Review of Soil Moisture Flux Studies at the Nevada Test Site Nye County, Nevada

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ft DESERT RESEARCH INSTITUTE <11 UNIVERSITY OF NEVADA SYSTEM REVIEW OF SOIL MOISTURE FLUX STUDIES AT THE NEVADA TEST SITE NYE COUNTY, NEVADA WATER RESOURCES CENTER

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REVIEW OF SOIL MOISTURE FLUX STUDIES AT THE NEVADA TEST SITE NYE COUNTY, NEVADA

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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 528 5 Port Royal Road

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REVIEW OF SOIL MOISTURE FLUX STUDIES AT THE NEVADA TEST SITE NYE COUNTY, NEVADA Water Resources Center Desert Research Institute University of Nevada System 1 Research Scientist, Water Resources Center The work upon which this report is based was supported in part by the U.S. Department of Energy under Contract #DE-AC08“8 5NVI0384.

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This report documents almost 30 years of research on soil moisture move ment and recharge at the Department of Energy, Nevada Test Site. Although data is scarce, three distinct topographic zones are represented: alluvial valleys, inundated terrains, and upland terrain. Recharge in alluvial valleys was found to be very small or negligible. Ponded areas such as playas and subsidence craters showed significant amounts of recharge. Data in the upland terrains is very scarce but one area, Rainier Mesa, shows active recharge of up to three percent of the annual average precipitation in fractured volcanic tuff. The report summarizes the results.

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The author wishes to thank the Desert Research Institute library staff— Rachel Melvin and Leslie Anastassatos for their tireless help in locating the references used in this document. The author appreciates the helpful review and comments of Drs. M. Campana and P. Fenske. Thanks also go to Deborah Wilson for her transcription and word processing skills.

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II. GENERAL RECHARGE CONSIDERATIONS 4 Geography and Surficial Conditions 4 III. RESULTS OF MOISTURE FLUX STUDIES AT NTS 7 Flux Analysis Using Soil HydraulicProperties 8 Flux Estimates Using Tritium Tracers 15 Soil Moisture Tritium 16 Recharge Studies in Fractured Terrain 36 Up land Areas 44V.

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3.1 Estimated Flux and Velocity from Rock Valley Soil. 13 3.3 Tritium Profiles from Area 5 Surface Soils. 23 3.4 Tritium in NTS Ground Waters. 25

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3.1 Soil Hydraulic Property Study Sites on NTS. 9 3.2 Conductivity and Pressure Head Data for Rock Valley Alluvium. 11 3.3 Water Retention Data for Rock Valley Alluvium. 12 3.4 Distribution of Tritium in Precipitation in Ottawa, Canada. 17 3.5 Locations of Soil Tritium Sampling Studies at NTS. lb 3.6 Soil Water Tritium Profile from Yucca Flat #YF-10. 20 3.7 Soil Water Tritium Profile from Frenchman Flat #FFI-A1. 21 3.8 Soil Water Tritium Profiles from Cambric RNM Site. 24 3.9 Location of In-Situ Study Areas on or near NTS. 27 3.10 Matric Potential Profile from Frenchman Flat. 29 3.11 Neutron Water Content Log from Frenchman Flat. 30 3.12 Matric Gradients Measured at Frenchman Flat. 31 3.13 Soil Water Velocities at Frenchman Flat. 33 3.14 Capillary Pressure from Core Samples at Yucca Flat. 35 3.15 Hydrostratigraphy of Rainier Mesa. 37 3.16 Rainier Mesa Showing Potential Catchment Areas for

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Since 1963, all U.S. nuclear weapons testing has been conducted underground, with a majority of these tests being held at the U.S. Department of Energy (DOE), Nevada Test Site (NTS) in southern Nevada. By conducting each detonation in the subsurface, the likelihood of imme diate release of radioactivity to the environment is greatly reduced. Subsur face detonation, however, does not negate all of the potential environmental pathways for radionuclide release. One of the most likely pathways for release to the accessible environment from testing is through the ground water. Since many of the tests are conducted above the zone of ground-water saturation, partially saturated flow or soil water flow must be understood if these path ways are to be fully evaluated. In addition, other waste materials, both hazardous and radioactive, are stored or disposed of at NTS. An understanding of soil water flow is also critical to risk assessment from these practices. At first glance, NTS appears to offer little, if any, areas for deep soil water movement. The area is one of the driest areas in the continental United States (Winograd and Thordarson, 1975). Mean daily maximum temperatures may range from 13 to 40°C. Combining the low precipitation and the high potential evapotranspiration into a water budget model, it has been generally thought that little or no ground-water recharge can occur. Recent studies (Gee and Kirkham, 1984 and Tyler et al., 1986) have shown, however, that simple water balance approaches to soil water movement and ground-water recharge may be insensitive in arid regions. These insens itivities arise from the conflict between the long time scales used in water budget models and the short time

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frames over which precipitation, evaporation, and soil water movement occur in coarse-grained surface soils or fractured rock found in many arid regions. These time scales may be on the order of minutes to a few hours, while most modeling approaches use daily, weekly, or even monthly estimates. In addition, studies by Clebsch (1961), Thordarson (1965), and Tyler et al. (1986), have suggested that deep soil water movement (recharge) is occurring in selected areas of the NTS. These studies indicate that areas such as mesa tops, frac tured bedrock surfaces, wash bottoms, and man-made depressions may allow deep soil water migration. It is interesting to note the two similarities of each of these study areas. In the first,, each locale receives or concentrates pre cipitation in small areas (ponds, channels, or fractures). In the second, each locale allows for the rapid migration of potential recharge below the depth of active evaporation and transpiration because the surficial materials are highly permeable. These two criteria, source water and rapid transport, are crucial to understanding the potential for recharge in arid areas. Although many of the studies already mentioned have dealt with aspects of soil water movement, none have been reviewed in terms of generating an overall recharge picture at NTS. The goal of this study is to review the current data base regarding soil water movement on or near the NTS. The results of this study will provide a clearer understanding of the potential recharge areas, an estimate of the magnitude of recharge, and directions for future studies. These results may also be used to design controls to prevent radionuclide mig ration to the ground-water system. In Section 2 of this report, the general hydrologic and climatological conditions of the NTS are described. In Section 3, the results of soil mois ture-related recharge studies are described in detail. The section describes three methodologies used by previous investigations to estimate recharge and flux. In addition, a discussion is presented of studies conducted at Rainier Mesa, an area of fractured tuff used for nuclear weapons testing. In Section 4, the results are summarized. Although not included in this review, a considerable research effort on soil moisture flux is presently being conducted for the Nevada Nuclear Waste Storage Investigation. These studies are focused on characterization of a potential high level nuclear waste repository in fractured, partially satu rated, tuffaceous rock. The repository site is located on the western boundary of NTS. Much of the data generated from this study have not been formally

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published; although, preliminary results (DOE, 1984) indicate that moisture flux at the Yucca Mountain Site is small or negligible. Since this project has only recently begun, it will not be included in this review. It is hoped that in the near future, data from this intensive effort will shed considerable light on the processes of moisture movement in fractured rock.

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GENERAL RECHARGE CONSIDERATIONS Various aspects of water flow through the unsaturated zone at NTS have been investigated for the past 25 years. Over 20 technical reports on the subject appear in this report's bibliography. Due to the wide variety in ter rain and scope of studies, these reports indicate a wide variety of soil mois ture conditions. This report describes the key studies, which may be related to soil water movement, and analyzes the results and suggests ways to determine recharge potential at NTS. Recharge to ground water can come from numerous sources: infiltration and percolation of natural precipitation; seepage from canals and streams; and artificial infiltration through wells or surface ponding (Freeze and Cherry, p. 211, 1979). The primary source for recharge in the testing areas will be meteoric water percolating through the soils and rock. In other areas, dis posal of wastewater may provide an additional source of recharge. GEOGRAPHY AND SURFICIAL CONDITIONS The NTS encompasses 3500 km2 of Basin and Range topography in southwestern Nevada. The eastern part of the NTS is characterized by parallel Cenozoic topography and structural elements generally associated with the Basin and Range province (U.S.G.S., 1976). The range and ridge tops consist of either bare rock or thin soil mantling fractured bedrock. The basins are character ized by thick sequences of Quaternary alluvium. (Soil pedogenesis in both locales is limited due to the scarce rainfall.)

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The western part of the NTS is made up of late Miocene and early Pliocene volcanism whose topographic and structural effects partly cover the typical Basin and Range topography (U.S.G.S., 1976). As in the eastern half, soils are thin in most highland areas while thick sequences of Quaternary alluvium fill the valleys. Soils and sediments are generally derived from volcanic rocks with little derived from the carbonate bedrock. Throughout the NTS, however, it is typical to find calcic horizons either near the surface or as buried soil horizons. Precipitation in the form of intense convective summer storms and winter frontal storms are the common weather patterns for the NTS. Mean annual preci pitation ranges from 7 to 15 cm on the valley floors to 25 to 40 cm on the mesa tops. The potential annual evaporation from lake and reservoir surfaces in the area was estimated by Meyers (1962) to range from 150 to 200 cm or roughly 5 to 25 times the annual precipitation. The mean daily maximum temperature at Las Vegas (station alt. 665 m) ranges from 13°C in January to 40.5°C in July. The mean daily minimum temperature for the same two months ranges from 0.5 to 24.5°C. The higher valleys, such as central Yucca Flat (1254 m), are as much as 3 to 8.5°C lower. Variations in precipitation and temperature cause marked differences in plant life. Creosote bush, burro bush, and a variety of yuccas dominate the basins below 1200 m. These give way to blackbrush and Joshua trees at slightly higher altitudes. Juniper, pinyon pine, and sagebrush dominate above 1800 m and are, in turn, replaced by white fir and yellow pine above 2300 m (Bradley, Winter precipitation originating in the west is usually associated with transitory low pressure systems and, therefore, moves over large areas (Quiring, 1965). The summer precipitation occurs predominantly as convective storms, which can be intense over a few square kilometers and which vary in location from one storm to the next. Summer moisture usually originates from the southeast or south (Winograd and Thordarson, 1975). Data collected by Winograd and Thordarson (1975) indicate up to 10 sepa rate hydrostratigraphic units that are present in various locations in the

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saturated zone underlying the NTS. The units range from fractured carbonates and clastic sediments to clastic volcanic and sedimentary rocks, each with different and variable hydrologic properties. From a vadose zone standpoint, the NTS also contains some of the thickest unsaturated zones in the United States. Depth to water in certain locales may The great depth to the water table in these areas is a combination of the following: areas of moderate to high relief; relatively permeable strata with the saturated zone; regional aquifers with topographically low discharge points; and little recharge. These thick unsaturated zones can be made up of any of the hydrostratigraphic units, but are typically made of layered Tertiary volcanics and Quaternary alluvial sediments. The climatic and hydrologic conditions found at the NTS indicate that water movement through the unsaturated zone should be slight. The lack of perennial streams and the small number of springs found in the area tend to support this conclusion. In any other locale, there would be little incentive to study soil moisture movement and recharge. At NTS, however, the activities conducted require a reasonable knowledge of soil water flux to assess its impacts on radionuclide transport. In the next section, reviews of studies conducted for DOE and its predecessor agencies will be used to assess the mag nitude of water flow in the unsaturated zone of the NTS.

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RESULTS OF MOISTURE FLUX STUDIES AT NTS The measurement of water movement in the unsaturated zone has received significant attention in the last few years. The need to estimate the impact of manfs activity on ground-water resources has driven this effort. At NTS, at least three methodologies have been applied to this end. In the first method, soil hydraulic properties have been combined with measured soil water energy data in Darcy's equation to estimate fluid flux and velocity. This technique assumes that the moisture conditions remain constant with time. Environmental tracers, such as tritium or chlorine-36, have also been used to estimate the velocity of soil water. These tracers, introduced during atmospheric testing of nuclear weapons, are assumed to move as discrete packets with each precipitation event. By knowing their position in the soil profile and their first appearance in the precipitation, the soil water velo city may be estimated. Direct measurement of soil water movement is, of course, the optimal tech nique for understanding ground-water recharge. In arid environments, however, this methodology encounters many difficulties. The difficulties arise from estimation of the small net volume of water which may be moving downward below the root zone. Several studies of this nature have been conducted at NTS and provide detailed information on soil water movement. A fourth, but not entirely independent, methodology has been applied to estimate the magnitude of water flux in fractures in the unsaturated zone. These studies, centered at Rainier Mesa, have utilized a combination of frac ture discharge measurements, environmental tracers, and geochemical data to

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understand the complex phenomenon of water flow in partially saturated frac In each of the following sections, a detailed review of the results of each methodology will be presented. The conclusions reached in these studies will be critically reviewed and alternative conclusions will be presented. By reviewing all the studies …