This hydrology and geochemistry volume is a companion volume to the 2007 Geological Society of America Memoir 199, The Geology and Climatology of Yucca Mountain and Vicinity, Southern Nevada and California , edited by Stuckless and Levich. The work in both volumes was originally reported in the U.S. Department of Energy regulatory document Yucca Mountain Site Description , for the site characterization study of Yucca Mountain, Nevada, as the proposed U.S. geologic repository for high-level radioactive waste. The selection of Yucca Mountain resulted from a nationwide search and numerous committee studies during a period of more than 40 yr. The waste, largely from commercial nuclear power reactors and the government's nuclear weapons programs, is characterized by intense penetrating radiation and high heat production, and, therefore, it must be isolated from the biosphere for tens of thousands of years. The extensive, unique, and often innovative geoscience investigations conducted at Yucca Mountain for more than 20 yr make it one of the most thoroughly studied geologic features on Earth. The results of these investigations contribute extensive knowledge to the hydrologic and geochemical aspects of radioactive waste disposal in the unsaturated zone. The science, analyses, and interpretations are important not only to Yucca Mountain, but also to the assessment of other sites or alternative processes that may be considered for waste disposal in the future. Groundwater conditions, processes, and geochemistry, especially in combination with the heat from radionuclide decay, are integral to the ability of a repository to isolate waste. Hydrology and geochemistry are discussed here in chapters on unsaturated zone hydrology, saturated zone hydrology, paleohydrology, hydrochemistry, radionuclide transport, and thermally driven coupled processes affecting long-term waste isolation. This introductory chapter reviews some of the reasons for choosing to study Yucca Mountain as a repository site.

The unsaturated zone at Yucca Mountain was investigated as a possible site for the nation's first high-level nuclear waste repository. Scientific investigations included infiltration studies, matrix properties testing, borehole testing and monitoring, underground excavation and testing, and the development of conceptual and numerical models of the hydrologic processes at Yucca Mountain. Infiltration estimates by empirical and geochemical methods range from 0.2 to 1.4 mm/yr and 0.2–6.0 mm/yr, respectively. Infiltration estimates from numerical models range from 4.5 mm/yr to 17.6 mm/yr. Rock matrix properties vary vertically and laterally as the result of depositional processes and subsequent postdepositional alteration. Laboratory tests indicate that the average matrix porosity and hydraulic conductivity values for the main level of the proposed repository (Topopah Spring Tuff middle nonlithophysal zone) are 0.08 and 4.7 × 10 −12 m/s, respectively. In situ fracture hydraulic conductivity values are 3–6 orders of magnitude greater. The permeability of fault zones is approximately an order of magnitude greater than that of the surrounding rock unit. Water samples from the fault zones have tritium concentrations that indicate some component of postnuclear testing. Gas and water vapor movement through the unsaturated zone is driven by changes in barometric pressure, temperature-induced density differences, and wind effects. The subsurface pressure response to surface barometric changes is controlled by the distribution and interconnectedness of fractures, the presence of faults and their ability to conduct gas and vapor, and the moisture content and matrix permeability of the rock units. In situ water potential values are generally less than −0.2 MPa (−2 bar), and the water potential gradients in the Topopah Spring Tuff units are very small. Perched-water zones at Yucca Mountain are associated with the basal vitrophyre of the Topopah Spring Tuff or the Calico Hills bedded tuff. Thermal gradients in the unsaturated zone vary with location, and range from ~2.0 °C to 6.0 °C per 100 m; the variability appears to be associated with topography. Large-scale heater testing identified a heat-pipe signature at ~97 °C, and identified thermally induced and excavation-induced changes in the stress field. Elevated gas-phase CO 2 concentrations and a decrease in the pH of water from the condensation zone also were identified. Conceptual and numerical flow and transport models of Yucca Mountain indicate that infiltration is highly variable, both spatially and temporally. Flow in the unsaturated zone is predominately through fractures in the welded units of the Tiva Canyon and Topopah Spring Tuffs and predominately through the matrix in the Paintbrush Tuff nonwelded units and Calico Hills Formation. Isolated, transient, fast-flow paths, such as faults, do exist but probably carry only a small portion of the total liquid-water flux at Yucca Mountain. The Paintbrush Tuff nonwelded units act as a storage buffer for transient infiltration pulses. Faults may act as flow boundaries and/or fast pathways. Below the proposed repository horizon, low-permeability lithostratigraphic units of the Topopah Spring Tuff and/or the Calico Hills Formation may divert flow laterally to faults that act as conduits to the water table. Advective transport pathways are consistent with flow pathways. Matrix diffusion is the major mechanism for mass transfer between fractures and the matrix and may contribute to retardation of radionuclide transport when fracture flow is dominant. Sorption may retard the movement of radionuclides in the unsaturated zone; however, sorption on mobile colloids may enhance radionuclide transport. Dispersion is not expected to be a major transport mechanism in the unsaturated zone at Yucca Mountain. Natural analogue studies support the concepts that percolating water may be diverted around underground openings and that the percentage of infiltration that becomes seepage decreases as infiltration decreases.

In 2002, Yucca Mountain, Nevada, was selected as the proposed site for the U.S. high-level nuclear waste repository. Yucca Mountain lies within a large topographically closed basin, in which surface water is internally drained. Groundwater, however, can and does flow into and out of this basin at depth through a regional carbonate-rock aquifer (commonly referred to as the lower carbonate-rock aquifer). Most groundwater recharge (water infiltrating downward through the unsaturated zone into the water table) originates in the highlands north of Yucca Mountain and flows generally southward. Some groundwater discharges within the basin, as in Oasis Valley and the southern Amargosa Desert, but the ultimate discharge is in Death Valley, where water is returned to the atmosphere by evapotranspiration. Groundwater flows through a heterogeneous medium produced by a complex geologic history including both compressional and extensional tectonics. For hydrologic purposes, the rocks and alluvium are divided into 25 hydrogeologic units. Regionally, the most important unit for regional groundwater flow is composed of Paleozoic carbonate rocks, which are locally separated into two aquifers by an intervening shale. Rocks of the southwestern Nevada volcanic field form thick deposits in the northern part of the basin, and these rocks host both aquifers and confining units. The potentiometric surface of the site-scale flow system contains areas of large hydraulic gradient (as great as 0.13) and small hydraulic gradient (as small as 0.0001). Both extremes are found within the Yucca Mountain site area, where they are well constrained by numerous boreholes. At Yucca Mountain, a single borehole penetrates to the regional carbonate-rock aquifer, and, at this locality, the hydraulic head at depth is 20 m greater than in the overlying volcanic rocks. This head difference is likely widespread, as indicated by thermal highs at the groundwater table in the vicinity of block-bounding faults, where upward leakage of water from the regional carbonate-rock aquifer is postulated. Since the early 1980s, numerous two- and three-dimensional flow models have been developed to depict regional groundwater flow. A 2004 transient flow model of the Death Valley region has 16 layers and a 1500 m/side horizontal grid; it is composed of 194 rows and 160 columns. The model was first calibrated to a steady-state condition and then to transient conditions. The model matches observed flow patterns well, and it generally agrees with measured water levels except in areas of large hydraulic gradient. The regional model provides the boundary conditions for a detailed site-scale flow model. The finite-element heat and mass transfer code, FEHM v2.24, was used to simulate flow through the saturated zone at Yucca Mountain. Cells in the site-scale model are 250 m/side in the horizontal grid; it is composed of 181 rows and 121 columns. The model may use as many as 67 layers, but the framework model allows a stair-stepped ground surface, so the number of layers is variable. Layer thickness ranges from 600 m at the bottom of the model to 10 m south of Yucca Mountain. The site-scale flow model was constructed and calibrated, matching observed hydrologic data well. The site-scale flow model provides a means for assessing the hypothetical flow path for any radioactive materials originating from the proposed repository.

Yucca Mountain, a site in southwest Nevada, has been proposed for a deep underground radioactive waste repository. An extensive database of geochemical and isotopic characteristics has been established for pore waters and gases from the unsaturated zone, perched water, and saturated zone waters in the Yucca Mountain area. The development of this database has been driven by diverse needs of the Yucca Mountain Project, especially those aspects of the project involving process modeling and performance assessment. Water and gas chemistries influence the sorption behavior of radionuclides and the solubility of the radionuclide compounds that form. The chemistry of waters that may infiltrate the proposed repository will be determined in part by that of water present in the unsaturated zone above the proposed repository horizon, whereas pore-water compositions beneath the repository horizon will influence the sorption behavior of the radionuclides transported toward the water table. However, more relevant to the discussion in this chapter, development and testing of conceptual flow and transport models for the Yucca Mountain hydrologic system are strengthened through the incorporation of natural environmental tracer data into the process. Chemical and isotopic data are used to establish bounds on key hydrologic parameters and to provide corroborative evidence for model assumptions and predictions. Examples of specific issues addressed by these data include spatial and temporal variability in net fluxes, the role of faults in controlling flow paths, fracture-matrix interactions, the age and origin of perched water, and the distribution of water traveltimes.

Surface, unsaturated-zone, and saturated-zone hydrologic conditions at Yucca Mountain responded to past climate variations and are at least partly preserved by sediment, fossil, and mineral records. Characterizing past hydrologic conditions in surface and subsurface environments helps to constrain hydrologic responses expected under future climate conditions and improve predictions of repository performance. Furthermore, these records provide a better understanding of hydrologic processes that operate at time scales not readily measured by other means. Pleistocene climates in southern Nevada were predominantly wetter and colder than the current interglacial period. Cyclic episodes of aggradation and incision in Fortymile Wash, which drains the eastern slope of Yucca Mountain, are closely linked to Pleistocene climate cycles. Formation of pedogenic cement is favored under wetter Pleistocene climates, consistent with increased soil moisture and vegetation, higher chemical solubility, and greater evapotranspiration relative to Holocene soil conditions. The distribution and geochemistry of secondary minerals in subsurface fractures and cavities reflect unsaturated-zone hydrologic conditions and the response of the hydrogeologic system to changes in temperature and percolation flux over the last 12.8 m.y. Physical and fluid-inclusion evidence indicates that secondary calcite and opal formed in air-filled cavities from fluids percolating downward through connected fracture pathways in the unsaturated zone. Oxygen, strontium, and carbon isotope data from calcite are consistent with a descending meteoric water source but also indicate that water compositions and temperatures evolved through time. Geochronological data indicate that secondary mineral growth rates are less than 1–5 mm/m.y., and have remained approximately uniform over the last 10 m.y. or longer. These data are interpreted as evidence for hydrological stability despite large differences in surface moisture caused by climate shifts between the Miocene and Pleistocene and between Pleistocene glacial-interglacial cycles. Secondary mineral distribution and δ 18 O profiles indicate that evaporation in the shallower welded tuffs reduces infiltration fluxes. Several near-surface and subsurface processes likely are responsible for diverting or dampening infiltration and percolation, resulting in buffering of percolation fluxes to the deeper unsaturated zone. Cooler and wetter Pleistocene climates resulted in increased recharge in upland areas and higher water tables at Yucca Mountain and throughout the region. Discharge deposits in the Amargosa Desert were active during glacial periods, but only in areas where the modern water table is within 7–30 m of the surface. Published groundwater models simulate water-table rises beneath Yucca Mountain of as much as 150 m during glacial climates. However, most evidence from Fortymile Canyon up gradient from Yucca Mountain limits water-table rises to 30 m or less, which is consistent with evidence from discharge sites in the Amargosa Desert. The isotopic compositions of uranium in tuffs spanning the water table in two Yucca Mountain boreholes indicate that Pleistocene water-table rises likely were restricted to 25–50 m above modern positions and are in approximate agreement with water-table rises estimated from zeolitic-to-vitric transitions in the Yucca Mountain tuffs (less than 60 m in the last 11.6 m.y.).

Characteristics of host rocks, secondary minerals, and fluids would affect the transport of radionuclides from a previously proposed repository at Yucca Mountain, Nevada. Minerals in the Yucca Mountain tuffs that are important for retarding radionuclides include clinoptilolite and mordenite (zeolites), clay minerals, and iron and manganese oxides and hydroxides. Water compositions along flow paths beneath Yucca Mountain are controlled by dissolution reactions, silica and calcite precipitation, and ion-exchange reactions. Radionuclide concentrations along flow paths from a repository could be limited by (1) low waste-form dissolution rates, (2) low radionuclide solubility, and (3) radionuclide sorption onto geological media. The chief sources of radioactivity in spent nuclear fuel are americium, plutonium, and neptunium. Therefore, studies have concentrated on their geochemical mobility. Uranium-233, uranium-234, iodine-129, technetium-99, and other radionuclides also have been included in some experiments. Solubilities were determined experimentally in representative Yucca Mountain waters. Sorption coefficients were determined using water, rock, and pure mineral samples from Yucca Mountain. Batch experiments were performed at several pH levels and oxidizing conditions. Dynamic transport-column experiments, diffusion experiments, and solid-rock beaker experiments also were conducted. The batch tests gave slightly lower retardation factors than those derived from column-breakthrough experiments. This finding indicates that using batch-sorption coefficients to predict radionuclide transport will yield conservative results in a performance assessment. Understanding of unsaturated-zone transport is based on laboratory and field-scale experiments. Fractures provide advective transport pathways. Sorption and matrix diffusion may contribute to retardation of radionuclides. Conversely, sorption onto mobile colloids may enhance radionuclide transport.

Heat from radionuclide decay causes coupled thermal (T), hydrological (H), chemical (C), and mechanical (M) processes in the rock mass. These coupled processes impact the ability of a repository to isolate waste by affecting water seepage into waste-emplacement drifts and by affecting radionuclide transport. The U.S. Department of Energy's Thermal Testing Program at the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, began in the mid-1990s and consisted of three large-scale in situ thermal tests. Although in 2010, the U.S. government decided to pursue alternative solutions to geologic disposal of radioactive waste at Yucca Mountain, the work reported throughout this volume refers to “the proposed repository” at Yucca Mountain, which was the status at the time the chapters were written (2009). The main objective of these thermal tests was to gain an in-depth understanding of the coupled THCM processes that would occur in the repository rock. Numerical models that capture coupled processes were constructed for the respective thermal tests, and the predictions from these numerical models, when compared to measured data, enabled the evaluation of processes occurring in the thermal tests. In turn, analysis of the thermal tests, particularly of the drift-scale test (the largest of these tests), has provided information on THCM processes that were incorporated in drift-scale and mountain-scale numerical models for the proposed repository at Yucca Mountain to predict repository performance during thermal loading. Such coupled-processes models for the proposed repository show that TH processes would produce a vaporization barrier, which would prevent water from seeping into the drifts when the temperature near the drifts rises above boiling. THC and THM processes cause permeability changes that modify flow paths near the drifts and, in turn, seepage of water into drifts. The impact of thermally driven coupled processes is largest near the drifts, where the increase in temperature is the greatest. Further away (tens of meters) from the drifts, the impact of THCM processes on radionuclide transport is insignificant. The detailed THCM studies at Yucca Mountain indicate that, overall, the effects of heating due to radioactive decay would not degrade the long-term ability of the proposed repository to isolate waste. On the contrary, the THCM coupled processes lead to more diversion of water around and less seepage into the waste-emplacement drifts than that at ambient conditions, thus making Yucca Mountain a more effective natural barrier to potential release of radionuclides to the biosphere.