1. INTRODUCTION
DOE is investigating whether Yucca Mountain, Nevada, is suitable for development as a permanent repository for the disposal of spent nuclear fuel and high-level radioactive waste. The Secretary of Energy may determine that the Yucca Mountain site is suitable for development of the potential repository. This decision would be based on the ability of the natural geologic environment and the engineered barrier system (EBS) to prevent migration of radionuclides from a repository to the accessible environment, within applicable radiation protection limits proposed by the NRC and EPA. The evaluation of the SZ for the potential repository must consider the potential transport of radionuclides from their introduction at the water table below the repository to a hypothetical well located 20 km (12.5 mi), or at an alternative location specified in regulations, downgradient from the site. CRWMS M&O (2000c) provides details regarding the location of the hypothetical well and the characteristics of the hypothetical receptor, who is an average member of the critical group in the proposed NRC standard (10 CFR 63 [64 FR 8640]), or is the reasonably maximally exposed individual in the proposed EPA standard (10 CFR 197 [64 FR 46976]). The DOE will also use information about the saturated zone to demonstrate compliance with the EPA’s proposed groundwater standard (10 CFR 197 [64 FR 46976]). The most likely pathway for radionuclides to reach the accessible environment is through the uppermost groundwater aquifers below the potential repository. These aquifers, collectively referred to as the (SZ), delay the transport of radionuclides to the accessible environment and reduce the concentration of radionuclides before they reach the accessible environment. The DOE anticipates developing, after public comment, a Site Recommendation Report (SR) for the site. The SR will document the information to be considered by the Secretary of Energy in deciding whether the site is suitable and whether to recommend the Yucca Mountain site to the President for development as a geologic repository for spent nuclear fuel and high-level radioactive waste. A series of documents that include the Yucca Mountain Site Description (YMSD) (CRWMS M&O 1998d), the Total System Performance Assessment (TSPA), and a set of nine Process Model Reports (PMRs) support the SR. The PMRs summarize the technical basis for the process models supporting the TSPA. The PMRs are supported by a series of lower-level Analysis Model Reports (AMRs). The relationships among these documents are shown schematically (Figure 1-1). The nine PMRs encompass the following areas:1.1 OBJECTIVE
The main objectives of the SZ PMR are:1.2 SCOPE
This section explains the information presented in, and the content of, the SZ PMR. It uses flowcharts to show the evolution of information from data to TSPA output and the evolution of information within the SZ PMR. The section also describes where to find relevant subject matter not included in the SZ PMR. References to related discussions in Chapter 2 are provided.1.2.1 Scope of the Saturated Zone Flow and Transport Process Model Report
The purpose of the site-scale SZ flow and transport model is to describe the spatial and temporal distribution of groundwater as it moves from the water table below the potential repository, through the SZ, and to the point of uptake by the receptor of interest. The SZ processes that control the movement of groundwater and the movement of dissolved radionuclides and colloidal particles that might be present, and the processes that reduce radionuclide concentrations in the SZ, are described. Summary information from other PMRs (e.g., the unsaturated zone [UZ] PMR and the Integrated Site Model [ISM] PMR), the interface between this PMR and the TSPA document, and the conclusions of this PMR are described. The discussion of model inputs and outputs, as implemented in the AMRs, is restricted to information needed for the assessment of postclosure performance of the potential repository.1.2.2 Principal Factors and Other Factors Considered
The principal factors are those factors most important or integral to the potential RSS that supports the postclosure repository safety case. The principal factors are those central to determining and demonstrating long-term safety of the repository system. In the RSS (CRWMS M&O 2000a), 7 principal factors, and 20 Other Factors of second-order importance, have been identified. The selection of the principal factors was based on preliminary TSPA analyses and expert judgement. The 7 principal factors, the 20 other factors, and the PMR to which each factor belongs are listed in the RSS (CRWMS M&O 2000a, Table 3-1). The principal and other factors provided the focus of planning documents in support of the SR and license application (LA) decisions discussed in Volume 4 of the Viability Assessment (VA) (DOE 1998a). Since the VA, SR work has been underway following the planning documents. Four important features of the SZ were identified during the RSS (CRWMS M&O 2000a) analyses. For the site-scale SZ flow and transport model, two principal factors (retardation of radionuclide and dilution of radionuclide concentrations during migration) and two other factors (advective pathways and colloid-facilitated transport) were identified.1.2.2.1 Principal Factors
The principal factors of the post closure case are those central to determining and demonstrating long-term safety of the repository system (CRWMS M&O 2000a). The two factors given below are the only principal factors relevant to the SZ (CRWMS M&O 2000a, Table 3-1). Retardation of Radionuclide Migration–This factor describes the effect of processes that delay the transport of dissolved or colloidal radionuclides in groundwater moving through the volcanic aquifers and alluvial valley fill of the SZ. This factor focuses on radionuclides that are important to system performance but could experience substantially retarded migration in the SZ. In the current approach to the postclosure safety case:1.2.2.2 Other Factors
Advective Pathways in the Saturated Zone–This factor describes the pathways for water that might transport radionuclides in the SZ and includes the pathways through the volcanic aquifers and the valley-fill alluvium in Amargosa Valley. In general, the site-scale SZ flow and transport model is used to estimate the advective pathways below the water table. Colloid-Facilitated Transport in the Saturated Zone-This factor describes the transport of radionuclides sorbed to colloids, in moving groundwater. Preliminary analyses for the VA (DOE 1998a) indicate that colloid-facilitated transport of plutonium provided an important contribution to annual dose, and this has been confirmed by additional ongoing work by the YMP. For the postclosure safety case, this evaluation is conducted using simplifications in the representation of colloid-facilitated transport of radionuclides as explained in the CRWMS M&O 2000l.1.3 Features, Events, and Processes
An initial set of FEPs was developed for the YMP TSPA by combining lists of FEPs identified as relevant to the YMP (CRWMS M&O 1999b). This combined list contains 1,261 FEP entries from the Nuclear Energy Agency working group, 292 FEPs from YMP literature and site studies, and 82 FEPs identified during YMP project staff workshops. The FEPs were identified by a variety of methods including expert judgment, informal elicitation, event tree analysis, stakeholder review, and proposed regulations. All potentially relevant FEPs have been included, regardless of origin. In addition, the compilation includes 151 layers, categories, and headings, resulting in a final list of 1,786 FEPs that is documented in a database of FEPs (CRWMS M&O 2000x). This approach led to considerable redundancy in the FEP list because the same FEPs frequently are identified by multiple sources, but it also ensures that a comprehensive review of narrowly defined FEPs will be performed. Each FEP has been classified as either primary or secondary. This classification resulted in the identification of 310 primary FEPs for which detailed screening arguments are developed. The classification and description of primary FEPs strives to capture the essence of all secondary FEPs that map to the primary. Secondary FEPs are either FEPs that are completely redundant or that can be aggregated into a single primary FEP. This resulted in 310 primary FEPs and 1,476 secondary FEPs (total of 1,786 FEPs). The primary FEPs have been assigned to associated PMRs. The assignments were based on the nature of the FEPs so that the analysis and resolution for screening decisions reside with the subject-matter experts in the relevant disciplines. The resolution of other than system-level FEPs are documented in AMRs prepared by the responsible PMR groups. In this section, a summary of the screening decisions associated with the FEPs for the SZ PMR group is presented. Details of the screening processes are documented by CRWMS M&O (2000q). The purpose of FEPs screening is to document and justify the treatment of the primary FEPs identified as potentially affecting the SZ. The FEPs that are deemed potentially important to repository performance are evaluated, either as components for the TSPA or as separate analyses in the AMR. The scope for this activity involves two tasks:1.4 QUALITY ASSURANCE
Pursuant to evaluations (CRWMS M&O 1999d, 1999e) performed in accordance with QAP-2-0, Conduct of Activities (Superceded by AP-2.21Q, Quality Determinations and Planning for Scientific, Engineering, and Regulatory Compliance Activities), it was determined that activities supporting development of the SZ PMR and its component models and their documentation are quality-affecting activities that are subject to the QA requirements of the Quality Assurance Requirements and Description (DOE 2000). The SZ PMR was prepared in accordance with AP-3.11Q, Technical Reports, and reviewed in accordance with AP-2.14Q, Review of Technical Products. The QA procedures under which the component AMRs were developed are identified in the respective AMRs and associated planning documents. The AMRs were prepared and reviewed in accordance with AP-3.10Q, Analyses and Models. This technical product was planned in accordance with AP-2.13Q, Technical Product Development Planning, under a document development plan (CRWMS M&O 2000u), and in accordance with AP-2.15Q, Work Package Planning Summaries, under four work package planning summaries (CRWMS M&O 1999c; 1999f; 1999g; and 1999h).1.4.1 Acquired and Developed Data
The status of the acquired and developed data that support this PMR is included in the supporting AMRs and in the DIRS database. The data are incorporated in the Technical Data Management System. Data verification and qualification were carried out in accordance with procedures AP-3.15Q, Managing Technical Product Inputs, and AP-SIII.2Q, Qualification of Unqualified Data and the Documentation of Rationale for Accepted Data.1.4.2 Software
No software codes or routines were used directly in the development of the PMR. However, all software codes and routines used in the analyses and models supporting this PMR, and the quality status of those codes, are listed in the appropriate AMRs. These codes were managed in accordance with AP-SI.1Q, Software Management, or used under Section 5.11 of AP-SI.1Q. The primary software codes used in supporting the SZ models and analysis are:1.5 RELATIONSHIP TO OTHER PROCESS MODEL REPORTS AND KEY PROJECT DOCUMENTS
The overall relationship between this SZ PMR and other key project documents is shown in Figure 1-1. More specifically, the SZ PMR interfaces directly with three other PMRs: the ISM PMR (CRWMS M&O 2000e), the UZ Flow and Transport Model PMR (CRWMS M&O 2000b), and the Biosphere PMR (CRWMS M&O 2000c). The SZ PMR summarizes inputs from the ISM and UZ PMRs, and discusses outputs of the SZ PMR to the TSPA-SR calculations. In this section, a summary-level statement of purpose and a description of each of these PMRs are given. In addition, the way in which the SZ PMR interfaces and overlaps with these three PMRs is discussed.1.5.1 Integrated Site Model Process Model Report
The ISM PMR (CRWMS M&O 2000e) describes the overall framework for discussing the geologic properties (e.g., stratigraphy, structural characteristics, rock properties, and mineralogic properties) of the Yucca Mountain site. The ISM, which incorporates three other models, merges details of the Yucca Mountain geology into a single model that can be used for subsequent simulation (e.g., hydrologic flow and radionuclide transport models) and design of the potential repository. The ISM PMR summarizes the outputs that are input to the UZ flow and transport model, the SZ flow and transport model, the tectonic hazards analysis, and the EBS design. The three components of the ISM are the geologic framework model (GFM), the rock properties model (RPM), and the mineralogic model (MM). The GFM is a description of the distributions of rock layers and faults in the subsurface at Yucca Mountain, and it is the framework into which rock properties and mineralogic distributions are placed, and thus serves as the framework of the ISM. The GFM is a 3-D interpretation of the geology surrounding the location of the potential repository, an area of 170 km2 (66 mi2) and a volume of 771 km3 (185 mi3) (Figure 1-3). The boundaries of the GFM were chosen to encompass a widely distributed set of exploratory boreholes and the area of interest for hydrologic flow and radionuclide transport modeling through the UZ. The depth of the model is constrained by the estimated depth of the Tertiary-Paleozoic unconformity that reaches a depth of 3,962 m (13,000 ft) in the area considered by the model. The GFM was constructed primarily from geologic map and borehole data, but information from measured stratigraphy sections, gravity profiles, and seismic profiles were incorporated where appropriate. The GFM is incorporated into the hydrogeologic framework model (HFM) for the site-scale SZ flow and transport model, which encompasses a much larger volume than the GFM. The hydrogeologic framework development is discussed in the HFM AMR (USGS 2000a). The HFM provides a representation of the spatial distribution of hydrologic properties for the 3-D site-scale SZ flow and transport model domain. This representation, in turn, is founded on the underlying geologically defined stratigraphic and structural framework. The RPM is a description of the rock material properties including matrix porosity, whole-rock bulk density, matrix-saturated hydraulic conductivity, lithophysal porosity, and whole-rock thermal conductivity for most of the stratigraphic intervals described in the GFM. The RPM results in a spatial distribution of the rock properties based on the locations and uncertainty in measured values within the UZ at Yucca Mountain. Values of matrix porosity and bulk density from the RPM are incorporated in the site-scale SZ flow and transport model. The MM is a 3-D weighted, inverse distance model that enables project personnel to calculate mineral abundance at any position, within any region, or within any stratigraphic unit in the model volume to support the analyses of hydrologic properties, radionuclide transport, mineral health hazards, potential repository performance, and potential repository design. The MM is referenced to the stratigraphic framework defined in the GFM and was developed from mineralogic data obtained from boreholes. The MM supports the analyses of hydrologic properties, radionuclide transport, mineral health hazards, potential repository performance, and potential repository design.1.5.2 Unsaturated Zone Flow and Transport Process Model Report
The UZ flow and transport PMR (CRWMS M&O 2000b) describes the processes affecting the amount of water entering and flowing through the UZ above the potential repository, contacting wastes in the potential repository, and the movement of water with dissolved radionuclides or colloidal particles through the UZ below the potential repository. The purpose of the model is to describe the spatial and temporal distribution of water flow through the UZ and the spatial and temporal distribution of water seepage into the underground openings of the potential repository. The UZ flow and transport PMR also describes inputs from other PMRs and outputs from the UZ flow and transport model to the SZ flow and transport model. The emphasis of the discussion of model inputs and outputs is on information needed for the assessment of postclosure performance. The SZ flow and transport model receives inputs of spatial and temporal distribution of recharge and radionuclide mass from the UZ flow and transport model for the purpose of TSPA calculations. Recharge boundary conditions for the SZ model (UZ domain) are specified based on the groundwater flux simulated at the base of the UZ model. Coupling of radionuclide transport between the UZ and the SZ is accomplished using the convolution integral method. The convolution integral method is used to combine the unit breakthrough curves calculated by the SZ flow and transport model with the time-varying radionuclide sources from the UZ.1.5.3 Biosphere Process Model Report
The Biosphere PMR (CRWMS M&O 2000c) describes the processes affecting the movement of radionuclides after they have been released from the geosphere to the environment. The biosphere model describes the lifestyle and habits of individuals living in present-day Amargosa Valley and produces dose conversion factors that are used by TSPA to estimate the annual dose to which those persons might be exposed. The model considers three basic exposure pathways (inhalation, ingestion, and external exposure) by which radionuclides could travel from groundwater (via a well that is located 20 km [12.5 mi] from Yucca Mountain) to a receptor. Radionuclides considered include those expected to be responsible for contributing the most to human dose. The environmental pathways, radionuclide inventory, and unit concentrations (1 pCi/L) of these radionuclides are used to calculate biosphere dose conversion factors. Biological dose conversion factors based on unit concentrations permit researchers to calculate doses expected to be received from any concentration.1.5.4 Relationship with Other Key Yucca Mountain Project Documents
Proposed regulatory requirements and SR/LA design feed the allocation analysis and the RSS (CRWMS M&O 2000a). The RSS determines the most important factors (principal factors) that affect performance of the potential repository. The RSS provides input to the AMRs and the SZ PMR; the AMRs also feed the SZ PMR. The SZ PMR, along with eight other PMRs, provide input to the TSPA and the YMSD.1.6 Issues for Saturated Zone Flow and Transport
1.6.1 Summary of Current Understanding of Saturated Zone Flow and Transport at Yucca Mountain
The SZ PMR considers many parameters, processes, and models that describe the flow of water and transport of materials from under Yucca Mountain to the proposed compliance boundary in the Amargosa Valley. An understanding of SZ flow and transport has been gained through the collection of site data and the modeling of relevant processes. Hypotheses that reflect the current understanding of flow and transport features and processes are summarized in Table 1-4, and pointers are provided from each statement to the detailed discussions in relevant sections of the PMR. The statements listed in the "Current Understanding" column of Table 1-4 are a mixture of observations, hypothesis, and modeling insight and only should be considered brief summaries of complex information. The YMP has received extensive and detailed input from internal and external peers, experts, and oversight groups concerning the credibility and defensibility of the YMP in general and the TSPA prepared for the VA (DOE 1998b). Many of the issues concern how the YMP is addressing SZ flow and transport (Appendix A). Issues were identified, and a workshop was held in February 1999 (Kuzio 1999) to identify how these issues would be addressed in future iterations of the TSPA. The YMP also must address the KTIs that have been identified by the NRC as the basis for acceptance criteria for a LA. These issues, and others raised at and since the February 1999 workshop, are discussed and identified in the following sections and in Appendices A and B.1.6.2 Issues from the Total System Performance Assessment-Viability Assessment
The data and analyses in the TSPA-VA (DOE 1998b) provide part of the information needed to evaluate performance of the potential repository. The analyses include an assessment of model enhancements and analyses that could improve future assessments of performance of the potential repository, including the representation of SZ flow and transport. Issues that were identified as having the potential to enhance confidence in the future assessments of SZ flow and transport, and suggestions that were made for additional work, are summarized in this section. Issues for SZ flow and transport that were recognized in the TSPA-VA (DOE 1998b) include:1.6.3 Issues from the Total System Performance Assessment-Viability Assessment Peer Review
The YMP convened a peer review panel to provide a formal, independent evaluation and critique of the TSPA-VA for the potential high-level waste repository at Yucca Mountain (DOE 1998b). The objectives of the panel were to describe the technical strengths and weaknesses of the TSPA-VA and to provide suggestions for its improvement. The panel issued three interim reports prior to the completion of the TSPA-VA and a final report (Budnitz et al. 1999) that was based on the completed TSPA-VA (DOE 1998b), its supporting Technical Basis Document (CRWMS M&O 1998e), and on documents cited as references to the TSPA-VA. Issues developed by the panel regarding SZ flow and transport are tabulated in Appendix A, and the sections of this document and other documents that provide information to address these issues are identified.1.6.4 Issues from the Saturated Zone Flow and Transport Expert Elicitation Project
The DOE sponsored the SZ Flow and Transport Expert Elicitation Project (CRWMS M&O 1998b) and asked experts to characterize the knowledge and uncertainties associated with certain key issues related to the SZ system in the Yucca Mountain area and the downgradient region. A major goal of the project was to capture the uncertainties involved in assessing the SZ flow processes, including uncertainty in models used to represent the physical processes controlling SZ flow and transport, and the parameter valuesused in the models. To capture a wide range of perspectives in the analyses, multiple individual judgments were elicited from members of an expert panel. The panel members, experts from within and outside the YMP, represented a range of experience and expertise. During the elicitation process, the experts identified a range of issues regarding the SZ. These issues are tabulated (Appendix A), and the sections of this document and other documents that provide information to address these issues are identified.1.6.5 Issues from the Advisory Committee on Nuclear Waste
The NRC established the Advisory Committee on Nuclear Waste to provide independent reviews of, and advice on, topics that include disposal of high-level radioactive wastes in geologic repositories. Transcripts and summaries of transcripts from Advisory Committee on Nuclear Waste meetings for the period from December 1997 to December 1999 were examined to identify SZ flow and transport issues. These issues are tabulated in Appendix A, and the sections of this document and other documents that provide information to address the concerns are identified.1.6.6 Issues from the Nuclear Waste Technical Review Board
The NWTRB was created by Congress in 1987 to review DOE scientific and technical activities pertaining to the management and disposal of commercial spent nuclear fuel. The activities reviewed include characterizing Yucca Mountain as a potential repository site as well as packaging and transporting commercial spent nuclear fuel and defense high-level wastes. The NWTRB monitors the YMP to ensure that site characterization is technically sound and scientifically credible. The NWTRB reports to Congress on issues involved in characterizing Yucca Mountain, and points out the concerns of outside parties that are of interest to the scientific community. Transcripts, and summaries of transcripts, from NWTRB meetings, NWTRB reports to Congress, and NWTRB letters to the Office of Civilian Radioactive Waste Management for the period from December 1997 to December 1999 were examined to identify SZ flow and transport issues that were developed during the meetings and provided to the Office of Civilian Radioactive Waste Management or Congress. These issues are tabulated in Appendix A, and the sections of this document and other documents that provide information to address the concerns are identified.1.6.7 Issues from the U.S. Nuclear Regulatory Commission
The NRC has identified KTIs that are considered most important to performance of the potential repository. The NRC has developed IRSRs that provide criteria for evaluating each of these KTIs. Evaluation of the criteria in these IRSRs indicate that the following KTIs have aspects pertaining to SZ flow and transport:2. EVOLUTION OF THE SATURATED ZONE PROCESS MODEL
In this chapter, the evolution of data-collection activities and the development of models for describing the characteristics, understanding the processes, assessing the viability of Yucca Mountain as a potential site for a nuclear waste repository, and key issues identified by overseeing bodies, peer review groups, and others are summarized. The Yucca Mountain Project has evaluated the Yucca Mountain site for over two decades. Data-collection activities have evolved from intensive surface-based investigations in the early 1980s to the current focus on underground drift tests, the SZ testing complex (C-wells Complex), and the SZ Alluvium Testing Complex. The models have evolved from early conceptual descriptions of the site to current site-scale SZ flow and transport model representations. In Section 2.1, an overview and discussion of the approach to developing the SZ PMR are presented. In Section 2.2, specific activities associated with geologic mapping, hydrologic studies, geochemical sampling, and hydraulic and tracer testing are described. In Section 2.3, the evolution of the SZ flow and transport modeling, and previous modeling at the regional, site, and sub-site scales are discussed. In Section 2.4, previous TSPA modeling (including previous SZ TSPA abstraction), coupling with other components of TSPA, a summary of the TSPA-Viability Assessment (VA) modeling results, and improvement to the TSPA-VA model are discussed. In Section 2.5, an overview of the current site-scale SZ flow and transport model is presented.2.1 OVERVIEW AND DEVELOPMENT APPROACH
Each PMR describes the information used to develop a process-level model. The PMRs summarize the technical bases that support the TSPA model. In this role, the PMRs identify, document, and describe the information needed to demonstrate postclosure performance. The process used to develop PMRs ensures that each PMR provides transparency and traceability of data, information, and references that relate to the process model and support the TSPA. The Nuclear Waste Policy Act of 1982 was amended in 1987 to focus effort on the Yucca Mountain site. A site characterization plan was completed in 1988 for systematic surface-based investigations, underground testing, laboratory testing, and modeling activities (DOE 1988). The Yucca Mountain Site Description (CRWMS M&O 1998d) summarizes the results of the site characterization program up to 1998. The Viability Assessment of a Repository at Yucca Mountain (DOE 1998a) applies the available data to, and discusses the uncertainties in, a TSPA. This SZ PMR, together with eight other PMRs and associated analysis and model reports, is being developed for updating the TSPA and for supporting the Site Recommendation and possibly the License Application (LA) with traceable and verifiable data. The VA and the Repository Safety Strategy (CRWMS M&O 2000a) identify four attributes for safe disposal: limited water contact with waste package, long waste package life time, low rate of radionuclide release from breached waste package, and reduction in radionuclide concentration during transport. Retardation of radionuclides and dilution of radionuclide concentrations during migration are two principal factors evaluated by this SZ PMR for assessment of the potential repository performance and safety. In addition, the SZ PMR includes the SZ flow and transport processes from the water table beneath the potential repository to the accessible environment at the proposed compliance boundary.2.2 Site Characterization and Data Collection
Prior to investigations of Yucca Mountain as a potential geologic repository, information on the geology and hydrology of the area was limited to surface geologic mapping and hydrologic investigations of localized groundwater development areas such as in the Amargosa Desert and Oasis Valley (Figure 2-1). The only subsurface information available was from wells J-12 and J–13, located on the Nevada Test Site (NTS) near Fortymile Wash, that were drilled in the 1960s to supply water to NTS facilities in Jackass Flats (Figure 2-2). Most of the hydrologic information about the Yucca Mountain site has been obtained from studies conducted since 1978. During the first few years of these studies, the emphasis was on describing the hydrogeology of the SZ. As additional data became available, consideration was given to locating the potential repository within the unsaturated zone (UZ), which became a new focus of site characterization activities. Beginning in 1981, hydrologic test holes, some as deep as 1.8 km (1.1 mi), were drilled into the SZ at Yucca Mountain. The holes were logged to determine lithology and stratigraphy of the rocks that were penetrated, and tested to determine hydrologic parameters such as depth to water, total water yield, water yield as a function of stratigraphic horizon, hydraulic conductivity, transmissivity, water chemistry, and apparent carbon-14 ages of some of the waters. The upper kilometer or more of the SZ penetrated by the wells consists of extensively fractured volcanic tuffs that appear to derive most of their permeability from fractures rather than from the porosity of the matrix. The borehole-drilling program at Yucca Mountain originally was designed to systematically characterize the subsurface geology and hydrology of the potential repository site area. Limitations were imposed by project directives on where boreholes could be drilled and by accessibility to drilling equipment. Drilling within the potential repository block was limited. Therefore, a plan was devised that would allow for the characterization of the potential repository block by constructing boreholes around the periphery of the block. Subsurface geologic and hydrologic information from these boreholes would be used to construct cross sections to interpret the subsurface geology and hydrology of the potential repository site area. Beginning in 1978 with the drilling of borehole UE-25a#1, 33 boreholes have been drilled in the Yucca Mountain area that penetrate the SZ; numerous other boreholes terminate in the UZ. The borehole designations indicate the general area, purpose of the hole, and general sequence of drilling. All boreholes were geologically and geophysically logged. The geophysical logs are identified in the Yucca Mountain Site Description (CRWMS M&O 1998a, Table 5.3-11). Cores were taken continuously or intermittently for laboratory analysis in 20 boreholes. Cores were analyzed for bulk density, porosity, particle density, volumetric water content, saturation, water potential, and saturated hydraulic conductivity (CRWMS M&O 1998a, Table 5.3-14). Groundwater samples were collected from all boreholes and analyzed for major-ion content and isotopic content (generally deuterium, oxygen-18, tritium, carbon-13, and carbon-14). Recently the county government of Nye County, Nevada carried out additional borehole drilling in cooperation with the DOE. This drilling program, termed the Nye County Early Warning Drilling Program, generally is located along Highway US 95 south of Yucca Mountain (Figure 2-3). Although the program still is in progress and data continue to be analyzed, some data have been included in Civilian Radioactive Waste Management System Management and Operating Contractor reports. Borehole location coordinates and water-level measurements are presented in USGS (2000b), chemical analyses are presented in CRWMS M&O (2000m), results of hydraulic tests in three boreholes are given in CRWMS M&O (2000n), and laboratory-derived transport properties (sorption coefficients and bulk density) are given in CRWMS M&O (2000r).2.2.1 Hydraulic Testing
More than 150 individual hydraulic tests have been conducted on boreholes on and around Yucca Mountain. Almost all of these were single-borehole tests that were located in specific depth intervals and may have included single-well, constant–discharge pumping test; slug-injection (falling-head) tests; pressure-injection tests; temperature surveys; and tracer injection flow surveys. Multiple-well pumping tests were conducted only at the C-wells Complex (Luckey et al. 1996, p. 32).2.2.1.1 Single-Borehole Tests
Transmissivity values were estimated from tests for the hydrogeologic units (upper volcanic aquifer, upper volcanic confining unit, lower volcanic aquifer, lower volcanic confining unit, and carbonate aquifer), and apparent hydraulic conductivity values were calculated from transmissivity. Apparent hydraulic conductivity values were based on reported single-borehole hydraulic tests and generally were calculated by dividing the reported transmissivity of the tested interval by the saturated thickness in the borehole. Hydraulic conductivity values for individual intervals in a borehole could vary by several orders of magnitude depending on whether or not water-bearing fractures were present. In intervals that contained no open fractures, hydraulic conductivity tended to be low and reflected the hydraulic conductivity of the rock matrix or of small fractures. In intervals that contained water-bearing fractures, the apparent hydraulic conductivity values may be somewhat misleading. Most, if not all, of the water produced in such an interval could have been produced by a few thin, highly conductive fractures in an otherwise thick, essentially nonproductive rock matrix (Luckey et al. 1996, p. 32). Flow surveys were conducted in most of the deeper boreholes at Yucca Mountain. Flow surveys are useful to determine the intervals of the borehole, and possibly the fractures, that produce water. Most of the flow surveys were conducted using a tool developed for oil-field use that releases small quantities of radioactive iodine-131. As the iodine moves up or down the borehole, it is sensed by gamma-ray detectors. Most surveys were conducted while water was being pumped from, or injected into, the borehole. Static tests also were conducted occasionally. Flow surveys are useful in determining the parts of the borehole that produce (or accept) most of the flow. This information is useful when subdividing a system into aquifers and confining units and for determining the location of, and spacing between, flowing intervals.2.2.1.2 Multiple-Well Tests
C-Wells Complex–In 1983 and 1984, three boreholes (UE-25c#1, UE-25c#2, and UE-25c#3; collectively called the C-wells Complex) were drilled to conduct aquifer and tracer tests. The C-wells Complex is located at the northern end of Bow Ridge on the west side of Midway Valley. This complex consists of three orthogonally spaced boreholes that are 30.4 to 76.6 m (99.7 to 251.3 ft) apart at the land surface, and each was drilled to a depth of 914 m (3,000 ft). Below the water table, which is 400 to 402 m (1,312 to 1,319 ft) deep at the site, the C-wells penetrate the Calico Hills Formation, and the Prow Pass, Bullfrog, and Tram Tuffs of the Crater Flat Group. In 1983 and 1984, 16 falling-head slug tests and 9 pressure-injection tests were conducted in UE-25c#1. A constant-head injection test was conducted in UE-25c#2 and was later converted into a constant-flux injection test. Two unsuccessful pumping tests were attempted in UE-25c#1. One pumping test was conducted in UE-25c#2, and two pumping tests were conducted in UE-25c#3. Testing resumed in 1995, and hydraulic tests were conducted in May 1995, June 1995, February 1996, and May 1996 to November 1997. In all of these tests, borehole UE-25c#3 was used as the pumping well, and boreholes UE-25c#1, UE-25c#2, ONC-1, USW H-4, UE-25 WT#3, and UE-25 WT#14 were used as observation wells. Hydraulic tests conducted at the C-wells Complex from 1995 to 1997 were designed to:2.2.2 Water Level Monitoring Program
Drilling for site characterization at Yucca Mountain began in 1978, but the first hydrologic test well was completed in 1981. Although water levels were measured as each well was completed, water level monitoring, in terms of a long-term water level network, did not begin until the initiation of regularly scheduled periodic water level measurements during 1983. Periodic measuring of water levels continues through the present and generally is conducted monthly using calibrated steel tapes or electronic cable units. The continuous measurement of water levels was accomplished using pressure transducers, and water level data was collected every hour. Though referred to as continuous measuring of water level data, data loggers were programmed to receive transducer pressure hourly. This frequency of data collection is sufficient to document daily, monthly, and yearly water level changes and trends. However, to detect water level fluctuations due to seismic events, data loggers that record all transducer pressure changes were installed in several wells during 1992, and data were collected from these wells until 1996. Since 1981, water-level data in the Yucca Mountain area have been collected and reported from 33 wells that monitor 41 depth intervals. Several wells monitor more than one depth interval. These intervals are isolated by inflatable packers or cement plugs.2.2.3 Data Reduction and Analysis
Data resulting from the SZ field and laboratory testing program were reduced and analyzed, and the results were summarized by Luckey et al. (1996, pp. 16 to 47). These data were collected and analyzed to reduce the uncertainties in the conceptual model. The key uncertainties, in descending order, include recharge, storage properties of SZ materials, transmissive properties, discharge, and hydraulic head (Luckey et al. 1996, p. 53). Details of the data and model uncertainties are discussed in Section 3.5.2.2.4 Modeling
Numerical models have been developed at various scales to simulate groundwater flow in the vicinity of Yucca Mountain. As data have been accumulated, and as modeling technology has advanced, the models have been refined and improved. Reviews of the principal SZ flow modeling efforts are presented in Luckey et al. (1996, pp. 6 to 7) and NRC (1999a, Section 4.5.2.14, pp. 141 to 150), and are summarized in Section 2.3. Flow and transport models are linked to other process models in a TSPA to construct a computer model for various aspects of the potential repository system and the biosphere that are important to an assessment of the overall performance of the potential repository system. A summary of previous TSPA modeling efforts is provided in Section 2.4.2.3 Previous Saturated Zone Modeling
Yucca Mountain lies within the Death Valley regional flow system (Figure 2-1). Groundwater flow models that approximately encompass this entire region or that are of larger extent are here termed regional models, whereas groundwater flow models of a sub-region of this area are termed sub-regional models. Models encompassing an area of only few hundred square kilometers are called local or site models; and those for smaller scale are called sub-site models. Boundaries of the regional-scale (D’Agnese et al. 1997) and site-scale SZ flow and transport models are shown in Figure 2-1. Regional-scale SZ models use a coarser grid to describe the flow system parameters and are generally less accurate in predicting the hydraulic head than sub-regional models, but are important in describing the boundary conditions for sub-regional models and in understanding the overall hydrological conditions of the area. Local or site models are important for making transport calculations because transport depend on fine scale permeability variations. An overview of regional (Table 2-1), sub-regional (Table 2-2), and site and sub-site (Table 2-3) scale models that were used to simulate SZ flow in the Yucca Mountain area is provided, with emphasis on the assumptions of each model. The technology of groundwater flow simulation has changed since the first flow models were applied at Yucca Mountain. Most of the models listed in the tables simulate the Yucca Mountain SZ flow system under present climatic and geologic conditions. The models are important because they improve understanding of the flow system and parameters of the area. Some of the models are used to investigate changes to the water table and hydraulic gradient due to possible climate change or disruptive geologic events. Changing climatic conditions may result in a change in the water table beneath Yucca Mountain. Possible disruptive geologic (tectonic or volcanic) conditions may result in an increase or decrease in the water table. An increase in the water level under Yucca Mountain is of concern because it will decrease the UZ barrier between the potential repository and the SZ, and it may increase the hydraulic gradient resulting in shorter radionuclide transport times through the SZ to the downgradient receptors. Oberlander (1979) performed the first numerical modeling of the Yucca Mountain area by modeling a two-dimensional (2-D) cross section between Pahute Mesa and Yucca Flat. Flow in the unmodeled third dimension was adjusted with a "correction" matrix. Oberlander (1979) estimated the flow rate through Paleozoic rocks beneath Yucca Mountain and found an upper bound to the flow from Yucca Flat to Ash Meadows. Waddell (1982) used a 2-D steady-state finite element model to simulate the regional-scale NTS groundwater flow in the SZ. The main goals were to estimate flow fields (to be used for transport predictions of radionuclides) and to study the effects of model parameters uncertainty on these estimates. The model encompassed an area of approximately 175-km by 175-km (109 mi by 109 mi) with boundaries along topographic highs to the north and northeast and along topographic lows to the southwest. The model was calibrated by adjusting transmissivities, recharge, and discharge to minimize the weighted sum of the squared head residuals (observed head minus simulated head). Most of the absolute head residuals were less than 30 m (98 ft) in the final model. The model estimated recharge in most areas, but considered discharge in the Ash Meadows area as a known constant and modeled other areas using a constant head condition. The results from this model substantiated conclusions of some earlier conceptual models of the site. The model was not accurate in the area of Pahute Mesa, possibly due to vertical flow effects. The model was able to identify areas in which hydrologic properties were key in defining the flow direction and magnitude of adjacent regions. For example, hydrologic properties in the Eleana Formation had a major effect on the flow beneath Timber Mountain and Jackass Flats, while hydrologic properties in Fortymile Canyon and Fortymile Wash affected fluxes beneath Jackass Flats and Yucca Mountain. Groundwater barriers to the north of Yucca Mountain, recharge on Pahute Mesa, and underflow from regions north of the Pahute Mesa had a significant impact on the model. The model also identified areas in which Waddell (1982) felt that no further study was needed. Czarnecki and Waddell (1984) developed a sub-regional model for the area near Yucca Mountain that was about one-third of the size of the Waddell (1982) regional-scale model. Some of the boundaries coincided with the Waddell (1982) model. On other boundaries, specified heads or fluxes were taken from the Waddell (1982) model. The subregional model was used to obtain a better understanding of the groundwater flow system beneath Yucca Mountain and for later use in determining the change in the hydraulic head due to increased recharge under assumed future climatic conditions. The model provided a good match to observed hydraulic head data except in areas where vertical flow components were present, such as Franklin Lake Playa (located about 8 km (5 mi) west of Ash Meadows) and where there were steep gradients such as directly north of Yucca Mountain. This model showed that groundwater tends to flow from north to south. In the region of the potential Yucca Mountain repository, groundwater flow tends to be southeasterly and then southerly. Rice (1984) constructed a 2-D, regional-scale, steady-state model covering an area approximately the same as that of more recent model by D’Agnese et al (1997). The objectives of the model were to understand the distribution of recharge, discharge, and hydraulic head within the model area. The hydraulic head distribution obtained from this model generally is in agreement with other regional-scale 2-D models. Czarnecki (1984) used the Czarnecki and Waddell (1984) 2-D model to assess potential effects of changes in future climatic conditions in the area of Yucca Mountain on the water table below the potential repository and the surrounding flow field. Czarnecki (1984) found that the simulated position of the water table rose by as much as 130 m (426 ft) in response to a 100 percent increase in precipitation over current conditions. A 100 percent increase in precipitation resulted in a factor of 15 increase in the model recharge rate. Czarnecki (1984) further found that changes in the flow direction at Yucca Mountain would be small and that the magnitude of the groundwater flux would increase by a factor of 2 to 4 over that found in simulations, assuming present-day climate conditions. Czarnecki (1990) also used this model to examine water level changes due to pumping wells in Franklin Lake Playa. Sinton (1987) modeled the same area as Waddell (1982), but with a quasi-three-dimensional (3-D), steady-state, finite-difference model. The two-layer model included a shallow upper layer of volcanic, alluvial, and carbonate rock over a deep lower layer of carbonate rock. Vertical flow was allowed between layers through a transmissive leaky unit. A sensitivity analysis was performed, and the flow system was found to be particularly sensitive to transmissivity values in the Crater Flat and Amargosa Desert areas. Haws (1990) constructed a 2-D, vertical, steady-state, flow model along an assumed flow line extending from Timber Mountain to the north of Yucca Mountain, southward to Alkali Flat (Figure 2-2). An objective of this model was to examine vertical flow between the volcanic and carbonate aquifers. Results of the model suggest that upward leakage from the deep carbonate aquifer to the shallow volcanic aquifer must occur in order to maintain the water table at its observed elevation. Carrigan et al. (1991) investigated the effects of earthquakes and dike intrusions on water table height below Yucca Mountain using a 2-D finite element flow model coupled with a boundary element solver used to predict volumetric strain and displacement. The possibility of high vertical-permeability anisotropy, and modeling of the UZ, were considered in the model. A number of tectonic scenarios were considered and worst-case scenarios produced excursions in the water table elevation of less than 20 m (66 ft) (Carrigan et al. 1991, p. 11). The results were of similar magnitude to observed excursions after large earthquakes. An earlier study by Barr and Miller (1987) examined the effect of increases in vertical leakage (either from the surface or from the underlying carbonate aquifer) on flow and transport in the local area surrounding Yucca Mountain. Ahola and Sagar (1992) studied the effect of increased recharge due to climate change and volcanic and tectonic scenarios on the water table level below Yucca Mountain using regional-scale and sub-regional models. They used a regional-scale transient 2-D model in which the SZ was modeled as a free surface (unconfined aquifer) that covered the same area as that of Rice (1984). The parameters were modified to incorporate the coarser grid used by Ahola and Sagar (1992). Ahola and Sagar (1992, pp. 6-1, 6-2, and 6-5) found water table increases of about 45 m and 87 m (148 ft and 285 ft) resulted from increases in regional recharge by factors of 10 and 20, respectively. The transition time to change from the initial steady-state water level to the perturbed steady state was about 400 to 700 yrs for these simulations. This time increment does not consider the lag time for infiltration through the UZ, which will retard the onset of the climate change response and increase the transition time. An approximately linear relation between the water level rise and the factor increase in recharge was shown. This study did not consider the possibility of additional discharge (due to streams) that was produced by rises in the water table or the possibility of increased evaporation, and it was performed before the benefit of an extensive climate change analysis was available. Ahola and Sagar (1992) also used a sub-regional model to investigate possible changes to the water table due to a reduction of hydraulic conductivity produced by an intrusion of a volcanic dike downgradient from the potential repository or by an increase in hydraulic conductivity due to increased fracturing of the rock from tectonic activity in the postulated low conductivity region north and northeast of the potential repository. These simulations yielded water table increases from a few meters to 275 m (902 ft) (Ahola and Sagar 1992). The greatest water table rise resulted from a scenario where tectonic activity caused the hydraulic conductivity to increase by three orders of magnitude north and northeast of Yucca Mountain. Prudic et al. (1993) developed a two-layer, pseudo 3-D, steady-state model of the carbonate rock province of the Great Basin (covering much of eastern Nevada and western Utah). This model was used to simulate regional-scale flow in the carbonate rocks that, in the vicinity of Yucca Mountain, underlie Tertiary volcanic rocks. This model suggests that flow in the carbonates is from north to south within the domain of the site-scale SZ flow and transport model. D’Agnese et al. (1997) developed a three-layer 3-D steady-state flow model of the SZ for the Death Valley region. This model incorporated large quantities of data from the Yucca Mountain and NTS sites compiled over the past 30 yrs. Geological data, including descriptions of important faults, were considered. Ten hydrogeologic units (a hydrologic unit is an area with distinct hydrological and geological properties and considerable lateral extent) were described for the area. Well logs from over 700 wells and cross sections were used to define a 3-D framework model of the individual hydrogeologic units, and a 3-D grid was generated taking into account the shape of these units. The numerical model grid consisted of 163 rows, 153 columns, and 3 layers. The row and column grid dimension was 1,500 m (4,900 ft), and the depth to the bottom of each of the three layers was 500 m (1,640 ft), 1,250 m (4,100 ft), and 2,750 m (9,020 ft), respectively from the water table surface. Hydraulic conductivity values were assigned to specific grid blocks in the D’Agnese et al. (1997) model using data published by Bedinger et al. (1989), and conductivity was assumed to decrease with depth. Initial recharge in the area was set using a modification of the Maxey and Eakin (1950) method. Discharge due to evapotranspiration at the Death Valley saltpan was modeled using a constant head boundary at the southwest boundary. All other boundaries were set as no-flow except for four locations in the northern part of the bottom model layer. Other sources of discharge include springs, wet playas, and irrigation pumpage. Discharge from these areas was estimated from available data. During the initial calibration process, hydraulic conductivities and recharge parameters were set to four classes for parameter estimation. Nonlinear regression was used to adjust uncertain model parameters by minimization of the squared residuals based on the difference between the observed and computed spring flows and heads. During calibration, the estimated parameter set was expanded to include layer anisotropy in hydraulic conductivity, the evapotranspiration rate factor, spring conductance, and groundwater pumpage. The initial conceptual model, permeability zone, boundaries, number of conductivity zones, boundary conditions, and recharge were changed in the calibration process. A map of residuals and weighted head residuals (weighted residuals are the residual weighted by the expected error at a location) showed reasonable agreement between computed and measured head data in the upper model layer, except in the northwest and northeast portions where data were sparse and of poorer quality. In the region extending from the Amargosa Valley to the Pahrump Valley, the simulated hydraulic gradient was somewhat higher than the observed hydraulic gradient. Most of the computed spring flows were less than the observed flows. However, the overall water balance was comparable to that estimated from data. A parameter sensitivity study indicated that flow was most sensitive to the highest recharge zone and the high conductivity zones prevalent in the area where wells are concentrated in the upper aquifer layer. More recent SZ flow modeling efforts were undertaken to evaluate the impact of long-term future climate forecasts on regional flow in the Yucca Mountain area. Analysis by USGS (2000b) forecast monsoonal and glacial-transition climates during the next 10,000 years. D’Agnese et al. (1999) uses the model developed in D’Agnese et al. (1997) to evaluate the effect of full-glacial and global-warming climates on the regional flow at Death Valley, which includes Yucca Mountain. One result of this model was that under the assumption of past (full-glacial) climate, the water table rises but maintains a shape similar to that found under present climate conditions. Assuming past full-glacial conditions, D’Agnese et al. (1999, p. 2) found that simulated water levels rose between 60 m (197 ft) and 150 m (492 ft). Assuming future global-warming conditions in which atmospheric carbon dioxide doubles, the forecast recharge in the Yucca Mountain area was greater than present, but less than the full-glacial model. In this case, the simulated water level beneath Yucca Mountain was estimated to rise less than 50 m (165 ft). A recent study by Czarnecki et al. (1997) uses the 3-D code FEHM (Zyvoloski et al. 1997a) to model flow in the SZ at the site scale. The model incorporates 16 zones and is calibrated using observed water levels at wells and approximations of the flux at the boundaries. A large gradient area at the north of the site is modeled by assuming a barrier of very low conductivity perpendicular to the high gradient area. The model boundaries were set to be coincident with model elements of D’Agnese et al. (1997). This model used a nonlinear regression to estimate subsets of parameters. Estimated permeability values differed from those measured values at the C-wells Complex, and the groundwater flux at the southern end of the boundary was twice (Czarnecki et al. 1997, p. 103) that found in the D’Agnese et al. (1997) regional-scale model. A sub-site-scale SZ model was used to simulate 3-D steady flow in a 150-km2 (58-mi2) area including Yucca Mountain (Cohen et al. 1997). These efforts simulated the detailed geologic structure and included major faults as independent elements. The effects of various vertical fault property scenarios (for faults in the Bullfrog Tuff) were tested through the transport of a tracer. Minimal transverse and vertical dispersion was observed when the faults are assumed to have high permeability, and considerable mixing occurred when the faults were assumed to have low permeability. A bifurcation of the tracer was observed when the faults were assumed to be displacement-only. A scale analysis was performed that showed that natural heat convection in high permeability units in the Bullfrog Tuff would occur on the order of 20 yrs, which implies that heat flow modeling could be important in transport. Lehman and Brown (1998) developed another sub-site-scale model. This 2-D model represents an alternate conceptual view on flow through the SZ. The model assumes a high conductivity contrast between wide fault zones and matrix rock, and it also assumes that these fault zones are long in extent. As expected under these assumptions, water moved quickly through the SZ. A large number of viewpoints have been presented by a variety of researchers, agencies, and organizations, and many of these viewpoints are represented in the site-scale SZ flow and transport model. This diversity of input provided improved understanding of the hydrology of the site and the expected response of the potential repository to varying climate and disruptive geologic scenarios.2.4 Previous TOTAL SYSTEM PERFORMANCE ASSESSMENT Modeling
In this section, a summary of the manner in which the transport of radionuclides in the SZ was handled in previous TSPA analyses, and some of the results of those analyses, are presented. Implications of the present SZ methodology relative to previous approaches are discussed.2.4.1 Total System Performance Assessment Modeling Before the Total System Performance Assessment-Viability Assessment
Initial performance assessments of a potential repository at Yucca Mountain (e.g., Sinnock et al. 1984; Sinnock et al. 1986; Barnard and Dockery 1991) either completely ignored or gave only cursory attention to the SZ by using simple 1-D approximations to the SZ. At the time, estimates of radionuclide transport through the UZ typically were more than 10,000 yrs, while the estimates of transport through the SZ were on the order of 1,000 yrs. In addition, the EPA radiation protection standard that guided post-closure PA for Yucca Mountain until 1992, 40 CFR 191 (50 FR 38066), used a metric of cumulative releases of radionuclides to the accessible environment over a period of 10,000 yrs. A cumulative release of a radionuclide is the total amount of that radionuclide that crosses a boundary over a given time period. The proposed boundary to the accessible environment was specified at a location that is 5 km (3 mi) from the potential repository, and the area within the boundary was called the controlled area. Because the transport time in the UZ appeared to be greater than the regulatory period, and because the transport time in the SZ appeared to be small compared to that in the UZ and the regulatory period, any need for detailed studies of the SZ were unnecessary. For the first TSPA, TSPA-1991 (Barnard et al. 1992, pp. 4.51 to 4.94), it was recognized that fast paths might exist through the UZ, and two alternative conceptual models of flow in the UZ were used: a model similar to the previous models that described flow predominately in the matrix, and a new model (the weeps model) that described flow as occurring predominately in fractures. This 2-D fracture-flow model was based on modeling by Czarnecki (1984) and Czarnecki and Waddell (1984) and encompassed the controlled area around the potential repository and the 5-km (3-mi) distance between the potential repository and the accessible environment. Boundary conditions for the TSPA model were taken from the Czarnecki (1984) model. A particle-tracking method was implemented to determine radionuclide transport times and velocities from the potential repository to the accessible environment for a conservative tracer. Particles were placed in the "footprint" of the potential repository (the area in the SZ immediately below the potential repository) and transport times to the proposed 5-km (3-mi) boundary were determined. Differences in initial position and paths taken produced a distribution of transport times. In general, transport in the SZ was in a southeasterly direction with transport times varying between 900 and 1,500 yrs (velocities of 3.25 to 5.7 m/yr [10.7 to 18.7 ft/yr]), and a mean transport of about 1,200 yrs. The results of the 2-D model were abstracted for the TSPA-1991 calculations into 1-D, horizontal flow tubes in a single porous medium. For the TSPA abstraction, dispersivities were assumed to be log-uniformly distributed between 50 m and 500 m (164 ft and 1,640 ft), and porosity was set to 17.5 percent. During the TSPA calculations, the 1-D model was used directly to solve the transport of radionuclides over the 5-km (3-mi) distance to the accessible environment. By the time of TSPA-1993 (Wilson et al. 1994, pp. 14-4, 14-5; CRWMS M&O 1994, pp. 3-4, 3-6), it was apparent that an upcoming regulation for a potential repository at Yucca Mountain might involve a metric of radiation dose to an individual. Calculation of dose requires a method of accurately determining the concentration of radionuclides at the interface of the geosphere and biosphere where the biological environment becomes exposed to the contaminants. Cumulative releases can be estimated using only the transport times. A 3-D, confined-aquifer model was constructed for TSPA-1993 for the expressed purpose of determining whether three dimensions were necessary to properly define the SZ flow system. The analysts concluded that incorporation of 3-D geologic structures was necessary to match observed hydraulic heads (water-table heights) and that geologic structures, to a large degree, determined the direction and velocity of groundwater flow. For TSPA-1993 (Wilson et al. 1994), as with TSPA-1991 (Barnard et al. 1992), transport and velocities were determined by calculating the transport of a conservative tracer from various locations in each of the three geologic units that intersected the potential repository footprint to a location 5 km (3 mi) downgradient from the potential repository. Porosity was set to 20 percent. Two conceptual models were investigated: one that only allowed water to leave and enter the SZ domain from the sides (the non-diversionary model), and another that incorporated a drain out of the volcanic aquifer (the diversionary model). Results showed significant variability as compared to the TSPA-1991 results. Transport times in the SZ for the various conceptual models and transport paths ranged from 230 to 1,700 yrs (velocities of 3 to 22 m/yr [10 to 72 ft/yr]), with averages ranging from 500 to 800 yrs (average velocities of 6 to 10 m/yr [20 to 33 ft/yr]). For TSPA-1993 (Wilson et al. 1994), the detailed outflow from the UZ transport model was used as input to a 1-D, single porosity model. Longitudinal dispersivity was assumed to be uniformly distributed between 100 m and 500 m (330 ft to 1,640 ft) to match the distribution of velocities, and porosity was kept at 20 percent. Dilution, and thus final concentration, was calculated from an estimate of the cross-sectional area of the transport plume at the proposed boundary to the accessible environment; the estimate was based on assumed transverse dispersivities and mixing depths (typically 50 m [165 ft]), and ranged between 34,000 m2 and 2,200,000 m2 (370,000 ft2 to 24,000,000 ft2). Climate change was incorporated in TSPA-1993 as a jump from one steady-state condition to another. For the SZ, future climate change was approximated by an increase in the water table; no changes were made to flow or transport parameters resulting from flow through previously unsaturated areas of the formation. A water-table rise of between 50 m and 120 m (165 ft to 395 ft) was specified using a uniform distribution. The rise in the water-table elevation caused radionuclides in the UZ, but now in the SZ, to be immediately introduced to the SZ, resulting sometime later in a pulse of higher releases and doses at the accessible environment. Cumulative releases and radiation doses (drinking-water doses only) were considered as performance metrics in TSPA-1993 (Wilson et al. 1994). For TSPA-1995 (CRWMS M&O 1995), the emphasis was on a more accurate depiction of the EBS of the potential repository at Yucca Mountain, and no new modeling of the SZ was conducted. Velocities from the TSPA-1993 3-D model (Wilson et al. 1994) were used again in 1-D abstractions. A constant longitudinal dispersivity of 500-m (1,640-ft) was assumed. Various cross-sectional areas of the contaminant plume were estimated as in TSPA-1993. From these cross-sectional areas and the groundwater flux, dilution factors were calculated, and the radionuclide concentrations from the 1-D TSPA abstracted model were reduced by these dilution factors. In some calculations, additional dilution was assumed for mixing with other groundwater basins, and for mixing with uncontaminated water during well withdrawal. As with TSPA-1993, water-table rise was taken from a uniform distribution, however, the rise was more continuous over time. Sensitivity studies were conducted for TSPA-1993 (Wilson et al. 1994) and TSPA-1995 (CRWMS M&O 1995) to look at how much the uncertainty and variability in parameter distributions influenced the uncertainty and variability of the results. Both TSPAs found performance of the potential repository to be sensitive to SZ parameters. For the radiation dose metric in particular, results were sensitive to parameters influencing radionuclide dilution in groundwater. In the simplified TSPA models, these parameters were the groundwater flux and the cross-sectional flow area of the contaminant plume (or the dilution factor). In the actual flow system, these parameters correspond to groundwater flux, transverse dispersion, transport path lengths, and possible well-withdrawal effects. Parameters primarily affecting radionuclide transport times (i.e., sorption coefficients and longitudinal dispersion) were found to be of lesser importance.2.4.2 Total System Performance Assessment-Viability Assessment Modeling
The TSPA-VA, published in 1998 (DOE 1998a), focused on calculating dose 20 km (12.5 mi) downgradient from the potential repository due to changes in DOE guidance that were based upon recommendations from the National Research Council (National Research Council 1995). A series of abstraction and testing workshops were conducted to compile a prioritized list of important technical issues related to flow and transport in the SZ (DOE 1998b, p. 3-138, Table 3-19). An expert elicitation also was conducted to obtain opinions from five experts on groundwater flow and transport regarding important issues related to the SZ (CRWMS M&O 1998b). The approach for conducting the TSPA-VA analysis was based upon input from the workshops and the expert elicitation. For the TSPA-VA, numerical simulations were performed for the base case, representing the most likely future evolution of the site, and sensitivity analyses were performed to study variations of the base case. The base case SZ flow and transport component of the TSPA-VA evaluated the migration of radionuclides from their introduction at the water table below the potential repository to the release point to the biosphere. A hierarchy of models was used to simulate the transport of radionuclides in the SZ. Explicit, 3-D modeling was not used to simulate radionuclide concentrations because it can generate numerical dispersion, which artificially lowers concentration. The TSPA 3-D SZ flow model was used only to determine flowpaths through the SZ (DOE 1998b, Section 4.1.12, p. 4-16). The TSPA 1-D SZ transport model (DOE 1998b, Section 3.7.2.1, p. 3-139) was developed based on the flowpaths from the 3-D flow modeling and used to determine concentration breakthrough curves at a distance of 20 km (12.5 mi) for unit releases of radionuclides from six streamtubes (DOE 1998b, Section 4.1.12, p. 4-16). The SZ transport component of the analysis was coupled to the transport calculations for the UZ that bring contaminants in downward percolating groundwater from the potential repository to the water table (e.g., the spatial and temporal distributions of simulated mass flux at the water table). The coupling was accomplished by using the convolution integral method (CRWMS M&O 1998e, Section 8.3.4) to combine the unit breakthrough curves calculated by the TSPA 1-D SZ transport model with the time-varying radionuclide sources from the UZ. Changes in the SZ flow and transport system in response to climatic variations were incorporated for the three discrete climate states (dry, long-term average, and superpluvial) considered in the other components of the TSPA-VA. Specific discharge and volumetric groundwater flow-rate in the SZ streamtubes were scaled in transport simulations to reflect climate state. The SZ transport results were linked to the biosphere analysis by the simulated time history, or system response as a function of time, of radionuclide concentration in groundwater produced from a hypothetical well located at the biosphere interface. The biosphere was assumed to be located 20 km (12.5 mi) from the potential repository. Radionuclide concentrations in the hypothetical well water were then used in the biosphere component to calculate doses received by the public. For the base case, uncertainty in the SZ system was evaluated through Monte Carlo variation in the input parameters (Wilson et al. 1994, pp. 3-19 to 3-21) that were used in the TSPA 1-D SZ transport model. Primarily, the uncertainty in radionuclide transport parameters was evaluated. The TSPA 1-D SZ transport model was used to calculate 101 unit breakthrough curves (100 Monte Carlo simulations and the expected-value case). The results of the 1-D SZ transport calculations were stored in a "library" of unitradionuclide concentration breakthrough-curves, and for each TSPA-VA realization, a SZ unit breakthrough curve was randomly drawn from the library for use in the convolution integral method. The convolution integral method (DOE 1998b, p. 3-141) was used in the TSPA-VA calculations to determine radionuclide concentrations in the SZ, 20 km (12.5 mi) downgradient of the potential repository, as a function of the transient radionuclide mass flux at the water table beneath the potential repository. This coupling method made full use of the detailed SZ flow and transport simulations for a given realization of the system, but without requiring complete numerical simulation of the SZ for the duration of each TSPA-VA realization. The two input functions to the convolution integral method are a unit concentration breakthrough curve (in response to a step-function mass flux source as simulated by the TSPA 1-D SZ transport model) and the radionuclide mass flux history as simulated by the UZ transport model. The TSPA-VA sensitivity studies were designed to examine five of the key issues related to assumptions about the base case SZ analysis (DOE 1998b, p. 4-2) and the importance of these issues with respect to performance of the potential repository. The effects of dilution in the SZ and vertical transverse dispersivity were investigated to address concerns from the expert elicitation panel. The impact of including heterogeneity and large-scale flow channelization in a 3-D flow and transport model was studied. A 2-D dual-porosity transport model was used to calculate radionuclide concentrations to examine the effects of the two base case assumptions: a single continuum and using effective porosity as a surrogate for the matrix diffusion process. A study to investigate the effect of calculating a population dose, compared to the dose to an individual, was performed for the base case. Finally, alternative conceptual models of colloid-facilitated plutonium transport were developed and implemented for sensitivity analysis. Colloid-facilitated transport of plutonium in the SZ was simulated based on a conceptual model using the assumptions of equilibrium, reversible sorption of plutonium onto colloids, and the potential for irreversible sorption of plutonium onto some colloidal particles (DOE 1998b, pp. 3-92, 3-99, and 3-104). A large fraction of the plutonium mass released was simulated to move assuming chemical equilibrium among dissolved plutonium, plutonium sorbed onto colloids, and plutonium sorbed onto the aquifer material (DOE 1998b, pp. 3-140 to 3-141). A small fraction of the plutonium mass was simulated to be irreversibly attached onto colloids and transported relatively rapidly in the SZ. The relative fraction of plutonium mass that was subject to reversible vs. irreversible sorption onto colloids was treated as uncertain and was included as a stochastic parameter in the analyses. The three alternative climate states considered in TSPA-VA analyses (DOE 1998b, pp. 3-8 to 3-9) were dry (present-day conditions), long-term average (corresponding to oscillating pluvial and glacial conditions), and superpluvial (corresponding to severe glacial conditions). An abstraction method for three climate states, as they affect flow and transport in the SZ, was developed for use with the convolution integral method. A primary simplifying assumption was that groundwater flow in the SZ immediately changed to a new steady state following climate change. Changes in the magnitude of SZ groundwater flux were represented, based on results from the regional-scale SZ flow model (D’Agnese et al. 1997; 1999), by a scaling factor for the three climate states. Changes in the volumetric groundwater flow rate through each of the 1-D SZ streamtubes for the three climate states were based on the site-scale UZ flow model simulations for these climatic conditions. Results of the TSPA-VA model may be expressed as dose rates in millirem per year (mrem/yr) at a point 20 km (12.5 km) down-gradient from the potential repository (the breakthrough point) for the three time periods (104, 105, and 106 yrs) studied. Breakthrough is defined as the first appearance of a radionuclide in sufficient quantity to provide a dose of 0.001 mrem/yr. During the first 10,000 yrs after emplacement, the only radionuclides expected to reach the biosphere are technetium-99, iodine-129, and carbon-14 (DOE 1998b, p. 4-25 and Figure 4-29). The breakthrough of technetium-99 is calculated to occur at about 3,500 yrs, while that of iodine-129 occurs at about 4,200 yrs. Carbon-14 provided a relatively small contribution to dose. High solubility and the assumption of no sorption account for the early breakthrough of these radionuclides compared to other radionuclides in the inventory over the first 10,000-yr period. From 10,000 yrs to 100,000 yrs after emplacement, technetium-99 and iodine-129 continue to dominate the dose to about 50,000 yrs, after which neptunium-237 begins to dominate (DOE 1998b, Figure 4-16). During this period, breakthrough of neptunium-237 occurs at about 30,000 yrs. Breakthrough of uranium-234 and plutonium-239 follow at about 50,000 and 80,000 yrs, respectively (DOE 1998b, Figure 4-16). For the time period from 100,000 to 200,000 yrs, neptunium-237 dominates the dose (DOE 1998b, Figure 4-21). From about 200,000 yrs to one million years, neptunium-237 and plutonium-242 dominate the dose rate. Notable peaks occur in the results during this last period due to assumed super-pluvial climate peaks and large numbers of cladding and no-drip package failures over large time steps in the simulations (DOE 1998b, p. 4-50).2.5 Current SATURATED ZONE Flow and Transport Model
The current site-scale SZ flow and transport model was developed in support of the upcoming TSPA-SR. The current model was built upon the model used for TSPA-VA (DOE 1998b), but includes a number of modifications to reflect the current understanding of the SZ flow and transport, address deficiencies in the TSPA-VA SZ model that were identified by reviews, and incorporate new data collected since TSPA-VA. Changes introduced since the TSPA-VA iteration include:3. SATURATED ZONE FLOW AND TRANSPORT MODEL AND ABSTRACTIONS FOR TOTAL SYSTEM PERFORMANCE ASSESSMENT FOR SITE RECOMMENDATION
The DOE has conducted investigations to characterize and model groundwater flow and chemical transport in the vicinity of the potential Yucca Mountain repository because groundwater transport is expected to be the main means of radionuclide transport from the potential repository to the proposed compliance point. Modeling of groundwater flow in the SZ has been carried out at regional and site scales. The regional-scale model (D’Agnese et al. 1997), encompassing the Death Valley regional groundwater flow system, an area of about 50,000 km2 (19,300 mi2), and the potential repository, was developed to model groundwater flow. The site-scale model, encompassing an area of 1,350 km2 (521 mi2), the potential repository, and the proposed 20 km (12.5 mi) compliance point (proposed 10 CFR 63.115(b)(1) [64 FR 8640]), was developed to model groundwater flow and chemical transport. In this section, the site-scale SZ flow and transport model, abstractions, analyses, uncertainties, and limitations of the output are discussed. Discussions are provided concerning characterization of SZ flow and transport (Section 3.1); the conceptual model of site-scale flow and transport (Section 3.2); mathematical and numerical modeling approach (Section 3.3); model validation activities (Section 3.4); assumptions, uses, and limits of the site-scale SZ flow and transport model (Section 3.5); synthesis of the site-scale SZ flow and transport model and model abstractions (Section 3.6); the site-scale SZ flow and transport base case (Section 3.7); and other views and alternative models (Section 3.8).3.1 Saturated Zone Flow and Transport Characterization
Discussions in Section 3.1 primarily are focused on historical data collection and characterization of groundwater flow and radionuclide transport in the SZ. Descriptions of regional and site-scale physiography, climate, soil and vegetation, geology, and groundwater flow are provided in Section 3.1.1. Also in this section is a discussion of groundwater chemistry at a scale slightly larger than that of the site-scale model domain. Hydrologic data (water levels, well testing, and recharge) are summarized in Section 3.1.2. Hydrochemical data pertinent to transport (specified patterns, results of tracer test, and oxidation potential) are summarized in Section 3.1.3. Laboratory data from sorption, diffusion, and colloidal transport experiments are summarized in Section 3.1.4.3.1.1 Description of the Saturated Zone System
The following subsections describe the regional and site-scale geologic setting, hydrologic setting, and hydrogeochemistry. The regional description, except for the discussion of hydrochemistry, is abstracted from D’Agnese et al. (1997). The regional-scale discussion on hydrochemistry is based on CRWMS M&O (2000m).3.1.1.1 Regional Description
Physiography–The Death Valley Region is situated within the southern Great Basin, a subprovince of the Basin and Range physiographic province. Altitudes range from 86 m (282 ft) below sea level at Badwater in Death Valley to 3,600 m (11,800 ft) above sea level at Mount Charleston in the Spring Mountains. The relief between valleys and adjoining mountains locally exceeds 1,500 m (4,920 ft). Most of the principal mountain ranges have distinct northwest-southeast trends, although the trends of intermediate-scale topographic features are variable. The uplands occupy about 25 percent of the landscape in the region, while the remainder of the landscape is occupied by broad intermontane basins formed from tectonically down-dropped grabens. The basins are filled with alluvium and locally interbedded volcanic deposits. These deposits gently slope from the valley floors to the bordering mountain ranges, forming piedmonts. Climate–The northern part of the region, including the Cactus, Kawich, and Timpahute Ranges (Figure 2-2), is characterized by warm, dry summers and cold, dry winters. The southern part of the region, including Death Valley and the Eastern Mojave Desert, is characterized by hot, dry summers and warm, dry winters. The central region around the NTS and Yucca Mountain has been called a transition desert, and represents a combination of the two climates. The upland areas receive the bulk of the annual precipitation, with mean values exceeding 700 mm/yr (28 in./yr) in the Spring Mountains, but the valleys receive much less, with a mean annual precipitation in Death Valley of only 50 mm/yr (2 in./yr). Average annual lake-evaporation values range from about 1,100 mm (43 in.) in the north to more than 2,000 mm (79 in.) in Death Valley (Grasso 1996, Table 8). Studies of past climates indicate that in the Death Valley region, climate oscillated between glacial and interglacial episodes, although glaciers were limited to colder regions of North America. The current climate generally is typical of interglacial periods, although paleoclimate records suggest that in the Yucca Mountain region, the present interglacial period is hotter and drier than earlier interglacial episodes (D’Agnese et al. 1997). In contrast to the current climate, periods of more extensive glaciation dominated the climate of North America for most of the past 500,000 yrs. Glacial episodes (also termed pluvial episodes south of the glacier border) were characterized by wetter and colder conditions than at present, and they prevailed over approximately 80 percent of that time. The principal effect of a cooler, wetter climate during glacial times on the SZ hydrology was increased recharge. This, in turn, resulted in larger gradients of hydraulic head, more total groundwater flow, a rise in the water table, and increased surface discharge in the form of streams, springs, and wetlands. A variety of physical evidence suggest that the water table in the Yucca Mountain region has been up to 120 m (30 ft to 394 ft) higher than the current water table (CRWMS M&O 2000w, Table 9.4-1), whether due to climate change or some other cause. Conversely, the preponderance of field data suggest that the water table has risen no more than 120 m (394 ft) above present levels for extended periods of time in the Yucca Mountain area since deposition of the volcanic units over 10 my ago (NRC 1999a, p. 30). Similarly, various hydrologic models that incorporate climate-induced changes predict that the water table was from less than 50 m to 150 m higher than the current water table (CRWMS M&O 2000w, Table 9.4-2) during the Pleistocene glacial maximum. Soil and Vegetation