Abstract The removal of a large dam requires special engineering considerations not normally required for the removal of smaller dams. Large dams generally provide greater project benefits, but they represent a higher downstream hazard in the event of failure and greater challenges to fish passage. The reservoirs associated with large dams can impound more sediment and affect downstream water quality to a greater degree. The environmental compliance and decision-making process for a large dam removal project can take many years and will normally require the evaluation of a full range of project alternatives with estimated costs, the performance of a comprehensive environmental impact analysis, and the identification and implementation of extensive mitigation measures. Streamflow diversion and demolition plans for the removal of a large dam must ensure acceptable construction risks from start to finish, and produce reservoir drawdown at a controlled rate for sediment management purposes and to prevent instability of natural or embankment slopes. Large dams require more time for removal, at a higher cost, and contain greater volumes of materials for which disposal sites must be found.

Abstract The removal of obsolete and unsafe dams for safety, environmental, or economic purposes frequently involves the exploration of sediments trapped within the impoundment and the subsequent assessment of sediment management needs and techniques. Sediment management planning requires a thorough understanding of the watershed’s surficial geology, topography, land cover, land use, and hydrology. The behavior of sediments is influenced by their age, consolidation, and stratigraphy. All watersheds have a history that helps forecast sediment loads, quality, gradation, and stratigraphy. Impounded sediment deposits may include coarse deltas and foreset slopes, fine or coarse bottom deposits, cohesive or organic matter, and wedge deposits immediately behind the dam. Some watersheds have anthropogenic pollutants from agricultural activities, mining, industries, or urban runoff. The volume and rate of sediment release during and after small dam removal can be limited by active management plans to reduce potential downstream impacts. Management strategies include natural erosion, phased breaches and drawdowns, natural revegetation of sediment surfaces, pre-excavation of an upstream channel, hazardous waste removal or containment, flow bypass plans, and sediment dredging.

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.

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.

Abstract Enhanced scrubber technology for coal-fired boilers, to be brought online at the Alcoa Warrick Operations Plant in Newburg, Indiana, USA, will generate increased volumes of coal-combustion products. This paper and field trip examine hydrogeological characteristics at the site of a management facility for coal-combustion products currently in development. An exposed highwall strip pit (Y-pit) on abandoned mine lands will be the site for coal-combustion product placement. Hydrogeological monitoring and characterization data for mined area spoils and unmined Paleozoic rocks will be reviewed in the field. Mine spoils at this site display horizontal hydraulic conductivities that range from 10 -3 to 10 -8 cm/s, and unmined bedrock varies from 10 -7 to 10 -8 cm/s. Groundwater at the B-2 site in shallow spoils near the Y-pit displays responses to precipitation events, and groundwater levels at the B-5 site in the unmined bedrock display Earth-tide responses. The Y-pit has variable hydrogeologic regimes along its length, including areas of flow-through and discharge. Groundwater discharge gradients increased markedly during periods of drought. Groundwater recharge gradients in the flow-through area decreased during drought conditions. Spoils groundwater is dominantly a highly mineralized water rich in calcium sulfate, and bedrock groundwater is highly mineralized water rich in sodium sulfate/chloride. Y-pit surface water most closely resembles spoils groundwater. Both the mine spoils and unmined bedrock groundwater samples have concentrations of arsenic that exceed the primary drinking water standard.

Abstract In the Asse salt mine, a system with relatively small pillars and stopes was excavated between 1909 and 1964. From 1965 until 1978, low- and intermediate-level radioactive wastes were disposed there permanently. Most of the chamber volume (∼3.5 million m 3 ) was exposed to free convergence until 1995, when a backfilling campaign was started using pneumatic transportation of granular salt material. The barrier to the overburden rocks is formed by rock salt that has a minimal thickness of only 15 m in the upper part. The flank dips ∼70°SW. The Asse site has been monitored for decades by displacement observations, stress and strain measurements in the pillars, and recording of the backfill pressure built up in the chambers. Softening and damaging in the pillars and stopes of the mining horizon have led to stress redistributions into the overburden rocks, where rupture processes have occurred. Hence, because of the small dimension of the bearing elements on the southern flank and the close distance to the overburden, far-reaching geomechanical interactions exist.

Abstract Spontaneous combustion commonly occurs in storage and waste dumps from coal mining. It has been suggested that this can be prevented by the use of a protective layer (possibly compacted) of a reactive material. As the coal ages (becomes oxidized), the coal reactivity toward oxygen decreases and the oxygen profile changes. Analyses of the oxygen concentration profiles in a 3-m-long, 20 cm inside diameter plastic column filled with coal were performed for a period of up to 8 mo. The results were modeled, and they can be adequately fitted to a simple diffusion/reaction model.