Site-scale Flow Research Articles

Securing Environmental Flows Through System Reoperation and Management: Lessons From Case Studies of Implementation

Water-management infrastructure, such as dams, diversions, and levees, provides important benefits to society, including energy, flood management and water supply, but this infrastructure is a primary cause of the decline of freshwater ecosystems and the services they provide. Due to these declines, recent attention has focused on improving the environmental performance of water infrastructure, such as modifying the location, design or operation of infrastructure to maintain or restore environmental flows. Despite growing attention to the importance of environmental flows, and continued advancement in flow assessment methods, implementation of flow protection or restoration has lagged expectations. In this paper we describe how pursuing environmental flows at the scale of infrastructure systems, rather than individual sites, such as a dam, offers two pathways to increased implementation of environmental flows. First, policy and management mechanisms that apply to large areas—river basins or political jurisdictions—can catalyze large-scale implementation of flow protection or restoration. We provide two examples of system-scale policy and management mechanisms: flow protection policies and system-scale hydropower planning and management. Although system-scale policy and management offer a clear path to large-scale implementation, there will continue to be a need for flow implementation that occurs at smaller scales, such as a high priority river reach. The second pathway focuses on implementation at that scale—such as environmental flow releases from a dam or small set of dams—but embeds dam reoperation or site-scale flow implementation within reoperation of the larger systems of resource management within which the dam or ecosystem is located. These systems of resource management can encompass various sectors and here we provide examples of dam reoperation or flow implementation facilitated by solutions that included changes to the management of (1) water supply systems; (2) floodplains; and (3) irrigation systems. We illustrate both of these system-scale pathways through a set of case studies, drawn primarily from North America, each of which includes an example of current implementation.

A hybrid modeling approach to evaluate the groundwater flow system at the low- and intermediate-level radioactive waste disposal site in Gyeong-Ju, Korea

The development and implementation of a hybrid discrete fracture network/equivalent porous medium (DFN/EPM) approach to groundwater flow at the Gyeong-Ju low- and intermediate-level radioactive waste (LILW) disposal site in the Republic of Korea is reported. The geometrical and hydrogeological properties of fractured zones, background fractures and rock matrix were derived from site characterization data and implemented as a DFN. Several DFN realizations, including the deterministic fractured zones and the stochastic background fractures, whose statistical properties were verified by comparison with in-situ fracture and hydraulic test data, were suggested, and they were then upscaled to continuums using a fracture tensor approach for site-scale flow simulations. The upscaled models were evaluated by comparison to in-situ pressure monitoring data, and then used to simulate post-closure hydrogeology for the LILW facility. Simulation results demonstrate the importance of careful characterization and implementation of fractured zones. The study highlighted the importance of reducing uncertainty regarding the properties and variability of natural background fractures, particularly in the immediate vicinity of repository emplacement.

Grid Resolution Study of Ground Water Flow and Transport

Three-dimensional grids representing a heterogeneous, ground water system are generated at 10 different resolutions in support of a site-scale flow and transport modeling effort. These grids represent hydrostratigraphy near Yucca Mountain, Nevada, consisting of 18 stratigraphic units with contrasting fluid flow and transport properties. The grid generation method allows the stratigraphy to be modeled by numerical grids of different resolution so that comparison studies can be performed to test for grid quality and determine the resolution required to resolve geologic structure and physical processes such as fluid flow and solute transport. The process of generating numerical grids with appropriate property distributions from geologic conceptual models is automated, thus making the entire process easy to implement with fewer user-induced errors. The series of grids of various resolutions are used to assess the level at which increasing resolution no longer influences the flow and solute transport results. Grid resolution is found to be a critical issue for ground water flow and solute transport. The resolution required in a particular instance is a function of the feature size of the model, the intrinsic properties of materials, the specific physics of the problem, and boundary conditions. The asymptotic nature of results related to flow and transport indicate that for a hydrologic model of the heterogeneous hydrostratigraphy under Yucca Mountain, a horizontal grid spacing of 600 m and vertical grid spacing of 40 m resolve the hydrostratigraphic model with sufficient precision to accurately model the hypothetical flow and solute transport to within 5% of the value that would be obtained with much higher resolution.

Performance assessment model development and analysis of radionuclide transport in the unsaturated zone, Yucca Mountain, Nevada

This paper describes the development and use of a particle-tracking model to perform radionuclide-transport simulations in the unsaturated zone at Yucca Mountain, Nevada. The goal of the effort was to produce a computational model that can be coupled to the project's calibrated 3D site-scale flow model so that the results of that effort could be incorporated directly into the Total System Performance Assessment (TSPA) analyses. The transport model simulates multiple species (typically 20 or more) with complex time-varying and spatially varying releases from the potential repository. Water-table rise, climate-change scenarios, and decay chains are additional features of the model. A cell-based particle-tracking method was employed that includes a dual-permeability formulation, advection, longitudinal dispersion, matrix diffusion, and colloid-facilitated transport. This paper examines the transport behavior of several key radionuclides through the unsaturated zone using the calibrated 3D unsaturated flow fields. Computational results illustrate the relative importance of fracture flow, matrix diffusion, and lateral diversion on the distribution of travel times from the simulated repository to the water table for various climatic conditions. Results also indicate rapid transport through fractures for a portion of the released mass. Further refinement of the model will address several issues, including conservatism in the transport model, the assignment of parameters in the flow and transport models, and the underlying assumptions used to support the conceptual models of flow and transport in the unsaturated zone at Yucca Mountain.

Radionuclide transport simulation and uncertainty analyses with the saturated-zone site-scale model at Yucca Mountain, Nevada

Evaluation of radionuclide transport in the saturated zone (SZ) to the accessible environment is an important component of performance assessment for the proposed radioactive waste repository at Yucca Mountain. Simulations of radionuclide migration in the SZ have been performed using the calibrated three-dimensional (3-D) SZ site-scale flow and transport model. An innovative particle-tracking method was used to simulate transport that includes the processes of advection, dispersion, matrix diffusion, and sorption. The uncertainties in groundwater flow and radionuclide transport were quantitatively evaluated to develop uncertainty distributions for key model parameters, and multiple realizations of the SZ system were simulated using the SZ site-scale model. The results of multiple realizations of radionuclide transport indicate significant aggregate uncertainty in transport times through the SZ. The simulated radionuclide mass breakthrough curves in the SZ have been coupled with other components of the repository system in Total System Performance Assessment (TSPA) analyses and constitute the means by which uncertainty in the SZ is incorporated into regulatory analyses. Regression analysis has been used to determine the sensitivity of radionuclide transport simulation results to the uncertainty of individual model input parameters. Results of the sensitivity analysis indicate that median radionuclide transport times were dominantly controlled by uncertainty in the specific discharge in the SZ, with sorption and retardation in the alluvium playing important roles for some radionuclides.

Overview of scientific investigations at Yucca Mountain—the potential repository for high-level nuclear waste

The US Department of Energy (DOE) has been evaluating the unsaturated zone (UZ) at Yucca Mountain, Nevada, as a potential repository site for high level nuclear waste. A large number of boreholes have been drilled at the site, and an underground tunnel, the exploratory studies facility (ESF), has been constructed. A variety of boreholes and underground tests have been conducted that have provided large amounts of data from the UZ. This paper summarizes some of the current knowledge of geological and hydrological characteristics of the site and provides the framework for a series of technical papers that follow in this issue. A brief discussion is also given to the repository safety strategy for Yucca Mountain and the UZ site-scale flow model.

Migration of Solutes in Unsaturated, Fractured Rock at Yucca Mountain: Measurements, Mechanisms, and Models

AbstractCharacterization and performance assessment (PA) studies for the potential high-level nuclear waste repository at Yucca Mountain require an understanding of migration mechanisms and pathways of radioactive solutes. Measurements of 36C1 in samples extracted from boreholes at the site are being used in conjunction with recent infiltration estimates to calibrate a site-scale flow and solute transport model. This exercise using the flow and solute transport model, FEHM, involves testing different model formulations and two different hypotheses to explain the occurrence of elevated 36Cl in the Calico Hills unit (CHn) which indicates younger water than in the overlying Topopah Spring unit (TSw). One hypothesis suggests fast vertical transport from the surface via fractures in the TSw to the CHn. An alternative hypothesis is that the elevated 36C1 concentrations reflect rapid horizontal flow in the CHn or at the interface between the CHn and the TSw with the source being vertical percolation under spatially isolated regions of high infiltration or at outcrops of those units. Arguments in favor of and against the hypotheses are described in conjunction with the site-scale transport studies.