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GROUND WATER MODEM Scientific and Regulatory Applications Water Science and Technology Board Committee on Ground Water Modeling Assessment Commission on Physical Sciences, Mathematics, and Resources National Research Council NATIONAL ACADEMY PRESS Washington, D.C. 1990

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National Academy Press 2101 Constitution Avenue, N.W. ; Washington, D.C. 20418 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Frank Press is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Robert M. White is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Samuel O. Thier is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Frank Press and Dr. Robert M. White are chairman and vice chairman, respectively, of the National Research Council. Support for this project was provided by the Electric Power Research Institute under Contract No. RP2485-10, the U.S. Nuclear Regulatory Commission under Contract No. NRC-04-87-096, the U.S. Environmental Protection Agency under Contract No. CR-814067 the National Science Foundation under Grant No. CES/8708081, and the U.S. Army under Purchase Order No. DAAD05-88-M-M061. Library of Congress Cataloging-in-Publication Data Ground water models: scientific and regulatory applications / Committee on Ground Water Modeling Assessment. Water Science and Technology Board, Commission on Physical Sciences, Mathematics, and Resources, National Research Council. p. cm. Includes bibliographical references. ISBN 0-309-03993-2 1. Groundwater Bow. 2. Liability for water pollution damages. I. National Research Council (U.S.). Committee on Ground Water Modeling Assessment. TC176.G76 1989 363.73'94-dc2089-14033 CIP Copyright (~)1990 by the National Academy of Sciences No part of this book may be reproduced by any mechanical, photographic, or electronic process, or in the form of a phonographic recording, nor may it be stored in a retrieval system, transmitted, or otherwise copied for public or private use, without written permission from the publisher, except for the purposes of official use by the U.S. government. Printed in the United States of America First Printing, January 1990 Second Printing, September 1990

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COMMITTEE ON GROUND WATER MODELING ASSESSMENT FRANK W. SCHWARTZ, Ohio State University, Columbus, Chairman CHARLES B. ANDREWS, S. S. Papadopulos & Associates, Inc., Rockville, Maryland DAVID L. FREYBERG, Stanford University, Stanford, California CHARLES T. KINCAID, Battelle, Pacific Northwest Laboratory, RichIand, Washington LEONARD F. KONIKOW, U.S. Geological Survey, Reston, Virginia CHESTER R. McKEE, In Situ, Inc., I,aramie, Wyoming DENNIS B. McLAUGHLIN, Massachusetts Institute of Technology, Cambridge JAMES W. MERGER, GeoTrans, Inc., Herndon, Virginia ELLEN J. QUINN, Northeast Utilities, Hartford, Connecticut P. SURESH CHANDRA RAO, University of Florida, Gainesville BRUCE E. RITTMANN, University of Illinois, Urbana-Champaign DONALD D. RUNNELLS, University of Colorado, Boulder PAUL K. M. van der HEl]DE, HoIcomb Research Institute, Indianapolis, Indiana WlI.I`lAM J. WALSH, Pepper, Hamilton & Scheetz, Washington, D.C. Staff WENDY L. MELGIN, Project Manager JEANNE AQUlLINO, Project Secretary Agency Liaisons to Committee EDWARD H. BRYAN, National Science Foundation DONALD L. CHERY, JR., U.S. Nuclear Regulatory Commission STEPHEN R. CORDLE, U.S. Environmental Protection Agency IRA MAY, U.S. Army Toxic and Hazardous Materials Agency ISHWAR P. MURARKA, Electric Power Research Institute ~e 111

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WATER SCIENCE AND TECHNOLOGY BOARD MICHAEL C. KAVANAUGH, James M. Montgomery Consulting Engineers, Oakland, California, Chairman NORMAN H. BROOKS, California Institute of Technology, Pasadena STEPHEN J. BURGES, University of Washington, Seattle (through 6/30/89) RICHARD A. CONWAY, Union Carbide Corporation, South Charleston, West Virginia JAMES P. HEANEY, University of Florida, Gainesville R. KEITH HIGGINSON, Idaho Department of Water Resources, Boise (through 6/30/89) HOWARD C. KUNREUTHER, University of Pennsylvania, Philadelphia LUNA B. LEOPOLD, University of California, Berkeley (through 6/30/89) G. RICHARD MARZOLF, Murray State University, Murray, Kentucky ROBERT R. MEGLEN, University of Colorado at Denver .lAMFS W MFIRC]F~R GeoTrans Herndon. Virginia (through 6/30/89) DONALD J. O'CONNOR, Manhattan College, Bronx, New York BETTY H. OLSON, University of California, Irvine P. SURESH C. RAO, University of Florida, Gainesville GORDON G. ROBECK, Consultant, Laguna Hills, California (through 6/30/89) PATRICIA L. ROSENFIELD, The Carnegie Corporation of New York DONALD D. RUNNELLS, University of Colorado, Boulder A. DAN TARLOCK, Chicago Kent College of Law HUGO F. THOMAS, Department of Environmental Protection, Hartford, Connecticut JAMES R. WALLIS, IBM T.~. Watson Research Center, Yorktown Heights, New York M. GORDON WOLMAN, The Johns Hopkins University, Baltimore, Maryland 1V

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Staff STEPHEN D. PARKER, Director SHEILA D. DAVID, Staff Officer CHRIS ELFRING, Staff Officer WENDY L. MELGIN, Staff Officer JEANNE AQUlLINO, Administrative Assistant ANITA A. HALL, Senior Secretary RENEE A. HAWKINS, Adrffinistrative Secretary v

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COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS, AND RESOURCES NORMAN HACKERMAN, Robert A. Welch Foundation, Chairman ROBERT C. BEARDSLEY, Woods Hole Oceanographic Institution B. CLARK BURCHFIEL, Massachusetts Institute of Technology GEORGE F. CARRIER, Harvard University RALPH J. CICERONE, University of California, Irvine HERBERT D. DOAN, The Dow Chemical Company (retired) PETER S. EAGLESON, Massachusetts Institute of Technology DEAN E. EASTMAN, IBM T.~. Watson Research Center MARYE ANNE FOX, University of Texas, Austin GERHART FRIEDLANDER, Brookhaven National Laboratory LAWRENCE W. FUNKHOUSER, Chevron Corporation (retired) PHILLIP A. GRIFFITHS, Duke University NEAL F. LANE, Rice University CHRISTOPHER F. McKEE, University of California, Berkeley RICHARD S. NICHOLSON, American Association for the Advancement of Science JACK E. OLIVER, Cornell University JEREMIAH P. OSTRIKER, Princeton University Observatory PHILIP A. PALMER, E.~. du Pont de Nemours & Company FRANK L. PARKER, Vanderbilt University DENIS J. PRAGER, MacArthur Foundation DAVID M. RAUP, University of Chicago ROY F. SCHWITTERS, Superconducting Super Collider Laboratory LARRY L. SMARR, University of Illinois, Urbana-Champaign KARL K. TUREKIAN, Yale University MYRON F. UMAN, Acting Executive Director V1

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Preface The issue of how ground water models should be used to address legal and regulatory concerns was brought to the attention of the Water Science and Technology Board in 1986. The U.S. Army re- quested assistance in assessing the efficacy of a specific modeling effort focused on an evaluation of contamination at a specific Army facility. The Army wanted to know to what extent that particu- lar mode! could be used to apportion liability among several pos- sible sources. The board concluded that investigation of such a site-specific problem was not appropriate for the NRC and decided instead to initiate a broader study dealing with the scientific basis and applicability of ground water models. This initiative probably could not have come at a better time. Hydrogeologists are being caught in the middle between some major advances in science and increasing pressure from legal and regulatory bodies to use models to provide answers to specific questions. On the scientific side, there has been an explosion in knowledge in the past 10 years concerning the processes that control flow and mass trans- port in all kinds of hydrogeologic settings. This new understanding of how ground water systems behave has been incorporated in a va- riety of models. On the practical side, there is a community of users, employed by engineering consulting firms, government agencies, and national laboratories, who are being asked to solve increasingly com- plicated legal and regulatory problems. It is not at all clear whether . V11

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~. V111 PREFACE existing models axe appropriate for the tasks being set for them, nor is it clear to what extent new knowledge has changed modeling practice. These issues provided the backdrop for an 18-month study sup- ported by the Electric Power Research Institute, the U.S. Nuclear Regulatory Commission, the Environmental Protection Agency, the U.S. Army, and the National Science Foundation. The goal of this study was to address two questions: "To what extent can the cur- rent generation of ground water models accurately predict complex hydrogeologic and chemical phenomena?" and "Given the accuracy of these models, is it reasonable to assign liability for specific ground water contamination incidents to individual parties or make regula- tory decisions based on long-term prediction?" The members of the committee formed to address these topics came from universities, government, and private industry with broad experience related to the scientific and legal aspects of modeling. We benefited from the guidance and expert assistance provided by the staff of the Water Science and Technology Board. Our sponsors, to be sure, supplied money for the study, but, far more than that, they provided yet other points of view for us to consider and assistance in gathering information. Our intention in writing this report was to produce a document that was stimulating, readable, and comprehensive. All of us came away from this task with some different views about models and modeling issues as the result of some very stimulating and provocative meetings. This report represents our best attempt at addressing some of the most controversial issues in ground water science today. FRANK W. SCHWARTZ, Chairman Committee on Ground Water Modeling Assessment

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Contents OVERVIEW, CONCLUSIONS, AND RECOMMENDATIONS....1 Overview, 1 Conclusions and Recommendations, 7 1 INTRODUCTION c c The Growth in the Use of Models, 23 References, 27 2 MODELING OF PROCESSES.. Introduction, 28 What is a Model?, 52 References, 73 FLOW PROCESSES c Introduction, 79 Saturated Continuum Flow, 80 Flow in the Unsaturated Zone, 88 Concepts of Water Flow in the Unsaturated Zone, 91 Fracture Flow, 97 Issues in Modeling, 104 References, 109 LX 22 . 28 79

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x CONTENTS 4 TRANSPORT e . e e e e e e e e e e 113 Introduction, 113 Transport of Conservative Solutes, 115 Nonconservative Solutes, 123 Transport in the Unsaturated Zone, 138 Multiphase Transport, 148 References, 156 EXPERIENCE WITH CONTAMINANT FLOW MODELS IN THE REGULATORY SYSTEM.ee...ee..eeeeeeeeeeeeeeee 160 Introduction, 160 UeSe Nuclear Regulatory Commission Regulations and Guidance, 161 Environmental Protection Agency Regulations and Guidance, 164 Selected Case Studies, 172 Notes, 207 Bibliography, 208 6 ISSUES IN THE DEVELOPMENT AND USE OF MODELS e e e e e e e e e e e e e e e e e e e e e e e e e e e . e e e e e e e e e e e e e e e . e . e e e Introduction, 211 The People Problem, 213 Uncertainty and Reliability, 216 Assuring the Quality of Models, 233 Quality Assurance Procedures for Code Development, 235 Mode! Application, 239 Bibliography, 243 RESEARCH NEEDSee eeeeeeeeeeeeee e Introduction, 249 Use of Models, 252 Scientific mends and Research, 255 Policy Mends and Support for Research, 276 References, 281 APPENDIX: BIOGRAPHICAL SKETCHES OF COMMITTEE MEMBERS ....................................... INDEX. 211 249 285 ....291

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Tables and Figures TABLES 1.1 Estimated Numbers of Contarn~nation Problems That Need to Be Addressed Under Various Statutes ... 2.1 A Summary of the Processes Important in Dissolved Contaminant Transport and Their Impact on Contaminant Spreadin g .. .. ..... .. .. ... .. .. .... ... .. .. . 2.2 Classes of Xenobiotic Organic Compounds Known to Be Biodegraded by Aerobic Bacteria.................. Classes of Xenobiotic Organic Compounds Known to Be Biodegraded Under Strictly Anaerobic Conditions 51 Summary of the Steps Involved in Developing the Unsaturated Flow Equation................................. Summary of the Steps Involved in Developing Two-Component Flow Equations...... 2.6 Typical Boundary Conditions for Ground Water Flow and Transport Problems e ~ - 65 A Summary of the Common Solution Techniques for Problems of Fluid Flow and Dissolved Mass Transport 69 Some O(h) Relationships Reported in the Literature.. Some K (~) or K (h) Relationships Reported in the Literature .................................................... 2.3 2.4 2.5 -.. 24 - 38 .51 58 . .59 X1 .93 94

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. X11 3.3 4.3 5.1 5.2 5.3 5.4 FIGURES TABLES AND FIGURES Some Expressions Developed to Describe the Time Dependence of Infiltration Rate (i) Following Ponding of Water e e e 3.4 Major Differences Between Saturated and Transient Unsaturated Water Flow in Porous Media e 98 ..... 94 Summary and Evaluation of Thermodynarn~cs of Abiotic Transformation Mechanisms for Aqueous Species eeeeeeeee Summary and Evaluation of Kinetics of Abiotic Transformation Mechanisms e e e e e e e . e 127 Summary and Evaluation of the Thermodynamics of Phase Transfers............................................. 126 Synopsis of Case Studies e RCRA Delisting Data for Gould, Inc. Facility, McConnelsville, Ohio.............................. Calculated Estimates of Trichloroethene Releases from Source Areas ~ ~ Calculated Relative Contributions from Individual Source Areas e Capillary Pressure and Relative Permeability Data for ADL Simulation 1 131 .174 .179 197 199 . 204 Data Used in ADL Simulation 1 . e e 204 2.1 An example of a regional ground water flow system 30 2.2 Dependence of the pattern of ground water flow on the recharge rate, as reflected by the configuration of the water table 31 2.3 Dependence of the pattern of ground water flow on the hydraulic conductivity distribution 2.4 Example of the relationships between pressure head and hydraulic conductivity for an unsaturated soil 2.5 The distribution of water in the unsaturated zone can be described in terms of pressure head and moisture content ~ The flow of a nonaqueous-phase liquid that is (a) less dense than water (oil) and (b) more dense than water (chlorohydrocarbon) in the unsaturated and saturated zones.. ...... 31 ...... 34 .34 ..36

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TABLES AND FIGURES e. X111 2.7 Plume produced (a) by advection alone and (b) by advection and dispersion 40 2.8 Idealized pattern of plume spreading in three dimensions is characterized by a longitudinal and two transverse dispersion components 41 2.9 (a) Microscale and (b) macroscale variability contributing to the development of dispersion 42 2.10 Many of the geochemical processes like radioactive decay and sorption attenuate the spread of 2.11 contaminants 44 The components of a model: input data, a governing equation solved in the code, and the predicted distribu tion, which for this example is hydraulic head 53 2.12 An example of how (a) a "real" system is represented by (b) a mode} system, which is defined by a region shape, boundary conditions, and hydraulic parameters 54 2.13 Mode! system from the previous figure subdivided by a rectangular grid system 3.1 Relationship between pore size (r) on capillary rise 3.2 .68 and pressure head (h)89 Examples of soil-water characteristic curves O(h), for several soils 3.3 Examples of K(h) relationships for several soils 3.4 Examples of the changes in infiltration rate (i) with time for different water application scenarios 3.5 Variations in soil water content as a result of several episodes of water inputs at the surface ,, 97 Conceptualization of discontinuities in a fractured medium. . . e ~ 3.7 Idealization of a natural fracture as parallel plates with an aperture of 2b -. 100 Experimental data that illustrate how flux in the fracture and aperture change as a function of effective stress for a sandblasted, sawcut fracture surface 101 Three different conceptualizations of fracture networks: (a) a two-dimensional system of line segments; (b) a three-dimensional system of rectangular fractures; and (c) a three-dimensional system of "penny-shaped" .90 ........ 92 3.6 . .95 3.8 3.9 ... t 99 cracks................................... 3.10 Variation in hydraulic conductivity as a function of the averaging volume. . .103 .. 105

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XIV 3.11 Examples of percolating and noncercolatinz networks in two dimensions.. TABLES AND FIGURES Variation of dispersivity with distance (or scale of ..106 measurement) 117 Flow through fractures and diffusion of contaminants from fractures into the rock matrix of a dual-porosity medium..................................................... .118 Effect of sampling scale on estimation of dispersivity 119 Overview of the role of simulation models in evaluating ground water contamination problems 123 4.5 Sequential leaching of two sorbed contaminants (1,2) and one nonsorbed contaminant (3) through the root zone as a result of rainfall shown in (a) 142 4.6 Attenuation of three contaminants during their transit through the unsaturated zone 143 4.7 (a) Episodes of pesticide loadings to shallow water table located beneath a citrus grove. (b) Daily pesticide loadings predicted by PROM and used in saturated zones simulations 144 4~8 Measured and simulated movement of aldicarb residues. ~ eats e 145 .149 4~9 Seawater intrusion in an unconfined aquifer....... 4~10 Organic liquid contamination of unsaturated and saturated porous media 151 5~1 Location of case studies 5~2 5~3 .174 Schematic of vertical-horizontal spread model 177 Predicted dilution factor as a function of waste amount ~ 179 5~4 Time-drawdown curves for mode! node near hypo thetical pumping wells in the Madison limestone183 5~5 Calculated probability distribution of drawdowns at the Niobrara well field ~ 184 5~6 Observed distribution of waste chloride in ground water in the Snake River plain aquifer, Idaho, ICPP-TRA vicinity in 1972 ~188 5~7 Model-projected distribution of waste chloride in the Snake River plain aquifer for 1980 (ICPP-TRA vicinity), assuming disposal continues at 1973 rates and the Big Lost River recharges the aquifer in ocId numbered years 189

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TABLES AND FIGURES 5.8 xv Distribution of waste chloride in the Snake River plain aquifer (ICPP-TRA vicinity), October 1980 190 5.9 Location of Tucson Airport 192 5.10 Example calculation of relative contribution to aquifer contamination 196 5.11a Simulated individual trichIoroethene plumes 198 5.1Ib Simulated combined trichloroethene plumes 198 5.12 A conceptual cross section of the hydraulic contain ment system to be implemented at the S-Area landfill 202 5.13 NAPL saturation profiles at one time for the two-layer simulation..................................... Conceptual framework for ground water model accuracy analysis.......................................... 7.1 The relationship of reasonable assurance to bounding analysis, regulatory limit, and realistic estimates. .205 .217 278

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