4 Modeling Fate and Transport of Chlorinated Organic Compounds in the Subsurface prepared by Brent E. Sleep with contributions by Neal D. Durant, Charles R. Faust, Joseph G. Guarnaccia, Mark R. Harkness, Jack C. Parker, Lily Sehayek CONTENTS 4.1Introduction and Overview 4.2Basic Concepts and Equations 4.2.1Multiphase Fluid Flow 4.2.1.1Darcy’s Law for Multiphase Flow 4.2.1.2Capillary Pressure and Relative Permeability Relations 4.2.2Multicomponent Mass Transport 4.2.2.1Mass Balance and Transport Flux Equations 4.2.2.2Interphase Mass Transfer 4.2.3Modeling Biotic and Abiotic Transformations 4.2.3.1Review of Chlorinated Solvent Transformation Mechanisms 4.2.3.2Petroleum Hydrocarbon Biodegradation Models 4.2.3.3Chlorinated Solvent Biodegradation Models 4.2.4Computational Issues 4.2.4.1Spatial Discretization 4.2.4.2Temporal Discretization 4.2.4.3Linearization of Nonlinear Equations of Multiphase Flow and Transport 4.2.4.4Solution of Linear Equations 4.3Modeling DNAPLs — State of Practice 4.3.1Roles of Modeling 4.3.1.1Research and Education L1667_book.fm Page 179 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC©2004 CRC Press LLC Appendix A: Workshop Panels PANEL 1 MIXING ZONE: DISCHARGE OF CONTAMINATED GROUND WATER INTO SURFACE WATER BODIES E XPERT P ANEL L EADER Miguel A. Medina, Jr., Duke University A SSISTANT E XPERT P ANEL L EADER Nancy R. Grosso, DuPont Company E XPERT P ANEL M EMBERS Robert L. Doneker, Oregon Graduate Institute of Science and Technology Henk Haitjema, Indiana University D. Michael Johns, Windward Environmental LLC Wu-Seng Lung, University of Virginia Steven C. McCutcheon, USEPA National Exposure Research Laboratory Farrukh Mohsen, Gannett Fleming, Inc. Aaron I. Packman, Northwestern University Philip J. Roberts, Georgia Institute of Technology J. Bart Ruiter, DuPont Company PANEL 2 CONTAMINATED SEDIMENT: ITS FATE AND TRANSPORT E XPERT P ANEL L EADER Danny D. Reible, Louisiana State University A SSISTANT E XPERT P ANEL L EADER Richard H. Jensen, DuPont Company L1667_book.fm Page 259 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC E XPERT P ANEL M EMBERS Sam Bentley, Louisiana State University Mimi B. Dannel, USEPA Headquarters Joseph V. DePinto, Limno-Tech, Inc. James A. Dyer, DuPont Company Kevin J. Farley, Manhattan College Marcelo H. Garcia, University of Illinois David Glaser, Quantitative Environmental Analysis John M. Hamrick, Tetra Tech, Inc. Wilbert J. Lick, University of California at Santa Barbara Robert A. Pastorok, Exponent Environmental Group Richard F. Schwer, DuPont Company C. Kirk Ziegler, Quantitative Environmental Analysis PANEL 3 OPTIMIZATION MODELING FOR REMEDIATION AND MONITORING E XPERT P ANEL L EADER George F. Pinder, University of Vermont A SSISTANT E XPERT P ANEL L EADER Robert B. Genau, DuPont Company E XPERT P ANEL M EMBERS Robert M. Greenwald, GeoTrans, Inc. Hugo A. Loaiciga, University of California at Santa Barbara George P. Karatzas, Technical University of Crete Peter K. Kitanidis, Stanford University Reed M. Maxwell, Lawrence Livermore National Laboratory Alexander S. Mayer, Michigan Technological University Dennis B. McLaughlin, Massachusetts Institute of Technology Richard C. Peralta, U.S. Air Force Reserve and Utah State University Christine A. Shoemaker, Cornell University Brian J. Wagner, U.S. Geological Survey Kathleen M. Yager, USEPA Technology Innovation OfÞce William W G. Yeh, University of California at Los Angeles L1667_book.fm Page 260 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC PANEL 4 SIMULATION OF HALOGENATED HYDROCARBONS IN THE SUBSURFACE E XPERT P ANEL L EADER Charles R. Faust, GeoTrans, Inc. A SSISTANT E XPERT P ANEL L EADERS Neal D. Durant, GeoTrans, Inc. Craig L. Bartlett, DuPont Company E XPERT P ANEL M EMBERS Robert C. Borden, North Carolina State University Ronald J. Buchanan, Jr., DuPont Company Randall Charbeneau, University of Texas Eva L. Davis, USEPA Kerr Laboratory Joseph G. Guarnaccia, CIBA-Geigy Specialty Chemicals Mark R. Harkness, General Electric Corporation Jack C. Parker, Oak Ridge National Laboratory Hanadi Rafai, University of Houston Lily Sehayek, Penn State Great Valley Brent E. Sleep, University of Toronto Jon F. Sykes, University of Waterloo Albert J. Valocchi, University of Illinois L1667_book.fm Page 261 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC 4.3.1.2Policy Development 4.3.1.3Site Assessment and Remedial Design 4.3.2Overview of Existing Models 4.3.2.1Three-Phase Models 4.3.2.2Two-Phase Models 4.3.2.3Single-Phase Models 4.3.2.4Chlorinated Solvent Biodegradation Models 4.3.3Model Selection and Limitations 4.4Site Applications 4.4.1Gulf Coast EDC DNAPL Release 4.4.2DNAPL Source Area Characterization Coastal Plain of New Jersey 4.5Research Needs 4.5.1Constitutive Relationships for Multiphase Flow, Transport, and Interphase Mass Transfer 4.5.1.1k–S–P Relationships 4.5.1.2Mass Transfer Relationships 4.5.1.3Summary 4.5.2DNAPLs In Fractured Media 4.5.2.1Unsaturated Water Flow 4.5.2.2NAPLs 4.5.2.3Summary 4.5.3Impact of Biodegradation on DNAPL Dissolution 4.5.3.1Model Development 4.5.3.2Reductive Dechlorination Rates at High Concentrations of Dissolved PCE 4.5.3.3Reductive Dechlorination in the Presence of DNAPL 4.5.3.4Summary 4.5.4Plume Attenuation 4.5.4.1Biotransformation Kinetics 4.5.4.2Halorespiration 4.5.4.3Spatial Variability in Redox Conditions 4.5.4.4Complex Mixtures 4.5.4.5Bioavailability and Mass Transfer from Sorbed Phase 4.5.4.6Summary 4.6Technology Transfer 4.6.1Approach 4.6.1.1QA Standards 4.6.1.2Expert Decision Support System 4.6.1.3Model Application Archive and Database Support 4.6.1.4Training Support 4.6.2Implementation Recommendations 4.7Summary, Conclusions, and Recommendations 4.7.1Multiphase Flow and Transport 4.7.2Chlorinated Hydrocarbon Biodegradation 4.7.3Technology Transfer L1667_book.fm Page 180 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC Acknowledgments References 4.1 INTRODUCTION AND OVERVIEW Halogenated organic compounds such as chlorinated aliphatic and aromatic com- pounds have been used widely as solvents since the early 1940s. As a result of widespread production, transportation, use, and disposal, these compounds are com- mon ground water contaminants. Due to their limited but environmentally signiÞcant aqueous phase solubilities, spills of these compounds typically result in the formation and migration of a separate organic phase that is denser than water. This dense nonaqueous phase liquid (DNAPL) can move signiÞcant distances in the subsurface, contaminating large volumes of the subsurface environment. The residual DNAPL left in the wake of DNAPL ßow can persist as a source of contamination for decades, slowly dissolving into the water phase and volatilizing into the soil–gas phase in the vadose zone. Chlorinated organic compounds are among the most serious ground water con- taminants because of their mobility and persistence in the subsurface, their wide- spread use, and their health effects. Consequently, billions of dollars are being spent in efforts to remediate ground water contamination from chlorinated organic com- pounds. Developing and applying reliable, accurate, and readily available fate and transport models is greatly needed to assess the risks posed by spills of these compounds to the subsurface and to aid in evaluating and designing remediation programs to address these spills. A variety of sophisticated research level models to predict chlorinated organic compound fate and transport in the subsurface have been developed. The use of these models is limited, however, due to mathematical com- plexity, signiÞcant data demands, and the inability to validate the many assumptions inherent in these models. Research on the physics, chemistry, and biology of these compounds and the prediction of their fate and transport in a complex, heterogeneous subsurface is ongoing, with many questions yet to be answered. The panel discussed issues associated with simulating chlorinated organic com- pound behavior in the subsurface. Presentations by panel members focused on modeling chlorinated organic compound fate and transport under natural conditions rather than under enhanced remediation conditions. The uncertainty associated with constitutive relationships appropriate for use in Þeld-scale modeling was identiÞed as an area for continued research. Aspects of modeling DNAPL source dissolution and volatilization were covered, and practical techniques for modeling source-term behavior at the Þeld scale were presented. The problems of dealing with subsurface heterogeneities in simulating Þeld-scale DNAPL behavior were identiÞed, and the need for upscaling methods and robust inverse modeling techniques were empha- sized. Several presentations addressed the current practices for modeling the natural attenuation of chlorinated compounds. Chlorinated compound biodegradation mod- els of varying levels of complexity were reviewed. The need for a framework for choosing the appropriate level of simpliÞcation in modeling was discussed, and the need for technology transfer to end users of models was emphasized. L1667_book.fm Page 181 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC The objective of this chapter is to review the state of the art with respect to the simulation of chlorinated organic compounds in the subsurface and to present the conclusions and recommendations of the panel with respect to basic research needs and issues of technology transfer. Basic concepts and equations underlying models for chlorinated organic compound behavior in the subsurface are reviewed in Section 4.2. The current state of practice with respect to modeling chlorinated organic compound fate and transport is described in Section 4.3. Section 4.4 contains Þeld applications presented by the panel. Research needs identiÞed by the panel are discussed in Section 4.5, while Section 4.6 examines aspects of technology transfer required for the effective promulgation of simulation models for chlorinated organic compounds. 4.2 BASIC CONCEPTS AND EQUATIONS Halogenated organic compounds exhibit a broad range of physical and chemical properties. Those of particular concern as ground water contaminants include indus- trial solvents, such as tetrachloroethylene (also known as perchloroethylene or PCE), trichloroethylene (TCE), and carbon tetrachloride (CT), and a variety of polychlo- rinated biphenyl (PCB) oils used in various industrial applications (Pankow and Cherry, 1996). These compounds are liquids at normal subsurface temperatures and have limited solubility in water and speciÞc gravities greater than water. They thus fall under the deÞnition of DNAPLs (Schwille, 1988). The speciÞc gravity of chlo- rinated aliphatic hydrocarbons can be as high as 1.6 (Mercer and Waddell, 1993). Low solubility and relatively high density are key properties that lead to the com- plexity of their distribution in the subsurface. In this section, these and other impor- tant factors are discussed. The transport and fate of halogenated organic compounds in the subsurface are controlled by complex phenomena in a wide variety of hydrologic settings. Halo- genated organic compounds can occur as a nonaqueous phase liquid (NAPL) and as species in the aqueous, vapor, and soil phases (Figure 4.1). The phenomena that govern the behavior of halogenated organic compounds in the subsurface can be classed into two broad categories that control the following: • Fluid-phase distributions and bulk ßow of NAPL, water, and vapor phases in the subsurface as affected by gravity, capillary and buoyancy forces, pore geometry, and larger-scale heterogeneity • Interphase mass transfer, transport, and attenuation of halogenated com- pounds and their by-products through dissolution, volatilization, sorp- tion/desorption, colloidal transport, diffusion–dispersion, and chemical and biological reactions In general, modeling halogenated organic compounds involves simulating sub- surface systems composed of more than one ßuid phase and requires a conceptual understanding of the relevant chemical, physical, and biological processes that con- trol the distributions, interactions, and reactions within all phases. L1667_book.fm Page 182 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC Mathematical models for multiphase migration of organic contaminants in soils and aquifers were presented by numerous authors beginning in the 1980s (e.g., Abriola and Pinder, 1985; Faust, 1985; Osborne and Sykes, 1986; Kuppusamy et al., 1987; Faust et al., 1989; Sleep and Sykes, 1989, 1993) based on earlier models developed for petroleum reservoir engineering (Aziz and Settari, 1979). 4.2.1 M ULTIPHASE F LUID F LOW 4.2.1.1 Darcy’s Law for Multiphase Flow When an organic ßuid enters the subsurface, it ßows downward due to gravity and capillary forces and moves laterally due to capillary forces. In the vadose zone, the organic displaces the air and water as it moves through the soil pores. DNAPLs can move below the water table, whereas LNAPLs pool on the water table. Below the water table, a DNAPL displaces the water phase as it moves downward. The move- ment of the organic phase through the soil pores is affected by the organic ß uid density, viscosity, interfacial tension with water and air, contact angle of the phase interface with the aquifer solids (i.e., wettability), and by the soil porosity, perme- ability, and pore-size distribution. The inßuence of these factors is manifested in the following generalized form of Darcy’s law that can be used to describe continuum level multiphase ßow in porous media: (4.1) FIGURE 4.1 Fate and transport of chlorinated organic compound releases in the subsurface. Ground Water Flow DNAPL Vapor DNAPL Release Dissolved DNAPL Plume Residual DNAPL Low-permeability layer DNAPL Pool DNAPL q k Pgz r b b b bb m r=- — + — () k L1667_book.fm Page 183 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC In this equation, q b is the Darcy velocity of phase b (b = g for gas, w for water, and n for NAPL) , k r b is the relative permeability of phase b, k is the intrinsic permeability tensor , m b is the viscosity of phase b, P b is the phase pressure, r b is the phase mass density, g is the gravitational acceleration constant, and z is the elevation. Intrinsic permeability is so called because it is generally assumed to be an inherent characteristic of the porous medium. This assumption is usually valid in coarse-grained media but can be a poor assumption in certain cases in which inter- actions between ßuids and soil grains result in temporal changes in the pore structure or if the porosity or pore structure are affected by changes in pore pressure (e.g., consolidation, shrink-swell, hydraulic fracturing). Although in most applications permeability is assumed to be constant over time for a given porous medium, in certain cases consideration of possible relationships to present or historical phase pressures or species concentrations may be necessary for accurate predictions. The tensorial nature of permeability reßects anisotropy that may develop due to a non- random spatial orientation of the pore structure or ßuid distributions or of larger scale heterogeneities (e.g., fractures, layering). Fluid densities vary as a function of respective ßuid pressures. For liquids subjected to small pressure variations, ßuid compressibility can often be safely disregarded. Gas-phase compressibility is signiÞcantly greater than that of liquids, and, if gas ßow is modeled, compressibility should be considered. Gas compress- ibility can be easily modeled based on ideal gas theory. Density effects may be signiÞcant in the vapor transport of dense volatile organics in permeable porous media (Sleep and Sykes, 1989). The mobility of NAPLs is inßuenced by viscosity. Less viscous NAPLs tend to migrate farther and more rapidly than others (Cohen and Mercer, 1993). Capillary pressures (i.e., the pressure differences between phases) are related to interfacial tension between the phases, wettability, and pore geometry. Decreases in interfacial tension or increases in contact angle decrease capillary pressures between phases for a given pore geometry. Decreases in interfacial tension or increases in contact angle thus decrease entry pressures for nonwetting phases into Þne-grained media, increasing the mobility of the nonwetting phase. In general, ßuid density, viscosity, and interfacial tensions vary as functions of temperature and phase composition (Ma and Sleep, 1997). In isothermal, noncompo- sitional models, the dependence of ßuid properties on temperature and ßuid compo- sition and their temporal and spatial variations are disregarded. However, where these effects are expected to be signiÞcant, they should be taken into account. Temperature effects on these properties are well understood, and mixture theories exist to compute compositional effects. In cases where the effects of surfactants may be under consid- eration, speciÞc experimental studies are required to characterize the behavior. 4.2.1.2 Capillary Pressure and Relative Permeability Relations In multiphase systems, pressure differences (i.e., capillary pressures) exist between phases as a result of interfacial tension between phases and curvature of the phase interfaces. The capillary pressure across a curved interface with principal radii r 1 and r 2 is given by the following Laplace–Young Equation (Hunter, 1991): L1667_book.fm Page 184 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC (4.2) where p ≤ and p ¢ are pressures on opposite sides of the interface. The principal radii for an interface in a soil pore are related to pore shape, the location of the phase interface in the pore, and the contact angle between the phase interface and the aquifer solids. The contact angle, measured through the denser ßuid, is determined by the chemical nature of the ßuids and the solid. Wetting ßuids have contact angles less than 90°, while nonwetting ßuids have contact angles greater than 90°. In the vadose zone, liquids (i.e., NAPLs or water) are usually wetting ßuids com- pared with air. In the saturated zone, most natural porous media are strongly water-wet (Anderson, 1986); the exception may be when signiÞcant quantities of natural organic matter, graphite, silicates, and many sulÞdes are present in the porous medium. When determining the wettability of multiphase systems con- taining NAPLs, several factors should be considered, including water chemistry, NAPL chemical composition, presence of natural organic matter, presence of other agents (e.g., surfactants), aquifer saturation history, and mineral composi- tion of the porous medium. For a three-phase (i.e., gas, water, NAPL) system, water is usually the most wetting phase, gas the least wetting, and NAPL the intermediate (Parker et al., 1987). The physically relevant capillary pressures are thus: p gn = p g – p n (4.3a) p nw = p n – p w (4.3b) where subscripts g , w , and n designate gas, water, and NAPL, respectively. For monotonically changing ßuid saturations, the gas–NAPL interface curvature, and, thus, the gas–NAPL capillary pressure, is expected to be a function of gas-phase saturation. The gas-phase saturation controls gas-relative permeability. Similarly, NAPL–water capillary pressure is expected to be a function of water saturation, which controls water-relative permeability. Various mathematical functions have been proposed to describe ßuid satura- tion–capillary pressure (k–S–P) relationships for two-phase and three-phase ßuid systems (Aziz and Settari, 1979; Corey, 1986; Parker et al., 1987). For example, for a two-phase NAPL–water system, the Brooks–Corey relationship for NAPL–water capillary pressure is as follows: (4.4a) (4.4b) ppp rr c = ¢¢ - ¢ =+ È Î Í ù û ú g 11 12 S p p for p p ew dow cow cow dow = Ê Ë Á ˆ ¯ ˜ > l S for p p ew cow dow ==1 L1667_book.fm Page 185 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC [...]... potential; 1,1-DCE = 1,1-Dichlorethene; t-DCE = trans-1,2-Dichloroethene; c-DCE = cis-1,2-Dichloroethene; VC = Vinyl chloride; PCA = Tetrachloroethane; 1,1,2-TCA = 1,1,2-Trichloroethane; 1,1,1-TCA = 1,1,1-Trichloroethane; 1,1-DCA = 1,1-Dichloroethane; 1,2-DCA = 1,2-Dichloroethane; CA = Chloroethane; CF = Chloroform; DCM = Dichloromethane; CM = Chloromethane Source: After McCarty and Semprini, 19 94; McCarty,... rPCE = - kX PCE Í ú úÍ Î K PCE + cPCE û Í K H2 + c H2 ú û Î (4. 19) where XPCE is the biomass concentration of PCE degraders, cPCE and cH2 are the water- phase concentrations of PCE and hydrogen, respectively, and KPCE and KH2 are the half-saturation constants for PCE and hydrogen, respectively The production of hydrogen from the fermentation of ethanol to acetic acid and propionic acid and ©20 04 CRC... ©20 04 CRC Press LLC L1667_book.fm Page 211 Tuesday, October 21, 2003 8:33 AM TABLE 4. 4 Overview of Unsaturated Flow and True Two-Phase Flow and Transport Models Program CHAIN_2D HBGC123D + FEMWATER 3DMURF +3DMURT R-UNSAT SUTRA TOUGH2 +T2VOC VLEACH VS2DI Description 2-D unsaturated ßow and multispecies transport; chain decay 3-D variably saturated ßow; multispecies transport; heat transport; bio- and. .. three ßuid-phase soil and ground water systems made their Þrst appearance in the mid-1980s (e.g., Faust, 1985) Facilitated by advances in basic understanding of multiphase ßow and transport and numerical techniques (e.g., Abriola and Pinder, 1985; Parker, 1989; Mercer and Cohen, 1990) and by advances in computer hardware, numerous multiphase numerical models were introduced in the 1990s (Table 4. 3) Multiphase... following: 0 Sh = 12(f - q n ) Re 0.75 q n Scw.5 w (4. 16) where f is porosity, qn is the nonwetting-phase volume fraction, Rew is the waterphase Reynolds number, and Scw is the water- phase Schmidt number (i.e., ratio of water- phase velocity to water- phase diffusion coefÞcient of organic species) Miller et al (1998) summarized correlations found by others such as Powers et al (1992, 19 94) It is likely that... investigations have found that sorption and desorption processes can take from months to years to reach equilibrium Cornelissen et al (1997) examined the temperature dependence of slow adsorption and desorption in batch experiments with chlorobenzenes, PCBs, polycyclic aromatic hydrocarbons (PAHs), and laboratory- and Þeld -contaminated sediments The laboratory -contaminated sediments were maintained in contact... associated with ground water movement A variety of investigations have shown chloroethenes to be resistant to abiotic degradation A growing body of evidence suggests, however, that PCE; TCE; PCA; 1,1,1-TCA; and CT can be degraded abiotically in the presence of ferrous sulÞdes that are common in reduced aquifers and wetlands (Kriegman-King and Reinhard, 1992; Curtis and Reinhard, 19 94; Devlin and Muller,... aqueous, NAPL, and solid (i.e., adsorbed) phases In the simplest models, local equilibrium phase parti- TABLE 4. 3 Overview of Available Three-Phase Flow and Transport Models Program FEHM MAGNUS MOFAT MUFTE NAPL NUFT STOMP COMPFLOW COMPSIM UTCHEM Description 3-D three-phase ßow; multispecies transport; heat transport; dual porosity 3-D three-phase ßow; single species transport 2-D three-phase ßow; multispecies... degradation 1,1,1-TCA and 1,2-dichloroethane (1,1-DCA) are both susceptible to degradation via hydrolysis 1,1,1-TCA can also be degraded via an abiotic elimination (alkane => alkene) reaction to form 1,1-DCE (Figure 4. 2) This abiotic degradation of 1,1,1-TCA likely is the primary source of 1,1-DCE in contaminated aquifers PCA degrades to TCE by the same type of abiotic reaction (Lorah and Olsen, 1999)... concentrations and assumptions about boundary-layer thickness and microcolony geometry The third class of bioÞlm models (Sykes et al., 1982; Borden and Bedient, 1986; Kindred and Celia, 1989; MacQuarrie et al., 1990; Wood et al., 19 94; McClure and Sleep, 1996) makes no assumption about bioÞlm conÞguration and assumes that diffusion limitations in stagnant boundary layers and in the bioÞlm may be neglected and . DNAPL 4. 5.3.4Summary 4. 5.4Plume Attenuation 4. 5 .4. 1Biotransformation Kinetics 4. 5 .4. 2Halorespiration 4. 5 .4. 3Spatial Variability in Redox Conditions 4. 5 .4. 4Complex Mixtures 4. 5 .4. 5Bioavailability and. Phase 4. 5 .4. 6Summary 4. 6Technology Transfer 4. 6.1Approach 4. 6.1.1QA Standards 4. 6.1.2Expert Decision Support System 4. 6.1.3Model Application Archive and Database Support 4. 6.1.4Training Support 4. 6.2Implementation. potential; 1,1-DCE = 1,1-Dichlorethene; t-DCE = trans-1,2-Dichloroethene; c-DCE = cis-1,2-Dichloroethene; VC = Vinyl chloride; PCA = Tetrachloroethane; 1,1,2-TCA = 1,1,2-Trichloro- ethane; 1,1,1-TCA =