University of Nevada Reno Tegrated Geochemical and Hydraulic of Nevada Test Site Ground Water Analyses Systems a Thesis Submitte
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University of Nevada Reno tegrated Geochemical and Hydraulic Analyses of Nevada Test Site Ground Water Systems A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Hydrogeology by Carol Jean Boughton HI May 1986 Mines Library University of Nevada - Reno Reno, Nevada 89557-0044 MINES UBRARY t h e s i s The thesis of Carol Jean Boughton is approved: University of Nevada Reno May 1986 IX ACKNOWLEDGEMENTS I would first like to thank Desert Research Institute for its financial assistance provided under DOE Contract #DE-AC08-81NV10162. This thesis could not have been at tempted without this assistance. My particular appreciation is extended to my advisor and committee members: Dr. Paul Fenske, Dr. Roger Jacobson, Dr. Steve Wheatcraft, and Dr. Jack Hess whose assistance and encouragement were invaluable. I also thank Bert Elliott, Kevin Sullivan, and Lee Huckins for their field assistance. Without them, the 1 ‘♦c carboys would never have made it to the lab. Further thanks are extended to the personnel of the Las Vegas EPA office, particularly Dan Wait and Frank Reed, who helped in identification and location of pumping wells and who helped obtain samples which would have been difficult to obtain without their help. Last, but certainly not least, I couldn't have done without the emotional support of my many friends who had more faith in me than I had in myself. I trust that faith was not misplaced. My daughter, Kim King, and my good friend and office mate, Tom Panian, who were always there when I needed them. iii ABSTRACT Major ion, tritium, and stable isotope analyses and radio-carbon age dating have been utilized in conjunction with hydraulic data to further understanding of the ground water flow regimes at the Nevada Test Site. Tritium is found in regional aquifers having uncorrected radiocarbon ages from 22,700 to >39,920 years before present. Temporal geochemical fluctuations occur within deep aquifers pointing to rapid recharge mechanisms. Stable isotope samples from ground water lie below and parallel to the Craig meteoric water line, <$D = 8<$ 1 80+1 0 . 5 1 80 ratios ranae from -14.1 to -12.4°/oo. Sp ratios range from -109 to -96°/oo. Radiocarbon ages are corrected based upon S13C analyses of various media which ranqe from +4.1 to -24.2°/oo. Evidence points to rapid 613c dilution within the soil zone toward -12.0°/oo, with no additional dilution in tuffa- ceous aquifers. Overall evidence points to complex mixing between the various saturated units located within the study area. IV TABLE OF CONTENTS Acknowledgements ii Abstract iii List of Figures vi List of Tables vii List of Appendices iix 1 . INTRODUCTION 1 2. ENVIRONMENTAL SETTING 5 2.1 Geographic Setting 5 2.1.1 Physiography 5 2.1.2 Climate 7 2.1.3 Flora 11 2.2 Geologic Setting 13 2.2.1 Stratigraphy 13 2.2.2 Structural Geology 14 2.3 Hydrologic Setting 15 2.3.1 Surface Water 15 2.3.2 Principal Aquifers and 15 Aquitards 2.3.3 Regional Ground Water Flow 19 3. THEORY OF HYDROCHEMICAL INVESTIGATIONS 26 3.1 Major Ion Chemistry 26 3.2 Radiocarbon Dating of Groundwater 29 3.3 Stable Isotope Chemistry 36 3.4 Tritium Dating of Groundwater 41 4. FIELD AND LABORATORY METHODS 45 4.1 Sampling Overview 45 4.2 Soil Gas Samples 46 4.3 Soil Samples 47 4.4 Caliche Samples 49 4.5 Core Samples 49 4.6 Discharge Area Sampling 52 4.6.1 Spring Sampling 52 4.6.2 Well Sampling 53 4.7 Laboratory Methods 56 V 5. RESULTS AND DISCUSSION 57 5.1 Water Table Map 57 5.2 Major Ion Chemistry 64 5.3 Radiocarbon Dating 71 5.3.1 Results of Soil and Rock Carbon Analyses 71 5.3.2 Caliche 613C Results 77 5.3.3 Soil Water and Soil Gas S13C Results 78 5.3.4 Radiocarbon Results 83 5.4 Tritium 87 5.5 Stable Isotope Chemistry 94 6. CONCLUSIONS 101 6.1 Overview 101 6.2 Proposals for Further Study 109 REFERENCES 112 APPENDICES 120 VI LIST OF FIGURES 2.1 Setting of the Nevada Test Site Area within the Basin and Range Physiographic Province 6 2.2 Climatic Distribution In and Near the Study Area 8 2.3 Mean Annual Precipitation 10 2.4 General Distribution of Major Aquifers and Aquitards in the Southern Great Basin 17 3.1 Expected Deviations from Craig MWL 42 5.1 NTS Water Table Map 58 5.2 Regional Perspective - NTS Water Table Map 62 5.3 Trilinear Diagram of NTS Samples 58 5.4 Relative <$13C Ratios of Samples from this Study 72 5.5 Tritium Versus Corrected Radiocarbon Ages 89 5.6 Oxygen Versus Hydrogen Isotopes from this Study 97 5.7 Oxygen Versus Hydrogen Isotopes from In and Near NTS 99 6.1 Comparison of Parameters Evaluated in this Study 102 vii LIST OF TABLES 3.1 Natural Abundance of Carbon Species 29 3.2 Effect of Contamination by Modern Carbon on True Age 37 3.3 Environmental Isotopes of Hydrogen and Their Relative Abundance in Water of the Hydrologic Cycle 37 3.4 Characteristic Constants of H20 and D20 33 4.1 Physical Characteristics and Environmental Setting of Caliche Samples 50 5.1 Chemical Analyses of Selected Samplesfrom NTS 65 5.2 Percent Carbonate of Soil and Rocks 73 5.3 Radiocarbon Results 84 5.4 Tritium Results 88 ■ 11X LIST OF APPENDICES 1. Stratigraphic and Hydrogeologic Units at Nevada Test Site and Vicinity 120 2. Locations of NTS Sampling Sites. 121 3. ^ R e s u l t s 122 4. Aquifer Characteristics - Desert Research Institute Sampling Sites 124 5. Summary of EPA Analyses 126 6. Tritium Analyses 128 7. Stable Isotope Data from in and near Study Area 131 1 CHAPTER 1 INTRODUCTION Since 1951 approximately 500 underground nuclear tests have taken place on the Nevada Test Site (NTS). Records indicate that at least 95 of these tests took place below the water table. The remainder were conducted in the unsat urated zone (USDOE, 1983). Those tests conducted in the saturated zone have caused localized radioactive contamination of the ground water and the possiblity of future contamination from the unsaturated zone exists. Because the saturated zone is a dynamic system, radionuclides may be transported with the ground water as it moves from the zone of contamination. This provides the necessity for definition of the flow regime to determine whether the contaminated water may at some point in space and time leave the confines of the Neva da Test Site and enter the biosphere. Theoretically a hydrologic test hole network could provide the definition required in the saturated zone. However, there are major drawbacks to this type of program. 1) It would be prohibitively expensive. 2) If funds were available for drilling, packer tests, and geochemical and environmental isotope sampling studies such as that con ducted on Pahute Mesa (Blankennagel and Weir, 1972) would 2 greatly enhance the working knowledge of the Nevada Test Site hydrologic regime. In that study, the drilling network provided an in-depth three dimensional understanding of the flow system. However, all lithologies are not conducive to such tests. Slaking, bridging, and caving can make it impossible to isolate intervals of interest and conduct the desired hydrologic tests. 3) Finally, an extensive drilling program could in itself alter the hydrodynamics of the flow system. Since this so-called "Swiss cheese" approach was not practicable, other, more indirect and less expensive methods were needed to provide insight into the NTS hydrologic re gime. This study has implemented several such methods. 1. A water table map was developed using existing drill hole, hydrologic test hole, and well data. This tool is characteristically used for identifying recharge and discharge areas, direction of flow, and gradients within the flow system. 2. An environmental isotope sampling program was im plemented utilizing existing wells as sampling sites. Radiocarbon and tritium, a radioactive isotope of hydrogen, produced in the upper atmos phere by cosmic ray bombardment of 14N and 2H, respectively, decay at known rates. This characteristic allows these radioisotopes to be used to estimate the age of ground water and the velocity 3 of various aquifers within the regional system. Radiocarbon age dating is accomplished by measuring the activity of carbonate species dissolved in ground water and comparing it with modern activity. Bicarbonate in ground water comes primarily from two separate sources: a. carbon dioxide gas and soil carbonate from the soil zone in the recharge areas and, b. carbonate from the rocks encountered as water passes through the aquifer. Samples of soil zone C02, soil from recharge and discharge areas, and carbonate core samples and ^ein fillings were analyzed for 513c ratios to provide applicable correction factors for radio carbon age-dating. The presence of tritium usually indicates that recent waters, less than 50 years old, have entered the flow regime. Underground thermonuclear testing may also account for tritiated ground water. Stable isotopes of oxygen and hydrogen were also sampled and analyzed. Use of these isotopes can be valuable in describing evaporation and conden sation processes which have taken place both prior to and after recharge occurs. These isotopes can help identify possible recharge areas and periods of recharge, fingerprinting waters from various source areas and identifying zones of mixing. 4 3. Major ion chemistry was included in the sampling program since aqueous geochemistry can reveal valuable information about the source area and the lithologic environment which the water has passed through.