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s Experience With Contaminant Flow Models in the Regulatory System INTRODUCTION This chapter is divided into two parts: (~) a review of federal regulations and guidance concerning the use of contaminant trans- port models, and (2) five case studies illustrating the site-specific application of such models. These sections are based on the com- mittee's review and interpretation of these regulations and guidance, existing reports on the use of such models, discussions with agency personnel, and the personal experience of the committee members. This chapter focuses on the regulations, guidance, and prac- tices of the U.S. Nuclear Regulatory Commmsion (USNRC) and the U.S. Environmental Protection Agency (EPA). These two regulatory agencies deal with contaminant transport from historic or proposed disposal facilities and recognize the need to evaluate present condi- tions and predict potential migrations. Both agencies have programs in place that require mocleling. However, each agency suffers from unique problems that reflect its particular regulatory concerns. The USNRC has had a number of years to prepare for an am plication for a high-level radioactive waste disposal. As a result, the agency has had the opportunity to develop detailed procedures on reviewing mode} applications. Unfortunately, because of changes in federal programs, the developed procedures are largely untested. In 160

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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 161 contract, EPA has had to evaluate a large number of modeling stud- ies as part of the Superfund program. Because of the rapid increase in sites being evaluated, EPA has not had an opportunity to develop a systematic plan for mode] review or application. In the following sections the two agencies' approaches to the use and review of models are summarized. U.S. NUCLEAR REGULATORY COMMISSION REGULATIONS AND GUIDANCE One of the USNRC's responsibilities is the licensing of facili- ties for the disposal of low-level and high-level radioactive wastes (see 10 CFR Part 61, relicensing Requirements for Land Disposal of Radioactive Waste," and 10 CFR Part 60, "Disposal of High-Leve! Radioactive Wastes in Geologic Repositories; Licensing Procedures," respectively). To be licensed, a facility must meet certain require- ments. For example, one requirement is that the site be capable of being modeled (10 CFR Part 61~. Thus the USNRC has embed- ded into its regulations and guidance general principles concerning contaminant transport modeling. However, this guidance is largely untested because the USNRC has performed only limited licensing for waste disposal facilities. Low-leve} radioactive waste (~LW) is generated by a number of institutions including industries, laboratories, hospitals, and facilities involved in the nuclear fuel cycle. Wastes are packaged and placed in shallow excavations or engineered structures that are then back- fi~led and capped to limit infiltration. The USNRC LLW disposal regulations specify performance objectives and specific technical re- quirements for site suitability that are designed to adequately protect public health (Siefken et al., 1982~. One of the requirements is that "the disposal site shall be capable of being characterized, modeled, analyzed, and monitored" (U.S. Nuclear Regulatory Commission, 1987~. The purpose of this requirement is to ensure that the hydro- geological conditions of the site are adequately understood through field studies. The USNRC has also developed standard review plans (SRPs) (U.S. Nuclear Regulatory Commission, 1987) that direct the USNRC staff in evaluating the potential for migration for a disposal facility. Review plans have been issued to evaluate a number of potential migration pathways including radionuclide movement through the

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162 GROUND WATER MODELS ground water and movement of radionuclides resulting from infiItra- tion through the ground surface. The SRPs for ground water and infiltration contain information on the amount of modeling planned by the regulatory agency as well as the type of issues that will be reviewed by the agency. The SRP indicates that the license application will be reviewed to determine whether the use of the input parameters has been justified and whether the data are sufficient to provide a reasonably accurate or conservative analysis regarding ground water pathways. The transport models will also be evaluated for their defensibility, suitability, and basic conservatism. The codes must be based on sound physical, chemical, and mathematical principles and must be correctly applied and sufficiently documented. The applicant must supply the following: a complete description of the contaminant transport path- ways between the engineered disposal unit and the site boundary and existing or known future ground water user locations; ~ estimates and justification for the physical and chemical input parameters used in the transport models to calculate radionuclide concentrations; a description of the contaminant transport models used in the analysis, including modeling procedures and complete documen- tation of the codes as required in NUREG-0856 (U.S. Nuclear Regu- latory Commission, 1987, p. 6.1.5.1-4~; ~ the justification, documentation, verification, and calibration of any equations or program codes used in the analyses; and ~ the description of data and justification for the manipulation of any data used in the analyses (p. 6.1.5.2-3~. The SRP does not attempt to quantify the level of information re- quired to adequately characterize the potential ground water trans- port at the sites nor does it outline the acceptance criteria for ad- equate site modeling. To evaluate the applicant's submittal, the USNRC will use "simple analytical modeling techniques with demon- strably conservative assumptions and coefficients" (p. 6.~.5.1-3~. The SRP does not outline which codes will be used, and no other support- ing documentation was provided that outlined the codes planned for use by the USNRC. The SRP guidance states that If the applicant's results are more realistic than conservative, then the applicant must clearly justify the application and results of the models (p. 6.~.5.2-3~. More

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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 163 sophisticated numerical modeling will be performed by the USNRC when the issues relating to the applicant's modeling studies cannot be resolved. The SRP does not discuss the apparent disparity between requesting field information to characterize the site and the use of conservative data in the modeling process. The LLW program appears to have developed a systematic plan for incorporating modeling into the site evaluation process. The plan has attempted to consider, in a general way, the reliability of the data input, as well as the documentation and reliability of the computer codes. The program, as outlined in the USNRC guidance and the SRPs, appears to emphasize conservatism, although the regulations place equal emphasis on collecting adequate field information. Also, the program has not attempted to direct applicants toward partic- ular computer codes because the codes the USNRC will use are not defined. The USNRC also has published extensive documentation on the codes that are planned for potential use in evaluation of license applications. The publication of this documentation allows license applicants to consider using USNRC codes or to review their code choices against the USNRC-distributed tools. The USNRC guidance is designed to evaluate whether the models accurately simulate the phenomena that are considered and to de- termine whether the numerical approximations accurately solve the mathematical equations. The test problems include analytical and semianalytical solutions, as well as problems based on laboratory or field studies. By providing a standardized process of mode} evaluation, the USNRC is attempting to limit the amount of code comparison that will be required at the time of license application. The USNRC (1982) outlines the level of documentation deemed adequate, i.e., The documentation of mathematical models and numerical methods will provide the basis for USNRC's review of the theory and means of solution used in the code. It should contain derivations and justification for the model. The documentation will help the USNRC in understanding modeling results that are submitted by the applicant during the licensing process and permits the USNRC to install and use the code on its own computer. The USNRC guidance also outlines a computer software man- agement system that will provide a software storage system to ensure

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164 GROUND WATER MODELS future retrievability of computer codes and will provide a standard- ized testing process for applied codes. The storage system will include a catalog of modifications and the most updated version of the codes in use. The USNRC is (1) assembling mathematical models for assessing Department of Energy (DOE) demonstrations; (2) cleveloping com- puter software for use in assessing the long-term risk from disposal of radioactive wastes in deep geologic formations, in estimating dose commitments and potential adverse health effects from released ra- dionuclides, and in performing sensitivity and uncertainty analyses; and (3) developing a quality assurance program to ensure adequate quality in computer codes developed and in data generated by these codes, as well as for maintenance of the programs. This program re- quires peer review and management approval to ensure a systematic record of calculations and analyses that are performed. In summary, the USNRC has attempted to define a process that considers not only the problems in evaluating mode! results but also the issues surrounding code selection and application. The guidance documents have attempted to direct applicants to the appropriate level of code review without limiting the choice of code selection. U.S. ENVI:ELONMENTAL PROTECTION AGENCY REGULATIONS AND GUIDANCE The U.S. Environmental Protection Agency uses a wicle variety of contaminant transport moclels en c} has a large number of specific sites where such models are used and will be used. The key EPA regulations and guidance affecting the use of contaminant transport moclels e.g., those in the SuperfuncI, hazardous waste management, ant} underground injection programs-are incluclec! in the following · ~ c .lscusslon. Sup erfund Law and Regulations Superfund is the environmental law that authorizes EPA to ~ identify sites where hazardous substances have been releaser} into the environment; ~ clean up such contamination and recover the costs from the responsible private parties; or

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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 165 ~ in the alternative, (~) order a private party to perform the cleanup or (2) obtain a voluntary agreement from such private party (called potentially responsible party [PRP]) to perform the cleanup.) Superfund Is primarily directed at cleanup of inactive hazardous waste sites. Courts generally have resolved legal uncertainties and issues of statutory interpretation in favor of the government in order to hold private parties liable, because Enliven the remedial nature of . . . iSuperfund] its provisions should be afforded a broad and liberal construction so as to aneroid frustration of prompt response efforts or so as to limit the liability of those responsible for clean-up costs beyond the limits expressly provided.2 If EPA performs the remedy with money from Superfund, the remedy is selected after a review of remedial alternatives. This pro- cess is subject to public comment. At other sites, EPA negotiates the remedy necessary for the site with the PRPs, such as in the S- Area I~ndfi~! case (see Case Study 5, this chapter). These negotiated remedies are then incorporated into a consent decree (a legal docu- ment that resolves a lawsuit without a determination of liability, but requires the defendant to perform an action, e.g., installation of tile drains and a cap, and/or monetary payment). Guidance Modeling may be used in the Superfund program to (1) guide the placement of monitoring wells (Environmental Protection Agency, 1988b); (2) predict concentrations in ground water for an assessment of the present and future risks at the site (Environmental Protection Agency, 19X6a, 198Sb); (3) assess the feasibility and efficacy of re- medial alternatives (Environmental Protection Agency, 1988b); (4) predict the concentration for an assessment of the residual risk after implementation of the preferred remedial action (Environmental Pro- tection Agency, 198Bb); or (5) apportion liability among responsible parties. Contaminant transport modeling is important in the process of estimating exposure and therefore risk. Regardless of the toxicity of the chemical, no injury can occur unless there is exposure. The chemicals must migrate from the source of contamination to a point where they come into contact with humans and interact biologically with the human body. Modeling can be "used as a too} . . . to estimate plume movement . . ." (Environmental Protection Agency,

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166 GROUND WATER MODELS 1988b). Models are most helpful when rough estimates are required.3 A worst-case estimate (an estimate where all assumptions are chosen so as not to underestimate the possible exposure) may indicate that little risk exists if significant exposures are not predicted. However, [ads more resources are devoted to an exposure assessment and more studies conducted, a refined assessment is generated. Often there will be several stages of refinement of an assessment, and the degree of refinement and accuracy finally required will be related to the certainty needed to enable risk management decisions [e.g., selecting a ground water cleanup level versus evaluating the most cost-effective method of achieving that leveli.4 The EPA Superfund Public Health Evaluation Manual guidance (Superfund guidance) specifies that realistic exposure assumptions based on the best data available should be used.5 Superfund guidance requires EPA to consider systematically the extent of chemical fate and transport in each environmental medium in order to account for the behavior of all released chemicals (Environmental Protection Agency, 1986a, p. 39; see also 40 CFR §§300.68te]~], 300.68th]~2~tiv], 300.68ti]~1~. A ground water concentration, based on such mode! estimation, is then compared to levels of public health concern, e.g., a drinking water standard or a risk-based cleanup level (Zamuda, 1986~. EPA advises that "caution should be used when applying models at Superfund sites because there is uncertainty whenever subsurface movement is modeled, particularly when the results of the mode! are based on estimated parameters" (Environmental Pro- tection Agency, 1988b, p. 3-22~. Superfund guidance provides a general framework for selecting and applying models (Environmental Protection Agency, 1988b, p. 3-33; 1988c). Superfund modeling guidance recognizes the potential problem posed by the large range of models available and attempts to support users by providing guidelines for mode] choice (Environ- mental Protection Agency, 198Sc). These criteria allow users to more easily justify code choice during discussions with regulators and may provide some common ground for discussing the use of alternative codes. Three types of criteria are recommended for use in mode] se- lection: objective, technical, and implementation. The objective criteria used relate to the level of modeling detail needed to meet the objectives of the study, i.e., (1) performing a screening study or (2) performing a detailed study (Environmental Protection Agency, J

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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 167 1988c). Because the purpose of a screening study would be to ob- tain a general understanding of site conditions or to make general comparisons between sites, a simple mode} may be suitable at that stage. The technical criterion used for mode! selection relates to the model's ability to simulate site-specific transport and fate phenom- ena of interest at the site. There are three areas where technical cri- teria should be developed: transport and transformation processes, domain configuration, and fluid media properties. The third type of criterion used for mode} selection relates to the ability to implement the model. Issues that must be considered include the difficulty of obtaining the model, the level of documen- tation and testing associated with the model, and the ease of mode! use. The budget and the schedule for any project will affect the type of criterion used and ultimately the mode} selected (Environmental Protection Agency, 1988c). The 1988 guidance represents a significant advance in the EPA modeling program because it provides structure to the mode! selec- tion process and will avoid mixing discussions of mode} applicability and mode} results. Dividing these two processes could help simplify interactions between the regulators and the regulated community. Even this guidance represents only a small step toward simplifying the regulatory process. A number of codes used in EPA programs are described in the latter portion of the report. However, information on the level of complexity of these codes and the criteria for their application are not included. Additional clarification of EPA mode! use will be needed to help direct code selection in mode! applications that will be submitted to the agency. If problems arise, EPA personnel are directed to EPA's Center of Exposure Assessment Modeling and the International Groundwa- ter Modeling Center for specific advice (Environmental Protection Agency, 1988b, p. 3-33; 1988c). Ultimately, however, EPA personnel must rely upon their own skills. Resource Conservation and Recovery Act Law and Regulations There are tens of thousands of facilities that handle hazardous waste and therefore must obtain a permit. The Resource Compensa

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168 GROUND WATER MODELS tion and Recovery Act (RCRA) establishes comprehensive, "cradIe- to-grave~ hazardous waste management programs. RCRA forbids waste treatment or disposal and limits waste storage for facilities not holding appropriate permits from EPA or a state agency (Section 3005 [a] of RCRA, 42 USC §6925 [a] ~ . The very foundation of any regulatory program is the definition of what is regulated versus what is not. The EPA definition of a hazardous waste determines "whether a waste, if mismanaged, has the potential to pose a significant hazard to human health or the environment due to its propensity to leach toxic compounds" (51 Fed. Reg. 21,653 [1986~6~. EPA has listed industrial waste streams as hazardous based on a lirn~ted sampling of a representative number of plants in the industry. Also, a waste is considered hazardousif it is ignitable, corrosive, or explosive or if the leachable concentra- tions of certain chemicals exceed regulatory health-based lignite (i.e., the extraction procedure [EP] test). A waste is hazardous based on this EP test if chemicals will leach out of the waste in quantities that may cause the ground water concentrations 500 It downgradi- ent to exceed drinking water standards after the waste is placed in a municipal landfill. EPA's original definition of hazardous waste assumed arbitrarily that the leachable concentration of a chemical would decrease by a factor of 100 in the 500 It (45 Fed. Reg. 33,084 t1980171. In 1986, EPA proposed to modify the EP test used to define haz- ardous waste by, among other things, (1) adding 38 organic chemical constituents, (2) substituting a more rigorous leaching test, (3) ap- plying compound-specific attenuation and dilution factors for each organic constituent to evaluate the worst-case potential impact on ground water 500 It downgradient of the location of deposal, and (4) using a risk-based concentration when no drinking water standard is available (51 Fed. Reg. 41,082 t1986~. EPA's proposed new defini- tion uses a subsurface fate and transport model, called EPASMOD (or the Composite Landfill Model), to derive compound-specific at- tenuation and dilution factors. EPASMOD considers the dilution, hydrolysis, and soil adsorption that occur as a chemical migrates from the bottom of a landfill to a drinking water source 500 It away (see 51 Fed. Reg. 1,602 [1986~9 for a more detailed discussion of the EPASMOD). The Environmental Protection Agency has revised EPASMOD and its input data and is considering additional revisions to EPAS- MOD and its input data so that the predicted concentrations would

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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 169 be less overpredictive (53 Fed. Reg. 28,892 t1988~°~. The proposal therefore would incorporate a contaminant transport into the defini- tion of hazardous waste. Contaminant transport models also have been used in other aspects of the RCRA program. For example, EPA uses the vertical- horizontal spread (VHS) mode! to determine when a listed hazardous waste from a particular facility would no longer be subject to RCRA hazardous waste requirements because the particular characteristics of the waste from that facility make the waste nonhazardous wherever it may be disposed (50 Fed. Reg. 48,886 t1985~; see Case Study 1, below). The RCRA regulations also require the permittee to perform ground water monitoring (40 CFR §264.97, 264.98, 264.99) and to clean up contaminated conditions at active facilities in any area where there was historic disposal of either hazardous or solid wastes (40 CFR §264.100~. The corrective action requirements, in essence, convert RCRA into a Superfund-type cleanup statute and expand RCRA's jurisdiction to cover all inactive waste disposal areas on operating facilities. The RCRA regulations require a permitter to clean up the ground water to (1) background levels, (2) the concentrations spec- ified by EPA for drinking water, or (3) a site-specific risk-based action level (the alternate concentration limit, or ACL) (40 CFR §264.94~. To evaluate the potential adverse effects on ground water quality, the permitter must provide information on, among other things, the wastes' potential for migration; the hydrogeological char- acteristics of the facility and surrounding land; the existing quality of ground water, including other sources of contamination and their cumulative impact on the ground water quality; and the potential for health risks caused by human exposure to waste constituents (40 CFR §264.94tb]~. The permittee must also submit an exposure and risk assess- ment. The two key concepts in the ACL process are that (1) the cleanup level must protect the public at the point of exposure (i.e., where ground water is withdrawn to use as drinking water), and (2) the point of compliance (i.e., the point where ground water is moni- tored) must be at the boundary of the regulated unit (Environmental Protection Agency, 1987a). It is necessary to set the ACL at a level (usually monitored at the boundary of the regulated unit) that, based on predictions, will result in ground water exposures that are below health protective

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170 GROUND WATER MODELS levels at some distant point (e.g., the nearest drinking water well) and some future time. Guidance Contaminant transport modeling can be used for the same pur- poses as in the Superfund program. EPA's general exposure guidance concerning the use of models (above) is equally applicable here. The RCRA guidance encourages using conservative assumptions "where time and/or resources are limited" (Environmental Protec- tion Agency, 1986b, p. 150~. Numerical models are preferred over analytical models (p. 158~. EPA's RCRA guidance lists publicly available models (p. 158~. The Environmental Protection Agency's RCRA guidance is con- tradictory, however. EPA's RCRA alternative concentration limit guidance (Environmental Protection Agency, 1987a, p. 4-6) states that [although not required for an ACL demonstration, mathematical sim- ulation models of ground water flow and contaminant transport can be extremely useful tools for the applicant. Models are more appropriate for relatively simple geologic environments where conditions do not vary widely; in complex geologic areas, modeling may be less useful. The permit applicant is responsible for ensuring that the mod- els used simulate as precisely as possible the characteristics of the site and the contaminants and minimize the estimates and assump- tions required.... Whenever possible, input parameters and ae~umpt~ona should be conservatioc in nature; worst-case secr~ar~os may eauc much effort. [Emphasis added.] The RCRA ground water monitoring guidance, on the other hand, states "modeling results should not be unduly relied upon in guiding the placement of assessment monitoring wells or in designing cor- rective actions" (Environmental Protection Agency, 1986b, p. 156; emphasis added). Recently, EPA has considered standardizing the steps in the risk assessment/modeling process by "prescribing the types of models that can be used or the assumptions that are incorporated into models" (Environmental Protection Agency, 1987b). Among the standard models being considered is the VHS model. The standard mode! would guide the decision "based on only minimal site-specific data" (Environmental Protection Agency, 1987b). The use of a nationwide database would be contrary to EPA site-specific use on the selection of models.

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200 GROUND WATER MODELS ~ Eighteen remedial alternatives were designed and analyzed for the Air Force property. Ground water extraction and recharge were chosen as the preferred remedy. This remedy for the Air Force property, which began operation in 1988, involves pumping, treating, and recharging approximately 26 billion gal. of water over a 10-yr period (Environmental Protection Agency, 1988a). This remedy is described in the Record of Decision issued for the site on July 25, 1988. No remedy has yet been selected for the area north of Los Reales Road. The modeling study conducted by CH2M Hill for EPA that attempted to assign liabilities was severely criticized by several of the potentially responsible parties, and as a result no agreement has yet been reached on an appropriate remedial action for this area. Discussion Ground water models are frequently used to determine sources of observed contamination. In general, the information on the current distribution of contaminants and hydrogeologic conditions is insuffi- cient to allow a unique solution for the location of sources and the timing of source releases. This is particularly true when all poten- tial sources are located along the same stream line and there are no marker chemicals for a specific source. Ground water models, however, can be used to help set bounds on the range of possible contributions from individual sources. ~Area, Niagara Falls, New York Background The S-Area landfi~! is located on the southeast corner of Oc- cidental Chemical Corporation's Buffalo Avenue Plant in Niagara Falls, New York. Approximately 63,100 tons of chemical waste was deposited at the site. The S-Area landfill is one of four landfi~Is in the Niagara Falls, New York, area that were operated by Occidental Chemical Corporation (OCC), formerly known as Hooker Chemicals Plastics Corporation. The other landfi~Is are Love Canal, Hyde Park, and the 102nd Street landfill. Ground water flow and transport models have been used ex- tensively at all of these sites, and the use of these models is par- ticularly well documented (Mercer et al., 1985; C. Faust, affidavits in Civil Action Nos. 79-988 and 79-989 in the U.S. District Court for the Western District of New York, 1984 and 1985, respectively).

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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 201 These contaminant transport models have also been incorporated into legally enforceable documents and have been evaluated and ap- proved by a court. For simplicity, this case study focuses primarily on the models used at the S-Area site, because they illustrate the complex processes that can be simulated with the current generation of ground water transport models. A major concern at the landfill is discontinuities in an underlying confining bed that allow dense nonaqueous-phase liquids (NAPEs) to contaminate a bedrock aquifer. The chemicals in the lands! will be contained after remediation by an integrated system of barrier walls, plugs, drains, and a cap that is designed to prevent off-site migration. A conceptual hydrogeologic cross section of the landfill! before and after remediation is shown in Figure 5.12. Prior to remediation, hydraulic gradients are downward, and ground water and NAPL flow into the bedrock where the clay and fill are missing. After remediation, the drains, walls, and cap on the site are intended to create a sufficient upward hydraulic gradient to reverse the flow of ground water and NAPL into the bedrock. A one-dimensional, tw~phase flow mode! was developed by Arthur D. Little, Inc. (ADL) to establish what upward hydraulic gradient would prevent downward migration of NAPL at the S-Area lands! (Arthur D. Little, Inc., 1983; Guswa, 1985; C. Faust, Af- fidavit in Civil Action No. 79-988 in the U.S. District Court for the Western District of New York, 1984 (particularly paragraphs 42-44~. The mode! considers, among other things, the effects of Ethology-dependent capillary pressure functions, hydraulic gradients, and permeability variations. Subsequently, a two-dimensional, two- phase flow mode} was developed by EPA's consultant to ensure that the one-dimensional mode! was appropriate for selecting a remedy for the site. After the initial remedies were selected for the site, a three-dimensional mode! was developed and is currently being used to evaluate conditions at the site and the potential effectiveness of additional remedies at the site. The model's use to design the remedy is discussed in this case study. Site Conditions The NAPL found at S-Area has a specific gravity of approxi- mately 1.5 and consists primarily of trichIorobenzene, tetrachioro- benzene, pentachIorobenzene, tetrachIoroethylene, hexachIorocyclo- pentadiene, and octachiorocyclopentene (S. Fogel, Affidavit in Civil Action No. 79-988 in the U.S. District Court for the Western District

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202 Before Rain Lagoon Leaks GROUND WATER MODELS Rain .: . :$ : ;: 8~ ~( t~ NApL : 2 . : ~ ~ ~ ~ ~,,~und W~r ~ NAPL i. · . . . . . %~ ~.. ... ..== ~ ~ ~ j ~j ~ ~ . ~ ~ ~j . j. ... ... . , .: it. ~,.~$::~:~; ~ ~ ~- ' ~ ~ ~ ~ ~ After `~ ` ~ rat Wall ~ ~ I_ ~ G .~ .'.' '.~ ~2 ~.'2. . Mimi ~ ~` ~ ,' .: . ' . . . . .. ~; ~ ! r ,; ~ ~r ~ ~ ~ 7~ ~ ~ ~ ~ ~ ~ ~ ~ ~ -~ ~ ~ _~ ~ it! ~ ~ ~ ~ ;* ; ;; ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ _ r ~ ~ ~ ~ ~' ~ ~ ~ ~ ~ Ground Water & NAPE FIGURE 5.12 A conceptual cross section of the hydraulic containment system to be implemented at the S-Area landfill. SOURCE: Cohen et al., 1978. of New York, 1984~. These liquids have been observed in discrete discontinuous zones in the landfill. Geologic logs indicate a litho- logic contact between unconsolidated glacial deposits and bedrock (Lockport dolomite) at an elevation of about 541 ft. The base of the unconsolidated glacial deposits is a clay ranging in thickness from about 0.25 to 15 ft. The clay is overlain by a relatively thick (up to 16 It) fine sand layer containing scattered zones of silt and fine gravel. This is overlain by about 14 It of artificial fill. Bedrock water- leve} measurements indicate a potentiometric elevation of about 561 ft. Water levels measured in the overlying unconsolidated deposits

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CONTAMINANT FLO W MODELS IN THE REGULATORY SYSTEM 203 indicate a positive head difference between the overburden and the underlying bedrock of between 2 and 5.5 ft. Under these conditions therefore, a vertical downward flow component exists. Mode} Formulation The two models developed to design the remedies used the method of finite differences. The ADL mode! employed the im- plicit pressure-explicit saturation (IMPES) method to solve the two coupled equations of flow for an immiscible nonaqueous phase and water. The air phase is neglected. The ADL mode} also used a mesh-centered grid, whereas the other model, referred to as SWAN- FLOW (simultaneous water and NAPL flow), used a block-centered approach (GeoTrans, 1985~. To evaluate the potential for downward NAPL flow, a vertical column 23 It long was divided into 24 blocks (nodes). The mode! was constructed with a 2-ft negative head difference (downward flow) between the water table and bedrock potentiometric level. The do- main contains three different porous materials. The upper 20 It consists of a fine sand with a hydraulic conductivity of 10-5 cm/s (k = 1.02 x 10-~4 mid. The fine sand is underlain by 1 It of clay (K = 10-7 cm/s; kin 1.02 x 10-~6 mid. The clay is underlain by the Lockport dolomite bedrock (K = 10-3 cm/s; k = 1.02 x 10-~2 mat. The residual saturation values for water and NAPL were assumed to be 20 and 10 percent, respectively. Other simulation data are given in Tables 5.5 and 5.6. Results and Conclusions The results show that a barrier to downward migration of NAPL is provided by capillary pressure differences between the sand and clay (Figure 5.13~. This condition has been confirmed in recent field investigations at the S-Area site (Faust and Guswa, 1989~. A comparison between the results of the two numerical models is shown in Figure 5.13. The saturations calculated by SWANFLOW and the ADL code at approximately 250 days are shown. The results from the two models compare favorably; however, there are some differences, especially just above the clay layer. The differences are probably caused by some combination of instability in the IMPES technique, alternative "ridding and time steps used in the two codes, and slight differences in the relative permeability relationships (the ADL [1983] mode! provided for hysteresis in capillary pressure).

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204 GROUND WATER MODELS TABLE 5.5 Capillary Pressure and Relative Permeability Data for ADL Simulation 1 Capillary Water Relative Permeabilities Pressure (N/m2)a Saturation Water NAPL Fine sand and bedrock 103,425.0 0.00 0.00000 1.00000 103,425.0 0.10 0.00000 0.82000 103,425.0 0.20 0.00000 0.68000 27,580.0 0.30 0.04000 0.55000 10,343.0 0.40 0.10000 0.43000 7,585.0 0.50 0.18000 0.31000 7,447.0 0.60 0.30000 0.20000 7,309.0 0.70 0.44000 0.12000 7,171.0 0.80 0.60000 0.05000 7,033.0 0.90 0.80000 0.00000 6,895.0 1.00 1.00000 0.00000 Clay 206,850.0 0.00 0.00000 1.00000 206,850.0 0.10 0.00000 0.82000 206,850.0 0.20 0.00000 0.68000 165,480.0 0.30 0.04000 0.55000 134,453.0 0.40 0.10000 0.43000 110,320.0 0.50 0.18000 0.31000 93,082.0 0.60 0.30000 0.20000 82,740.0 0.70 0.44000 0.12000 75,845.0 0.80 0.60000 0.05000 72,398.0 0.90 0.80000 0.00000 68,950.0 1.00 1.00000 0.00000 aN = newton (i.e., kg-n~s2). SOURCE: Arthur D. Little, Inc.,1983. TABLE 5.6 Data Used in ADL Simulation 1 Parameter Value Porosity Permeability Fine sand Clay Bedrock Density of water Density of NAPL Water viscosity NAPL viscosity Dz (vertical dispersion length) - SOURCE: Arthur D. Little, Inc.,1983. 0.2 1.02 x 10-~4m2 1.02 x 10- ~6 m2 1.02 x 10- ~2 m2 1,000 kg/m3 1,500 kg/m3 0.001 kg/m-e 0.001 kg/m-e 0.3048 m

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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 205 575 llJ r ~ 570 _ > ~ z 560 _ LIJ O 550 _; i O 540 _ 535 _ BY In 44 ' / '/ ,,, or/ ... :e- ·:~:-: ::: ·:~:-: :-:-: .... _ Original water table elevation Modeled column (e = node locations) hSR-tWT = 9 It ky fine sand = 10 5 cm/s Initial NAPL distribution ADL ~ 250 days SWANFLOW ~ 275 days Multiple points Model boundary nodes ~ jI "\ L- Fine sand ; ~ 0 20 40 60 NAPL DISTRIBUTION, percent Bedrock FIGURE 5.13 NAPL saturation profiles at one time for the two-layer simula- tion. The effects of a water-phase hydraulic gradient on NAPL migra- tion were also examined via these simulations, where the clay layer was assumed to be musing. As shown in Figure 5.13, the results of this series of simulations indicated that a minimum upward head difference of 9 It between the water table elevation and bedrock po- tentiometric level in the vicinity of a clay layer discontinuity could be sufficient to prevent downward migration of NAPL into the bedrock (Guswa, 1985~. This figure indicates NAPL saturations at about 250 days. As shown, there is a noticeable upward movement of NAPL. Data have been collected as part of a remedy designed to lower the hydraulic head in the overburden sand. These data will be used to confirm the remedy as well as modeling results. Regulatory Context In December 1979 the federal government filed four lawsuits to obtain cleanup of four OCC landfills. EPA, the state of New

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206 GROUND WATER MODELS York, and OCC negotiated an extensive set of remedies. The S- Area Consent Decree incorporated these remedies, including the one-dimensional, two-phase containment transport mode} discussed above. The consent decree was lodged with the court on January 10, 1984.23 Consent decrees are subject to a Sunday public comment period. If the consent decree is adequate, proper, and in the public interest, the Department of Justice and the court finally approve it (see 28 CFR §50.7~. In this particular case, the province of Ontario requested and the court granted a formal evidentiary hearing to re- view the consent decree. These models were subject to close, critical scrutiny during the public comment period and court hearing, in- cluding scrutiny by consultants hired by the province of Ontario.24 The court held that the consent decree was "fair, adequate, and con- sistent with public policy . . . [and] wall adequately protect the public L J Interest In neaten ano tne environment."25 Two consultants employed by EPA, as well as EPA and state personnel, oversaw and peer-reviewed the development of the ADL mode! (G. Pinder, Affidavit in United States v. Hooker Chemicals and Plastics (~5~ Area Landfill}, Civil Action No. 79-988 in U.S. District Court for the Western District of New York, 1984, particu- larly paragraphs 23-25~. The two-dimensional, two-phase flow mode} was developed by one of EPA's consultants to ensure that the one- dimensional mode! was appropriate at the site (C. Faust, Affidavit in Civil Action No. 79-989 in the U.S. District Court for the Western District of New York, 1985~. Discussion This case study illustrates the use of relatively complex models of ground water and NAP L flow to help design a remedial action for a hazardous waste site. Field studies have shown that both of these models were able to simulate observed field conditions. The results of these mode} studies have demonstrated that the current generation of ground water models can be used to investigate the migration of an immiscible, denser than water fluid within an aquifer. Interestingly, this study also shows that a one-dimensional mode} can be just as useful as a two-dimensional mode! in the investigation of the appropriateness of a remedial action.

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CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM 207 NOTES 1. 42 USC §9601 et seq. and 40 CFR Part 300. National Oil and Hazardous Substances Pollution Contingency Plan, 53 Fed. Reg. 51,394 (1988), contains the proposed new Superfund regulations. 2. United Statue v. Mottolo, 605 F. Supp. 898, 902 (DNH 1985~. 3. Chemical Carcinogens; A Review of the Science and Its Associated Principles, 1985, 50 Fed. Reg. 10,372 (1985~. 4. Guidelines for Estimating Exposures, 51 Fed. Reg. 34,042 (1986~. 5. Ibid. 6. Hazardous Waste Management System; Identification and Listing of Hazardous Waste; Final Exclusion and Final Organic Leach ate Model (OLM). 7. Hazardous Waste Management System; Identification and Listing of Hazardous Waste. 8. Hazardous Waste Management System; Identification and Listing of Hazardous Waste; Notification Requirements; Reportable Quantity Adjust- ments. 9. Hazardous Waste Management System; Land Disposal Restrictions. 10. Hazardous Waste Management System; Identification and Listing of Hazardous Waste; New Data and Use of These Data Regarding the Establish- ment of Regulatory Levels for the Toxicity Characteristic; and Use of the Model for the Delisting Program. 11. Underground Injection Control Program: Hazardous Waste Disposal Injection Restrictions; Amendments to Technical Requirements for Class I Hazardous Waste Injection Wells; and Additional Monitoring Requirements Applicable to All Class I Wells. 12. Citing D. Morganwalp and R. Smith, 1987, Modeling of Representative Injection Sites, EPA report in progress. 13. This discussion of contamination in the Tucson Airport area is largely extracted and paraphrased from the report by CH2M Hill (1987) and the Remedial Investigation prepared for the Arizona Department of Health Services by Schmidt (1985) and Mock et al. (1985~. 14. Hazardous Waste Management System; Identification and Listing of Hazardous Waste: Use of a Generic Dilution/Attenuation Factor for Establish- ing Regulatory Levels and Chronic Toxicity Reference Level Revisions. 15. Hazardous Waste Management System; Identification and Listing of Hazardous Waste. 16. Hazardous Waste Management System; Identification and Listing of Hazardous Waste; Notification Requirements; Reportable Quantity Adjust- ments. 17. McLouth Steel Products flora. v. Thomas, 838 F.2d 1317, 1320 (D.C. Cir. 1988). 18. Hazardous Waste Management System; Identification and Listing of Hazardous Waste. 19. Hazardous Waste Management System; Identification and Listing of Hazardous Waste; Final Exclusion Rule. 20. Hazardous Waste Management System; Identification and Listing of Hazardous Waste; Final Denials. 21. See supra, note 10. 22. The Washington Post, p. 1 (October 9, 1981~. 23. United States v. Hooicr Chemicals ~ Plastics Corp. (S-ArcaJ, Civ. Act. No. 79-988 (filed January 10, 1984~.

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208 GROUND WATER MODELS 24. Unfed Stated v. Hoofer Chcaucal~ ~ Plastics Corp., 607 F. Supp. at 1061. 25. Ibid. at 1070. BIBLIOGRAPHY Arizona Department of Health Services. 1986. Responsiveness Summary, Re- sults of the Tucson Airport Area Remedial Investigation, 15 pp. Arthur D. Little, Inc. 1983. S-Area to Phase Flow Model. Prepared for Wald, Harkrader & Ross (now merged with Pepper, Hamilton & Scheetz), Washington, D.C. CH2M Hill. 1987. Assessment of the Relative Contribution to Groundwater Contamination from Potential Sources in the Tucson Airport Area, Tucson, Arizona. Prepared for U.S. EPA Region IX. Cohen, R. M., R. R. Rabold, C. R. Faust, J. O. Rumbaugh III, and J. R. Bridge. 1978. Investigation and hydraulic containment of chemical migration: Four landfills in Niagara Falls. Civil Engineering Practice (Spring), 33-58. Cooley, R. L. 1977. A method for estimating parameters and assessing reliability for models of steady-state ground-water flow, 1. Theory and numerical properties. Water Resources Research 13, 318-324. Domenico, P. A., and V. V. Palciauskas. 1982. Alternative boundaries in solid waste management. Ground Water 20, 301-311. Downey, J. S., and E. J. Weiss. 1980. Preliminary Data Set for Three- Dimensional Digital Model of the Red River and Madison Aquifers. U.S. Geological Survey Open-File Report 80-756, Denver, Colo. Environmental Protection Agency. 1986a. Superfund Public Health Evaluation Manual. OSWER Directive No. 9285.4-1, Washington, D.C. Environmental Protection Agency. 1986b. RCR~A Ground-water Monitor- ing Technical Enforcement Guidance Document. OSWER Directive No. 9950.1, Washington, D.C. Environmental Protection Agency. 1987a. Alternate Concentration Limit Guid- ance Part 1: ACL Policy and Information Requirements. Interim Final. OSWER Directive No. 9481.00-6C, EPA/530-SW-87-017. Washington, D.C. Environmental Protection Agency. 1987b. Evaluation of Risk-Based Decision- making in ACRE. Annotated Briefing. Internal document, p. 5. Environmental Protection Agency. 1988a. Evaluation of Hughes Aircraft, U.S. Air Force Plant No. 44, Tucson, Ariz. EPA/700-8-87-037, Hazardous Waste Ground-Water Task Force, Washington, D.C. Environmental Protection Agency. 1988b. Final Review Draft Guidance on Remedial Actions for Contaminated Ground Water at Superfund Sites. OSWER Directive No. 9283, Washington, D.C., pp. 3-22. Environmental Protection Agency. 1988c. Selection Criteria for Mathematical Models Used in Exposure Assessments: Ground-water Models. EPA/600/8- 88/075. Washington, D.C. Faust, C. R., and J. H. Guswa: 1989. Simulation of three-dimensional flow of immiscible fluids within and below the unsaturated zone. Submitted to Water Resources Research. In preset Fenneman, N. M. 1931. Physiography of the Western United States. McGraw- Hill, New York.

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CONTAMINANT FLO W MODELS IN THE REGULATORY SYSTEM 209 GeoTrans, Inc. 1985. SWANFLOW: Simultaneous Water, Air, and Nonaqueous Phase Flow, Version 1.0. Documentation prepared for Environmental Protection Agency. Grove, D. B. 1977. The Use of Galerkin Finite-Element Methods to Solve Ma~s-Transport Equations. U.S. Geological Survey Water Resources In- vestigation 77-49, 55 pp. Guswa, J. H. 1985. Application of Multi-Phase Flow Theory at a Chemical Waste Landfill, Niagara Falls, New York. Pp. 108-111 in Proceedings of the Second International Conference on Groundwater Quality Research, published by the National Center for Ground Water Research, Stillwater, Okla. Konikow, L. F. 1976. Preliminary Digital Model of Ground-Water Flow in the Madison Group, Powder River Basin and Adjacent Areas, Wyoming, Montana, South Dakota, North Dakota, and Nebraska. U.S. Geological Survey Water Resources Investigation 63-75, 44 pp. Konikow, L. F., and J. D. Bredehoeft. 1978. Computer Model of Two- Dimensional Solute Transport and Dispersion in Ground Water: Tech- niques of Water-Resources Investigations of the United States Geological Survey. Book 7, Chapter C2, U.S. Geological Survey, 90 pp. Lewis, B. D., and F. J. Goldstein. 1982. Evaluation of a Predictive Ground Water Solute-Transport Model at the Idaho National Engineering Labora- tory, Idaho. U.S. Geological Survey Water Resources Investigation 82-55, 71 pp. Mercer, J., C. R. Faust, R. M. Cohen, P. F. Andersen, and P. S. Huyakorn. 1985. Remedial action assessment for hazardous waste sites via numerical simulation. Waste Management and Research 3, 377-387. Mock, P. A., B. C. Travers, and C. K. Williams. 1985. Results of the Tucson Airport Area Remedial Investigation, Volume III, Contaminant Transport Modeling. Arizona Department of Water Resources, 106 pp. Rampe, J. 1985. Results of the Tucson Airport Area Remedial Investigation, Phase I, Volume III, Evaluation of the Potential Sources of Groundwater Contamination near the Tucson International Airport. Arizona Depart- ment of Health Services. Reeves, M., D. S. Ward, N. D. Johns, and R. M. Cranwell. 1986a. Data Input Guide for S WIFT II; the Sandia Waste-Isolation Flow and Transport Model for Fractured Media. Release 4.84. NUREG/CR-3162, U.S. Nuclear Regulatory Commission, Washington, D.C. Reeves, M., D. S. Ward, N. D. Johns, and R. M. Cranwell. 1986b. Theory and Implementation for SWIFT II; the Sandia Waste-Isolation Flow and Transport Model for Fractured Media. Release 4.84. NUREG/CR-3328, U.S. Nuclear Regulatory Commission, Washington, D.C. Reeves, M., D. S. Ward, P. A. Davis, and E. J. Bonena. 1987. SWIFT II Self- Teaching Curriculum; Illustrative Problems for the Sandia Waste-Isolation Flow and Transport Model for Fractured Media (revised). NUREG/CR- 3925, U.S. Nuclear Regulatory Commission, Washington, D.C. Robertson, J. B. 1974. Digital Modeling of Radioactive and Chemical Waste Transport in the Snake River Plain Aquifer at the National Reactor Testing Station, Idaho. U.S. Geological Survey Open-File Report ID0-22054, 41 PP

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210 GROUND WATER MODELS Ross, B., J. W. Mercer, S. D. Thomas, and B. H. Lester. 1982. Benchmark Problems for Repository Siting Models. NUREG/CR-3097, U.S. Nuclear Regulatory Commission, Washington, D.C. Schmidt, K. D. 1985. Results of the Tucson Airport Area Remedial Investi- gation, Vol. I, Summary Report. Arizona Department of Health Services, 113 pp. Science Advisory Board. 1984. Report on the Review of Proposed Environmen- tal Standards for the Management and Disposal of Spent Nuclear Fuel, High Level and Transuranic Radioactive Wastes (40 CFR §191~. Report to the EPA by the High-Level Radioactive Waste Subcommittee. Siefken, D., G. Pangburn, R. Pennifill, and R. J. Starmer. 1982. U.S. Nu- clear Regulatory Commission Low Level Waste Licensing Branch Technical Position Site Suitability, Selection, and Characterization. NUREG-0902, U.S. Nuclear Regulatory Commission, Washington, D.C. Silling, S. A. 1983. Final Technical Position or Documentation of Computer Codes for High-Level Waste Management. NUREG-0856, U.S. Nuclear Regulatory Commission, Washington, D.C. Trescott, P. C., G. F. Pinder, and S. P. Larson. 1976. Finite-Difference Model for Aquifer Simulation in Two Dimensions with Results of Numerical Experiments. Techniques of Water-Resources Investigations of the U.S. Geological Survey, Book 7, Gh. C1. U.S. Government Printing Office, Washington, D.C., 116 pp. U.S. Nuclear Regulatory Commission. 1982. Final Technical Position on Documentation of Computer Codes for High Level Waste Management. NUREG-0856, Washington, D.C., p. 2. U.S. Nuclear Regulatory Commission. 1988. Standard Review Plan for the Review of a Licensed Application for a Low Level Radioactive Waste Disposal Facility: Safety Analysis Report 1988, NUREG-1200, Revision 1. Washington, D.C. Wilkinson, G. F., and G. E. Runkle. 1986. Quality Assurance (QA) Plan for the Computer Software Supporting the U.S. Nuclear Regulatory Commission's High-Level Waste Management Program. NUREG/CR-4369, U.S. Nuclear Regulatory Commission, Washington, D.C. Woodward-Clyde Consultants, Inc. 1981. Well-Field Hydrology Technical Re- port for ETSI Coal Slurry Pipeline Project. U.S. Bureau of Land Manage- ment. Zamuda, C. 1986. The Superfund record of decision process: Part 1 The role of risk assessment. Chemical Waste Litigation Reporter 11, 847.