The saturated zone hydrology of Yucca Mountain and the surrounding area, southern Nevada and...

mwr209-03 1st pgs page 1

1

The Geological Society of AmericaMemoir 209

2012

The saturated zone hydrology of Yucca Mountain and the surrounding area, southern Nevada and adjacent areas of

California, USA

Wayne R. BelcherU.S. Geological Survey, 160 North Stephanie Street, Henderson, Nevada 89074, USA

John S. StucklessU.S. Geological Survey, MS 963, Box 25046, Denver Federal Center, Denver, Colorado 80225, USA

Scott C. JamesSandia National Laboratories, P.O. Box 969, Livermore, California 94551, USA

ABSTRACT

In 2002, Yucca Mountain, Nevada, was selected as the proposed site for the U.S. high-level nuclear waste repository. Yucca Mountain lies within a large topo-graphically closed basin, in which surface water is internally drained. Groundwater, however, can and does fl ow into and out of this basin at depth through a regional carbonate-rock aquifer (commonly referred to as the lower carbonate-rock aquifer). Most groundwater recharge (water infi ltrating downward through the unsaturated zone into the water table) originates in the highlands north of Yucca Mountain and fl ows generally southward. Some groundwater discharges within the basin, as in Oasis Valley and the southern Amargosa Desert, but the ultimate discharge is in Death Val-ley, where water is returned to the atmosphere by evapotranspiration. Groundwa-ter fl ows through a heterogeneous medium produced by a complex geologic history including both compressional and extensional tectonics. For hydrologic purposes, the rocks and alluvium are divided into 25 hydrogeologic units. Regionally, the most important unit for regional groundwater fl ow is composed of Paleozoic carbonate rocks, which are locally separated into two aquifers by an intervening shale. Rocks of the southwestern Nevada volcanic fi eld form thick deposits in the northern part of the basin, and these rocks host both aquifers and confi ning units.

The potentiometric surface of the site-scale fl ow system contains areas of large hydraulic gradient (as great as 0.13) and small hydraulic gradient (as small as 0.0001). Both extremes are found within the Yucca Mountain site area, where they are well constrained by numerous boreholes. At Yucca Mountain, a single borehole pen-etrates to the regional carbonate-rock aquifer, and, at this locality, the hydraulic head at depth is 20 m greater than in the overlying volcanic rocks. This head difference

Belcher, W.R., Stuckless, J.S., and James, S.C., 2012, The saturated zone hydrology of Yucca Mountain and the surrounding area, southern Nevada and adjacent areas of California, USA, in Stuckless, J.S., ed., Hydrology and Geochemistry of Yucca Mountain and Vicinity, Southern Nevada and California: Geological Society of America Memoir 209, p. 1–71, doi:10.1130/2012.1209(03). For permission to copy, contact [email protected]. © 2012 The Geological Society of America. All rights reserved.

2 Belcher et al.


INTRODUCTION

Characterization of the saturated-zone hydrology for Yucca Mountain, in Nye County, southern Nevada, is necessary to understand whether radioactive waste stored in the proposed high-level radioactive waste repository for the United States at Yucca Mountain (Fig. 1) can be isolated. Groundwater mov-ing within the saturated zone provides the principal means by which radioactive materials that may be released from the pro-posed repository, either as dissolved or suspended constituents, may be transported to the accessible environment. The acces-sible environment is defi ned by the U.S. Nuclear Regulatory Commission in 10 CFR Part 63 as an area 18 km downgradient from the proposed repository (the compliance boundary). The saturated zone affects the performance of the proposed reposi-tory in two principal ways. First, the saturated zone delays the arrival of radionuclides at the compliance boundary, providing more time for radionuclide decay. Second, radionuclide con-centrations can be attenuated during transport to the compli-ance boundary (this effect may not be important to regulatory compliance depending on how dilution at the biosphere is cal-culated). The most recent site-scale saturated-zone fl ow model (Sandia National Laboratories, 2007d) builds on previous ver-sions produced by Bechtel SAIC Company (2001, 2003, 2004a) for the U.S. Department of Energy.

The site-scale groundwater fl ow system at Yucca Mountain, which encompasses a few hundred square kilometers centered on Yucca Mountain, cannot be evaluated in isolation, but, rather, it must be considered in the context of the Death Valley regional groundwater fl ow system within which it is contained, referred to here as the Death Valley region, consisting of ~70,000 km2 in Nevada and California (Belcher et al., 2010) (Fig. 1).

Because groundwater tends to acquire the chemical signa-ture of the materials through which it moves, hydrochemical data from boreholes, wells, and springs (Marshall et al., this vol-ume) provide an independent body of data against which pre-dicted groundwater fl ow patterns and rates of fl ow can be tested. Although the areal distribution of data is somewhat restricted and the density of data points is highly variable, a large amount of major-ion, trace-element, and isotopic data are available from the Yucca Mountain and downgradient areas.

To understand the regional and site-scale fl ow systems, vari-ous components of the system need to be defi ned, such as the hydrogeologic framework, hydraulic properties of the aquifers and confi ning units, locations of boundaries, and locations and rates of recharge and discharge. Once a conceptual model of the groundwater fl ow system is understood, a numerical fl ow model can be constructed. Numerical fl ow models have been used to simulate both the regional and site-scale fl ow systems at Yucca Mountain. These numerical fl ow models can further our under-standing of the fl ow system by integrating data, identifying data gaps, and simulating future effects on the fl ow system. The site-scale numerical model is a subset of the regional model, so the regional model provides boundary conditions for the site-scale model. The site-scale, saturated-zone fl ow model is used for sol-ute transport modeling and quantitative simulations of possible transport of radioactive materials from the proposed repository to the accessible environment.

This chapter describes the saturated zone hydrology based on data and interpretations of the regional and site-scale fl ow systems and modeling efforts available through the end of 2006. The U.S. Department of Energy continues to refi ne the under-standing of the fl ow system and improve the fl ow models as new data are available. The Death Valley region is in the southern part

is likely widespread, as indicated by thermal highs at the groundwater table in the vicinity of block-bounding faults, where upward leakage of water from the regional carbonate-rock aquifer is postulated.

Since the early 1980s, numerous two- and three-dimensional fl ow models have been developed to depict regional groundwater fl ow. A 2004 transient fl ow model of the Death Valley region has 16 layers and a 1500 m/side horizontal grid; it is composed of 194 rows and 160 columns. The model was fi rst calibrated to a steady-state condi-tion and then to transient conditions. The model matches observed fl ow patterns well, and it generally agrees with measured water levels except in areas of large hydraulic gradient. The regional model provides the boundary conditions for a detailed site-scale fl ow model.

The fi nite-element heat and mass transfer code, FEHM v2.24, was used to simu-late fl ow through the saturated zone at Yucca Mountain. Cells in the site-scale model are 250 m/side in the horizontal grid; it is composed of 181 rows and 121 columns. The model may use as many as 67 layers, but the framework model allows a stair-stepped ground surface, so the number of layers is variable. Layer thickness ranges from 600 m at the bottom of the model to 10 m south of Yucca Mountain. The site-scale fl ow model was constructed and calibrated, matching observed hydrologic data well. The site-scale fl ow model provides a means for assessing the hypothetical fl ow path for any radioactive materials originating from the proposed repository.

The saturated zone hydrology of Yucca Mountain and the surrounding area 3


450000 500000 550000 600000 650000

3900

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4000

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4050

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4100

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San Bernardino Co

Inyo Co

Clark Co

Lincoln Co

Nye Co

Esmeralda Co

CALIFORNIA

NEVADAM o j ave D e s e rt

G r e a t B a s i n D e s e r t

IndianWellsValley

Inyo Mts

Cottonwood M

ts

TuckiMtn

SearlesValley

OwensValley

Saline Valley

Fish Lake Valley

StoneCabinValley

MagruderMtn

StonewallMtn Gold

Flat

ReveilleValley

PenoyerValley

GoldfieldHills

Railr

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Palmetto MtsMon

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Hiko

Ran

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Eureka Valley

Nopah

Range

ChicagoValley

Greenwater

Valley

Quail Mts

MesquiteFlat

DesertValley

Pint

wat

er R

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Desert Range

Shadow Mts

Ivan

pah

Valle

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Grapevine Mts

AmargosaFlat

Quin

n

Cany

on

Ran

ge

GardenValley

CoalValley

WhiteRiverValley

Mt. Irish

Mt. Helen

Abbreviations

AF = Amargosa FarmsAM = Ash MeadowsBB = Busted ButteBM = Bare MountainBW = Badwater BasinCF = Crater FlatDVH = Devils Hole HillsDVNP = Death Valley National ParkEM = Eagle MountainFC = Fluorspar CanyonFCW = Furnace Creek WashFMC = Fortymile CanyonFW = Fortymile WashGC = Grapevine CanyonGWR = Greenwater RangeIH = Ibex HillsJF = Jackass FlatsMM = Mt. Montgomery, Montgomery MountainsMP = Mormon PointMQM = Mesquite MountainsPV = Pahranagat ValleyRSR = Resting Spring RangeRV = Rock ValleySH/LSM = Striped Hills/ Little Skull MtnSPH = Sperry HillsSR = Specter RangeSTV = Stewart ValleySV = Shadow ValleyYM = Yucca Mountain

PV

Mine Mtn

BM CF JF

Shoshone Mtn

YuccaFlatTimber

Mtn

BB

RV

EMGWR

SilurianValley

ValjeanValley

SV

MesquiteValley

ER

C

CCBB

CC

Amargosa River

YM

AMDVNP

OasisValley FMC

GC

Tonopah

FurnaceCreek

Beatty

LasVegas

IndianSprings

Tecopa

Baker

Shoshone

Pahrump

DevilsHole DVH

FCW

CA

LIFOR

NIA

NEVADA

MAP AREA

118°W 117° 116° 115°

38°N

37°

36°

35°

0

40200

40 80

MILES

KILOMETERS

EXPLANATION

50,000-meter grid based on Universal Transverse Mercator projection, Zone 11.Shaded-relief base from 1:250,000-scale Digital Elevation Model; sun illumination from northwest at 30 degrees above horizon

Va

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Kawich Range

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Panamint Range

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MP

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Desert boundary

Underground Test Area ground-water flow model boundary (IT Corporation, 1996a)

Prepumping Death Valley regional ground-water flow system model boundary (D'Agnese and others, 2002)

Death Valley regional ground-water flow system model boundary

Nevada Test Site boundary

Yucca Mountain Project ground-water flow model boundary (D'Agnese and others, 1997) Populated place

FCBullfro

gHills

Figure 1. Geographic and prominent topographic features of the Death Valley region (from Belcher et al., 2010, their fi gure A-1).

4 Belcher et al.


of the Great Basin subprovince of the Basin and Range phys-iographic province. The regional geology and physiography of this region are summarized by Stuckless and O’Leary (2007). The topography of the region is controlled by linear, generally northwest-southeast–trending, fault-bounded mountain ranges that are separated by broad (20–30 km) intermontane alluvial basins that developed during Tertiary extension of the region. The relief between valley fl oors and adjacent mountain tops locally exceeds 1500 m (Bedinger et al., 1989a), and altitudes in the region range from 86 m below sea level at Badwater in Death Valley to 3600 m above sea level in the Spring Mountains. Overall, the mountain ranges occupy ~40%–50% of the surface area within the region (Stuckless and O’Leary, 2007). The topo-graphic valleys and basins that constitute the remaining area are fi lled with alluvial and colluvial deposits that, generally, descend from the fl anks of the bordering mountain ranges to the valley fl oors as gently sloping alluvial fans or piedmont slopes. Basin-fi ll deposits commonly attain thicknesses of several hundred meters in the central parts of the basins (Sweetkind et al., 2010).

The valley and basin fl oors are local depositional centers, and the basins usually contain playas that act as catchments for surface-water runoff (Grose and Smith, 1989). Playas occupy ~10% of the region (Bedinger et al., 1989a), and, although they are seldom occupied by perennial surface water, numerous pla-yas contain saline deposits that indicate the evaporation of sur-face water or shallow groundwater from the playa surface.

Several major mountain ranges are located in or bound the region. The Panamint Range and the Amargosa Range border Death Valley on the west and the east, respectively, and the Last Chance Range, the Montezuma Range, the Cactus Range, the Kawich Range, the Quinn Canyon Range, and the Timpahute Range defi ne the northern extent of the region. The Pahranagat Range, the Sheep Range, and the Spring Mountains form the east-ern boundary, and the Kingston Range is near the southern bound-ary. Pahute Mesa is a prominent upland area extending ~75 km from Rainier Mesa on the Nevada National Security Site west-ward to Stonewall Mountain. With altitudes generally exceeding 1700 m, Pahute Mesa has a major infl uence on the local climate and, therefore, on recharge to the groundwater fl ow system in the northern part of the region, including Yucca Mountain.

The hydrology of the region is dominated by the present-day arid to semiarid climatic conditions (Sharpe, 2007), which restrict the quantities of water available to sustain surface-water drainage systems and recharge underlying groundwater fl ow sys-tems. The Amargosa River and its tributaries (Fig. 1) constitute the major fl uvial system within the region, which, under present-day climatic conditions, is an ephemeral system in which streams fl ow only in response to infrequent heavy precipitation. The surface-water and groundwater fl ow systems terminate in hydro-logic sinks consisting of discharge areas from which water is returned to the atmosphere by evaporation or plant transpiration. The region contains several large valleys, such as the Amargosa Desert and Oasis Valley, which are major intermediate discharge areas for the regional groundwater fl ow system. The salt pan that

occupies the fl oor of Death Valley is the ultimate discharge area, or sink, for the regional hydrologic system.

REGIONAL GROUNDWATER FLOW SYSTEM

Hydrogeologic Framework

The Death Valley region has a long and complex geologic history that includes marine and nonmarine sedimentation and compressive and extensional tectonics (Sweetkind et al., 2010; Stuckless and O’Leary, 2007). Consequently, diverse rock types and deformational structures are generally juxtaposed such that subsurface conditions are variable and complex. Knowledge of the geologic diversity beneath alluvial basins is scant and indi-rect, which complicates understanding of the hydrogeologic framework. Much of the following description of the hydrogeo-logic framework of the Death Valley region has been modifi ed from Sweetkind et al. (2010). At a coarse scale, the regional geology can be summarized as consisting of the following major lithostratigraphic groups:

1. Proterozoic and Early Cambrian crystalline and siliciclas-tic rocks,

2. Paleozoic carbonate and fi ne-grained siliciclastic rocks,3. Mesozoic siliciclastic and intrusive rocks of age,4. Tertiary tuffs, lava fl ows, and volcaniclastic rocks, and5. Cenozoic basin-fi ll deposits, dominantly alluvial and col-

luvial deposits and lesser amounts of basalt cones and fl ows and eolian, paludal, and playa sediments.

The hydrologic basement of the Death Valley region is formed by low-permeability crystalline and siliciclastic rocks. These rocks consist of Early to Middle Proterozoic crystalline rocks and some Late Proterozoic siliciclastic rock. Although some Early Proterozoic rocks are exposed in the region, most Proterozoic rocks are of Late Proterozoic age.

The eastern and southern parts of the Death Valley region lie within the carbonate-rock province of the Great Basin (Prudic et al., 1995), which is characterized by thick sequences of car-bonate rocks. These rocks form a generally deep regional aquifer within the groundwater fl ow system. Winograd and Thordarson (1975, p. C53) attributed the deep water table in the Yucca Flat and Frenchman Flat (Fig. 1) areas to drainage from saturated basin-fi ll materials into the underlying and surrounding carbon-ate rocks. These carbonate rocks also allow transfer of ground-water between basins in the Death Valley region (D’Agnese et al., 1997, p. 5). In valleys such as the Amargosa Desert and southern Indian Springs Valley (Fig. 1), and possibly in eastern Jackass Flats (Fig. 1), interbasin movement of groundwater generally (groundwater fl ow through bedrock mountain ranges between fault-block basins) results in upward fl ow from the deep carbon-ate rocks into the overlying basin-fi ll materials (Winograd and Thordarson, 1975).

Rocks of Mesozoic age are of minor importance near Yucca Mountain and are restricted to a few small Cretaceous plutons in the northeastern part of the Nevada National Security Site.



Mesozoic-age intrusive rocks are more common in the south-western and western parts of the Death Valley area.

The northwestern part of the Death Valley region generally is underlain by Tertiary silicic volcanic rocks that are part of the southwestern Nevada volcanic fi eld (Laczniak et al., 1996, p. 15, their fi gure 4). The hydraulic properties of the volcanic rocks are governed chiefl y by the mode of eruption and cooling, by the extent of primary and secondary fracturing, and by the degree to which secondary alteration (crystallization of volcanic glass and alteration to zeolites) has affected primary permeability (Lacz-niak et al., 1996, p. 15). On a regional scale, the volcanic rocks generally are in hydraulic connection with overlying basin-fi ll deposits and may be in hydraulic connection with underlying carbonate rocks as well.

These rocks and unconsolidated deposits form the system through which groundwater fl ows. They can be grouped into units of similar hydrogeologic character called hydrogeologic units. A hydrogeologic unit has considerable lateral extent and has reasonably distinct hydrologic properties because of its phys-ical (geological and structural) characteristics.

The basic pre-Cenozoic hydrogeologic framework for the Death Valley region, particularly in the vicinity of the Nevada National Security Site, was defi ned by Winograd and Thordar-

son (1975). Pre-Cenozoic sedimentary rocks were grouped into four units: a lower clastic aquitard (confi ning unit), composed of Late Proterozoic through Middle Cambrian siliciclastic rocks; a regional or lower carbonate-rock aquifer, composed of Middle Cambrian through Devonian mostly carbonate rocks; an upper clastic aquitard, composed of Devonian and Mississippian silici-clastic rocks; and an upper carbonate-rock aquifer, composed of Pennsylvanian and Permian carbonate rocks that overlie the rocks of the upper clastic aquitard, but which are only present in the vicinity of the Nevada National Security Site (formerly the Nevada Test Site). Most subsequent descriptions of hydrogeo-logic units and groundwater fl ow models of the region (Waddell, 1982; Luckey et al., 1996; Laczniak et al., 1996) have honored these subdivisions of the pre-Cenozoic sedimentary section. Sim-ilar treatment of these units in the most recent regional ground-water fl ow models is shown in Table 1 (IT Corporation, 1996a; D’Agnese et al., 1997; Belcher et al., 2002a) and in Table 2 (Sweetkind et al., 2010).

In contrast to the general consistency in the treatment of the pre-Cenozoic section, several different approaches have been taken to subdivide the Cenozoic section, particularly the volcanic rocks at the Nevada National Security Site, into units. Past approaches have differed in the number of units used and in the treatment of

TABLE 1. HYDROGEOLOGIC UNITS IN THE YUCCA MOUNTAIN AREA USED IN U.S. DEPARTMENT OF ENERGY MODELS (MODIFIED FROM SWEETKIND ET AL., 2010)

Primary component of hydrogeologic unit Belcher et al.

(2002a)* D’Agnese et al.

(1997)† IT Corporation

(1996a)§ Cenozoic units

AA fvTQ laTQ stisoped llif-nisaB pQ pTQ stisoped ayalP –

UV vT ,vTQ UV skcor cinaclov detaitnereffidnU AV vT ,vTQ AV etiS tseT adaveN nrehtuos ,refiuqa kcor-cinacloV

Volcanic-rock confining unit, southern Nevada Test Site VCU QTv, Tv VCU AMT vT ,vTQ AMT refiuqa niatnuoM rebmiT

CT vT ,vTQ CT enoc ffut slliH ocilaC–hsurbtniaP BCT vT ,vTQ BCT sffut dedlewnon tinu gninifnoc gorflluB ABT vT ,vTQ ABT sffut dedlew refiuqa egnaR detleB

UCBT vT ,vTQ UCBT sffut dedlewnon tinu gninifnoc lasaB QBT vT ,vTQ QBT sffut dedlew refiuqa lasaB

SVDST svT SVDST noitces yellaV htaeD–stnemides yraitreT Pre-Cenozoic units

svM skcor yratnemides dna citsalcinaclov ciozoseM # Mvs – ACU refiuqa kcor-etanobrac reppU – LCA3

UCCU UCE UCCU tinu gninifnoc kcor-citsalcicilis reppU **ACL 2P **ACL refiuqa kcor-etanobrac rewoL

UCCL tinu gninifnoc kcor-citsalcicilis rewoL †† P1, pCgm LCCU†† UCCL mgCp mgCp skcor cihpromatem dna etinarg nairbmacerP

Tertiary–Jurassic intrusive rocks TJi TJg I Note: Dash indicates unit not named in that model. *Steady-state Death Valley regional groundwater flow system units. †Yucca Mountain Project regional model units. §Underground Test Area (UGTA) program regional model units. #Thrusted units of Mvs in the Lee Canyon thrust and the Keystone thrust modeled as separate hydrogeologic units, owing to software limitations. **Thrusted units of lower carbonate-rock aquifer (Lee Canyon thrust, Gass Peak thrust, Schwaub Peak thrust, Specter Range thrust, and Wheeler Pass thrust) modeled as separate hydrogeologic units, owing to software limitations. ††Thrusted units of lower clastic-rock confining unit (Gass Peak thrust, Specter Range thrust, and Wheeler Pass thrust) modeled as separate hydrogeologic units, owing to software limitations.

6 Belcher et al.


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spatially variable material properties in the volcanic-rock units. Winograd and Thordarson (1975, their table 1) assigned the volca-nic rocks at Nevada National Security Site to hydrogeologic units based upon lithology and inferred hydrologic signifi cance—for example, tuff aquitard, bedded tuff aquifer, welded tuff aquifer, lava fl ow aquifer. The geologic units described and their strati-graphic position, however, were based upon older 1960s-era geo-logic mapping, and the designations did not necessarily account for spatial variability of properties in a single hydrogeologic unit. Laczniak et al. (1996, their table 1) extended the work of Wino-grad and Thordarson (1975) to produce a more detailed descrip-tion of volcanic-rock hydrogeologic units in the area around the Nevada National Security Site. The updated designations were based on new volcanic-rock stratigraphic unit descriptions (Saw-yer et al., 1994); each formation was designated as a welded-tuff aquifer, lava-fl ow aquifer, or tuff confi ning unit, and the area on the Nevada National Security Site where the units were impor-tant aquifers or confi ning units. Both of these studies provided essential descriptions of the volcanic-rock units; however, neither study was suffi ciently detailed to defi ne stratigraphic complexities throughout the Death Valley region.

Regional interpretations have differed signifi cantly in the way in which the Cenozoic section of the Death Valley region has been grouped into hydrogeologic units, both in terms of the number of units and in the way the spatial variability of material properties in the volcanic units was addressed (IT Corporation, 1996a; D’Agnese et al., 1997; Table 1). The volcanic rock units defi ned by D’Agnese et al. (1997) were based on a hydrogeo-logic map compilation (Faunt et al., 1997) and geologic cross sections (Grose, 1983) in which all volcanic rocks were desig-nated as Tertiary volcanic rocks or Tertiary-Quaternary volcanic rocks (Table 1). Spatial variability in hydrologic properties in the volcanic-rock section was addressed by assigning zones of variable hydraulic conductivity (D’Agnese et al., 1997, 2002). Volcanic-rock hydrogeologic units were described by IT Corpo-ration (1996a) on the basis of abundant borehole data from the Nevada National Security Site and are considerably more detailed than units described by D’Agnese et al. (1997). Spatial variation in the volcanic units was handled in part by developing differ-ent schemes for different parts of the Nevada National Security Site; specifi c aquifers (primarily lava fl ow and welded tuff) and confi ning units were assigned for each geographic area. Belcher et al. (2002a) merged these two schemes by using the IT Cor-poration (1996a) units in the immediate vicinity of the Nevada National Security Site and the volcanic-rock unit of D’Agnese et al. (1997) outside of the Nevada National SecuritySite (Table 1).

In the most recent and comprehensive description of the hydrogeologic units within the Death Valley region, Sweetkind et al. (2010) defi ned 25 hydrogeologic units (Table 2). The pre-Cenozoic sedimentary section still generally follows the previous subdivisions; the lowermost unit is the crystalline-rock confi n-ing unit (XCU). This unit contains Early to Middle Proterozoic crystalline rocks and some Late Proterozoic siliciclastic rock. Groundwater may be present locally in fractures, but the frac-

tures are poorly connected, and, therefore, the unit is considered a barrier to fl ow (D’Agnese et al., 1997). Closely related to the crystalline-rock confi ning unit, there is the intrusive-rock con-fi ning unit (ICU). This discontinuous unit contains all plutonic rocks of Mesozoic age and Tertiary intrusive rocks that are asso-ciated with volcanism related to the formation of the southwest-ern Nevada volcanic fi eld. Small quantities of water may pass through the intrusive-rock confi ning unit, but, in general, the fractures are poorly connected, and the unit impedes groundwa-ter fl ow (Winograd and Thordarson, 1975).

The lower siliciclastic-rock confi ning unit (LCCU) con-tains siliciclastic rocks and some dolomite of Late Proterozoic through Lower Cambrian age (Table 2). Sweetkind et al. (2010) reported thicknesses for the lower siliciclastic-rock confi ning unit of 2500–3300 m. The lower siliciclastic-rock confi ning unit has long been considered a major confi ning unit in the Death Valley region (Winograd and Thordarson, 1975) and, along with the crystalline-rock confi ning unit, represents the hydraulic base-ment for the Death Valley region (D’Agnese et al., 1997).

The lower carbonate-rock aquifer (LCA; Table 2) is the major regional aquifer for the eastern two-thirds of the Great Basin, including the Death Valley region (Winograd and Thor-darson, 1975; Bedinger et al., 1989a; Harrill and Prudic, 1998). The rocks are a dominantly carbonate sequence of Middle Cam-brian to Middle Devonian age with minor interbedded silici-clastic rocks. In the northwestern part of the region, the rocks of the lower carbonate-rock aquifer are somewhat thicker than elsewhere in the Death Valley region and represent a deeper-water facies (shale and impure carbonate rocks) (Cornwall, 1972; Burchfi el et al., 1982).

The rocks of the lower carbonate-rock aquifer have an aggre-gate thickness of as much as 8000 m and are generally the most permeable rocks in the region (Bedinger et al., 1989b; Belcher et al., 2001). Where hydraulically connected, the carbonate rocks provide a path for interbasin fl ow (Dettinger and Schaefer, 1996; D’Agnese et al., 1997; Harrill and Prudic, 1998; Winograd et al., 2005), although Nelson et al. (2004, 2005) and Anderson et al. (2006) questioned the extent of interbasin fl ow. Winograd and Thordarson (1975) noted that most of the springs in the Death Valley region are associated with the carbonate rocks. Fractures, faults, and solution channels create large hydraulic conductivity, whereas intergranular fl ow is relatively insignifi cant (Winograd and Thordarson, 1975). Faulting in eastern and southern Nevada has apparently increased the transmissivity through carbonate rock by a factor of 25 or more (Dettinger et al., 1995).

Thrust faults within the lower carbonate-rock aquifer com-plicate the fl ow patterns in the groundwater system. The area between the southern Funeral Mountains and the Spring Moun-tains contains several hydrologically important thrust faults: the Schwaub Peak, Specter Range, and Wheeler Pass thrusts. In these thrust faults, the lower siliciclastic-rock confi ning unit overlies the lower carbonate-rock aquifer. Other hydrologically important thrust faults in the region are the Keystone thrust in the Spring Mountains and the Gass Peak thrust in the Sheep Range.

8 Belcher et al.


Although the lower carbonate-rock aquifer is regionally extensive, its hydraulic characteristics are regionally variable. For this reason, Sweetkind et al. (2010, p. 59–61) subdivided the lower carbonate-rock aquifer into 11 zones on the basis of pri-mary and secondary features. Because the degree of fracturing determines the transmissivity of the lower carbonate-rock aqui-fer, the degree of structural disruption is of greater importance in defi ning zones within the lower carbonate-rock aquifer than dep-ositional features. The region has been affected by two roughly east-west–directed events: a Mesozoic compressional event (the Sevier orogeny) and a Tertiary extensional event. Either event, or the latter compounding the former, may have disrupted hydro-logic continuity.

In places, the lower carbonate-rock aquifer is overlain by the upper siliciclastic-rock confi ning unit (UCCU), which consists of the Eleana Formation and Chainman Shale of Upper Devonian through Mississippian age (Table 2). The upper siliciclastic-rock confi ning unit is largely made up of argillites and shales of low porosity and permeability and may be as much as 2000 m thick (Trexler et al., 1996). The upper siliciclastic-rock confi ning unit pinches out to the east, where it is replaced with a number of shelf-facies limestones (Stuckless and O’Leary, 2007).

Although the upper siliciclastic-rock confi ning unit is het-erogeneous in distribution, it strongly infl uences groundwater fl ow where present. It may account for many of the steep hydrau-lic gradients observed in the eastern and central Nevada National Security Site (Winograd and Thordarson, 1975; Fridrich et al., 1994; D’Agnese et al., 1997). Winograd and Thordarson (1975) also argued that the confi nement of the lower carbonate-rock aquifer by upper siliciclastic-rock confi ning unit is responsible for hydraulic head at Devils Hole.

Where the upper siliciclastic-rock confi ning unit is pres-ent, the regional carbonate-rock aquifer is divided into lower (LCA) and upper carbonate-rock aquifer (UCA) parts, and the upper siliciclastic-rock confi ning unit is overlain by the upper carbonate-rock aquifer, which includes Pennsylvanian and Mis-sissippian dolomites, limestones, and calcareous shales. Where these rocks lie directly on the lower carbonate-rock aquifer, they are treated as part of that unit (Sweetkind et al., 2010). In the area of Yucca Flat, the upper carbonate-rock aquifer is a separate aquifer (Winograd and Thordarson, 1975).

The youngest of the pre-Cenozoic hydrogeologic units is the sedimentary-rock confi ning unit (SCU). The sedimentary-rock confi ning unit consists of unmetamorphosed Mesozoic cra-tonic sedimentary rocks in the southeastern corner of the region and Mesozoic metasedimentary and metavolcanic rocks in the southwestern part of the region; the latter metamorphic rocks are poorly exposed. Hydraulic properties of the sedimentary-rock confi ning unit are highly variable. In terms of the regional fl ow system, most of the sedimentary-rock confi ning unit is too little volume to affect the regional groundwater fl ow system (Sweet-kind et al., 2010).

The hydrogeologic units of Cenozoic age consist both of rocks and of unconsolidated sediments, and Sweetkind et al.

(2010) grouped them into unconsolidated basin-fi ll sediments and local younger volcanic rocks, consolidated basin-fi ll depos-its, and volcanic rocks of the southwestern Nevada volcanic fi eld. Volcanic-rock units are defi ned by group-level stratigraphic des-ignations that are based on geologic map compilations (Slate et al., 2000; Workman et al., 2002), geologic cross sections (Sweet-kind et al., 2001), and borehole lithologic data. The spatial vari-ability of material properties is defi ned for each volcanic-rock unit based on geologic properties, primarily the effects of the amount of fracturing and alteration in the rocks on hydraulic conductivity. Brittle rocks (such as lava fl ows and welded tuffs) tend to have greater conductivities owing to their greater poten-tial to fracture, whereas nonbrittle rocks (such as nonwelded tuffs) tend to have lesser hydraulic conductivities owing to their lesser potential to fracture. Increasing alteration tends to decrease hydraulic conductivity in the rock owing to increased clay con-tent. Various combinations of zones of brittleness and alteration were used to defi ne zones of enhanced or decreased hydraulic conductivity in the volcanic-rock units (Sweetkind et al., 2010).

The oldest Cenozoic unit is the older volcanic-rock unit (OVU), which is Oligocene to Miocene in age. It consists of a vari-ety of types of tuff and volcaniclastic sediments and is restricted to the northern half of the area around Yucca Mountain (Fig. 1). Part of the older volcanic-rock unit is related, at least geographically, to the southwestern Nevada volcanic fi eld, a volcanic area that was the source for the younger volcanic rocks at Yucca Mountain. The older volcanic-rock unit rocks are common in the northeast-ern part of the Nevada National Security Site and within a few tens of kilometers to the north of the Nevada National Security Site (Slate et al., 2000). They also have been identifi ed in drill core at Yucca Mountain (Keefer et al., 2007). Similar units are known from boreholes in Pahute Mesa, and Yucca and Frenchman Flats. Most of these units are nonwelded to partly welded tuffs, with the exception of some densely welded units (Sawyer et al., 1995). The nonwelded tuffs typically are devitrifi ed and altered to zeo-lites (Drellack, 1997; Prothro et al., 1999).

The remainder of the older volcanic-rock unit originated from volcanic centers to the north of the Nevada National Secu-rity Site and is known only in the extreme northern and northeast-ern part of the Nevada National Security Site and regions to the north of the Nevada National Security Site (Ekren et al., 1971). This part of the older volcanic-rock unit is predominantly partly to densely welded ash-fl ow tuffs that have an aggregate thickness of as much as several hundred meters in large parts of western Lincoln County and central Nye County, Nevada (Ekren et al., 1971) (Fig. 1).

The large number of volcanic-rock units included in the older volcanic-rock unit leads to widely varying material proper-ties. Older volcanic-rock unit rocks north of the Nevada National Security Site commonly consist of a series of regionally exten-sive ash-fl ow tuffs; these tuffs are locally fractured and serve as aquifers throughout a large part of southern Nye County (Plume and Carlton, 1988). In most places in the southwestern Nevada volcanic fi eld, the older volcanic-rock unit rocks likely



act as a confi ning unit because they generally are nonwelded to partially welded with widespread zeolitic alteration (Sawyer et al., 1995; Drellack, 1997; Prothro et al., 1999). Lava fl ows and densely welded tuffs exist within the southern part of the older volcanic-rock unit and also can form fracture-fl ow aquifers. However, these units are generally too localized or too deep to be of regional importance. The older volcanic-rock unit is important in Yucca and Frenchman Flats, where it separates the overlying fractured volcanic-rock aquifers from the underlying regional carbonate-rock aquifer. In the central part of Yucca Flat, the older volcanic-rock unit is saturated, and measured transmissivities are very low (Sweetkind et al., 2010).

The Belted Range unit (BRU) is made up of rocks of the Belted Range Group of Miocene age, which is a voluminous assemblage of peralkaline volcanic rocks erupted as ash fl ows, air-fall tuffs, and related lava fl ows (Slate et al., 2000). Volcanic-rock units on the eastern part of Pahute Mesa are highly fractured and form the principal aquifer unit (Blankennagel and Weir, 1973; Laczniak et al., 1996; Prothro and Drellack, 1997). The Belted Range unit is not present at or to the south of Yucca Mountain.

Each of the three principal geologic units that make up the Crater Flat Group, the Tram Tuff, Bullfrog Tuff, and the Prow Pass Tuff (W.J. Carr et al., 1986; Sawyer et al., 1994), is classi-fi ed as a separate hydrogeologic unit within the regional setting (Sweetkind et al., 2010). The Crater Flat Group rocks are present at Pahute Mesa, Yucca Mountain, and Crater Flat.

The Crater Flat–Tram aquifer (CFTA) consists of the Tram Tuff, which is mostly a nonwelded to partially welded, ash-fl ow tuff, except at Tram Ridge (one of the ridges making up Yucca Mountain), where it is densely welded (Fridrich et al., 1999). It is exposed in northern Crater Flat and Yucca Mountain and has been encountered in boreholes in the Crater Flat and Yucca Mountain areas (W.J. Carr et al., 1986; Keefer et al., 2007). The Tram Tuff is known to extend as far west as the Grapevine Mountains and to the east beneath Jackass Flats (W.J. Carr et al., 1986). Hydro-geologic zones of the Crater Flat–Tram aquifer are nonbrittle and altered in areas roughly north of the center of Yucca Mountain and Crater Flat and nonbrittle and unaltered to the south.

The Crater Flat–Bullfrog confi ning unit (CFBCU) consists of the Bullfrog Tuff, a widespread unit that occupies more than half of the area of the southwestern Nevada volcanic fi eld. The thickness of the Crater Flat–Bullfrog confi ning unit is 100–150 m in Jackass Flats, Yucca Mountain, and the Bullfrog Hills, but it may be more than 400 m thick in Crater Flat. Where it exists as an intracaldera tuff, it is ~680 m thick (Ferguson et al., 1994; Sawyer et al., 1994). The Crater Flat–Bullfrog confi ning unit is nonwelded to poorly welded throughout most of the southwestern Nevada volcanic fi eld and is nonbrittle and altered, thereby form-ing a confi ning unit (Blankennagel and Weir, 1973; Laczniak et al., 1996). At Yucca Mountain, the Bullfrog Tuff is a compound-cooling unit; the interior is moderately to densely welded and devitrifi ed, and the margins are nonwelded to partly welded.

The Crater Flat–Prow Pass aquifer (CFPPA) is formed by the Prow Pass Tuff and local age-equivalent tuffs and rhyolite-

lava fl ows present in the subsurface beneath Pahute Mesa. The Prow Pass Tuff is exposed to the northwest of Yucca Mountain and at the south end of Crater Flat (Potter et al., 2002). Drilling indicates that it exists in the subsurface in Crater Flat (W.J. Carr et al., 1986; Moyer and Geslin, 1995), and at Yucca Mountain (Keefer et al., 2007), where the unit is thickest (194 m) and most densely welded; it thins (74–50 m) westward into Crater Flat and southward (Potter et al., 2002). Hydrogeologic zones for the Cra-ter Flat–Prow Pass aquifer are largely nonbrittle and unaltered. Nonwelded to partly welded parts of the unit, such as beneath Crater Flat and north of Pahute Mesa, contain zeolitic alteration (Sweetkind et al., 2010).

In the southeastern corner of the southwestern Nevada vol-canic fi eld, the Crater Flat Group is overlain by the Wahmonie and Salyer Formations of Miocene age (Table 2), which make up the Wahmonie volcanic-rock unit (WVU). The Wahmonie volcanic-rock unit consists of andesitic to dacitic lava fl ows, tephra, and related volcaniclastic deposits that were likely erupted from a volcanic center north of Skull Mountain (Fig. 1), where they are ~1300 m thick (Poole et al., 1965a). Regionally, this tuff extends east to Yucca Flat, north to Rainier Mesa, and southwest to Little Skull Mountain and the southern part of Yucca Moun-tain (Ekren and Sargent, 1965; Poole et al., 1965a, 1965b). The hydraulic nature of this unit, as to whether it acts as an aquifer or confi ning unit, varies spatially.

The Calico Hills volcanic-rock unit (CHVU) is formed by the Calico Hills Formation (Sawyer et al., 1994), which is a sequence of rhyolitic lava fl ows, variably welded ash-fl ow depos-its, and nonwelded ash-fall deposits that lie above the Crater Flat Group at Yucca Mountain and northward at Pahute Mesa. Near the eruptive source areas (Calico Hills–Fortymile Canyon area and beneath Pahute Mesa), rhyolite lavas are common (Dicker-son and Drake, 1998); farther away, the Calico Hills volcanic-rock unit is dominated by nonwelded pyroclastic fl ows, which commonly contain zeolitic alteration.

Fractured lava fl ows in the Calico Hills volcanic-rock unit commonly provide zones of increased permeability. For exam-ple, in the central and western parts of Pahute Mesa, thick accu-mulations of rhyolite lava fl ows in the Calico Hills volcanic-rock unit function as a single fractured aquifer (Blankennagel and Weir, 1973; Laczniak et al., 1996). Conversely, in the northeast-ern part of Pahute Mesa and beneath the southern part of Yucca Mountain, the Calico Hills volcanic-rock unit is characterized by thick intervals of nonwelded ash-fl ow tuff that serve as a confi ning unit (Blankennagel and Weir, 1973; Moyer and Ges-lin, 1995; Laczniak et al., 1996; Prothro and Drellack, 1997). Other hydrologic zones of reduced permeability are related to alteration of nonwelded ash-fl ows and bedded tuffs. The non-welded tuffs of the Calico Hills volcanic-rock unit are zeoliti-cally altered throughout most of the southern part of Pahute Mesa (Blankennagel and Weir, 1973; Laczniak et al., 1996) and Yucca Flat (Winograd and Thordarson, 1975). Tuffs in the Calico Hills volcanic-rock unit are zeolitically altered beneath the northern part of Yucca Mountain but are vitric beneath the

10 Belcher et al.


southern and southwestern parts of Yucca Mountain (Moyer and Geslin, 1995). In the Calico Hills, hydrothermal alteration of lava fl ows produced argillic alteration, silicifi cation, and pyriti-zation (Simonds, 1989) that have reduced transmissivity.

Because the densely welded and fractured tuffs dominate the hydrologic character of the rocks of the Paintbrush Group of Miocene age, the group is treated as a single hydrogeologic unit, the Paintbrush volcanic-rock aquifer (PVA). Volumetrically, the Paintbrush volcanic-rock aquifer is the largest volcanic-rock unit at Yucca Mountain, where it is ~610 m thick. The rocks of the Paintbrush volcanic-rock aquifer generally are above the water table at Yucca Mountain and in the eastern and central parts of Pahute Mesa. For these two locations, alteration generally is minimal and consists of primarily argillic or zeolitic alteration of the nonwelded intervals (Moyer et al., 1996). Rocks of the Paintbrush volcanic-rock aquifer are below the water table in the western part of Pahute Mesa, where they are zeolitically altered in downfaulted blocks (Laczniak et al., 1996).

The uppermost unit of the southwestern Nevada volcanic fi eld is the Thirsty Canyon–Timber Mountain volcanic-rock aquifer (TMVA), which is composed of the Miocene-age Timber Mountain Group, Thirsty Canyon Group, and Stonewall Moun-tain Flat Tuff (Sweetkind et al., 2010, p. 43). Rocks of the Tim-ber Mountain volcanic-rock aquifer crop out throughout most of the southwestern Nevada volcanic fi eld and generally are located above the water table. However, caldera collapses in the vicinity of Timber Mountain created topographic lows that were fi lled by pyroclastic materials. One such area is located north of Timber Mountain, where one borehole penetrated 1200 m of rocks from the Timber Mountain Group. Like other hydrogeologic units of the regional fl ow system, hydrologic properties vary spatially on the basis of the presence of lava fl ows, the degree of welding of the ash-fl ow tuffs, and the type and degree of alteration.

The volcanic- and sedimentary-rock unit (VSU) consists of consolidated Cenozoic basin-fi ll deposits that range in age from Eocene to Pliocene. Where the hydrogeologic units of the south-western Nevada volcanic fi eld are named, they separate unit vol-canic- and sedimentary-rock unit into upper and lower units. The volcanic- and sedimentary-rock unit is very heterogeneous; it con-tains volcanic and sedimentary rocks such as lavas and welded and nonwelded tuffs, and sediments of alluvial, fl uvial, colluvial, eolian, paludal, and lacustrine origin (Sweetkind et al., 2010).

The younger volcanic-rock unit (YVU) is roughly equivalent in age to rocks of the southwestern Nevada volcanic fi eld, but it con-sists of tuffs and other volcanic rocks that are not associated with the sources for the southwestern Nevada volcanic fi eld. The younger volcanic-rock unit tends to overlie rocks associated with the south-western Nevada volcanic fi eld. Individual volcanic units within this hydrogeologic unit are not laterally extensive and are outliers of much more extensive outcrops northeast of the Death Valley region (Sweetkind et al., 2010). Most of the younger volcanic-rock unit within the Death Valley region is above the water table.

The lava-fl ow unit (LFU) consists of local Pliocene and Pleistocene basalt and rhyolite lava fl ows. The volcanism that

produced these fl ows is expressed in the vicinity of the Nevada National Security Site by isolated, relatively small, basaltic cinder cones and associated lava fl ows. Individual lava fl ows are not lat-erally extensive and are typically above the water table. Because of this, the lava-fl ow unit is not a regional aquifer (Sweetkind et al., 2010).

The limestone aquifer (LA) consists of Pliocene to Holo-cene lacustrine and spring deposits that are interfi ngered with the basin-fi ll deposits. Typically, these are dense, crystalline deposits of limestone or travertine. The hydrologic properties of these deposits can differ greatly within short distances because of abrupt changes in grain size, fracturing, and consolidation. These deposits can be productive local aquifers, as in parts of the Amargosa Desert. In general, the LA does not crop out and is identifi ed only in drill core in the basin-fi ll deposits (Sweetkind et al., 2010).

Two basin-fi ll confi ning units are called the younger and older alluvial confi ning units (YACU and OACU, respectively). The older alluvial confi ning unit is mostly Pliocene to Pleisto-cene in age, and the younger alluvial confi ning unit is mostly Pleistocene to Holocene in age. Both units consist of fi ne-grained basin-axis deposits. The late Holocene playa or salt-pan deposits are commonly underlain by older playa or lacustrine sequences of middle to early Holocene and Pleistocene age. Both units typically are mixtures of moderately stratifi ed to well-stratifi ed silt, clay, and fi ne sand (Sweetkind et al., 2010). The thickness is poorly constrained but may range from 1 to 10 m in Holocene deposits and may be greater than 300 m in the older deposits (Workman et al., 2002).

Two basin-fi ll aquifer units are referred to as the younger and older alluvial aquifer (YAA and OAA, respectively). The older alluvial aquifer consists of mostly Pliocene to Pleistocene alluvium, colluvium, and minor eolian and debris-fl ow sediments associated with alluvial geomorphic surfaces (Workman et al., 2002). The older alluvial aquifer is generally thicker than the younger alluvial aquifer. In general, fl uvial deposits are predom-inantly sandy gravel with interbedded gravelly sand and sand, whereas the grain size in alluvial fans decreases gradationally from proximal to distal fan. Local eolian accumulations consist of Holocene sand sheets or dune fi elds or relict upper to middle Pleistocene sand-ramp deposits that are banked along the fl anks of some ranges. Although these units tend to be aquifers, fi ner-grained sediments and intercalated volcanic rocks locally can impede groundwater movement. The younger alluvial aquifer is mostly Holocene in age and is generally thinner and less well indurated than the older alluvial aquifer, but it has the same sedi-mentary components as the older alluvial aquifer (Sweetkind et al., 2010).

Hydraulic Properties of the Hydrogeologic Units

Bedinger et al. (1987) compiled and analyzed hydraulic-property data from the literature for the Basin and Range Province and for rocks with similar characteristics outside the province.



They tabulated ranges of values for hydraulic conductivity and porosity (Bedinger et al., 1987, their table 1), and calculated the distributions of expected hydraulic conductivity values for 14 rock types (Bedinger et al., 1987, their fi gure 2).

Belcher et al. (2001, 2002b) compiled published and unpub-lished hydraulic-property data from aquifer tests to estimate hydraulic properties for the major units defi ned for the Death Val-ley region except for unit lower siliciclastic-rock confi ning unit. Permeameter tests on rock cores were used to estimate hydraulic properties for the lower siliciclastic-rock confi ning unit (Belcher et al., 2001). The hydraulic-property estimates included those for transmissivity, hydraulic conductivity, storage coeffi cient, and anisotropy ratios. Hydraulic conductivity was the only property with a suffi cient number of estimates to generate statistical distri-butions for specifi c units (Belcher et al., 2001, 2002b).

The variability inherent in the hydrogeologic units of the Death Valley region increases the uncertainty of the estimated hydraulic conductivity values. Lithologic factors, such as facies changes in sedimentary rock, changes in welding in volcanic rock, and degree of fracturing, can cause hydraulic conductivity values to vary substantially within relatively short distances. Vari-ability also can result from sampling bias. Estimates of matrix permeability commonly depend upon the variable lithology and interval penetrated by a well within a particular unit. Sampling variability also can be a factor in fractured rocks if boreholes intersect rocks with different degrees of fracturing.

Data from Belcher et al. (2001) were used to estimate prob-ability distributions and reasonable ranges of hydraulic conduc-tivity for the major units in the Death Valley region (Belcher et

al., 2002b). Table 3 presents probability distributions of hydrau-lic conductivity for the major hydrogeologic units in the Death Valley region. Fracturing appears to have the greatest infl uence on the permeability of bedrock units: the greater the degree of fracturing, the greater the permeability. Alteration and welding in the Cenozoic volcanic rocks also greatly infl uence hydraulic conductivity. Alteration decreases hydraulic conductivity; weld-ing forms brittle rocks that fracture more easily, thereby increas-ing hydraulic conductivity (Belcher et al., 2001, 2002b).

A compilation and analysis of the results of hydraulic test-ing in Miocene volcanic rocks at Pahute Mesa and Yucca Moun-tain in the southwestern Nevada volcanic fi eld were published by Geldon (2004). Geldon (2004) concluded that cross-hole scale tests provided the best data for analysis for these rocks. At Yucca Mountain, cross-hole scale hydraulic conductivity ranged from 1.4 to 32 m/d, whereas at Pahute Mesa it ranged from 0.62 to 20 m/d (Geldon, 2004, p. 73). Geldon (2004), however, used a different set of hydrogeologic units than were used by Belcher et al. (2001, 2002b).

The permeability of the rocks in the Death Valley region is variable within and among rock types. The faulted and karstic Paleozoic carbonate-rock aquifer units tend to have the largest hydraulic conductivity values, but the alluvial and fractured vol-canic units also may permit large hydraulic conductivity values. Of the volcanic units, the tuff breccias tend to be the most perme-able, and the ash-fl ow tuffs, bedded tuffs, and lava fl ows tend to be the least permeable. The hydraulic conductivity of the welded, usually fractured, tuffs tends to be greater than that of the non-welded tuffs (Belcher et al., 2001, 2002b).

TABLE 3. PROBABILITY DISTRIBUTIONS OF ESTIMATES OF HORIZONTAL HYDRAULIC CONDUCTIVITY FOR HYDROGEOLOGIC UNITS IN THE DEATH VALLEY REGION (MODIFIED FROM BELCHER ET AL., 2002b)

Hydrogeologic unit or subunit

Hydraulic conductivity (m/d) 95% confidence interval (m/d)

Measurements (number)

naeMGeometric Arithmetic Minimum Maximum

YAA/OAA 1.5 10.8 0.00006 130 0.005–430 52 YACU/OACU 3 10.5 0.003 34 0.02–470 15

2 AN 4 200.0 AN AN UFLYVU/VSU 0.06 1.5 0.00004 6 0.00005–80 15

TMVA 0.01 2 0.0002 20 0.00001–18 11 PVA 0.02 4 0.000007 17 0.0000003–1300 9 CHVU 0.2 0.55 0.008 2 0.007–5 14 BRU 0.3 1.03 0.01 4 0.006–17 6 CFTA 0.05 0.4 0.003 2 0.0004–5.3 11 CFBCU 0.4 6.8 0.0003 55 0.0006–240 34 CFPPA 0.3 13 0.001 180 0.000006–2.4 19 OVU 0. 004 0.07 0.000001 1 0.00002–5 46 ICU 0.01 0.3 0.0006 1.4 0.00002–5 7 SCU 0.002 0.02 0.0002 0.3 0.00004–0.09 16 UCA/LCA 2.5 90 0.0001 820 0.0008–7700 53 UCCU/LCCU 0.00002 0.2 0.00000003 5 0.0000000001–3 29 Note: Geometric mean and standard deviation were determined from log-transformed distribution. Abbreviations: BRU—Belted Range unit; CFBCU—Crater Flat–Bullfrog confining unit; CFPPA—Crater Flat–Prow Pass aquifer; CFTA—Crater Flat–Tram aquifer; CHVU—Calico Hills volcanic-rock unit; ICU—intrusive-rock confining unit; LCA—lower carbonate-rock aquifer; LCCU—lower siliciclastic-rock confining unit; LFU—lava-flow unit; NA—not applicable; OAA—older alluvial aquifer; OACU—older alluvial confining unit; OVU—older volcanic-rock unit; PVA—Paintbrush volcanic-rock aquifer; SCU—sedimentary-rock confining unit; TMVA—Thirsty Canyon–Timber Mountain volcanic-rock aquifer; UCA—upper carbonate-rock aquifer; UCCU—upper siliciclastic-rock confining unit; VSU—volcaniclastic- and sedimentary-rock unit; YAA—younger alluvial aquifer; YACU—younger alluvial confining unit; YVU—younger volcanic-rock unit.

12 Belcher et al.


Overview of Regional Groundwater Flow

Groundwater movement in the regional fl ow system gen-erally originates underfl ow across the lateral boundaries of the Death Valley regional fl ow system, mostly through the carbon-ate rocks (Fig. 2). It may also originate as localized asymmetric radial fl ow of recharge from precipitation on mountains and other highlands located principally along the periphery of the system. The overall fl ow system, therefore, can be thought of as consist-ing of a set of relatively shallow, localized fl ow systems that are superimposed on a deeper regional system (Fig. 2). Within the system, the overall movement of groundwater is from the source areas near the margins of the system to the regional hydrologic sink in the fl oor of Death Valley (Figs. 2 and 3). Groundwater fl ow from the Panamint Range on the western boundary of the regional system is generally east to northeast. Progressing clock-wise from the northwestern end of Death Valley, the regional fl ow directions change from south to southwest, and then to approxi-mately west from the Spring Mountains and the southeast quad-rant of the system (Faunt et al., 2010a). Groundwater may also fl ow out of the Death Valley region into the Las Vegas Valley (Faunt et al., 2010a).

The geographic distribution of rainfall and snowfall is prin-cipally a function of the altitude of the land surface, so the high-land areas receive most of the precipitation and provide most of the recharge to the groundwater fl ow system. This distribution of recharge is refl ected as highs in the potentiometric surface (Fig. 3). The most prominent recharge mound in the region is associated with the Spring Mountains (Fig. 3) in the southeast-ern part of the fl ow system, where the potentiometric-surface altitude is estimated to exceed 2300 m. At the regional sink in Death Valley (Fig. 3), in the southwestern part of the fl ow sys-tem, the potentiometric altitude is almost 100 m below sea level, providing ~2400 m of total relief on the regional potentiometric surface (D’Agnese et al., 1998). Regional groundwater fl ow also discharges at intermediate areas as spring fl ow (such as at Ash Meadows), as evapotranspiration (a combination of groundwater evaporation and plant transpiration) at playas and valley fl oors (such as Alkali Flat and Oasis Valley), as pumping in Pahrump Valley and in the Amargosa Desert (San Juan et al., 2010; Faunt et al., 2010a), and as underfl ow across the lateral boundaries and out of the Death Valley region into the Las Vegas Valley.

Fractures and faults within the hydrogeologic units con-stitute the dominant pathways for regional groundwater fl ow

Figure 2. Schematic block diagram of Death Valley regional groundwater fl ow system illustrating the structural relations among mountain blocks, valleys, and groundwater fl ow (after Eakin et al., 1976).



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50,000-meter grid based on Universal Transverse Mercator projection,Zone 11. Shaded-relief base from 1:250,000-scale Digital Elevation Model;sun illumination from northwest at 30 degrees above horizon

Potentiometric surface contour, In meters above sea level, contour interval 100 meters (D'Agnese et al., 1998)

Spring Mts

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Meadows

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Yucca Mtn.

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Groom Range

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Figure 3. Potentiometric surface of the Death Valley region showing major potentiometric features (from D’Agnese et al., 1998).

14 Belcher et al.


(Faunt, 1997). The presence, orientation, and type of faults pro-vide major controls on groundwater fl ow according to three prin-ciples (Faunt, 1997, p. 24–31):

1. Large-scale folding and block faulting formed the major topographic features and sedimentary basins that defi ne the groundwater recharge and discharge areas.

2. Faulting and intense folding have induced fracturing in the rock masses and created highly permeable channels that have been enlarged by dissolution in the lower and upper carbonate-rock aquifers.

3. Faulting and folding in some rock types have created bar-riers to groundwater fl ow by placing permeable strata against low-permeability strata and by emplacing low-permeability materials within faults and fractures. These circumstances locally may cause groundwater to be forced to the surface as springs and diffuse discharge.

Tensional faults and fracture zones develop normal to regional extension (least-stress direction) and enhance large-scale permeability in that direction (Riggs et al., 1994; Faunt, 1997). This increase has been documented at Devils Hole (Fig. 1) (Carr, 1987). Compressional and shear features intersect the prin-cipal stress direction at moderate to large angles, and they com-monly are associated with mechanical and chemical effects that produce low-permeability gouge in and near faults. In the north-ern and central parts of the Death Valley regional groundwater fl ow system, the north-south to northeast-southwest orientation of major normal faults is approximately perpendicular to the least principal stress. Those normal faults thus increase fl ow in the southward direction, which is already favored by the regional topographic slope.

Interbasin Flow

Interbasin fl ow in the Great Basin region has been estab-lished by scientifi c studies over the past century. Interbasin fl ow, although it is not uniform between all basins, is common and is a function of the hydraulic gradient between basins and hydraulic conductivity of the intervening rocks. Several decades of geo-logic and hydrologic work in the Great Basin region has led to the conclusion that groundwater fl ow results from an interconnected, complex groundwater fl ow system (Mendenhall, 1909; Carpen-ter, 1915; Eakin and Moore, 1964; Eakin and Winograd, 1965; Eakin, 1966; Hunt et al., 1966, p. B40; Miffl in, 1968; Winograd and Thordarson, 1975; Miffl in and Hess, 1979; Harrill et al., 1988). Knowledge of interbasin groundwater fl ow through bed-rock and basin-fi ll deposits of the region is the basis for regional groundwater management and water-resource planning in the Great Basin. Several groundwater fl ow models have been con-structed by various workers who used this conceptual model of interbasin fl ow (D’Agnese et al., 1997, 2002; Belcher and Sweet-kind, 2010; Belcher et al., 2010).

In this prevailing conceptual model, water enters the sys-tem as interbasin underfl ow and as recharge from precipitation in upland areas. Because of present-day arid conditions, present-

day recharge is restricted to higher altitudes; virtually no recharge occurs and no perennial surface water fl ows in the lowlands and valley fl oors (Winograd et al., 2005). Groundwater fl ow paths within the system diverge from the highlands and are superim-posed on deeper regional fl ow paths that are controlled largely by fl ow in the regional carbonate-rock aquifer. The overall direction of fl ow is toward the hydrologic sink in Death Valley, although there are a number of local discharge areas.

Hydrologic Evidence of Interbasin FlowIn the 1960s, major hydrogeologic investigations (drilling,

hydraulic testing, and hydrogeochemical studies in conjunction with geologic mapping and geophysical surveys) at the Nevada Test Site demonstrated a regional carbonate-rock aquifer that fl anks and underlies most of the intermontane basins from east-central Nevada southward. Water levels measured in many test holes demonstrate that the potentiometric surface in the regional carbonate-rock aquifer generally is uninterrupted by the ridges that separate the many topographically closed basins of the region. These fi ndings have been interpreted as evidence that interbasin fl ow integrates Yucca Flat, Frenchman Flat, Mercury Valley, and other adjacent topographically closed basins into a single groundwater basin tributary to the springs at Ash Meadows in the east-central Amargosa Desert (Winograd and Thordarson, 1975; Laczniak et al., 1996; Thomas et al., 1996).

Interbasin Flow Inferred from Hydrochemical EvidenceThe hydrochemistry of the saturated zone is discussed by

Marshall et al. (this volume), and only a few major conclusions are presented here. Major-ion chemistry and certain radiogenic isotopes (such as strontium-87 [87Sr]) mainly indicate interac-tions between rock and water along a groundwater fl ow path. The composition of lighter isotopes (such as hydrogen and oxygen) provides information regarding location, timing, and environ-ment of groundwater recharge, as well as mixing of groundwater within and between various aquifer systems.

The major-ion chemistry of groundwater in the region defi nes two principal hydrochemical facies: a relatively dilute sodium-potassium bicarbonate facies associated with the Ter-tiary volcanic-rock aquifers and with volcanic-rock detritus derived from the volcanic rocks, and a more concentrated calcium-magnesium bicarbonate facies associated with the underlying lower and upper carbonate-rock aquifers and detrital material derived from these rocks. Where these two hydrochem-ical facies mix, the groundwater forms a calcium- magnesium-sodium bicarbonate facies. In addition, a playa facies of vari-able hydrochemistry associated with near-surface evaporation is recognized locally within groundwaters in the basin-fi ll aqui-fers, and a distinct sodium-sulfate bicarbonate facies is recog-nizable in waters that are discharged from the regional system in springs in Death Valley.

Major fi ndings from the hydrochemical and isotopic data for groundwater in the central Death Valley region (Marshall et al., this volume) are as follows:



1. Deuterium (D) and delta oxygen-18 (δ18O) appear to be conservative constituents in the regional groundwater fl ow sys-tem; they indicate the conditions of recharge from land-surface precipitation. The values for δD and δ18O from groundwater in the Death Valley region differ markedly from the values for mod-ern precipitation in the region, which is heavier. This difference is interpreted to indicate that major recharge to the fl ow system from precipitation occurred under pluvial conditions that pre-vailed during the Pleistocene and early Holocene. At that time, humidity near the eastern Pacifi c Ocean likely would have been higher than present-day humidity.

2. Uranium 234U/238U activity ratios measured in groundwa-ter beneath Yucca Mountain support the concept that at least the upper part of the saturated zone is recharged by downward per-colation through the mountain.

3. Hydrochemical data obtained from groundwater in the Fortymile Wash drainage system support the interpretation of generally southward regional groundwater fl ow derived from potentiometric data (Marshall et al., this volume).

4. The chemical composition of the water discharged from major springs in the Furnace Creek Wash area on the eastern fl ank of Death Valley suggests that these springs constitute a major discharge area for regional groundwaters fl owing southwest-ward through unit lower carbonate-rock aquifer. These regional groundwaters may have mixed with waters in the southern central Amargosa Desert alluvium. The strontium isotope data further suggest that these spring waters have reacted with Precambrian rocks or detritus derived from the Precambrian rocks.

Overall, the regional hydrochemical and isotopic data (Mar-shall et al., this volume) support the conceptual model of inter-basin fl ow.

Boundaries of Death Valley Regional Flow System

The upper, lower, and lateral fl ow boundaries have been defi ned for the saturated zone in the Death Valley region as either physical boundaries (caused by changes in bedrock characteris-tics) or hydraulic boundaries (caused by potentiometric-surface confi gurations). The upper bounding surface is the water table (D’Agnese et al., 2002; Faunt et al., 2010a). Groundwater moves across this boundary in one of two ways: vertically downward across the upper bounding surface into the fl ow system as regional groundwater recharge; or vertically upward across this boundary out of the system at natural groundwater discharge sites (springs, evapotranspiration areas, and wet playas). The lower bounding surface is at a depth where groundwater fl ow is dominantly hori-zontal or parallel to the lower surface and moves with such small velocities that the volumes of water involved are only a very small percentage of the total estimated regional fl ow. The depth of this surface varies and generally corresponds with the upper surface of low-permeability basement rock (crystalline-rock con-fi ning unit [XCU]; Table 2).

The boundaries of the Death Valley region have been variably defi ned and named by several investigators (Harrill

et al., 1988; Harrill and Prudic, 1998; Bedinger et al., 1989a; Bedinger and Harrill, 2010; D’Agnese et al., 1997) (Fig. 1). The lateral boundary of the Death Valley region is a combina-tion of no-fl ow boundaries resulting from physical barriers or hydraulic separation of fl ow regimes (groundwater divides or regional fl ow lines) and of arbitrary lateral-fl ow (throughfl ow) boundaries where water fl ows across the system boundary. No-fl ow conditions exist where groundwater movement across the boundary is impeded by physical barriers, and fl ow is parallel to the boundary, or by groundwater divides that force groundwater fl ow paths to diverge. The location of fl ow paths and ground-water divides may shift if the hydraulic head changes (Faunt et al., 2010a). An estimated regional potentiometric-surface map was developed for the Death Valley region to delineate areas that contribute infl ow to or receive outfl ow from the region across the regional fl ow-system boundary (Bedinger and Har-rill, 2010).

Subregional Flow Systems

Groundwater fl ow in the Death Valley region is described in terms of the northern, central, and southern Death Valley sub-regions (Fig. 4) of D’Agnese et al. (1997, p. 62–67). The sub-regions are further subdivided into groundwater sections, and the sections in the central Death Valley subregion are grouped into groundwater basins (Table 4). These subregions, basins, and sections are used for descriptive purposes only, and the bound-aries do not defi ne independent fl ow systems. The subregions, basins, and sections are delineated primarily by (1) location of recharge areas; (2) regional hydraulic gradients; (3) distribu-tion of aquifers, structures, and confi ning units that affect fl ow; (4) location of major discharge areas; and (5) hydrochemical composition of the groundwater. Flow directions across the sys-tem boundary are based on the lateral fl ow estimates provided in Harrill and Bedinger (2010). Much of this description is taken from, and more detail on the subregional fl ow systems can be found in, Faunt et al. (2010a).

Northern Death Valley SubregionGroundwater in the northern Death Valley subregion is

derived from precipitation on the Montezuma and Panamint Ranges, Slate Ridge, and the Palmetto, Stonewall, and Gold Mountains (Fig. 5). Groundwater also may enter the northern subregion across the boundary for the Death Valley regional fl ow system from Eureka Valley, from the southern part of Saline Val-ley, and possibly across the northern part of the Panamint Range (Harrill and Bedinger, 2010). Much of the groundwater fl ow is controlled by northeast-southwest–trending structural zones (Carr, 1984; Sweetkind et al., 2010). Deep regional fl ow is unlikely because the relatively low-permeability, shallow, intrusive-rock confi ning unit (ICU), the lower clastic-rock confi ning unit (LCCU), and the crystalline-rock confi ning unit (XCU) under-lie most of the subregion. The lower carbonate-rock aquifer (LCA) crops out extensively in the Grapevine and Cottonwood

16 Belcher et al.


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Subregion boundary (within model domain)

Southern Railroad ValleyReveille ValleyStone Cabin ValleyRalston Valley (Mud Lake)Fish Lake and Eureka ValleysSaline Valley

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EXPLANATION

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shear zone

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Figure 4. Interpreted subregions and associated fl ow paths of the Death Valley region (from Faunt et al., 2010c, their fi gure D-5).

Mountains (Fig. 5) in the southern part of the subregion. The lower carbonate-rock aquifer has been interpreted to exist in the subsurface in the southern part of the subregion (Grose, 1983; Sweetkind et al., 2001), including the southern part of Sarcobatus Flat and the vicinity of Grapevine Springs (Fig. 5) in the north-ern part of Death Valley. Pumpage in the northern Death Valley subregion has been negligible, and the change in the volume of

groundwater storage relative to the total amount in storage is neg-ligible (Moreo et al., 2003).

Central Death Valley SubregionIn the central Death Valley subregion, the dominant fl ow

paths have been interpreted to be associated with major regional or intermediate discharge areas and have been grouped into three



groundwater basins on the basis of the major discharge areas (Fig. 6): Pahute Mesa–Oasis Valley basin, Ash Meadows basin, and Alkali Flat–Furnace Creek basin (Waddell, 1982; D’Agnese et al., 1997, 2002).

The Pahute Mesa–Oasis Valley groundwater basin is the smallest and northernmost of the three basins, and its extent is not well defi ned (Fig. 6). Groundwater is derived primar-ily from recharge in Pahute Mesa and the Kawich, Cactus, and Belted Ranges (D’Agnese et al., 1997). Additional recharge from within the basin may originate at Black and Quartz Mountains. Throughfl ow into the Pahute Mesa–Oasis Valley basin may come from the southern part of Railroad, Reveille, and Stone Cabin Valleys (Harrill and Bedinger, 2010).

At Oasis Valley, groundwater is diverted upward by the con-fi ning units along faults to discharge by evapotranspiration and spring fl ow at and along the fl ood plain of the Amargosa River and tributary drainages (Fig. 6) (White, 1979; Laczniak et al., 1996). Mass-balance calculations indicate that about half the water that fl ows to Oasis Valley discharges through evapotrans-piration (White, 1979). Groundwater that does not discharge within Oasis Valley fl ows through a veneer of alluvium or the low-permeability basement rocks at Amargosa Narrows south of Beatty, Nevada (Fig. 6), and into the Alkali Flat–Furnace Creek basin (Waddell, 1982; Laczniak et al., 1996). Some groundwater may not reach Oasis Valley and may fl ow around the northern part of Bare Mountain toward Crater Flat (Fig. 6). Likewise, some groundwater in the northwestern part of the section (parts of Cactus and Gold Flats) may fl ow toward the eastern part of Sarcobatus Flat. The Oasis Valley section contains the basin’s major discharge area. Water is withdrawn for irrigation and for

Subregion of Death Valley

Groundwater basin Section

TABLE 4. DIVISIONS OF THE DEATH VALLEY REGIONAL GROUNDWATER FLOW SYSTEM (MODIFIED FROM FAUNT ET AL.,

2010c, THEIR TABLE D-1)

Northern Lida-Stonewall Sarcobatus Flat Grapevine Canyon–Mesquite Flat Oriental Wash

Central Pahute Mesa– Oasis Valley

Southern Railroad Valley–Penoyer ValleyKawich Valley Oasis Valley

Ash Meadows Pahranagat Tikaboo Valley Indian Springs Emigrant Valley Yucca–Frenchman Flat Specter Range

Alkali Flat– Furnace Creek

Fortymile Canyon Amargosa River Crater Flat Funeral Mountains

Southern Pahrump Valley Shoshone–Tecopa California Valley Ibex Hills

domestic and public supply in upper Oasis Valley. The Pahute Mesa–Oasis Valley basin part of the Nevada National Security Site has been pumped periodically since the 1950s for water sup-plies and for long- and short-term aquifer tests to help character-ize the fl ow system. Most of this development has been small in scale and likely has had little long-term effect on the system. Similarly, the relatively small amount of pumpage in the area of Penoyer Valley for irrigation likely has had little long-term effect (Moreo et al., 2003).

The Ash Meadows basin is the largest basin in the central Death Valley subregion (Fig. 6) (Waddell, 1982). Much of the groundwater in this basin is derived from recharge on the Spring Mountains and the Sheep, Pahranagat, and Belted Ranges. Recharge also may be derived within the basin on the Spotted, Pintwater, and Desert Ranges (Laczniak et al., 1996). The Ash Meadows basin is subdivided into six sections: Pahranagat, Tika-boo Valley, Indian Springs, Emigrant Valley, Yucca–Frenchman Flat, and Specter Range.

The Ash Meadows discharge area (Fig. 6) represents the terminus of the Ash Meadows basin. Water entering Ash Mead-ows encounters a northwest-southeast–trending fault that jux-taposes fi ne-grained basin-fi ll sediments and the more perme-able regional carbonate-rock aquifer (Dudley and Larson, 1976, p. 9–10). Groundwater discharges in Ash Meadows through ~30 springs along a 16-km-long spring line that generally coincides with the trace of the buried fault. All major springs emerge from circular pools, are relatively warm, and discharged at nearly con-stant rates from 1953 until agricultural development began in the area in 1969 (Dettinger et al., 1995, p. 79). Although most of the spring discharge at Ash Meadows likely reinfi ltrates and recharges the basin-fi ll aquifers, much of this recharge may dis-charge as evapotranspiration from alluvium along the Amargosa River, Carson Slough, and Alkali Flat (Figs. 1 and 6) (Czarnecki and Waddell, 1984; Czarnecki, 1997).

Groundwater is pumped from wells scattered throughout the Ash Meadows basin. Wells near Ash Meadows tap the basin-fi ll aquifers adjacent to the regional carbonate-rock aquifer. Wells on the Nevada National Security Site within the basin are used to supply ~50% of the water demand at the Nevada National Secu-rity Site (Laczniak et al., 1996). Pumping from basin-fi ll aquifers around Devils Hole (Fig. 1), a tensional and collapse feature in the carbonate rock (Riggs et al., 1994) supporting an endemic species of desert pupfi sh (Cyprinidon diabolis), caused the water level to decline in Devils Hole and fl ow to decrease or temporarily stop at several major springs issuing from the regional carbonate-rock aquifer (Dudley and Larson, 1976). After pumping ceased, water levels and spring fl ow gradually recovered. The effect of pumping on individual springs differed, indicating that a variable degree of hydraulic connection exists between the basin fi ll and the regional carbonate-rock aquifer (Dettinger et al., 1995, p. 80).

Previous conceptual models of the Ash Meadows basin indi-cate substantial amounts of fl ow from Pahranagat Valley into Ash Meadows (Winograd and Thordarson, 1975). Evaluations of hydrochemical data, however, indicate that the volume of this

18 Belcher et al.


infl ow could be negligible (J.M. Thomas and William Sicke, Des-ert Research Institute, Reno, Nevada, 2003, written commun.). Analysis of calcite veins precipitated at Devils Hole (Winograd et al., 1992) suggests that most, and perhaps all, of the ground-water in Ash Meadows originates from the Spring Mountains. This conclusion is strengthened by the synchroneity of δ18O time series obtained from vein calcite in Devils Hole with time series

of sea-surface temperature off southern California during the past 160,000 yr (Winograd et al., 2006). This synchroneity can be interpreted to support a single principal source for the Ash Meadows spring discharge—that is, recharge from the Spring Mountains (I.J. Winograd, 2007, written commun.).

Groundwater that bypasses the springs at Ash Meadows may fl ow to Furnace Creek (Fig. 6) or may recharge the basin-fi ll

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CentralDeath Valley

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Regional springs

Spring



Grapevine Canyon- Mesquite FlatOriental Wash

Sarcobatus FlatLida-Stonewall

General direction of groundwater flow associated with groundwater section

Populated place

Groundwater section boundary and name

Potential flow into or between subregions


ABC

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Cottonwood M

ts

Figure 5. Northern Death Valley subregion of the Death Valley regional groundwater fl ow system showing groundwater sections and fl ow directions (from Faunt et al., 2010c, their fi gure D-6).



sediments and join other groundwater in the basin-fi ll sediments to fl ow southward toward Alkali Flat, where it either discharges or continues south to the southern Death Valley subregion. Three springs at the southern end of the Ash Meadows spring line (Big, Bole, and Last Chance) have elevated strontium values, which may indicate that they receive some fl ow from a different ori-

gin, such as the Pahrump Valley (Peterman and Stuckless, 1993a, p. 70; Peterman and Stuckless, 1993b, p. 712). High-resolution aeromagnetic surveys over the Amargosa Desert and Pahrump indicate a possible hydraulic connection between Pahrump Val-ley and the Amargosa Desert through Stewart Valley (Blakely and Ponce, 2001).

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50,000-meter grid based on Universal Transverse Mercator projection, Zone 11. Image is false-color composite combining LANDSAT 7 spectral bands 2, 5, and 7 on shaded-relief base from 1:250,000-scale Digital Elevation Model; sun illumination from northwest at 30 degrees above horizon

Pint

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ange

Las Vegas Valley shear zone

Pahute Mesa–Oasis Valley groundwater basin

Section name:

Section name:

Section name:

Southern Railroad Valley- Penoyer ValleyKawich ValleyOasis Valley

Ash Meadows groundwater basin

PahranagatTikaboo ValleyIndian SpringsEmigrant ValleyYucca–Frenchman FlatSpecter Range

Alkali Flat–Furnace Creek groundwater basin

Fortymile CanyonAmargosa RiverCrater FlatFuneral Mountains

a

bc

abcdef

abcd

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3

Regional spring


Populated place


Groundwater basin boundary


Spotted Range–Mine Mountain structural zone

1





EXPLANATION

Figure 6. Central Death Valley subregion of the Death Valley regional groundwater fl ow system showing groundwater basins, sections, and fl ow directions (from Faunt et al., 2010c, their fi gure D-7).

20 Belcher et al.


The basin-fi ll and volcanic-rock aquifers in Emigrant Valley and Yucca and Frenchman Flats (Fig. 6) provide recharge (from the local highland areas) to the regional carbonate-rock aquifer by downward percolation (Winograd and Thordarson, 1975; Lac-zniak et al., 1996). The water chemistry at Indian Springs Valley indicates that these waters have had little contact with volcanic rock or basin-fi ll sediments composed of volcanic rocks; thus, the groundwater beneath Tikaboo and Emigrant Valleys and Yucca and Frenchman Flats is not moving southward toward Indian Springs Valley. The water in the regional carbonate-rock aquifer in these locations may be moving toward the Amargosa Desert, where the groundwater is generally of mixed chemical character and has high levels of sodium (Schoff and Moore, 1964; Wino-grad and Thordarson, 1975). Ultimately most of the groundwater discharges at Ash Meadows.

The Pahranagat section is recharged partly by throughfl ow from Tikaboo Valley. Regional groundwater recharged on the Sheep Range and Spring Mountains fl ows into the Indian Springs section (Fig. 6) from the south and east (Fig. 3). Recharge also may originate on higher mountains of the Spotted, Pintwater, and Desert Ranges (Laczniak et al., 1996), most of which are underlain by carbonate rocks. Most of the water has had little opportunity for contact with volcanic rock or basin-fi ll sediments composed of volcanic rocks. As a result, hydrochemical data can be useful in delineating fl ow paths to and from this region.

The Alkali Flat–Furnace Creek basin lies south and west of the Ash Meadows and Pahute Mesa–Oasis Valley basins; it occupies a large part of the western half of the Nevada National Security Site and contains Yucca Mountain (Fig. 6). Groundwa-ter in this basin is derived from recharge on Pahute and Rainier Mesas, Timber and Shoshone Mountains, and the Grapevine and Funeral Mountains. Additional recharge to this basin may be throughfl ow from Sarcobatus Flat, Oasis Valley, and Ash Meadows. Recharged groundwater from throughfl ow and local recharge moves through volcanic-rock aquifers in the north and basin-fi ll and regional carbonate-rock aquifers in the south toward discharge areas in the southern and southwestern parts of the basin. Subsurface outfl ow follows the general course of the Amargosa River drainage (Fig. 1) through a veneer of alluvium near Eagle Mountain into the southern Death Valley subregion (Walker and Eakin, 1963). As with the other basins, the location of the boundary of the Alkali Flat–Furnace Creek basin is neither well established nor fully understood. The Alkali Flat–Furnace Creek basin is divided into four sections: the Fortymile Canyon, Amargosa River, Crater Flat, and Funeral Mountains sections.

The main discharge for the basin is the springs in the Furnace Creek area (Fig. 6). Hydrochemical data indicate that spring fl ow in the major springs in the Furnace Creek area likely derives from the regional carbonate-rock aquifer (Winograd and Thordarson, 1975, p. C95). However, the δ87Sr data indicate some complexity beyond this simple conceptual fl ow model (Marshall et al., this volume). Similar hydrochemistry between spring waters at Ash Meadows and the Furnace Creek area (Czarnecki and Wilson, 1991; Steinkampf and Werrell, 2001) indicates a hydraulic con-

nection between these two discharge areas through the regional carbonate-rock aquifer by way of large-scale fractures or channels in the carbonate rocks (Winograd and Pearson, 1976). Downgra-dient from the Furnace Creek springs, groundwater not exiting at the springs and reinfi ltrated spring fl ow move toward the Death Valley salt pan, and this water is transpired either by stands of mesquite on the lower part of the Furnace Creek fan or is evapo-rated from the salt pan in Badwater Basin (Fig. 6). The Death Valley salt pan is the largest playa in the region, and despite the low rate of evapotranspiration from the salt pan proper, the large area of this feature results in a substantial amount of discharge (DeMeo et al., 2003).

Hydraulic and hydrochemical data indicate that water in the regional fl ow system in the southern part of Amargosa Desert (Fig. 6) may fl ow southwest toward Death Valley through frac-tures in the southeastern end of the Funeral Mountains or fl ow southward and toward the surface at Alkali Flat (or Franklin Lake playa), defl ected by the low-permeability quartzites of the Resting Spring Range (Fig. 6) (Czarnecki and Waddell, 1984; Czarnecki and Wilson, 1991). The carbonate rocks beneath the Funeral Mountains also might provide preferential conduits or drains for fl ow from the basin-fi ll sediments beneath the Amar-gosa Desert toward Death Valley (Czarnecki and Waddell, 1984; Luckey et al., 1996, p. 14).

Southern Death Valley SubregionGroundwater in the southern Death Valley subregion is

derived primarily from recharge on the Spring Mountains and to a lesser extent from recharge on the Nopah, Kingston, and Gre-enwater Ranges (Fig. 7). Groundwater also may be entering the system as throughfl ow in the basin-fi ll sediments of the Silurian Valley and valleys adjacent to the Owlshead Mountains (Harrill and Bedinger, 2010). Additional minor groundwater infl ow may move across the boundary from the Alkali Flat–Furnace Creek basin south of Alkali Flat (Fig. 7). The largest discharge area in the subregion is in Pahrump Valley, which contains a broad playa with several springs. The subregion contains four sections: Pah-rump Valley, Shoshone-Tecopa, California Valley, and Ibex Hills, each with an area of considerable discharge (Faunt et al., 2010a).

Before extensive development, the playa area in Pahrump Valley contained some phreatophytic vegetation and was sur-rounded by sparse shrubland vegetation rising into alluvial fans. Groundwater withdrawals accompanying large-scale agricultural development in the Pahrump Valley section caused some major springs in the area to stop fl owing during withdrawal; spring fl ow gradually recovered after some withdrawal stopped (Faunt et al., 2010a). Historically, Manse and Bennetts Springs (Fig. 7) dis-charged along the base of the broad alluvial fans at the foot of the Spring Mountains. Groundwater withdrawal in the valley caused these springs to cease fl owing in the 1970s. In the late 1990s, Manse Spring began to fl ow again, perhaps owing to changes in the amount of agriculture and agricultural practices in the valley. Withdrawal in the valley does continue for domestic uses and small-scale agricultural uses (Moreo et al., 2003).



Regional Flow System Water Budget

Many investigators have estimated water budgets for local areas within the Death Valley region (Malmberg and Eakin, 1962; Walker and Eakin, 1963; Malmberg, 1967; Winograd and Thordarson, 1975; Miller, 1977; Harrill, 1986). These previous budgets were based on estimates of recharge and discharge using

the Maxey-Eakin method (Maxey and Eakin, 1950), groundwa-ter evapotranspiration and spring discharge, and estimated inter-basin underfl ow, which was calculated as the difference between precipitation recharge and evapotranspiration.

A water budget is developed to evaluate the balance between the fl ow into and fl ow out of a groundwater fl ow system. The primary components of the regional water budget described

CALIFORNIA

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SaratogaSprings

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Badw

ater Basin

IbexHills

Salt Springs

Hills

Death Valley

OwlsheadMts

Ash

MeadowsAlkaliFlat

EagleMtn

Nopah Range

Resting

Spring Range

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Spring Mts

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Californ

iaValle

y

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TecopaShoshone

ChinaRanch

Greenwater

Range

KingstonRange

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ValjeanValley

ShadowValley


Subregion


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Am

argosa River

Manse Spring

Bennetts Spring

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4020 60 KILOMETERS50,000-meter grid based on Universal Transverse Mercator projection, Zone 11. Image is false-color composite combining LANDSAT 6 spectral bands 2, 5, and 7 on shaded-relief base from 1:250,000-scale Digital Elevation Model; sun illumination from northwest at 30 degrees above horizon

Pahrump Valley Shoshone-TecopaCalifornia ValleyIbex Hills

Regional springs


Populated place



ABCD

EXPLANATIONDeath Valley regional groundwater flow systemmodel boundary




Figure 7. Southern Death Valley subregion of the Death Valley regional groundwater fl ow system showing groundwater sections and fl ow directions (from Faunt et al., 2010c, their fi gure D-8).

22 Belcher et al.


herein are natural discharge (evapotranspiration and spring fl ow), pumpage, recharge, and lateral fl ow into and out of an area (esti-mated by using Darcy calculations or existing water budgets). The introduction of pumping as a discharge from the fl ow sys-tem initially decreases hydraulic heads and ultimately affects one or more fl ow components by decreasing natural discharge or by increasing recharge, as well as by removing groundwater from aquifer storage. Much of the following information is taken from the San Juan et al. (2010) summary of the water budget for the Death Valley region. The volumetric fl ows estimated for the major water-budget components of the Death Valley region are summarized in Table 5.

For prepumped conditions, annual recharge accounted for ~87% of the total infl ow (143.4 million cubic meters [Mm3]), and natural discharge (evapotranspiration and spring fl ow) accounted for ~93% of the total outfl ow (133.8 Mm3). The remainder of the infl ow and outfl ow is accounted for by lateral fl ows into and out of the Death Valley region. The unaccounted for difference between estimated prepumped infl ows and outfl ows is less than 7% of the estimated infl ow. By 1998, pumpage was ~93.5 Mm3, which equates to ~70% of the total outfl ow estimated for pre-pumped conditions. Note that this pumpage estimate is not adjusted for any potential return fl ow and that Table 5 does not account for return fl ow as a potential infl ow to the 1998 water budget (San Juan et al., 2010, p. 117).

Natural DischargeThe quantity of groundwater discharging from most of

the major discharge areas in the Death Valley region (Fig. 8) has been estimated primarily from spring-fl ow measurements,

groundwater evapotranspiration estimates, or a combination of the two. Usually, groundwater discharge has been estimated only for an individual discharge area or at a specifi c location and not for the entire fl ow system: Malmberg and Eakin (1962), Walker and Eakin (1963), Pistrang and Kunkel (1964), Hunt et al. (1966), Malmberg (1967), Glancy (1968), Rush (1968), Van Denburgh and Rush (1974), Winograd and Thordarson (1975), Miller (1977), Harrill (1986), Czarnecki (1997), D’Agnese et al. (1997), Laczniak et al. (1999), Reiner et al. (2002), and DeMeo et al. (2003). Discrepancies in discharge estimates between more recent and earlier reports typically refl ect differences in the delin-eation of the area contributing to evapotranspiration, the num-ber of springs measured, evapotranspiration rates estimated for vegetation types, or some combination thereof (Laczniak et al., 2001, p. 31; D’Agnese et al., 2002, p. 26).

Less than 5% of the Death Valley region is covered by dis-charge areas (Fig. 8; San Juan et al., 2010, p. 101) such as wet playas, wetlands with free-standing water or surface fl ow, nar-row drainages lined with riparian vegetation, and broad areas of phreatophytic shrubs and grasses. The discharge areas with the greatest fl ow volumes are Death Valley, Ash Meadows, and Sarcobatus Flat (Fig. 8; Table 6). Each of these discharge areas represents a unique environment, and together they account for most of the types of local habitat supported by groundwa-ter discharge throughout the Death Valley region. Death Val-ley is dominated by a salt pan surrounded by alluvial fans and by numerous locally and regionally fed springs fringed with a variety of desert shrubs, trees, and grasses. Ash Meadows is a unique desert oasis that consists of broad wetlands fed by orifi ce-type springs. These large-volume springs are surrounded

TABLE 5. ESTIMATES OF ANNUAL VOLUMETRIC FLOW FOR MAJOR WATER-BUDGET COMPONENTS OF DEATH VALLEY REGIONAL GROUNDWATER FLOW SYSTEM MODEL DOMAIN BEFORE PUMPING

AND IN 1998 (MODIFIED FROM SAN JUAN ET AL., 2010, THEIR TABLE C-5)

Water-budget component

Estimated annual volumetric flow (Mm3) Prepumped conditions 1998

wolfnI 521 521 )noitartlifni ten( egrahceR – 4.81 *wolfni yradnuoB 4.341 LATOT wolftuO

Natural discharge: ET† 5.701< 5.701 §

Natural discharge: Spring flow# 5 8.61 – 5.9 *wolftuo yradnuoB

Pumpage** 5.39 0 8.331 LATOT

ecnereffiD 6.9 wolftuo sunim wolfnI 7.6 ecnereffid egatnecreP

Note: – indicates no estimate was made or available; ET—evapotranspiration; Mm³—million cubic meters. *See Table C-4 in San Juan et al. (2010) for more details. Boundary flux was estimated only for steady-state conditions and is unknown for 1998. †Estimate for prepumped conditions not included in estimate of San Juan et al. (2010, their table C-1) or in Table 6 for Pahrump Valley. §“Less than” symbol is intended to indicate only that the value likely is less than the prepumped natural discharge. #Bennetts and Manse Springs were reported dry after 1975. See San Juan et al. (2010, their table C-2) for a discussion of these values. **See San Juan et al. (2010, their table C-3) for more details.



Spring

MountainsSpring

MountainsSpring

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36º

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EXPLANATION

Death Valley

Pahrump Valley

Ash Meadows

AmargosaDesert

Staininger and Grapevine

Springs area

SarcobatusFlat

OasisValley

Texas, Travertine, and Nevares Springs area

FranklinWell

FranklinLake

PenoyerValley

Indian and CactusSprings area

Manse andBennetts

Springs areaStewartValley

ChicagoValleyShoshone

Tecopa/California

Valley

Spring-flow-based discharge estimate



Area of natural groundwater discharge

Evapotranspiration-based discharge estimate, assumed prepumped conditions (Van Denburgh and Rush, 1974; Laczniak et al., 2001; Reiner et al., 2002; DeMeo et al., 2003)Evapotranspiration-based discharge estimate, 1959–61 conditions (D'Agnese et al., 2002)

Figure 8. Major areas of natural groundwater discharge in the Death Valley region (from San Juan et al., 2010, their fi gure C-2).

24 Belcher et al.


by extensive grass meadows interspersed with moderately dense to sparse stands of trees and shrubs. Sarcobatus Flat is a broad playa surrounded by moderately dense grasses and sparse shrubs that are supported by a few small springs and seeps and a moderately shallow water table.

Recent investigations of natural groundwater discharge in the Death Valley region estimated discharge by calculating groundwater evapotranspiration (San Juan et al., 2010, p. 101). The underlying assumption of this approach is that most of the groundwater issuing from springs and seeps within the dis-charge area ultimately is evaporated or transpired locally in the Death Valley region and therefore is accounted for in estimates of evapotranspiration (Laczniak et al., 1999, 2001; Reiner et al., 2002; DeMeo et al., 2003). Laczniak et al. (2001) provided the most comprehensive evaluation of groundwater discharge in that they estimated groundwater discharge for 9 of the 15 major evapotranspiration-dominated discharge areas in Death Valley region (Fig. 8). Their estimate of discharge in Oasis Valley was revised in a subsequent study (Reiner et al., 2002). Laczniak et al. (2001) made no attempt to revise estimates of natural dis-charge from Pahrump and Penoyer Valleys because groundwater withdrawn for irrigation had locally altered the distribution of native vegetation and decreased local spring fl ow. D’Agnese et al. (2002, p. 26) provided an estimate of natural discharge from Pahrump Valley but stated that their estimate was based on an

evapotranspiration analysis that used a map delineating the native phreatophyte distribution in 1959–1961 (Malmberg, 1967, their plate 3)—a time by which vegetation already had been signifi -cantly affected by local pumping. These same authors presented an estimate of natural discharge from Penoyer Valley that was fi rst documented in a reconnaissance report by Van Denburgh and Rush (1974, p. 23). DeMeo et al. (2003) was the primary source used to develop estimates of groundwater discharge from the fl oor of Death Valley.

The more recent investigations (Laczniak et al., 1999, 2001; Reiner et al., 2002; DeMeo et al., 2003) were similar in that continuous micrometeorological data were collected to estimate local evapotranspiration rates, and remotely sensed multispectral data were used to distribute measured evapotranspiration rates throughout the area evaluated. Micrometeorological data were collected continuously at 15 stations for 1–3 yr each in Ash Mead-ows and Oasis Valley (Laczniak et al., 1999; Reiner et al., 2002) and at six sites in Death Valley during a 4 yr period (DeMeo et al., 2003). Remotely sensed images, aerial photographs, and soils and wetland maps were integrated by using geographic informa-tion system techniques and were used in these studies to delineate evapotranspiration units (areas of similar vegetation and mois-ture conditions) and distribute calculated evapotranspiration rates throughout respective discharge areas. This process resulted in more consistent and generally improved estimates of groundwater

TABLE 6. ESTIMATES OF MEAN ANNUAL GROUNDWATER DISCHARGE BASED ON EVAPOTRANSPIRATION-BASED DISCHARGE FROM MAJOR EVAPOTRANSPIRATION-DOMINATED DISCHARGE AREAS IN DEATH VALLEY REGIONAL GROUNDWATER FLOW

SYSTEM (MODIFIED FROM SAN JUAN ET AL., 2010, THEIR TABLE C-1)

Discharge area Estimate mean annual ET rate

(m/yr)

Area (km2)

Annual precipitation rate

(m/yr)

Estimate precipitation-adjusted annual ET rate

(m/yr)

Estimated mean groundwater

discharge (m3/yr)

000,302,22 44.0 11.0 5.05 55.0 *swodaeM hsA 000,035 32.0 11.0 84.2 43.0 *yellaV ogacihC 000,432,1 31.0 01.0 34.9 32.0 *ekaL nilknarF

Franklin Well area* 0.46 1.20 0.11 0.35 432,000

Oasis Valley† 000,104,7 55.0 51.0 9.31 07.0 Pahrump Valley§ 000,280,8 76.0 21.0 2.21 97.0 #

000,056,4 60.0 – 9.67 – **yellaV reyoneP 000,530,61 21.0 51.0 6.831 72.0 *talF sutabocraS 000,095,2 64.0 90.0 26.5 55.0 *aera enohsohS 000,432,1 90.0 11.0 2.21 02.0 *yellaV trawetS

Tecopa–California Valley area* 0.64 14.2 0.09 0.55 7,894,000 Death Valley floor†† 000,271,34 10.0 – 5.544 – §§

Total 115,457,000 Note: Groundwater discharge is rounded to the nearest thousand. Rates are rounded to the nearest hundredth. Mean annual groundwater discharge may not equal product of precipitation-adjusted ET rate and area because of rounding. Dashes indicate that no value was reported in referenced source or that the information given was insufficient to compute a value. ET—evapotranspiration; Mm³—million cubic meters. *Laczniak et al. (2001, their tables 5 and 10). †Reiner et al. (2002, their table 5). §D’Agnese et al. (2002, their table 3). #Estimate represents annual groundwater discharge during period 1959–1961. **Van Denburgh and Rush (1974, their table 8 and p. 23); D’Agnese et al. (2002, p. 26). ††DeMeo et al. (2003, their table 4). §§Estimate varies from about 27.1 Mm3 to 43.2 Mm3 as adjusted for different flood recurrence intervals (DeMeo et al., 2003, p. 24). Flood-adjusted ET estimate reported by DeMeo et al. (2003, p. 24) is 40.7 Mm3.



discharge than were provided by previous studies. It is important to note that the evapotranspiration described here is derived from the regional groundwater fl ow system and not from precipitation and is designated as “groundwater evapotranspiration” to distin-guish it from evapotranspiration derived from precipitation.

Limitations inherent in an evapotranspiration-based approach for estimating groundwater discharge can be attributed to errors in delineating the extent of evapotranspiration units and errors in calculating evapotranspiration rates (Laczniak et al., 2001, p. 31). Other factors potentially affecting the accuracy of evapotranspiration-based estimates of groundwater discharge include (1) the assumption that all spring fl ow ultimately is evap-orated or transpired within the discharge area, (2) the assump-tion that surface-water infl ow is minimal, (3) the short period of record used to compute mean annual evapotranspiration rates, (4) the small number of local sites used to estimate mean annual evapotranspiration rates, (5) uncertainties associated with esti-mating evapotranspiration on the basis of relative differences in vegetation density, and (6) uncertainties in the amount of water contributed by precipitation and surface fl ow to the evapotranspi-ration estimates (Laczniak et al., 2001, p. 31).

Most of the groundwater discharged naturally from the Death Valley region fl ows from springs and seeps. Regional high-volume springs having fl ows greater than 1500 m3/d discharge in Oasis Valley, Ash Meadows, Pahrump Valley, the Shoshone and Tecopa areas, and on the fl oor of Death Valley (Fig. 8; San Juan et al., 2010, p. 104). Typically, these regional springs dis-charge water with temperatures greater than 30 °C (U.S. Geo-logical Survey, 2003) directly from the rocks that make up the regional aquifer. Because most fl ow from springs and seeps in major evapotranspiration-dominated discharge areas is evapo-rated and (or) transpired by the local riparian vegetation, ground-water evapotranspiration estimates are assumed to include spring and seep fl ow (Table 6; Laczniak et al., 2001; Reiner et al., 2002).

Spring discharge cannot always be quantifi ed accurately by using evapotranspiration-based methods. For example, evapotranspiration-based methods are not well suited for estimat-ing discharge in areas where springs support limited vegetation or where local pumping has decreased spring fl ow. Estimates of groundwater discharge from areas of spring fl ow not estimated by an evapotranspiration technique were derived solely on the basis of spring-fl ow measurements and are presented in Table 7. Areas of discharge not included in evapotranspiration-based estimates are the Staininger and Grapevine Springs areas near Scotty’s Castle in Death Valley; Texas, Travertine, and Nevares Springs areas near Furnace Creek Ranch in Death Valley; Indian and Cactus Springs areas near Indian Springs, Clark County, Nevada; and the Manse and Bennetts Springs areas in Pahrump Valley (Fig. 8). All discharge estimates, except those for Pahrump Valley (Bennetts and Manse Springs), were based on fl ow mea-surements made or compiled by C.S. Savard (U.S. Geological Survey, 2001, written commun.). Thus, any nonreferenced dis-charge values in the following sections are attributed to Savard’s unpublished work. The total annual discharge from spring fl ow summarized in Table 7 is ~16.8 Mm3. Uncertainties in the mea-surement of the spring fl ow are an expression of the accuracy of the measurement method and tools used. Accuracy of the spring fl ow measurements ranged from 10% to 25% (San Juan et al., 2010, their table C-2).

PumpageSubstantial quantities of groundwater have been pumped

from the Death Valley region. Groundwater pumping started around 1913 in Pahrump Valley to support a small agricultural community and has continued throughout the region to support local agriculture, mining, industry, and rural and urban growth. The number of pumping wells in the area encompassed by the Death Valley regional groundwater fl ow system increased

TABLE 7. ESTIMATES OF MEAN ANNUAL NATURAL GROUNDWATER DISCHARGE FROM MAJOR SPRING AREAS NOT INCLUDED IN EVAPOTRANSPIRATION-BASED DISCHARGE ESTIMATES (TABLE 6) IN THE DEATH VALLEY

REGIONAL GROUNDWATER FLOW SYSTEM (SAN JUAN ET AL., 2010, THEIR TABLE C-2)

General location Spring name or area Estimated discharge

rate (m3/d)

Estimated mean discharge

(m3/yr)

Estimated accuracy

(%) Death Valley, California Scotty’s Castle Staininger Spring* 51 000,873 5301

sgnirpS eniveparG *

2450 894,900 20

Furnace Creek Ranch Texas Spring* 51 006,544 0221 gnirpS enitrevarT * 01 001,196,1 0364

gnirpS seraveN *

1885 688,500 –

Clark County, Nevada Indian and Cactus Springs*

2240 818,200 10

Pahrump, Nevada Bennetts and Manse Springs† 32,400 11,834,100 25 004,057,61 068,54 latoT

Note: Volumetric rates are rounded to the nearest five; discharge is rounded to the nearest hundred; – indicates no value reported. *Estimate based on flow measurements made or compiled by C.S. Savard (U.S. Geological Survey, 2001, written commun.). †Estimate of groundwater discharge based on flow measurements from Bennetts and Manse Springs made before 1913, when groundwater pumping began (Maxey and Jameson, 1948; Malmberg, 1967; Harrill, 1986).

26 Belcher et al.


HA 146HA 146HA 146

HA 144HA 144HA 144

HA 243HA 243HA 243

HA 257, 258HA 257, 258HA 257, 258

HA 240HA 240HA 240

HA 241HA 241HA 241

HA 211HA 211HA 211

HA 168HA 168HA 168

HA 210HA 210HA 210HA 169BHA 169BHA 169B

HA 169AHA 169AHA 169A

HA 230P 638W 437

HA 230P 638W 437

HA 230P 638W 437

HA 242P 1W 2

HA 242P 1W 2

HA 242P 1W 2

HA 242P 1W 2

HA 225P 8W 1

HA 162P 2210W 7859

HA 162P 2210W 7859

HA 162P 2210W 7859

HA 161P 25W 85

HA 161P 25W 85

HA 161P 25W 85

HA 211P 7W 3

HA 211P 7W 3

HA 211P 7W 3

HA 163P 1

W 19

HA 163P 1

W 19

HA 163P 1

W 19

HA 228P 18W 28

HA 228P 18W 28

HA 228P 18W 28

HA 229P 1W 6

HA 229P 1W 6

HA 229P 1W 6

HA 227AP 9W 7

HA 227AP 9W 7

HA 227AP 9W 7

HA 227BP 9W 4

HA 227BP 9W 4

HA 227BP 9W 4

HA 159P 20W 11

HA 159P 20W 11

HA 159P 20W 11

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HA 160P 34W 7

HA 160P 34W 7

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EXPLANATION

DomesticMining, public supply, and commercialIrrigation

Hydrographic areas where pumpage exceeds 100 million cubic meters (Mm3) Hydrographic areas where pumpage exceeds 1 Mm3— HA, hydrographic area; P, total pumpage in Mm3; W, number of wells.

Amargosa Desert

Pahrump Valley

PenoyerValley

Death Valley regional groundwater flow system model boundaryNevada Test Site boundary

Pumping wells by water-use class (Moreo et al., 2003)

Hydrographic area boundary (modified from Cardinalli et al., 1968)

Spring

MountainsSpring

MountainsSpring

Mountains

Death Valley

HA 242 Hydrographic areas where pumpageis less than 1 Mm3— HA, hydrographic area

Figure 9. Spatial distribution of pumping wells by hydrographic area, water-use class, and total pumpage, 1913–1998, in the Yucca Mountain area (from San Juan et al., 2010, their fi gure C-4).



substantially from only a few wells in 1913 to nearly 9300 wells in 1998 (Moreo et al., 2003). About 8600 of the ~9300 wells investigated by Moreo et al. (2003) are located in the Death Valley region.

About 97% of the pumping wells are in the southern part of the Death Valley region (Fig. 9; Table 8). These wells are concen-trated primarily in the southern part of Amargosa Desert and in Pahrump Valley. Penoyer Valley has the greatest concentration of pumping wells in the northern part of the region. About 95% of the pumpage estimated from 1913 to 1998 was withdrawn from these three hydrographic areas (Fig. 9; Table 8) delineated by Cardinalli et al. (1968) on the basis of topographic basins. Of the 38 hydrographic areas in the Death Valley region, 16 have no reported pumping during this period.

Irrigation accounted for 90% of the groundwater pumped from the Death Valley region during 1913–1998. Irrigation gradu-ally declined from ~100% (~4940 Mm3) of the groundwater used in 1913 to ~80% (~74,710 of 93,450 Mm3) in 1998 (Fig. 10). Moreo et al. (2003) estimated annual irrigation by multiplying an irrigated acreage by a crop application rate. Mining, public supply, and commercial pumpage accounted for ~8% of all the

groundwater pumped from 1913 to 1998. By 1998, pumpage in this category had increased to nearly 13% of the annual total (Fig. 10). Pumpage for domestic use accounted for ~2% of the total groundwater pumped from 1913 to 1998. The percentage of water pumped for domestic use has gradually increased, and by 1998 it accounted for more than 7% of the annual total (Fig. 10). The minimum estimate of domestic pumpage was based on an annual per household consumption of 616.5 m3, and the maxi-mum estimate was based on an annual per household consump-tion of 1233 m3 (Moreo et al., 2003).

Estimates of annual groundwater pumpage in the Death Val-ley region increased from ~5 Mm3 in 1913 to ~93.5 Mm3 in 1998 (Fig. 10; Table 8). The greatest number of wells and the larg-est withdrawals are in Pahrump Valley, Amargosa Desert, and Penoyer Valley (Fig. 9). In the years 1913–1945, groundwater was used primarily for irrigation and was supplied by ~30 fl owing wells in Pahrump Valley (Moreo et al., 2003). After 1945, local water use relied on pumps and continued to increase as access to the region improved (Fig. 10; Moreo et al., 2003). The percentage of groundwater pumped for nonirrigation uses (domestic, min-ing, public supply, and commercial) increased from only a small

TABLE 8. NUMBER OF WELLS AND ESTIMATED TOTAL PUMPAGE, 1913–1998, BY HYDROGRAPHIC AREA FOR DEATH VALLEY REGIONAL GROUNDWATER FLOW SYSTEM (MODIFIED AFTER SAN JUAN ET AL.,

2010, THEIR TABLE C-3)

Hydrographic area Number of wells

1913–1998

Estimated pumpage Number (see Fig. 9)

Name 1913–1998 (m3)

1998 (m3)

068 000,21 1 yellaV adiL 441 061,52 000,058 51 talF sutabocraS 641 071,34 000,165,4 8 talF dloG 741 047,65 000,668 2 talF sutcaC 841 083,543 000,691,51 4 yellaV tnargimE A851 082,19 000,320,02 11 talF accuY 951 001,435 000,272,43 7 talF namhcnerF 061

161 Indian Springs Valley 85 25,422,000 789,680 162 Pahrump Valley 7859 2,210,135,000 43,855,360

080,13 000,950,1 91 yellaV etiuqseM 361 097,966,51 000,093,272 66 yellaV reyoneP 071

0394 000,791 2 yellaV daorliaR A371

211 Three Lakes Valley (southern part)

3 6,986,000 410,750

0073 000,974,8 1 yellaV yrucreM 522 068 000,83 1 yellaV kcoR 622

227A Fortymile Canyon (Jackass Flats)

7 8,510,000 184,650

227B Fortymile Canyon (Buckboard Mesa)

4 8,674,000 117,180

006,903 000,088,71 82 yellaV sisaO 822 054,171 000,490,1 6 talF retarC 922

230 Amargosa Desert 437 637,619,000 30,729,610 242 Lower Amargosa Desert 2 1,132,000 33,300

007,04 000,794 1 yellaV htaeD 342 033,944,39 000,298,572,3 9658 latoT

Note: Annual pumpage estimates computed from data in Moreo et al. (2003) for 22 hydrographic areas (Cardinalli et al., 1968) having reported pumpage.

28 Belcher et al.


percentage in 1960 to ~20% of the annual total in 1998. This trend is expected to continue as the population of Pahrump Val-ley and Amargosa Desert continues to increase. The total amount of groundwater pumped from the Death Valley region during the period 1913–1998 is estimated at ~3276 Mm3 (Table 8).

Moreo et al. (2003) in their estimate of pumpage expressed uncertainty as a range between a minimum and maximum esti-mate. Accordingly, their estimate of total pumpage from the Death Valley region during the period 1913–1998 ranges from 1616 to 6081 Mm3. This large uncertainty is attributed to incom-plete pumping records, misidentifi cation of crop type, and errors associated with estimating annual domestic consumption, the irrigated area, and crop application rates (Moreo et al., 2003). The error associated with the uncertainty in the application rate, which differs spatially and temporally with variations in potential evapotranspiration, length of growing season, irrigation systems, crop type, and management practices, exceeds that of all other uncertainties combined (Moreo et al., 2003).

RechargeGroundwater recharge is defi ned as water that infi ltrates

downward through the unsaturated zone into the water table and thus adds water to the saturated zone. Most of the groundwater recharge in the Death Valley region originates from precipitation that falls on mountainous areas throughout the region. The dis-tribution and quantifi cation of recharge for basins in the Death Valley region have been evaluated by using empirical (Maxey and Eakin, 1950; Malmberg and Eakin, 1962; Walker and Eakin, 1963; Malmberg, 1967; Winograd and Thordarson, 1975; Miller, 1977; Harrill, 1986; IT Corporation, 1996c; D’Agnese et al., 1997); water-balance (Rice, 1984; West, 1989); chloride mass-balance (Dettinger, 1989; Lichty and McKinley, 1995; Russell and Minor, 2002); and distributed-parameter (Hevesi et al., 2002; Hevesi et al., 2003) methods. Each of these methods attempts to capture the complex array of factors that control recharge.

The distributed-parameter method described by Hevesi et al. (2003) provided an estimate of the potential recharge on the

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DomesticMining, public supply, and commercialIrrigation

YEAR

Figure 10. Annual groundwater pumpage estimates by water-use class from Death Valley regional groundwater fl ow system model domain, 1913–1998 (from San Juan et al., 2010, their fi gure C-5).



basis of net infi ltration. Hevesi et al. (2003) estimated poten-tial recharge by using a net-infi ltration model, INFILv3. Net infi ltration is considered a reasonable indicator of groundwater recharge because most of it, as well as surface runoff, eventually migrates downward through the unsaturated zone to recharge the groundwater fl ow system (Hevesi et al., 2003). In gen-eral, the uncertainty of estimating potential recharge from net infi ltration increases as the thickness and heterogeneity of the unsaturated zone increase (Hevesi et al., 2003). INFILv3 simu-lates surface-water fl ow, snowmelt, evapotranspiration (derived from precipitation), and downward groundwater movement in a multilayered root zone and has a climate algorithm that simu-lates daily climate conditions in local watersheds. Topography, geology, soils, and vegetation data are entered to represent local drainage-basin characteristics.

INFILv3 was used to simulate daily major components of a mass-balance equation within the unsaturated zone to a maxi-mum depth of 6 m, the depth at which the seasonal effects of precipitation-derived evapotranspiration were estimated to become insignifi cant. The root zone was represented by two to fi ve soil layers, allowing for variable soil thickness, and by an underlying bedrock layer also having variable thickness, and allowing for precipitation-derived evapotranspiration from frac-tured bedrock at locations with thin soil cover. Net infi ltration equaled the sum of snowmelt, rain, and infi ltrating surface fl ow minus the sum of evapotranspiration, runoff, and changes in root-zone storage. Runoff was generated in the model when and where available water exceeded the root-zone storage capacity or the saturated hydraulic conductivity of the soil or bedrock.

An average annual net infi ltration of 2.8 mm was estimated over the entire model domain by averaging simulated daily net infi ltration over the 50 yr simulation period (Fig. 11). This esti-mate is less than 2% of the average annual precipitation computed for the same period (Hevesi et al., 2003). An annual potential recharge of ~125 Mm3 was computed by multiplying the average annual infi ltration by the area of the model domain.

The uncertainty in model-generated net infi ltration estimates was related to uncertainties associated with the representation of the near-surface environment and the unsaturated zone processes. Hevesi et al. (2003) presented model uncertainty qualitatively because the results of their study could not support a rigorous quantifi cation of uncertainty. Model uncertainty remained high for many INFILv3 model inputs, such as bedrock permeability, soil thickness, root density as a function of depth, stream-channel properties, spatial distribution of climate by month (computed from daily records), and potential evapotranspiration coeffi cients.

Lateral Flow across Death Valley Regional BoundaryAreas of potential infl ow and outfl ow, or lateral groundwa-

ter fl ow, along the Death Valley regional boundary were defi ned for prepumped conditions (Bedinger and Harrill, 2010; Harrill and Bedinger, 2010). Lateral fl ow was estimated using the Darcy equation with hydraulic gradients defi ned by regional water lev-els (Bedinger and Harrill, 2010), and using estimates of hydraulic

conductivity (San Juan et al., 2010) and the cross-sectional area of hydrogeologic units along a segmented version of the Death Valley region boundary (Harrill and Bedinger, 2010) (Fig. 12). Where possible, lateral-fl ow estimates were constrained by infl ows and outfl ows estimated from available water-budget information for areas adjacent to the Death Valley region.

On the basis of these estimates of lateral fl ow, nearly 18.4 Mm3 of groundwater fl ow into the Death Valley region annually, primarily along the western and northern parts of the Death Val-ley region, and 9.5 Mm3 fl ow out, primarily along the eastern part of the Death Valley region (Tables 5 and 9). The greatest infl ow comes from the area west of Death Valley, and the greatest out-fl ow moves to the area east of the Sheep Range. The estimated annual net lateral fl ow is ~8.8 Mm3 into the Death Valley region (Harrill and Bedinger, 2010; San Juan et al., 2010). The lateral fl ow estimates incorporate a great deal of uncertainty.

Potentiometric Surface and Hydraulic Gradients

Several potentiometric-surface maps have been developed for basins within the Death Valley region (D’Agnese et al., 1998, p. 3). Several are generalized contour maps of shallow basin-fi ll aquifers, such as those for Sarcobatus Flat and Oasis Valley (Malmberg and Eakin, 1962, p. 13 and 23), Amargosa Desert (Walker and Eakin, 1963, p. 16–17), and Pahrump Valley (Malm-berg, 1967, p. 25). Winograd and Thordarson (1975) provided several potentiometric maps of basins, or parts of basins, in the area of the Nevada National Security Site.

A regional potentiometric-surface map (Fig. 3) was con-structed (D’Agnese et al., 1998) to aid in conceptualizing the Death Valley regional groundwater fl ow system. The regional potentiometric-surface map (Fig. 3) was generated primarily from regional water-level data as defi ned by D’Agnese et al. (1998). However, where needed, these data were supplemented by additional comparative information derived from the hydro-graphic data, hydrogeologic maps, and interpretations of the distribution of regional discharge and recharge to improve inter-polations in areas of sparse water-level data. Details of map con-struction are given by D’Agnese et al. (1998).

Mounds in the potentiometric surface are commonly associ-ated with groundwater recharge areas (Fig. 3). The most promi-nent mound (and the largest recharge area) is located beneath the Spring Mountains; smaller recharge mounds are present beneath other mountains or mountain ranges (Fig. 3).

Two prominent troughs are controlled by geologic struc-ture (D’Agnese et al., 1998, p. 12). A trough north of the Spring Mountains (Fig. 3) has been described as the result of high- permeability, faulted, and fractured rock present along the axis of the Spotted Range–Mine Mountain structural zone (Faunt, 1997, plate 1) (Fig. 4). The trough located at Pahute Mesa is the result of a linear feature believed to be a fault. Less-prominent troughs at Amargosa Desert and Stonewall Pass (Fig. 3) may be structur-ally controlled. Troughs that may be controlled by topography and lithology are located at Yucca Flat and Emigrant Valley. A

30 Belcher et al.


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TimpahuteRange

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FortymileWash

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RainierMesa

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CottonwoodMountains Death

Valley

Amargosa River

Net infiltration, in millimeters per year (based on model 1 simulation of Hevesi et al., 2003)

0–0.1

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EXPLANATION



Figure 11. Simulated net infi ltration used to estimate recharge to the Death Valley region, 1950–1999 (from San Juan et al., 2010, their fi gure C-8).



NevadaTest Site

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LakeMohave

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SilurianOwlshead

Panamint

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Stone Cabin-Railroad

Clayton5

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EXPLANATION

Color-coded straight line segment that approximates the boundary of the Death Valley regional groundwater flow system model

Populated location

Figure 12. Death Valley regional groundwater fl ow system model boundary segments used to estimate lateral fl ow in the model do-main (Harrill and Bedinger, 2010, their fi gure A2-3).

32 Belcher et al.


trough that may be associated with all three controls is located at Fortymile Canyon, although specifi c geologic or structural con-trols have been sought but not found.

In order to improve quantitative analysis of regional and interbasin fl ows in the Death Valley, hydraulic head values were used to develop a spatially interpolated contour map of the regional groundwater potential by Bedinger and Harrill (2010). The map was used to delineate the areas outside the Death Val-ley regional groundwater fl ow system (Belcher and Sweetkind, 2010) that contribute groundwater fl ow to the system, to estimate the regional hydraulic gradient on the lateral boundary, and to estimate the amount of fl ow by the Darcy method (Harrill and Bedinger, 2010). The number of deep wells that can be mea-sured, however, is insuffi cient for the spatial distribution of data points needed to map the regional potentiometric surface of the Death Valley region. Because of the regional hydraulic continu-ity between deep and shallow fl ow in the Death Valley region, additional data points for the regional groundwater potentiomet-ric surface can be inferred from fl ow-net analysis of the shallow groundwater system (Bedinger and Harrill, 2010).

Water-Level TrendsThe most recent, and perhaps the most comprehensive,

analysis of water-level trends in the Death Valley region was by Fenelon and Moreo (2002). Fenelon and Moreo (2002, p. 64–66) summarized trend analysis of water-level data collected from 1960 to 2000, emphasizing the period from 1992 to 2000. For water levels measured from 1992 to 2000, statistically signifi -cant upward trends were determined for 12 water-level sites, and statistically signifi cant downward trends were determined for 14 water-level sites and one spring-discharge site. In general,

the magnitude of change in water levels from 1992 to 2000 was small (less than 0.6 m), except where they were infl uenced by pumping or affected by local aquifer conditions near a well site. Fenelon and Moreo (2002) also found that seasonal trends are superimposed on some of the long-term trends in water levels and spring discharge caused by changes in barometric pressure, evapotranspiration, and pumping. Four wells had water levels that appeared to be responding to evapotranspiration (three in Ash Meadows and one near Death Valley Junction).

Fenelon and Moreo (2002) concluded that groundwater withdrawals from several areas, such as Las Vegas Valley, Pah-rump Valley, the Amargosa Farms area, and the Nevada National Security Site, may account for long-term water-level declines in the Ash Meadows area (Fig. 6). The largest area of consistent trends in the study area is in the Amargosa Farms area, where water levels declined 3–9 m from 1964 to 2000. The Amargosa Farms area is the largest center of pumping in the Death Valley region. Pumping in the Amargosa Farms area may affect water levels in some wells as far away as 3–9 km (Dudley and Larson, 1976; Fenelon and Moreo, 2002).

Large Hydraulic GradientsHydraulic gradient is the rate of change of the water level

over a given distance in a given direction (Wilson and Moore, 1998). Large hydraulic gradients (gradients greater than 0.1) are a common feature of the Death Valley region. One of the two major areas of large hydraulic gradient separates Death Val-ley from adjoining areas to the northeast and east (Fig. 3). This abrupt drop in potentiometric-surface altitude is associated with contacts between low-permeability rocks in the Amargosa Range and higher-permeability rocks that are associated with the Death

TABLE 9. REGIONAL ESTIMATES OF FLOW ACROSS LATERAL BOUNDARY SEGMENTS OF DEATH VALLEY REGIONAL GROUNDWATER FLOW SYSTEM MODEL DOMAIN FOR PREPUMPED CONDITIONS

(MODIFIED FROM SAN JUAN ET AL., 2010, THEIR TABLE C-4)

Boundary segment (see Fig. 12)

Boundary flow estimate (m3/d) Estimate of annual boundary flow*

(m3) Darcy

calculation Water-budget

calculation Most reasonable

estimate 005 004,11– 521– nairuliS †

Spring-Mesquite –782 – 0§ 0 000,176,1– 5754– – 5754– sageV saL

Sheep Range –18,747 – –18,747 –6,847,000 000,610,1– 3872– – 3872– taganarhaP 000,215,1 9314 – 9314 laoC-nedraG

Stone Cabin-Railroad 12,476 – 12,476 4,557,000 000,442 766 – 766 notyalC

Eureka and Saline# 20,813 15,600 15,100 5,515,000 000,974,5 000,51 000,61–000,41 050,41 tnimanaP 000,078 2832 – 2832 daehslwO 000,628,8 391,42 306,72 latoT

Note: + values indicate flow into model domain; – values indicate flow out of model domain; – indicates no value was reported or estimate was unreliable. *Volume calculated using most reasonable estimate of boundary flow from data analysis in Harrill and Bedinger (2010) rounded to the nearest 1000 m3. †See Harrill and Bedinger (2010) for an explanation of method used to determine most reasonable estimate. §No significant flow estimated across boundary because segment closely coincides with natural no-flow boundary. #Estimate is sum of flows across Eureka and Saline boundary segments.



Valley fault zone. This large hydraulic gradient also may result from the large change in land-surface altitude between the Ama-rgosa Desert (~700 m) and Death Valley (~–80 m). The second major area of large hydraulic gradient lies along the northern and western margins of the Spring Mountains (Fig. 3). The large hydraulic gradient in this area is associated with the low-perme-ability clastic confi ning unit present in the northwestern Spring Mountains (Fig. 3). North of the Spring Mountains, the large hydraulic gradient also appears to be related to the Las Vegas Val-ley shear zone and may be due to low-permeability fault gouge developed in this major structural zone.

Another zone of large hydraulic gradient extends from the east side of the southern Grapevine Mountains across Yucca Mountain, and northeastward to Emigrant Valley and the Groom Range (Figs. 3 and 6). This hydraulic gradient may be the result of three or more large-scale features, such as the following:

1. A mostly buried contact of less-permeable rocks to the north and thick, permeable units to the south;

2. A generally southward decrease of elevation in the topog-raphy that is mirrored approximately by the regional potentiometric surface; and

3. Large regional recharge areas to the north and west, for example, from the vicinity of Pahute Mesa to the Groom Range.

The large hydraulic gradient zone beneath northern Yucca Mountain is characterized by a gradient of 0.13 and possibly as large as 0.15 (Luckey et al., 1996, p. 21). However, an alternative interpretation, that water levels in this area refl ect perched water bodies, results in a smaller gradient of 0.06–0.07 (U.S. Geologi-cal Survey, 2004a).

Vertical Hydraulic GradientsVertical hydraulic gradients, both upward and downward,

exist throughout the region. Kilroy (1991) considered that upward gradients imply hydraulic connection with the lower carbonate-rock aquifer, whereas downward gradients imply hydraulic isolation from it. In several boreholes (or sets of bore-holes), upward gradients exceed 0.1, and one extreme value of 1.2 is given (Kilroy, 1991). The highly transmissive regional carbonate-rock aquifer is relatively isolated from the overlying shallow groundwater fl ow systems, and water-table altitudes in saturated basin-fi ll materials in basins upgradient from Ash Meadows (Yucca Flat, Frenchman Flat, and eastern Emigrant Valley; Fig. 6), as well as beneath the intervening topographic divides, are higher than the potentiometric surface in the lower carbonate-rock aquifer (Winograd and Thordarson, 1975, p. C93). Thus, these valleys may contribute recharge to the under-lying lower carbonate-rock aquifer. However, the vertical per-meabilities of the volcanic and basin-fi ll units are small, and these units are expected to impede downward fl ow. From mea-surements of hydraulic heads in boreholes in Yucca Flat, Wino-grad and Thordarson (1975) suggested that the maximum verti-cal gradient is ~0.2 and that downward leakage into the lower carbonate-rock aquifer in Yucca Flat amounts to ~31,000 m3/yr.

Upward gradients commonly are observed in the eastern and southern parts of the Amargosa Desert (Kilroy, 1991). Down-ward gradients are found at several localities but particularly near the Amargosa River east of the Funeral Mountains (western Amargosa Desert). Downward gradients are as large as 0.4. Dud-ley and Larson (1976) reported numerous fl owing wells at Ash Meadows, which is to be expected in a major regional discharge area that contains many large springs. Dudley and Larson (1976) also provided a contour map of the water table showing a ground-water mound beneath the Devils Hole Hills (Fig. 1), which are composed of faulted and intensely fractured Paleozoic carbon-ate rocks that receive little recharge. The warm temperature of the water in and discharging from these rocks, 33 °C to 34.5 °C (Dudley and Larson, 1976), implies that the groundwater mound is supported by upward fl ow of water that has circulated deeply within the regional fl ow system.

Water levels in 17 boreholes in the Yucca Mountain area indicate vertical gradients. There were only fi ve boreholes with downward gradients and 12 with upward gradients. Several of the boreholes that have positive (upward) head differences are located near Yucca Mountain, and a few others are located south and southwest of Yucca Mountain. The latter group of bore-holes indicates upward vertical head differences of ~0.1–7.6 m. Upward vertical head differences range from 0.1 to almost 55 m (the latter in drill-hole USW H-1 between the Crater Flat–Prow Pass aquifer and older volcanic-rock unit; Luckey et al., 1996). In borehole UE-25p#1, the vertical head gradient is ~20 m upward between the regional carbonate-rock aquifer and the overlying volcanic rocks (Stuckless et al., 1991). Borehole NC-EWDP-2DB penetrated Paleozoic carbonate rocks toward the bottom of the borehole, and it also shows a greater hydraulic head in the carbonate rocks (Stuckless et al., 1991). Leakage upward from the lower carbonate-rock aquifer may contribute to some anoma-lously heavy carbon near the Solitario Canyon fault (Stuckless et al., 1991).

A downward component is expected in recharge areas, which here, as in most arid to semiarid environments, are in uplands. A downward gradient also exists in the Ash Meadows groundwater basin, part of the central Death Valley subregion (Fig. 6). Downward vertical head differences range from 0.5 to 38 m (gradients generally less than 0.01 but as large as 0.09) (Stuckless et al., 1991).

Numerical Modeling of the Regional Groundwater Flow System

Regional-scale groundwater fl ow models developed during the last two decades have provided new insights into groundwa-ter fl ow in the Death Valley region. The Department of Energy (Nevada Test Site and Yucca Mountain Project programs) has supported the construction of several such models to evaluate groundwater fl ow in parts, or all, of the Death Valley regional groundwater fl ow system. Successive models have incorpo-rated additional hydrogeologic complexity and computational

34 Belcher et al.


sophistication in an effort to simulate regional hydrologic pro-cesses and conditions. With each successive model, investigators have refi ned the understanding of the three-dimensional (3-D) nature of the Death Valley regional groundwater fl ow system.

Previous WorkEarly numerical groundwater modeling was based on simpli-

fi ed conceptual models of the geology and hydrology known to exist in the region. Two- and three-dimensional groundwater fl ow models developed in the 1980s contained considerable abstrac-tions of the natural hydrogeologic conditions and depended on combining hydrogeologic units and averaging hydraulic proper-ties (Waddell, 1982; Czarnecki and Waddell, 1984; Rice, 1984; Czarnecki, 1985; Sinton, 1987). Although these models were considered adequate for their intended purposes, the results of these investigations indicated that lumped-parameter representa-tions do not necessarily adequately depict vertical groundwater fl ow components, subbasin groundwater fl ow, large hydraulic gradients, and physical subbasin boundaries.

In contrast, the more complex groundwater fl ow models developed in more recent investigations allow for the examina-tion of the spatial and process complexities of the 3-D hydro-geologic system (Prudic et al., 1995; IT Corporation, 1996a; D’Agnese et al., 1997, 2002; Belcher and Sweetkind, 2010). These more geologically and hydrologically representative fl ow models were constructed with a computer-based 3-D geologic framework model to defi ne the complexities of the hydrogeo-logic unit geometry, composition, and structure.

Digital hydrogeologic framework models providing a computer-based description of the geometry, composition, and structure of the hydrogeologic units were constructed for several of the regional-scale groundwater fl ow models completed in the 1990s and early 2000s as part of the Nevada Underground Test Area program at the Nevada National Security Site, and for the Yucca Mountain Project. Two regional framework models, the Department of Energy–Nevada model (IT Corporation, 1996b) for the Underground Test Area Phase I work, and the Yucca Moun-tain Project/Hydrologic Resources Program model (D’Agnese et al., 1997), were merged by Belcher et al. (2002a) to produce a single, integrated hydrogeologic framework model for use with the steady-state prepumping groundwater fl ow model by D’Agnese et al. (2002). Because of project-scope limitations, a few interpre-tations were made where these two framework models disagree (mostly with respect to the units defi ned for each fl ow model). During the merging process, the Cenozoic volcanic units and the more detailed Cenozoic basin-fi ll units from the Underground Test Area model (IT Corporation, 1996b) were used, augmented by the Cenozoic playa-deposits units from D’Agnese et al. (1997).

The Death Valley region steady-state prepumping fl ow model (D’Agnese et al., 2002) simulated the fl ow system by using a 3-D steady-state model that incorporates a nonlinear least-squares regression technique to estimate aquifer-system parameters. This model incorporates a vertical discretization that resulted in 15 model layers. The accuracy of the fi nal calibrated

Death Valley region steady-state model was tested by compar-ing simulated values with measured (observed) values for heads and groundwater discharges and by comparing expected (fi eld-estimated) values for parameters such as hydraulic conductivity (D’Agnese et al., 2002).

Development of the 2004 Transient Regional-Scale Groundwater Flow Model

The most recent Death Valley regional groundwater fl ow model simulates transient conditions from 1913 through 1998 by using the modular groundwater fl ow model, MODFLOW-2000 (Harbaugh et al., 2000; Hill et al., 2000), and a simulated steady-state hydraulic-head distribution representing prepumping condi-tions (D’Agnese et al., 2002). Transient stresses imposed on the regional groundwater fl ow system include groundwater pump-age from 1913 through 1998 and fl ows from springs affected by pumping; simulated areal recharge is held constant at average annual values.

The digital hydrogeologic framework model used as input for the groundwater fl ow model of the Death Valley region defi nes the 3-D geometries of 27 hydrogeologic units by using surface and subsurface geologic information, such as digital-elevation models, geologic maps, borehole information, geologic and hydrogeologic cross sections, and other 3-D computer mod-els (Faunt et al., 2010a). The areas of Pahute Mesa and Yucca Mountain contain more detail because these areas are of interest for local-scale modeling (Faunt et al., 2010c).

The main hydrologic components defi ned for the regional-scale groundwater fl ow system model are the amount and locations of water entering (recharge and lateral boundary fl ow) and exiting (discharge and lateral boundary fl ow) the system, the ability (with depth) of each hydrogeologic unit to transmit and store water, and the location and depth to groundwater. Groundwater entering the Death Valley region is represented in the model by recharge from infi ltration of precipitation and runoff on high mountain ranges (mostly in the mountains of central and southern Nevada) and by a small amount of fl ow across the system boundary from some adja-cent groundwater basins. Groundwater exiting the Death Valley region is represented in the model (1) naturally, by spring fl ow and evapotranspiration in Ash Meadows and Oasis Valley (south and west of the Nevada National Security Site; Fig. 3), and in Death Valley, California, and by fl ow across the system boundary to some adjacent groundwater basins, and (2) since 1913, by pumpage for agricultural, commercial, and domestic use, mostly in Pahrump Valley, Amargosa Desert, and Penoyer Valley (Fig. 1). A series of studies was completed to measure or defi ne these values as inputs for the model by compiling previously existing information, by acquiring additional information when needed, and by refi ning earlier ideas on groundwater fl ow in the Death Valley region to refl ect the current understanding (Faunt et al., 2010b).

The regional-scale groundwater fl ow model was divided into a grid of 16 layers with 194 rows and 160 columns of cells, with 1500 m on a side (Fig. 13). Measurements of steady-state



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Death Valley

Death Valley

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Tikaboo ValleyPahranagatPahranagatPahranagat

RangeRangeRange

Pahrump ValleyPahrump ValleyPahrump Valley

AmargosaDesert

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AmargosaDesert

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Shee

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YuccaMountain

YuccaMountain

0 40 MILES20

0 40 80 KILOMETERS50,000-meter grid based on Universal TransverseMercator projection, Zone 11. Shaded-relief basefrom 1:250,000-scale Digital Elevation Model;sun illumination from northwest at 30 degreesabove horizon

Active cells

Inactive cells


Death Valley regional groundwater flow system model grid boundary

EXPLANATION

Figure 13. Model grid for the Death Valley regional groundwater fl ow system (from Faunt et al., 2010b, their fi gure F-1).

36 Belcher et al.


groundwater levels, the amount of spring fl ow, and evapotrans-piration were used to calibrate the model for initial steady-state conditions (before pumping began in the region in 1913). The model then was calibrated for transient conditions (1913–1998) using transient water levels, spring fl ow, evapotranspiration, and pumping (Faunt et al., 2010c).

The model simulations of groundwater fl ow in the Death Val-ley region match the observed groundwater fl ow patterns. Flow generally moves from north to south, originating from recharge of precipitation in the mountains in central and southern Nevada and discharging at regional discharge areas in Ash Meadows, Oasis Valley, Sarcobatus Flat, and Death Valley (Figs. 14 and 15). The model fi t was evaluated in terms of both unweighted and weighted residuals of hydraulic head (water levels). Unweighted residuals are the difference between the simulated value and the measured value of head. Weighted residuals are the differences between the measured and simulated values multiplied by a sta-tistical weight to account for the level of confi dence in the accu-racy of the measurements (Hill et al., 2000; Faunt et al., 2010b; San Juan et al., 2010). Weighted residuals are a better indica-tion of the calibration of the model than residuals because they take into account the accuracy of the measured water levels. The simulated groundwater levels generally match measured water levels (Figs. 14 and 15, green data points) except in areas of large hydraulic gradients, such as the Eleana Range, the western part of Yucca Flat, the southern part of the Owlshead Mountains, the southern part of the Bullfrog Hills, and the north-northwestern part of the model domain. The simulated water levels generally match the observed decline of water levels with time in the Pah-rump Valley, Amargosa Desert, and Penoyer Valley areas. The model adequately simulated the observed decline of spring fl ow because of pumping in Pahrump Valley during the twentieth cen-tury, though the simulation tended to underestimate the decline. The simulated hydraulic properties that defi ne the ability of a hydrogeologic unit to transmit and store water match the range of values measured in the fi eld.

Limitations of the Regional-Scale ModelThree basic limitations of the regional-scale fl ow model

are inherent in its construction: inaccuracies in the represen-tation of (1) the physical framework, (2) the hydrologic con-ditions, and (3) the representation of time. This discussion of limitations of the regional-scale model is taken from Faunt et al. (2010b, p. 340).

Although the 1500 m resolution of the fl ow model grid is appropriate to represent regional-scale conditions, higher resolu-tion would improve the accuracy of the simulation, particularly in areas of geologic complexity. The large grid cells tend to gen-eralize local-scale complexities that affect regional hydrologic conditions. To represent local-scale variability, smaller grid cells throughout the model, or local refi nement around selected fea-tures or in critical areas in the model domain, would be required.

The model represents hydrologic conditions as boundary conditions such as recharge, lateral boundary fl ows, discharge

from evapotranspiration and springs, and pumpage. Of these boundary conditions, the most important is recharge (Faunt et al., 2010b, p. 340). The main limitation in the representation of recharge is the inaccurate assumption that net infi ltration results in regional recharge. The net-infi ltration model likely overesti-mates recharge in many parts of the model domain, because it is assumed that all infi ltrating water that passes the root zone ulti-mately reaches the water table (Hevesi et al., 2003). This assump-tion ignores the possibility that infi ltrating water could be inter-cepted and either diverted or perched by a lower- permeability layer in the unsaturated zone, or the possibility of deep evapora-tion from the unsaturated zone. This limitation may be resolved by including in the fl ow model a means to account for deep, unsaturated zone processes that may act to reduce or redistribute infi ltrating water (Faunt et al., 2010b, p. 340).

Limitations in the defi nition of lateral boundary fl ow are the result of incomplete understanding of natural conditions (Faunt et al., 2010b, p. 340). Because very little data exist in the areas defi ned as lateral fl ow-system boundary segments, all aspects of the assigned boundary conditions are poorly known. Despite these uncertainties, the data used to characterize these boundary fl ows have been thoroughly analyzed for this model. The model does not simulate the complex process of evapotranspiration but accounts for the groundwater discharge attributed to evapotrans-piration through use of the Drain package for MODFLOW-2000 (Harbaugh et al., 2000). Evapotranspiration by native vegetation has been studied extensively. Future revisions of the Death Val-ley regional model might be improved by using a more com-plex evapotranspiration package instead of the Drain package. This more complex package could incorporate spatially varying parameters to simulate direct recharge, soil moisture, and vegeta-tive growth.

The representation of time by yearlong stress periods simu-lated in the model allows the model to address only those dynam-ics that change in the course of at least several years. Simula-tion of seasonal dynamics by use of shorter stress periods could be advantageous to account for the seasonal nature of irrigation pumpage. Such a simulation would require seasonal defi nition of hydrologic conditions.

This model is intended to be used to (1) provide the bound-ary conditions for the site-scale models at Yucca Mountain and the underground test area Corrective Action Units on the Nevada National Security Site, (2) evaluate the effects of changes in sys-tem fl ux, regardless of whether the changes are natural or human induced, (3) provide a technical basis for decisions on the quan-tity of water available for defense and economic development activities on the Nevada National Security Site, (4) determine the potential effects of increased offsite water use on Nevada National Security Site water supplies, (5) provide a framework for deter-mining effective source plume, ambient trend, and point-of-use groundwater-quality monitoring locations, and (6) facilitate the development of a management tool for groundwater resources in the Death Valley region (Belcher and Sweetkind, 2010; Belcher et al., 2010; Faunt et al., 2010b).



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36

37

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117 116 115

AshMeadows

PahuteMesa

Solitario Canyon horizontal-flow barrier

EXPLANATION


Horizontal-flow barrier

Death Valley regional groundwater flow system model grid boundary

Simulated discharge greater than observed discharge

Head residual—In meters200200 Simulated potentiometric-surface contour for uppermost active model layer—Contour interval 200 meters. Datum is NAVD 88.

Less than –100 –100 to –50

–50 to –20

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–10 to 10

10 to 20

20 to 50

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Simulated discharge less than observed discharge

Spring Mountains

Spring Mountains

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ountainsPanam

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Death Valley

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IndianSprings

Sarcobatus

Flat

PenoyerValley

OasisValley

Bullfro

gHills

YuccaFlat

Owlshead Mts

Elea

na R

ange

Figure 14. Steady-state stress period hydraulic-head residuals (observed minus simulated) and simulated potentiometric surface for uppermost active model layer in the Death Valley region area (from Faunt et al., 2010b, their fi gure F-46).

38 Belcher et al.


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EXPLANATION

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Death Valley regional groundwater flow system model grid boundary

36

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Less than –6 –6 to –4

–4 to –2

–2 to –1

–1 to 1

1 to 2

2 to 4

4 to 6

Greater than 6

Weighted head residual 200200

Horizontal-flow barrier

Simulated discharge greater than observed discharge

Simulated discharge less than observed discharge

Solitario Canyon horizontal-flow barrier

Simulated potentiometric-surface contour for uppermost active model layer—Contour interval 200 meters. Datum is NAVD 88

PahuteMesa

AshMeadows

Owlshead Mts

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na R

ange

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Spring Mountains

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ountainsPanam

int Mountains

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ountains

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Black Mountains

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Death Valley

Death Valley

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YuccaMountain

RangeRangeRange

PahuteMesa

IndianSprings

Sarcobatus

Flat

PenoyerValley

OasisValley

Bullfro

gHills

YuccaFlat

Figure 15. Steady-state stress period hydraulic-head weighted residuals (observed minus simulated) and simulated potentiometric surface for uppermost active model layer in the Yucca Mountain area (from Faunt et al., 2004b, their fi gure F-47).



GROUNDWATER FLOW WITHIN THE YUCCA MOUNTAIN AREA

Overview of Hydrogeology and Groundwater Flow

The Yucca Mountain area generally is considered to extend from central Jackass Flats on the east (longitude 116°18′W) to western Crater Flat on the west (longitude 116°37′30″W), and from the headwaters of Beatty Wash on the north (latitude 36°56′N) to the southern Amargosa Desert on the south (latitude 36°34′N) (Fig. 16). The southern boundary is ~30 km south of the proposed repository.

Yucca Mountain is a north-trending ridge formed by eastward-tilted, fault-bounded blocks of layered volcanic rocks (Keefer et

al., 2007) of Miocene age near the southern limit of the south-western Nevada volcanic fi eld. The volcanic rocks, mainly ash-fl ow tuffs and lavas, are thickest in the north nearer the source areas and decrease in thickness to the south. They are ~2 km thick beneath Yucca Mountain. Southward from the proposed repository area, depositional thinning, post-Miocene erosion, and faulting produce an increasingly discontinuous distribution of the volcanic rocks into the northern Amargosa Desert, allow-ing exposure of the underlying Paleozoic and Proterozoic rocks at the surface or beneath surfi cial basin-fi ll deposits (Potter et al., 2002). Numerous north-striking faults border and intersect the Yucca Mountain area. Northwest-striking faults lie just north, south, and southeast of the area. Northeast-striking faults, region-ally important east and south of the site, project into the southern

Shoshone Mtn

Pahute Mesa

Fortymile C

anyon

UE-25 p#1

Universal Transverse Mercator projection, Zone 11

Fran Ridge

Drill H

ole Wash

36°56′ 00″

116°18′ 30″ 116°18′ 00″

36°34′ 00″

Figure 16. Important physiographic features near Yucca Mountain. Rectangle delineates the site-scale saturated zone (SZ) fl ow model boundaries.

40 Belcher et al.


part of the Yucca Mountain area. The faults probably greatly infl uence the movement of groundwater, either as preferential pathways or impediments to fl ow (Faunt, 1997).

Yucca Mountain is located within the Alkali Flat–Furnace Creek groundwater basin of the central Death Valley subregion (Fig. 6, number 3) and is positioned between regional areas of recharge and discharge. Groundwater fl ow in the saturated zone beneath Yucca Mountain is part of the dominant regional north-to-south fl ow within the northern part of the Alkali Flat–Furnace Creek groundwater basin.

In the Yucca Mountain area, the Paleozoic rocks, which are the regional carbonate-rock aquifer (LCA), are covered by Mio-cene volcanic rocks, but the extent to which the lower carbonate-rock aquifer is present beneath the volcanic rocks is uncertain. Borehole UE-25p#1, located near the eastern fl ank of Yucca Mountain (Fig. 16), penetrates the lower carbonate-rock aquifer (M.D. Carr et al., 1986) and, together with a seismic line across Crater Flat and Yucca Mountain (Brocher et al., 1998), indicates that the regional carbonate-rock aquifer underlies the volcanic rocks throughout the Yucca Mountain area. However, north of Yucca Mountain, in the calderas of the southwestern Nevada vol-canic fi eld, the regional carbonate-rock aquifer likely is absent or exists only as isolated blocks that lack hydraulic continuity (Sweetkind et al., 2010).

The upper part of the Miocene volcanic sequence and the surfi cial Quaternary–Tertiary sediments compose an unsaturated zone that is 400–750 m thick in the Yucca Mountain area (LeCain and Stuckless, this volume). This variation is principally because of topographic irregularity above a relatively fl at water table. From west to east between block-bounding faults, the generally eastward dip of the layered volcanic rocks causes successively younger strata to be submerged beneath the more gently sloping water table. The small rate of infi ltration (averaging 2.9 mm/yr in the Yucca Mountain area and 4.5 mm/yr at the location of the proposed repository; Flint et al., 2001) produces a correspond-ingly small groundwater fl ux (Flint et al., 2001), which is readily transmitted through the rocks, which constitutes most of the site area to the saturated zone (Flint et al., 2001).

Although infi ltration through the unsaturated zone contrib-utes some recharge at the water table, most of the groundwater fl ux in the saturated zone beneath the proposed repository prob-ably is throughfl ow (Flint et al., 2001), which, in a regional con-text, generally is from north to south. At Yucca Mountain, the local saturated-zone fl ow direction is southeast toward Jackass Flats, but it turns southward as it approaches Fortymile Wash adjacent to the mountain (Fig. 16) (Tucci and Burkhardt, 1995).

Groundwater in the Yucca Mountain area, especially in the Fortymile Wash drainage system to the east of Yucca Mountain (Fig. 16), probably originates primarily as recharge in eastern-most Pahute Mesa, Rainier Mesa, Timber Mountain, and Sho-shone Mountain, which are the principal uplands of the north-ern Fortymile Canyon area (Savard, 1998; Flint et al., 2001, 2002). Hydrochemical and isotopic evidence (Marshall et al., this volume) indicates that recharge to the groundwater sys-

tem underlying Yucca Mountain, at least near the water table, has occurred predominantly as downward percolation of water through the overlying unsaturated zone with relatively little contribution by subsurface infl ow from the volcanic highlands to the north. On the basis of the confi guration of the potentio-metric surface (Fig. 3), groundwater in the Yucca Mountain area is considered to fl ow generally southward through the aquifers of the volcanic sequence underlying Crater Flat, Yucca Moun-tain, and Jackass Flats and, ultimately, into basin-fi ll material in the Amargosa Desert.

Groundwater at Yucca Mountain fl ows primarily through a sequence of fractured volcanic rocks and underlying carbon-ate rocks. Where the volcanic rocks pinch out to the south of Yucca Mountain, groundwater also fl ows in Tertiary alluvial and carbonate deposits. Luckey et al. (1996) divided the Mio-cene volcanic sequence into four hydrogeologic units different from Winograd and Thordarson’s (1975) previous work and those commonly used (see later section, “Site-Scale Saturated-Zone Flow Model”). Luckey et al.’s (1996) units are described briefl y here because much of the work done at Yucca Mountain refers to them. Luckey et al. (1996) divided the Miocene volcanic rocks into four units (from top to bottom): an upper volcanic-rock aquifer (consisting of the Topopah Springs Tuff of the Paintbrush Group), an upper volcanic-rock confi ning unit (consisting of the Calico Hills Formation), a lower volcanic-rock aquifer (consist-ing of the Crater Flat Group), and a lower volcanic-rock confi n-ing unit (consisting of older tuffs, fl ow breccias, and lava fl ows).

Generally, only a few intervals within the volcanic-rock aquifers, usually associated with fractures or faults, produce water. The volcanic-rock confi ning units also may transmit water but to a lesser extent than the aquifers.

The regional carbonate-rock aquifer appears to underlie the lower volcanic-rock confi ning unit beneath Yucca Moun-tain (Luckey et al., 1996). Several lines of evidence indicate that the lower volcanic-rock confi ning unit effectively isolates the regional carbonate-rock aquifer from the volcanic-rock aquifers. However, the altered mineralogy of the tuffs, particularly in bore-hole UE-25p#1, indicates past upward fl ow of groundwater, pos-sibly localized along faults (M.D. Carr et al., 1986, p. 24). Greater temperatures at the water table aligned along existing faults may indicate upward fl ow, probably from the regional carbonate-rock aquifer (Fridrich et al. 1994). Beneath the northern part of Yucca Mountain, the regional carbonate-rock aquifer also may be con-fi ned by the Eleana Formation (upper siliciclastic-rock confi ning unit [UCCU]; Table 2). If the volcanic-rock aquifers were essen-tially isolated from the regional carbonate-rock aquifer, then the potentiometric surface in the volcanic-rock aquifer would be independent of the potentiometric surface in the regional carbon-ate-rock aquifer. If, however, the confi ned area is breached by faults, the potentiometric surface in one system may affect the potentiometric surface in the other system. An analysis of fault permeability at borehole UE-25p#1 indicates that the regional carbonate-rock aquifer is well confi ned by a layer of rocks of low-hydraulic conductivity (Bredehoeft, 1997, p. 2459).



Hydraulic Properties

Hydraulic properties most commonly estimated for the Yucca Mountain area are hydraulic conductivity at the rock-matrix scale (by tests on core samples) and transmissivity at the rock-mass scale (by tests in boreholes). In addition, a few multiple-borehole tests provide information on storage and large-scale hydraulic conductivity. Intrinsic permeability was also estimated using both single-borehole and multiple-borehole tests. The discussion in this section emphasizes fi eld-scale measurements in the Yucca Mountain area. Additional details of the core-scale data are pre-sented by LeCain and Stuckless (this volume).

Matrix properties in the saturated volcanic tuffs are not important for understanding the large-scale fl ow system, because most of the water transmitted for large distances in the saturated zone is transmitted through fractures and faults. How-ever, estimates of fl uid storage need to account for the volume in the rock matrix, and diffusion into the pore space within the rock matrix is an important mechanism for solute transport. A more complete understanding of matrix properties (such as permeability) is based on fi eld tests in the unsaturated zone (LeCain and Stuckless, this volume) because matrix properties exert greater control on the behavior of the hydrologic system under unsaturated conditions.

The specifi c discharge downgradient from the proposed repository, along with effective porosity, determines the rate at which groundwater moves beneath and away from Yucca Moun-tain. The specifi c discharge, in turn, is a function of the perme-ability of the rocks and basin-fi ll deposits and of the hydraulic gradient in this area.

More than 150 hydraulic tests have been conducted in bore-holes on and around Yucca Mountain. Several methods have been used to determine the hydraulic properties of the saturated

zone. Field tests to estimate transmissivity and storativity may be (1) single-borehole, constant-rate discharge tests, (2) single-borehole, variable-rate discharge tests (conventional pumping tests), (3) single-borehole, slug-injection, and slug-withdrawal tests; and single-borehole, pressure-injection tests, (4) constant-rate injection tests in single-borehole and multiple-borehole con-fi gurations, and (5) multiple-borehole pumping tests, in which observation boreholes are used (Fig. 17).

Borehole fl ow and temperature surveys during aquifer tests provide information on the vertical distribution of the hydraulic properties. Because of the depth of the saturated zone at Yucca Mountain, most of the tests in the volcanic tuffs were single-borehole tests of the entire open saturated interval or of specifi c depth intervals in a borehole. Multiple-borehole tests have been conducted mainly at the C-Wells complex (Geldon et al., 1998, 2002), but borehole pairs (UE-25a#1 and UE-25b#1) also have been tested in Drill Hole Wash (Moench, 1984, p. 831–846). Test sites are located in the basin-fi ll deposits at the Alluvial Testing Complex (see Fig. 18).

Hydraulic properties of faults near Yucca Mountain are largely unknown. Faunt (1997, p. 24–31), who summarized current understanding of the effect of faulting on groundwater movement in the Death Valley region, developed three general principles regarding the properties of faults in the region (dis-cussed in “Overview of Regional Groundwater Flow”):

1. Large-scale folding and block faulting formed major top-ographic features and sedimentary basins, thereby defi n-ing recharge and discharge areas.

2. Faulting and intense folding caused fracturing and created highly permeable channels that are enlarged by dissolu-tion in the regional carbonate-rock aquifer.

3. Faulting and folding in certain rock types can create bar-riers to groundwater movement by displacing segments

YAAYACUOAAVSU (lower)

TMVAPVA

LFUCHVUCFPPACFBCUCFTALCAICUXCU

Nevada Test Site core data

Figure 17. Measured and model- calibrated hydraulic conductivities in hydrogeologic units (refer to Table 2) of the Yucca Moun-tain area (Sandia National Laboratories, 2007d, their fi gure 7-4). Bars represent the 95% confi dence interval on the mean. Ab-breviations: CFBCU—Crater Flat–Bullfrog confi ning unit; CFPPA—Crater Flat–Prow Pass aquifer; CFTA—Crater Flat–Tram con-fi ning unit; CHVU— Calico Hills volcanic-rock unit; ICU— intrusive-rock confi ning unit; LCA—lower carbonate-rock aquifer; LFU—lava-fl ow unit; OAA—older allu-vial aquifer; PVA—Paintbrush volcanic-rock aquifer; TMVA—Timber Mountain volcanic-rock aquifer; VSU (lower)—low-er volcanic- and sedimentary-rock unit; XCU—crystalline-rock confi ning unit; YAA—younger alluvial aquifer; YACU—younger alluvial confi ning unit.

42 Belcher et al.


of permeable strata against low-permeability strata and by producing or emplacing low-permeability materials within faults and fractures. Locally, groundwater may be forced to the surface as springs and diffuse discharge.

Bredehoeft (1997) concluded that the regional carbonate-rock aquifer has a good tidal response at borehole UE-25p#1, indicating that the fault zones that cut through rocks beneath Jackass Flats east of Yucca Mountain (Fig. 16) are relatively impermeable. The hydraulic conductivity of the Fran Ridge fault

zone, if it is assumed that the zone is 3 m wide, is ~10−6 m/s, which is slightly greater than that of the tuff aquifer (Brede-hoeft, 1997). Varying the width of the fault would inversely affect the calculated permeability; the fl ux of the water would remain the same.

Single- and cross-borehole tests in the Calico Hills Forma-tion appear to produce different estimates of permeability for this formation. The mean value of intrinsic permeability estimated from single-borehole tests (k = 3.1 × 10−13 m2; Lahoud et al.,

EXPLANATIONSaturated zone borehole and numberNye County Early Warning Drilling Program (NC-EWDP)borehole and number

Highway 95

Yucca Mountain

C-Wells Complex

Drill Hole Wash

116° 20′00″W 116° 22′30″

36° 52′30″N

36° 45′00″

36° 37′30″

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m

Figure 18. Location of water-level measurement sites near Yucca Mountain.



1984) is larger than the upper part of the range of estimates from cross-borehole tests (k = 0.8–1.7 × 10−13 m2; Geldon et al., 2002). Although this difference is small, it may indicate that geologic factors other than the test method and scale may have infl uenced the results.

One factor might be zones of considerable fracturing in proximity to faults. The single-borehole tests in the Calico Hills Formation were conducted in the highly faulted area at borehole UE-25b#1, whereas faults were present only at deeper strati-graphic horizons in cross-borehole tests at the C-Wells complex. Geologic contacts with open partings may have enhanced per-meability in the Calico Hills Formation at the C-Wells complex (Geldon et al., 1998). The mean permeability of the Calico Hills Formation using Yucca Mountain data indicates the mean intrin-sic permeability for the Calico Hills Formation is either larger than the mean permeability estimated for the regional carbon-ate-rock aquifer (k = 2 × 10−13 m2; Craig and Robison, 1984, p. 49), or they are comparable to the mean intrinsic permeability estimated for the regional carbonate-rock aquifer using Nevada National Security Site data (k = 6.0 × 10−13 m2). In the northern part of Yucca Mountain, the apparently widespread presence of perched water on top of the Calico Hills Formation (Patterson, 1999) indicates that the formation generally has low permeability compared to the rate of water percolation through the unsaturated zone. Although the Calico Hills Formation may be more conduc-tive locally than the regional carbonate-rock aquifer, it is unlikely that this relation exists throughout the region. Mean hydraulic conductivity values reported by Belcher et al. (2001, their table 2) for the regional carbonate rock are much greater than those of the Calico Hills Formation.

Four hydraulic tests were conducted by the U.S. Geological Survey in the Miocene tuffaceous rocks at the C-Wells complex at Yucca Mountain between 1995 and 1997 (Geldon et al., 1998, 2002). Analyses of these data by Eddebbarh et al. (2003) indi-cated that the Miocene tuffaceous rocks at Yucca Mountain had horizontal hydraulic conductivity anisotropy ratios of 0.05–20. Values of permeability at the Yucca Mountain site differ depend-ing on scale of measurement, lithology, and proximity to faults and fractures. The approximate ranges of permeability values for the core, single-borehole, and cross-borehole tests indicate the role of scale and fracturing on the permeabilities measured or estimated for the Yucca Mountain tuffs (Fig. 17). Matrix perme-ability obtained from cores spans a range of values that extends to considerably lower values than fi eld-scale permeability mea-surements (Geldon, 2004; Vacher et al., 2006). Because it is likely that permeability values obtained from testing cores are more accurate indicators of the matrix permeability of unfrac-tured rock than fi eld tests (which measure bulk permeability), it is unlikely that unfractured rock transmits great quantities of fl uid. The “single-hole tests” category (Fig. 17) indicates the range of single-borehole permeability data (encompassing a greater test-ing volume than that measured by cores); single fractures and fracture networks control bulk permeability. Because some of these tests undoubtedly were performed in regions of low frac-

ture density, the range of permeability values overlaps with the range estimated on the basis of core measurements.

Potentiometric Surface Description

Water-level data in the Yucca Mountain area have been col-lected since at least 1960 (Fenelon and Moreo, 2002) and have been reported for a number of boreholes and depth intervals (Fig. 18). Water-level data have been collected by the U.S. Geo-logical Survey, Department of Energy programs at the Nevada National Security Site, and other federal agencies, Nevada State agencies, and Nye County.

Water-Level TrendsSeveral studies have examined water-level trends in the

Yucca Mountain area (Rice et al., 1990; Ervin et al., 1994; Tucci and Burkhardt, 1995; Luckey et al., 1996; Graves et al., 1997; Fenelon and Moreo, 2002), typically with differing conclusions. Some of these differences are due to the use of various analy-sis techniques or data from various time frames. Luckey et al. (1996, p. 29) reported that water levels in the Yucca Mountain area generally have been stable, whereas other workers have reported rising and falling water levels as well as some stable water levels. Rice et al. (1990) analyzed 1983–1986 water-level data from eight boreholes and concluded that water levels in fi ve boreholes may have a periodic component. The periodic behav-ior had a spatial distribution: boreholes west of Yucca Mountain had periods that differed from periods of boreholes on and east of Yucca Mountain, possibly owing to recharge differences that refl ect periodicity in precipitation (Rice et al., 1990). Ervin et al. (1994) analyzed water-level data collected from 23 boreholes during 1986–1989 for yearly trends; fi ve boreholes had statisti-cally signifi cant upward trends (rising 0.07–0.6 m/yr; Ervin et al., 1994, their Table 2).

Tucci and Burkhardt analyzed trends in water-level data collected from 1986 to 1993; three of 22 boreholes had sta-tistically signifi cant upward trends (rising 0.004–0.01 m/yr; Tucci and Burkhardt, 1995, p. 12–13). Ervin et al. (1994) and Tucci and Burkhardt (1995) did not fi nd the same trends in the same boreholes.

Graves et al. (1997) analyzed water-level data collected dur-ing 1985–1995 but found few uniform water-level trends. Data came from 28 boreholes that monitored 36 depth intervals. They concluded (1997, p. 58–68) that no seasonal trends were present, that regional groundwater pumping did not affect water levels, and that fl uctuations can be attributed to barometric, Earth-tide changes, and earthquakes (Fig. 19).

Fenelon and Moreo’s (2002) conclusions are diffi cult to compare with the others presented in this section, owing to the use of different wells, greater area, and different time frame. Within the Yucca Mountain area, considering wells common to earlier work mentioned previously herein, fi ve wells showed upward trends, and none showed downward trends. Considering all of the wells in the Yucca Mountain area analyzed by Fenelon

44 Belcher et al.


A

B

C

D

Wat

er L

evel

(m

abo

ve s

ea le

vel)

731.5

731.0

730.5

1036.5

1036.0

1035.5

1035.0

1034.5

1034.0

731.0

730.8

730.6

730.5

753.0

752.5

752.0

751.5

Figure 19. Water-level elevations in the Yucca Mountain area that may have been affected (before and after) by June 1992 earthquakes (after Graves et al., 1997).



and Moreo (2002), six showed upward trends, and three showed downward trends.

Potentiometric SurfaceSeveral potentiometric maps of the Yucca Mountain area

have been constructed. The U.S. Geological Survey (2001) pro-duced a site-scale potentiometric map that incorporates all water-level data within the Yucca Mountain area, including data from the Nye County Early Warning Drilling Program boreholes and information from detailed geologic mapping (Fig. 20). The map represented the early 1990s; pre-1990 data were used, however, to provide better areal coverage. For this map, it was assumed that the saturated-zone groundwater system was a hydraulically well-connected water body. However, the map area may contain perched water levels and composite heads from multiple zones. This map refl ects the assumption that faults strongly affect saturated-zone fl ow, and, in the fault blocks to the east and west of the Solitario Canyon fault, the map indicates nearly level hydrau-lic gradients extending ~10 km west and south of the proposed repository to a fault approximately paralleling U.S. Highway 95.

Figure 20 uses 25 m contour intervals, which permits defi -nition of the large- and medium-hydraulic-gradient areas; how-ever, the 25 m interval is too coarse to allow the small hydraulic gradient to be adequately portrayed on the map. This interpreted groundwater contour pattern suggests that the conditions similar to those in the area of the small hydraulic gradient previously defi ned between Fortymile Wash and Yucca Mountain extend southward throughout a large area.

Using an alternative conceptual model of the large hydrau-lic gradient and more recent water-level data, the U.S. Geo-logical Survey (2004a) updated the potentiometric surface south of Yucca Mountain (Fig. 21). The area covered by this potentiometric-surface map is the same as the area shown in the earlier map (Fig. 20), and most of the water-level data used to construct the maps were the same. Updated information and water levels from borehole USW WT-24 (north of Yucca Moun-tain) and fi ve Nye County Early Warning Drilling Project bore-holes (south of Yucca Mountain) were used in the updated map.

Overall, the two potentiometric maps (Figs. 20 and 21) are similar; however, they differ in several important ways. The por-trayal of the large hydraulic gradient is different. In Figure 21, the large hydraulic gradient is reduced from ~0.11 (Tucci and Burkhardt, 1995, p. 9) to 0.06–0.07. Moreover, the potentiomet-ric contours are no longer offset where they cross faults. Such offsets (visible in Fig. 20) are not expected where the contours are perpendicular or nearly perpendicular to fault traces. Direct evidence of offset would be provided by boreholes that straddle the fault, but none exists at Yucca Mountain. Faults were used, however, to guide the placement of contours oriented parallel or approximately parallel to some faults that may act as barriers. If groundwater fl ow is impeded across a fault (or fault zone) and instead fl ows parallel to the fault, the contour line should run approximately perpendicular to the fault. The contour intervals used for the two maps also differ. In the updated map (Fig. 21),

the contour intervals are not uniform: An interval of 50 m is used for contours greater than 800 m, and an interval of 25 m is used for contours less than 800 m. Two additional contours, 730 m and 720 m, were added to help in visualizing the effect of the fault along U.S. Highway 95 on the groundwater fl ow system. The more recent map also shows a moderate- to large-gradient (0.01–0.05) area southwest of Yucca Mountain (along Highway 95 near southern Crater Flat), where water levels range from 720 to 775 m (Figs. 20 and 21).

The potentiometric surface constructed by the U.S. Geologi-cal Survey (2004a) (Fig. 21) is believed to reasonably represent hydrologic conditions in the Yucca Mountain area. Lehman and Brown (1996) presented an alternative potentiometric surface that appears to be infl uenced by northwest-southeast–trending structures on the east side of Yucca Mountain. That map is based on invalid corrections made to water levels measured in wells with large open intervals. It is also diffi cult to directly compare work by Lehman and Brown (1996) to other work owing to its spatial distortion (Bechtel SAIC Company, 2004b, p. 6–36).

Large, Moderate, and Small Hydraulic GradientsThe potentiometric surface is characterized by four major

regions that can be inferred from potentiometric contours (Fig. 22).

1. A small-gradient (0.0001–0.0004; Tucci and Burkhardt, 1995, p. 9) area occurs at, east, and south of Yucca Mountain. In the area that lies beneath Yucca Mountain, water-level altitudes range from ~728 m to 732 m. Gradients in the Amargosa Desert, south of Yucca Mountain, are also small (0.001–0.0004; water levels range from ~690 m to ~720 m).

2. A moderate-gradient (0.02–0.04; Tucci and Burkhardt, 1995, p. 9) area lies west of Yucca Mountain, where water levels range from ~740 m to 800 m.

3. A moderate- to large-gradient area (0.01–0.05) lies south-west of Yucca Mountain (along U.S. Highway 95 near southern Crater Flat), where water levels range from 720 m to 775 m.

4. A large-gradient (0.06–0.07) area lies north of Yucca Mountain, where water levels range from ~738 m to 1200 m. This gradient assumes that water levels in boreholes USW G-2 and UE-25 WT#6 represent perched conditions, and so those measurements are not used. The hydraulic gradient in the large-hydraulic-gradient area north of Yucca Mountain previously had been reported as ~0.11 (Tucci and Burkhardt, 1995, p. 9).

These gradients are evident on detailed potentiometric-surface maps presented by Ervin et al. (1994) and by Tucci and Burkhardt (1995), as well as on maps with large contour intervals compiled by D’Agnese et al. (1997). The large-contour-interval maps do not portray the small or moderate gradients adequately because of limitations imposed by contour intervals; however, the large gradient is recognizable on all of these maps. This potentiometric surface on Figure 22 implies a hydraulically well-connected fl ow system (Tucci and Burkhardt, 1995).

Several explanations have been proposed to explain the pres-ence of the apparent large hydraulic gradient at the north end of

46 Belcher et al.


EXPLANATION

116°30′00″W 116°22′30″

36°52′30″N

36°45′00″

36°37′30″

Figure 20. Potentiometric surface map of the Yucca Mountain area (U.S. Geological Survey, 2001).




Highway 95

EXPLANATION

116°35′00″ 540W

36°5

5′00

″ N

116°35′00″550 560

732.2

.

.

.

Figure 21. Potentiometric surface map of the Yucca Mountain area, assuming perched conditions for the large hydraulic gradient (U.S. Geological Survey, 2004a).

48 Belcher et al.


EXPLANATION

000m000m000m

4

00

0m4

000m

4

00

0m4

000m

4

00

0m


.

Figure 22. Potentiometric surface and general location of four hydraulic gradient areas in the Yucca Mountain area (U.S. Geological Survey, 2004a).



Yucca Mountain. This large gradient could be the result of the Claim Canyon caldera and its associated alteration of hydrogeo-logic properties. Permeability changes in similar environments have been studied by economic geologists (Norton and Knapp, 1977). Many explanations posit some sort of permeability con-trast north and south of Yucca Mountain. Because the fl ow through the saturated volcanic rocks is assumed to be uniform and continuous from north to south and to be consistent with Darcy’s law, the hydraulic gradient must be larger through the less permeable rocks to the north than it is through the more per-meable formations to the south.

There is no single explanation for the large hydraulic gradi-ent. Some explanations proposed for it are:

1. Faults that contain nontransmissive fault gouge (Czar-necki and Waddell, 1984, p. 19);

2. Faults that juxtapose transmissive tuff against nontrans-missive tuff (Czarnecki and Waddell, 1984, p. 19);

3. The presence of a less fractured lithologic unit (Czarnecki and Waddell, 1984, p. 19);

4. A change in the direction of the regional stress fi eld and a resultant change in the intensity, interconnectedness, and orientation of open fractures on either side of the area of the large hydraulic gradient (Czarnecki and Waddell, 1984, p. 19); and

5. The apparent large gradient represents a disconnected, perched, or semiperched water body, so that the high water-level altitudes refl ect only local hydraulic condi-tions and are not part of the regional saturated-zone fl ow system (Ervin et al., 1994).

Fridrich et al. (1994) suggested two hydrogeologic explana-tions for the large hydraulic gradient: (1) a highly permeable buried fault that drains water from the volcanic rock units into a deeper regional carbonate-rock aquifer or (2) a buried fault that forms a “spillway” in the volcanic rocks. Their second explanation, in effect, juxtaposes transmissive tuff against nontransmissive tuff and is therefore the same as that of Czarnecki and Waddell (1984, p. 19).

On a regional basis, other large-hydraulic-gradient areas are associated with a contact in the Paleozoic rocks between clas-tic confi ning-unit rocks and the regional carbonate-rock aquifer; however, the cause and nature of the large gradient near Yucca Mountain are not evident. The Claim Canyon Caldera is an area of extensive hydrothermal alteration north of Yucca Mountain. The southern edge of the Claim Canyon caldera produces a large hydraulic gradient where caldera rocks have been hydrother-mally altered and thus reduces permeability in the hydrogeologic units (Bechtel SAIC Company, 2004b).

The cause of the moderate hydraulic gradient is better under-stood than that of the large hydraulic gradient. Luckey et al. (1996, p. 25) suggested that the Solitario Canyon fault and its splays func-tion as a barrier to fl ow from west to east, owing to the presence of low-permeability fault gouge or to the juxtaposition of more-permeable units against less-permeable units. The small hydraulic gradient occupies most of the area of the proposed repository and the downgradient area eastward to Fortymile Wash (Fig. 22). In a

distance of 6 km between the crest of Yucca Mountain and For-tymile Wash, the hydraulic head declines only ~2.5 m. The small gradient could indicate highly transmissive rocks or little ground-water fl ow in this area (Ervin et al. 1994, p. 15).

Vertical GradientsInformation on vertical hydraulic gradients in the saturated

zone is available from the Nye County Early Warning Drill-ing Program (NC-EWDP) wells (Nye County, 2007), for wells in the Amargosa Desert (Kilroy 1991), and from Luckey et al. (1996). The information presented in this section is summarized from U.S. Geological Survey (2004a) and Bechtel SAIC Com-pany (2004b). Multiple-depth-interval data are available at 17 boreholes near Yucca Mountain. Vertical differences at various depth intervals in the same borehole indicated upward gradients that ranged from as little as 0.10 m in borehole USW H-4 and NC-EWDP-9SX to as much as 54.7 m in USW H-1 (Table 10). Downward gradients were also observed with a maximum head difference of 38 m at NC-EWDP-1DX. Aside from well NC-EWDP-1DX located along U.S. Highway 95 south of Cra-ter Flat, the largest head differences were between the regional carbonate-rock aquifer or the adjoining lower volcanic-rock confi ning unit and the overlying lower volcanic-rock aquifer. Between the upper part of the lower volcanic-rock confi ning unit and the lower volcanic-rock aquifer, the differences in potentio-metric levels generally were 1 m or less.

Some potentiometric levels were higher in the lower inter-vals of the volcanic rocks than in the upper intervals, indicating a potential for upward groundwater movement. Of 17 wells in which it was possible to measure a vertical gradient, six showed a marked (>5 m) upward gradient (boreholes USW H-1, USW H-3, UE-25p#1, NC-EWDP-2D/-2DB, NC-EWDP-4PA/-4PB, and NC-EWDP-19P/-19D), fi ve showed essentially no (less than 2 m) head differences between uppermost and lowermost moni-tored intervals (boreholes USW H-4, USW H-5, USW H-6, UE-25c#3, NC-EWDP-9SX), and fi ve showed a downward gradient (boreholes UE-25b#1, USW G-4, UE-25 J-13, NC-EWDP-1DX, NC-EWDP-3S/3D, NC-EWDP-12PA/-12PC), although only slightly downward except for NC-EWDP-1DX.

Overall, a notable upward vertical gradient is observed between the lower and upper volcanic-rock aquifer at loca-tions nearest Yucca Mountain (USW H-1 and USW H-3). Away from Yucca Mountain, the direction of the vertical hydraulic gradient differs from location to location. For example, loca-tions UE-25 J-13, NC-EWDP-1DX, and NC-EWDP-3S, have a downward gradient in the upper part of the volcanic units. For wells in lower Fortymile Wash, such as NC-EWDP-2D/2DB, NC-EWDP-4PA/-4PB, NC-EWDP-9SX (probes 1 and 2), NC-EWDP-12PA/-12PB, and NC-EWDP-19P/-19D, the gradients are slightly to moderately upward.

Although no obvious spatial patterns in the distribution of vertical hydraulic gradients around Yucca Mountain are appar-ent, some generalizations can be made about the distribution of potentiometric levels in the lower sections of the volcanic rocks.

50 Belcher et al.


TABLE 10. VERTICAL POTENTIOMETRIC LEVELS IN MONITORING BOREHOLES AT YUCCA MOUNTAIN (MODIFIED FROM BECHTEL SAIC COMPANY, 2004b, THEIR TABLE 6-4)

Borehole Open interval*

(m)

Potentiometric level† (m)

Head difference§

(m) Remarks

USW H–1 tube 4 573–673 730.94 54.7 1991 mean level; Luckey et al. (1996, their table 3)

USW H–1 tube 3 716–765 730.75 1991 mean level; Luckey et al. (1996, their table 3)



USW H–3 upper 762–1114 731.19 28.9 1996 mean level; Graves (1998, p. 59)

USW H–3 lower 1114–1219 760.07 1996 mean level; Graves (1998, p. 59)

USW H–4 upper 525–1188 730.49 0.1 1991 mean level; Luckey et al. (1996, their table 3)

USW H–4 lower 1188–1219 730.56 1991 mean level; Luckey et al. (1996, their table 3)





USW H–6 1193–1220 778.18 1/84–5/84 mean level; Luckey et al. (1996, their table 3)

UE–25 b#1 upper 488–1199 730.71 –1.0 1991 mean level; Luckey et al. (1996, their table 3)

UE–25 p#1 (volcanic) 384–500 729.90 21.4 Luckey et al. (1996, their table 3)

UE–25 p#1 (carbonate) 1297–1805 751.26 Luckey et al. (1996, their table 3)

UE–25 c#3 692–753 730.22 0.4 1990 mean level; Luckey et al. (1996, their table 3)

UE–25 c#3 753–914 730.64 1990 mean level; Luckey et al. (1996, their table 3)

)3 elbat rieht ,6991( .la te yekcuL 5.0– 3.037 747–516 4–G WSU

)3 elbat rieht ,6991( .la te yekcuL 8.927 519–747 4–G WSU

UE–25 J–13 upper 282–451 728.8 –0.8 Luckey et al. (1996, their table 3)

UE–25 J–13 471–502 728.9 Luckey et al. (1996, their table 3)



NC–EWDP–1DX (shallow) WT–419 786.8 –38.0 5/99–2/00; Nye County (2007)

NC–EWDP–1DX (deep) 658–683 748.8 8/99–2/00; Nye County (2007)

NC–EWDP–2D (volcanic) WT–493 706.1 7.2 1/99; Nye County (2007)

NC–EWDP–2DB (carbonate) 820–937 713.3 11/15/00–11/22/00; Nye County (2007)

NC–EWDP–3S probe 2 103–129 719.8 –1.5 5/06/99–12/06/00; Nye County (2007)

NC–EWDP–3S probe 3 145–168 719.4 5/06/99–12/06/00; Nye County (2007)

NC–EWDP–3D WT–762 718.3 3/99–8/99; Nye County (2007)

NC–EWDP–4PA 124–148 717.9 5.7 1/13/00–10/26/00; Nye County (2007)

NC–EWDP–4PB 225–256 723.6 1/21/00–10/26/00; Nye County (2007)

NC–EWDP–9SX probe 1 27–37 766.7 0.1 5/13/99–12/06/00; Nye County (2007)

NC–EWDP–9SX probe 2 43–49 767.3 5/13/99–12/06/00; Nye County (2007)

NC–EWDP–9SX probe 4 101–104 766.8 5/13/99–12/06/00; Nye County (2007)

NC–EWDP–12PA 99–117 722.9 2.2 4/18/00–11/15/00; Nye County (2007)

NC–EWDP–12PB 99–117 723.0 4/18/00–11/15/00; Nye County (2007)

NC–EWDP–12PC 52–70 720.7 4/27/00–11/15/00; Nye County (2007)

NC–EWDP–19P 109–140 707.5 5.3 3/13/00–6/17/00; Nye County (2007)

NC–EWDP–19D 106–433 712.8 6/14/00–6/22/00; Nye County (2007) Note: WT—water table. *Meters below land surface. †Meters above sea level. §Deepest interval minus shallowest interval.



Potentiometric levels in the lower volcanic-rock confi ning unit are relatively high (altitude greater than 750 m) in the western and northern parts of Yucca Mountain and are relatively low (alti-tude ~730 m) in the eastern part of Yucca Mountain. On the basis of potentiometric levels that were measured in borehole UE-25p#1 and NC-EWDP-2D/-2DB, the potentiometric levels in the lower volcanic-rock confi ning unit in boreholes USW H-1, USW H-3, USW H-5, and USW H-6 may refl ect the potentiometric level in the regional carbonate-rock aquifer (Luckey et al., 1996). Boreholes UE-25b#1 and USW G-4, however, do not seem to fi t the pattern established by the other boreholes. These two boreholes penetrated only 31 m and 64 m, respectively, into the lower volcanic-rock confi ning unit and had potentiometric levels (~730 m) that were similar to potentiometric levels in the lower volcanic-rock aquifer. Penetration of the other four boreholes into the lower volcanic-rock confi ning unit ranged from 123 m in borehole USW H-3 to 726 m in borehole USW H-1. The water levels measured in USW UZ-14 and USW H-5 are anomalous because they are infl uenced by a splay in the Solitario Canyon fault (Bechtel SAIC Company, 2004b).

Potentiometric levels in the regional carbonate-rock aquifer at borehole UE-25p#1 are ~20 m higher than levels in the lower volcanic-rock aquifer (Craig and Robison, 1984), indicating the potential for upward groundwater fl ow (U.S. Geological Survey, 2004a; Bechtel SAIC Company, 2004b). Vertical hydraulic gra-dients could have an important effect on the analysis of the effec-tiveness of the saturated zone as a barrier to radionuclide trans-port in that they keep the fl ow path from the proposed repository in the shallow groundwater only if the gradient is upward. On the basis of available data, a spatially extensive upward gradient can be inferred between the regional carbonate-rock aquifer and the volcanic-rock aquifers, which indicates that, at least for the immediate Yucca Mountain area, radionuclide transport would be restricted to the volcanic system.

Earthquake Effects on Water LevelsEarthquakes have caused water-level fl uctuations in the

Yucca Mountain area. Fenelon and Moreo (2002, p. 54) found that earthquakes are known to affect spring discharge and water levels in the Yucca Mountain area, as at Ash Meadows. Some of these effects, observed in discharge records for the regional carbonate-rock aquifer, appear to last for years.

Four earthquakes in California during late April 1992 pro-duced small water-level fl uctuations (0.26–0.53 m) in borehole USW H-5 (O’Brien, 1992) that generally lasted less than 2 h. No long-term effects on water-level fl uctuations due to these earth-quakes were seen in other boreholes in the Yucca Mountain area. Short-term effects could have occurred in other boreholes, but only borehole USW H-5 was instrumented to monitor for earth-quake activity during that time.

On 28 June 1992, earthquakes were recorded in Califor-nia near Landers (magnitude 7.5) and Big Bear (magnitude 6.6; O’Brien 1993, his fi gure 1 and table 1). Both locations are ~300 km south of Yucca Mountain. On 29 June 1992, an earth-

quake was recorded at Little Skull Mountain (~23 km from Yucca Mountain) with a magnitude of 5.6 (O’Brien, 1993, his fi gure 1 and table 1). Boreholes USW H-5 and USW H-6 were moni-tored continuously (hourly) at that time and recorded the effects of seismic ground motion on water levels and fl uid pressures (O’Brien, 1993, his fi gures 2 and 3, respectively).

The earthquakes caused short-term (less than 2 h) responses in 17 depth intervals of 14 boreholes that were being moni-tored hourly. These responses were small and of short duration (O’Brien, 1993). Longer-term effects on water-level fl uctuations from the earthquakes (Fig. 19) were thought to be present in boreholes UE-25 WT#4, UE-25 WT#6, USW WT-11, and UE-25p#1 (Graves et al., 1997).

Thermal Characteristics

Heat fl ow usually refl ects the tectonic history of an area. For example, the geothermal gradient and values for heat fl ow in the Basin and Range Province are both relatively large, which is mainly due to the tectonically thinned crust and consequent decrease in the depth to the mantle. Heat-fl ow values in the province are typically between ~60 mW/m2 and ~100 mW/m2. However, Sass et al. (1971) reported a large area between Eureka and Mercury in southeastern Nevada (Fig. 23, inset) that had subnormal heat fl ow (less than 63 mW/m2) and designated it as the Eureka Low. Subsequent hydrologic and geothermal studies (Winograd and Thordarson, 1975; Sass and Lachenbruch, 1982) concluded that the anomaly was hydrologic in origin, the result of lateral fl ow to depths of 3–4 km and a small component of downward seepage with a velocity of a few millimeters per year (Sass et al., 1995). This latter component may be the cause of other local anomalies in permeable units.

The most representative heat fl ow for the Yucca Mountain area is probably 84 mW/m2, which was obtained from a borehole drilled into the Precambrian section ~30 km south-southeast of the mountain (Sass and Lachenbruch, 1982). The geologic setting of this drill hole is important because it is outside of the Eureka Low and in a formation for which little movement of water is expected. Therefore, there should be no hydrologic disturbance of the heat-fl ow regime.

Sass et al. (1988) reported heat-fl ow values measured in the unsaturated section of 33 wells at and near Yucca Mountain. Values for 28 of these wells are less than 63 mW/m2, and they are, therefore, included within the area of the Eureka Low. The remaining fi ve wells form an arcuate pattern southwest, south, and southeast of Yucca Mountain (Fig. 23), but even these fi ve wells have lower values for heat fl ow (67–73 mW/m2) than the regional value of ~85 mW/m2 (Sass et al., 1988). Although the heat defi ciency could be accounted for by downward percolation of several millimeters per year of recharge, hydrologic data do not support this mechanism. Heat fl ow was measured in both the saturated and unsaturated zones in nine wells, and average values are 40 mW/m2and 41 mW/m2, respectively. Thus, the downward movement of water would have to persist in the saturated zone,

52 Belcher et al.


Water-table contour,m above sea level,contour interval 100 m

Well number and heat-flow value,mega-Watt per square meter (mW m-2),contour interval 10 mW m-2UE-25 WT#12

Figure 23. Heat fl ows for boreholes in the vicinity of the Nevada National Security Site, Nye County, Nevada (from Sass et al., 1988).



but the hydraulic potential of at least three wells is upward (Robi-son, 1984). This leaves lateral loss of heat as the only reasonable explanation for the low heat fl ow at Yucca Mountain.

The heat-fl ow defi ciency beneath Yucca Mountain area extends at least to near the base of the Tertiary volcanic rocks, on the basis of logs for deep boreholes. The tentatively reconstructed heat fl ow in unit lower carbonate-rock aquifer at borehole UE-25p#1 of ~62 mW/m2 may indicate that the anomaly penetrates deeply into Paleozoic rocks. From these two lines of evidence, Sass et al. (1988, p. 48) concluded that groundwater fl ow in unit lower carbonate-rock aquifer intercepts crustal heat fl ow and transports it laterally toward discharge areas. From the proposed repository area, the deep carbonate-rock aquifer becomes shal-lower southward. Although no water discharges to the surface at present, nonemergent discharge into younger Tertiary volcanic rocks and the basin-fi ll deposits is probable in this hydrogeologic setting. Heat being transported in the regional groundwater sys-tem should be revealed by anomalously high geothermal gradi-ents, groundwater temperatures, and heat fl ow in the direction of water fl ow (to the south of Yucca Mountain). Such anoma-lously warm water has been reported ~15 km farther south, in the vicinity of U.S. Highway 95, where the water table is shallower and water-table temperatures range from the near-surface mean-annual temperature of ~18 °C to ~30 °C (Paces and Whelan, 2001). Even higher temperatures were measured in boreholes from the Nye County’s Early Warning Drilling Program (http://www.nyecounty.com/ewdpmain.htm) (Fig. 24).

Temperatures at the water table (Sass et al., 1995) near Yucca Mountain range from ~28 °C to 38 °C (Fig. 2). The lowest tem-peratures are along and east of Fortymile Wash, which may sug-gest a small amount of downward fl ow to the water table during times when there is surface-water fl ow. High-temperature anom-alies are associated with Solitario Canyon and Midway Valley, which overlie zones of major north-trending faults. This spatial association indicates a relation between faulting and temperature anomalies at the water table, which is consistent with leakage upward from the regional carbonate-rock aquifer (Fridrich et al., 1994). The upward leakage is reasonable because the hydrau-lic head in unit lower carbonate-rock aquifer is ~20 m greater in borehole UE-25p#1 than it is in the overlying tuff (Craig and Robison, 1984) (Table 10).

Temperature gradients at the water table are common (Fig. 24) and, in the vicinity of Yucca Mountain, reach ~5 °C/km on the east and west sides of the north-trending high- temperature anomalies. Between the anomalies (that is, within the fault-bounded structural blocks), small gradients parallel the north-south structural trend. The water table generally warms south-ward from 31 °C at borehole USW G-1 to 33 °C at borehole UE-25 WT#12 (a distance of ~10 km), but the trend is not uni-form. The depth to the water table decreases in the same dis-tance from 572 m to 345 m. Farther south, two areas contain large thermal gradients and anomalously elevated groundwater tem-peratures. These attributes seem to correlate with the Highway 95 fault, which is oriented roughly perpendicular to the regional fl ow

and likely allows deep fl ow to leak upward. The westernmost of the temperature anomalies corresponds to an area of deposits left by ancient groundwater (Paces and Whelan, 2001, this volume).

The hypothesis that a fault underlying the large hydrau-lic gradient (Fridrich et al., 1994) (Fig. 22) provides a path for downward fl ow of cool water into the Paleozoic rocks is partly consistent with geothermal data (Fig 23). In the northern part of the area of large hydraulic gradient, heat fl ow is 48 mW/m2. Near the southern part of the large hydraulic gradient area, values are ~40 mW/m2; this trend suggests the introduction of south-fl owing cooler water, possibly from a fault. However, the lowest heat fl ows, ~29 to 34 mW/m2, were measured for the boreholes directly south of the large hydraulic gradient area. The decrease of deep, primarily conductive, heat fl ow southward from the large gradient area is not consistent with a simple conceptual model of cool recharge to unit lower carbonate-rock aquifer and southward warming of that fl ow as heat is collected. However, a somewhat more realistic, 3-D model might prove to be consistent with the observations. No alternative for introducing cool water to the deep aquifer south of the area of large hydraulic gradient has been proposed.

The regional carbonate-rock aquifer has a large tidal response that is about in phase with the tidal potential and is, therefore, well confi ned. If the fault zone intersected by borehole UE-25p#1 were transmissive, the regional carbonate-rock aquifer would not be well confi ned. Bredehoeft (1997) estimated that, for a deeper fault with an assumed width of 3 m and a length of 10 km, a fault-zone hydraulic conductivity through the saturated tuffs of ~1 × 10−6 m/s would provide suffi cient mass and heat transfer to approximate the water-table temperature anomaly near borehole UE-25p#1. Bredehoeft (1997) estimated that the volume of water leaked from the carbonate aquifer would be ~365,000 m3/yr.

Geothermal Features of the Surrounding AreaOne objective of the Yucca Mountain studies was to assess

if, in southern Nevada near Yucca Mountain, development of geothermal resources might be precluded by the development of a nuclear-waste repository. Evaluation of geologic, geophysi-cal, and geochemical fi ndings and the low thermal gradient led to the conclusion that “the evaluation clearly indicates that there is no potential for geothermal development in this area” and “the absence of a geothermal anomaly and the extreme depth to low-temperature fl uids essentially rule out geothermal exploration or development on a commercial scale in the Yucca Mountain area” (Flynn et al., 1996, p. 92).

Hill et al. (1995) supported a hydrothermal origin for calcite and opal deposits at Yucca Mountain. They cited thermal springs as being characteristic of the region and heat fl ow “as high as 130” (p. 71) as being characteristic of the Yucca Mountain area. Descriptions of thermal springs in Nevada (Garside and Schil-ling, 1979, their plate 1) show that thermal springs and tempera-tures in boreholes in southern Nye County are classifi ed as low temperature (between the mean-annual surface temperature and 90 °C). Stuckless et al. (1998, p. 70) observed that the highest

54 Belcher et al.


ExplanationWater temperature contour interval 2°C

Yucca Mountain Exploratory Shaft Facility

Amargosa Desert

USW G-1

Fort

ymile

Was

h

Mid

way

Val

ley

Solit

ario

Can

yon

Yucca M

ountain

NEVADACALIFORNIA

UE-25 p#1

Figure 24. Temperatures at the water table in the vicinity of Yucca Mountain (compiled by R.W. Spengler, U.S. Geo-logical Survey, using data from Sass et al. [1988] and Nye County [ 2007]).



spring temperatures in the vicinity of Yucca Mountain are mea-sured in Oasis Valley (Fig. 1), west of Crater Flat, where deeply circulating groundwaters from Pahute Mesa and Timber Moun-tain (Fig. 1) emerge with a maximum temperature of 41 °C. That temperature is only ~25 °C warmer than the mean-annual surface temperature. It is evident from several temperature logs (Sass et al., 1988) that temperatures no greater than 40 °C to 60 °C are attainable within 1–2 km of the surface in the Yucca Mountain area. Furthermore, the area is characterized by a heat-fl ow defi ciency.

The heat fl ow of 130 mW/m2 cited by Hill et al. (1995) was measured in borehole UE-25a#3 (Sass et al., 1980), which is ~20 km east of Yucca Mountain (note that Sass et al. [1980] cal-culated heat fl ow values of 76 and 35 mW/m2 deeper in the same hole). This borehole penetrates argillite of the Eleana Formation before entering carbonate rocks that probably are part of unit lower carbonate-rock aquifer, which is structurally high beneath the Calico Hills. It is likely that the high heat fl ow in the argillite is a local anomaly driven by deeper warm water rising along a fault zone, as occurs in borehole UE-25p#1, and at the northern edge of the Amargosa Desert along the Highway 95 fault.

Site-Scale Saturated-Zone Groundwater Flow

On a regional scale, groundwater generally fl ows from north of Yucca Mountain (in the higher mountains and plateaus) to south of the mountain (in the lower desert valleys), and it then continues south and westward to lower areas of discharge in Death Valley (Fig. 6). Flow paths through the Timber Mountain area are not clear because of a lack of potentiometric data. To evaluate likely fl ow paths beneath Yucca Mountain, hydrochemi-cal and isotopic data were analyzed to identify areas with similar concentrations of conservative species. Chloride data, as well as other chemical and isotopic data, suggest that groundwater from beneath the proposed repository site may fl ow initially southeast-ward, then generally south-southwestward (Sandia National Lab-oratories, 2007d, their appendixes A and B). For areas downgra-dient from the proposed repository, fl ow paths simulated by using the site-scale fl ow and transport model are generally consistent with the fl ow paths estimated from hydrochemical data. The fl ow paths shown in Figure 25, which are inferred from hydrochemi-cal data, for areas south of the repository also are consistent with the potentiometric-surface map (Fig. 21).

Vertical hydraulic gradients could infl uence the analysis of the effectiveness of the saturated zone as a barrier to contaminant transport. If the vertical hydraulic gradients are upward, con-taminants from the proposed repository would be restricted from entering the regional carbonate-rock aquifer. On the other hand, if the vertical hydraulic gradients are downward, contaminants from the proposed repository possibly could migrate from the volcanic-rock aquifers into the regional carbonate-rock aquifer. On the basis of sparse data (six boreholes), an areally extensive upward gradient can be inferred between the regional carbonate-rock and volcanic-rock aquifers.

The volume of groundwater that fl ows beneath Yucca Moun-tain is unknown. Most conceptual models interpret the large gra-dient as a region of small groundwater fl ux caused by a local or a distributed zone of low permeability. The low-gradient region downgradient from Yucca Mountain is probably due to low groundwater fl ow rates. However, because there is no direct way to measure groundwater fl ow, the possible causes of the small and large hydraulic gradients depend on indirect evidence.

Infl uence of FaultsSome faults in the Yucca Mountain area have been described

as barriers to groundwater fl ow, whereas others have been described as conduits. The infl uence of faults on groundwater movement in the Death Valley region has been characterized and related their infl uence to crustal stress, fracture mechanics, and structural geologic data (Faunt, 1997). In the 2004 regional groundwater model, some faults were considered barriers (fea-tures trending northwest to southeast) to groundwater fl ow (Faunt et al., 2010b). Locally at Yucca Mountain, however, the features trending northwest to southeast may not be barriers to fl ow, at least not in their trend direction. Hydraulic and tracer tests in Drill Hole Wash (Ogard et al., 1983; Lahoud et al., 1984) indicate that the underlying structure, whether a fault or a frac-ture zone, provides continuous, permeable fl ow paths. Also, as inferred from drawdown in observation boreholes (during pump-ing of the C-Wells complex), the northwest-southeast structure could provide a permeable connection to borehole USW H-4 (Geldon et al., 1997).

Recharge

Four sources supply recharge in the Yucca Mountain area:1. Downward and possible lateral recharge may occur from

episodic fl ooding of Fortymile Wash. On the basis of fi eld studies of stream loss, the total recharge in Fortymile Wash is estimated at ~108,000 m3/yr (Savard, 1998, his table 5). This estimate rep-resents a minimum value based on the inability to account for all reaches of Fortymile Wash (Fig. 1), which may have received unobserved runoff and recharge, coupled with the minimum period of streamfl ow observations.

2. Throughfl ow may occur from Pahute and Rainier Mesas and from the northwestern part of the Amargosa Desert (Faunt et al., 2010a) (Fig. 6). Few data constrain the conceptual models or the numerical values for this recharge. Initial work by DeMeo et al. (2006) to estimate evapotranspiration can be used to esti-mate infi ltration on Rainier Mesa. In addition, some groundwater fl ows into the Yucca Mountain area through its eastern boundary.

3. Minor recharge may occur from episodic fl ooding of the Amargosa River (Fig. 1) channel. Streamfl ow in the Amargosa Farms area generally is sparse (Osterkamp et al., 1994; Ston-estrom et al., 2003). On the basis of channel-morphology mea-surements, the composite average recharge is estimated to be 0.2 × 106 m3/yr (Osterkamp et al., 1994). Stonestrom et al. (2003), by using chloride mass-balance methods, estimated that (even

56 Belcher et al.


Universal Transverse Mercator, Zone 11 Easting, meters

Nor

thin

g, m

eter

s

Figure 25. Groundwater fl ow paths in the Yucca Mountain area inferred from geochemical data (after Sandia National Laborato-ries, 2007d, their fi gure 7-5); solid fl ow paths indicate a relatively high degree of confi dence in the interpretations; dashed fl ow paths indicate relatively less confi dence; black fl ow paths indicate fl ow through volcanic rocks; and blue fl ow paths indicate fl ow through carbonate rocks. Symbols are boreholes with numbered designator.



though much uncertainty exists) ~8%–16% of ephemeral fl ow becomes deep percolation through the river channel.

4. Net infi ltration may occur primarily along the northern part of Yucca Mountain. Precipitation becomes recharge to the saturated zone beneath Yucca Mountain (Czarnecki et al., 1997, p. 43; Flint et al., 2001).

A detailed description of net infi ltration to the water table at Yucca Mountain has been developed (Flint et al., 1996, 2001; U.S. Geological Survey, 2004b). That description shows that, in gen-eral, recharge to the saturated zone is highest at the northern end of Yucca Mountain and below some of the major surface-water drainages (Flint et al., 1996; U.S. Geological Survey, 2004b). Net infi ltration is an episodic process at Yucca Mountain that is likely only under wetter-than-average conditions or in response to iso-lated but intense storms (Flint et al., 1996, 2001; U.S. Geological Survey, 2004b). During an average precipitation year (~170 mm annual precipitation), an average annual net infi ltration rate of 3.6 mm/yr has been estimated for modern climate scenarios throughout the 123.7 km2 area considered in the Yucca Mountain infi ltration-model domain (U.S. Geological Survey, 2004b).

Regional recharge to the saturated zone near Yucca Moun-tain was estimated by using the chloride mass-balance method (Flint et al., 2002). For groundwater in the immediate vicinity of Yucca Mountain, chloride concentrations range from 5 to 9 mg/L (Marshall et al., this volume), from which local recharge rates were estimated at ~7–14 mm/yr. These values are consis-tent with values from unsaturated-zone models (LeCain and Stuckless, this volume).

DischargeNatural surface discharge does not occur near Yucca

Mountain. The nearest natural discharge areas connected to the saturated-zone fl ow system beneath Yucca Mountain are Alkali Flat, the major springs at Furnace Creek, and the fl oor of Death Valley (Fig. 6). Although most regional models (Rice, 1984, his fi gures 6 and 12; Czarnecki and Waddell, 1984, their plate 2; D’Agnese et al., 1997, p. 113–116; Belcher and Sweetkind, 2010) simulated a groundwater fl ow path from Yucca Moun-tain to Death Valley, Czarnecki and Wilson (1991) suggested that groundwater from Yucca Mountain ultimately discharges at Alkali Flat through evapotranspiration.

Within the Yucca Mountain area, groundwater is withdrawn in the Amargosa Desert (for agricultural and domestic use) and in Jackass Flats (industrial use). In the Amargosa Desert, most groundwater discharges in the southwestern corner of the Yucca Mountain area (D’Agnese et al., 1997). Pumpage from boreholes UE-25 J-12 and UE-25 J-13, just east of Yucca Mountain, was ~1,200,000 m3 from each borehole from 1985 to 1995 (Graves et al., 1997, p. 40–41).

Site-Scale Saturated-Zone Flow Model

The fi nite-element heat and mass transfer code FEHM v2.24 (Zyvoloski et al., 1999) was used to simulate fl ow through the

saturated-zone site-scale model domain. The site-scale model uses the 2004 regional fl ow model (Belcher and Sweetkind, 2010) to specify calibration targets for lateral fl uxes. Cells in the site-scale saturated-zone model are 250 × 250 m2 with 181 rows and 121 columns, in the same orientation as the regional fl ow model. The model contains as many as 67 layers, but the frame-work model allows a stair-stepped ground surface, so the number of layers is variable. Layer thicknesses range from 600 m at the bottom to 10 m near the water table south of the proposed reposi-tory (~700 m elevation).

The site-scale saturated-zone hydrogeologic framework model was constructed to provide a simplifi ed characterization of the complex lithostratigraphic conditions beneath the Yucca Mountain area suitable for the site-scale saturated-zone fl ow model. This hydrogeologic framework model represents the hydrogeologic setting for a Yucca Mountain area of ~1350 km2 and a thickness of ~6 km (Sandia National Laboratories, 2007a). The boundaries of the site-scale saturated-zone hydrogeologic framework model (Fig. 16) were chosen to coincide with grid cells in the 2004 Death Valley regional hydrogeologic frame-work model (Faunt et al., 2010c). The base of the site hydro-geologic framework model is consistent with the base of the regional hydrogeologic framework model (Faunt et al., 2010c) at −4000 m altitude. Although the site hydrogeologic framework model is based primarily on the Faunt et al. (2010c) regional hydrogeologic framework model, it also incorporates data from the previous Yucca Mountain geologic framework model, exist-ing boreholes in the general area, and recent lithostratigraphic information from the Nye County Early Warning Drilling Pro-gram boreholes (Sandia National Laboratories, 2007a). The site and regional hydrogeologic framework models use the same hydrogeologic units (Sandia National Laboratories, 2007a). An important goal for the site hydrogeologic framework model was for it to fi t seamlessly within the 2004 regional hydrogeologic framework model without an abrupt transition at the site-scale model boundary, thus allowing more direct comparisons with regional conditions and parameters and direct use of boundary fl uxes extracted from the regional model (Sandia National Labo-ratories, 2007a).

The site-scale saturated-zone fl ow model (Sandia National Laboratories, 2007d) was constructed to:

1. Estimate groundwater fl ow directions and magnitudes,2. Characterize the complex three-dimensional behavior of

fl ow through heterogeneous porous and fractured media,3. Identify the potential role of faults as barriers or conduits

to groundwater fl ow,4. Provide a model of the fl ow system to use for subsequent

assessment (by modeling) of transport performance, and5. Assess the conceptual model and parameter uncertainties

with respect to their infl uence on total system perfor-mance of the proposed Yucca Mountain repository.

Several important simplifi cations were made during devel-opment of the site-scale saturated-zone fl ow model (Sandia National Laboratories, 2007d):

58 Belcher et al.


1. Hydrologic properties of each hydrogeologic unit or of each fault or feature are uniform.

2. The hydrogeologic units represented in the hydrogeologic framework model (Sandia National Laboratories, 2007a) and faults infl uence the magnitude and direction of groundwater fl ow.

3. Groundwater fl ow conditions in the saturated zone are approximately steady state (Sandia National Laboratories, 2007d, section 1).

4. A confi ned aquifer solution is adequate and allows for recharge to be modeled as distributed infl ow on the top layer.

5. Horizontal anisotropy of permeability in the volcanic units southeast of the proposed repository is adequately repre-sented by a 5:1 permeability tensor preferentially oriented in the north-south direction. This tensor is implemented in the model by multiplying north-south permeabilities by 2.236 and east-west permeabilities by 0.447, thereby preserving two aspects: an equivalent geometric-mean permeability (Sandia National Labo-ratories, 2007d, section 6.4.3.7; Sandia National Laboratories, 2007c, appendix C6) and a vertical-to-horizontal anisotropy ratio of 0.1, which is appropriate for most of the hydrogeologic units (Sandia National Laboratories, 2007d, section 6.4.3.11).

6. Fluid in the matrix of volcanic rocks is stagnant.7. Boundary fl uxes simulated by the regional fl ow model are

similar enough to actual fl uxes so that any inaccuracies in the calculated regional fl uxes do not adversely affect site-scale fl ow and transport calculations.

8. For purposes of model simulation of potential future conditions, changes in the water-table altitude owing to climate changes will have a negligible effect on the direction of ground-water fl ow.

Faults and fault zones are hydrogeologic features that require special treatment in the site-scale saturated-zone fl ow model. Faults and fractures are pervasive at Yucca Mountain, and they may affect groundwater fl ow patterns by acting as either conduits for or barriers to groundwater fl ow. The role that faults play in facilitating or inhibiting groundwater fl ow depends on the nature of the fault (whether the faults are in tension, compression, or shear) and other factors, such as the juxtaposition of various geo-logic units along the fault plane, rock types involved, the pres-ence of fault gouge, secondary mineralization, and depth below land surface.

Faults were simulated in the site-scale model that are interpreted to have an effect on groundwater fl ow (as indi-cated by water-level measurements and potentiometric-surface contours). Faults simulated in the model include the Solitario Canyon, Highway 95, Crater Flat, and Bare Mountain faults (Fig. 26), all of which appear to act as barriers to groundwater fl ow. Faults in relative tension are more likely to be preferen-tial conduits for groundwater movement, and faults in shear or compression are more likely to be barriers (Sandia National Laboratories, 2007d, section 6.3.1.10). Other geologic features in the site-scale saturated-zone fl ow model are Fortymile Wash and Sever Wash faults, and splays of the Solitario Canyon fault (Fig. 26).

The Lower Fortymile Wash alluvial zone (Fig. 26) was added as a geologic feature affecting fl ow because of the dis-tinct character of the Fortymile Wash alluvium in the southern part of the model. Field observations, drill logs, and geophysi-cal surveys indicate possible channelization and textural con-trasts within alluvial material (Oatfi eld and Czarnecki, 1991). A quadrilateral-shaped zone (Sandia National Laboratories, 2007d, their table 6–7) near Yucca Mountain defi nes the extent of an anisotropic zone in the volcanic units and a zone of greater permeability in the Crater Flat tuffs that better approximates the small hydraulic gradient in the area (Fig. 26). The zone was defi ned on the basis of the responses of boreholes USW H-4, UE-25 C#1, UE-25 WT#14, and UE-25 WT#3 to pumping at the C-Wells complex from May 1996 to November 1997 (San-dia National Laboratories, 2007c, section 6.2.2). Although wells USW H-5, USW G-1, and UZ-14 are located east of the Soli-tario Canyon fault, they were not included in the zone because they had heads closer to those observed in wells located west of Solitario Canyon fault (USW H-6, WT-7, and WT-10) during the pumping. These heads indicate that some uncharacterized feature or process affects the water levels just to the east of Solitario Can-yon fault, and the newly defi ned zone allows the model to better represent these data.

The Claim Canyon caldera (which is expressed as a topo-graphic moat around Timber Mountain, Fig. 1) is an area of extensive alteration (designated as the “altered northern region” or “thermally altered zone”) that seems to have reduced per-meability in many of the hydrogeologic units in this area. The concept of a thermally altered zone allows various permeability values to be assigned to the same hydrogeologic unit depending on location with respect to the altered northern region (Fig. 26). Deeper units (crystalline-rock confi ning unit [XCU], intrusive-rock confi ning unit [ICU], lower siliciclastic-rock confi ning unit [LCCU], and the lower carbonate-rock aquifer) are assumed to be excluded from this alteration because their origin predates the caldera. Conceptually, this altered zone facilitates modeling of the large-hydraulic-gradient area. In the site-scale saturated-zone model, faults located in the altered northern region did not have a great effect on groundwater fl ow in the model and were not used, except for the Sever Wash fault. The Sever Wash fault appears to be important in facilitating southeasterly fl ow near the proposed repository area.

Simulated total recharge in the model area is ~60 kg/s (1.9 × 106 m3/yr). Figure 27 illustrates the net infi ltration applied as recharge to the water table. Most of the groundwater fl ux in the saturated zone beneath the proposed repository probably is lat-eral throughfl ow, which, in a regional context, is generally north to south. Flow into the model domain crosses both east and west boundaries. At Yucca Mountain, the local saturated-zone fl ow direction is southeast toward Jackass Flats, but it turns southward as it approaches Fortymile Wash.

Figure 28 shows the estimated potentiometric surface within the domain of the site-scale saturated-zone fl ow model. This potentiometric surface (Sandia National Laboratories, 2007d,



appendix E) builds on the potentiometric surface as represented by contour lines presented by the U.S. Geological Survey (2004a, their fi gure 6-1) as modifi ed by Bechtel SAIC Company (2004b, their fi gure 6-2), which also incorporates data from two additional recently completed wells, NC-EWDP-24P and NC-EWDP-29P. Data from this potentiometric surface were extracted and sup-plied as constant-head boundary conditions along the lateral boundaries of the fl ow model. Insuffi cient data are available to specify hydraulic head gradients at the model boundary condi-tions (of the 17 wells showing a vertical head gradient [Sandia National Laboratories, 2007d, section 6.3.1.5], six showed a substantial upward gradient, six showed essentially no gradient,

and fi ve showed a downward gradient). The zone of decreased permeability applied in the site-scale saturated-zone model in the altered northern region in the Claim Canyon caldera (Fig. 26) is important to accurately represent the large-hydraulic-gradient area in the numerical model in order to understand how it affects estimates of groundwater fl ow and fl ow paths from below the proposed repository.

The model formulation and the FEHM code (Zyvoloski et al., 1999) require a specifi ed permeability at each node. Sets of nodes are grouped into specifi c permeability zones on the basis of similar permeability characteristics as identifi ed in the site-scale hydrogeologic framework model (Sandia National

Universal Transverse Mercator, Zone 11

Proposed Repository

Southern Boundary ofThermally Altered Zone

Nor

thin

g, m

eter

s

Easting, meters

Yucca Mountain

Jackass Flats

Amargosa Desert

Figure 26. Geologic features of the Yucca Mountain area represented in the site-scale saturated zone fl ow model (Sandia National Labo-ratories, 2007d, their fi gure 6-12).

60 Belcher et al.


Laboratories 2007a). A single permeability value is assigned to each zone, and these zonal permeabilities are the parameters optimized during model calibration by using PEST (Watermark Numerical Computing, 2004). Permeability zones correspond to hydrogeologic units identifi ed in the site-scale saturated-zone hydrogeologic framework model, which is conceptually iden-tical to the regional hydrogeologic framework model (Sandia National Laboratories, 2007a), or to specifi c hydrogeologic features (Fig. 26). Vertical anisotropy is assigned a value of 10:1 (horizontal to vertical) in the volcanic and basin-fi ll units. Permeability that is lower in the vertical direction than in the horizontal direction is typical of stratifi ed media, and the ratio of 10:1 is in the generally accepted range (Civilian Radioactive

Waste Management System Management and Operation, 1998, their table 3-2). For example, the relatively high vertical gradi-ent observed in borehole UE-25p#1 suggests that permeability is lower in the vertical direction, an observation that is incorpo-rated into the model by use of a vertical anisotropy factor. As noted earlier, 11 out of 17 wells within the domain of the site-scale saturated-zone model had vertical gradients.

Site-Scale Saturated-Zone Model ResultsTwenty-three permeability zones were established on the

basis of hydrogeologic units within the site-scale saturated-zone model domain (Sandia National Laboratories, 2007b). Figure 17 compares estimated permeability data with the calibrated model

Easting, meters

Nor

thin

g, m

eter

s

Jackass Flats

Yucca Mountain

Amargosa Desert

Was

h

Fort

ymile


Figure 27. Infi ltration constant-fl ux boundary conditions applied at the wa-ter table as recharge, in kilograms per second (kg/s), of the site-scale saturated zone fl ow model. Approximate outline of the proposed repository is shown in green (Sandia National Laboratories, 2007d, their fi gure 6-14).



parameters. Additional (usually low) permeability zones refl ect-ing the altered northern region were added to the model to help establish known system characteristics (such as the large hydrau-lic gradient). These permeability zones were established by divid-ing existing (base) permeability values of geologic units in the altered northern region with fractional permeabilities defi ned by multipliers. These permeability multipliers are calibration param-eters that modify the permeability values assigned to geologic units in the altered northern region (Fig. 17). Eleven additional permeability zones representing faults and the Lower Fortymile

Wash alluvium were established because they were identifi ed as important structural features (for example, the Solitario Canyon fault) or were necessary for some conceptual feature, such as the large hydraulic gradient north of Yucca Mountain or anisotropic zones (Fig. 26).

The site-scale fl ow model was calibrated to measured water levels and heads in wells and to residual head differences (Fig. 29). The greatest residuals (~100 m) were in the northern part of the model domain. They largely result from the low weighting assigned to these calibration targets (that is, they are not likely to


EXPLANATION

Potentiometric surface contour. Variable contour interval.Proposed repository footprint.

Amargosa Desert

Jackass FlatsW

ash

Fort

ymile

Yucca M

ou

ntain

Nor

thin

g, m

eter

s

Easting, meters

Figure 28. Contour map of the poten-tiometric surface in the Yucca Moun-tain area constructed on the basis of water-level data within the domain of the site-scale saturated zone fl ow model (Sandia National Laboratories, 2007d, their fi gure 6-16).

62 Belcher et al.


infl uence a fl ow path leaving the proposed repository) and of the uncertainty in these measurements owing to perched conditions that may exist in this area. The next largest group of head residu-als borders the Crater Flat and Solitario Canyon faults.

The site-scale saturated-zone model was calibrated to bound-ary fl uxes extracted from the regional-scale model. The weighted root-mean-squared error for fl uxes is 35.3 kg/s (1.1 × 106 m3/yr), and fl ux through the southern boundary of the site-scale model is 528.1 kg/s (1.7 × 107 m3/yr), which is ~23% less than the corre-sponding value of fl ux through the regional-scale model. Table 11 lists the target fl ux data and the calibrated model results.

Advective (nondispersive) fl ow paths from the proposed repository were simulated by use of the calibrated site-scale model (Fig. 30). Flow paths for hypothetical water particles were simulated for the steady-state condition. From the proposed repository area, the pathways generally travel south- southeasterly to near Fortymile Wash, then trend south-southwest and gener-ally follow Fortymile Wash southward (Sandia National Labo-ratories, 2007d). Some of the pathways follow fault zones along Fortymile Wash. These predicted paths generally agree with the fl ow paths interpreted on the basis of hydrochemistry data (San-dia National Laboratories, 2007d, appendixes A and B) (Fig. 24).

535000 540000 545000 550000 555000 560000

4050000

4055000

4060000

4065000

4070000

4075000

4080000

4085000

4090000 Head residual (m)

-150 to -20 -20 to -5 -5 to -2 -2 to 2 2 to 5 5 to 20 20 to 120

725

Nor

thin

g, m

eter

s

Easting, metersUniversal Transverse Mercator, Zone 11

EXPLANATION

Potentiometric surface contour. Variable contour interval.

Figure 29. Simulated potentiometric surface in the Yucca Mountain area in-cluding residuals (observed minus sim-ulated), in meters above sea level (San-dia National Laboratories, 2007d, their fi gure 6-15).



Example fl ow paths simulated by the site-scale saturated-zone fl ow model are shown in Figure 30.

Specifi c discharge (groundwater velocity) values of 0.36 and 0.55 m/yr were estimated for a nominal fl ow path leaving the proposed repository area and traveling 5 and 18 km, respectively. These values agreed well with those estimated by an expert elici-tation panel, which estimated a specifi c discharge of 0.66 m/yr at a distance of 5 km (Civilian Radioactive Waste Management System Management and Operation 1998, their fi gure 3-2e).

NUMERICAL MODELS AS PREDICTORS OF WATER-TABLE ALTITUDES GIVEN FUTURE CLIMATE

Several modeling simulations have evaluated the effects of known past and hypothetical future changes in climate on the height of the water table in the Yucca Mountain area. Czarnecki (1984) evaluated the effects of increased precipitation (similar to past pluvial conditions) on the groundwater system in the vicin-ity of Yucca Mountain using a two-dimensional fi nite-element groundwater fl ow model of the region. Input parameters and initial conditions for the model were based on previous models (Czarnecki and Waddell, 1984) to derive a baseline potentiomet-ric surface of current water-table levels. Modern precipitation was assumed to be doubled, based on previous investigations by Spaulding et al. (1984). That assumption increased recharge by 15 times in the model domain. At Yucca Mountain, recharge increased from the modern rate of 0.5 mm/yr (Sandia National Laboratories, 2007e) to a simulated rate of ~8 mm/yr. Results from the model showed a maximum increase in water-table alti-tude of ~130 m beneath the site of the proposed repository (Czar-necki, 1984, p. 21). These results commonly have been cited as being consistent with water-table rises estimated from studies of paleodischarge deposits and subsurface mineralogy. However,

more complete hydrologic property data and new potentiometric-surface data at the Nye County paleodischarge sites show that such a rise is highly unlikely (Nye County, 2007).

Ahola and Sagar (1992) used a regional groundwater fl ow model to evaluate the effect of a 10-fold increase in recharge within the Yucca Mountain area, as at Fortymile Wash. Simu-lations indicated that the water table might rise ~75–100 m; however, the model was preliminary, required the use of inter-polated data based on previous calibrated models, and was not fully calibrated.

D’Agnese et al. (1998) modeled the effects of climate change on the Death Valley region by using a past climate representative of the late Pleistocene full glacial condition (21,000 yr ago), for which maximum water-table altitudes are documented by stud-ies of paleodischarge sites and mineralogic characteristics in the subsurface at Yucca Mountain. An initial simulation was evalu-ated by comparing the results with information obtained from paleodischarge sites around the region. The cooler and wetter conditions present 21,000 yr ago were also considered to repre-sent a wet climate that might recur in the future during the opera-tional life of the proposed repository (Sharpe, 2007). A second simulation assumed a wetter and warmer future climate due to increased carbon dioxide concentrations in the atmosphere, and recharge to the Death Valley regional groundwater fl ow system was increased.

In each of these simulations, the changes in climatic condi-tions were simulated by changing the distribution and rates of groundwater recharge throughout the model grid. Average annual recharge maps for past-climate and future-climate scenarios were reassigned to the model grid. Results of the simulations were evaluated through analyses of simulated discharge areas, water-level changes, potentiometric-surface confi gurations, and water budgets. The simulated recharge for the past climate (21,000 yr

TABLE 11. LATERAL BOUNDARY FLUX TARGETS AND CALIBRATED RESULTS FOR THE YUCCA MOUNTAIN SITE-SCALE MODEL

Boundary zone (geographic coordinate range in m)*

Target flow from regional-scale flow

Simulated flow Calibration

weight Mass flow (kg/s)

Volume flow (m3/yr)

Mass flow (kg/s)†

Volume flow (m3/yr)

North (533,000–563,000) –158.9 –5.0 × 106 –57.1 –1.8 × 106 5

West (4,046,500–4,091,500) –120.3 –3.8 × 106 –101.0 –3.2 × 106 5

East1 (4,046,500–4,052,500) –273.1 –8.6 × 106 –232.1 –7.3 × 106 1

East2 (4,052,501–4,058,500) 33.3 1.0 × 106 –97.4 –3.1 × 106 1

East3 (4,058,501–4,069,000) –127.8 –4.0 × 106 260.9 8.2 × 106 1

East4 (4,069,001–4,079,500) 30.2 9.5 × 105 –206.6 6.5 × 106 1

East5 (4,079,501–4,091,500) –0.4 –1.2 × 104 –30.7 –9.7 × 105 1

South (533,000–563,000) 681.9 2.2 × 107 528.1 1.7 × 107 NA

Notes: Negative values indicate flow into the model. South boundary volumetric/mass flow rates were not used as targets for the calibration of the saturated zone site-scale flow model; rather, they were calculated from the balance of infiltration and the volumetric/mass flow rates across north, west, and east boundaries. NA—not applicable. *Coordinates are Universal Transverse Mercator, Zone 11. †Mass flows are approximate because of the technique FEHM modeling uses in the flxz macro to sum boundary flows.

64 Belcher et al.


Universal Transverse Mercator, Zone 11 EXPLANATIONSimulated potentiometric surfacecontour. Variable contour interval.Compliance boundaryParticle flow paths

Nor

thin

g, m

eter

s

Water table

Lower F

ortymile W

ashalluvial zone

Highw

ay 95 fault

730730720

Yucca Mountain

Special geologic features (see Fig. 26)

Jackass Flats

Was

h

Fort

ymile

Amargosa Desert

Easting, meters

Figure 30. Simulated fl ow paths in the Yucca Mountain area for particles released (uniformly but randomly distributed) from the proposed repository area and graph of elevations (Sandia National Laboratories, 2007d, their fi gure 6-16).



ago) was estimated to have increased fi vefold, which increased the altitude of the potentiometric surface near Yucca Mountain ~60–150 m. These changes presumed a much greater depth to water at the Nye County paleodischarge sites than is now known to exist (Nye County, 1997; Paces and Whelan, this volume).

Simulated water levels for past-climate conditions in the Yucca Mountain area rose most substantially to the north and northeast of Yucca Mountain in the Timber and Shoshone Moun-tain areas (Fig. 1). Enough groundwater was simulated in the system to maintain ancient lake levels in the northern parts of the model domain and at ancient Lake Manly in Death Valley (Fig. 1). Groundwater discharge was simulated at most of the observed paleodischarge sites, which indicated that the recharge distribu-tion used in the simulation generally was valid. The higher water-table levels that were simulated between Rainier Mesa and Yucca Flat (Figs. 1 and 6), north of Yucca Mountain, increased the areal extent and steepness of the present-day large hydraulic gradient in this area. However, the large gradient appeared to remain fi xed geographically—that is, it did not migrate southward toward the proposed repository block (D’Agnese et al., 1999, p. 27).

Under future-climate conditions, simulated recharge both increased and decreased throughout the model domain (D’Agnese et al., 1999, p. 2). The confi guration of the simulated potentio-metric surface changed slightly relative to simulated present-day conditions to indicate depressions at discharging playas. Simu-lated discharge to these playas, however, was not as great as dur-ing the past full-glacial climate, and perennial lakes probably would not exist at these locations. Simulated discharge increased as compared with present-day discharge at Ash Meadows, Oasis Valley, and Death Valley (Fig. 8). Under future-climate condi-tions, large hydraulic gradients were maintained and steepened in some areas, but their position did not migrate. Simulated recharge throughout the modeled area increased by a factor of ~1.8 com-pared with simulated present-day recharge levels and caused the water table beneath the proposed repository to rise ~50 m.

Numerical models have many limitations in evaluating the effects of climate change on regional groundwater fl ow systems (D’Agnese et al., 1999). Limitations relevant in estimating future water-table altitude are as follows:

1. The predictive simulations can be no more accurate than the present-day, steady-state regional groundwater fl ow model on which they are based.

2. Paleohydrologic evidence is critical to the validity of the assumptions, and this evidence, such as evidence of past- discharge areas, is subject to interpretation.

3. The size of the model grid from which the average annual precipitation distributions for past-climate and future- climate conditions were extracted was greater than the size of the grid required for the regional groundwater fl ow model.

Future-climate scenarios are simulated with the site-scale saturated-zone fl ow model, and results were generally consistent with those from the regional-scale model (Sandia National Labo-ratories, 2007d, section 6.6.4).

CONCLUSION

Yucca Mountain is located within a large topographically closed basin. All surface water drains internally, and groundwa-ter fl ow largely mimics this feature, except that individual topo-graphic lows are commonly connected at depth by a regional carbonate-rock aquifer. Most recharge originates in the high-lands north of Yucca Mountain and fl ows generally southward. Some groundwater discharges within the basin (such as in Oasis Valley and the southern Amargosa Desert), but the ultimate dis-charge is in Death Valley, where water is returned to the atmo-sphere by evapotranspiration. Groundwater fl ows through a heterogeneous medium that refl ects a complex geologic history including both compressional and extensional tectonics. The potentiometric surface for the fl ow system has areas of large hydraulic gradient (as great as 0.13) and of small hydraulic gra-dient (as small as 0.0001). Both extremes are found within the Yucca Mountain site area, where they are well constrained by numerous boreholes.

Since the early 1980s, numerous two- and three-dimensional fl ow models have been developed to describe regional ground-water fl ow. After nearly 40 yr of study of the saturated zone at Nevada National Security Site and the surrounding area, numeri-cal models of the regional fl ow system now match well with the observed data except in areas of large hydraulic gradients. A detailed site-scale fl ow model for the Yucca Mountain area again was constructed and calibrated that matches well with observed hydrologic data and provides a means for assessing the hypo-thetical fl ow path for any radioactive materials originating from the proposed repository.

ACKNOWLEDGMENTS

We wish to thank Patrick Tucci and Amjad Umari (both of the U.S. Geological Survey) for detailed and helpful comments, which greatly improved this manuscript. The results reported in this chapter are a summary of work by numerous scientists over more than 40 yr, as indicated by the abundant references to previous work. The authors wish to particularly acknowledge the pioneering work of I.J. Winograd (U.S. Geological Survey, retired) and to thank him for his encouragement in producing this summary report.

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