. Constant-head cells were used to simulate paleolakes present 21,000 years ago. 

2. Constant-head cells along the northern and northeastern edge of the lateral model 

boundaries were assigned to all 3 layers in the past model, as opposed to only layer 3 in 

the present-day model. This allowed for potentially larger underflow of groundwater into 

the model domain. 

3. Recharge rates were increased to reflect glacial climate conditions. 
2-3 

4. Recharge distribution was changed because of different precipitation patterns. 

5. The drain package was used to convert evapotranspiration areas into wetlands, and to 

simulate surface runoff in mountain-top areas where potential recharge water exceeds 

hydraulic conductivity. 

6. Pumping from wells was eliminated. 

In mountain ranges with low hydraulic-conductivity units, the increased recharge exceeded the 

hydraulic conductivity of the formation; in those cases, the excess portion of recharge was 

assumed to become surface runoff, referred to by D’Agnese, et al. (1999) as “rejected 

recharge.” For the wetter past-climate scenario, simulated recharge increased 5.4 times over 

the model domain and 7 percent of the assigned recharge was rejected. This model produced a 

similarly shaped but higher regional potentiometric surface, with more pronounced large 

hydraulic gradients. Groundwater discharge was predicted at most of the observed 

paleodischarge sites, indicating simulated recharge distributions were reasonable. Water levels 

were 60–150 m [200–490 feet] higher beneath Yucca Mountain. The proposed repository level 

is located 200–400 m [655–1,310 ft] above the present day water table and is a minimum of 

50 m [165 ft] above the past-climate simulation water table elevation. 

Future (global warming) climate conditions represent an anthropogenic “worst-case” scenario 

with a doubling of atmospheric carbon dioxide due to burning of all estimated fossil-fuel 

reserves (D’Agnese, et al., 1999). The warming condition had less precipitation than the 

past-climate (glacial) scenario resulting in generally less recharge input to the model. The 

present-day model parameters and boundaries (D’Agnese, et al., 1997) were used in the 

future-climate simulations, except as noted below: 

1. Constant-head cells were assigned to all three layers at lateral boundary areas where 

interbasinal flow was assumed to occur, as opposed to only layer 3 in the present-day 

model. This allowed for potentially larger underflow of groundwater into the model domain. 

Note, however, that most of the lateral boundaries were treated as no-flow to represent the 

predominantly closed-basin characteristics of the DVRFS. 

2. Total recharge was greater than for present-day simulations. Recharge rates were 

increased for higher altitude areas, and less than or equal to present-day for the rest 

of the model. 

3. Recharge distribution was changed because of different precipitation patterns. A much 

larger portion had zero recharge compared to the past-climate simulation. 

4. The drain package was used to convert evapotranspiration areas into wetlands, and to 

simulate surface runoff in mountain tops where recharge exceeds hydraulic conductivity. 

5. Future pumping remained constant at present-day rates. 

Under future-climate conditions, simulated recharge increased 1.8 times over parts of the 

model domain, or remained constant or even decreased in other parts of the domain, relative 

to present day. In mountain-top areas where recharge exceeded hydraulic conductivity, 

1.2 percent of recharge was rejected, representing surface runoff. This increase in recharge 

resulted in only slight changes in the configuration of the potentiometric surface. Large 

hydraulic gradients observed in present day were maintained or enhanced in some areas. 
2-4 

Simulated water levels were less than 50 m [165 ft] higher beneath Yucca Mountain. The 

proposed repository horizon is 200–400 m [655–1,310 ft] above the present-day water table and 

at least 150 m [490 ft] above the future climate (global warming) scenario water-table elevation. 

This 1999 modeling analysis provided the basis for estimating the increase in water flow rates in 

the Yucca Mountain area under future wetter climatic conditions, the change in upward 

hydraulic gradients between the underlying regional Paleozoic carbonate aquifer and the 

overlying volcanic rocks, and simulated the increased recharge under glacial-transition climates 

(DOE, 2008b). 

D’Agnese, et al. (2002)

Following development of the D’Agnese, et al. (1997) model, the DOE [Office of Environmental 

Restoration and Waste Management, and National Nuclear Security Administration (NNSA)] 

requested the USGS to develop a steady-state model to determine predevelopment conditions 

of the DVRFS. The conceptual model and geologic interpretations from the 1997 and 1999 

models were incorporated into the 2002 model (D’Agnese, et al., 2002) and the model was 

updated to use MODFLOW-2000 (Harbaugh, et al., 2000). The 2002 model was designed to 

(i) provide the foundation and boundary conditions for the site-scale models at Yucca Mountain 

and the Underground Test Areas on the NTS, (ii) define regional three-dimensional groundwater 

flow paths, (iii) determine discharge and recharge locations, (iv) estimate magnitude of 

subsurface flux, and (v) account for the effect of regional geologic structural features on 

regional flow. 

A net infiltration model was developed to update estimates of regional ground-water recharge. 

All water that infiltrates past the “root zone” was assumed to eventually become groundwater 

recharge. Precipitation on the higher mountains in the region is the main source of recharge to 

the regional groundwater flow system (Figure 2-2). Interbasin flow from outside of the DVRFS 

also contributes to recharge, as well as recycled irrigation and domestic waters, and spring 

discharge seepage. 

Groundwater discharge occurs locally and is mainly through evapotranspiration due to the arid 

climate in the DVRFS (Figure 2-3). Water stored in shallow unconfined aquifers accessible to 

vegetation and at the water table is connected to the atmosphere. Most of the water 

evapotranspiring in these discharge zones originates from spring or seep flow or as diffuse 

upflow from lower regional confined aquifers. 

New techniques available in MODFLOW-2000 allowed for improved calibration with observed 

hydraulic heads and spring flows. During calibration, four types of conceptual models were 

analyzed using the regression methods in MODFLOW-2000. These include modifications to 

(i) the location and type of flow-system boundary conditions, (ii) the delineation of discharge 

areas, (iii) the definition of recharge areas, and (iv) the configuration of hydrogeologic 

framework features. For each change, a new set of parameters was estimated employing 

MODFLOW-2000, and simulated heads and groundwater discharges were compared with 

observed values. Conceptual model changes with >10 percent improvement in model fit, as 

demonstrated by a reduction in the sum of squared errors, were incorporated into the final 

model. Refinements to the hydrogeologic framework interpretation did the most to improve the 

numerical model fit. 
2-5 

Figure 2-2. Recharge Simulated in the Death Valley Regional Flow System Model 

(D’Agnese, et al., 2002, Figure 21) 
2-6 

Figure 2-3. Groundwater Discharge Areas in the Death Valley Regional Flow System 

(D’Agnese, et al., 2002, Figure 13) 
2-7 

The steady-state prepumping DVRFS model was deemed by USGS to be an acceptable 

representation of the prepumping conditions for the DVRFS and an improvement over previous 

models (Belcher, et al., 2004). Uncertainties and errors are a consequence of (i) the quality of 

interpretation and representation of flow-model observations, (ii) geometry and spatial variability 

of hydrogeologic materials and structures in the hydrogeologic-framework and groundwater flow 

models, and (iii) the physical framework and the hydrologic conditions in the flow model 

(D’Agnese, et al., 2002). Belcher, et al. (2004) noted that some uncertainty also arises from 

treating the flow system as confined in locations where an unconfined water table is predicted. 

Belcher (2004)

In 2004, the USGS merged an IT Corporation (1996) model (designated DOE/NV-UGTA) with 

the 1997 ground-water flow model to produce a transient flow model of the Death Valley 

regional ground-water flow system (Belcher, 2004) for DOE (Office of Environmental 

Management, NNSA). The DOE/NV–UGTA flow model was used to evaluate the transport of 

radionuclides from Nevada Test Site underground nuclear weapons tests. The purpose of 

Belcher’s (2004) model was to assimilate and reevaluate decades of groundwater flow system 

study and previous groundwater flow models and incorporate new data to develop and calibrate 

a more complex, transient model simulation. The intended uses of the model were to provide 

boundary conditions for site-scale models; assess the consequences of changes in system flux; 

provide a technical basis for decisions on the quantity of water available for development 

activities; determine source plumes, ambient trend, and point-of-use ground-water quality 

monitoring locations; and help initiate a regional Death Valley ground-water management district 

(Belcher, et al., 2004). Belcher (2004) contains several chapters by other authors, which are 

referred to individually throughout this report. 

MODFLOW-2000 (Harbaugh et al., 2000) was again used in this model. Updates included use 

of a three-dimensional digital hydrogeologic framework model (HFM) developed from digital 

elevation models, geologic maps, borehole information, geologic and hydrogeologic cross 

sections, faults and fractures, and other three-dimensional models to represent the geometry of 

27 different hydrogeologic units (HGUs) (Sweetkind, et al., 2004). The detail afforded by the 

HFM and by MODFLOW-2000 improved spatial accuracy for every model parameter. To further 

enhance accuracy, additional data were collected on the lithostratigraphy and structural effects 

of the hydrogeologic framework and recharge and discharge estimates. Boundary inflow and 

outflow estimates also were reevaluated (Belcher, 2004). 

The 2004 model was cited in the DOE license application for Yucca Mountain (DOE, 2008b) 

and provided recharge and discharge boundary conditions (locations where water enters or 

leaves the domain of the Yucca Mountain site-scale model). These estimates of underflow were 

used to calibrate the Yucca Mountain site-scale model lateral boundary inflows and outflows to 

the flows predicted in the regional model. In addition, the site-scale saturated-zone flow model 

(Sandia National Laboratories, 2007a) uses 23 of the 27 hydrostratigraphic units identified in the 

DVRGFM hydrogeologic framework model. The 2004 model update, while more complex than 

the previous version, did not result in significant changes to the input, calibration, or output 

relating to the previous Yucca Mountain site-scale model (BSC, 2004a). 

The model was first calibrated to prepumped steady-state flow conditions. The calibrated 

steady-state model then provided the initial conditions for the transient-flow model, which 

was then calibrated to simulate transient-flow conditions that included pumping for the years 

1913–1998. 
2-8 

The largest pumping center included in the model is in Pahrump Valley, in the southern 

Death Valley subregion, with as many as 7,876 wells pumping 2.21 × 109

 m3 [7.80 × 1010 ft3] 

from 1913 through 1998 (Figure 2-4). The central Death Valley subregion has a total of 

675 wells pumping 1.06 × 109

 m3 [3.74 × 1010 ft3], focused in the Alkali Flat- Furnace Creek 

groundwater basin, Ash Meadows groundwater basin, and Pahute Mesa-Oasis Valley 

groundwater basin. The northern Death Valley subregion only has 16 wells pumping 

1.11 × 106

 m3 [3.92 × 107 ft3] (Faunt, et al., 2004b). 

Faunt, et al. (2004b) concluded that the three-dimensional model simulated observed values 

reasonably well with downward hydraulic gradients in recharge areas and upward hydraulic 

gradients in discharge areas. Some model limitations recognized by Faunt, et al. (2004b) 

include (i) inadequacies or inaccuracies in observations used in the model (i.e., clustering 

causing overemphasis of observations in isolated areas, and difficulty assessing prepumping 

conditions for steady-state calibration in agricultural areas with pumping wells); (ii) the 

hydrogeologic interpretation (geometric complexity of hydrogeologic materials and structures 

simplified in HGUs result in the removal of the effects of small-scale variability); and (iii) the 

scale represented by observations in the flow model compared to the 1,500-m [4,920-ft] 

resolution of the flow model grid. Faunt, et al. (2004b) also noted that the net-infiltration model 

may overestimate recharge somewhat because water passing the root zone is assumed to 

reach the water table and is not able to be diverted, perched by a lower permeability layer in the 

unsaturated zone, or evaporated from deep within the unsaturated zone. Other reasons for 

overestimating recharge include: (i) lateral-flow boundary conditions are poorly understood 

due to lack of data, (ii) use of the Drain package to simulate discharge attributed to 

evapotranspiration could be improved by using a more accurate evapotranspiration package, 

and (iii) seasonal irrigation pumpage could be accounted for by decreasing the length of the 

year-long stress periods (Faunt, et al., 2004b). 

Belcher and Sweetkind (2010)

Belcher and Sweetkind (2010) formalizes the material originally presented in Belcher 

(2004) with errors and inconsistencies within and between chapters corrected and the 

clarity and accuracy of a number of the figures improved. Other than these relatively minor 

changes, it is essentially the same as document as Belcher (2004). Note that DOE cites the 

Belcher (2004) version of the document in their Yucca Mountain Repository License Application 

(DOE, 2008a,b). 

2.3 Insights on Groundwater Flow Patterns and Directions in the 

Yucca Mountain Area Gained From Results of the Death Valley 

Regional Groundwater Flow Model 

The DVRFS is a major regional flow system in which groundwater flows between recharge 

areas in the mountains of central and southern Nevada and discharge areas of wet playas and 

springs, south and west of the NTS and in Death Valley. The DVRFS model domain is divided 

into northern, central and southern Death Valley subregions, as shown in Figure 2-5. 

The central subregion is divided into the Pahute Mesa-Oasis Valley, Ash Meadows, and 

Alkali Flat-Furnace Creek groundwater basins, as shown in Figure 2-6. The Alkali Flat-Furnace 

Creek basin is further divided into the Fortymile Canyon, Amargosa River, Crater Flat, and 

Funeral Mountains sections. Yucca Mountain is located in the central Death Valley subregion, 

the Alkali Flat- Furnace Creek basin, and borders the Fortymile Canyon and Crater Flat 

sections. The boundaries of these subregions, basins, and sections do not define independent 
2-9 

Figure 2-4. Pumping Locations in the Belcher (2004) Death Valley Regional Groundwater 

Flow Model (Faunt, et al., 2004b, Figure F–9) 
2-10 

Figure 2-5. Interpreted Subregions, Groundwater Basins, and Associated Flow Paths of 

the Death Valley Regional Groundwater Flow System (D’Agnese, et al., 2002, Figure 7)
2-11 

Figure 2-6. Central Death Valley Subregion of the Death Valley Regional Groundwater 

Flow System Showing Groundwater Basins, Sections, and Flow Directions 

(Faunt, et al., 2004a, Figure D-7)

flow systems and were based on location of recharge areas; regional hydraulic gradients; 

distribution of aquifers, structures, and confining units; location of major discharge areas, and 

hydrochemical composition of groundwater (Faunt, et al., 2004a). 
2-12 

The Alkali Flat-Furnace Creek basin includes Fortymile Canyon, the Amargosa River, Crater 

Flat, and Funeral Mountains. Groundwater originates as recharge on Pahute Mesa, Timber and 

Shoshone Mountains, and the Grapevine and Funeral Mountains, and as throughflow from 

Sarcobatus Flat, Oasis Valley, and Ash Meadows (Figures 2-6 and 2-7). Groundwater flows 

through volcanic-rock aquifers in the north and basin-fill and carbonate-rock aquifers in the 

south toward discharge areas in the southern and southwestern parts of the basin (Faunt, et al., 

2004a). Discharge is mainly through the springs in the Furnace Creek area, with some water 

discharging at Ash Meadows springs (Faunt, et al., 2004a). Furnace Creek and Ash Meadows 

springs have similar hydrochemistry suggesting a hydraulic connection between these areas 

(Faunt, et al., 2004a), or perhaps a similar source region. Spring discharge is via large-scale 

fractures or channels in the carbonate-rock aquifer (Winograd and Pearson, 1976). Farther 

downgradient, groundwater, including reinfiltrated spring flow, is either transpired by mesquite 

on the lower part of the Furnace Creek fan, or is evaporated from the saltpan in Badwater Basin 

(Faunt, et al., 2004a), the lowest elevation in North America at 86 m [282 feet] below sea level. 

The following four paragraphs are summarized from Faunt, et al. (2004a). The Fortymile 

Canyon and Fortymile Wash section is recharged predominantly by throughflow from volcanic 

rocks of eastern Pahute Mesa and western Rainier Mesa. Another source of local recharge is 

infiltration of surface runoff in the alluvium of Fortymile Canyon and Fortymile Wash during 

moderate to intense precipitation. Groundwater flows south from eastern Pahute Mesa and 

western Rainier Mesa. There are insufficient data to evaluate whether flow continues south 

beneath Timber Mountain or is redirected toward Shoshone Mountain, Yucca Mountain, and 

Jackass Flats. At Yucca Mountain, hydraulic gradients are directed upward in the volcanic-rock 

units from the carbonate aquifer. Flow continues south as throughflow from Fortymile Wash into 

the Amargosa River section. 

An important source of recharge to the Amargosa River section is from throughflow in basin-fill 

sediments from the Oasis Valley, Crater Flat, Fortymile Canyon and Wash, and Specter Range 

sections. The carbonate-rock aquifer is recharged by throughflow from the Specter Range and 

Fortymile Canyon and Wash sections. In the northwest Amargosa River section, groundwater 

flow is primarily lateral and downward toward the regional flow system. In the south-central 

portion of the basin, regional groundwater moves upward from the carbonate aquifer into the 

intermediate system and toward discharge areas along the Amargosa River, Carson Slough, 

and Alkali Flat. Water flows from the carbonate aquifer to the north and northeast and from the 

volcanic-rock aquifers to the north and northwest toward the Amargosa Desert. Groundwater in 

the Amargosa Desert is of mixed chemical composition and contains a large amount of sodium. 

In the southern Amargosa Desert, hydraulic and hydrochemical data suggest that regional flow 

is either (i) to the southwest through fractures in southeastern Funeral Mountains toward Death 

Valley or (ii) to the south diverted by low-permeability quartzites of the Resting Spring Range 

toward the surface at Alkali Flat. 

Recharge to the Funeral Mountains section is mainly from throughflow in the carbonate 

aquifer in the southern Funeral Mountains, with some additional throughflow from Panamint 

Valley and the Owlshead Mountains. Local precipitation in the Panamint Range, Black 

Mountains, Funeral Mountains, and Greenwater Range supports mountain-front recharge as 

surface water infiltrates alluvial fans circling the floor of Death Valley. The Furnace Creek area 

of the Alkali Flat-Furnace Creek basin is the predominant discharge area in the Funeral 

Mountains section. 
2-13 

Figure 2-7. Geographic and Prominent Topographic Features of the Death Valley 

Regional Groundwater Flow System Region, Nevada and California 

(Belcher, et al., 2004, Figure A–1)
2-14 

Based on the modeled directions of flow in the DVRFS, groundwater passing beneath 

Yucca Mountain under current conditions is most likely captured by agricultural pumping in the 

northern Amargosa Desert. In the absence of agricultural pumping, it is not clear whether water 

passing beneath Yucca Mountain would flow to the Ash Meadows area or the Badwater Basin 

area, as the hydraulic gradient in the southern Amargosa Desert regions is nearly flat and may 

be influenced by groundwater pumping. Pumping in the western Amargosa Desert for 

domestic, agricultural, recreational, and municipal purposes has resulted in local water level 

declines, and withdrawal for mining operations south of Beatty has lowered water levels in the 

northwestern Amargosa Desert (Faunt, et al., 2004a). 

2.4 Inputs to the DOE Site-Scale Saturated Zone Flow Model for 

 Yucca Mountain Derived From the Death Valley Regional 

 Groundwater Flow Model 

The DVRGFM model provides the hydrogeological setting for the Yucca Mountain site-scale 

saturated-zone flow model. The location of the Yucca Mountain model within the DVRFS model 

boundaries is shown in Figure 2-8. The 1997 (D’Agnese, et al., 1997), 2002 (D’Agnese, et al., 

2002), and 2004 (Belcher, et al., 2004) versions of the DVRFS model supplied recharge 

boundary conditions to successive versions of the Yucca Mountain site-scale flow models. 

Distributed recharge from the DVRFS models was used for all areas of the Yucca Mountain 

site-scale model domain except for (i) the area beneath the unsaturated zone site-scale flow 

model (Wu, et al., 1997; BSC, 2004b) and (ii) the area along Fortymile Wash. 

The 1997 DVRGFM lateral boundary fluxes were used as calibration targets in the initial 

base-case calibration of the Yucca Mountain model. Flow rates from the 2002 DVRGFM model 

were used in an alternate site-scale model to evaluate sensitivity to alternative interpretations 

(BSC, 2004a). Comparison of the Yucca Mountain site-scale model boundary flows calibrated 

to the 1997 versus the 2002 DVRGFM models indicated that the differences were sufficient to 

cause some differences in the resulting saturated zone flow fields for Yucca Mountain 

(BSC, 2004a). The main difference was that the alternative model calibrated to the 2002 

DVRGFM targets had a relatively small net outflow from the western vertical model boundary, 

compared to a small net inflow for the model calibrated to the 1997 DVRGFM targets. 

For the updated version of the Yucca Mountain site-scale saturated zone flow model used to 

support the DOE license application (Sandia National Laboratories, 2007a), the prepumped, 

steady-state boundary flows from the Belcher (2004) version of the DVRGFM were used as 

calibration. Although these boundary flux calibration targets do not include effects of current 

pumping, BSC (2004a) explains that the potentiometric data to which the site-scale model was 

calibrated represent the effects of drawdown due to pumping; the net effect of the lateral 

boundary flow targets is minor and local to the southwest corner of the base-case saturated 

zone site-scale model; and the modeled flow paths emanating from the proposed repository are 

virtually unchanged as a result of this calibration target. 
2-15 

Figure 2-8. Map Showing the Boundaries of Regional and Site-Scale Models. 

NOTE: HFM = Hydrogeologic Framework Model (DOE, 2008b, Figure 2.3.9-3). 
3-1 

3 MODELS OF THE YUCCA MOUNTAIN AREA 

The following paragraphs provide brief summaries of numerical groundwater flow models for the 

Yucca Mountain area and some relevant supporting studies. This set of 10 bibliographic 

summaries is not intended to be exhaustive as there are hundreds, perhaps thousands, of 

publications available that document various aspects of the saturated zone flow system at the 

Yucca Mountain site. Rather, this section provides a list of a few key documents that chronicle 

the current understanding of the saturated zone flow system and would provide a useful starting 

point for a qualified hydrogeologist attempting to become familiar with the topic in the context of 

geologic disposal of nuclear waste. 

Winterle, et al. (2002) documented the development of a three-dimensional, site-scale 

groundwater flow model for the saturated zone beneath Yucca Mountain, Nevada. The 

30-layer model grid covers a 28 × 41-km [17.4 × 25.5-mi] area surrounding Yucca Mountain 

and explicitly includes 6 hydrostratigraphic layers and 13 structural features. The model 

was developed for use with the MODFLOW code using the Groundwater Modeling System, 

Version 3.1, user interface. Model calibrations to observed hydraulic heads were conducted 

by varying hydraulic conductivity values assigned to the model layers and structural features. 

This model is similar in scale to the DOE model used to support the Yucca Mountain license 

application (Sandia National Laboratories, 2007a), and the same potentiometric data were 

used in the calibration. The main difference from the DOE model is the assignment of 

hydrogeologic features, which was based on an independent interpretation of hydrostratigraphy 

and structure in the region. Results of the Winterle, et al. (2002) model indicate that a 

reasonable model calibration requires consideration of geologic structure and some type of 

barrier to lateral flow in the northern portion of the model and along the Solitario Canyon fault 

zone immediately west of Yucca Mountain. This conclusion is generally consistent with the 

DOE model, which also implemented low-permeability features to the north and west of the 

proposed repository footprint. 

Winterle (2003) used the model by Winterle, et al. (2002) to evaluate the effects of conceptual 

model uncertainty regarding recharge and boundary conditions on calculations of saturated 

zone flow paths, groundwater travel times, and potential magnitudes of water table rise during 

future climate conditions. Four modeling case studies were presented. The results indicate that 

consideration of recharge over the proposed repository area can result in an order of magnitude 

decrease in calculated groundwater travel times for the effective porosities assumed for these 

modeling studies by driving flow paths deeper through more permeable pathways. Increased 

recharge in the model area north of Yucca Mountain and water table rise that may accompany 

a future wetter climate and the addition of recharge in the Fortymile Wash area did not 

substantially affect flow path or travel time calculations in the case studies presented in 

this report. 

Winterle, et al. (2003) evaluated the effect of improved model calibration on the prediction of 

flow paths away from Yucca Mountain. The geometries and hydraulic conductivities of fault 

zones and HGUs were adjusted from the initial Winterle, et al. (2002) model to achieve a 

reduction in root-mean-square calibration error from 27 m [89 ft] in the initial model to only 

1.1 m [3.6 ft] in the adjusted model. A comparison of results for the two models indicated that 

predicted flow paths and mean groundwater travel times were not significantly changed by the 

improved calibration, although the variance in groundwater travel times was somewhat less for 

the adjusted model. 
3-2 

Painter, et al. (2003) investigated a thermal anomaly pattern near the Paintbrush fault. Ground 

water temperatures in the fractured volcanic aquifer near Yucca Mountain, Nevada, have 

previously been shown to have significant spatial variability with regions of elevated 

temperatures coinciding roughly with near-vertical north-south trending faults. Previous 

investigators had suggested upwelling along faults from an underlying aquifer as a likely 

explanation for this ground water temperature pattern. Using a three-dimensional coupled flow 

and heat-transport model, they showed that the thermal high coinciding with the Paintbrush fault 

zone can be explained without significant upwelling from the underlying aquifer. Instead, the 

thermal anomaly could be explained by thermal conduction enhanced slightly by vertical ground 

water movement within the volcanic aquifer sequence. 

Winterle (2005) developed a model for future wetter climate conditions—an extension of one of 

the modeling cases in Winterle (2003)—to evaluate the effects of water table rise during 

potential future wetter climate conditions. Water table rise was included in the model by 

increasing potentiometric head values at the model side boundaries by a fixed percentage and 

doubling the rate of surface recharge. A 5-percent increase in boundary heads from the 

estimated present-day values caused the calculated water table elevation to first reach the land 

surface in an area coincident with evaporite deposits that indicate the past occurrence of spring 

flows. To model their effect on flow paths, spring discharges were simulated using the 

MODFLOW DRAIN package, total spring discharge was varied by using different values for 

drain conductance and elevation. Particle-tracking analyses of flow paths from beneath 

Yucca Mountain were then performed for different spring discharge rates. The modeling 

analysis included maximum spring discharge in excess of 10,000 m3

/d [3,000 acre-ft/yr] from the 

area of observed evaporite deposits. Results suggest that calculated flow paths from beneath 

Yucca Mountain do not change appreciably as a result of spring discharges at this location. 

Reverse particle tracking indicated that simulated spring discharges at this location originate 

from the Crater Flat area, west of Yucca Mountain. 

Inyo County (2006) conducted studies to evaluate potential groundwater transport pathways 

from the Yucca Mountain region into Inyo County, California, including Death Valley, and to 

evaluate the connection between the regional lower carbonate aquifer (LCA) and the biosphere. 

This work included a cooperative drilling program to better characterize the potential for 

interbasin flow through the LCA. Inyo County was interested in the LCA because (i) the upward 

gradient in the LCA at Yucca Mountain provides a natural barrier to radionuclide transport; 

(ii) the LCA is a necessary habitat resource for the endangered Devil’s Hole pup fish; and 

(iii) the LCA is the primary water supply and source of water to the major springs in Death Valley 

National Park. The results of geophysical surveys, combined with the geological framework 

model of the region, were used to site three exploratory monitoring wells that penetrate the LCA. 

A numerical groundwater model of the LCA through the Southern Funeral Mountain range was 

developed to demonstrate potential flow through spillways in the LCA through the Southern 

Funeral Mountain Range. A second numerical groundwater model of the water table aquifer 

system at the Texas and Travertine Spring complexes was developed to simulate the potential 

effect of water production wells on spring flows. According to Inyo County (2006), this second 

model indicated that impacts of pumping on spring flows in this area would not be significant. 

Sandia National Laboratories (2007a) documents the Yucca Mountain site-scale saturated zone 

flow model. This three dimensional model was developed using the FEHM Version 2.24 

software and is the culmination of an iterative process of model development spanning more 

than 15 years. DOE used this model to describe the steady-state flow of groundwater as it 

moves from the water table below the proposed repository, through the saturated zone, and to 

the accessible environment in its flow and transport model abstraction of the total-system 
3-3 

performance assessment for the license application (DOE, 2008b). The model grid is 

composed of 500-m [1,640 ft] square cells oriented in a north-south and east-west direction. 

The model domain was selected to be: (i) coincident with grid cells of the DVRGFM where 

site-scale model nodes correspond to regional model cell corners in the horizontal plane; 

(ii) sufficiently large to reduce the effects of boundary conditions on estimating permeabilities 

and calculated flow fields near Yucca Mountain; (iii) sufficiently large to assess groundwater 

flow at distances beyond the 18-km [11.2-mi] compliance boundary from the proposed 

repository area; (iv) small enough to minimize the model size for computational efficiency and to 

include structural feature detail affecting flow; (v) thick enough to include part of the regional 

Paleozoic carbonate aquifer (the bottoms of the site- and regional-scale models are equal at 

−4,000 m [−13,000 ft] below sea level); and (vi) large enough to include borehole data from the 

Amargosa Desert at the southern end of the modeled area. The model was calibrated to match 

161 hydraulic head measurements throughout the model domain, and successfully reproduces 

the general pattern of observed hydraulic gradients. Results indicate flow paths that converge 

on the Fortymile Wash area from the north, east, and west, and nearly all of the modeled flow 

exits the model domain through the south lateral boundary. 

Sandia National Laboratories (2007b) documents the hydrogeologic framework model, 

referred to as HFM 2006 that was used as the basis for defining hydrogeologic layers in 

the Yucca Mountain site-scale saturated zone flow model. HFM 2006 provides a static 

three-dimensional, simplified conceptual model with geometric elements that represent the 

location of differentiated HGUs within the site-scale saturated zone flow model domain. Similar 

to the flow model, HFM 2006 was the result of a long term iterative process of data collection, 

interpretation, and model revision over many years. 

Sandia National Laboratories (2007c) documents the results and interpretations of field 

experiments, mainly hydraulic and tracer testing, used to develop and validate the conceptual 

models for flow and radionuclide transport models in the saturated zone near Yucca Mountain. 

The test interpretations summarized in this document provide the technical basis for most of the 

flow and transport input parameter distributions used in the total system performance 

assessment for the license application. Information is also provided to facilitate traceability of 

data sources. 

Sun and Bertetti (2007) summarized numerical model experiments conducted at the Center for 

Nuclear Waste Regulatory Analyses to quantify the effects of subgrid physical and chemical 

heterogeneities in saturated alluvium of Fortymile Wash, south of Yucca Mountain. Monte Carlo 

simulations were performed using realizations of a fine-scale block model for which the 

properties are generated stochastically based on a hierarchical alluvium facies model. The 

results indicated that the upscaled block hydraulic conductivities have similar magnitudes as 

those assigned to the alluvium in site-scale models, which has grid blocks that are 500 m 

[1,600 ft]. The simulated longitudinal macrodispersivities at the scale of a model grid block were 

on the order of 10 m [32.8 ft], depending on the variance of hydraulic conductivities. The results 

of reactive transport modeling showed that retardation introduces at most a two-fold increase in 

solute spread in the longitudinal direction. Additional sensitivity studies show that including 

correlation in surface area normalized distribution coefficient may reduce the amount of plume 

spread caused by chemical heterogeneity. 

Sun, et al. (2008) evaluated data from field investigations and borehole cuttings logs to develop 

a three-dimensional hydrofacies model of the sedimentary architecture of the alluvium deposits 

in Fortymile Wash, Nevada, using a hierarchical transition probability geostatistical approach. 

This alluvial aquifer comprises a segment of the groundwater flow pathway from the proposed 
3-4 

high-level nuclear waste repository at Yucca Mountain, Nevada to the downstream accessible 

environment. This work demonstrated the link between the alluvium spatial variability and 

solute dispersion at different spatiotemporal scales using the stochastic-Lagrangian transport 

theory. They showed that, over a scale of several kilometers, the longitudinal macrodispersivity 

continues to increase and can be on the order of hundreds to thousands of meters, and it may 

not reach its asymptotic value until after 1,000 years of travel time. 
4-1 

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