Contaminated Ground Water and Sediment - Chapter 1 ppt

LEWIS PUBLISHERS
A CRC Press Company
Boca Raton London New York Washington, D.C.
Contaminated
Ground Water
and
Sediment
Modeling for Management
and Remediation
Edited by
Calvin C. Chien
Miguel A. Medina, Jr.
George F. Pinder
Danny D. Reible
Brent E. Sleep
Chunmiao Zheng

This book contains information obtained from authentic and highly regarded sources. Reprinted material
is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable
efforts have been made to publish reliable data and information, but the author and the publisher cannot
assume responsibility for the validity of all materials or for the consequences of their use.
Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic
or mechanical, including photocopying, microÞlming, and recording, or by any information storage or
retrieval system, without prior permission in writing from the publisher.
All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or
internal use of speciÞc clients, may be granted by CRC Press LLC, provided that $1.50 per page
photocopied is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923
USA. The fee code for users of the Transactional Reporting Service is ISBN 0-56670-667-
X/04/$0.00+$1.50. The fee is subject to change without notice. For organizations that have been granted
a photocopy license by the CCC, a separate system of payment has been arranged.
The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for

creating new works, or for resale. SpeciÞc permission must be obtained in writing from CRC Press LLC
for such copying.
Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431.

Trademark Notice:

Product or corporate names may be trademarks or registered trademarks, and are
used only for identiÞcation and explanation, without intent to infringe.

Visit the CRC Press Web site at www.crcpress.com

© 2004 by CRC Press LLC
Lewis Publishers is an imprint of CRC Press LLC
No claim to original U.S. Government works
International Standard Book Number 0-56670-667-X
Library of Congress Card Number 2003061194
Printed in the United States of America 1 2 3 4 5 6 7 8 9 0
Printed on acid-free paper

Library of Congress Cataloging-in-Publication Data

Contaminated ground water and sediment : modeling for management and remediation/
edited by Calvin C. Chien … [et al.].
p. cm.
Includes bibliographical references.
ISBN 0-56670-667-X (alk. paper)
1. Ground water—Pollution—Mathematical models. 2. Contaminated
sediments—Mathematical models. 3. Organochlorine compounds—Environmental
aspects—Mathematical models. I. Chien, Calvin C.
TD426.C657 2003

628.1

¢

68—dc22 2003061194

L1667_C00.fm Page iv Tuesday, October 21, 2003 3:50 PM

Introduction

The use of models to provide additional details on contaminant fate and transport
has rapidly increased in the past 3 decades. The increasing global recognition of the
potential risks associated with surface water or ground water contamination and
speciÞc environmental regulations implemented after 1980 have demanded a more
accurate understanding of these risks as they relate to human health and the envi-
ronment. Modeling has become an invaluable tool in providing the necessary infor-
mation to understand the risks associated with contaminants in complex environ-
ments with complicated environmental processes.
Although improvements in computing power provided by modern personal com-
puters and various new computational methods have allowed the development of
more sophisticated environmental models, many technical issues and disagreements
on particular modeling approaches and methods remain. Because the public, gov-
ernment agencies, and industry all have a high level of interest and stake in envi-
ronmental protection and remediation, and because billions of dollars are spent every
year for remediation, the need for a comprehensive review of the theory, practice,
and future direction of modeling technology is becoming more urgent. The forma-
tion, requested by the U.S. Environmental Protection Agency (USEPA), of the
Environmental Modeling Subcommittee of the Science Advisory Board in 2000, the
effort ordered by the USEPA Administrator in 2003 to revitalize the agency’s Council
for Regulatory Environmental Modeling (CREM), and a panel study on the same

issue recently planned by the National Research Council (NRC) best explain the
increasing urgency to better understand modeling technology development and appli-
cation so that a more reasonable and defensible decision-making process for envi-
ronmental issues can be achieved.
The DuPont Company provided a forum and necessary support for this purpose.
A workshop,

Modeling and Management of Emerging Environmental Issues —
Expert Workshop 2000

, was planned, organized, and chaired by Calvin C. Chien,
leader of environmental modeling technology and development for the DuPont
Corporate Remediation Group (CRG). Approximately four dozen modeling experts
from the U.S. and Canada were carefully selected and invited to participate in this
effort. Four panels were formed, with each addressing one of the following primary
environmental contamination and remediation issues involving modeling: (1) Mixing
Zone: Discharge of Contaminated Ground water into Surface Water Bodies, (2)
Contaminated Sediment: Its Fate and Transport, (3) Optimization Modeling for
Remediation and Monitoring, and (4) Simulation of Halogenated Hydrocarbons in
the Subsurface. Although the details of these issues vary, all involve technical and/or
regulatory challenges and a high Þnancial stake.
Each panel had a panel leader who worked with the CRG to select the panel
members, outline the panel discussion, facilitate the discussion at the workshop, and

L1667_C00.fm Page v Tuesday, October 21, 2003 3:50 PM
©2004 CRC Press LLC

help prepare the manuscripts for the chapters presented in this book. The workshop
was held from July 25 to 27, 2000, at the campus of Penn State University Great
Valley in Malvern, PA. An assistant panel leader supported each leader and took

discussion notes, which helped panelists in the preparation of this book. A complete
list of panelists and their afÞliations are provided in Appendix A.
This book was prepared using the information generated from workshop discus-
sions and additional materials provided by the panelists. The primary objectives of
this book were to provide information on the state of the art and current practice
and identify the research and development needs of the modeling technologies
discussed. It should be noted that the discussions herein are based not only on
technical analysis but also on regulatory acceptance and cost effectiveness.
This book comprises four chapters. Each chapter addresses one of the four topic
areas discussed at the workshop. In most cases, a section of each chapter was
prepared by a panel member and, in some cases, includes materials offered by other
members. The panel leaders either assembled the material submitted by the panelists
or further edited the manuscripts prior to overall editing. During the editing process,
original submitted materials were modiÞed, expanded, and reorganized. As a result,
it is impossible to accurately allocate credits to individual contributors. However,
those individuals who made signiÞcant contributions are mentioned. The person
responsible for assembling and editing each chapter manuscript is listed at the
beginning of the chapter, followed by the names of signiÞcant contributors. Calvin
C. Chien was responsible for the overall planning, preparation, and publication of
the book.
DuPont and the book contributors want to express their deep appreciations to
Penn State Great Valley and Elayna McReynolds, the conference coordinator, for
the support provided during the workshop. Special credit must be offered to Kathleen
O. Adams, DuPont contract technical writer, for her deep involvement, dedication,
and signiÞcant contributions throughout the editing process.

L1667_C00.fm Page vi Tuesday, October 21, 2003 3:50 PM
©2004 CRC Press LLC

Editors

Calvin C. Chien

, a senior environmental fellow with the Corporate Remediation
Group, joined DuPont in 1981 and transferred to DuPont Engineering in 1986. With
an undergraduate degree in hydraulic engineering, he earned his Ph.D. from the
State University of New York at Buffalo in 1974 with research in the modeling of
hydrologic systems.
In his near 30 years of practice, Dr. Chien has focused on performing ground
water investigations and facilitating environmental remediation technology develop-
ment. As a company leader for technology development, he has concentrated in the
areas of environmental modeling and containment technology since 1986. Besides
serving as the leader of the Groundwater Work Group of the Chemical Manufacturers
Association (CMA, now American Chemistry Council) in the late 1990s, he was an
appointed member of the U.S. Environmental Protection Agency (USEPA) Science
Advisory Board for four terms and served on the Environmental Engineering Com-
mittee and Environmental Modeling Subcommittee from 1993 to 2000. He was also
appointed to serve on its Science and Technology Achievement Award (STAA)
Committee. Dr. Chien has served on many national ground water modeling technical
and review committees. He has advocated for the collaboration among industry,
university, and government agencies through a number of major national expert
workshops in the past 10 years. Dr. Chien is recognized as the pioneer in a new
approach in solving problems in environmental remediation and as one of the leading
modelers in the industry.

Miguel A. Medina, Jr.

earned a Ph.D. degree in water resources and environmental
engineering sciences from the University of Florida in 1976 and joined the Duke
faculty thereafter. He is director of the International Honors Program of the School

of Engineering and director of the Center for Hydrologic Sciences. He has been a
registered professional hydrologist by the American Institute of Hydrology (Minne-
sota) since 1983 and was its vice president for institute development from 1998 to
2000. He was named External Evaluator of the UNESCO International Hydrological
Programme from 2002 to 2003.
Professor Medina has conducted funded research in hydrologic and water quality
mathematical modeling for the U.S. Environmental Protection Agency (USEPA),
the National Science Foundation, the OfÞce of Water Research and Technology, the
U.S. Air Force, the U.S. Army Waterways Experiment Station, the Naval Oceano-
graphic OfÞce, DuPont Engineering, the North Carolina Water Resources Research
Institute, and the State of North Carolina. His current research focuses on ßow and
solute transport surface/ground water interactions and he has published numerous
articles on this topic in peer-reviewed journals.

L1667_C00.fm Page vii Tuesday, October 21, 2003 3:50 PM
©2004 CRC Press LLC

Dr. Medina is a former president of the Universities Council on Water Resources,
Inc. and the North Carolina Section of the American Water Resources Association.
He is a consultant to the USEPA, the World Health Organization, the Research
Triangle Institute of North Carolina, the Inter-American Development Bank, the Pan
American Health Organization, UNESCO, the Ministries of Water Resources in
Venezuela and Spain, the Technical Advisory Service for Attorneys, and other private
enterprises. He is a past chairman of the International Technical Advisory Committee
of the International Ground Water Modeling Center (Colorado School of Mines, and
Delft, the Netherlands). In 1989, the Governor of North Carolina appointed Dr.
Medina to the Environmental Management Commission.

George F. Pinder

received his Bachelor of Science degree in geology at the Uni-
versity of Western Ontario (London) and his Ph.D. in geology, civil engineering,
and agriculture at the University of Illinois at Urbana. After 4 years as a research
hydrologist with the U.S. Geological Survey in Washington, he joined the Civil
Engineering Department at Princeton University as an associate professor. He was
promoted to full professor 5 years later. He served as chairman of the Department
of Civil Engineering and Operations Research from 1980 to 1989. He served as
dean of the College of Engineering and Mathematics at the University of Vermont
from 1989 to 1996 and is currently head of the Research Center for Groundwater
Remediation Design at the University of Vermont.
Dr. Pinder has published more than 200 papers and reports in the area of
quantitative ground water models. He has also published seven books. The latest,

Groundwater Modeling Using Geographical Information Systems

, was published in
2002 by John Wiley & Sons. In addition to his responsibilities as founding editor
of the journals

Advances in Water Resources

and

Numerical Methods for Partial
Differential Equations

, he is also on the editorial board of

Applied Numerical
Mathematics

and

Numerical Methods in Fluids

.
Dr. Pinder served as dean of the Division of Engineering, Mathematics, and
Business Administration at the University of Vermont from 1992 to 1996; he was
named a 1993–1994 University Scholar in recognition of his contributions to
research and scholarship. The American Geophysical Union (AGU) presented their
Horton Award to Dr. Pinder in 1969 and in 1993 invited him to become an AGU
fellow. In 1975, The Geological Society of America presented him with the O.E.
Meinzer Award for an outstanding contribution to the Þeld of hydrology. He received
the Hinds medal of the American Society of Civil Engineers in 2002.

Daniel D. Reible

has provided national and international leadership on environmen-
tal matters to students, colleagues, and his profession. He is currently Chevron
Professor of Chemical Engineering and director of the Hazardous Substance
Research Center at Louisiana State University. He joined LSU after receiving a
B.S. degree in chemical engineering from Lamar University (1977) and an M.S. and
Ph.D. in chemical engineering from the California Institute of Technology (1979
and 1982, respectively). As a teacher he has developed several graduate-level courses
in chemical engineering and remains active in teaching both undergraduate and
advanced-level chemical engineering courses. His teaching efforts have also

L1667_C00.fm Page viii Tuesday, October 21, 2003 3:50 PM
©2004 CRC Press LLC

extended far beyond the university, for example, with his direction of Advanced
Study Institutes in Prague with NATO support in 2001 and in Rio de Janeiro with
NSF support in 2002. Both institutes involved more than 100 attendees focused on
the current science of environmental assessment and remediation. He is the author
of two books,

Fundamentals of Environmental Engineering

and

Diffusion Models
of Environmental Transport

, which are widely used as both course texts and reference
books.
Dr. Reible has been active in both environmental research and its implications
for policy. He has edited two books over the past 2 years on the state of the art in
assessment and remediation of contaminated sites. He served on the National
Research Council Committee on PCB-contaminated sediments, which has had a
profound impact on the management of contaminated sediments in this country, and
currently serves on the National Research Council Committee for Remediation of
Navy Sites. He recently provided congressional testimony before the U.S. House
Subcommittee on Water Resources and the Environment on strategies for the man-
agement of contaminated sediment sites. His leadership role in environmental
research and its policy implications has been recognized by the American Institute
of Chemical Engineers from whom Dr. Reible received the Lawrence K. Cecil Award
in 2001.

Brent E. Sleep

is a professor in the Department of Civil Engineering at the Uni-
versity of Toronto, where he teaches courses in contaminant hydrogeology, environ-
mental chemistry, and engineering mathematics. Dr. Sleep’s research interests and
publications are in the area of remediation of organic contamination of ground water,
including experimental studies and numerical modeling. Current projects include
laboratory studies of anaerobic biodegradation of DNAPL source zones,

in situ

chemical oxidation and biodegradation of DNAPL source zones, biodegradation of
mixtures of halogenated organic compounds, isotopic fractionation associated with
biological processes, bioÞlm growth in fractures, and biological processes in low
permeability media. Numerical modeling is focused on modeling nonisothermal
multiphase ßow and multicomponent transport in the subsurface incorporating bio-
logical processes, parameter estimation, and optimization of remediation processes.
Previous studies have included pilot-scale studies and numerical modeling of sub-
surface LNAPL and DNAPL transport, free-phase recovery, bioventing, air sparging,
and bench-scale studies of vapor transport in soils, sequential anaerobic/aerobic
biodegradation of chlorinated ethenes, and steam ßushing for DNAPL removal.
Dr. Sleep holds a Ph.D. in civil engineering from the University of Waterloo.
He also holds a B.A.Sc. and M.Eng. in chemical engineering from the University
of Waterloo. Dr. Sleep is a member of the American Geophysical Union and the
National Ground Water Association and an associate editor of

Advances in Water
Resources

.

Chunmiao Zheng

is professor of hydrogeology in the Department of Geological
Sciences at the University of Alabama. He holds a Ph.D. in hydrogeology from the
University of Wisconsin–Madison. From 1988 to 1993, he was a senior hydroge-
ologist and director of software development at S.S. Papadopulos & Associates, Inc.,

L1667_C00.fm Page ix Tuesday, October 21, 2003 3:50 PM
©2004 CRC Press LLC

an environmental and water-resource consulting Þrm. Since 1993, he has been
leading the interdisciplinary hydrogeology program at the University of Alabama.
Dr. Zheng is developer of MT3D/MT3DMS, a widely used contaminant fate and
transport simulation model, and co-author of the textbook

Applied Contaminant
Transport Modeling

, published by John Wiley & Sons and currently in the second
edition. Dr. Zheng has published over 50 papers and book chapters on both applied
and theoretical aspects of hydrogeology, contaminant transport, and optimal ground
water management. He is recipient of the 1998 John Hem Excellence in Science
and Engineering Award from the National Ground Water Association for outstanding
contributions to the understanding of ground water, and is a fellow of the Geological
Society of America. Dr. Zheng serves on the Groundwater Committee of the
American Geophysical Union, the Standing Committee on Hydrologic Information
Systems of the Consortium of Universities for Advancement of Hydrologic Science,
and the editorial boards of

Ground Water

and

Hydrogeology Journal

.

L1667_C00.fm Page x Tuesday, October 21, 2003 3:50 PM
©2004 CRC Press LLC

Contributors

CHAPTER 1

Miguel A. Medina, Jr.

Chapter Editor
Duke University

Robert L. Doneker

Oregon Graduate Institute of Science
and Technology

Nancy R. Grosso

DuPont Company

D. Michael Johns

Windward Environmental, LLC

Wu-Seng Lung

University of Virginia

Farrukh Mohsen

Gannet Fleming, Inc.

Aaron I. Packman

Northwestern University

Philip J. Roberts

Georgia Institute of Technology

CHAPTER 2

Danny D. Reible

Chapter Editor
Louisiana State Univeristy

Sam Bentley

Louisiana State University

Mimi B. Dannel

U.S. Environmental Protection
Agency 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.

Richard H. Jensen

DuPont Company

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

L1667_C00.fm Page xi Tuesday, October 21, 2003 3:50 PM
©2004 CRC Press LLC

CHAPTER 3

George F. Pinder

Chapter Editor
University of Vermont

David E. Dougherty

Subterranean Research, Inc.

Robert M. Greenwald

GeoTrans, Inc.

George P. Karatzas

Technical University of Crete

Peter K. Kitanidis

Stanford University

Hugo A. Loaiciga

University of California at Santa
Barbara

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

Donna M. Rizzo

Subterranean Research, Inc.

Brian J. Wagner

U.S. Geological Survey

Kathleen M. Yager

U.S. Environmental Protection Agency,
Technology Innovation OfÞce

William W G. Yeh

University of California at Los Angeles

CHAPTER 4

Brent E. Sleep

Chapter Editor
University of Toronto

Neal D. Durant

Geotrans, Inc.

Charles R. Faust

GeoTrans, Inc.

Joseph G. Guarnaccia

CIBA-Geigy Specialty Chemicals

Mark R. Harkness

General Electric Corporation

Jack C. Parker

Oak Ridge National Laboratory

Lily Sehayek

URS Corporation
Penn State Great Valley

L1667_C00.fm Page xii Tuesday, October 21, 2003 3:50 PM
©2004 CRC Press LLC

Contents

Chapter 1

Surface Water–Ground Water Interactions
and Modeling Applications

prepared by Miguel A. Medina, Jr.
with contributions by Robert L. Doneker, Nancy R. Grosso, D. Michael Johns,
Wu-Seng Lung, Farrukh Mohsen, Aaron I. Packman, Philip J. Roberts

Chapter 2

The Role of Modeling in Managing
Contaminated Sediments

prepared by Danny D. Reible
with contributions by Sam Bentley, Mimi B. Dannel, Joseph V. DePinto,
James A. Dyer, Kevin J. Farley, Marcelo H. Garcia, David Glaser, John M. Hamrick,
Richard H. Jensen, Wilbert J. Lick, Robert A. Pastorok, Richard F. Schwer,
C. Kirk Ziegler

Chapter 3

Optimization and Modeling for Remediation and Monitoring

prepared by George F. Pinder
with contributions by David E. Dougherty, Robert M. Greenwald,
George P. Karatzas, Peter K. Kitanidis, Hugo A. Loaiciga, Reed M. Maxwell,
Alexander S. Mayer, Dennis B. McLaughlin, Richard C. Peralta, Donna M. Rizzo,
Brian J. Wagner, Kathleen M. Yager, William W G. Yeh

Chapter 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

Appendix A: Workshop Panels

L1667_bookTOC.fm Page xiii Tuesday, October 21, 2003 3:51 PM
©2004 CRC Press LLC

1

Surface Water–Ground
Water Interactions and
Modeling Applications

prepared by Miguel A. Medina, Jr.

with contributions by
Robert L. Doneker, Nancy R. Grosso,
D. Michael Johns, Wu-Seng Lung, Farrukh Mohsen,
Aaron I. Packman, Philip J. Roberts

CONTENTS

1.1Introduction and Overview
1.1.1Overview of Issues IdentiÞed
1.1.2Ground Water–Surface Water Interaction Technical Background
1.2The User’s Perspective
1.2.1Point Source Discharge Regulations
1.2.1.1The Zone of Initial Dilution (ZID)
1.2.1.2The Toxic Dilution Zone (TDZ)
1.2.2National Pollutant Discharge Elimination System (NPDES)
Permitting Technical Issues
1.2.2.1Two-Stage Mixing
1.2.2.2Federal Guidelines
1.2.2.3Acute Toxicity
1.2.2.4Dimensions of Regulatory Mixing Zones
1.2.3Nonpoint Sources
1.2.3.1State of Michigan Mixing Zone Rules
1.3Current State of Knowledge
1.3.1Problem-Oriented Perspective
1.3.1.1Ecological and Health Risk Aspects
1.3.1.2Environment Boundaries and Scope
1.3.1.3Ground Water–Surface Water Connections
1.3.1.4Stream–Subsurface Exchange Processes
1.3.1.5Implications for Controlled and Uncontrolled
Contaminant Discharges
1.3.2Enabling Technologies Perspective — Simulation Models

L1667_book.fm Page 1 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

1.3.2.1Introduction and Policy Implications
of Technological Limits

1.3.2.2Modeling Stream–Subsurface Exchange Processes
1.3.2.3Tidal Exchanges and Oscillations
1.3.2.4Mathematical Formulation — Tidally Inßuenced Case
1.3.2.5Exit Concentration
1.3.3Emerging Technologies
1.3.3.1Mathematical Models
1.3.3.2New Laboratory Techniques
1.3.3.3New Field Techniques
1.3.4Alternative Approaches
1.3.4.1InsigniÞcant Momentum-Induced Dilution
1.3.4.2Modeling Approach
1.3.4.3Case Studies of Model Applications
1.4Acceptance of Methodology
1.5Summary, Conclusions, and Recommendations
Acknowledgments
References

1.1 INTRODUCTION AND OVERVIEW

The interaction between surface water and ground water bodies traditionally has
been idealized as a simple unidirectional transport process. More recent detailed
examination has shown that ßow systems can be complicated. Complicated ßow and
mixing patterns can have signiÞcant implications for physical, biogeochemical, and
biological processes within the system and for contaminant transport. Ultimately,
the effects of these complex processes on the risk to human health and the environ-
ment must be assessed.
This panel examined the technical complexities of surface water and ground
water interaction on a spatial and temporal scale. The regulatory framework of
mixing zones was reviewed, and the policy implications of mixing zones on ground
water and surface water interaction were discussed. The panel focused on mathe-

matical modeling of these processes and reviewed the state-of-the-art technology in
aqueous mixing simulation models. Advantages and disadvantages of different mod-
eling approaches, time and spatial resolution disparities, and aggregation–disaggre-
gation of data were also discussed.
The U.S. Environmental Protection Agency (USEPA, 1998b) considers the primary
exchange processes between the sediment and the overlying surface water to occur
within the upper 2 in. of sediment deposits.



Important elements in estimating the ground
water contribution are distinguishing and characterizing the various inputs to the surface
water–sediment system, which, in some cases, can be contaminated sediment.



The
speciÞc role of modeling in managing contaminated sediments is reviewed in Chapter 2.
Until recently, methods for quantifying the local extent and quality of contam-
inated ground water discharges and their pollutant load to surface waters consisted
primarily of hydrologic and physicochemical techniques (USEPA, 1998a).



Promis-
ing new research is focusing on the use of biological indicators (organisms that

L1667_book.fm Page 2 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

spend all or part of their life cycle in contact with ground water) to characterize
zones of ground water–surface water interaction, reviewed later in this chapter.
This chapter attempts to present the best understanding of the underlying
hydrodynamic, chemical, and biological processes required to describe contami-
nant transport between ground water and surface water and the limitations of
numerical modeling.
Figure 1.1 (Minsker et al., 1998) shows some of the interactions between ground
water and surface water bodies, including atmospheric exchange and exchanges
between ground water, sediment, the water column, and the larger surface water body.

1.1.1 O

VERVIEW


OF
I

SSUES

I

DENTIFIED

The expert panel identiÞed several technical issues, including speciÞc modeling
issues that deserve further discussion.



Among the most salient technical issues that

need resolution with regard to surface water–ground water interactions (i.e., the
mixing zone) are as follows:
•DeÞning conceptual models of sufÞcient detail for aquifer, transition zone,
and water column interactions (including biologic, geologic, hydrologic,
and geochemical processes)
•DeÞning the relevance of the ecology in the transition zone (e.g.,
hyporheic zone, which is usually deÞned in terms of the biota only)

FIGURE 1.1

Atmospheric–surface water–ground water–sediment interactions. (ModiÞed
after Minsker et al., 1998.)

L1667_book.fm Page 3 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

• Locating a discharge area or upwelling
• Characterizing a discharge
• Locating and characterizing a plume
• Identifying and characterizing all other contaminant sources
• Identifying not only signiÞcant ground water–surface water interactions,
exchanges, and processes but deÞning these within both spatial and tem-
poral frameworks
•DeÞning data quality objectives (DQOs)
Closely associated with these technical issues are the following speciÞc model-
ing issues that are required to resolve the issues listed above adequately:
•Formulate screening and tiered approaches using modeling. A tiered
approach addresses two important technical issues. First, simple analytic
solutions increasing in complexity allow for a greater understanding of
the system. Second, less complex sites require less complex models, and

the tiered approach allows the evaluation to stop at an appropriate level
of complexity.
•Develop research and operational models for the mixing zones, transition
zones, and interfaces (e.g., contaminated sediments layers).
•Develop linkage techniques to couple ground water and surface water
models to address unique temporal and spatial scaling for ßow, transport,
and biogeochemical processes.
• Apply veriÞcation procedures (including peer review), benchmarking,
validation, and Þeld testing.
• Use and develop scientiÞc process models to deÞne various types of mixing
zones and transition zones in support of conceptual model development.
• Obtain new data sets to develop and validate modeling approaches to
address regulatory requirements.
• Establish feedback mechanisms between data collection, modeling, and
resource decisions.
•Develop methods to account for uncertainty and heterogeneity.
For complex surface water sites where an unacceptable risk to human health
and the environment is likely, sophisticated mathematical modeling may be neces-
sary. A framework can be developed to achieve the following:
• Apply hydrodynamic modeling principles while incorporating key chem-
ical and biological criteria to deÞne more quantitatively the mixing zone
regulatory boundaries and target goals (e.g., ecological impacts on a
localized scale, large-scale ecological or human health concerns).
•Evaluate alternative control or management strategies to achieve sound
risk-based decisions even under conditions of uncertainty.
All major factors central to the transport and fate of contaminants (physical,
chemical, biological) and ecological risk should be identiÞed properly in the model

L1667_book.fm Page 4 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

for complex sites. In addition, these models should have the capability to address
current and potential regulatory deÞnitions of the various types of mixing zones and
transition zones. Accounting for parameter uncertainty can permit key regulatory
policies to be addressed in the presence of technical uncertainty, perhaps encouraging
a review of the policy or the granting of a variance.
The broad technical aspects can be lumped into three major categories, as
illustrated in Figure 1.2.
For an improved understanding of the ecological relevance of the biological
community in the transition zone, the following needs were identiÞed:
• Compare site chemical data to the appropriate ecological benchmark
criteria.
•Perform basic research in community structure, life histories, faunal struc-
ture, functional structure.
• Improve sampling techniques.
•Improve evaluation techniques, including community analysis and toxicity
assessment.
•Incorporate into the risk paradigm.
•Evaluate risk presented by a discrete plume in terms of the risk posed to
overall ecologic and environmental health of the system.
The policy and management issues below remain to be resolved by the regulatory
agencies. Changes in policy can alter not only the regulatory landscape but also the
technical analysis.
• Should the geometry of regulatory mixing zones be based on the hydro-
dynamic mixing zone?
• Under what conditions are mixing zones acceptable in terms of risk to
ecological receptors?
• Can mixing zones be integrated into total maximum daily loads (TMDLs)
such as storm water discharges?

FIGURE 1.2

Broad technical aspects.
Physical and
hydrodynamic
aspects
Surface water
system
Ground water
system
Transition zone
Mixing zones
Streambed
Bed sediment
Hydraulic
exchange
Toxicity
Bioaccumulation
Trophic transfer
Geochemical and
biogeochemical
reactions
Bioavailability
Bioaccessibility
Ecological
aspects
Biogeochemical
ground water
and
surface water

L1667_book.fm Page 5 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

•Are antidegradation policies consistent or overly conservative with respect
to ecological risk?
•What are the policy implications of technological limits?

1.1.2 G

ROUND

W

ATER

–S

URFACE

W

ATER

I

NTERACTION


T

ECHNICAL

B
ACKGROUND

Winter (1995, 1998) and Winter et al. (1998) note that surface water bodies are integral
parts of ground water ßow systems, and ground water interacts with surface water in
nearly all landscapes — from small streams, lakes, and wetlands in headwater areas
to major river valleys and seacoasts. On a relatively large scale, characterization of
water mass transfer between ground water and surface water bodies is relatively well
understood. For example, streams are either gaining or losing. However, the ßow
system on a smaller scale near the interface of the surface water column and the
sediment bed can be complicated. At this scale, ground water–surface water interac-
tions are probably best thought of as a superposition of ßows that occur at a number
of different spatial scales and often change seasonally or in response to a climatic event.
Complex small-scale ßows can result from a variety of physical aspects and
processes such as seasonally high surface water levels, evaporation and transpiration
of ground water from around the perimeter of surface water bodies, rifße and pool
dynamics in streams, tidal ßuctuations, limited hydraulic exchange due to imperme-
able sediment, and streamßow and velocity.
Ground water and surface water interaction or mixing can be divided into the
following zones (see Figure 1.1 [Minsker et al., 1998]):
• The surface water column (both near the discharge area and further out
into the larger part of the surface water body)
• The bank storage zone or the shallow sediment section near the sediment
bed, also referred to as the biologically active zone
• The zone of transition from ground water to surface water below the
sediment–water interface but not into the aquifer proper
Within various surface water body environments, speciÞc processes can play a more

signiÞcant role than in other environments. For instance, streams present a very
special case of ground water and surface water interaction. Within streams, a portion
of the biologically active zone and the transition zone is called the hyporheic zone
based on biological environment. The process of water and solute exchange in both
directions across a streambed is usually termed the hyporheic exchange. The direc-
tion of seepage through the streambed is commonly related to abrupt changes in
bed slope or to meanders in the stream channel.



The dimensions of the hyporheic
zone depend on the type of sediment in the streambed and banks, streambed slope
and variability, and hydraulic gradients. The hyporheic zone is a potentially signif-
icant zone of biological activity in aquatic systems. Because of ground water and
surface water mixing within the hyporheic zone, the chemical and biological char-
acteristics of water within the zone can differ considerably from those of adjacent
surface water and ground water systems.

L1667_book.fm Page 6 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

The ecological and health risk factors associated with mixing zones, whether in
the surface water column or in the sediment bed and deeper, are reviewed in more
detail later in this chapter. Examples of modeling the exchange of ground water and
surface water are given for the case of a streambed and a tidal estuary.

1.2 THE USER’S PERSPECTIVE

While mixing zones have been applied to point source discharges, application to
nonpoint sources has not been widely addressed in regulatory management. Fur-

thermore, the spatial deÞnition of a regulatory mixing zone and the actual physical
mixing zone resulting from a hydrodynamic sequence of events are not usually
the same. Thus, a distinction is made throughout this chapter between regulatory
mixing zones and hydrodynamic mixing zones

.

The regulatory deÞnition of the
mixing zone describes it as an allocated impact zone where numeric water quality
criteria can be exceeded as long as acutely toxic conditions are prevented. Cur-
rently, the USEPA (2001) is conducting a review to consider a potential nationwide
phase-out of mixing zones for the most persistent, toxic, and bioaccumulative
chemicals of concern (BCCs) such as mercury, dichlorodiphenyltrichloroethane
(DDT), polychlorinated biphenyls (PCBs), and dioxins. BCCs will be phased out
of permitted discharges to the Great Lakes.
Because mixing zone regulations have been applied primarily to point sources
of contamination, that perspective is reviewed Þrst, even though it is the nonpoint
source aspect of the problem (surface water–ground water interaction) that is of
primary focus in this chapter. Although states have the Þnal say on mixing zones,
the USEPA (1993) does provide some guidance that may be applicable to nonpoint
sources. For example, the handbook (USEPA, 1993) does not explicitly exclude
nonpoint sources in the mixing zone deÞnition. Furthermore, the handbook indicates
that the mixing zone can be deÞned in terms of volume and that the location and
shape should be deÞned using biological criteria.
Some examples of the Michigan Department of Environmental Quality (MDEQ)
mixing zone rules are presented in the following text because they provide some
interpretation for nonpoint source application. A discussion of the nonpoint source
regulatory framework as it applies to mixing zones is also presented.

1.2.1 P

OINT

S

OURCE

D

ISCHARGE

R

EGULATIONS

The USEPA’s Water Quality Standards (WQS) regulation (40 CFR 131, Federal
Register, Subpart B) allows states to adopt provisions authorizing mixing zones.
Thus, individual state law and policy determine whether a mixing zone is permitted.
The mixing zone is deÞned as an allocated impact zone where numeric water quality
criteria can be exceeded as long as acutely toxic conditions are prevented. A mixing
zone can be thought of as a limited area or volume where the initial dilution of a
discharge occurs. Water quality standards apply at the boundary of the mixing zone,
not within the mixing zone itself. The USEPA has published numerous documents
providing guidance for determining mixing zones (e.g., USEPA, 1991



and 1993;
USEPA Region VIII, 1994).

L1667_book.fm Page 7 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

In terms of location, biologically important areas are identiÞed and protected
(i.e., Þsh spawning areas) as well as all drinking water intakes. With regard to size,
the area or volume of an individual zone or group of zones must be limited to the
smallest practicable that will not interfere with the designated uses of the waterway.
The shape is a simple conÞguration that is easy to locate in a body of water while
avoiding impingement of biologically important areas. In general, a mixing zone
should be free from the following:
• Acutely toxic conditions
• Materials that settle to form objectionable deposits
• Substances such as ßoating debris and oil that form nuisances
• Substances that produce objectionable color, odor, and taste
• Substances that produce undesirable aquatic life or result in a dominant
nuisance species
The USEPA rules for mixing zones recognize that the state has discretion
whether to adopt a mixing zone and to specify its dimensions. The USEPA allows
the use of a mixing zone in permit applications except where one is prohibited in
state regulations. State regulations addressing streams or rivers generally limit mix-
ing zone widths or cross-sectional areas and allow lengths to be determined on a
case-by-case basis. According to a report prepared for the Chemical Manufacturers
Association by The Advent Group (1994), 23 states have a narrative mixing zone
language and 27 states have speciÞc mixing zone area and/or volume deÞnitions in
their regulations. SpeciÞc examples from the MDEQ Surface Water Quality Division
administrative rules on mixing zones are presented in the section after point source
regulations, for it is clear that in those regulations some thought was given to
nonpoint source compliance as well.
In the case of lakes, estuaries, and coastal waters, some states specify the surface
area that can be affected by the discharge. The surface area limitation usually applies

to the underlying water column and benthic area. In the absence of speciÞc mixing
zone dimensions, the actual shape and size is determined on a case-by-case basis.

1.2.1.1 The Zone of Initial Dilution (ZID)

Special mixing zone deÞnitions have been developed for the discharge of municipal
wastewater into the coastal ocean, as regulated under Section 301(h) of the Clean
Water Act. Frequently, these same deÞnitions are used for industrial and other
discharges into coastal waters or large lakes, resulting in a plurality of terminology.
For those discharges, the mixing zone is labeled as the ZID in which rapid mixing
of the waste stream (usually the rising buoyant fresh water plume within the ambient
saline water) occurs. The USEPA requires that the ZID be a regularly shaped area
(e.g., circular or rectangular) surrounding the discharge structure (e.g., submerged
pipe or diffuser line) and encompassing the regions of high (exceeding standards)
pollutant concentrations under design conditions. In practice, limiting boundaries
deÞned by dimensions equal to the water depth measured horizontally from any

L1667_book.fm Page 8 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

point of the discharge structure is accepted by the USEPA, provided no other mixing
zone restrictions are violated.
The ZID is often denoted differently in common use and regulatory management.
In common use, the ZID often refers to the initial dilution of a discharge. Initial
dilution is the process of forced entrainment of ambient water into a discharge plume
(ßow) through both momentum and buoyancy-induced turbulent and shear processes.
As the discharge ßow propagates into the ambient water, it entrains water that dilutes
the discharge. Through the entrainment process, the plume density approaches the
ambient density (neutral buoyancy) and, depending on the location at which this
occurs, the plume either reaches the surface or becomes trapped at some intermediate

level. Therefore, the spatial extent of the mixing region is sometimes referred to as
the ZID because it is where initial dilution is achieved. Beyond the ZID, ambient
mixing processes tend to control further dilution of the plume. However, in a 1994
USEPA technical support document (USEPA, 1994), the ZID is deÞned as follows:

The zone of initial dilution (ZID) is the region of initial mixing surrounding or adjacent
to the end of the outfall pipe or diffuser ports and includes the underlying seabed. The
ZID describes an area in which inhabitants, including the benthos, can be chronically
exposed to concentrations of pollutants in violation of water quality standards and criteria
or at least to concentrations more severe than those predicted for critical conditions.
The ZID is not intended to describe the area bounding the entire mixing process for all
conditions or the total area impacted by the sedimentation of settleable material.

1.2.1.2 The Toxic Dilution Zone (TDZ)

The USEPA maintains the following two water quality criteria for the allowable
concentration of toxic substances: a criterion maximum concentration (CMC) to
protect against acute or lethal effects and a criterion continuous concentration (CCC)
to protect against chronic effects (USEPA, 1991). The CMC value is greater than
or equal to the CCC value and is usually more restrictive. The CCC must be met at
the edge of the same regulatory mixing zone speciÞed for conventional and non-
conventional discharges. Lethality to passing organisms within the mixing zone can
be prevented in one of the following four ways:
• The Þrst alternative is to meet the CMC criterion within the pipe itself.
• The second alternative is to meet the CMC within a short distance from
the outfall. If dilution of the toxic discharge in the ambient environment
is allowed, a TDZ that is usually more restrictive than the legal mixing
zone for conventional and nonconventional pollutants can be used. The
revised 1991 toxic technical support document (USEPA, 1991) recom-
mends a minimum exit velocity for new discharges of 10 ft/s in order to

allow sufÞciently rapid mixing that minimizes organism exposure time to
toxic material. The document does not set a requirement in this regard,
recognizing that the restrictions listed in the following paragraph can in
many instances also be met by other designs, especially if the ambient
velocity is large.

L1667_book.fm Page 9 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

• The third alternative is making the outfall design meet the most restrictive
of the following geometric restrictions for a TDZ:
• The CMC must be met within 10% of the distance from the edge of
the outfall structure to the edge of the regulatory mixing zone in any
spatial direction.
• The CMC must be met within a distance of 50 times the discharge
length scale in any spatial direction. The discharge length scale is
deÞned as the square root of the cross-sectional area of any discharge
outlet. This restriction is intended to ensure a dilution factor of at least
10 within this distance under all possible circumstances, including
situations of severe bottom interaction and surface interaction.
• The CMC must be met within a distance of Þve times the local water
depth in any horizontal direction. The local water depth is deÞned as
the natural water depth (existing prior to the installation of the dis-
charge outlet) prevailing under mixing zone design condition (e.g., low
ßow for rivers). This restriction prevents locating the discharge in very
shallow environments or very close to shore, which results in signiÞ-
cant surface and bottom concentrations.
•A fourth alternative is to show that a drifting organism would not be
exposed more than 1 h to average concentrations exceeding the CMC.

1.2.2 N

ATIONAL

P

OLLUTANT

D

ISCHARGE

E

LIMINATION

S

YSTEM


(NPDES) P

ERMITTING

T

ECHNICAL

I

SSUES

During the NPDES permit evaluation process, regulators assess many dischargers
with stringent efßuent limits. When the efßuent concentration must meet the ambient
water quality standard, no mixing zone is allowed for the discharge. On the other
hand, many states allow mixing zones in rivers and streams under certain conditions.
QuantiÞcation of mixing zones to comply with regulations is an urgent topic facing
many regulatory staff, water quality engineers, and water quality management deci-
sion makers. More speciÞcally, determining how to combine the regulatory aspects
with technical issues in quantifying mixing zones is the key to this water quality
problem in receiving waters.
There are a number of water quality constituents related to mixing zones, ranging
from conventional pollutants (e.g., temperature, fecal coliform bacteria,
viruses/pathogens) to nonconventional contaminants (e.g., metals, synthetic organ-
ics, chlorine residual, color, whole efßuent toxicity).

1.2.2.1 Two-Stage Mixing

When wastewater is discharged into the receiving water, its transport can be divided
into two stages with distinct mixing characteristics.



The initial momentum of the
discharge determines mixing and dilution in the Þrst stage.



The design of the

discharge outfall should provide ample momentum to dilute the concentrations in
the immediate contact area as quickly as possible.



(It should be noted that many
existing outfalls with small ßows do not have sufÞcient momentum for initial

L1667_book.fm Page 10 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

dilution.)



The second stage of mixing covers a more extensive area in which the
effect of initial momentum is diminished and the waste is mixed primarily by residual
plume buoyancy and ambient turbulence.

1.2.2.2 Federal Guidelines

For toxic discharges, the USEPA maintains two water quality criteria for allowable
magnitude of toxic substances: a CMC to protect against acute or lethal effects and
a CCC to protect against chronic effects.



Thus, the CMC should be met at the edge
of the zone of initial dilution, and the CCC should be met at the edge of the overall
mixing zone.

In some states, this zone of initial dilution is referred to as the allocated
impact zone.



In rivers or tidal rivers that have a persistent through-ßow in the
downstream direction and do not exhibit signiÞcant natural density stratiÞcation, the
1-day, 10-year low ßow (1Q10) and 7-day, 10-year low ßow (7Q10) for the CMC
and CCC, respectively, are recommended in steady-state mixing-zone modeling
analysis (USEPA, 1991).

1.2.2.3 Acute Toxicity

The CMC is used as a means to prevent lethality or other acute effects.



It is deÞned
as one half of the Þnal acute value (FAV) for speciÞc toxicants and 0.3 acute toxic
unit (TU

a

) for whole efßuent toxicity (USEPA, 1991).

The acute toxicity unit is
expressed as TU

a

= 100/LC

50

, where LC

50

is the percentage of efßuent that causes
50% of the organisms to die through the exposure period.



For example, an efßuent
that is found to have an LC

50

of 5% is evaluated as 20 TU

a

.

1.2.2.4 Dimensions of Regulatory Mixing Zones

The dimensions of a mixing zone in a river are usually determined by state regula-
tions.



Doneker and Jirka (1990) provide a summary of state mixing zone regulations.
In general, regulatory mixing zones are speciÞed by a distance, area, or volume
around the discharge point. For example, Virginia water quality standards state that
the overall mixing zone shall not constitute more than one half of the width of the
receiving watercourse or one third of the area of any cross section of the receiving
watercourse (Lung, 1995).



In addition, it shall not extend downstream for a distance
more than Þve times the width of the receiving watercourse at the point of discharge.
The dimensions of the allocated impact zone within which the CMC is met depend
on the size of the overall mixing zone (Lung, 1995).



It appears that the river width
is the key factor determining the sizes of the allocated impact zone and the overall
mixing zone in a river or tidal river system.

1.2.3 N

ONPOINT

S

OURCES

The contribution of ground water to total streamßow varies widely among streams,
but hydrologists estimate the average contribution to be between 40 and 50% in
small- and medium-sized streams (Alley et al., 1999).



Extrapolation of these numbers
to large rivers is more complicated; however, the ground water contribution to all

L1667_book.fm Page 11 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

streamßow can be as large as 40%. This does not include ground water contributions
to lakes and wetlands. According to Tomassoni (2000), the ground water–surface
water interaction zone is important from federal statutory, regulatory, and policy
perspectives because 75% of Superfund and Resource Conservation and Recovery
Act (RCRA) sites are located within a half mile of a surface water body. In 47% of
these Superfund sites, there have been recorded impacts to surface water. Although
progress has been achieved in controlling point sources within the past 25 years
through the Clean Water Act, the USEPA now needs to consider nonpoint sources.
The USEPA supports sound science and risk-based decision making (RBDM).
RBDM requires a multidisciplinary approach; an understanding of requirements;
and ßexibility in applicable statutes, regulations, and policies (Tomassoni, 2000).
As noted earlier, there are many technical and policy issues regarding ground
water–surface water interactions, and good policy depends on good technical infor-
mation. Recently, greater attention has been placed by the USEPA (2000) on these

interactions. The goal of Superfund is to return usable ground water to beneÞcial
uses (current and future) where practical. When this is not practical, Superfund
strives to prevent further migration and exposure and to evaluate opportunities for
further risk reduction.



Preliminary remedial goals are set at levels that protect
resources, including surface waters that receive contaminated ground water, taking
into account Clean Water Act requirements or state standards, whichever are more
stringent. Final cleanup levels are attained throughout the plume and beyond the
edge of any wastes left in place, where the point of compliance for a surface water
body is where the release enters the surface water. Alternate concentration limits
(ACLs) can be considered where contaminated ground water discharges to surface
water, where contaminated ground water does not lead to increased contaminants
in surface water, where enforceable measures are available to prevent exposure to
ground water, or where restoring ground water is “not practicable.”
Tomassoni (2000) further points out that RCRA has similar requirements to
Superfund with respect to the following: returning usable ground water to beneÞcial
uses, points of compliance for ground water and surface water, protection of surface
water from contaminated ground water, and provisions for ACLs and treatment of
principal threats. Therefore, if current human exposures are under control and no
further migration of contaminated ground water is expected, primary near-term goals
are established using two environmental indicators. Thus, surface water becomes
the boundary if the discharge of contaminated ground water is within protective
limits.



It is estimated that the majority of contaminated sites have serious potential

to affect surface waters. Although the federal framework allows for RBDM with
respect to ground water–surface water interaction, the expectation of restoring
ground water to beneÞcial use and ensuring that discharges of ground water to
surface water are protective must be achieved. The following are among the key
policy issues to ponder:
•How to achieve short- and long-term protection
• Where, how, and how often to measure compliance
• Whether to restore ground water even if it has no impact on surface water
•How to address the diversity of surface bodies consistently

L1667_book.fm Page 12 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

•How to address cleanup goals in relation to the Clean Water Act’s NPDES
approach
•How to account for, track, and translate TMDLs in watersheds
In theory, nonpoint sources could be managed within existing mixing zone
deÞnitions and regulation. However, particularly for TDZs, some of the deÞnitions
and control strategies that apply to point sources are not relevant to nonpoint sources.
Furthermore, it is difÞcult to generalize the actual practice of implementing the
mixing zone regulations given the large number and diverse types of jurisdictions
and permit-granting authorities involved. By and large, however, current procedure
falls into one of the following approaches or can involve a combination thereof:
• The mixing zone is deÞned by some numerical dimension. The applicant
must demonstrate that the existing or proposed discharge meets all appli-
cable standards for conventional pollutants or for the CCC of toxic pol-
lutants at the edge of the speciÞed mixing zone.
• No numerical deÞnition for a mixing zone applies. In this case, the
applicant proposes a mixing zone dimension. To do so, the applicant
generally uses actual concentration measurements for existing discharges,

dye dispersion tests, or model predictions to show at what plume distance,
width, or region the applicable standard will be met. Further, ecologically
or water use-oriented arguments are used to demonstrate that the size of
the predicted region provides reasonable protection. The permitting
authority calculates the proposal for a mixing zone. This approach resem-
bles a negotiating process with the objective of providing optimal protec-
tion of the aquatic environment consistent with other uses.

1.2.3.1 State of Michigan Mixing Zone Rules

The MDEQ Surface Water Quality Division (1999) contains language that allows
nonpoint source mixing. The administrative rules deÞne a mixing zone as “the portion
of a water body in which a point source discharge or venting ground water is mixed
with the receiving water.” As a minimum restriction, the FAV for aquatic life shall not
be exceeded when determining a



wasteload allocation for acute aquatic life protection
unless it is determined by the MDEQ that a higher level is acceptable or it can be
demonstrated to the MDEQ that an acute mixing zone is acceptable consistent with
subrule (7). Subrule (7) is quite detailed and intended for site-speciÞc investigations,
including items such as whether overlapping mixing zones exist and the following:
•“A description of the amount of dilution occurring at the boundaries of
the proposed mixing zone and the size, shape, and location of the area of
mixing, including the manner in which diffusion and dispersion occur.”
• “The mixing zone demonstration shall be based on the assumption that
environmental fate or other physical, chemical, or biological factors do
not affect the concentration of the toxic substance in the water column
within the proposed mixing zone.”

L1667_book.fm Page 13 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC

Mixing zone boundaries should be determined on a case-by-case basis. With
regard to surface runoff, “a watercourse or portions of a watercourse that without
one or more point source discharges would have no ßow except during periods of
surface runoff may be considered as a mixing zone for a point source discharge.”
The Michigan administrative rules also have speciÞc provisions for temperature at
the edge of the mixing zone: “monthly maximum temperatures, based on the nine-
tieth percentile occurrence of natural water temperatures plus the increase allowed
at the edge of the mixing zone and in part on long-term physiological needs of Þsh,
may be exceeded for short periods when natural water temperatures exceed the
ninetieth percentile occurrence.” Of particular interest are the provisions made at
the ground water–surface water interface:
•If a remedial action plan (RAP) allows for a mixing zone for discharges of
ground water venting to a surface water, then the ground water discharge
must comply with the same mixing zone rules as those of point source
discharges.
•If a mixing zone is not provided in the RAP or permit, the ground water
quality must meet the generic ground water–surface water interface (GSI)
criteria.
GSI criteria can be summarized as follows:
•Chronic criteria are calculated based on dilution and ambient surface water
data in order to meet water quality criteria after mixing.
•Final acute criteria are calculated as maximum concentrations not to be
exceeded at the GSI in order to prevent immediate harm to aquatic life.
•Mixing zones for BCCs are allowed for existing discharges until March
23, 2007.
•More stringent provisions apply to the Great Lakes throughout the admin-

istrative rules.

1.3 CURRENT STATE OF KNOWLEDGE
1.3.1 P

ROBLEM

-O

RIENTED

P

ERSPECTIVE

1.3.1.1 Ecological and Health Risk Aspects

The hyporheic zone represents a zone of transition from ground water to surface
water and can extend up to approximately 40 in. (100 cm) below the sediment–water
interface. Figure 1.3 shows the approximate position of the hyporheic zone during
low ßow conditions (Williams, 2000). The size of the hyporheic zone can vary
seasonally and in response to ßooding or drought. The relative contributions of
ground water and surface water to this transition depend on the geologic character-
istics within the zone and the prevailing hydraulic heads. While this zone represents
an interesting hydrogeologic feature, it can also represent a potentially signiÞcant
zone of biological activity in aquatic systems.

L1667_book.fm Page 14 Tuesday, October 21, 2003 8:33 AM
©2004 CRC Press LLC