Regional Groundwater Flow and Tritium Transport Modeling and Risk Assessment of the Underground Test Area, Nevada Test Site, Nevada

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U.S. Department of Energy Nevada Operations Office Regional Groundwater Flow and Tritium Transport Modeling and Risk Assessment of the Underground Test Area, Nevada Test Site, Nevada Approved for public release; further dissemination unlimited.

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This report has been reproduced directly from the best available copy. Available to DOE and DOE contractors from the Office of Scientific and Technical Information, P.O. Box 62, Oak Ridge, TN 37831; prices available from (423) 576-8401. Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161, telephone (703) 487-4650.

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REGIONAL GROUNDWATER FLOW AND TRITIUM TRANSPORT MODELING AND RISK ASSESSMENT OF THE UNDERGROUND TEST AREA, NEVADA TEST SITE, NEVADA DOE Nevada Operations Office Las Vegas, Nevada Approved for public release; further dissemination unlimited.

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REGIONAL GROUNDWATER FLOW AND TRITIUM TRANSPORT MODELING AND RISK ASSESSMENT OF THE UNDERGROUND TEST AREA, NEVADA TEST SITE, NEVADA Underground Test Area Subproject . 1 Nevada Environmental Restoration Project

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This document presents the results of the data analysis activities conducted during the regional investigation of the Underground Test Area Subproject. These activities have been led by IT Corporation; Tetra Tech, Inc.; GeoTrans, Inc.; and Daniel B. Stephens Corporation; however, the investigation could not have been completed without the cooperative efforts of many other participants involved in work at the Nevada Test Site. Their organizations are listed here in • Desert Research Institute • Lawrence Livermore National Laboratory • Los Alamos National Laboratory • Professional Analysis, Inc. • Raytheon Services Nevada • U.S. Geological Survey, Water Resources Division • U.S. Geological Survey, Yucca Mountain Project The data analysis activities have been reviewed by a panel of subject matter experts who provided technical input that has been incorporated into the document packages. Their invaluable contributions are reflected in this report. The reviewers and their respective organizations are alphabetically listed as follows: C Anthony Armstrong Oak Ridge National Laboratory National Risk Management Research Laboratory C Joe Downey Downey & Gutentag LLC C Lynn Gelhar Massachusetts Institute of Technology C Gerry Grisak Intera, Inc. C Brad Hill Computer Data Sytems, Inc. - Kerr Lab

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The groundwater flow system of the Nevada Test Site and surrounding region was evaluated to estimate the highest potential current and near-term risk to the public and the environment from groundwater contamination downgradient of the underground nuclear testing areas. The highest, or greatest, potential risk is estimated by assuming that several unusually rapid transport pathways as well as public and environmental exposures all occur simultaneously. These conservative assumptions may cause risks to be significantly overestimated. However, such a deliberate, conservative approach ensures that public health and environmental risks are not underestimated and allows prioritization of future work to minimize potential risks. Historical underground nuclear testing activities, particularly detonations near or below the water table, have contaminated groundwater near testing locations with radioactive and nonradioactive constituents. Tritium was selected as the contaminant of primary concern for this phase of the project because it is abundant, highly mobile, and represents the most significant contributor to the potential radiation dose to humans for the short term. It was also assumed that the predicted risk to human health and the environment from tritium exposure would reasonably represent the risk from other, less mobile radionuclides within the same time frame. Other contaminants will be investigated at a later date. Existing and newly collected hydrogeologic data were compiled for a large area of southern Nevada and California, encompassing the Nevada Test Site regional groundwater flow system. These data were used to develop numerical groundwater flow and tritium transport models for use in the prediction of tritium concentrations at hypothetical human and ecological receptor locations for a 200-year time frame. A numerical, steady-state regional groundwater flow model was developed to serve as the basis for the prediction of the movement of tritium from the underground testing areas on a regional scale. The groundwater flow model was used in conjunction with a particle-tracking code to define the pathlines followed by groundwater particles originating from 415 points associated with 253 nuclear test locations. Three of the most rapid pathlines were selected for transport simulations. These pathlines are associated with three nuclear test locations, each representing one of the three largest testing areas. These testing locations are: BOURBON on Yucca Flat, HOUSTON on Central Pahute Mesa, and TYBO on Western Pahute Mesa.

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One-dimensional stochastic tritium transport simulations were performed for the three pathlines using the Monte Carlo method with Latin hypercube sampling. For the BOURBON and TYBO pathlines, sources of tritium from other tests located along the same pathline were included in the simulations. Sensitivity analyses were also performed on the transport model to evaluate the uncertainties associated with the geologic model, the rates of groundwater flow, the tritium source, and the transport parameters. Tritium concentration predictions were found to be mostly sensitive to the regional geology in controlling the horizontal and vertical position of transport pathways. The simulated concentrations are also sensitive to matrix diffusion, an important mechanism governing the migration of tritium in fractured carbonate and volcanic rocks. Source term concentration uncertainty is most important near the test locations and decreases in importance as the travel distance increases. The uncertainty on groundwater flow rates is as important as that on matrix diffusion at downgradient locations. The risk assessment was performed to provide conservative and bounding estimates of the potential risks to human health and the environment from tritium in groundwater. Risk models were designed by coupling scenario-specific tritium intake with tritium dose models and cancer and genetic risk estimates using the Monte Carlo method. Estimated radiation doses received by individuals from chronic exposure to tritium, and the corresponding human health risks at hypothetical point-of-use locations along each of the pathlines were calculated for six potential land-use scenarios. Conservative land-use scenarios were postulated to ensure that the calculated exposures would bound any realistic dose received by individuals. Based on the human-health risk estimates, tritium exposures associated with the HOUSTON and BOURBON pathlines do not present a human health hazard off the Nevada Test Site in the present, the near term, or in the future. However, the estimates show that the TYBO pathline has the greatest potential for off-site release with a projected groundwater discharge at Oasis Valley. Using the most conservative scenario for tritium exposure demonstrates that dose could exceed the 100-mrem/yr limit at locations along the TYBO pathline. However, the risk predictions for the TYBO pathline are not supported by results from the current environmental monitoring network. Water samples from the Oasis Valley springs and wells, west and south of Pahute

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Mesa, do not show tritium is present in levels above background. These monitoring results confirm the premise that the conservative modeling approach was likely to overestimate tritium transport. Results also indicate that ecological risks due to tritium exposure are not anticipated to occur outside of federal lands.

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Table of Contents List of Plates ............................................................ xxiii List of Tables ............................................................ xxiv List of Acronyms and Abbreviations ......................................... xxviii

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Table of Contents (Continued) 1.2.2 History of Operations ......................................... 1-2 1.2.3 Underground Nuclear Testing ................................... 1-4 1.2.4 Impact on Groundwater ........................................ 1-6 1.3 Purpose and Scope ................................................ 1-7 1.4 Regional Evaluation Completion Process ................................ 1-7 2.0 Site Physical Features .................................................. 2-1

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Table of Contents (Continued) 2.2 Topography and Terrain ............................................ 2-1 2.3 Climate and Meteorology ............................................ 2-4 2.5.1 Precambrian and Paleozoic ..................................... 2-5 2.6.2 Groundwater Occurrence and Movement .......................... 2-11 2.6.3 Groundwater Radiological Monitoring ............................ 2-14 2.9 Archaeological and Historical Resources ............................... 2-21 3.2 Contaminant of Potential Concern ..................................... 3-1 3.4 Hydrologic Modeling Approach ...................................... 3-3 3.5 Risk Assessment Approach .......................................... 3-8 3.5.1 Human Health Risk Assessment ................................. 3-9 3.5.2 Ecological Risk Assessment ................................... 3-10

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Table of Contents (Continued) 4.0 Geologic Data Analysis ................................................. 4-1 4.1 Geologic Model Domain ............................................ 4-1 4.4 Geologic Data Compilation .......................................... 4-4 4.5 Conceptual Geologic Model Development ............................... 4-5 4.5.1 Stratigraphic Cross Section Construction ........................... 4-5 4.5.2 Hydrostratigraphic Unit Definition ................................ 4-5 4.5.3 Hydrostratigraphic Cross Section Development ..................... 4-10 4.6 Digital Geologic Model Development ................................. 4-10 4.6.1 Data Integration into ERMA .................................. 4-10® 4.6.2 Construction of Surface Contour Maps ........................... 4-11 4.6.3 Gridding of Contoured Surfaces ................................ 4-12 4.7 Geologic Uncertainty and Model Revisions ............................. 4-13 5.0 Hydrologic Data Analysis ............................................... 5-1 5.3.3 Recharge and Discharge ....................................... 5-3 5.5.1.1 Data Compilation and Evaluation .......................... 5-4 5.5.1.3 Hydrostratigraphic Unit Assignment ........................ 5-8 5.5.1.5 Distribution of Hydraulic Conductivity with Depth ............. 5-9 5.5.2.1 Porosity Data From Tracer Migration Studies ................ 5-12

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Table of Contents (Continued) 5.5.2.3 Summary of Porosity Data ............................... 5-15 5.6.1 Data Compilation and Evaluation ................................ 5-16 5.6.2 Identification of Predevelopment Hydraulic Heads ................... 5-17 5.6.4 Site Hydrostratigraphic Unit Definition ........................... 5-20 5.6.5 Dataset and Map Generation ................................... 5-21 5.7 Recharge and Discharge ........................................... 5-21 5.7.1.2 Modified ME Method .................................. 5-24 5.7.1.2.2 Preliminary Recharge Distribution ............... 5-25 6.0 Conceptual Groundwater Flow Model ..................................... 6-1 6.1 Groundwater Flow System Boundary ................................... 6-1 6.2 Regional Groundwater Flow System ................................... 6-2 6.2.1.1.1 East of the NTS ................................ 6-5 6.2.1.1.2 North of the NTS ............................... 6-5 6.2.1.1.3 West of the NTS ............................... 6-8 6.2.1.1.4 Amargosa Desert and Death Valley Areas ........... 6-10 6.2.2 Regional Groundwater Flow ................................... 6-12 6.2.2.1 Groundwater Occurrence and Movement ................... 6-14

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Table of Contents (Continued) 6.3 Groundwater Flow System at the NTS and Vicinity ....................... 6-17 6.3.1.1 Yucca Flat and Vicinity ................................. 6-18 6.3.1.3 Pahute Mesa/Timber Mountain Caldera Complex .............. 6-24 6.3.2 Groundwater Occurrence and Movement .......................... 6-25 6.3.2.3 Pahute Mesa/Timber Mountain Caldera Complex .............. 6-29 6.4 Conceptual Model Uncertainties ...................................... 6-30 6.4.1 Geologic Model Uncertainties ................................... 6-30 7.0 Numerical Groundwater Flow Model ...................................... 7-1 7.2.1 Numerical Code Description ..................................... 7-3 7.2.3 Modeling Data Requirements .................................... 7-4 7.3.4 Hydraulic Conductivity and Vertical Conductance Calculations .......... 7-18 7.3.4.1 Hydraulic Conductivity Parameters ........................ 7-19