Conceptual Models of Flow and Transport in the Fractured Vadose Zone (2001) / Chapter Skim
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Parameterization and Upscaling in Modeling Flow and Transport in the Unsaturated Zone of Yucca Mountain
Parameterization and Upscaling in Modeling Flow and Transport in the Unsaturated Zone of Yucca Mountain
Pages 335-366
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The unsaturated zone consists of a series 1 Earth Sciences Division, Lawrence Berkeley National Laboratory 335
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of the unsaturated zone at Yucca Mountain. We will first briefly describe the available data and the basic elements of the conceptual model of the unsaturated zone flow and transport.
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The zeolitic rocks have low matrix permeabilities, on the order of microdarcies, and some fracture permeabilities. The current conceptual model, primarily based on perched water data, favors a small amount of water flow through the zeolitic units, with most of the water flowing laterally in perched water bodies and then vertically down through highly permeable faults (Wu et al., l999b; Kwicklis et al., 1999; Flint et al., 1999~.
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A variety of approaches are available for constructing subsurface heterogeneous hydraulic property distributions, as reviewed by Koltermann and Gorelick (1996~. A geology-based, deterministic approach, in which entire model layers are assigned uniform hydraulic properties, has been mainly used for characterizing the unsaturated zone of Yucca Mountain (e.g., Bodvarsson et al., 1997a)
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It is therefore reasonable to assume that matrix liquid water distribution within a sitescale grid block, similar to that at the core scale, is mainly controlled by capillary force under the steady-state flow condition. In this case, the unscaled retention curve can be simply obtained by averaging the retention curves measured from core samples for a given model layer.
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It is a very challenging task to develop an approach to describe large-scale fracture flow, which incorporates fracture flow mechanisms observed at smaller scale. While different approaches are available, we believe that a continuum approach is a suitable and robust approach for the large-scale fracture flow in the unsaturated zone of Yucca Mountain.
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For example, very similar matrix saturation distributions were observed from different deep boreholes. Calcite coatings, signatures of water flow history, are found in many fractures within the welded units.
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have indicated that in many cases subsurface heterogeneities can be characterized by stochastic fractals. The air permeability data seem to support this reasoning.
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Different fracture permeability values should be used for modeling flow and transport at different scales. The permeability values for the site-scale model were determined by calibrating the model against field observations, including ambient pneumatic signals, while the small-scale (<10 m)
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In this case, the van Genuchten relations may not be valid. Recently, we developed an "active fracture model," which is based on the hypothesis that not all connected fractures actively conduct liquid water in the unsaturated zone at Yucca Mountain because of fingering (Liu et al., 1998~.
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However, a large-scale tracer test, which is needed to determine the dispersivity value for the site-scale model, is not considered to be a realistic task for the unsaturated zone of Yucca Mountain considering the temporal scale required for conducting the test. Since fracture dispersivity values from field observations are currently not available, in order to evaluate the importance of fracture dispersivity to the overall chemical transport behavior, we simulate tracer transport along a vertical column extracted from the three-dimensional model for the unsaturated zone of Yucca Mountain (Bodvarsson et al., 1997a)
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Data inversion involves using numerical models to predict conditions in the unsaturated zone and then comparing them to observations of these conditions from the field (e.g., saturation data and gas pressure data)
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Saturation and moisture tension data inversion is used to estimate the matrix and fracture flow parameters, including permeability, a fracture-matrix interaction parameter, and unsaturated flow parameters (van Genuchten's or and m)
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estimation of kf, km' OCf, OCm, mf, mm, and ~ for faults. 3-D site-scale model inversion of perched water pump test data estimation of kf for model layers in perched water verification against saturation, water potential, pneumatic pressure, temperature, perched water extent and elevation, and ambient geochemistry data FIGURE 11-5 Flow chart showing inversion/calibration/verification approach.
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1o2 FIGURE 11-6 Calibrated simulation match to saturation and moisture tension data at borehole USW SD-9.
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FIGURE 11-7 Calibrated simulation match to pneumatic pressure data at borehole USW SD-12.
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Where pneumatic data is used to calibrate fracture permeability, the uncertainty of the fracture permeability is much less than the uncertainty of the matrix permeability. This is because there are substantially more pneumatic data points than saturation and water potential data points.
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The presence of perched water indicates that the vertical percolation flux locally exceeds the saturated hydraulic conductivity at those perching layers and suggests that flow paths may not be vertical through the entire thickness of the unsaturated zone to the water table. Instead, water may be diverted laterally to a fault zone or another high-permeability channel that serves to focus flow down
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To capture these phenomena of lateral flow caused by water perching conditions, threedimensional model calibrations are needed. The genesis of perched water at Yucca Mountain is much debated among Yucca Mountain Project scientists, and several conceptual models have been discussed (Rousseau et al., 1998; Wu et al., l999b)
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The modeling study and sensitivity analysis of this work concludes that it is necessary to conduct three-dimensional model calibrations in order to describe perched water occurrences at Yucca Mountain. Several key factors were identified for creating a perched-water zone in a numerical model.
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in the unsaturated zone can be used as an important constraint on net infiltration rates, percolation fluxes, and flow pathways, and as an indicator of the extent of fracture-matrix and water-rock interaction. Mineral paragenesis, distribution, and composition are also useful, yet complex, systems for quantifying flow and transport processes.
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356 1 500 1 400 1 300 1 200 __ .° 1 1 00 ct 1 000 900 800 CONCEPTUAL MODELS OF FLOWAND TRANSPORT H-5 . \ H-5 Observed Data H-5 Simulated Data Hydro.
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In contrast to directly using a method such as chloride mass balance and making assumptions as to the absence of diffusion and lateral flow, limiting the system to one-dimensional piston-flow, it is preferable to incorporate directly such tracers into the calibrated flow model. An example using chloride as a conservative tracer in UZ flow and transport simulations of Yucca Mountain is presented in Figure 11-9.
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, and the average i4C age of air at this level (~6,000 years; Yang et al., 1996~; however, data from this particular zone will be needed for model validation. The large-scale continuum model has proven remarkably effective in predicting the range in concentrations and their trends in the unsaturated zone (Sonnenthal and Bodvarsson, 1999~.
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. Perched waters that are in contact with zeolitized rocks have Sr concentrations one to two orders of magnitude lower.
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All of these uncertainties have been addressed to a certain extent in past studies; however, a comprehensive study is still needed to reduce these parameter uncertainties further by continuous field, laboratory, and modeling efforts. SUMMARY AND CONCLUSIONS The unsaturated zone flow and transport model of Yucca Mountain, Nevada.
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This work was supported by the Director, Office of Civilian Radioactive Waste Management, U.S. Department of Energy, through Memorandum Purchase Order EA9013MC5X between TRW Environmental Safety Systems and the Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab)
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Using Chloride to Trace Water Movement in the Unsaturated Zone at Yucca Mountain. In: Proceedings of the Eighth International Conference on High-Level Radioactive Waste Management.
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Inferences for Yucca Mountain unsaturated zone hydrology from secondary minerals. In: Proceedings of the Eighth Annual International Conference, High Level Radioactive Waste Management, May 1114, 1998.
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Alternative Concepts and Approaches for Modeling Unsaturated Flow and Transport in Fractured Rocks. Chapter 24 of The Site-Scale Unsaturated Zone Model of Yucca Mountain, Nevada, for the Viability Assessment.
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A modeling study of perched water phenomena in the unsaturated zone at Yucca Mountain. Journal of Contaminant Hydrology 38(1-3)
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