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Ground Water Models: Scientific and Regulatory Applications (1990)
Commission on Physical Sciences, Mathematics, and Applications (CPSMA)

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1 Introduction This report addresses the use of ground water flow and contam- inant transport modeling in the regulatory process. Its goals are to (~) examine the scientific bases upon which existing models are founded; (2) communicate the philosophies and approaches routinely used in the application of models to decisionmaking for regulatory purposes; and (3) provide guidelines concerning how models should be developed and applied in the regulatory process so that their util- ity and credibility are enhanced. This study is particularly timely because there are both increasing reliance on models and increasing uncertainty about the extent to which models can be and should be used. Because the subsurface environment is not easily observed or accessible, models have become the tools employed to understand ground water systems and simulate and predict their behavior. Mod- els are nothing more than mathematical representations of complex phenomena (McGarity, 1985~. They are used to do the following: ~ evaluate the understanding of physical processes in a quanti- tative way; ~ identify the key issues needing further theoretical or field research; educate a nontechnical audience such as a government poli 22

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INTROD ACTION 23 cymaker or the public, including a judge or a jury, by illustrating a phenomenon or concept; ~ select optimal sampling locations and otherwise enhance field ., . monitoring; . simulate the past or future response of water levels to pump- ing, or the pattern of spreading of a plume of chemicals from a landfill, spill, leaking underground storage tank, or other source; design a ground water remedial program; or ~ optimize efficiency in industrial processes, such as secondary and tertiary methods of recovering oil. slon Both flow and transport models have been used in an equally wide variety of regulatory and legal contexts, such as the following: ~ a federal or state environmental impact statement (ENS) to assess the potential impact of a particular project before it is im- plemented, e.g., the likelihood and severity of leakage of radioactive wastes from a long-term nuclear waste depository; ~ an administrative record to support the technical standards required pursuant to federal or state regulations; an administrative record supporting a remedial action deci an administrative record for a permit at a particular site; and evidence at a trial, e.g., to establish causation in a Superfund contribution action by one private party against another private party or to establish exposure in a personal injury action. THE GROWTH IN THE USE OF MODELS The growth in the use of models in the United States stems from a series of ever more stringent and comprehensive environmen- tal statutes developed since the early 1970s. The most important statutes for the purposes of this report include the Comprehensive Environmental Response, Compensation, and Liability Act (CER- CLA or ~Superfund"), the Resource Conservation and Recovery Act (RCRA), the Safe Drinking Water Act (SDWA), and the National Environmental Policy Act (NEPA) (see Table A.. The galvaniz- ing force for these statutes came from highly publicized pollution incidents, such as the relocation of residents from the vicinity of con- tamination sources at Love Canal in New York, and Times Beach, Missouri. There is a very large number of potential sources of ground water contamination (see Table 1.1~. Virtually all of these sources could

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24 GROUND WATER MODELS TABLE 1.1 Estimated Numbers of Contamination Problems That Need to Be Addressed Under Various Statutes Type of Potential Ground Water Contamination Source Superfund hazardous waste National Priority List (NPL) sites (currently on the NPL or proposed)a Potential Superfund NPL sites that must be assessed preliminarily and inspected by 1989b Superfund remedial investigations and feasibility studies at Superfund sites that must be commenced By 1989 By 1991C Number Nationwide (unless otherwise noted) 951 27,000 Superfund remedial actions that must be commenced By 1989 By 1992C RCRA hazardous waste facilities Operating landfills Closing landfills Operating and closing incinerator and other treatment and storage facilities Projected number of RCRA facility investigations RCRA nonhazardous waste facilities (e.g., municipal and identical landfills RCRA nonhazardous waste facilities with a high likelihood of containing hazardous wastese Mining waste sitese Underground storage tanks e Pesticide-contaminated sitese Underground injection wellsf Class I wells (hazardous waste injected below a U.S. drinking water supply) Class II wells (secondary oil and gas production) Class III wells (mining) Class IV wells (hazardous waste injected into or above a drinking water supply now essentially banned) Class V wells (all other miscellaneous wells) TOTAL Estimated number of abandoned and unplugged oil and gas wells" Sites with releases of radioactive materialse Environmental impact statements per yearh (1985) (it is estimated that between 15 and 40 percent of these documents may involve projects that require the use of ground water or contaminant flow modeling) Surface impoundments' Industrial Municipal Agricultural Mining 275 650 175 375 393 1,095 3,338 2,938 70,419 to 261,930 70,419 22,339 10,820 3,920 533 153,126 249 25 46,271 200,204 1,200,000 1,502 549 25,749 36,179 19,167 24,451

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INTRODUCTION TABLE 1.1 Continued Type of Potential Ground Water Contamination Source Oil/gas brine pits Other TOTAL Petroleum product pipelines miles (19761' (carrying 10 billion barrels) Liquid petroleum and nonhazardous waste underground storage tanks (as of 1984)k 25 Number Nationwide (unless otherwise noted) 64,951 5,748 176,245 175,000 2,500,000 a National Priorities List for Uncontrolled Hazardous Waste Sites, 52 Fed. Reg. 27,620, 27,621 (1987) (Final Rule). b Section 116(d)(1) of CERCLA, 42 USCA § 9616(d)(1); and Surveys and Investigations Staff, House Committee on Appropriations, Report on the Status of the Environmental Protection Agency's Superfund Program 31 (March 1988) (hereinafter House Staff Report). C Section 116(e)(1) of CERCLA, 42 USCA § 9616(e)(1). d General Accounting Office, Hazardous Waste: Corrective Action Cleanups Will Take Years To Complete Table II.1, at 31 (1987) (GAO/RCED-88-48). e General Accounting Office, Supe~fund: Extent of Nation's Potential Hazardous Waste Problem Still Unknown Table 2.1, at 14 (1987) (GAO/RCED-88-44). fGeneral Accounting Office Report to the Chairman, Environment, Energy, and Natural Resources Subcommittee, Committee on Government Operations, House of Representatives, Hazardous Waste: Controls Over injection Well Disposal Operations Table 1.2, at 13 (1987) (GAO/RCED-87- 170). gEPA, Report to Congress on the Management of Wastes and the Exploration, Development, and Production of Crude Oil, Natural Gas, and Geothermal Energy, Executive Summaries 14 (De cember 1987). h Council on Environmental Quality, The Sixteenth Annual Report of the Council on Environmental Quality Table 4-4, at 173 (1986). 'Geophysics Study Committee, Geophysics Research Forum, Commission on Physical Sciences, Mathematics, and Resources, National Research Council, Groundwater Contamination Table 1, at 4 (1984). Patrick, R., E. Ford, and J. Quarles, Groundwater Contamination in the United States 269 (2d ed. 1987). k House Staff Report, supra note b, at 13; also see G. Lucero, Director of the Office of Waste Programs Enforcement, EPA, Son of Supe~fund, Can the Program Meet Expectations, Environ mental Forum 5, 5-9 (March/April 1988). There is a very large number of potential sources of ground water contamination (see Table A.. Virtually all of these sources could require the use of a contaminant transport model. The use of models is increasing at an accelerated rate. Guidance on the investigation of hazardous waste sites by federal agencies will encourage the use of contaminant flow models in the future (see Chapter 6~. Many of the responsibilities mandated by federal and state leg- islation cannot adequately be carried out without models. Yet, the majority of federal and state agencies have no overall strategy for de- veloping, using, disseminating, and maintaining these valuable tools

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26 GROUND WATER MODELS (Office of Technology Assessment, 1982~. As we will see throughout this report, the key scientific question affecting whether a mode! can be used is: How good are the predictions made by the model? There are undeniable scientific uncertainties inherent in mode! predictions, e.g. (National Research Council, 1988), Where is no model that will adequately describe all ground water quality problems because the assumptions and simplifications gener ally associated with models do not adequately mimic all the processes that influence the movement and behavior of the water and/or the chemicals of interest. Legal issues can also determine whether a mode! is used properly. How good do the predictions need to be as a matter of law and/or policy? It is within this context that the Water Science and Technology Board assembled the Committee on Ground Water Modeling As- sessment to examine the current state of knowledge in ground water models and the role of contaminant transport in the regulatory arena. This 21-month study was supported by the Electric Power Research Institute, the U.S. Nuclear Regulatory Commission (USNRC), the U.S. Environmental Protection Agency (EPA), the National Science Foundation, and the U.S. Army. The remainder of this report is divided into six parts. Chapter 2 describes how models are classified, the mathematical formulation and solution of the flow and mass transport equations, and the steps that are followed in code selection and mode} development. Chapters 3 and 4 provide basic background information in the form of an overview of the unportant physical, chemical, and bio- logical processes that provide the scientific framework for models. The intent of these chapters is to give the reader a clear apprecia- tion of how water and contaminants move in flow systems and which parameters control their behavior. Chapter 5 reviews the agency regulations and guidelines that require or give guidance on the use of modeling and provides five case studies. This chapter demonstrates how the concepts of modeling, developed in the previous chapters, have been applied to practical problems. Chapter 6 reviews the USNRC and EPA experience in applying models and discusses other issues in the development and use of mod- els. For example, quality assurance, the lack of qualified modelers, and the role of modeling in management are discussed. Chapter 7 focuses on what the committee perceives to be the

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INTROD ACTION 27 emerging scientific, engineering, and policy trends as they relate to modeling. Trues examined In this chapter include linking geochem- ical and physical transport models, developing new modeling capa- bilities to handle complex processes, and the emerging new model approaches. The committee attempts to bring together the varied concepts and ideas that were developed throughout the report in a way that will be useful to regulators and modelers alike. As the reader will discover, there are inherent limitations in what models can accom- plish, but there are ways in which the developers and consumers of these models can enhance their usability. REFERENCES McGarity, T. 1985. The Role of Regulatory Analysis in Regulation Decision- Making, (background report for Recommendation 85-2 of the Administra- tive Conference). Published by Administrative Conference of the United States, p. 241. National Research Council. 1988. Hazardous Waste Site Management: Water Quality Issues. Report on a colloquium sponsored by the Water Science and Technology Board. National Academy Press, Washington, D.C., p. 14. Office of Technology Assessment. 1982. Use of Models for Water Resources Management, Planning, and Policy. U.S. Government Printing Office, Washington, D.C., pp. 9-10.

Representative terms from entire chapter:

hazardous waste