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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE 4 Evaluation of Conceptual and Quantitative Models of Fluid Flow and Chemical Transport in Fractured Media Brian Berkowitz,1 Ronit Nativ,2 and Eilon Adar3 ABSTRACT Patterns of fluid flow and chemical transport in heterogeneous porous and fractured media tend to be highly complex. Models of flow and transport processes 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. Recent introduction of advanced theoretical and experimental techniques is providing new insight into our understanding of these highly non-uniform patterns. Statistical network models can be used to characterize the structure of pore and fracture systems, and to define power laws that quantify flow and transport processes within them. In a similar spirit, a random walk formalism can quantify anomalous (non-Gaussian) patterns of chemical transport that are frequently observed in laboratory and field experiments. 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. 1 Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot, Israel 2 Department of Soils and Water, The Hebrew University of Jerusalem, Rehovot, Israel 3 Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Israel
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE INTRODUCTION Major efforts have been devoted over the last two decades to the development of realistic theoretical models capable of simulating flow and transport processes in fractured and heterogeneous porous formations. These efforts have led to significant understanding of the dynamics of flow and transport processes in these systems. However, predictive capabilities related to real fractured and heterogeneous media remain limited. This is due, in part, to the very complex nature of fracture networks in the subsurface, and to the virtual impossibility of obtaining detailed structural, hydraulic, and geochemical characterizations of fractures in situ, in diverse geological and lithological settings. As a result, studies must often rely on extrapolation of exposed features and indirect measurements, together with subjective considerations, to generate a statistical characterization of fracture systems (e.g., Berkowitz and Adler, 1998). These analyses are demanding because fractures exist in a broad range of geological formations and rock types, and are produced under a variety of geological and environmental processes. As a result of the variability in rock properties and structures, as well as the variety in fracturing mechanisms, fracture sizes range from microfissures of the order of microns to major faults of the order of kilometers, while fracture network patterns range from relatively regular polygonal arrangements to apparently random distributions. The hydraulic and transport properties of these formations vary considerably, being dependent largely on the degree of fracture interconnection, aperture variations in the fractures, and chemical characteristics of the fractures and host rock. With regard to fluid flow and chemical transport processes in fractured vadose zones, our ability to predict actual flow and transport behavior —and even our understanding of the basic dynamics—is severely limited, and in some cases rudimentary. Several factors force development of specific conceptual pictures and models tailored to dealing with these processes: (1) fracture walls are rough; (2) fractures often contain filling material; (3) this filling material and the walls themselves are subject to processes of chemical reaction, dissolution, precipitation, mineralization, and/or particle detachment and trapping; (4) fluid migration in partially saturated conditions occurs in preferential paths and channels, with a complex interplay between the fractures and the porous host rock; and (5) fluid and chemical migration patterns are often temporally and spatially unstable. Quantitative models of fluid flow and chemical transport in the fractured vadose zone must therefore be predicated on conceptual pictures of fractured, heterogeneous, and otherwise “disordered” porous media, which account for a variety of non-uniform flow and transport behaviors. 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. Other models spe-
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE cific to fluid flow and chemical transport in unsaturated fractured media, such as film flow and fracture-matrix interactions, are discussed in other chapters in this volume. Given the wide variety of flow and transport processes, and the different perspectives that arise as a function of the actual problem of interest, the quantitative models that we examine are problem-specific. Clearly, there is no single generic model that is appropriate, and so as a consequence, a range of probabilistic and statistical approaches must be introduced. In the next section, we discuss conceptual pictures and modeling approaches for fluid flow and chemical transport. We consider these processes at the scale of individual fractures and in fracture networks. Several experimental efforts are then examined, and in light of these results, we address the issue of how to evaluate conceptual and quantitative models at the laboratory and field scales. 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) indicate that the classical view of a rock fracture as a pair of smooth, parallel plates is not adequate to fully quantify fluid flow. However, though more advanced conceptual models have been introduced in recent years, an alternative model for flow in fractures has yet to be generally accepted. Moreover, some very basic aspects of the theoretical problem remain largely unstudied. A critical, intimately related issue is the validity of the local cubic law. To date, the majority of theoretical and simulation studies of single-fracture flow (Iwai, 1976; Brown, 1987; Moreno et al., 1988; Thompson and Brown, 1991; David, 1993; Unger and Mase, 1993; Amadei and Illangasekare, 1994) have postulated that local flow magnitudes are well described by the Reynolds equation. From this equation, local flow magnitudes are proportional to the cube of the local aperture; hence the name “local cubic law” (LCL). The LCL, a key feature in conceptual models, states that the volumetric flow through a fracture varies as the cube of the fracture aperture. Thus definition of fracture aperture, and application of the LCL, strongly influence quantitative analyses and interpretation of laboratory and field measurements on flow in fractures. The Reynolds equation originates from lubrication theory, which was formulated for narrow void spaces between artificially smoothed surfaces. However, the roughness of rock fracture surfaces is usually more significant and irregular, and has been shown to possess self-affine fractal properties, with a roughness, or
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Hurst exponent H 0.8 (Brown and Scholz, 1985; Poon et al., 1992; Schmittbuhl et al., 1995). A typical self-affine fracture surface with H = 0.8 is shown in Figure 4-1. Moreover, computer simulations (Mourzenko et al., 1995; Brown et al., 1995) have compared predictions from the Stokes and Reynolds equations, and suggest that the Reynolds equation is of limited validity for rough fractures. The adequacy of the LCL for rough geometries remains, essentially, an open question. A detailed review of these studies is beyond the scope of this report. Here we focus on a new approach that we have proposed recently, which addresses the above issues. Oron and Berkowitz (1998) used a two-dimensional order-of-magnitude analysis of the Navier-Stokes equations for a general rough-wall geometry to examine the LCL assumption. This study hypothesized that LCL behavior should not be assumed to be correct or incorrect a priori for the entire fracture; rather, it should be studied on a local basis. As a result, three conditions can be defined for the applicability of LCL flow, as a leading-order approximation, in a local fracture segment with parallel or nonparallel walls. Consider a local cross section of some fracture segment of length L, with a typical halfaperture B. We define a geometric aspect ratio parameter, δ ≡ B/L, and a nondimensional local roughness parameter ε ≡ max(σu/B, σl/B) << 1, where σu and σl are the root-mean-square roughnesses (deviations from the means) of the upper and lower walls, respectively, averaged over the length L. Oron and Berkowitz (1998) demonstrated that the LCL is an adequate first-order approximation FIGURE 4-1 A typical self-affine fracture surface, generated numerically on a 128 × 128 grid with H = 0.8. After Oron, A. P., and B. Berkowitz, 1998. Flow in rock fractures: The local cubic law assumption reexamined. Water Resources Research 34: 2811-2825. Copy-right by American Geophysical Union.
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE of the flow under three conditions: (1) the local segment must be long enough relative to its aperture, so that δ2 << 1; (2) over this segment length, ε << 1; and (3) inertial effects are limited via Re·max(δ,ε) << 1, where Re is the Reynolds number in the segment. These conditions define dynamic-geometric conditions for LCL flow, which are a function of fluid velocity, relative to the fracture aperture and the (relative) wall roughness. From these conditions it follows that the cubic law aperture should be measured as an average over a distance L, normal to the mean trend of the walls, and not with point-by-point methods as suggested by, for example, Ge (1997) or Mourzenko et al. (1995). An illustration of various methods for defining the aperture is given in Figure 4-2. The orientation of measurement over L is set by the general orientation of the segment. Under the above conditions, the LCL, even though it is only a first-order approximation, provides second-order accuracy for estimates of local discharge and of mean fluid velocity. The LCL may also be adapted to pathways with nonparallel walls, as long as the mean path half-angle is moderate. Moreover, the aspect-ratio condition (1) sets a finite resolution in the x-y plane to all analyses based on the LCL. Also, these conditions suggest that even when Re is as low as ~1-10, transition to non-LCL (nonlaminar) flow may occur. Extending to the third dimension, in addition to defining apertures over segment lengths, Oron and Berkowitz (1998) 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. Accounting for the self-affine nature of fracture walls, typical (fractal) fracture wall contact areas generated numerically are illustrated in Figure 4-3. We find a wide variability among the hydraulic conductivity maps, in terms of the shape of the regions in which LCL flow exists, due to the varying distribution and morphology of contact and non-LCL regions in the fractures. While contact ratios of 0.1-0.2 are usually assumed to have a negligible effect, our calculations suggest that contact ratios as low as 0.03-0.05 can be significant. Analysis of computer-generated fractures with self-affine walls demonstrates a nonlinear increase in contact area, and a faster-than-cubic decrease in the overall hydraulic conductivity, with decreasing fracture aperture. These results are in accordance with existing experimental data on flow in fractures (Durham and Bonner, 1994; see also the summary in Figure 14 of Mourzenko et al., 1997). We conclude this section by considering the fundamental ill-posedness of this problem. In view of the complexity of local flow behavior, a global cubic law need not exist for every fracture; the cubic law results from the parabolic velocity profile in perfect Poiseuille flow, and not from any dimensional considerations. Flow, or permeability, versus aperture curves are derived indirectly, on the basis of displacement measurements on the perimeter of laboratory fracture samples. At best, one can measure the mechanical aperture, that is, the apparent, average,
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE FIGURE 4-2 A demonstration of the various methods for measuring the LCL aperture, Shown are the vertical aperture (short dashes), the normal-to-local-centerline aperture of Ge (1997) (dots), and the ball aperture of Mourzenko et al, (1995). The approach of Oron and Berkowitz (1998) indicates that the cubic law aperture is not a point-by-point property but an average over a segment; the bold dashes on the pathway walls indicate a possible division into cubic law segments. The location marked with an arrow is inadequate for local cubic law because of a rapidly varying wall geometry. The rightmost, diagonal segment has a diverging angle that makes this region marginal for cubic law flow. After Oron, A. P., and B, Berkowitz, 1998. Flow in rock fractures: The local cubic law assumption reexamined. Water Resources Research 34: 2811-2825. Copyright by American Geophysical Union.
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE FIGURE 4-3 Maps of the estimated local hydraulic conductivity for a numerically generated fracture with self-affine walls, with H = 0.8 and a grid size of 128 × 128, for flow in the x-direction (left to right). The length of each side of the map is equivalent to 6.4 mm. Black indicates fracture wall contact, while grey indicates regions where LCL is not applicable; LCL flow exists only in the white regions. Average fracture apertures are (a) 100 µm, (b) 150 µm, (c) 175 µm, and (d) 200 µm. After Oron, A. P., and B. Berkowitz, 1998. Flow in rock fractures: The local cubic law assumption reexamined. Water Resources Research 34: 2811-2825. Copyright by American Geophysical Union. separation between the fracture walls. 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), and we have only relative displacement
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE measurements rather than the actual aperture values. Any conversion of these measurements to local apertures, and definition of an effective aperture, is based on assumptions and convenience. 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. In general, fracture wall roughness and fracture infilling evolve over time, as a function of a variety of dynamic mechanical and geochemical processes (e.g., Steefel and Lichtner, 1994). There is evidence that these changes may also take place over very short time intervals. It is critical that we understand these processes, which can occur naturally or be induced artificially (e.g., by injection of grouting in efforts to seal underground repository walls). The highly complex interactions between the mass transport mechanisms and the changing properties of a fractured porous medium make quantitative analysis of such systems very difficult. Precipitation and dissolution can significantly modify the physical and chemical properties of fractured media. Physical changes in fracture aperture, tortuosity, and thus effective mass diffusivities and permeabilities are coupled to the subsequent fluid flow and solute transport and the precipitation and dissolution reactions. As such, a variety of closure patterns can be observed in the field, which range from fully filled (mineralized) fractures to those with pockets or irregular layers on the walls. Chemical changes in the fractures, such as sorption and reaction capacities, also have a significant effect on chemical transport and fracture wall evolution. As a first step to quantifying precipitation/dissolution processes in fractures, we first consider modeling of reactive transport. A considerable body of literature deals with the transport of conservative contaminants in individual fractures. In contrast, however, little attention has been devoted to the analysis of the transport of reactive contaminants in fractures. The existing literature has been reviewed by Berkowitz and Zhou (1996), who developed a model to quantify the transport of reactive contaminants in a simplified fracture. The model, which accounts for advection, molecular diffusion, and interphase mass transfer between the aqueous phase and the fracture walls, demonstrates that the solute transport is controlled by the interplay between the Damköhler number (characterizing the effect of reaction relative to that of molecular diffusion on solute transport) and the Peclet number (characterizing the effect of advection relative to that of molecular diffusion on solute transport). Clearly, sorption reactions in groundwater systems involve many kinds of chemical constituents and different chemical and physical processes. Thus no single model can successfully describe all kinds of sorption reactions. Berkowitz
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE and Zhou (1996) considered several surface reaction models, including irreversible first-order kinetics, instantaneous reversible reactions, and reversible first-order kinetics. They determined criteria under which one can model reactive solutes as nonreactive, and reversible reactions as irreversible, in terms of time evolution of concentration distributions. They also gave a criterion for applicability of the local equilibrium assumption and subsequent use of a retardation coefficient. Numerical models describing precipitation and dissolution in porous media and fractures were presented by Sallès et al. (1993) and Békri et al. (1995), who investigated a variety of idealized geometries that included parallel plate channels and channels with opposite protrusions on each wall. Similar numerical models for precipitation in deterministic and stochastic fractures were presented by Mourzenko et al. (1996) and Békri et al. (1997). Numerical models for precipitation and dissolution in a constant aperture fracture and the surrounding evolving porous matrix were given by Savage and Rochelle (1993) and Steefel and Lichtner (1994). Realistic parameters were used to simulate a fracture originating from a cement-bearing nuclear waste repository. These studies examined effects on the surrounding medium but did not study the evolution of the fracture geometry itself. A model for the evolution of fracture network maze patterns, for fractures with initially large apertures, was presented by Palmer (1975, 1991), while Dreybrodt (1996) analyzed the time needed for the development of mature karst channels. A numerical model to investigate the minimum hydrogeochemical conditions for fracture dissolution in limestone was given by Groves and Howard (1994a). Their results indicate that the selective enlargement of fracture flow passages may be strongly influenced by their initial sizes. Groves and Howard (1994b) and Howard and Groves (1995) then developed numerical models for fracture dissolution in limestone for laminar and turbulent flow. Their results show that for laminar flow, passage enlargement taking place early in the conduit network development is highly selective, while the transition to turbulent flow often results in more general passage enlargement. Dijk and Berkowitz (1998) examined the evolution of fracture aperture over the entire length of the fracture, as a function of time, due to solute precipitation and dissolution. Irreversible first-order kinetic surface reactions were considered. The effect of the relevant transport mechanisms, the fracture geometries, and the initial and boundary conditions on the evolution of the fracture properties, fluid flow, and solute transport for geological systems and timescales were investigated. Calculations were based on a wide range of parameter values estimated from data available in the literature. Results showed the evolution of the solute transport and fracture geometry as a function of the Damköhler and Peclet numbers. Figure 4-4 illustrates typical simulated fracture mineralization patterns. Fracture closure times are found to be of the order of days to millions of years, for half-life reaction times (Langmuir and Mahoney, 1984) of the order of seconds to years, and for fluid residence times of the order of minutes to days. These closure times are compatible with typical hydrogeological time scales.
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE FIGURE 4-4 Simulated fracture mineralization patterns for three fractures and two rates of flow and chemical reaction. Predicted dimensionless half-aperture b** as a function of dimensionless location within the fracture, x**, and of dimensionless time t**. For initially flat (a), (d); linearly constricted (b), (e); and sinusoidal (c), (f) fracture walls. Line of symmetry (fracture center) in each fracture is given by the x-axis. Cases (a), (b), (c): large flow rate relative to rate of reaction; cases (d), (e), (f): low flow rate relative to rate of reaction. Modified from Dijk, P. E., and B. Berkowitz, 1998. Precipitation and dissolution of reactive solutes in fractures. Water Resources Research 34(3): 457-470. Copyright by American Geophysical Union.
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Fluid Flow and Chemical Transport in Partially Saturated Fracture Systems The conceptual pictures and quantitative models of fluid flow and chemical transport in the fully saturated fractures that we have discussed above are not directly applicable to fluid flow and chemical transport in the fractured vadose zone. However, our established understanding of these processes in saturated systems forms a valuable knowledge base from which we can work. 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. However, even at this stage, we must define what, precisely, we mean by infiltration, or fluid flow and chemical transport, in unsaturated (or partially saturated) fractures. Consider a fracture that is, initially, completely unsaturated. As shown by, for example, Di Pietro et al. (1994) and Di Pietro (1996), using numerical simulations with lattice gas automata, the pattern of infiltration and fluid distribution is largely a function of the rate of infiltration (relative to the dimensions of the fracture). In other words, for high rates of fluid entering the fracture, the fracture becomes fully saturated, and we need consider only the transient advance of the wetting front as partially saturated (e.g., Nicholl et al., 1992, 1993a, 1993b). For slower rates of fluid infiltration, film flows can develop along the fracture walls. The significant advance of fluid along such films, even with thicknesses of the order of 1 µm, has been demonstrated by Tokunaga and Wan (1997). Similar observations about saturation patterns can be made for mixed fracture/porous matrix systems (e.g., Glass and Norton, 1992). Models that account for film flow and fracture-matrix interactions are discussed in detail in other chapters in this volume, and we therefore do not dwell on them here. A critical feature in these cases is that medium heterogeneity and irregular distributions of fluid lead to the development of preferential flow paths. Because fluid flow patterns may be highly ramified and sensitive to small-scale details, use of simulation models that yield only averaged, “effective” behavior may be of highly limited value, and will in many instances yield meaningless results. We can, however, conceptualize the fracture/porous matrix system as a heterogeneous porous medium, particularly if the fracture has very rough walls, and/or contains filling material. Several modeling approaches can then be considered, depending upon whether the emphasis is on (1) the steady-state fluid flow and transient chemical transport through a partially saturated domain, or (2) the transient evolution of either the advancing front of fluid or contaminant, or the actual flow paths. In the former case, flow and transport models for heterogeneous, fully saturated domains can be applied simply by restricting the conducting, saturated portion of the domain to account for the air phase (e.g., Birkholzer and Tsang, 1997). Alternatively, in the latter case, we can introduce a variety of pore network
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE tracer concentration. The active flow paths along the fracture plane were resolved by two tracer tests. In each test, water in the percolation ponds was tagged, for 6 h and 4.5 h during the first and second tests, respectively. The first tracer test (test A) began 43 h after the beginning of the experiment, and the second test (test B) started 29 h later. The flow trajectories were defined by relating the tracers found in the various sampler cells in the borehole to their source ponds on the surface where they were applied. Details of the entire experiment are given in Dahan et al. (1999). Flow rates into the upper opening of the fracture from each of the percolation ponds and into the lower opening in each of the sampler cells are presented in Figure 4-9. The temporal flow rates reflect the variability in fluid flow from each of the ponds to each of the sampler cells. The high variability in maximum flow rates observed at the inlet and outlet sides of the fracture indicates that only about 50 percent of the length of the fracture opening allowed any appreciable flow, whereas less than about 20 percent of the fracture opening transmitted high flow rates. Significantly, the temporal variations in flow rates—with no clear pattern—persisted throughout the duration of the five-day experiment; a steady flow pattern was not reached in any of the ponds or sampler cells during this period. Both abrupt and gradual temporal variations were observed. Turbidity measurements in the drained effluents reflect the transport of suspended material through the fracture. Abrupt changes in turbidity (with maximum values of about 1500 nephelometric turbidity units [NTU]) were observed in samples from most cells throughout the duration of the experiment. These fluctuations reflect the unstable structure of the fracture void and its filling material (as noted by Weisbrod et al., 1998, 1999), and are linked to flow rate changes and stability. Flow trajectories through the fracture—between the percolation ponds and the sampler cells—were estimated on the basis of the tracer tests, as well as by correlating fluxes between ponds and cells. The connecting lines in Figure 4-10 represent the relative contribution from each pond to each sampler cell. This relative contribution was determined by the percentage of the maximum relative concentration of a tracer derived from a pond and observed in the sampler cell. Not surprisingly, the flow trajectories between the ponds and the sampler cells—separated by only 1 m—were often not vertical, but rather shifted horizontally along the fracture plane. Two important—and perhaps unexpected—observations can be made. First, the paths of tagged water from different source ponds often crossed without any observed mixing. For example, tagged water from pond 8 moving to sampler cells 4 and 5 crossed the zone where tagged water from ponds 3, 5, and 7 drained downward (Figure 4-10), 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. These flow paths could arise through the complex structure of the consolidated and unconsolidated
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE FIGURE 4-9 Temporal variations in the flow rate of each of the percolation ponds (above) and the sampler cells (below). The figure displays the exact position of the ponds with respect to the sampler cells. The flow-rate scale is variable according to the maximum measured flow rate in each individual pond/cell. The duration of the two tracer tests A and B is shown by dotted lines (A-A and B-B, respectively) on both graphs. After Dahan, O., R. Nativ, E, Adar, B, Berkowitz, and Z. Ronen, 1999. Field observation of flow in a fracture intersecting unsaturated chalk. Water Resources Research 35(11): 3315-3326. Copyright by American Geophysical Union.
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE FIGURE 4-10 Flow trajectories connecting the source ponds on land surface to the sampler cells in the borehole, as defined by the two tracer tests. The line styles represent the relative water contribution of a certain pond to its corresponding sampler cell. After Dahan, O., R. Nativ, E. Adar, B. Berkowitz, and Z. Ronen, 1999. Field observation of flow in a fracture intersecting unsaturated chalk. Water Resources Research 35(11): 3315-3326. Copyright by American Geophysical Union.
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE filling material with the fracture void, micro-dissolution channels along the fracture walls, or intersecting small fractures. Second, the relative contributions of fluid from each pond to each sampler cell changed significantly during the experiment. The temporal variations in flow paths during the two tracer tests (Figure 4-10) are remarkable—evidently due to dissolution of filling material and walls, as well as to particle mobilization, migration, and entrapment, as evidenced by the turbidity measurements. Analysis of the data also indicated that the instability of the flow regime observed on a small scale at each individual fracture segment did not decrease on larger scales, for several combined segments or for the entire flow domain. The observations of Dahan et al. (1998) suggest that large portions of the fracture plane play only a minor role, with small sections controlling most of the flow activity, probably within a few major discrete flow paths. Close examination of the fracture opening along the bottom of the ponds and along the borehole ceiling indicated that the hydraulically active sections of the fracture are related to dissolution channels along its plane. These dissolution channels were found to be closely related to fracture plane crossings between the main fracture plane and secondary fracture planes. CONCLUDING REMARKS We have considered aspects of fluid flow and chemical transport in the fractured vadose zone by building on our more established (though by no means complete) understanding of these processes in saturated systems. Other relevant experiments and models specific to unsaturated fractured media, such as film flow and fracture-matrix interactions, are discussed in other chapters in this volume. Fluid flow and chemical transport behavior in fractured porous formations is highly complex. 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. Analysis of chemical transport must also include parameters that describe chemical reaction, dissolution, and precipitation, and particle detachment and trapping, all of which lead to spatial and temporal variations in flow and transport patterns. These parameters are also fundamental to flow and transport models in saturated, highly heterogeneous formations, and similar conceptual frameworks and quantitative tools can often, though not always, be applied. For example, the finding that 3D flow paths within a fracture plane can consist of discrete and unconnected channels supports our
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE suggestion that fractures in the vadose zone may be treated as highly heterogeneous porous media. From the modeling point of view, it is important to distinguish between predictions of large-scale (average) and small-scale flow and transport behavior. Here we stress the importance of capturing essential fluid flow and chemical transport behavior, such as highly ramified fluid flow patterns; such behavior is often extremely sensitive to small-scale medium details. Large-scale (average) models are clearly irrelevant for analysis of small-scale fluid velocity and chemical concentration distributions, or, for example, for prediction of first arrival times of a contaminant. But significantly, models that attempt to apply effective, or homogenized, parameters to describe even the average flow and transport behavior in heterogeneous media often fail. Careful measurement in the laboratory and the field has indeed demonstrated the dominant influence of small-scale heterogeneity. 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. To date, there exist only a very limited number of large-scale laboratory and field experiments in the fractured vadose zone. A major difficulty lies in actually measuring the parameters of interest, particularly under unsaturated conditions. The other key difficulty is that we must account for high degrees of uncertainty, and for the interplay of flow and transport dynamics over a range of spatial and temporal scales. For example, if fluid flow in the fractured vadose zone follows a path analogous to a percolation system near the threshold, then the likelihood of measuring and actually delineating this path is very small. On the other hand, theoretical investigations of fluid flow and chemical transport, predicated on available experimental evidence, have yielded tremendous insight into the dynamics of flow and transport in the fractured vadose zone. The use of probabilistic and statistical tools allows us to characterize these dynamics, and to place quantitative bounds on fluid flow and chemical transport processes. New experiments with measurements that are sensitive over a range of scales to the geometrical, hydraulic, and chemical heterogeneity of fractured vadose zones are required to further refine these models. ACKNOWLEDGMENT Brian Berkowitz thanks the European Commission Environment and Climate Program (Contract No. ENV4-CT97-0456) for partial support.
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