A Deuterium - Calibrated Ground Water Flow Model of the Western Nevada Test Site and Vicinity

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ft DESERT RESEARCH INSTITUTE <11 UNIVERSITY OF NEVADA SYSTEM FLOW MODEL OF THE WESTERN NEVADA TEST SITE AND VICINITY WATER RESOURCES CENTER

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This report was prepared as an account of work sponsored by the United States Government, Neither the United States nor the United States Department of Energy, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately-owned rights. Reference herein to any specific commercial product, process, or service by trade name, mark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any Printed in the United States of America National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road

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FLOW MODEL OF THE WESTERN NEVADA TEST SITE AMD VICINITY Water Resources Center Desert Research Institute University of Nevada System 1 Graduate Research Fellow, Water Resources Center, Desert Research Institute Associate Research Professor, Water Resources Center, Desert Research The work upon which this report is based was supported in part by the U.S. Department of Energy under Contract #DE-AC08-8 5NV10384.

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DESCRIPTION OF THE STUDY AREA 2 PREVIOUS WORK ON REGIONAL FLOW 9 Effective Porosities and Cell Volumes 18 Cell Deuterium Concentrations 20 System Boundary Deuterium Recharge Concentration 21 RESULTS AND DISCUSSION 24 Calculated Versus Observed Deuterium Concentrations 24 Recharge Areas and Estimates 25 Vertical Recharge Estimates 27 Model Flow Paths 29 Flow Estimate Comparisons 32 Off-Site Migration From Areas 19 and 20 35 SUMMARY AND CONCLUSIONS 36 SUGGESTIONS FOR FUTURE RESEARCH 37 APPENDIX A - LIST OF WELLS AND SPRINGS IN STUDY AREA 42 APPENDIX B - LOCATION OF SAMPLED WELLS AND SPRINGS 45

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LIST OF FIGURES 1. Location and Major Features of the StudyArea 3 2. Nested Caldera Region in Vicinity of StudyArea 5 3. Types of Mixing Cells in the DSC Model 15 4. Study Area Isotope Samples 17 6. Model Flow Paths 30

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LIST OF TABLES 1. Geologic Column of Study Area 7 3. Observed and Simulated Deuterium Concentrations 24 4. Vertical Recharge Estimates 28 5. Internal Flow Paths 31 6. Flow Estimate Comparisons 3 2 7. Groundwater Minimum Mean Ages 3 4

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The authors wish to express their gratitude to those who helped make this investigation possible, John Hess and Brad Lyles carefully reviewed the manuscript and we appreciate their time, effort, and suggestions. Lee Huckins was of great assistance in locating information. Barbie Nauroth typed the final manuscript, and Karla Cosens provided her usual invaluable help in assuring the completion of the final report. We are indebted to JoAnne Burrows of the U.S. Department of Energy for her support and patience throughout our efforts. Finally, we wish to thank the Department of Energy for its financial

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A discrete-state compartraent model, consisting of nine aquifer subregions, is calibrated with the stable isotope deuterium to delineate groundwater flow in the vicinity of western Nevada Test Site. Calibrated deuterium concentrations agree very closely with observed ones, except on eastern Pahute Mesa and Timber Mountain (which had no data on observed deuterium concentra tions) • Model flow paths and amounts are in relative agreement with previous studies. The total groundwater flow through the modeled area is 32.4 x 10 m3/yr. Fortymile Canyon/Wash and Pahute Mesa are major recharge sites, accounting for 2.6 and 8.1 x 10 m /yr of recharge, respectively. Recharge, as a percentage of precipitation, is greatest in Fortymile and Stockade Washes, ranging between 5 and 12 percent. In Crater Flat, Timber Mountain, and eastern Pahute Mesa, recharge is between 1 and 2 percent of precipitation. Major dis charge areas occur in Oasis Valley and the Anargosa Desert, discharging 2.7 and 28.8 x 10 m /yr, respectively. Groundwater mean ages range from 2700 years at Ash Meadows to 9200 years at. Pahute Mesa, and represent minimum groundwater residence times. The reason these ages are ,,minimums,, stems from the inability of the current DSC computer code to assign non-zero ages to underflow recharge amounts when in reality these waters may be many thousands of years old. Research is underway to circumvent this drawback. The DSC deuterium model has not been verified, primarily because it is quite different from conventional groundwater flow models; hence, comparisons are difficult. However, it can be checked by constructing DSC models calibrated with other environmental tracers such as carbon-14, a task currently in progress.

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Environmental (i.e., naturally-occurring) isotopes can provide a wealth of information on groundwater flow systems. Radioisotopes, such as carbon-14, can be used to date groundwater; stable isotopes, such as deuterium and oxygen-18, can provide information on recharge areas and the mixing of water masses from different source areas. Deuterium (2H) and oxygen-18 ( are especially useful as tracers of water movement because these two nuclides are incorporated into the water molecule and, therefore, more accurately reflect the path taken by the water. In non-thermal systems, both deuterium and oxygen-18 behave conservatively once in the saturated zone, a feature that further enhances their usefulness as tracers. The hydrogeologic utility of the aforementioned environmental isotopes can be improved dramatically when their spatial and/or temporal distributions in groundwater are interpreted in the context of a quantitative flow or mixing model. In this use, environmental isotopes are used to calibrate a flow/mixing model; depending upon the particular model, estimates of parameters such as flow rates, groundwater ages, volumes of water in storage, and recharge rates This report documents the application of the stable isotope deuterium to groundwater flow model calibration. The model selected is a relatively simple area modeled is the western portion of the Nevada Test Site (NTS) and vicinity. The model will provide estimates of the following: 3) approximate groundwater flow directions; and 4) minimum groundwater mean ages. From the standpoint of potential anthropogenic radionuclide migration off-site via groundwater flow, knowledge of all the above is important. Items (1) and (2) relate to the movement of water (and any radionuclides contained therein) into and out of the flow system. Item (3) is important regarding possible paths taken by any radionuclides in the flow system, and item (4) will help assess travel times in the saturated zone. The latter item is significant because to our knowledge, it represents the first time a stable isotope has been used to calculate groundwater mean ages.

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DESCRIPTION OF THE STUDY AREA The study area (Figure 1), which encompasses 3740 km , is located in the Great Basin section of the Basin and Range physiographic province of the southwestern United States between 117 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 study area is arid; the average annual precipitation varies from 8 to 16 cm in the valleys and is generally 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 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. Several major ephemeral streams that exist in or discharge into the study area contain water only after intense storms. Ihe only perennial flows are drainages from the Ash Meadows springs near the southern part of the Amargosa River. The ephemeral Anargosa River originates in Oasis Valley and runs southeast through the Amargosa Desert valley until it veers south out of the study area between the Resting Springs Range and the Funeral Mountains. Fortymile 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 Mountian 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 comprise the discharge from the area. Springs within the study area's boundaries are located in the Amargosa Desert at Ash Meadows and in Oasis Valley. Evapotranspiration is great in these areas because of the agricultural areas in the Amargosa Desert. Evapotranspiration is also significant at Pahute Mesa and Timber Mountain where vegetation density increases with elevation. Pumpage has been most significant in Beatty and the Amargosa Desert; for example, water levels in alluvial wells

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in the Amargosa Desert have dropped 8 m between 1962 and 1984 (Nichols and Creosote bush (Larrea divaricata), burro bush (Franseria dumosa), and yucca plants (Yucca yuca) populate the valleys, giving way to blackbrush (Coleogyne ramosissima) and Joshua trees (Yucca brevifolia) at slightly higher altitudes. Above 1800 m elevation pinon pine (Pinus monophy11a), juniper (Juniperus osteosperma), and sage (Artemisia tridentata and A. nova) dominate, while above 2300 m white fir (Abies concolor) and yellow pine (Pinus echinata) are most abundant. The following is a brief review of the geology pertinent to this study and is based on Winograd and Thordarson (1975), to which the reader is referred for Lithology in the study area consists of late Precambrian to early Paleo zoic sedimentary 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; 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 thicknesses of the Precambrian and Paleozoic rocks, the upper half of which is carbonate, can approach 11,300 m (Winograd and Thordarson, 1975, p. C9). However, the study area encompasses a region of overlapping calderas originating from Tertiary volcanisra, which was subsequently responsible for the widespread deposition of thick sequences of tuff and other volcanic rocks in the calderas and in the vicinity of the Nevada Test Site. It is possible that much of the Paleozoic sedimentary rock has been obliterated or is insignificant in a regional context within the region of "nested11 calderas (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 carbonate 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 principal rock types

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of the Paleozoic assemblage are dolomite, siltstone, argillite, and quartzite. 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 m. Valley fill (alluvium) varies in thickness, but maximum thick ness is probably 610 m. Table 1 is a generalized geologic column of the study area; a more detail ed hydrogeologic column is in Winograd and Thordarson (1975, pp. C10-C11). Two major periods of deformation are distinguished: folding and thrusting of Paleozoic 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 Volcanism during the Tertiary formed a field of calderas which influenced local structure. Ihe calderas formed when deep seated magma bodies rose, caus ing 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 the 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 locally 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 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. Winograd and Thordarson (1975) divided the rocks in the NTS region into 10 hydrogeologic units based on aquifer test and porosity determinations. Due to

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Table L - GEOLOGIC COLUMN OF STUDY AREA in some areas basalts 61 Pliocene unit is an aquifer rhyolite flows 610 aquifer ash flow tuffs, lava flow tuff ash flow tuff, nonwelded to mod ash fall tuff ash flow tuff, and quartz latitic lavas with altered bedded and ash flow tuffs and conglomerate with granitic stocks of in study area only at eastern

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the nature of the DSC model, the effect of composite water sampling, lack of vertical head data, uncertainty in aquifer thicknesses, and the large variation in well depth throughout the area, saturated thicknesses approximated from nonfully penetrating wells are 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. Ihe study area generally contains three lithologies which control groundwater movement: alluvium (valley fill), volcanic rocks, and carbonate rocks. The 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 the 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 control ling regional groundwater flow where they are abundant. As a result of vol canism in the area (see Lithology section), carbonates are assumed to exist in significant'thickness only beneath the Amargosa Desert. All three lithologies communicate as aquifers to some degree at some locations …