Conceptual Models of Flow and Transport in the Fractured Vadose Zone (2001) / Chapter Skim
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Conceptual Model of Vadose-Zone Transport in Fractured, Weathered Shales
Conceptual Model of Vadose-Zone Transport in Fractured, Weathered Shales
Pages 87-114
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From page 87... ...
Fractured subsurface media that have weathered often depict the extreme case of pore-class heterogeneity because highly conductive voids surround low-permeability, high-porosity matrix blocks. In this chapter, the physical and chemical processes controlling water and solute transport in fractured, weathered shales are discussed.
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From page 88... ...
The investigation involves multiscale experimental and numerical approaches to address coupled hydrological and geochemical processes controlling the fate and transport of contaminants in fractured vadose zone saprolites. Novel tracer techniques and experimental manipulation strategies using laboratory-, intermediate-, and field-scale experiments helped unravel how coupled transport processes affect the nature and extent of secondary contaminant sources.
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From page 89... ...
The diverse mineralogy results in a highly reactive solid phase that can significantly alter the geochemical behavior and transport of solutes. In an effort to enhance our conceptual understanding of and predictive capability for solute transport processes in these unsaturated fractured soils, a multiscale experimental and numerical approach was used to quantify the rates and mechanisms of pore class interactions.
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From page 90... ...
using multiple tracers with grossly different sizes. Controlling Flow-Path Dynamics Variations in Pore-Water Flux A relatively simple technique for confirming and quantifying physical nonequilibrium in soil systems involves displacement experiments performed at a variety of experimental fluxes using a single representative tracer.
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From page 91... ...
Bromide breakthrough curves for a series of steady-state specific discharges in an undisturbed column of fractured, weathered shale. The largest and slowest flux experiments were conducted over periods of 6 and 2,200 h respectively.
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From page 92... ...
multiple tracers, flow interruption) described below, it can become a powerful means of quantifying physical nonequilibrium processes in structured media (see, for example, Hu and Brusseau, 1995; Reedy et al., 1996~.
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From page 93... ...
Reactive solute transport is also dramatically influenced by water content changes and pore regime connectivity. With enhanced preferential flow, important geochemical reactions such as sorption, oxidation/reduction, complexation/ dissociation, and precipitation/dissolution become increasingly limited due to a decrease in residence time of pore water with the soil matrix.
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From page 94... ...
The utility of the flow-interrupt method for confirming and quantifying physical nonequilibrium can be observed in Figure 3-3, which shows Br- breakthrough curves at two fluxes in an undisturbed column of fractured, weathered shale from the ORR. The observed concentration perturbations on the ascending and descending limbs of the breakthrough curves are the result of prolonged flow interrupt and the system approaching a new state of physical equilibrium.
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From page 95... ...
1 ) ~1 o q=475 cm d-1 oc=1.85d-1 _ = 1 1 1 1 B 0 2 4 6 8 10 Pore Volumes 95 12 14 FIGURE 3-3 Breakthrough curves with flow interruption, at two specific discharges, for a nonreactive Br- tracer in an undisturbed column of fractured, weathered shale.
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From page 96... ...
Colloidal tracers that are size-excluded from the matrix porosity of the media are not included in this type of experimental technique. When physical nonequilibrium processes are significant in porous media, tracers with larger molecular diffusion coefficients will be preferentially lost from advective flow paths (i.e., fractures)
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From page 97... ...
When colloidal tracers are coupled with dissolved solutes that can interact with the matrix porosity, a unique technique emerges for assessing physical nonequilibrium processes in subsurface media. The utility of using multiple tracers of different sizes can be seen in Figure 35, which shows the simultaneous injection of two strains of bacteriophage (PRD1 and MS-2)
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From page 98... ...
B Flux = 2.2 cm/d . Br~ PFBA · ~^ · .~ l l 0 2 4 6 Pore Volume 8 10 12 FIGURE 3-4 Breakthrough curves for the simultaneous injection of two nonreactive tracers, Br- and PFBA, at a flux of (A)
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From page 99... ...
The dissolved tracers, on the other hand, were influenced by diffusive mass-transfer processes between fractures and the matrix (Figure 3-5~. Besides providing visual evidence of physical nonequilibrium processes in structured media, the experiments provided advective flow velocities that were used to parameterize numerical models designed to simulate the observed data.
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From page 100... ...
These results were coupled with computed tomography imaging of the undisturbed core specimens, and the fracture network patterns were used to simulate tracer displacement experiments conducted on the undisturbed cores. INTERMEDIATE-SCALE ASSESSMENT OF TRANSPORT IN FRACTURED WEATHERED, SHALES A logical progression from laboratory-scale undisturbed columns is the use of intermediate-scale in situ pedons for assessing the interaction of coupled processes on the fate and transport of solutes in the fractured, weathered shales (Figure 3-6~.
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From page 101... ...
At an infiltration rate of 30 cm/d, Br- is transported exclusively through medium and small pore regimes indicative of secondary fractures and the soil matrix, respectively. Flow through the large pore regimes, indicative of primary fractures, is essentially excluded since the imposed
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From page 102... ...
FIGURE 3-7 Breakthrough curves for a nonreactive Br- tracer in discrete pore regimes during two infiltration experiments on the fractured, weathered shale soil block. At an infiltration rate of 30 cm/d (A)
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From page 103... ...
Both the column and pedon studies have shown that the rate of mass transfer between fractures and the soil matrix is relatively rapid, and thus it can be inferred that the soil matrix remains an integral part of the entire system during storm events and is not necessarily excluded from transport processes when preferential flow occurs in fractures. FIELD-SCALE ASSESSMENT OF TRANSPORT IN FRACTURED WEATHERED SHALES I, Waste migration issues in fractured, weathered shales are field-scale problems that are complicated by large-scale media heterogeneities that cannot be replicated at the laboratory or intermediate scale.
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From page 104... ...
. While the actual tracer release revealed a rapid transport through the fracture network of the soil, the mass transfer into the low-permeability matrix was significant since >50 percent of the applied tracer was found to reside within matrix porosity of the soil primarily at a depth of 1-1.5 m (Figure 3-9b)
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From page 105... ...
~ ~ ~ 1 ~ UPPER FLUME REV LC) WER FLUME 1 105 ORNL DWG ~998 FIGURE 3-8 Schematic illustration of the subsurface weirs that intercept lateral storm flow from the subcatchment that is used for field-scale tracer injection experiments in the fractured, weathered shale soil: (a)
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From page 106... ...
o.o l W ~ \ / /1 on V - ~ D3 & D4 A0 // IS? 1 1 1 1 1 1 1 1 1 Resident concentrations in the soil matrix Am D1 FIGURE 3-9 Storm flow and soil matrix tracer results following a release of Br- at the field-scale tracer injection facility in the fractured, weathered shale soil: (a)
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From page 107... ...
Clean rainwater infiltrating into secondary fractures and converging into primary fractures would bypass an initially contaminant-rich soil matrix. Thus, physical nonequilibrium that is, concentration gradients would exist between regions and cause diffusive transfer from the matrix to the mobile regions.
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From page 108... ...
Yeh, 1996. Using a multiregion model to study the effects of advective and diffusive mass transfer on local physical nonequilibrium and solute mobility in a structured soil.
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From page 109... ...
Modeling results suggested that mesoscale spreading of tracer in structured porous media may be largely attributed to interregion mass transfer. The multiregion model has also been used to numerically simulate storm-driven solute transport experiments conducted at the watershed scale within the fractured saprolites.
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From page 110... ...
Solute concentration differences between the various sized pores creates concentration gradients that drive time-dependent inter-region diffusive mass transfer. Storm-enhanced preferential flow in these weathered shales disrupts geochemical equilibrium between the solid, liquid, and gas phases.
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From page 111... ...
Yeh, 1996. Using a multiregion model to study the effects of advective and diffusive mass transfer on local physical nonequilibrium and solute mobility in a structured soil.
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From page 112... ...
Quantifying diffusive mass transfer in fractured shale bedrock. Water Resources Research 35: 2015-2030.
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From page 113... ...
Quantifying the diffusive mass transfer of nonreactive solutes in columns of fractured saprolite using flow interruption.
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From page 114... ...
Field-scale transport from a buried line source in variable saturated soil. Journal of Hydrology 145: 83-109.
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