71 2 Modeling of Fluid Transport through Barriers Prepared by* Brent E. Sleep University of Toronto, Toronto, Canada Charles D. Shackelford Colorado State University, Fort Collins, Colorado Jack C. Parker Oak Ridge National Laboratory, Oak Ridge, Tennessee 2.1 OVERVIEW As understanding of the mechanisms of contaminant transport through barrier tive approach to a performance design approach. It is expected that reliance on models for predictive-based design will increase in the future, as the need for predicting long-term barrier system performance increases. This chapter details the mechanisms and models for predicting the performance of components of passive barriers such as caps, permeable reactive barriers (PRBs), and walls and floors. The relevant regulatory drivers and current state of practice are summa- modeling while this chapter focuses on the performance of components that constitute containment systems. * With contributions by Calvin C. Chien, DuPont, Wilmington, Delaware; Thomas O. Early, Oak Ridge National Laboratory, Oak Ridge, Tennessee; Clifford K. Ho, Sandia National Laboratories, Albuquerque, New Mexico; Richard C. Landis, DuPont, Wilmington, Delaware; Alyssa Lanier, University of Wisconsin, Madison, Wisconsin; Michael A. Malusis, GeoTrans, Inc., Westminster, Colorado; Mario Manassero, Politecnico I, Torino, Italy; Greg P. Newman, Geo-Slope International Ltd., Calgary, Canada; Robert W. Puls, U.S. Environmental Protection Agency, Ada, Oklahoma; Terrence M. Sullivan, Brookhaven National Laboratory, Upton, New York 4040_book.fm Page 71 Wednesday, September 14, 2005 12:43 PM © 2006 by Taylor & Francis Group, LLC rized, and research needs are identified. Chapter 1 dealt with system performance systems improves, the design of containment systems is moving from a prescrip- 72 Barrier Systems for Environmental Contaminant Containment & Treatment 2.2 CAPS 2.2.1 F EATURES , E VENTS , AND P ROCESSES A FFECTING P ERFORMANCE OF C APS Covers and caps are engineered structures that must perform within a larger dynamic natural system and, as such, must be designed with consideration of natural system influences. Understanding these physical processes and applying appropriate numerical analyses to these processes can help the engineer to build an appropriate overall system that will perform with the desired objective. The primary processes acting on a cap are described in the subsections below. 2.2.1.1 Hydrologic Cycle The purpose of a cap is usually to minimize water infiltration into underlying waste, and sometimes to minimize gas transport to the atmosphere. As shown in the cap slope, cap soil properties, cap moisture conditions, and the duration and magnitude of precipitation, ponding and water run off can occur. Water that does not run off of the cap is either stored in depressions in the cap surface, or infiltrated into the surface layer of the cap. Water infiltrating into the surface layer of the cap is subject to evapo-transpiration. Rates of evapo-transpiration depend on surface vegetation, soil properties, surface temperatures, soil and air relative humidities, and net solar radiation. The remainder of the precipitation not trans- formed to run off or evapo-transpiration remains as storage in the cap, or, if the storage capacity of the cap is exceeded, the water percolates through the cap. Contaminant vapors can migrate through caps by advection or diffusion. Advection rates depend on gas-phase permeabilities and pressure gradients across the cap. Variations in barometric pressures can increase contaminant vapor advec- tion to the atmosphere. Vapor diffusion is driven by the gas-phase concentration gradient existing across the cap. Diffusion coefficients depend on soil porosity and water content, as well as contaminant molecular weight. It is often assumed that diffusion at the ground surface occurs across a stagnant surface boundary conditions (Thibodeaux, 1981). Water percolation and contaminant transport through the cap can also be the migration of water or contaminant vapors through the system. Natural events such as earthquakes, tornadoes, floods, and melting snow can also be disruptive can be significant and should therefore be considered. 4040_book.fm Page 72 Wednesday, September 14, 2005 12:43 PM © 2006 by Taylor & Francis Group, LLC Figure 2.1, water originates as precipitation that falls on the cap. Depending on Figure 2.2. Animal burrows or other passageways through the cap can accelerate altered by human or biointrusion into the cap and other natural events, leading to disparities between probable current and future percolation rates as shown in and processes as discussed in Chapter 1, their potential impact and consequence to the cap. Although a great deal of uncertainty is associated with these events layer the depth of which depends on surface topography, vegetation, and wind Modeling of Fluid Transport through Barriers 73 FIGURE 2.1 Features, events, and processes associated with a long-term cap. FIGURE 2.2 Cumulative probability distribution of water percolation reaching the mill tailings for present and future conditions. (From Ho, C.K. et al., 2001. Sandia National Laboratory Report SAND2001-3032; October.) Climate Transpiration Precipitation Run-on Run-off Gas release Evaporation Storage Lateral drainage Waste Percolation/leaching Human intrusion/ bio-intrusion 10 −13 10 −12 10 −11 10 −10 10 −9 10 −8 10 −7 10 −6 0 20 40 60 80 100 Present Future Percolation Flux through Cover (cm/s) Cumulative Probability 40 CFR Part 264.301 4040_book.fm Page 73 Wednesday, September 14, 2005 12:43 PM © 2006 by Taylor & Francis Group, LLC 74 Barrier Systems for Environmental Contaminant Containment & Treatment 2.2.1.2 Layers and Features In very rare cases, a cap comprises a single soil layer over waste material. Typically however, a cap is the unique combination of soils placed in layers on top of each other and in certain order that create the desired effect. This section briefly outlines the general performance objective of each potential cap layer. • Ground surface layer — The top few inches of any surface soil may need to be treated as a unique soil region since, due to desiccation and drying effects, this zone generally has a much higher hydraulic con- ductivity than the soil a few inches below surface. This zone is espe- cially important to include when simulating infiltration through cover systems using numerical models. • Vegetation layers — It is common to include a vegetation growth layer that may or may not be part of another cover layer. In many cases, the vegetation can be a key to cap performance, but based on According to energy balance accounting, the sum of actual evaporation and transpiration are always less than the potential evaporation. This means that for near-surface processes, the availability of water limits evapo-transpiration, and water that is not transpired through vegetation is removed through evaporation. In other words, if vegetation were not present, actual evaporation would remove a similar amount of water. The transpiration process becomes important when it is necessary to draw water from deeper beneath the surface, particularly when actual evaporation has significantly diminished at the surface due to drying of soils. Vegetation is also critical for stability purposes on sloped covers, as well as erosion control. • Capillary break layers — These layers are generally created with coarse materials next to fine materials because, at a common negative water pressure, two different soils have different water contents. Cap- illary breaks can be used in caps for various purposes. When placed beneath a compacted layer, the capillary break limits percolation through the compacted material. When placed above a compacted layer, the capillary break limits the evaporative drying of the compacted layer, because water cannot readily be drawn up in its liquid phase through the coarser capillary break layer when it is dry. For this type of cover design, a model that includes coupled vapor flow should be used to assess the impact of vapor flux on barrier layer drying in the event that upward liquid phase flow has shut down. • Barrier layers — Barrier layers are generally made of well-com- pacted, low-permeability fine-grained soils. A barrier layer should not be placed directly at the surface, or it will be subjected to effects such as extreme drying, desiccation, and freeze-thaw. It is common to place 4040_book.fm Page 74 Wednesday, September 14, 2005 12:43 PM © 2006 by Taylor & Francis Group, LLC the analysis presented in Section 1.4.1, this should not be assumed. Modeling of Fluid Transport through Barriers 75 a barrier layer over a coarser layer to create a capillary break effect, and then place it beneath a vegetation growth layer. It is not desired to have the root zone of the plant species extend into the barrier layer where damage can occur. While long-term barrier layer performance is unknown and cannot be predicted with precision, the use of dense, well-graded materials for these layers has shown the best resistance to long-term performance deterioration (Wilson, 2002). • Storage layers — These layers are generally made of loose well graded materials such that the hydraulic conductivity is sufficient to allow water to infiltrate and subsequently be drawn back out by evaporation and/or roots. The thickness of a storage layer becomes a critical ques- tion in its functionality. The cover must be thick enough to keep near- surface wetting and drying processes from interacting with the waste, and to withstand long-term erosion. If the cover is to limit gas fluxes as well, there must be a zone of continual near-saturation within this layer over time and over prolonged dry periods; either that, or the storage layer must protect a deeper near-saturation barrier layer. Long- term storage layer performance can be affected by coarse material breakdown, which can result in permeability loss. 2.2.2 C URRENT S TATE OF P RACTICE FOR M ODELING P ERFORMANCE OF C APS Water movement through soils can be thought of as a three-component system consisting of the soil-atmosphere interface, the near-surface unsaturated zone, and the deeper saturated zone. In the past, groundwater modeling has primarily focused on the saturated zone, which creates a discontinuity in the natural system because the unsaturated zone and the soil-atmosphere interface are not repre- sented. Advances in unsaturated soil technology during the past decade have led to the development of routine modeling techniques for saturated and unsaturated soil systems. However, modeling techniques for the third component, involving the detailed evaluation of the flux boundary condition imposed by the atmosphere, are not routinely available. This section discusses some of the available codes that can be used for the predictive modeling of processes associated with cap performance. A summary of the codes considered, and some of the key features different available software tools and their main solution processes, as well as feature overviews and source availability. Table 2.2 lists the individual program’s solution options and features that are built into the various codes. 2.2.2.1 Water Balance Method The estimation of the amount of water infiltrating through a cap is essentially the estimation of the water balance for the cap. The net percolation through the cap is the remainder from precipitation after run off, surface storage, evapo-transpiration, 4040_book.fm Page 75 Wednesday, September 14, 2005 12:43 PM © 2006 by Taylor & Francis Group, LLC and solution techniques are provided in Tables 2.1 and 2.2. Table 2.1 lists several 76 Barrier Systems for Environmental Contaminant Containment & Treatment TABLE 2.1 Available Software Overview Software Name Process Solved Parameters Technique Features/Limitations User Interface A vailability SoilCover 1D, Transient FEM Pressure, temperature, vapor pressure with pseudo gas Coupled, Simultaneous, nonlinear Pre- and post-processor included; code unavailable. Freeware Text in Excel with dialogues; requires Excel 97 or 2000 www.vadose- science.com Oxygen flux Subsequent VADOSE/W 2D, transient and steady- state FEM Pressure, temperature, vapor pressure; can be linked with slope stability software and contaminant transfer software Coupled, simultaneous, nonlinear Enhanced pre and post- processor included; climate and soils database included; user support included; commercially developed for cover/cap design Full CAD data input and mesh generation; Microsoft certified for XP and lower OS www.geo-slope.com Oxygen or radon diffusion, dissolution, decay Subsequent linear Earthquake seismic analysis using V ADOSE/W generated pore pressure data Supplemental Integrated with program QU AKE/W Slope stability analysis using VADOSE/W generated pore pressure data Supplemental Integrated with program SLOPE/W 4040_book.fm Page 76 Wednesday, September 14, 2005 12:43 PM © 2006 by Taylor & Francis Group, LLC Modeling of Fluid Transport through Barriers 77 Contaminant transfer, advection/dispersion, decay, particle tracking Subsequent nonlinear Integrated with program CTRAN/W HELP 1D, quasi 2D, Analytical Water balance Analytical Climate and soil database included; not physically based; limited design application; assumes unit gradient Text in editor or Windows dialogues www.wes.army.mil/el/ elmodels/helpinfo.html UNSAT-H 1D, transient FEM Pressure with vapor Nonlinear Pre- and post-processor available but excluded. Code available Text in editor or Windows dialogues www.hydrology.pnl.gov/ unsath.asp Temperature (optional) Subsequent linear HYDRUS-2D 2D, Transient and steady- state FDM Pressure, with vapor flow Nonlinear Pre- and post-processor included; CAD mesh generation add-on CAD and Windows dialogues www.ussl.ars.usda.gov/ models/hydrus2d.HTM Temperature Subsequent linear Contaminant transfer Subsequent nonlinear TOUGH 2 1D, 2D, 3D, transient and steady-state IFDM Pressure, temperature, vapor, gas in porous or fractured media Coupled, Simultaneous, nonlinear Limited pre- and post- processor available from independent suppliers. Code available; users can customize Limited CAD and text in editor www-esd.lbl.gov/ TOUGH2 4040_book.fm Page 77 Wednesday, September 14, 2005 12:43 PM © 2006 by Taylor & Francis Group, LLC 78 Barrier Systems for Environmental Contaminant Containment & Treatment TABLE 2.1 (continued) Available Software Overview Software Name Process Solved Parameters Technique Features/Limitations User Interface Availability FEHM 1D, 2D, 3D, transient FEM/FVM Multi-phase, multi- component heat, mass, gas, air including double porosity flow; can solve contaminant flow as advection/ dispersion or particle tracking Coupled, simultaneous, nonlinear Limited pre- and post- processor with 3D grid generator available from independent sources. Unix or PC based; code included; user can customize; USA only Limited CAD with text input www-lanl.gov/EES5/ fehm.html RAECOM 1D steady- state radon- gas diffusion Radon-gas concentration and flux through a multi-layer system Linear Can automatically optimize layer thickness Text entry RAECOM-cloned calculator available on the web: wise/uranium/ctc.html Coupled, physical coupling between equations; simultaneous, more than one equation solv ed at same time (must be coupled); subsequent, more than one equation solved one after the other at each time step; supplemental, data from completed analysis used in separate analysis; linear, mat erial properties not a function of variable 4040_book.fm Page 78 Wednesday, September 14, 2005 12:43 PM http://www.antenna.nl/ being solved; nonlinear, material properties change with variable being solved, so iterations required; analytical, no partial differential equation, one pass solution. © 2006 by Taylor & Francis Group, LLC Modeling of Fluid Transport through Barriers 79 TABLE 2.2 Available Software: Detailed Options Software Name Solved Parameters Solution Complexity Evapor -ation Transpir- ation Freez- ing Run Off Pond- ing Soil Properties SoilCover Pressure, temperature, vapor pressure with pseudo gas RP RP RE SE RE — FF, CF Oxygen flux A VADOSE/W Pressure, temperature, vapor pressure; can be linked with slope stability software and contaminant transfer software RP RP RE RE RE RE FF, CF Oxygen or radon diffusion, dissolution, decay RP Internally calculated Earthquake seismic analysis using VADOSE/W generated pore pressure data RP FF Slope stability analysis using VADOSE/W generated pore pressure data RP FF Contaminant transfer, advection/dispersion, decay, particle tracking RP FF HELP Water balance A SE SE E E — CF UNSAT-H Pressure with vapor RE SE RE — SE — CF Temperature (optional) RE 4040_book.fm Page 79 Wednesday, September 14, 2005 12:43 PM © 2006 by Taylor & Francis Group, LLC 80 Barrier Systems for Environmental Contaminant Containment & Treatment TABLE 2.2 (continued) Available Software: Detailed Options Software Name Solved Parameters Solution Complexity Evapor -ation Transpir- ation Freez- ing Run Off Pond- ing Soil Properties HYDRUS-2D Pressure, with vapor flow RE SE RE — SE — CF Temperature RE Contaminant transfer RE TOUGH2 Pressure, temperature, vapor, gas in porous or fractured media RP — — — — — CF FEHM Multi-phase, multi-component heat, mass, gas, air including double porosity flow; can solve contaminant flow as advection/dispersion or particle tracking RP SE — — SE SE CF RAECOM Radon-gas concentration and flux through a multi-layer system A—————CF RP, rigorous physically based with assumptions limited to current understanding of real ph ysical processes; RE, rigorous physically based but with empirical components or built-in limiting assumptions; SE, semi-empirical, equation based but user sets limits or there ar e limited built-in assumptions; E, empirically based, extreme limiting assumptions and little ph ysical bases for generated data; A, analytically based — no partial differential equations; FF, free-form functions, user can customize; CF , closed-form functions, curve-fit parameters. 4040_book.fm Page 80 Wednesday, September 14, 2005 12:43 PM © 2006 by Taylor & Francis Group, LLC [...]... 20 05 12: 43 PM 96 Barrier Systems for Environmental Contaminant Containment & Treatment 8 9 10 11 2 F 1 0 21 22 23 24 25 2 26 2 FIGURE 2. 3 Section of cemented core from a PRB at the USDOE Y- 12 plant in Oak Ridge, Tennessee appears to be restricted to a very thin zone at the influent face of the PRB In contrast, the highly mineralized water from the Y- 12 plant resulted in much more extensive formation of... boxes and custom menus, and the solver is a 3 2- bit Fortran executable file 2. 2 .2. 5 HYDRUS-2D HYDRUS-2D can be used to simulate two-dimensional water flow, heat transport, and the movement of solutes involved in consecutive first-order decay reactions in variably saturated soils HYDRUS-2D uses the Richards’ equation for simulating variably saturated flow and Fickian-based convection-dispersion equations for. .. 20 06 by Taylor & Francis Group, LLC 4040_book.fm Page 82 Wednesday, September 14, 20 05 12: 43 PM 82 Barrier Systems for Environmental Contaminant Containment & Treatment 2. 2 .2. 3 UNSAT-H UNSAT-H (WinUNSAT-H) is a model for calculating water and heat flow in unsaturated media The model was developed at Pacific Northwest National Laboratory in Richland, Washington, to assess the water dynamics of nearsurface,... Lepidocrocite (γ-FeOOH) (Maghemite (Fe2O3)) Magnetite (Fe3O4) Amorphous iron oxyhydroxides Mackinawite (Fe9S8) Amorphous ferrous sulfide (FeS) Aragonite (CaCO3, orthorhombic) Calcite (CaCO3, hexagonal) Siderite (FeCO3) GR-I (CO 32 ) (Fe 42+ Fe23+(OH) 12) (CO3 ⋅2H2O) GR-I (Cl–) (Fe 32+ Fe3+(OH)8Cl) GR-II (SO 42 ) (Fe 42+ Fe23+(OH) 12) (SO4⋅2H2O) Source: Liang, L., Sullivan, A.B., West, O.R., Kamolpornwijit, W and Moline,... 6H+ + CrO 42 = Fe3+ + Cr(OH )2+ + 2H2O Fe0 +4H+ + TcO4– = Fe3+ + TcO2 + 2H2O 2Fe0 + 3UO 22+ = 2Fe3+ + 3UO2 The reduction of these metals tends to make them less soluble and less mobile than the oxidized forms Because these contaminants generally are present in such low concentrations in groundwater, it has not been possible to identify specific solid phases where they are located within PRBs For example,... (Freedman and Gossett, 1991) The generated acetate can then serve as an electron donor Other examples of partially dechlorinated compounds that can serve as electron donors are cis-1 , 2- dichloroethylene (cis-1 , 2- DCE) and vinyl chloride, the daughter products of PCE and TCE (Bradley and Chapelle, 20 00), and 1,1-DCE, the daughter product of 1,1,1-TCA In aerobic aquifers, the biological transformation... models for changes in cap hydraulic properties due to dessication or freeze-thaw have not been developed Many caps are expected to provide environmental protection for decades or centuries Studies of cap stability and soil and geomembrane property stability © 20 06 by Taylor & Francis Group, LLC 4040_book.fm Page 90 Wednesday, September 14, 20 05 12: 43 PM 90 Barrier Systems for Environmental Contaminant Containment. .. tetrachloride and ethene from PCE and TCE Similarly, the treatment of 1,1,1-TCA with ZVI yields a significant amount of ethane, with lesser amounts of ethene, cis -2 - butene, and 2- butyne (Fennelly and Roberts, 1998) Typically, the daughter products of chlorinated solvents treated with ZVI are partially dechlorinated and therefore less highly oxidized than the parent compound Some of these partially reduced... 20 06 by Taylor & Francis Group, LLC 4040_book.fm Page 94 Wednesday, September 14, 20 05 12: 43 PM 94 Barrier Systems for Environmental Contaminant Containment & Treatment TABLE 2. 3 Examples of Precipitated Minerals Found in Fe(0) FieldInstalled PRBs and Column Studies Mineral Precipitate Group Iron oxides and oxyhydroxides Iron sulfides Carbonates Green Rusts Minerals Goethite (α-FeOOH) Akaganeite (β-FeOOH)... concentration gradient Desorption rate can be © 20 06 by Taylor & Francis Group, LLC 4040_book.fm Page 1 02 Wednesday, September 14, 20 05 12: 43 PM 1 02 Barrier Systems for Environmental Contaminant Containment & Treatment Theoretical Desorption Curve Normalized Concentration 1 0.8 0.6 0.4 0 .2 0 Time FIGURE 2. 4 Representative desorption curve modeled using the Langmuir isotherm and is often represented as a graph . 20 05 12: 43 PM © 20 06 by Taylor & Francis Group, LLC 82 Barrier Systems for Environmental Contaminant Containment & Treatment 2. 2 .2. 3 UNSAT-H UNSAT-H (WinUNSAT-H) is a model for. prescrip- 72 Barrier Systems for Environmental Contaminant Containment & Treatment 2. 2 CAPS 2. 2.1 F EATURES , E VENTS , AND P ROCESSES A FFECTING P ERFORMANCE . 20 05 12: 43 PM © 20 06 by Taylor & Francis Group, LLC and solution techniques are provided in Tables 2. 1 and 2. 2. Table 2. 1 lists several 76 Barrier Systems for Environmental Contaminant Containment