University of Nevada Reno a Deuterium-Calibrated Groundwater Flow Model of Western Nevada Test Site and Vicinity a Thesis Submit

University of Nevada Reno

A Deuterium-Calibrated Groundwater Flow Model of Western Nevada Test Site and Vicinity

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Hydrology and Hydrogeology

Thomas Aquinas Feeney t (*

Minos Library University of Nevada - Reno Reno, Nevada 89557-0044

February, 1987 IW cI ojI C. QU /C<- Thesis Advisor

THOMAS AQUINAS FEENEY

ACKNOWLEDGEMENTS

I would like to express my gratitude to the U.S. Department of Energy for funding this research project and making this thesis possible, and the staff of the Desert Research Institute for lending a helping hand whenever I needed one. Dr. Donald Tibbitts also deserves thanks for being on my committee and expressing an interest in my thesis. I am especially grateful to Professors Michael Campana and Roger Jacobson of the Desert Research Institute for giving me the opportunity to work on this project, for serving on my committee, and for their support which extended well beyond the thesis itself. Their knowledge and selves will be sorely missed. Finally, thank you Carol for helping me through the tough times during the writing of my thesis and making me realize that finishing it was not an end but a beginning. ABSTRACT

A discrete-state compartment model, consisting of nine aquifer sub- regions, is calibrated with deuterium to delineate groundwater flow in the vicinity of western Nevada Test Site. Model flow paths and fluxes are in relative agreement with previous studies. The total groundwater flux through the model is 32.4 x 10^ m^/yr. Fortymile Canyon/Wash and

Pahute Mesa are major recharge sites, respectively accounting for 2.6

recharge. Infiltration as a percent of precipitation is greatest in Fortymile and Stockade Washes, ranging between 5 and 12%. Major discharge areas occur in Oasis Valley and the Amargosa

Desert, discharging 2.7 and 28.8 x 10^ m^/yr, respectively. Model mean ages range from 2700 years at Ash Meadows to 9200 years at Pahute Mesa, and represent minimum groundwater residence times. A sensitivity analy

sis reveals that cell deuterium concentrations are most influenced by

local recharge. TABLE OF CONTENTS page Acknowledgements ii Abstract iii List of Figures V List of Tables vi Purpose and Scope 1 Description of Study Area 1 Regional Geology 4 Stratigraphy 4

Structure 6

Surface Hydrology 8 Hydrogeology 9 Previous Work 11 The Discrete State Compartment Model 15 Model Calibration 18 Results and Discussion 28 System Simulation 28 Recharge Areas and Estimates 29 Model Flow Paths 34 Model Flux Comparisons 38 Model Uncertainty after Calibration 40 Model Sensitivity 43 Summary and Conclusions 47 Suggestions for Future Research 48 References 50 Appendix A 53 Appendix B 55 Appendix C 56 V

LIST OF FIGURES Title page 1. Location of study area o 2. Nested Caldera Region in Vicinity of Study Area 5 3. Types of mixing cells in the DSC Model 17 4. Study Area Isotope Samples ig 5. DSC Model Network 26 6. Areas Eligible for Recharge Calculation 33 7. Model Flow Paths 35 n | iiiim n in iiii mrnwM

LIST OF TABLES

Title page 12345678

1. Geologic Column of Study Area 7 2. Cell Characteristics 27 3. Calibration Results 27 4. DSC Model Recharge Estimates 32 5. Flow Paths, Volumes 36 6. Model Fluxes and Comparisons 39 7. Model Mean Ages 41 8. Results of Sensitivity Analyses 44 1 PURPOSE and SCOPE of the STUDY The purpose of this study is to determine the hydrologic characteristics of groundwater flow systems in the study area with the aid of a Discrete State Compartment (DSC) mixing model and deuterium (2H), a stable isotope tracer. Because the application of this method is new to this area, results from the DSC model can be objectively com pared with those from previous hydraulic models and studies to assess the feasibility of all available quantitative flux estimates and regional flow characteristics in the study area. A properly calibrated DSC model simulating groundwater flow in a hydrologic sys tem can give the following results:

1) Recharge/discharge volumes;

2) Locations of recharge/discharge areas; 3) Indications of areas with significant vertical flow; 4) General flow directions; 5) Evaluation of groundwater barriers; and,

6) Groundwater residence times.

DESCRIPTION of the STUDY AREA

The study area (Figure 1) is located in the Great Basin section of the Basin and Range physiographic province of the southwestern United States between (l 17 degrees 50^ minutes and 116 degrees west longitude, and 37 degrees 30 minutes and 36 degrees 15 minutes north latitude in Nye County, Nevada. The area extends approximately 47 km north to south and 22 km east to west at its farthest points, encompassing 3740 km2. CJ \ 0 V/ ^ Fifty wells and springs were sampled for deuterium in the area; all wells are screened in alluvium or tuff except for one well on the eastern side of Yucca Mountain which penetrates carbonates.

Southern Nevada is arid; the average annual precipitation is from 8 to 16 cm in the valleys and mostly less than 25 cm on the ridges and mesas. The general distribution of annual rainfall in the study area is thought to be transitional in amount compared to 2

Figure 1, Location of Study Area 3 areas to the east which receive an excess of precipitation, and areas to the west which receive a deficit (French, 1983). Precipitation varies seasonally to a great extent with most precipitation falling in winter and summer. Summer precipitation events usually originate from storms from the south or southeast and localized convective storms. Winter precipitation is usually derived from broad low pressure weather systems from the west. Evidence indicates that periods of greater rainfall, or pluvials, have been more common than dry periods in the past 150,000 years (Winograd and Doty, 1980). South ern Nevada may have experienced significant changes in rainfall due to pluvial periods which have occurred in North America. It is believed that the study area received more precipitation during these periods than during dry periods such as present day condi tions.

Creosote bush (Larrea divaricata), burro bush (Franseria dumosa), and yucca plants ( Yucca yuca) populate the valleys, giving way to blackbrush (Coleogyne ramosis- sima) and Joshua trees ( Yucca brevifolia) at slightly higher altitudes. Above 1800 m elevation pinon pine (Pmus monophylla), juniper (Juniperus osteosperma), and sage (Artemisia tridentata and A. nova) dominate, and above 2300 m white fir (Abies con- color) and yellow pine (Pmus echmata) are most abundant. 4 REGIONAL GEOLOGY of the STUDY AREA The following is a brief review of the geology pertinent to this study based on Winograd and Thordarson (1975), to which the reader is referred for more information.

STRATIGRAPHY Lithology in the study area consists of late Precambrian to early Paleozoic sedi mentary rocks, extrusive rocks of Tertiary age, and minor sedimentary rocks of Tertiary and younger age. The Paleozoic rocks have been intruded by minor Mesozoic granitic stocks, and mafic dikes of Miocene or younger age have intruded both Paleozoic and Tertiary rocks. Basins are filled with lacustrine and alluvial sediments ranging in age from Tertiary in the deeper sections to Quaternary in the shallower sections. Local stratigraphic thickness of the Precambrian and Paleozoic rocks, the upper half of which is carbonates, can approach 11300 meters (Winograd and Thordarson, 1975). However, the thesis area almost exclusively encompasses a region of overlapping calderas originat ing from Tertiary volcanism, which was subsequently responsible for the widespread deposition of thick sequences of tuff and other volcanic rock in the calderas and in the vicinity of the Nevada Test Site (NTS). It is possible that much of the Paleozoic sedi mentary rock has been obliterated or is insignificant in a regional context within the region of ”nested” calderas (see Figure 2). No carbonates have been found within presently hypothesized caldera boundaries unless surrounding highlands contain fault blocks of Paleozoic rock. The lack of carbonate detritus may indicate the absence of car bonate strata within calderas, since eruptions from the calderas would be expected to bring to the surface rocks which were at depth inside the caldera. In any case, the princi pal rock types of the Paleozoic assemblage are dolomite, siltstone, argillite, and quart zite. The Tertiary rocks are predominantly tuff but basalt, rhyolite, and dacite also appear in this interval. The total thickness of the Tertiary sequence is about 3960 meters. Valley fill (alluvium) varies in thickness, but maximum thickness is probably 610 meters. 5

FIGURE 2. Nested Caldera Region in Vicinity of Study Area, (from Scott and Castellanos, 1984) 6 For a generalized geologic column of the study area see Table 1. A more detailed hydrogeologic column is found in Waddell et al. (1984).

STRUCTURE Two major periods of deformation are distinguished: folding and thrusting of Paleo zoic rock from late Mesozoic to early Tertiary, and normal (block) faulting associated with the Basin and Range during mid to late Cenozoic. Significant displacements along major strike slip faults occurred during both deformation periods. Volcanism during the Tertiary formed a field of calderas which influenced local structure. The calderas formed when deep seated magma bodies rose, causing the surface to bulge. Sets of arcuate fractures then formed along the circumference of the up-doming features, through which the magma erupted. Overlying rock then collapsed in on itself due to the loss of underlying material, forming the calderas. The southern tip of Nevada including NTS is part of a continental scale lineament known as the Walker Lane Belt. It crosses the Basin and Range Province from Texas to Oregon and extends from the Furnace Creek and Death Valley Fault Zones in California to the Las Vegas Shear Zone and Pahute Mesa in Nevada. It separates the northwest structural trends east of the Sierra Nevada in California from the predominantly north to south trends of typical Basin and Range structure in Nevada. This belt is a zone of diversely oriented relatively low relief hills and valleys dominated by lateral shear rather than vertical tectonics. The shear zones tend to be poorly exposed, mainly because in the southern Great Basin some of them are tectonically inactive and their traces are buried beneath young volcanics and alluvium. Many northwest trending faults in the belt are thought to be buried structures. Table 1. - GEOLOGIC COLUMN OF STUDY AREA

T im e Lithologic P erio d M axim um C om m en ts U nit Thickness(m) Quaternary, alluvium,fluvial, and T e rtia ry 610 thickness varies; lacustrine deposits not present Quaternary, basalts 61 T e rtia ry Pliocene unit

rhyolite flows 610 aquifer ash flow tuffs, 914 aquifer nonwelded to densely welded lava flow tuff 1100 low permeability and breccia, locally altered T e rtia ry rhyolite lavas 610 potential aquifer and tuffs ash flow tuff, 610 aquifer/aquitard nonwelded to mod erately welded, interbedded with ash fall tuff ash flow tuff, 610 aq u itard partially to densely welded, argillized altered rhyolitic ? ? and quartz latitic lavas with altered bedded and ash flow tuffs tuffaceous sandstone, 427 a q u itard silts tone,claystone, freshwater limestone and conglomerate with minor gypsum freshwater limestone, 305 aq u itard conglomerate tuff Cretaceous granitic stocks of 3 05 insignificant through Permian granodiorite, quartz in study area m onzonite Permian through lim estone 1100 aquifer, Pennsylvanian questionable occurrence in area Mississippian argillite, quartzite 2440 aquitard,significant through Devonian conglomerate, limestone only at eastern b o u n d ary Devonian through dolomite and 4880 aquifer(may be C am b rian limestone, shale, significant in silts to ne south only) Cambrian through quartzite, limestone, 4 57 0 aquitard, forms Proterozoic shale,dolomite, ’impervious’ lower (Precambrian) locally metamor boundary in phosed,sandstone th e region 8 SURFACE HYDROLOGY Several major ephemeral streams that exist in or discharge into the study area con tain water only after intense storms. The only perennial flows are drainages from the Ash Meadows springs near the southern part of the Amargosa River. The ephemeral Amargosa River originates in Oasis Valley. It runs southeast along the Amargosa Desert valley until it veers due south between the Resting Springs Range and the Funeral Mountains out of the study area. Forty mile Canyon originates at Buckboard Mesa, winds around to the east of Timber Mountain, and heads into Jackass Flats, where it becomes a wash. It is joined by Drill Hole and Yucca Washes off Yucca Mountain, and disperses near Lathrop Wells. Topopah Wash is the other major drainage in the model region. It runs from Shoshone Mountain to south central Jackass Flats where it diffuses in close proximity to Fortymile Wash. Springs, evapotranspiration, and pumpage are the surface discharge from this sys tem. Springs within the model boundary are located in the Amargosa Desert at Ash Meadows and in Oasis Valley. Evapotranspiration is great in these areas, and is increased due to agricultural areas located in the Amargosa Desert. Evapotransporation is also significant at Pahute Mesa and Timber Mountain, where vegetation density increases with elevation. Pumpage has been most significant in Beatty and the Amar gosa Desert due to the relatively high densities of continuously pumped wells at these locations. For example, water levels in alluvial wells in the Amargosa Desert have dropped 8 meters between 1962 and 1984 (Nichols and Akers, 1985). 9 HYDROGEOLOGY

Winograd and Thordarson (1975) divided the rocks in the NTS region into 10 hydrogeologic units based on aquifer test and porosity determinations. Due to the nature of the DSC model, the effect of composite water sampling, lack of vertical head data, uncertainty of aquifer thicknesses, and the large variation in well depth throughout the area, saturated thickness approximated from non-fully penetrating wells will be used in this report over previous subdivisions of lithologies. Composite well samples are taken from the entire water column in the well, and as a result distinct zones in the well bore become mixed. Saturated thickness was determined by subtracting the depth to water in each well from total well depth. The study area contains generally three lithologies which control groundwater movement: alluvium (valley fill), volcanic rocks, and carbonate rocks. Alluvium most likely contains small scale (local and intermediate) flow systems, although these systems can be superimposed on a regional system at discharge areas such as Alkali Flat and Ash Meadows. Volcanic rocks, mainly tuff, are thick in the northern part of the study area, and pinch out to the south, around the border of NTS near Lathrop Wells. Because of variation in tuff deposits from location to location, this lithologic unit probably harbors local and intermediate flow systems as well as a regional system. Carbonate rocks make up the bulk of the deepest aquifers in the region, thereby controlling regional groundwa ter flow where they are abundant. As a result of volcanism in the area (see Stratigraphy section), carbonates are assumed to exist in significant thickness only beneath the Amar- gosa Desert. All three lithologies communicate as aquifers to some degree at some loca tions in the modeled region, as in Oasis Valley and Amargosa Desert. Local fault juxta position and the stratigraphic location of clay and clastic aquitards influence flow between carbonates and tuff's in areas such as the Eleana Range, and between carbonates and alluvium in Ash Meadows. Precambrian quartzites and other rocks form the lower flow boundary at the deepest part of the geologic sections. Argillite of the Eleana For mation is present only at the northeastern boundary of the model area and is believed to restrict groundwater flow into Yucca Flat. Other clastic rocks which crop out in the 10 Kawich and Belted Ranges to the north may also impede interbasin flow. Groundwater flow is controlled by secondary porosity in the tuffs and carbonates and primary porosity in the alluvium. The faults, fractures, and solution channels creat ing the secondary porosity result in a heterogeneous, anisotropic aquifer system or sys tems. The majority of groundwater flux may converge along fracture zones in the vicin ity of the ring faults associated with subsided volcanic calderas in the study area. Blank- ennagel and Weir (1973) found very high transmissivities in the highly fractured ring fault zone of the Silent Canyon Caldera on Pahute Mesa which may apply to fracture zones associated with the other calderas in the region. Groundwater may be moving through these zones at higher velocities and in greater quantities than through the rest of the system, particularly since there is a high density of calderas in the study area (Figure 2). Regional tectonics may also affect the direction of groundwater flow in the study area. Northwest trending basement faults and other large scale lineaments (see Struc ture section) may influence regional flow because they trace through all of the permeable strata. The Eleana Formation, which outcrops in the Eleana Range on the west side of Yucca Flat, consists of low permeability clastic rocks that have been upthrusted against more permeable lithologies. Winograd and Thordarson (1975) believed that this forma tion acted as a barrier to eastward groundwater flow, although they noted that the elas tics were underlain by carbonates, and therefore might not restrict flow unless the car bonates were also discontinuous. The present database does not allow accurate determination of aquifer thickness in the study area because no wells fully penetrate to the Precambrian quartzite. The majority of wells are shallow irrigation or domestic type in the Amargosa Desert and Oasis Valley. All but one of the many deep wells on Yucca Mountain and Pahute Mesa penetrate Tertiary rock; well UE25p#l, between Yucca Mountain and Fortymile Wash, penetrates Paleozoic carbonates. 11 PREVIOUS WORK on REGIONAL FLOW in the STUDY AREA There has been an extensive amount of research on interbasin groundwater flow systems in the vicinity of the Nevada Test Site (NTS) since Hunt and Robinson (1960) observed that springs in Ash Meadows, southern Nevada, were geochemically similar to springs in Death Valley, California. They noticed that the Death Valley springs had discharges too large for their respective catchment areas. They hypothesized that since the springs were on fault zones flow may have taken place through these zones from nearby basins, suggesting that groundwater was flowing at high velocity through open conduits in the thick Paleozoic carbonate formations. Winograd (1962) inferred from low interbasin hydraulic gradients that groundwater flowed between basins, and from high intrabasin hydraulic gradients that downward flow occurred within basins. Horizontal groundwater flow occurred primarily at great depth in carbonate rock higher in permeability than the overlying rocks. Schoff and Moore (1964) concluded from groundwater chemistry in the NTS region that groundwater flowing from the Spring Mountains and other sources in the northwest made up the bulk of flow into the Amargosa Desert, with NTS contributing a relatively small volume. Eakin, Schoff, and Cohen (1964), by using water table contours, judged that underflow moves southward through NTS from areas north. The Amargosa Desert seemed to be a discharge area from its elevation and proximity. The NTS contribution of flux to the Amargosa was estimated to be 7 1/2 % of the groundwater discharged at the Amargosa Desert. The authors reported that the shear zone which trends northwest in the Amargosa Desert had potential for limiting groundwater movement, although water levels and chemistry were not conclusive. Las Vegas and Muddy River Springs were ruled out as potential discharge areas for NTS underflow on the basis of significant hydrologic barriers (mountain ranges), chemical quality, head levels, and water budgets. Grove et al. (1969) attempted to correlate carbon 14 ages in the eastern portion of the test site with groundwater age along a proposed flow path. Results were 12 inconclusive, as some waters up the flow path appeared older than water down the flow path. Blankennagel and Weir (1973) postulated that underflow travelled beneath Pahute Mesa from Gold Flat and Kawich Valley to Crater Flat, western Jackass Flats, and Oasis Valley. They estimated the underflow from Pahute Mesa through a 15 mile wide strip along its southern boundary. A hydraulic barrier and an adjacent ’groundwater drain’ said to exist at the western edge of this strip were indicated by potentiometric data and seemed to coincide with the boundary of the Silent Canyon Caldera which was mapped on Pahute Mesa. Winograd and Friedman (1972) suggested that 65% of Ash Meadows springflow in the Amargosa Desert originated as recharge to the Spring Mountains according to observed deuterium isotope and water level data in the hypothesized Ash Meadows Groundwater Basin, which is bordered on the west by the Eleana and Belted Ranges and on the east by the Sheep Range. Using a mass balance approach, they calculated that underflow from the rest of the basin northeast of the springs contributed 35% of the springflow. Underflow from other areas to the southeast was not supported by the data. The present data were insufficient to determine if the mean deuterium content was con stant during the residence times of waters in the carbonate aquifers. Nonparametric sta tistical tests of Spring Mountain, Ash Meadows, and Pahranagat Valley groundwater samples showed different deuterium populations at the 0.01 level of significance. The authors assumed the major sources of recharge to Paleozoic carbonates were the Spring Mountains and the Sheep Range, but the possibility of other minor recharge areas was noted. These ’minor’ sources were not thought to alter mass balance calculations of discharge, although they may have altered the distribution of recharge. Winograd and Thordarson (1975) divided the Paleozoic carbonates and elastics and the Cenozoic volcanic and sedimentary strata at the NTS into 10 hydrogeologic units. The lower clastic aquitard, lower carbonate aquifer, and the tuff aquitard were postu lated to control regional flow of groundwater in the area. Interstitial permeability in all lithologies was considered negligible. Solution caverns were locally present in the 13 carbonate aquifer (Ash Meadows), but regional movement was said to be controlled by fractures and transmissivity variations as well as by local stratigraphic position of the carbonate aquifer and the lower clastic aquitard. The authors proposed that groundwa ter flowed freely up to 1280 m below land surface in the NTS area, and probably 457 m below the top of the carbonate aquifer. The authors determined that a 11520 km2 area consisting of 10 intermontane valleys was hydraulically integrated into one groundwater basin designated the Ash Meadows Basin, and interbasin flow occurred through the car bonate aquifer. Discharge was observed to occur along a fault controlled spring line at Ash Meadows in the Amargosa Desert. Intrabasin movement between the tuff aquifers and the carbonate aquifer was controlled by the tuff aquitard, the basal Cenozoic hydrogeologic unit, which was apparent in the groundwater chemistry. For example, the groundwater in eastern Amargosa Desert was designated as a calcium-magnesium-sodium bicarbonate type, representing a mixture of the chemical composition of both carbonate and tuff waters. Winograd and Pearson (1976) observed that the carbon-14 content of one spring at the center of the Ash Meadows spring line was five times greater than the others along the lineament. These waters were similar in alkalinity, pH, 13C, lsO, deuterium, tritium (3H), and some ions. Ten possible explanations of this 14C anomaly were investigated using all available chemical and isotopic data from wells and upland springs tapping the regional carbonate aquifer. The four most plausible hypotheses required the presence of a major longitudinal heterogeneity in the distal portion of the groundwater basin. Mega channeling of groundwater flow was therefore indicated. White (1979) concluded that regional similarities and trends in aqueous chemistry indicated most recharge entering the Oasis Valley groundwater was the result of inflow from Pahute Mesa and Gold Flat. Mass balance calculations showed that half the water recharged to Oasis Valley discharged through evapotranspiration and the rest flowed to the Amargosa Desert. Locally, most of the water had moved from recharge areas in the highlands through the fracture system in the tuffaceous rocks to the alluvium in the val ley floors. 14 Waddell (1982) constructed a two dimensional finite element model of the groundwater flow system of NTS and vicinity using parameter estimation techniques. The model simulated flow in the area underlain by elastics and carbonates of Precambrian and Paleozoic age, volcanic rocks of Tertiary age, and Quaternary rocks (alluvium). Recharge areas used in the model were the Spring Mountains, and the Pahranagat, Tim- pahute, and Sheep Ranges. Underflow occurred from Gold Flat and Kawich Valley. Discharge areas included Ash Meadows, Oasis Valley, Alkali Flat, and Furnace Creek. Flux terms in the model having the most influence on output included recharge to Pahute Mesa, underflow to Gold Flat and Kawich Valley, and discharge at Ash Meadows. The rocks with the most significant transmissivities were the Amargosa Desert alluvium, the Eleana Formation just west of Yucca Flat, and the elastics under the Groom Range. Rice (1984) developed a two dimensional finite difference groundwater flow model for a large area encompassing the NTS and Death Valley, California. The model’s pur pose was similar to Waddell’s 1982 model in that it simulated regional flow through the hypothesized carbonate sequence. Water balance within the model boundary agreed with published estimates of recharge and discharge according to the author. Recharge areas used in the model included Pahute Mesa, the Belted, Kawich, Pahranagat, Sheep 4 and Groom Ranges, Spring Mountains, and smaller unlabeled areas. It was noted that significant underflow entered the model area from the north and east. Czarnecki and Waddell (1984) constructed a finite element model of the groundwa ter flow system in the vicinity of Yucca Mountain. The model simulated steady state flow, assumed to occur through alluvium, tuffs and other volcanic rocks, and carbonate rocks. The most significant flux terms were found from a sensitivity analysis to be in the Alkali Flat (Franklin Lake Playa) area of the Amargosa Desert. Vertical flow and steep hydraulic gradients in areas such as Alkali Flat and Yucca Mountain were found to cause simulation difficulty due to the authors’ model assumption of horizontal flow. Bar riers to flow, particularly the area north of Yucca Mountain, greatly affected groundwa ter flow directions in the model. 15

THE DISCRETE STATE COMPARTMENT (DSC) MODEL The code for the DSC model (Simpson and Duckstein, 1976) was developed by Campana (1975) and applied to the Edwards aquifer in Texas by Mahin (1978) and Campana and Mahin (1985), and the Tucson Basin aquifer by Campana and Simpson (1984). The DSC model represents a hydrologic system as a group of cells through which water and dissolved constituents are transported in a manner designated by the user. Cells are assumed to represent hydrologically homogeneous areas within an aquifer or aquifers based on available data. Transport of the water is governed by recursive con servation of mass equations which describe the system as a series of distinct states. The DSC model can represent a hydrologic system in 1, 2, or 3 dimensions. A tracer is used to identify a parcel of water as it travels through the model, and the mass of tracer in a cell is known as the cell’s state. In this study the stable isotope deuterium (2H) will be used as the tracer. By definition, concentration gradients may exist between cells but never within cells. The state of each cell is calculated at every iteration using a discrete form of the continuity equation. Assuming a conservative tracer, equation 1 determines the state of the cell: (1) S(N) = S(N-l) + [BRV(N)*BRC(N)] - [BDV(N)*BDC(N)] where N = iteration number; S = cell state; BRV = boundary recharge volume (total volumetric inflow to cell); BRC = boundary recharge concentration (concentration of tracer in the BRV); BDV = boundary discharge volume (total volumetric outflow from cell); and, BDC = boundary discharge concentration (concentration of tracer in the BDV). 16 An iteration can represent a real time step whose length is chosen by the modeler and depends on the temporal distribution of available data. The model accounts for inflows from outside the system, or system boundary recharge volumes and concentra tions (SBRV, SBRC), as well as inflows from other cells, then discharges an outflow amount equal to the inflow, to other cells and or outside the system in the steady state case assumed for this study.

In equation 1, every term on the right hand side is known except for BDC. The DSC model offers a choice of two types of mixing cells to determine the BDC: the simple mixing cell (SMC) and the modified mixing cell (MMC). These cells are illustrated in Figure 3. The simple mixing cell simulates perfect mixing by expanding the cell to accommo date the recharge volume. The incoming volume is then completely mixed with the cell’s contents before the cell discharges. The BDC for a cell will equal the cell’s tracer con centration during an iteration. The equation for the BDC is: (2) BDC(N) = [S(N-l) + {BRV(N)*BRC(N)}] / VOL + BRV(N) where VOL = cell volume. This mixing cell type is also known as the ’in-mix-out’ cell. The modified mixing cell falls somewhere between perfect mixing and piston flow (no mixing). A recharge volume enters the cell and displaces an equal volume as the cell’s discharge. The incoming volume is then mixed with the remaining cell contents. The BDC for a cell at the Nth iteration is equal to the cell’s tracer concentration at Nth-1 iteration. Equation 3 describing the BDC is: (3) BDC(N) = S(N-l) / VOL This mixing cell type is known as the ’in-out-mix’ cell. A routine is included in the DSC code which involves using inflow volumes and rates to calculate the mean age of a cell (Campana, 1975). 17

DSC MIXING RULES

MODIFIED MIXING CELL

------1------“------n------1 i i l BRV VOL BDV S BRC BDC 1 1 1 ___ t ______- _l

BDC(N) = S(N-1) / VOL

SIMPLE MIXING CELL

BRV VOL BDV BRC S BDC

BDC(N) - [s(N-D + {BRV(N)*BRC(N)}] / VOL + BRV(N)

Figure 3, Types of Mixing Cells in the DSC Model (from Campana, 1975) 18 The modeler must have some idea of the flow system beforehand in order to specify recharge, discharge, and flow paths for the model. When the model is run, it is con sidered to reach steady state when cell states do not change from one iteration to the next. The system’s hydrogeologic characteristics such as recharge volumes, effective porosity, and flow directions, are adjusted during the calibration process to obtain an agreement between the observed and model calculated cell states. Deuterium, the stable isotope of hydrogen, was used as the tracer in calibrating the DSC model. This isotope is part of the water molecule and is assumed to mimic the flow path of water. The principle for using deuterium follows from the observation that recharge waters have unique deuterium signatures based on a number of factors affecting the isotope fractionation of meteoric waters including temperature of condensation, sea son, altitude, latitude, distance from ocean, amount of precipitation, evaporation history, and climatic cycles (Friedman et al., 1964). Although exchange may occur in some hydrogen-bearing clays, deuterium is thought to be relatively conservative. Its stable nature is well suited to extensive aquifers or aquifer systems with long groundwater flow paths and large residence times. When deuterium values from the study area are plotted versus lsO a pattern typical of southern Nevada groundwater is evident (see Figure 4). For a list of the wells and springs used to construct the plot and their associated isotope values, see Appendix A.

MODEL CALIBRATION Groundwater deuterium concentrations and well information were obtained from Benson and McKinley (1985), Boughton (1986), Claassen (1983), Waddell et al. (1984), Winograd and Friedman (1972), Winograd and Pearson (1976), and Desert Research Institute files. Additional deuterium data was obtained by the author on a sampling trip in June 1985. The initial recharge, discharge, and underflow estimates were taken from Blanken- nagel and Weir (1973), White (1979), and Winograd and Thordarson (1975). The initial del deuterium, permil iue . td Ae Iooe Samples Isotope Area Study 4. Figure e oye-8 permil oxygen-18, del e. o SMOW to rel. 19 mmm m sm sm m m m .

20 recharge estimate to Fortymile Canyon and Wash was crudely calculated using an estimated channel width and an annual stream stage based on the frequency of floods. Information on flooding and channel geometry in the Fortymile Canyon area was based on data from Squires and Young (1984).

The ranges of bulk porosities given by Blankennagel and Weir (1973) are 4 to 38 % for welded tuffs and 12 to 52 % for ashfall and non-welded tuffs. Winograd and Thor- darson (1975) estimated the lower carbonate aquifer to have an effective porosity range of 0 to 9 %. Total porosity of the valley fill deposits was given by the same authors as 16 to 42 %■ Effective porosity values investigated during calibration for specific cells were the following values:

- 38, 25, 10, 3, and 1% in Pahute Mesa cells 1,2,and 3, - 38, 35, 30, 20, 3, and 1% in Amargosa Desert cells 8 and 9, - 38, 25, 3, and 1% in Timber Mountain cell 4, Fortymile Wash cell 5, Stockade Wash cell 6, and - 38, 25, 3, 2, 1% in Crater Flat cell 7.

Best results were obtained with an effective porosity of 3 % in cells 1 through 5, 8 and 9, \% in cell 6, and 2 % in cell 7, as the model could not be calibrated with larger porosities. The author assumes the effective porosity range of 1 to 3 % to approximate the actual mean effective porosities of tuffaceous (and carbonate) rocks in the study area for the following reasons:

1) Effective porosity is not likely to be great at the depths to which groundwater is assumed to flow in the study area (up to 1800 meters below land surface). There fore the average effective porosity throughout the entire saturated thickness may not be larger than 3%. 2) Tuffaceous rocks are assumed not to have high effective porosities, since a large number of the stratigraphic units in the study area are considered aquitards and/or are not highly fractured (Waddell et. al., 1984). Carbonate rocks in the Ash Meadows Groundwater Basin, even with dissolution of the host rock potentially 21 increasing its permeability, are only estimated to have a mean effective porosity of 1% (Winograd and Thordarson, 1975). 3) Secondary porosity, in the form of fractures and solution channels, makes up only a small percentage of an aquifer’s total volume. Saturated well thickness, or the total well depth minus the depth to water, was used to estimate cell volumes as opposed to aquifer thickness or screened well interval for the following reasons: 1) Use of screened interval thickness would neglect part of the contributing reser voir in the calculation of cell volume; 2) Aquifer thickness is not well defined in the study area; 3) Observed deuterium values are composite samples from the water column in a well in many cases. A composite well sample represents a mixture of the well’s entire screened interval instead of being from a discrete zone in the well. After a representative or ’observed’ deuterium value was chosen for each cell, saturated thickness in each cell was determined from the well with a value equal to the observed value. Cells 1, 2, and 3 were assumed to have the same saturated thickness because they are all located on Pahute Mesa. In the Amargosa Desert, wells are com paratively shallow and were not assumed to represent the entire aquifer thickness, so a combination of permeable carbonate aquifer thickness (from Winograd and Thordarson, 1975), and desert alluvium thickness (from Walker and Eakin, 1963) , was used for cells 8 and 9. " Y 1 Observed 2H data were evaluated in cells 5 through 7 to arrive at a single value for h o^ , each cell. For cell 5 (Fortymile Canyon and Wash), well UE29a#2’s deuterium value was discarded because the well is shallow and is located up the canyon. This value is believed to closely resemble the 2H concentration of recharge to the canyon. This well’s isotope concentrations plot close to the meteoric water line (see Figure 4 and Appendix A), and points lying on this line are assumed to be of meteoric origin, showing no mixing with existing surface or subsurface water (Gat, 1971). This reinforces the assumption 22 that well UE29a#2’s sample concentration approximates that of local meteoric water. In cell 6, Stockade Wash, two values were identical and a third differed by 3 permil in well TW-8. When the three were averaged, the mean was within analytical error (+/- 1 per mil for H) of the twin value. This twin value was therefore chosen as the observed value. The heavier, discarded value is assumed to show more local influence. In cell 7, Crater Flat, ten wells have accompanying 2H values. Nine of the ten wells lie on or in close proximity to Yucca Mountain. These 9 wells are closer to the meteoric water line than all but three of the wells in the study area (see Figure 4). It is assumed that the deuterium values from the Yucca Mountain wells represent local flow systems and to a lesser extent the mixing of Crater Flat and Fortymile Canyon groundwaters. Therefore Yucca Mountain was assumed to be the border between these two areas. The wide range of values between the closely spaced Yucca Mountain wells (-99.5 to -106 per mil 2H) suggests these waters are of predominantly local origin. The isotope values also lie between that of Crater Flat and those of Fortymile Canyon and Wash in Figure 4. Well VH-1, located in the center of Crater Flat was chosen as representing the deu terium concentration of cell 7. A well sample from VH-1 contains hydrogen and oxygen isotope values which plot between Pahute Mesa values and those of the Amargosa Desert indicating that Pahute Mesa groundwater may be flowing through Crater Flat to the Amargosa Desert.

The observed values chosen for cells 8 and 9 (-102, -104 permil, respectively), Amar gosa Desert, are believed to more closely resemble the major flow system in the desert without masking any important sources of recharge. It can be argued that these observed values are the same, especially considering the analytical error of 1 permil. However, it is assumed that the two areas receive very different recharge sources and that the observed values reflect these differences. Several wells in the Amargosa Desert were discarded; heavy (-99 permil or higher) 2H sample values were disregarded because it is assumed that these values intermingled areally with significantly lighter values are indicative of the effect of irrigation return flow and local flood water infiltration. The wide scatter of data from this area in Figure 23 4 supports this assumption. An extremely light 2H value from a well in the northwest part of the desert was discarded because the well may have been used for injection pur poses. The DSC model could not be validated owing to its non-predictive nature. The long flow paths involving large time spans make it impossible to have historical observed data to reproduce, and the temporal distribution of the deuterium content of recharge in each locale of the study area is unknown. Therefore, model results such as inflow volumes (SBRVs) and tracer concentrations (SBRCs) are non-unique and may not be correct. However, model results can be verified qualitatively by comparing its results with data from other models. The isotopic content of actual recharge is difficult to obtain even with large amounts of data. In this study, the system boundary recharge tracer concentration (SBRC) was the most difficult parameter to estimate due to the almost non-existence of pertinent data and the lack of any previous estimates. The tracer concentrations of sys tem recharge used in this study contain a lot of uncertainty, which should be considered when evaluating results from the DSC model. Several methods were used to assign deuterium concentrations to each cell’s system recharge during the calibration process, because precipitation 2H data were almost non existent in the study area. Two methods which proved unsuccessful involved choosing a 2H value from wells within a cell to represent that cell’s recharge deuterium concentra tion, and having a single deuterium value for winter recharge and one for summer recharge. The method used in the calibrated model involved latitude, springs, and data from outside the system.

On Pahute Mesa (cells 1, 2, and 3), 2H concentrations of system recharge had to depict the combination of precipitation recharge and underflow because the DSC model only accounted for one source of recharge from outside the model (i.e., system recharge). The average of 2H values from wells in Cactus Flat were used because they were the only ones available upgradient of the mesa. 24 The deuterium concentration of system recharge for cells 4 and 6, -100 permil, was correlated with Captain Jack Spring which lies at approximately the same latitude as the two cells, and is assumed to be recharged mainly by snowmelt. Cells 5 and 7 were assigned an system boundary recharge concentration of -90 per mil. This value was taken from Topopah Spring at Shoshone Mountain, latitudinally at the middle of Fortymile Canyon and Wash. This ‘H value is assumed to be indicative of the recharge to these areas. Cell 7 was assigned this value only because it lies at the same latitude and no other data was available.

Cells 8 and 9, which receive several types of system recharge inflow, were under the same model constraints mentioned for cells 1, 2, and 3. Cell 8 (Amargosa Desert) system recharge must account for underflow from the northwest part of the desert, Amargosa River and Fortymile Wash recharge, and underflow from the Ash Meadows Basin (Wino- grad and Thordarson, 1975). A value of -100 permil was assumed by designating the Ash Meadows underflow -105 permil based on data from carbonate wells, -90 permil for the local recharge, and underflow from the northwest to be somewhere in between. Cell 9’s (Ash Meadows area of desert) system recharge deuterium concentration was arrived at by considering high elevation springs in the Spring Mountains, underflow from the Ash Meadows Groundwater Basin, and spring discharge infiltration. The first cell network was based on Waddell’s (1982) transmissivity distributions for the study area; each region with a distinct transmissivity was designated as a cell. This network consisted of fourteen cells, four without observed 2H values, with the fol lowing areas (or cells) receiving recharge: Oasis Valley, Pahute Mesa, Fortymile Canyon, Timber Mountain, and Amargosa Desert. However, reproduction of observed cell deu terium concentrations was poor, and deuterium variation within some cells was significant, so model dimensions were altered. The next model contained ten cells, with basically all areas receiving recharge. The new cell boundaries were drawn with more strict attention to the areal distribution of sample deuterium values. Cell volumes were also recalculated in some areas (such as Amargosa Desert) using wells which did not have an associated sample 2H value. Model calculated values did not match observed values, 25 which was interpreted as indicating that the observed values were from local flow sys tems. The model was expanded to three dimensions by the addition of a lower tier of cells which was identical in number and area to the existing network. This lower layer bot tomed at 700 meters below sea level, which corresponded with the deepest well penetra tion in the study area (well UE25p^l). The intent was to better simulate a larger, perhaps regional flow system because it was thought at this stage that the data might have been showing too much local influence. However, the lower tier was eventually eliminated for several reasons. First, because the maximum depth of groundwater flow is not documented throughout the study area, cell volumes in the lower tier had to be estimated roughly. Second, the only observed 2H data in the lower tier was below the east flank of Yucca Mountain in well UE25p#l, which penetrated carbonates. It was assumed that the carbonates were not continuous in the rest of the area. Thirdly, during the calibration process favorable results were obtained when the lower tier communi cated (in terms of vertical flow) less with the upper tier, to the point that the lower tier became insignificant. Finally, results in the lower tier were unrealistic. Reduction of the remaining tier to nine cells , small additional changes by trial and error in system recharge volumes, their associated 2H concentrations, and adjustments in cell volumes resulted in the calibrated version of the model. The calibrated network appears in Figure 5. Cell characteristics and associated data are listed in Table 2, and observed versus simulated deuterium values are displayed in Table 3. 26 27

Table 2. Cell Characteristics Cell Cell Cell Syst. Rechrg. In/Out Flow Syst. Disclirg. number area volume volumetric flow, 10am3/yr (l08m2) (10 V ) 1 7.2 361 3.22 6.81 2.72 2 2.3 115 2.46 3.32 _ 3 2.5 127 2.46 2.46 — 4 6.3 253 1.42 5.73 — 5 1.5 33.6 2.55 3.98 — 6 1.0 20.7 0.87 1.36 0.82 7 6.4 73.8 1.12 6.58 -- 8 6.4 205 4.32 14.9 7.44 9 3.9 124 14.0 21.6 21.6

Table 3. Observed vs. Simulated Deuterium Concentrations

CELL Cell Concent., permit delD,

observed calculated 1 -112 -112 2 -114 -114 3 -110 -114 4 - -109 5 -97 -97 6 -104 -104 7 -108 -108 8 -102 -102 9 -104 -104 28

RESULTS and DISCUSSION

SYSTEM SIMULATION Calculated deuterium values match the observed values in seven of the nine cells in the calibrated model (see Table 3). Timber Mountain (cell 4) has no observed values and east Pahute Mesa (cell 3) is a poor match; -114 permil calculated and -110 permil observed. The eastern portion of Pahute Mesa lies to the south of and down the hydraulic gradient from Kawich Valley, while the central and western parts lie downgradient from Gold and Cactus Flats. Only one system recharge Tl value was used for the entire mesa because there were no isotope data associated with Kawich Valley underflow. Therefore, the poor match in the Pahute Mesa area may be due to one or more of the following fac tors: 1) Eastern Pahute Mesa (cell 3) may receive an isotopically distinct inflow from the Kawich Valley; 2) Eastern Pahute Mesa may receive precipitation recharge and underflow in different proportions in comparison to the rest of the mesa; and, _==j?3) Downward vertical hydraulic gradients were found in eastern Pahute Mesa wells, including the two wells used for calibrating the eastern mesa area. The downward gradient may indicate the well water is mostly recharge water. If this were the case then the deuterium concentrations from these wells would be skewed towards local recharge values. The downward gradients in the eastern portion of the mesa were usually due to a more transmissive lithologic unit at depth, indicating that water was flowing from the less to the more transmissive unit (Blankennagel and Weir, 1973). The simulation in cell 3 is most likely a result of a combination of different rainfall amounts and a different underflow deuterium concentration to eastern Pahute Mesa. 29 As discussed in the Calibration section, the observed deuterium values in the Amar- gosa Desert lie in a narrow range equal to the range of analytical error for two samples (2 permil). This implies that the area acts as a single body of water hydrologically, but an apparent zonation is seen in the data and results. The heavier value, -102 permil, in central Amargosa Desert (cell 8), indicates influence from isotopically heavy Amargosa River and Fortymile Wash infiltration assumed to originate from summer storms, while the lighter value in the Ash Meadows area (cell 9), -104 permil, is associated mainly with underflow and spring discharge from the Ash Meadows Groundwater Basin (Winograd, 1962). In other words, cell 8 isotopic data show more influence from local hydrologic processes, and cell 9 shows the effects of a larger scale flow system or systems.

RECHARGE AREAS and ESTIMATES All nine cells receive recharge. As was noted previously, some cells receive recharge from several source types such as underflow from outside the system and infiltration from precipitation. Model results related to recharge, and a method for determining recharge percentages from DSC estimates, are discussed in the following section. Recharge estimates based solely on the DSC model for Pahute Mesa are unattain able due to the underflow component in the system recharge. However, recharge was cal culated using precipitation estimates in Blankennagel and Weir (1973). Timber Mountain recharge is important because it is indirectly related to flux through Oasis Valley and the Amargosa Desert. Unfortunately, the recharge estimate at Timber Mountain, which may be large owing to the highly fractured nature of the mountain as a result of dome resurgence, is not verifiable due to the lack of isotopic and hydrologic data in the Timber Mountain Caldera region. The relative magnitude of Timber Mountain model recharge, however, can be judged against the Blankennagel and Weir (1973) estimate of recharge to the entire area of western Jackass Flats, Crater Flat, and Timber Mountain, assuming that the aforementioned authors’ estimate is an upper limit for recharge to the Timber Mountain area. In this respect, the Timber Mountain (cell 4) estimate is reasonable. 30 Fortymile Canyon and Wash, and adjoining Yucca Wash comprise cell 5. Flood flow in these washes was probably more common during pluvials in the past 20,000 years, contributing potentially large volumes of recharge to the hydrologic systems of the study area. Results indicate that large volumes of recharge have been sustained over thousands of years to the Fortymile area, probably by localized intense storms. In fact, Fortymile Canyon-Wash receives more vertical recharge than any other area in the model if previous estimates of underflow to Pahute Mesa, the other major recharge area in the system, are realistic. This suggests that Fortymile Wash is a primary regional recharge source. The system recharge to western Amargosa Desert is assumed to be made up of Amargosa River infiltration, underflow from Oasis Valley, Fortymile Wash discharge, and possibly outflux from the Ash Meadows groundwater basin such as Topopah Wash outflow or upward groundwater flow from the carbonate aquifer at depth. Ash Meadows system recharge consists of underflow from the Spring Mountains and from the Ash Meadows Groundwater Basin and some recharge from the Ash Meadows springs. Interpretation of recharge sources in the Amargosa Desert is difficult due to the variety of potential sources, some of which may obscure the regional isotopic signature in the mostly irrigation-type wells in the desert. The possible sources are: 1) underflow from the carbonate aquifer of the Ash Meadows basin; 2) underflow from the northwest part of the desert; 3) Spring Mountains underflow; 4) infiltration from the Amargosa River; 5) Ash Meadows runoff during wet years; 6) irrigation return flow; or 7) surface discharge from Fortymile and Topopah Washes. Of these potential sources, 1, 2, 3, and 4 are significant in the desert judging from model fluxes, although the DSC model did not separate different recharge sources. Upward regional flow and streamflow infiltration were suggested as likely sources in western Amargosa during calibration because isotopically heavier system recharge concentrations 31 produced closer deuterium matches. The other components listed above probably obscure the isotopic signature of the large scale flow system in the area. The Ash Meadows area is significant because it is the intersection of the current study’s flow sys tem and that of the Ash Meadows Groundwater Basin. This area receives chiefly underflow recharge. An investigation was made into the percent of precipitation in each cell that results in recharge. Table 4 contains estimates of the percentage of precipitation that actually recharges each of the cells investigated. The cells used for this calculation were those with system recharge consisting only of vertical recharge from infiltration of precipita tion, as shown in Figure 6. The calculation was carried out for cells 4, 5, 6, and 7, but cells 1, 2, 3, 8, and 9 could not be investigated. When precipitation data were not avail able for a cell, data were extrapolated from the nearest area or areas. Such a method invites uncertainty, so an approximate range of percentages is given. The method entailed multiplying the cell area by the amount of annual precipitation. This product was then divided into the cell system recharge to get the percentage of precipitation that recharges the cell. This method is crude, as actual watersheds and areally weighted rain fall amounts must be taken into account to get more accurate estimates. A more exten sive data set is needed before such estimates can be computed in the study area. The highest infiltration percentages are in Fortymile Canyon and Wash (cell 5), and Stockade Wash (cell 6). This makes intuitive sense because of the increased potential for recharge in the gravel beds and wash channels which make up much of these areas. However, the range for cell 5 is certainly high because the entire watershed was not taken into account. The Timber Mountain (cell 4) estimate appears reasonable compared to Crater Flat (cell 7), considering the moderate elevation mountain ranges in these areas. This further suggests that the Timber Mountain area is adequately simulated in the calibrated model. Pahute Mesa cells were ineligible for the recharge calculation. However, Blanken- nagel and Weir (1973) suggest that 38% of the influx (i.e., system recharge) to eastern 32

Table 4. DSC Model Recharge Estimates

Geographic Area Precipitation Model Recharge % of Precip. name xl08m2 Range (cm/yr) xl06m3/yr as Recharge Stockade Wash 1.0 18 0.9 5 Fortymile Canyon 1.5 15 to 24 2.6 7 to 12 Timber Mountain 6.3 21 to 24 1.4 1 Crater Flat 6.4 9 to 14 1.1 1 to 2 E. Pahute Mesa 4.8 24 1.9* 1* • ' * based on a proportion of total flux to Pahute Mesa > 7

oJl 2 Sy$4 , (^<5 cM a fy

34 Pahute Mesa is precipitation. Using cells 2 and 3 and the method described above, if 38% of the system recharge to the mesa is precipitation, then the percent of the precipi tation recharging Pahute Mesa according to the DSC model can be calculated (see Table 4). As an extreme case, if one assumes all of the system recharge (to cells 2,3) to be from precipitation, then the resulting infiltrating percentage (4%) is not unrealistic. If this recharge percentage could be verified, then it would imply that precipitation is possibly the major source of the total influx to this area. Vertical recharge could not be taken from system recharge estimates for the Amargosa Desert due to the many possible recharge sources and the inability of the DSC code to distinguish between them.

MODEL FLOW PATHS Figure 7 dispays cell network flow paths which represent the direction of groundwa ter flow in the system, and Table 5 lists the model flow paths and associated discharge volumes. The simulated flow paths are in general agreement with those suggested in pre vious studies. The direction of groundwater flow is dominantly north to south, influenced by the region’s typical north trending Basin and Range normal faults. This is evident in all regions of the study area. However, there are eastward and westward com ponents of flow in the model, and the location of southwestern and northeastern system discharge areas indicate that groundwater is not traveling strictly southward. Ground- water movement may be influenced to an extent by northwest trending faults charac teristic of the Walker Lane Belt in areas such as the Amargosa Desert, where there is a southeast hydraulic gradient, and in Crater Flat, which is sheared in the northwest direction. The buried caldera complexes of the area may also influence groundwater flow directions. For example, the proposed caldera outlining Crater Flat (Carr et ah, 1984) may influence groundwater flow paths because there is an absence of an east-west com ponent of flow in this area, although regional springs lie to the west of this area. How ever, the model can only depict gross flow directions. Pahute Mesa, represented by cells 1, 2, and 3, is the major area of influx to the sys tem. Underflow enters from the northwest via Cactus Flat and Gold Flat, and from the 35

Figure 7. Model Flow Paths (Arrows indicate approximate flow direction,

numbers next to arrows are volumes in 1C)6m3/yr) 36

Table 5. Flow Paths, Volumes Outflow Fraction of Outflow Volume Inflow Cell Cell’s Outflow (xl08m3/yr) Cell 3 0.35 0.9 2 3 0.45 1.1 4 3 0.20 0.5 6 2 0.20 0.7 1 2 0.80 2.7 4 6 0.40 0.5 4 4 0.51 2.9 1 4 0.25 1.4 5 4 0.24 1.4 7 1 0.60 4.1 7 5 1.00 4.0 8 7 1.00 6.6 8 8 0.50 7.4 9 37 northeast via Kawich Valley, and local winter precipitation makes a significant contribu tion to recharge. Best results were obtained when there was no east-west flow path between Fortym- ile Canyon-Wash (cell 5) and Jackass Flats or Yucca Mountain, although well data show water levels in Yucca Mountain and Fortymile Wash to be very similar. Several authors use this type of observation as evidence for interbasin hydraulic connection in the NTS area. The connection appears likely from isotopes as well. Figure 4 shows the isotopic values from Yucca Mountain wells to lie between those of Fortymile Wash wells and that of well VH-1 (-108 permil delD) in Crater Flat, suggesting that Yucca Mountain groundwater is partly derived from these two sources, (see Appendix A for a list of these wells and isotope values). This apparent dichotomy, in which the model shows no flow but the isotopic data suggest a possible hydraulic link, may best be explained by ack nowledging the mixing of the two waters but in small enough volumes as to be relatively insignificant in comparison to the other inflows to these areas. Groundwater discharges from the system along the southwestern border of the model at Oasis Valley (border of cell 1), but the southern boundary of the model, the Amargosa Desert, is the major discharge area of the system. The western boundary to the Ash Meadows Groundwater Basin coincides approxi mately with the eastern DSC model boundary. Although there is minor system discharge from the Stockade Wash area (cell 6) moving eastward through the Eleana Range to Yucca Flat , the major portion of the eastern system boundary, Fortymile Canyon and W w Wash, Timber Mountain, and eastern Pahute Mesa, contribute no simulated flow to eastern NTS. Underflow moving southward through the study area initially enters the western part of the Amargosa Desert (cell 8) before either discharging as pumpage or evapotranspiration, leaving the system as underflow towards Death Valley, or moving into the southeastern part of the desert (cell 9). Groundwater flow enters the Ash Meadows area from the Ash Meadows Groundwater Basin and from the Spring Moun tains, before discharging as evapotranspiration, pumpage, and underflow to Death Val ley. 38 Several authors have concluded that the study area is part of a larger regional groundwater flow system (Blankennagel and Weir, 1973; Waddell, 1982; Rice, 1984; Czarnecki and Waddell, 1984). The DSC model system cannot be designated a separate and distinct groundwater basin because part of the flux entering the system is underflow, indicating recharge areas outside the system, and an underflow component also leaves the system, indicating at least one discharge area is not located within the model boun daries.

MODEL FLUX COMPARISONS DSC model flux estimates are similar to published estimates in most cases. Table 6 contains model calculated groundwater fluxes for specific regions within the study area and previously published estimates for the same areas when available. Estimates of southward flux through the Pahute Mesa region from the DSC model and Blankennagel and Weir (1973) differ by more than 35 %, although the model esti mate is not without uncertainty due to the poor fit of the eastern mesa (cell 3) in the calibration stage. However, the fluxes are within the same order of magnitude and are considered similar. Pahute Mesa underflow eastward into Yucca Flat is greater than Blankennagel and Weir’s estimate but is still r> negligible 1 compared to southward flow from the mesa as Blankennagel and Weir have maintained. This calculated flux may be affected by the poor model fit on the east mesa as it receives some discharge from that area (see Table 5, outflow cell 3, inflow cell 6). The estimate of southwestward flux from Pahute Mesa to Oasis Valley used by several authors (Blankennagel and Weir, 1973; White, 1979), is based on the work of Malmberg and Eakin (1962). The DSC model and Malmberg and Eakin estimates of discharge at Oasis Valley agree approximately, reinforcing the idea of a Pahute Mesa- Oasis Valley flow path. The model estimate of groundwater flow across the barrier and drain hypothesized for Pahute Mesa by Blankennagel and Weir (1973) is small (see Table 5, outflow cell 2, Table 6. MODEL FLUXES AND COMPARISONS (fluxes in 106m3/yr) Area Dschg. Rchg. Model Other Cell Cell

Pahute Mesa eastward flux 6 * 0.8 2.5 (B&W,73) westward flux 1 * 2.7 3.0 (M&E,’62) southward flux 2,3 4 3.8 6.2 (B&W,73) 1,2,3 4,7 7.9

Fortymile Canyon and Wash Flux 5 8 4.0 8.1 (C&W,’84)

Crater Flat-Timber Mtn- W . Jackass Flats Recharge * 4,7 2.5 2.5 (B&W,73) * 4,5,7 5.1

Amargosa Desert total flux 8,9 * 28.8 29.6 (W&E,’63) Ash Meadows flux 9 * 21.4 20.9 (W&E,’63) West/Central Desert flux *,9 14.9 12.3 (B&W,73) 8 Author Abbreviations: B&W - Blankennagel and Weir, M&E - Malmberg and Eakin, C&W - Czarnecki and Waddell, W&E - Walker and Eakin. * - into or out of system. 40 inflow cell 1), which concurs with Blankennagel and Weir’s conclusions about flow across the barrier as compared to along it. Model simulated groundwater outflux from Fortymile Wash to the Amargosa Desert was basically derived during calibration after starting with a rough recharge esti mate using simulated flood stages in the wash channel. DSC model constraints prohi bited the inclusion of surface discharge from Fortymile Wash in the outflux for cell 5 in this area, which may account for some of the disparity between the modeled and pub lished estimates. There is a very large amount of flux through Crater Flat in proportion to recharge to that area, which suggests that Crater Flat receives very little vertical recharge. Com bined, the model estimate of flux through the Amargosa Desert is more than half of the total flux to the system, although much of the volume flows in and out of the Ash Meadows area without involving the rest of the system. The DSC model and Walker and Eakin (1963) estimates for these areas are in relative agreement.

MODEL UNCERTAINTY after CALIBRATION As mentioned previously, if part of a cell’s system recharge was underflow, its age was not accounted for because the DSC model assigned an age of zero to all system recharge. This characteristic causes groundwater mean ages to be younger than actual ages if cell system recharge values have significant underflow components which may be relatively old, as is the case in this study. Therefore mean ages calculated by the model should not be interpreted as actual groundwater ages (see Table 7). The mean age cal culated by the model indicates the minimum, average amount of time that the water of a particular cell has spent inside the entire modeled system. For example, the Ash Meadows (cell 9) model calculated mean age is too young because its system recharge, assumed to be underflow, is incorrectly assigned an age of zero as it enters the cell, when it is actually tens of thousands of years old according to uncorrected carbon 14 ages from Ash Meadows springs. Table 7. Model Mean Ages cell age(yrs) 1 9200

2 4800

3 5200

4 8000

5 3700

6 3400

7 8500

8 6100

9 2700 42 The effect of paleoclimatic fluctuations on model system recharge effects the calcu lated groundwater mean ages. Ideally, residence times suggested by model mean ages would indicate if a hydrologic system contained groundwater that may have been recharged during one or more pluvial periods. Shifts in 180 concentrations of up to 10 permil were seen in ice and snow cores from Greenland (Moser and Stichler in Fritz and Fontes, 1980). These shifts purportedly occurred during periods of increased precipita tion, or pluvial periods, with the last one ending approximately 10,000 years ago. This implies that precipitation and therefore recharge during these periods may have had dis tinctly different isotopic concentrations due to the effect of climatic change on isotope fractionation. However, the climate was generally believed to have been only moderately different in the study area during the pluvial periods of the past (Winograd and Doty, 1980, p. 68). No paleoclimatic data exist in the vicinity of the study area which could document a paleo-shift in the deuterium content of recharge. Although model calculated ages suggest that groundwater in the study area was generally recharged before the last pluvial period , the model receives inflow from recharge areas located outside the model boundary, and mean ages do not represent actual ages. These factors combined make determining the effect of potential climatic shifts on the study area with the DSC model impossible from presently available data. 43 MODEL SENSITIVITY An evaluation of model sensitivity was performed during the final calibration to determine how a study (or ’inflow ’) cell’s deuterium concentration was affected when its inflows were varied systematically. During a run, each inflow entering the cell of interest was individually increased by 20% from calibrated values while the others were decreased by a total of 20% to maintain mass balance. In some situations cell outflow size constraints only permitted an increment of less than 20%, and an increment of 30% was used in cell 4 due to its many inflows (four). The basis of the analysis was to deter mine which inflow had the greatest influence on the cell’s state and deuterium concentra tion. The results of the analysis, listed in Table 8, generally show that the inflow with the isotope value furthest from the calibrated value of the study cell had the greatest effect regardless of how large the relative volumes of inflow were. This implies that the isotope concentration of a cell’s inflows rather than volumes are the most important. If this is the case, then the sensitivity analysis indicates that the model is most sensitive to local recharge. The computed 2H values in Table 8 for cells 1, 2, 3 representing Pahute Mesa are within the limit of analytical error which preludes any inflow from having a distinct influence on the calculated 2H for these cells. There are three possible explanations for the relatively small computed deuterium differences: 1) The increment of 20% was not large enough to show differences in the inflows. This is not likely because some inflows were doubled or tripled when increased by 20% of the total influx to the cell. 2) The DSC computer code is incapable of discerning the underflow or precipitation component in the system recharge to a cell. The cells on Pahute Mesa are assumed to receive precipitation and underflow, but these components are combined in a sin gle flux term. Perhaps a significant change in cell state would occur if the two com ponents could be altered separately. 3) The inflow concentrations are too similar in the Pahute Mesa cells. The changes in proportion of inflow volumes do not have an appreciable effect on cell state 44

T a b le 8. Results of Sensitivity Analyses

In flo w % of Inflow co m p u te d M ea n Recharge Concen. cell by cell, rchg. d el D A g e -1 + 1 p erm il 1 cells system (2) (4) recharge * 10 4 3 47 -1 1 2 .0 7 79 7 -1 1 2 .5 -1 1 1 .6 5 6 3 32 -1 1 1 .2 9 2 9 6 -1 1 1 .5 -1 1 0 .9 3 0 3 3 3 7 -1 1 2 .4 856 8 -1 1 2 .5 -1 1 1 .8 5 2 8 67 -1 1 2 .8 612 7 -1 1 3 .4 -1 1 2 .1

2 cell system (3) recharge * 2 6 7 4 -1 1 4 .0 5 00 6 -1 1 4 .8 -1 1 3 .3 4 6 54 -1 1 4 .0 5 95 1 -1 1 4 .5 -1 1 3 .5 6 94 -1 1 4 .0 4 07 6 -1 1 4 .9 -1 1 3 .1

3 s y s te m re c h a rg e

* x 1 -1 1 4 .0 4 6 7 5 -1 1 5 .0 -1 1 3 .0 x .5 -1 1 4 .0 935 0 -1 1 5 .0 -1 1 3 .0 x 2 -1 1 4 .0 2337 -1 1 5 .0 -1 1 3 .0

4 cells s y s te m (3) (2) (6) recharge * t 19 4 6 9 25 -1 0 9 .4 9 4 2 6 -1 0 9 .6 -1 0 9 .1 4 0 3 8 3 19 -1 1 0 .2 10000 -1 1 0 .4 -1 1 0 .0 2 0 56 5 19 -1 1 0 .7 9589 -1 1 0 .9 -1 1 0 .5 t 15 41 23 20 -1 0 9 .0 9951 -1 0 9 .2 -1 0 8 .8 10 3 6 5 49 -1 0 6 .5 7 9 7 5 -1 0 6 .9 -1 0 6 .0

5 cell system (4) recharge

* 3 6 64 -9 7 .0 4 5 1 2 -9 7 .6 - 9 6 .3 5 6 44 -1 0 0 .9 540 8 -1 0 1 .3 -1 0 0 .4 16 84 -9 3 .1 221 0 -9 3 .9 -9 2 .3

6 cell system (3) recharge

* 3 6 64 -1 0 4 .4 8 1 1 3 -1 0 5 .1 -1 0 3 .8 5 6 44 -1 0 7 .4 9 0 3 8 -1 0 7 .9 -1 0 7 .0 16 84 -1 0 1 .4 7 1 8 7 -1 0 2 .3 -1 0 0 .6

7 cells system (1) (4) recharge -1 0 7 .6 * 6 2 21 17 -1 0 7 .7 8591 -1 0 7 .9 -1 1 0 .1 8 2 11 7 -1 1 0 .2 9631 -1 1 0 .3 -1 0 9 .3 5 2 41 7 -1 0 9 .4 9 2 6 7 -1 0 9 .5 -1 0 4 .0 -1 0 3 .2 5 2 11 37 -1 0 3 .6 686 8

8 cells system (5) (7) recharge -1 0 2 .6 -1 0 2 .0 * 27 4 4 29 -1 0 2 .3 6 22 0 -1 0 3 .2 -1 0 2 .2 7 4 4 49 -1 0 2 .7 547 8 -1 0 1 .0 -1 0 0 .1 2 7 2 4 49 -1 0 0 .6 451 0

9 cell system (8) recharge 280 2 -1 0 4 .7 -1 0 3 .4 * 3 5 65 -1 0 4 .1 4 0 4 0 -1 0 4 .0 -1 0 3 .1 5 5 45 -1 0 3 .5 1611 -1 0 5 .4 -1 0 3 .7 15 85 -1 0 4 .6

* - refers to row of initial calibrated values t - % of inflow sums to 99 due to round off error because all inflow deuterium values fall in a narrow range. Interpretation of sensitivity runs for cell 4 must be done with caution due to the lack of calibration data in that area. Assuming that the calibrated results are reason able, vertical recharge increased by 20% of the total BRV to cell 4 had a dramatic effect, increasing the cell state by 3 permil. Increasing the percentage of cell 4 BRV from cell 2 increased the cell 4 state slightly, suggesting that Pahute Mesa inflow is subordinate to local precipitation in determining the isotopic content of groundwater under Timber Mountain. Cell 5 exhibits pronounced changes in cell state, because the cell has only two inflows. The results of successive 20% increases in each input are predictable. Note that when the precipitation inflow (i.e., system recharge) is increased, the cell’s concen tration becomes equal to the 2H concentration in well UE29a#2 in the upper reaches of Fortymile Canyon. This reinforces the assumption that the sample value from this well closely resembles recharge water to this cell. Cell 6 analysis shows similar results to cell 5. Well TW-8, used for cell 6, is in Stockade Wash and was assumed to receive a significant amount of vertical recharge despite the great depth to water in the cell. Increasing system recharge causes the largest change of state in cell 7. This indi cates that precipitation does not reach well VH-1 in great quantity. This may be the case, because well VH-1 lies in the center of Crater Flat and results show this area to receive the greatest amount of total influx in proportion to vertical recharge of any area in the system. In cell 8, inflows were reduced instead of increased because 100% of cell discharges already enter cell 8 in the calibrated model. The reduction of cell 7 inflow had more effect on cell state than cell 5’s decrease, possibly indicating that cell 7 inflow is as significant as that from cell 5. Cell 9 inflow variations produced no significant changes, probably because both inflows, from cell 8 and system recharge, have similar isotope values. 46 The second stage of sensitivity runs consisted of varying system recharge 2H values to each cell by 1 permil, the analytical error range for deuterium analyses. This was done for all of the previous sensitivity runs. The results are listed in the fifth and sixth columns of Table 8. For every cell the effect of changing system recharge ^ values amounted to a calculated cell concentration that deviated by 1 permil or less from the calibrated value. This suggests the model is relatively insensitive to small changes in the content of cell inflow which might occur from analytical error. These results have two implications. First, changes in climate which might alter recharge deuterium concentrations to a small degree would not affect the model significantly. Second, the DSC model may not be able to sufficiently distinguish between adjacent cells with only slightly different observed isotope values when the cells receive system recharge, particularly if the respective system recharge tracer values are similar. 47

SUMMARY and CONCLUSIONS The DSC model simulating groundwater flow in the study area represents the hydrologic system as a two dimensional network of 9 cells. The calibrated model’s deu terium values match their associated observed values except in the areas of Pahute Mesa and Timber Mountain (which has no observed ^ values against which to calibrate). The poor correspondence at Pahute Mesa is most likely due to the lack of a 2H value for Kawich Valley underflow and infiltration recharge. Results of flow simulation in the Amargosa Desert indicate that the Ash Meadows area and central Amargosa Desert receive the major portion of their inflows from different sources, although the calibrated values of 2H may be too similar in these respective areas to consider them separate and distinct hydrogeologic regions. The model could not be validated due to insufficient data. The results are therefore non-unique and may not be correct. All areas of the model receive recharge. The areas receiving the largest amounts of vertical recharge include cell 5 (Fortymile Wash), with 2.6xl06 m3/yr, and cells 1, 2, and 3 (Pahute Mesa), with approximately 38% of 8.1xl06 m3/yr of flux originating as precipitation-derived recharge on the mesa. Whether or not western Amargosa Desert receives a substantial portion of its large system recharge as infiltration is not discernible from the results. Infiltration percentages were estimated for areas in the model that only receive vertical recharge. The highest range, 5 to 12%, is in the Fortymile and Stockade Washes. Timber Mountain, Crater Flat, and Pahute Mesa fell into the range of 1 to 2%.

7>/ Groundwater flow paths in the model generally agree with previous concepts of flow directions in the study area. The majority of flux moves southward, influenced by north trending Basin and Range faulting. However, minor leakage of 0.8 x 10 m /yr occurs through the Eleana Range on the eastern border, and a significant discharge of 2.7 x 106 m3/yr is located at Oasis Valley on the western border, of the model. Pahute Mesa , receiving 8.1 x 108 m3/yr of model recharge, is the major area of system influx with the exception of Ash Meadows, which receives 14.0 x 108 m3/yr of mainly underflow but discharges without entering the rest of the model. 48 Model estimates of groundwater flux generally agree with published estimates. The largest disparities between model and published estimates exist on eastern Pahute Mesa, whose flow system is poorly represented, and through Fortymile Canyon-Wash, caused by excluding surface discharge from the DSC flux estimate for this wash. Model results indicate that the study area is part of a larger groundwater basin due to significant underflow volumes entering and leaving the system. Mean ages from this DSC model cannot be interpreted as actual groundwater ages because the age of underflow system recharge is not accounted for by the model pro gram. The mean age in this study does represent the minimum average amount of time that a parcel of water has spent within the modeled system. The effect of potential climatic variations on the 2H value of system recharge to the model could not be docu mented due to insufficient information on deuterium concentrations of meteoric water during the pluvial periods of the Pleistocene. A sensitivity analysis showed that the model was most sensitive to local recharge inflows in many of the cases, and least sensitive to cells with inflows that are all similar in deuterium concentration. Changes of one permil in the system recharge within the limit of analytical error for deuterium analyses indicates that the model is not sensitive to small changes in inflow concentration.

SUGGESTIONS FOR FUTURE RESEARCH Regional flow studies in the Nevada Test Site (NTS) area have produced similar results, which may be due in part to the common database and similar conceptual model shared by all of the studies since Winograd and Thordarson (1975). Key wells within, and in the vicinity of, the study area have not been sampled for isotopes or geochemis try, due to reasons ranging from inaccessibility of well location to tremendous depth to water which requires special sampling devices. Future studies will be better served if these previously neglected sampling sites in the hydrologic system are sampled and added to the present database, whose usefulness has been stretched to the limit. 49 A recharge study that considers the spatial and temporal characteristics of recharge volumes and their associated isotopic concentrations in the study area would greatly aid in quantifying the amount of groundwater flux through this region. These parameters had the least amount of information associated with them and are the source of most of the uncertainty in this study. Vertical groundwater flow in the study area needs to be better defined in order to obtain more accurate estimates of groundwater flux and better flow system delineation in the modeled region, particularly on Pahute Mesa and in the Amargosa Desert where vertical flow is probably very significant. To better understand if and to what extent volcanic calderas and their structural features influence groundwater flow within the NTS and vicinity, the hydrogeologic characteristics of boundary ring faults which are associated with calderas in the study area should be investigated. Several aspects of the DSC model could be investigated in order to produce more accurate results. The computer code should be adjusted so that several recharge sources can recharge a cell. This would allow better definition of the hydrologic system. Dating the system boundary recharge would increase the usefulness of model mean ages, because they would be more realistic. Also, the model could be verified to an extent with another tracer, for example carbon-14. 50 REFERENCES

Benson, L. V., and P. W. McKinley, 1985, ’’Chemical Composition of Ground Water and the Locations of Permeable Zones in the Yucca Mountain Area, Nevada,” U.S. Geol. Survey Open-File Report, no. 85-484, 10 p. Blankennagel, R. K. and J. E. Weir Jr., 1973, ’’Geohydrology of the eastern part of Pahute Mesa, Nevada Test Site, Nye Co., Nevada,” U.S. Geol. Survey Professional Paper, vol. 712-C, 35 P- Boughton, C., 1986, ’’Integrated Geochemical and Hydraulic Analyses of the Nevada Test Site Groundwater Systems,” unpublished M.S. thesis, Univ. of Nevada, Reno, 135 p. Campana, M. E., 1975, ’’Finite-State Models of Transport Phenomena in Hydrologic Systems,” Ph.D Dissertation, Univ. of Arizona, Tucson, 252 p. Campana, M. E., and E. S. Simpson, 1984, "Groundwater Residence Times and Recharge Rates Using a Discrete-State Compartment Model and C14 Data,” Journal of Hydrology, vol. 72, pp. 171-185. Campana, M. E., and D. A. Mahin, 1985, ’’Model-Derived Estimates of Groundwater Mean Ages, Recharge Rates, Effective Porosities and Storage in a Limestone Aquifer,” Journal of Hydrology, vol. 76, pp. 247-264. Carr, W. J., F. M. Byers, Jr., and P. P. Orkild, 1984, "Stratigraphic and volcano-tectonic rela tions of Crater Flat Tuff and some older volcanic units, Nye Co., Nevada,” U.S. Geol. Sur vey Open-File Report 84-114, 42 p. Claassen, H. C., 1983, "Sources and Mechanisms of Recharge for Ground Water in the West- Central Amargosa Desert, Nevada-A Geochemical Interpretation,” U.S. Geol. Survey Open- File Report 83-542, 58 p. Czarnecki, J. B. and R. K. Waddell, 1984, ’’Finite-Element Simulation of Ground-Water Flow in the Vicinity of Yucca Mountain, Nevada-California,” U.S. Geol. Survey Water-Resources Investig. Report 84-4349, 38 p. Eakin, T. E., S. L. Schoff and P. Cohen, 1963, "Regional Hydrology of a part of Southern Nevada: A Reconnaissance,” U.S. Geol. Survey TEI-833, 40 p. French, R., 1983, ”A Preliminary Analysis of Precipitation in Southern Nevada,” Desert Research Institute publ. 45031, pp. 3-17. Friedman, I., A. C. Redfield, B. Schoen, and J. Harris, 1964, "The Variation of the Deuterium Content of Natural Waters in the Hydrologic Cycle,” Reviews of Geophysics, vol. 2, no. 1, pp. 177-224. Gat, J. R., 1971, ’’Comments on the Stable Isotope Method in Regional Groundwater Investiga tions,” Water Resources Research, vol. 7, no. 4, pp. 980-993. Grove, D. B., M. Rubin, B. B. Hanshaw, and W. A. Beetem, 1969, ”Carbon-14 dates of ground water from a Paleozoic carbonate aquifer, south-central Nevada,” U.S. Geol. Survey Profes sional Paper, vol. 650-C, pp. c215-c218. Hunt, C. B. and T. W. Robinson, 1960, ’’Possible interbasin circulation of ground water in the southern part of the Great Basin,” U.S. Geol. Survey Professional Paper, vol. 400-B, pp. 51 b273-b274. Mahin, D. A., 1978, ’’Analysis of Groundwater Flow in the Edwards Limestone Aquifer, San Antonio, Texas,” M.S. Thesis, Univ. of Nevada, Reno, 49 p. Mai mb erg, G. T. and Tv E. Eakin, 1962, ’’Ground-water appraisal of Sarcobatus Flat and Oasis Valley, Nye and Esmeralda Counties, Nevada,” Nevada Dept, of Conserv. and Nat. Resources, Ground-Water Resources Reconnaissance Series Report, vol. 10, 39 p. Moser, H, and W. Stichler, 1980, ’’Environmental Isotopes in Ice and Snow,” in: Handbook of Environmental Isotope Geochemistry, Vol. 1, The Terrestrial Environment, A, Fritz,P. and J.Ch. Fontes, eds., Elsevier Scientific Publ. Co., pp. 141-178. Nichols, W. D. and J. P. Akers, 1985, ’’Water-Level Declines in the Amargosa Valley Area, Nye Co., Nevada 1962-84,” U.S. Geol. Survey Water-Resources Investig. Report 85-4273, 7 p. Rice, W. A., 1984, "Preliminary Two-Dimensional Regional Hydrologic Model of the Nevada Test Site and Vicinity,” Sandia contractor report SAND83-7466, 44 p. Schoff, S. L. and J. E. Moore, 1964, "Chemistry and Movement of Ground Water, Nevada Test Site,” U.S. Geol. Survey TEI-838, 74 p. Scott, R. B. and M. Castellanos, 1984, ’’Stratigraphic and Structural Relations of Volcanic Rocks in Drill Holes USW GU-3 and USW G-3, Yucca Mountain , Nye Co., Nevada,” U.S. Geol. Survey Open-File Report 84-491, 121 p. Simpson, E. S., and L. Duckstein, 1976, "Finite-state mixing-cell models,” in: Karst Hydrology and water resources, vol. 2, Yevjevich.V., ed, Fort Collins, Colorado, Water Resource Pub lications, pp. 489-508. Squires, R. R. and R. L. Young, 1984, ’’Flood Potential of Fortymile Wash and its Principal Southwestern Tributaries, Nevada Test Site, Southern Nevada,” U.S. Geol. Survey Water- Resources Investig. Report 83-4001, 33 p. Waddell, R. K., 1982, "Two-Dimensional, Steady-State Model of Ground-Water Flow, Nevada Test Site and Vicinity, Nevada-California,” U.S. Geol. Survey Water-Resources Investig. Report 81-4085, 71 p. Waddell, R. K., J. H. Robison, and R. K. Blankennagel, 1984, ’’Hydrology of Yucca Mountain and Vicinity, Nevada-California-Investigative Results Through Mid-1983,” U.S. Geol. Survey Water-Resources Investig. Report 84-4267, 72 p. Walker, G. E. and T. E. Eakin, 1963, "Geology and Ground Water of Amargosa Desert, Nevada- California,” Nevada Dept, of Cons, and Nat. Resources, Ground-Water Resources - Recon naissance Series Report 14, 45 p. White, A. F., 1979, "Geochemistry of Ground Water Associated with Tuffaceous Rocks, Oasis Valley, Nevada,” U.S. Geol. Survey Professional Paper 712-E, 25 p. Winograd, I. J., 1962, "Interbasin movement of ground water at the Nevada Test Site, Nevada,” U.S. Geol. Survey Professional Paper 450-C, pp. cl08-clll. Winograd, I. J. and I. Friedman, 1972, ’’Deuterium as a tracer of regional groundwater flow, southern Great Basin, Nevada-California,” Geol. Soc. of America Bulletin, vol. 83, no. 12, pp. 3591-3708. 52 Winograd, I. J. and W. lhordarson, 1975, ’’llydrogeologic and Ilydrochemical Framework, South-Central Great Basin, Nevada-California, with Special Reference to the Nevada Test Site,” U.S. Geol. Survey Professional Paper 712-C, 126 p. Winograd, I. J. and F. J. Pearson Jr., 1976, ’’Major carbon-14 anomaly in a regional carbonate aquifer: Possible evidence for megascale channeling, south-central Great Basin,” Water Resources Research, vol. 12, no. 6, pp. 1125-1143. Winograd, I. J. and G. C. Doty, 1980, ’’Paleohydrology of the Southern Great Basin, with Special reference to Water Table Fluctuations Beneath the Nevada Test Site During the Late(?) Pleistocene,” U.S. Geol. Survey Open-File Report 80-569, 91 p. 53 APPENDIX A List of wells, springs(*) in study area

Cell, area Well, Spring Location 18q 2H (isotope values in permil, relative to SMOW) Pahute Mesa U20a#2 N907395 E571439 -14.8 -114 cells 1,2,3 UEl9gs N931339 E587843 -14.5 -114 UEl9c -14.1 -109 UEl9e N927300 E596999 -14.0 -110 N0S/47E-14bab same -14.5,- -112,-112 Stockade Wash,cell 6 TW-8 N879468 E609999 -13.0,12.9,- -104,-101,-104 Fortymile Canyon J-12 N733509 E501011 -12.8 -97.5 and Wash, J-13 N749209 E579651 -13.0,-12.7 -97.5,-96 cell 5 UE29a#2 36 56’29” 116 22’26” -12.8 -93.5,-93.0 Crater Flat VH-1 36 47'32” 116 33’07” -14.2 -108 (inch Yucca H-l 36 51’58” 116 27’12” -13.4,-13.5 -103,-101 Mountain), H-3 36 49’42” 116 28’01” -13.9 -101 cell 7 H-4 36 50’32” 116 26’54” -14.0 -104 H-6 36 50’49” 116 28’55” -13.8,-14.0,-14.0 -106,-105,-107 UE25p#l -13.5,-13.5 -106,-106 UE25b#l 36 51’08” 116 26’23” -13.4,-13.5 -100,-99.5 USW G-4 -13.8 -103 Oasis Valley, 12S/47E-19adc same -13.3,- -104,-104 border of 12S/47E-20bbb same -13.6 -106 cells 1,7 12S/47E-7dba same -13.9 -108 12S/47E-6cdd same -13.3 -102 llS/47E-5cda same -14.1,— -108,-108 llS/47E-28dac same -14.1 -109 HS/47E-28acc same -14.1 -108 llS/47E-??dbb same -14.0 -108 10S/47E-27cba same -14.3 -110 Amargosa Desert 18S/50E-25ab same - -105 (inch Ash 17S/50E-19aab same - -105 Meadows), 17S/49E-8ddb same -13.0 -102 cells 8,9 17S/49E-7bb same -12.7 -104 17S/48E-lab same -13.0 -104 17S/49E-9aa same -12.8 -105 16S/49E-36aaa same -13.7 -104 16S/49E-19daa same -13.1 -101 16S/49E-16ccc same -13.2 -97.5 16S/48E-15dda same -13.4 -103 16S/49E-8abb same -13.2 -99.5 16S/49E-5acc same -13.2 -103 16S/50E-7bcd same -13.8 -105 15S/49E-22dc same -12.8 -102 16S/48E-25aa same -13.0 -102 16S/49E-23add same -13.2 -99.0 16S/49E-18dc same -12.6 -102 16S/48E-15aaa same -13.4 -103 16S/48E-7cbc same -13.1 -102 16S/48E-10cba same -13.4 -102 *Big Spring -13.5 -102 54 *King Spring -13.6 -104 *Fairbanks Spring -13.6,- -103,-103 *Crystal Pool -13.7 -102 *Point 'O Rocks Spring -105 Location is given in Nevada coordinates, e.g. N907395 E571439; or in latitude and longitude, latitude given first, e.g. 36 56 59 116 22’26"; or is labeled ’’same”, in which case the well name is in township and range coordinates, e.g. 10S/47E-14bab. 55

Appendix B. Location of Sampled Wells and Springs 56 Appendix C. Cell Deuterium Concentrations at Specific Iterations and Prescribed Cell Concentrations. (NC == no change, 1 it. = 100 yrs). Values in negative permil, relative to SMOW. Cell Observ. Iteration Number 0 100 200 300 400 500 600 700 800 900 1 112 1000 436.4 201.9 133.0 116.4 112.9 112.2 112.1 112.0 NC 2 114 1000 227.4 129.7 116.3 114.4 114.1 114.0 NC 3 114 1000 244.1 133.1 116.8 114.4 114.1 114.0 NC 4 — 1000 371.9 160.3 118.1 110.8 109.6 109.4 NC 5 97.0 1000 205.1 118.4 100.7 97.6 97.1 97.0 NC 6 104 1000 171.8 114.2 105.9 104.6 104.5 104.4 NC 7 108 1000 404.1 186.8 125.7 111.4 108.4 107.9 107.8 107.7 NC 8 102 1000 294.4 153.1 113.7 104.6 102.8 102.4 102.3 NC 9 104 1000 175.7 123.2 108.4 104.9 104.2 104.1 NC 1 112 120 115.0 112.8 112.2 112.1 112.0 NC 2 114 120 114.8 114.1 114.0 NC 3 114 120 114.9 114.1 114.0 NC 4 — 120 111.6 109.8 109.4 NC 5 97.0 120 97.9 97.1 97.0 NC ' 6 104 120 104.9 104.5 104.4 NC 7 108 120 110.4 108.4 107.9 107.8 107.7 NC 8 102 120 104.1 102.7 102.4 102.3 NC 9 104 120 104.7 104.2 104.1 NC 1 112 90.0 104.2 109.9 111.5 111.9 112.0 NC 2 114 90.0 111.1 113.6 114.0 NC 3 114 90.0 110.6 113.5 114.0 NC 4 — 90.0 103.0 108.1 109.2 109.4 NC 5 97.0 90.0 94.3 96.5 96.9 97.0 NC 6 104 90.0 102.7 104.2 104.4 NC 7 108 90.0 100.5 105.8 107.3 107.6 107.7 NC 8 102 90.0 97.7 101.1 102.1 102.3 NC 9 104 90.0 102.3 103.6 104.0 NC