Conceptual Models of Flow and Transport in the Fractured Vadose Zone (2001) / Chapter Skim
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Evaluation of Conceptual and Quantitative Models of Fluid Flow and Chamical Transport in Fractured Media
Evaluation of Conceptual and Quantitative Models of Fluid Flow and Chamical Transport in Fractured Media
Pages 115-148
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From page 115... ...
Analyses of transport of reactive chemicals, including effects of precipitation and dissolution, as well as changes in fracture morphology, also demonstrate highly non-uniform behavior. In the context of these results, we discuss some conceptual and quantitative models of fluid flow and chemical transport in the fractured vadose zone, and the laboratory and field data that are required to evaluate them.
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In this chapter, we consider conceptual pictures and quantitative models of fluid flow and chemical transport in fractured and heterogeneous porous media relevant to the fractured vadose zone. To this end, we build on our more established understanding of these processes in saturated systems.
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CONCEPTUAL PICTURES AND MODELING APPROACHES FOR FLUID FLOW AND CHEMICAL TRANSPORT Single Fractures Fluid Flow in Saturated Fractures Many laboratory experiments (e.g., Raven and Gale, 1985; Pyrak-Nolte et al., 1987; Durham and Bonner, 1994) and field studies (e.g., Novakowski et al., 1985,1995; Rasmuson and Neretnieks, 1986; Raven et al., 1988)
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Flow in rock fractures: The local cubic law assumption reexamined. Water Resources Research 34: 2811-2825.
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find that the geometry of the contact regions between fracture walls influences flow paths more significantly than might be expected from consideration of only the nominal area fraction of these contacts. Moreover, this effect is enhanced by the presence of non-LCL regions around these contacts.
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120 CONCEPTUAL MODELS OF FLOWAND TRANSPORT it/ At' ~1 ~ / 31 ~ i S> ~ h ~ — -\ .
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In general, we do not have exact information on the (varying) void space within the sample itself, as it changes under different applied stresses, as well as following geochemical reactions (see the next section and the section on Field-Scale Analysis of Fluid Flow and Chemical Transport in the Fractured Vadose Zone)
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As a consequence, although natural, the question of flow versus aperture, and validity of a cubic law, may be the wrong one to ask about rough fractures. Chemical Transport Processes in Saturated Fractures In addition to rough fracture walls, the presence of filling material further influences the hydraulic properties of a fracture.
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Numerical models describing precipitation and dissolution in porous media and fractures were presented by Salles et al.
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In the context of fluid flow and chemical transport in partially saturated fractured systems, we can consider flow in initially unsaturated (or partially saturated) single fractures, accounting only for the fracture itself, or accounting for the fracture together with the porous medium in which it is embedded.
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(1998~. Fracture Networks We consider now conceptual and quantitative models of fluid flow and chemical transport in fracture networks.
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Another approach to modeling flow and transport in fracture networks, which is able to capture distinct preferential flow paths and channeling, is based on percolation theory (e.g., Englman et al., 1983; Robinson, 1983, 1984; Hestir and Long, 1990; Balberg et al., 1991; Berkowitz and Balberg, 1993; Berkowitz, 1995~. In this framework, and near the percolation threshold the point at which the fractures are "just connected" across the entire domain network structures display channeling patterns and transport properties that are quantifiable by power law relationships.
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fluid flow and chemical transport even in fractured porous media with very low bulk saturation. In these contexts, dynamics of fluid flow and chemical transport in fracture networks can also be quantified using the modeling approaches outlined earlier.
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Water Resources Research 34~9~: 21032121. Copyright by American Geophysical Union.
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We analyzed a series of Monte Carlo transport simulations based on synthetic random fracture networks (Berkowitz and Scher, 1997, 1998~. The spatial and temporal behavior of the contaminant advance, based on particle tracking simulations in these networks, is shown in Figure 4-6a, while theoretical concentration profiles based on CTRW are shown in Figure 4-6b.
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, determined from numerical simulations of particle transport in a two-dimensional, random fracture network. The concentration profile, P(x, t)
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EVALUATION OF CONCEPTUAL AND QUANTITATIVE MODELS: FIELD AND LABORATORY DATA From the survey in the previous section, it is clear that a variety of theoretical models must be invoked in order to quantify the broad spectrum of fluid flow and chemical transport behaviors. Similarly, experiments at a variety of laboratory and field scales, under both fully and partially saturated conditions, are required in our efforts to understand and predict these behaviors.
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From page 133... ...
It is expected, moreover, that the asymmetries will have a significant influence on chemical solute transport, dissolution, and precipitation. The features discussed in these studies emphasize the strong heterogeneity and the highly 3D nature of the flow patterns in natural rock fractures, even for relatively small Reynolds numbers, and the need for 3D flow analyses.
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Water Resources Research 34: 1881-1887. Copyright by American Geophysical Union.
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chemical transport in fractured and heterogeneous porous media. We have used these solutions (Berkowitz and Scher, 1998)
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Evidently, infiltrating water and contaminants bypass the lowpermeability chalk matrix via preferential flow paths, possibly through the numerous vertical to subvertical fractures intersecting the chalk matrix (characterized by Bahat, 1988~. Indeed, preferential water flow and solute transport across the fractured chalk in the vadose zone was demonstrated using chemical and isotopic tracers (Nativ et al., 1995~.
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From page 137... ...
Thus packers attached to the top of the sampler divided the fracture opening at the borehole ceiling into 21 separate sections. The use of various tracers in the percolation ponds, and their detection in the drained effluents, enables delineation of flow trajectories connecting the ponds at the fracture inlet to the sampler cells in the horizontal borehole below.
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, and yet no indication of tracer from these ponds was found in sampler cells 4 and 5. This finding suggests that flow paths consist of discrete and unconnected channels within the main fracture plane.
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CONCEPTUAL AND QUANTITATIVE MODELS IN FRACTURED MEDIA 139 ~ en P Cal ~ ~ (a)
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~ " O ~ ~ \ CL \ CONCEPTUAL MODELS OF FLOWAND TRANSPORT m o ~ O of Z ~ = O ~ ~ — ~ oh O AL \ o .' t ~ G)
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Models of flow and transport in these systems must account for the inherent heterogeneity and scaling properties of porous rocks, as well as the typically sparse and uncertain field data that can be obtained to characterize a geological formation. In the fractured vadose zone, key physical parameters that must be considered include, over a range of scales, the geometrical and hydraulic characteristics of individual fractures, their interconnection within networks, and their interplay with the porous host rock.
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From page 142... ...
Thus, we consider alternative modeling approaches, depending on whether the emphasis is on steady-state fluid flow and transient chemical transport through a partially saturated domain, or on transient evolution of either the advancing front of fluid or contaminant, or of the actual flow paths. It is essential that we obtain additional field and laboratory data in order to determine the applicability of these various models.
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Anomalous transport in laboratory-scale, heterogeneous porous media. Water Resources Research 36(1)
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Three-dimensional flow measurements in rock fractures. Water Resources Research 35(12)
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Structure, flow, and generalized conductivity scaling in fracture networks. Water Resources Research 34(9)
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Flow in rock fractures: The local cubic law assumption reexamined. Water Resources Research 34: 2811-2825.
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From page 147... ...
Channel model of flow through fractured media. Water Resources Research 23(3)
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