Site-scale Model Research Articles

Coupled HM effects in a crystalline rock mass due to glaciation: indicative results from groundwater flow regimes and stresses from an FEM study

This paper highlights certain aspects of glacial impact on groundwater flow and rock mechanics, of particular interest in future scenarios for the geological disposal of nuclear waste. The investigation took the form of a generic exercise based on conditions at the underground research facility in Whiteshell, Canada. The site scale model domain boundaries were set up based on a number of major deformation zones. The surface boundary conditions comprised a transient ice sheet load and related hydraulic heads, generated by meltwater. It has thus been possible to compare glacial impact in relation to present-day climatological conditions. The main issues in the investigation were to evaluate the groundwater flow regime and the pre-requisites for underground jacking as well as shearing according to the prescribed geoscientific properties of a benchmark protocol. Special attention has been devoted to a solution to the hydromechanical (HM) problem (1) with or without process coupling, (2) with or without a transient approach, and (3) considering a two- or three-dimensional mode. The study underlines the need for transient analyses in 3D of these coupled phenomena.

Interbasin flow revisited: The contribution of local recharge to high-discharge springs, Death Valley, CA

Springs in the Furnace Creek area (Texas, Travertine, and Nevares Springs) of Death Valley National Park exhibit high discharge rates and depleted δ 18O VSMOW (∼−13‰) and δD VSMOW (∼−102‰) values. Isotopic depletion of this magnitude and large spring fluxes (∼10,000 L/min) suggests that modern local recharge in the arid Furnace Creek drainage cannot be responsible for spring fluxes. An alternate explanation, interbasin flow, is difficult to envisage due to the stratigraphic and structural relationships of bedrock in intervening ranges, although it is the most common conceptual model for Furnace Creek spring flows. High-flux springs at Furnace Creek nonetheless respond modestly to modern climate in terms of discharge rate and isotopic composition. Hydrographs show a climate response and variations in time-series stable isotope data of widely spaced springs track one another. Small, but measurable quantities of tritium (<0.2 TU) were found at Nevares Spring, also suggesting a component of modern recharge. Thus, whatever the main source of water for these springs may be, there appears to be a subtle, but recent climatic influence. Estimates of flow at nearby mountain springs produce discharge rates per square kilometer of catchment that, by analogy, could support from 20 to 300% of the flow at large Death Valley springs under the current climate. Yet, 14C model ages suggest valley-bottom springs at Furnace Creek (5500–14,500 yr) contain a large component of older water, suggesting that much of the water was recharged during a pluvial period (Younger Dryas?) when net infiltration would have been much higher and isotopically depleted. 14C model ages are also of similar age, or younger, than many ‘up gradient’ waters, rather than being older as would be expected for interbasin flow. Chemical evolution models of solutes are consistent with both local recharge and interbasin transfer from Ash Meadows. However, when considered with isotopic constraints, interbasin flow becomes obviously untenable. Estimates of the thickness of alluvium and semi-consolidated Tertiary units in the Furnace Creek drainage seem to provide adequate storage, confinement, and upward leakage to accommodate current discharge. Thus, although Death Valley is the ultimate discharge location for regional groundwaters in terms of potential, careful study of these springs suggests that most of their flux is supported by local pluvial recharge, suggesting that a careful re-evaluation of the interbasin transfers be conducted on a case-by-case basis. Furthermore, regional flow models that are built on the concept of interbasin flow provide boundary flux conditions for site-scale models for the proposed nuclear waste repository at Yucca Mountain, Nevada. Thus, site-scale models may over-predict the potential transport of waste from the Yucca Mountain facility.

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.

Field investigation into unsaturated flow and transport in a fault: model analyses

Results of a fault test performed in the unsaturated zone of Yucca Mountain, Nevada, were analyzed using a three-dimensional numerical model. The fault was explicitly represented as a discrete feature and the surrounding rock was treated as a dual-continuum (fracture–matrix) system. Model calibration against seepage and water-travel-velocity data suggests that lithophysal cavities connected to fractures can considerably enhance the effective fracture porosity and therefore retard water flow in fractures. Comparisons between simulation results and tracer concentration data also indicate that matrix diffusion is an important mechanism for solute transport in unsaturated fractured rock. We found that an increased fault–matrix and fracture–matrix interface areas were needed to match the observed tracer data, which is consistent with previous studies. The study results suggest that the current site-scale model for the unsaturated zone of Yucca Mountain may underestimate radionuclide transport time within the unsaturated zone, because an increased fracture–matrix interface area and the increased effective fracture porosity arising from lithophysal cavities are not considered in the current site-scale model.

A triple-continuum approach for modeling flow and transport processes in fractured rock

This paper presents a triple-continuum conceptual model for simulating flow and transport processes in fractured rock. Field data collected from the unsaturated zone of Yucca Mountain, a repository site of high-level nuclear waste, show a large number of small-scale fractures. The effect of these small fractures has not been considered in previous modeling investigations within the context of a continuum approach. A new triple-continuum model (consisting of matrix, small-fracture, and large-fracture continua) has been developed to investigate the effect of these small fractures. This paper derives the model formulation and discusses the basic triple-continuum behavior of flow and transport processes under different conditions, using both analytical solutions and numerical approaches. The simulation results from the site-scale model of the unsaturated zone of Yucca Mountain indicate that these small fractures may have an important effect on radionuclide transport within the 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.

The saturated zone at Yucca Mountain: an overview of the characterization and assessment of the saturated zone as a barrier to potential radionuclide migration

The US Department of Energy is pursuing Yucca Mountain, Nevada, for the development of a geologic repository for the disposal of spent nuclear fuel and high-level radioactive waste, if the repository is able to meet applicable radiation protection standards established by the US Nuclear Regulatory Commission and the US Environmental Protection Agency (EPA). Effective performance of such a repository would rely on a number of natural and engineered barriers to isolate radioactive waste from the accessible environment. Groundwater beneath Yucca Mountain is the primary medium through which most radionuclides might move away from the potential repository. The saturated zone (SZ) system is expected to act as a natural barrier to this possible movement of radionuclides both by delaying their transport and by reducing their concentration before they reach the accessible environment. Information obtained from Yucca Mountain Site Characterization Project activities is used to estimate groundwater flow rates through the site-scale SZ flow and transport model area and to constrain general conceptual models of groundwater flow in the site-scale area. The site-scale conceptual model is a synthesis of what is known about flow and transport processes at the scale required for total system performance assessment of the site. This knowledge builds on and is consistent with knowledge that has accumulated at the regional scale but is more detailed because more data are available at the site-scale level. The mathematical basis of the site-scale model and the associated numerical approaches are designed to assist in quantifying the uncertainty in the permeability of rocks in the geologic framework model and to represent accurately the flow and transport processes included in the site-scale conceptual model. Confidence in the results of the mathematical model was obtained by comparing calculated to observed hydraulic heads, estimated to measured permeabilities, and lateral flow rates calculated by the site-scale model to those calculated by the regional-scale flow model. In addition, it was confirmed that the flow paths leaving the region of the potential repository are consistent with those inferred from gradients of measured head and those independently inferred from water-chemistry data. The general approach of the site-scale SZ flow and transport model analysis is to calculate unit breakthrough curves for radionuclides at the interface between the SZ and the biosphere using the three-dimensional site-scale SZ flow and transport model. Uncertainties are explicitly incorporated into the site-scale SZ flow and transport abstractions through key parameters and conceptual models.

Modeling dispersion in three-dimensional heterogeneous fractured media at Yucca Mountain

Highly resolved numerical simulations are conducted to evaluate the longitudinal and transverse dispersivities proposed for use in the larger-scale Yucca Mountain saturated zone (SZ) site-scale model. Two different stochastic continuum models (SCM) that define the spatial variability of permeability are inferred from the observed fracture characteristics and the measured permeabilities. These models are created with a combination of indicator geostatistics and boolean simulation that allow for modeling different correlation lengths and anisotropy ratios at different permeability thresholds as well as the inclusion of large, high-permeability features. Longitudinal and transverse (horizontal and vertical) dispersion through the permeability realizations is evaluated for both distributed and focused source geometries using groundwater flow and streamline particle tracking. These numerical results are compared to behavior predicted by an analytical solution and to dispersivities estimated by an expert panel. Early time transport results are significantly non-Gaussian due to the strong heterogeneity of the fractured medium. At late times, travel distances of 23 correlation lengths, the longitudinal and transverse horizontal dispersivity results are well approximated by the analytical solution and the expert elicitation estimates. The calculated transverse vertical dispersivity values are smaller than those estimated from the analytical solution. Inclusion of high-permeability features of the same size as the model domain with a distributed planar source creates extreme values of the longitudinal and transverse horizontal dispersivity.

Coupling basin- and site-scale inverse models of the Española aquifer.

Large-scale models are frequently used to estimate fluxes to small-scale models. The uncertainty associated with these flux estimates, however, is rarely addressed. We present a case study from the Española Basin, northern New Mexico, where we use a basin-scale model coupled with a high-resolution, nested site-scale model. Both models are three-dimensional and are analyzed by codes FEHM and PEST. Using constrained nonlinear optimization, we examine the effect of parameter uncertainty in the basin-scale model on the nonlinear confidence limits of predicted fluxes to the site-scale model. We find that some of the fluxes are very well constrained, while for others there is fairly large uncertainty. Site-scale transport simulation results, however, are relatively insensitive to the estimated uncertainty in the fluxes. We also compare parameter estimates obtained by the basin- and site-scale inverse models. Differences in the model grid resolution (scale of parameter estimation) result in differing delineation of hydrostratigraphic units, so the two models produce different estimates for some units. The effect is similar to the observed scale effect in medium properties owing to differences in tested volume. More important, estimation uncertainty of model parameters is quite different at the two scales. Overall, the basin inverse model resulted in significantly lower estimates of uncertainty, because of the larger calibration dataset available. This suggests that the basin-scale model contributes not only important boundary condition information but also improved parameter identification for some units. Our results demonstrate that caution is warranted when applying parameter estimates inferred from a large-scale model to small-scale simulations, and vice versa.

Estimating recharge at Yucca Mountain, Nevada, USA: comparison of methods

Obtaining values of net infiltration, groundwater travel time, and recharge is necessary at the Yucca Mountain site, Nevada, USA, in order to evaluate the expected performance of a potential repository as a containment system for high-level radioactive waste. However, the geologic complexities of this site, its low precipitation and net infiltration, with numerous mechanisms operating simultaneously to move water through the system, provide many challenges for the estimation of the spatial distribution of recharge. A variety of methods appropriate for arid environments has been applied, including water-balance techniques, calculations using Darcy's law in the unsaturated zone, a soil-physics method applied to neutron-hole water-content data, inverse modeling of thermal profiles in boreholes extending through the thick unsaturated zone, chloride mass balance, atmospheric radionuclides, and empirical approaches. These methods indicate that near-surface infiltration rates at Yucca Mountain are highly variable in time and space, with local (point) values ranging from zero to several hundred millimeters per year. Spatially distributed net-infiltration values average 5 mm/year, with the highest values approaching 20 mm/year near Yucca Crest. Site-scale recharge estimates range from less than 1 to about 12 mm/year. These results have been incorporated into a site-scale model that has been calibrated using these data sets that reflect infiltration processes acting on highly variable temporal and spatial scales. The modeling study predicts highly non-uniform recharge at the water table, distributed significantly differently from the non-uniform infiltration pattern at the surface.

Multiple-scale habitat modeling approach for rare plant conservation

Multiple-scale habitat assessment for rare plants is an important component of conservation and development planning. It is challenging, however, due to lack of information synthesis on the ecology of rare plants, lack of effective approaches for habitat assessment at multiple spatial scales, and lack of spatial data for relevant environmental attributes and scales. A multiple-scale habitat modeling approach was developed to meet this need. Regional-, landscape-, and site-scale habitat models were developed for eight rare plant species found in southern Texas, USA. The models were partially validated and used for planning of rare plant conservation and highway construction. Regional-scale habitat models were used to predict, based on coarse-scale geographic information system (GIS) data, spatial distribution of areas containing potential habitat of rare plant species and the probability of encountering potential rare plant habitats. Site-scale models, based on synthesis of the literature and field investigations, were developed for field survey and mapping of rare plant habitats to enable accurate assessment of potential and present habitat suitability of specific locations using fine-resolution field data on soil, landform and vegetation structure. The greatest need for assessing the presence and potential habitat of rare plants is at the landscape scales. Thus, landscape-scale models were developed for spatially explicit assessment of potential and present habitat suitability, based on site-scale models but using GIS and remote sensing-based data. These models can be used as effective tools for conservation planning, monitoring and management of rare plant habitat, as well as for reduction of land use conflicts and development cost. The processes of model development and application synthesizes the diffuse literature, identifies knowledge and data gaps to guide future research, and provides a framework for assimilating new information acquired in the future to improve habitat assessment.

Calibrating hydrogeologic parameters for the 3-D site-scale unsaturated zone model of Yucca Mountain, Nevada

An important issue in the evaluation of the unsaturated zone (UZ) at Yucca Mountain, Nevada is the calibration of the parameters used in the 3-D site-scale numerical flow model. The hydrogeologic parameters are calibrated using an inversion code (ITOUGH2) to fit measured core sample and in situ data according to statistical criteria. The available data include saturations, water potentials, pneumatic pressure records, and pore water age estimates based on geochemical analyses. Five 1-D columns are extracted from the 3-D site-scale model and used in an inversion that matches data from the five boreholes simultaneously. This results in layer-averaged fracture and matrix permeabilities and van Genuchten fitting parameters. The columns are then refined and individual inversions are performed with each 1-D model to develop a spatially varying parameter set. A number of sensitivity studies are performed to evaluate the effects of modeling assumptions on the calibrated parameters. Several fracture-matrix interaction formulations are also investigated during the study. The results show that the calibrated parameter set concluded to be most consistent with the conceptual model provides a good fit to the ambient data. The study also shows that the choice of the fracture-matrix interaction formulation has a significant effect on the calibrated parameter set.

Investigation of conceptual and numerical approaches for evaluating moisture, gas, chemical, and heat transport in fractured unsaturated rock

We investigate the utility and appropriateness of various conceptual and numerical approaches for modeling several flow and transport processes in the unsaturated zone (UZ) at Yucca Mountain, NV, using a one-dimensional (1-D) column extracted from a three-dimensional (3-D) UZ site-scale model. Simulations of steady-state and transient moisture flow, transient gas flow, tracer transport, and thermal loading scenarios are performed, using a variety of numerical approaches to treat fracture–matrix (F–M) interactions, including an effective continuum model (ECM) and several varieties of dual-continua models. For the dual-continua models, we investigate the effect of varying the number of matrix gridblocks per fracture gridblock, the formulation applied for calculating F–M interface area, and whether or not global matrix-to-matrix flow occurs (dual-permeability vs. dual-porosity models). The key findings of the work based on a 1-D column are as follows. For steady-state moisture flow, most of the numerical and conceptual models provide similar results for saturation and fracture flow profiles. The ECM adequately models the steady-state processes because the system is not too far away from F–M equilibrium. For both transient moisture flow and transient transport in a steady flow field, the ECM is not adequate in general. Within the dual-continua models, the formulation for F–M interface area can have a major effect on the hydrodynamic response to an infiltration pulse and tracer arrival at various horizons, with fracture responses becoming earlier as F–M interface area decreases. The number of matrix blocks also has a significant effect on response time, with the more accurate multi-matrix-gridblock models yielding slower fracture response times. For transient gas flow arising from barometric pressure variations, the ECM adequately models the process, because F–M gas flow occurs rapidly compared to the time scale of the barometric pressure variations. For thermal loading, preliminary studies indicate that the ECM does not capture all the significant physical processes, because rapid fluid and heat flow can occur in the fractures before the matrix has a chance to equilibrate.

A site-scale model for fluid and heat flow in the unsaturated zone of Yucca Mountain, Nevada

A three-dimensional unsaturated-zone numerical model has been developed to simulate flow and distribution of moisture, gas and heat at Yucca Mountain, Nevada, a potential repository site for high-level radioactive waste. The model takes into account the simultaneous flow dynamics of liquid water, vapor, air and heat in the highly heterogeneous, fractured porous rock in the unsaturated zone (UZ). This model is intended for use in the prediction of the current and future conditions in the UZ so as to aid in the assessment of the system performance of the proposed repository. The modeling approach is based on a mathematical formulation of coupled multiphase, multicomponent fluid and heat flow through porous and fractured rock. Fracture and matrix flow is treated using both dual-permeability and effective-continuum modeling approaches. The model domain covers a total area of approximately 43 km 2, and uses the land surface and the water table as its top and bottom boundaries. In addition, site-specific data, representative surface infiltration, and geothermal conditions are incorporated into the model. The reliability and accuracy of the model have been the subject of a comprehensive model calibration study, in which the model was calibrated against measured data, including liquid saturation, water potential and temperature. It has been found that the model is generally able to reproduce the overall system behavior at Yucca Mountain with respect to moisture profiles, pneumatic pressure variations in different geological units, and ambient geothermal conditions.

Methodology for optimization of operation to reduce site-scale energy use in production plants

A methodology was developed to improve energy usage in the process industries by optimizing plant operation. The first step was to build fine tuned site-scale models of energy intensive production plants and of combined heat and power utility distribution networks (heat, power, cooling), and later to optimize them using various numerical techniques. The methodology not only comprises these models, but also the selected optimization methods, their implementation in algorithms suited for the problems to be solved and the evaluation of the results in terms of their applicability in real industrial problems. The detailed models were tuned on the basis of actual plant data. They were used to calculate accurate energy balances, and thus to identify sources of energy waste. Finally they allowed optimization of energy usage by adjusting operating conditions at a medium time scale (a few hours to a couple of days). Energy savings in the range 5%–10% could be obtained for energy intensive processes, where even a low percentage represents a considerable amount.

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.

Spatial Variability in Hydrologic Properties of a Volcanic Tuff

AbstractSpatial variability of hydrologic properties was quantified for a nonwelded‐to‐welded ash flow tuff at Yucca Mountain, Nevada, the potential site of a high‐level, nuclear waste repository. Bulk density, porosity, saturated hydraulic conductivity, and sorptivity were measured on core specimens collected from outcrops on a grid that extended vertically through the entire unit thickness and horizontally 1.3 km in the direction of ash transport from the volcanic vent. A strong, geologically determined (deterministic) vertical trend in properties was apparent that correlated with visual trends in degree of welding observed in the outcrop. The trend was accurately described by simple regression models based on stratigraphic elevation (vertical distance from the base of the unit divided by unit thickness). No significant horizontal trends in properties were detected along the length of the transect. The validity of the developed model was tested by comparing model predictions with measured porosity values from additional outcrop sections and boreholes that extended 3000 m north, 1500 m northeast, and 6000 m south of the study area. The model accurately described vertical porosity variations except for locations very close to the source caldera, where the model underpredicted porosity in the upper half of the section. The presence of deterministic geologic trends, such as those demonstrated for an ash flow unit in this study, can simplify the collection of site characterization data and the development of site‐scale models.

Entering the Era of Distributed Snow Models

Traditionally, snowmelt modelling has been governed by the operational need for runoff forecasts. Parsimony in terms of model complexity and data requirements was a major concern. More recently, the increased importance of analyzing environmental problems and extreme conditions has motivated the development of distributed snow models. Unfortunately, the use of this type of models is limited by a number of factors including a) the extreme heterogeneity of the hydrologic environment, b) the mismatch of scales between observed variables and model state variables, c) the large number of model parameters, and d) the observability/testability problem. This paper discusses the implications of these constraints on the use of site and catchment scale concepts, regionalisation techniques, and calibration methods. In particular, the point is made that in many cases model parameters are poorly defined or not unique when being optimized on the basis of runoff data. Snow cover depletion patterns are shown to be vastly superior to runoff data for discriminating between alternative model assumptions. The patterns are capable of addressing individual model components representing snow deposition and albedo while the respective parameters are highly intercorrelated in terms of catchment runoff. The paper concludes that site scale models of snow cover processes are fairly advanced but much is left to be done at the catchment scale. Specifically, more emphasis needs to be directed towards measuring and representing spatial variability in catchments as well as on spatially distributed model evaluation.