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Conceptual Models of Flow and Transport in the Fractured Vadose Zone
Pages 7-44

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From page 7...
... Panel Report 7
From page 9...
... When carefully developed and supported by field data, models can be effective tools for understanding complex phenomena and for making informed predictions for a variety of assumed future scenarios. However, model results are always subject to some degree of uncertainty due to limitations in field data and incomplete knowledge of natural pro9
From page 10...
... The Panel was charged with preparing a consensus report on the development and testing of conceptual models for fluid flow and transport in the fractured vadose zone. The Panel's conclusions and recommendations were based in large part on the workshop presentations and discussions.
From page 11...
... First, we discuss general considerations applicable to the development and testing of conceptual models. Second, we summarize the current state of knowledge of flow and transport processes in the fractured vadose zone.
From page 12...
... The more detailed statement reflects the complexity of the modeled processes, some of which may be poorly understood. In the present study, we define a conceptual model as follows: A conceptual model is an evolving hypothesis identifying the important features, processes, and events controlling fluid flow and contaminant transport of consequence at a specific field site in the context of a recognized problem.
From page 13...
... VADOSE ZONE CONCEPTUAL MODELS FIGURE 1-1 Flow chart illustrating the elements of the modeling process.
From page 14...
... For example, an initial conceptual model that is based on regional water budgets, general hydraulic properties, and sparse environmental tracer data is more likely to include relevant and significant processes than if only general hydraulic properties were considered. The conceptual model is the foundation upon which all aspects of the modeling process are constructed, and this should be clearly understood by all members of a project team, from data gatherers to modelers to managers.
From page 15...
... As shown in Figure 1-1, the calibration step can involve significant feedback to the conceptual model. Model calibration is often achieved by adjusting model parameters, either manually or by automated methods, until the model simulated results agree with field data to an acceptable level.
From page 16...
... In this section, we discuss some general issues that should be considered in conceptual model testing. We assume at the outset that the computer code used for simulation is already verified, in other words, that the program logic and numerical algorithm are correctly implemented, and the results are free of computational errors.
From page 17...
... As an example of evaluating a "feature" component of the conceptual model, consider the question of whether a geologic stratum that is represented in the model as a continuous layer underlying the entire model region may in fact be discontinuous and absent at certain locations. The first step in evaluating this alternative hypothesis is to consider its consequence.
From page 18...
... Fourth, solute transport in fractured rocks can exhibit complex behaviors that are difficult to interpret. Development of conceptual models may be more difficult for transport than for flow.
From page 19...
... , "film flow" occurs as a thin film of fluid flowing down the fracture wall. Unlike capillary flow, in which the fluid contacts both fracture walls, the film of fluid contacts only one fracture wall, with an air phase between itself and the opposing fracture wall.
From page 20...
... / C By/' CONCEPTUAL MODELS OF FLOWAND TRANSPORT a b d . ~ ~ , ~ L: ; FIGURE 1-2 Conceptualization of capillary flow in a fracture at increasing levels of saturation (plan view of fracture face)
From page 21...
... Thus, estimation of solute travel time based on the average advance of an infiltration front may not accurately represent the fastest solute travel. In capillary flow, heterogeneity and flow instability can cause large variations in velocity, resulting in "preferential flow" (discussed at greater length below)
From page 22...
... pressure head. The resultant curves appear analogous to the hydraulic conductivity and retention curves used in traditional models of capillary flow (discussed in next section)
From page 23...
... ~3 sCt Cal _.
From page 24...
... The consequence of preferential flowis that a portion of the infiltrating water can be concentrated to move along certain pathways at rates that are significantly faster than the rest of the infiltrating water. With the occurrence of preferential flow, contaminants carried by the infiltrating water can reach a given depth in less time than predicted by calculations assuming a uniform wetting front.
From page 25...
... van Genuchten, 1997. New piecewise-continuous hydraulic functions for modeling preferential flow in an intermittent-flood-irrigated field.
From page 26...
... indicate that in some situations, unsaturated flow pathways within fractured rock can be altered by chemical dissolution and precipitation. Preferential flow may also occur in the absence of macropores, in the form of unstable flow in which an initially subhorizontal, downward-moving wetting front breaks into "fingers." This type of flow pattern is sometimes referred to as "fingering." Wetting front instability can occur in a layered soil profile, as the front moves from a fine-textured layer into coarse-textured layer (Hill and Parlange, 1972~.
From page 27...
... A mechanistic model for water seepage through thick unsaturated zones in fractured rocks of low matrix permeability. Water Resources Research 34~4~: 10391051.
From page 28...
... This is achieved by using a composite hydraulic conductivity function that represents both matrix and fractures. The arrow in the schematic representation indicates fluid flow through the composite (matrix and fracture)
From page 29...
... Thus, the composite hydraulic conductivity curve may have a double hump appearance. By using such an unconventional hydraulic conductivity curve, a traditional model of unsaturated flow can be adapted for preferential flow.
From page 30...
... This topic was discussed in detail by the Committee on Fracture Characterization and Fluid Flow (NRC, 1996, Chapter 6~. When applied to the fractured vadose zone, discrete fracture models require the ability to simulate flow in the matrix as well as in the fractures (Figure 1-7f)
From page 31...
... Fracture-Matrix Interaction The extent to which fracture-matrix interaction controls unsaturated flow depends on a number of factors. Upon the initiation of fracture flow (e.g., after a precipitation event)
From page 32...
... By contrast, evidence indicating significant fracture flow at Yucca Mountain includes: detection of an environmental tracer (bomb-pulse chlorine-36) at depth, an estimated infiltration rate that is greater than the saturated matrix hydraulic conductivity of welded tuff units, and geochemical nonequilibrium between perched water and pore water in the rock matrix.
From page 33...
... Solute Transport Modeling solute transport in the fractured vadose zone adds several layers of complexity compared with modeling fluid flow. The following discussion is limited to modeling of nonreactive solute of sufficiently low concentration that density effects can be neglected.
From page 34...
... For the fractured vadose zone, solute transport models typically assume that fluid flow occurs in the matrix blocks and in the fractures. The conceptual flow model is that of the dual permeability model illustrated by Figure 1-7d.
From page 35...
... While this approach can lead to a detailed understanding of flow processes, key solute transport mechanisms can be overlooked. In some cases, interpretation of environmental tracers has lead to drastic revisions of a conceptual model based initially solely on hydrodynamic analysis.
From page 36...
... While numerous environmental tracers have been used in subsurface hydrology, the existence of both air and water phases in the vadose zone is likely to complicate the use of some tracers, especially those that utilize dissolved gases. Environmental tracers that are likely to be useful for directly investigating fluid flow in fractured vadose zones include tritium, stable isotopes of oxygen and hydrogen in the water molecule, halides, and chlorine-36; see Phillips (this report, Chapter 9)
From page 37...
... CONCLUSIONS AND RECOMMENDATIONS This report discusses the process through which conceptual models of flow and transport in the fractured vadose zone are developed, tested, refined, and reviewed. A conceptual model is defined as an evolving hypothesis identifying the important features, processes, and events controlling fluid flow and contaminant transport of consequence at a specific field site in the context of a recognized problem.
From page 38...
... These conclusions are followed by the Panel's suggestions for research activities that will contribute to the conceptual modeling process. Conclusions on Development and Testing of Conceptual Models 1.
From page 39...
... Conclusions on Flow and Transport in the Fractured Vadose Zone 1. There exists a body of field evidence indicating that infiltration through fractured rocks and structured soils does not always occur as a wetting front advancing at a uniform rate.
From page 40...
... The approach in many current models is to avoid explicitly simulating the mechanisms that cause preferential flow. Instead, the model is implemented to simulate fast and slow flow, by use of a composite hydraulic conductivity curve or by dual-permeability domains.
From page 41...
... These experiments should be designed to understand the controlling processes (capillary flow, film flow, and intermittent behavior) for a broad range of field conditions, to evaluate methods of parameter upscaling, and to test alternative conceptual models.
From page 42...
... Sampling fluids directly from fractures remains problematic. Improved sampling techniques will facilitate the use of environmental tracers and geochemical data for conceptual model building.
From page 43...
... New piecewise-continuous hydraulic functions for modeling preferential flow in an intermittentflood-irrigated field. Water Resources Research 33(9)
From page 44...
... Hydrologic mechanisms governing fluid flow in a partially saturated, fractured, porous medium. Water Resources Research 21(12)


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