Contaminated Ground Water and Sediment - Chapter 1 ppt

Four panels were formed, with each addressing one of the following primaryenvironmental contamination and remediation issues involving modeling: 1 MixingZone: Discharge of Contaminated G

LEWIS PUBLISHER S

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Contaminated Ground Water

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Contaminated ground water and sediment : modeling for management and remediation/

edited by Calvin C Chien … [et al.].

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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

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The use of models to provide additional details on contaminant fate and transporthas rapidly increased in the past 3 decades The increasing global recognition of thepotential risks associated with surface water or ground water contamination andspeciÞc environmental regulations implemented after 1980 have demanded a moreaccurate 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 puters and various new computational methods have allowed the development ofmore sophisticated environmental models, many technical issues and disagreements

com-on particular modeling approaches and methods remain Because the public, 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 everyyear 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 theEnvironmental Modeling Subcommittee of the Science Advisory Board in 2000, theeffort ordered by the USEPA Administrator in 2003 to revitalize the agency’s Councilfor Regulatory Environmental Modeling (CREM), and a panel study on the sameissue recently planned by the National Research Council (NRC) best explain theincreasing 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

gov-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 DuPontCorporate Remediation Group (CRG) Approximately four dozen modeling expertsfrom the U.S and Canada were carefully selected and invited to participate in thiseffort Four panels were formed, with each addressing one of the following primaryenvironmental contamination and remediation issues involving modeling: (1) MixingZone: Discharge of Contaminated Ground water into Surface Water Bodies, (2)Contaminated Sediment: Its Fate and Transport, (3) Optimization Modeling forRemediation and Monitoring, and (4) Simulation of Halogenated Hydrocarbons inthe Subsurface Although the details of these issues vary, all involve technical and/orregulatory challenges and a high Þnancial stake

Each panel had a panel leader who worked with the CRG to select the panelmembers, outline the panel discussion, facilitate the discussion at the workshop, andL1667_C00.fm Page v Tuesday, October 21, 2003 3:50 PM

help prepare the manuscripts for the chapters presented in this book The workshopwas held from July 25 to 27, 2000, at the campus of Penn State University GreatValley in Malvern, PA An assistant panel leader supported each leader and tookdiscussion notes, which helped panelists in the preparation of this book A completelist of panelists and their afÞliations are provided in Appendix A.

This book was prepared using the information generated from workshop sions and additional materials provided by the panelists The primary objectives ofthis book were to provide information on the state of the art and current practiceand identify the research and development needs of the modeling technologiesdiscussed It should be noted that the discussions herein are based not only ontechnical analysis but also on regulatory acceptance and cost effectiveness.This book comprises four chapters Each chapter addresses one of the four topicareas discussed at the workshop In most cases, a section of each chapter wasprepared by a panel member and, in some cases, includes materials offered by othermembers The panel leaders either assembled the material submitted by the panelists

discus-or further edited the manuscripts pridiscus-or 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 personresponsible for assembling and editing each chapter manuscript is listed at thebeginning of the chapter, followed by the names of signiÞcant contributors Calvin

C Chien was responsible for the overall planning, preparation, and publication ofthe book

DuPont and the book contributors want to express their deep appreciations toPenn State Great Valley and Elayna McReynolds, the conference coordinator, forthe 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

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In his near 30 years of practice, Dr Chien has focused on performing groundwater investigations and facilitating environmental remediation technology develop-ment As a company leader for technology development, he has concentrated in theareas of environmental modeling and containment technology since 1986 Besidesserving as the leader of the Groundwater Work Group of the Chemical ManufacturersAssociation (CMA, now American Chemistry Council) in the late 1990s, he was anappointed member of the U.S Environmental Protection Agency (USEPA) ScienceAdvisory Board for four terms and served on the Environmental Engineering Com-mittee and Environmental Modeling Subcommittee from 1993 to 2000 He was alsoappointed to serve on its Science and Technology Achievement Award (STAA)Committee Dr Chien has served on many national ground water modeling technicaland review committees He has advocated for the collaboration among industry,university, and government agencies through a number of major national expertworkshops in the past 10 years Dr Chien is recognized as the pioneer in a newapproach in solving problems in environmental remediation and as one of the leadingmodelers in the industry.

Miguel A Medina, Jr. earned a Ph.D degree in water resources and environmentalengineering sciences from the University of Florida in 1976 and joined the Dukefaculty 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 aregistered 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 HydrologicalProgramme from 2002 to 2003

Professor Medina has conducted funded research in hydrologic and water qualitymathematical modeling for the U.S Environmental Protection Agency (USEPA),the National Science Foundation, the OfÞce of Water Research and Technology, theU.S Air Force, the U.S Army Waterways Experiment Station, the Naval Oceano-graphic OfÞce, DuPont Engineering, the North Carolina Water Resources ResearchInstitute, and the State of North Carolina His current research focuses on ßow andsolute transport surface/ground water interactions and he has published numerousarticles on this topic in peer-reviewed journals

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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 ResearchTriangle Institute of North Carolina, the Inter-American Development Bank, the PanAmerican Health Organization, UNESCO, the Ministries of Water Resources inVenezuela and Spain, the Technical Advisory Service for Attorneys, and other privateenterprises He is a past chairman of the International Technical Advisory Committee

of the International Ground Water Modeling Center (Colorado School of Mines, andDelft, 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 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 researchhydrologist with the U.S Geological Survey in Washington, he joined the CivilEngineering Department at Princeton University as an associate professor He waspromoted to full professor 5 years later He served as chairman of the Department

Uni-of Civil Engineering and Operations Research from 1980 to 1989 He served asdean of the College of Engineering and Mathematics at the University of Vermontfrom 1989 to 1996 and is currently head of the Research Center for GroundwaterRemediation Design at the University of Vermont

Dr Pinder has published more than 200 papers and reports in the area ofquantitative 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, andBusiness Administration at the University of Vermont from 1992 to 1996; he wasnamed a 1993–1994 University Scholar in recognition of his contributions toresearch and scholarship The American Geophysical Union (AGU) presented theirHorton Award to Dr Pinder in 1969 and in 1993 invited him to become an AGUfellow 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 receivedthe Hinds medal of the American Society of Civil Engineers in 2002

Daniel D Reible has provided national and international leadership on tal matters to students, colleagues, and his profession He is currently ChevronProfessor of Chemical Engineering and director of the Hazardous SubstanceResearch Center at Louisiana State University He joined LSU after receiving aB.S degree in chemical engineering from Lamar University (1977) and an M.S andPh.D in chemical engineering from the California Institute of Technology (1979and 1982, respectively) As a teacher he has developed several graduate-level courses

environmen-in chemical engenvironmen-ineerenvironmen-ing and remaenvironmen-ins active environmen-in teachenvironmen-ing both undergraduate andadvanced-level chemical engineering courses His teaching efforts have alsoL1667_C00.fm Page viii Tuesday, October 21, 2003 3:50 PM

extended far beyond the university, for example, with his direction of AdvancedStudy Institutes in Prague with NATO support in 2001 and in Rio de Janeiro withNSF support in 2002 Both institutes involved more than 100 attendees focused onthe 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 referencebooks

Dr Reible has been active in both environmental research and its implicationsfor policy He has edited two books over the past 2 years on the state of the art inassessment and remediation of contaminated sites He served on the NationalResearch Council Committee on PCB-contaminated sediments, which has had aprofound impact on the management of contaminated sediments in this country, andcurrently serves on the National Research Council Committee for Remediation ofNavy Sites He recently provided congressional testimony before the U.S HouseSubcommittee on Water Resources and the Environment on strategies for the man-agement of contaminated sediment sites His leadership role in environmentalresearch 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 versity of Toronto, where he teaches courses in contaminant hydrogeology, environ-mental chemistry, and engineering mathematics Dr Sleep’s research interests andpublications are in the area of remediation of organic contamination of ground water,including experimental studies and numerical modeling Current projects includelaboratory studies of anaerobic biodegradation of DNAPL source zones, in situ

Uni-chemical oxidation and biodegradation of DNAPL source zones, biodegradation ofmixtures of halogenated organic compounds, isotopic fractionation associated withbiological processes, bioÞlm growth in fractures, and biological processes in lowpermeability media Numerical modeling is focused on modeling nonisothermalmultiphase 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/aerobicbiodegradation 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 theNational Ground Water Association and an associate editor of Advances in Water Resources

Sciences at the University of Alabama He holds a Ph.D in hydrogeology from theUniversity 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

an environmental and water-resource consulting Þrm Since 1993, he has beenleading the interdisciplinary hydrogeology program at the University of Alabama.

Dr Zheng is developer of MT3D/MT3DMS, a widely used contaminant fate andtransport simulation model, and co-author of the textbook Applied Contaminant Transport Modeling, published by John Wiley & Sons and currently in the secondedition Dr Zheng has published over 50 papers and book chapters on both appliedand theoretical aspects of hydrogeology, contaminant transport, and optimal groundwater management He is recipient of the 1998 John Hem Excellence in Scienceand Engineering Award from the National Ground Water Association for outstandingcontributions to the understanding of ground water, and is a fellow of the GeologicalSociety of America Dr Zheng serves on the Groundwater Committee of theAmerican Geophysical Union, the Standing Committee on Hydrologic InformationSystems of the Consortium of Universities for Advancement of Hydrologic Science,and the editorial boards of Ground Water and Hydrogeology Journal

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U.S Air Force Reserve

and Utah State University

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

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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

1.1 Introduction and Overview1.1.1 Overview of Issues IdentiÞed1.1.2 Ground Water–Surface Water Interaction Technical Background1.2 The User’s Perspective

1.2.1 Point Source Discharge Regulations1.2.1.1 The Zone of Initial Dilution (ZID)1.2.1.2 The Toxic Dilution Zone (TDZ)1.2.2 National Pollutant Discharge Elimination System (NPDES) Permitting Technical Issues

1.2.2.1 Two-Stage Mixing1.2.2.2 Federal Guidelines1.2.2.3 Acute Toxicity1.2.2.4 Dimensions of Regulatory Mixing Zones1.2.3 Nonpoint Sources

1.2.3.1 State of Michigan Mixing Zone Rules1.3 Current State of Knowledge

1.3.1 Problem-Oriented Perspective1.3.1.1 Ecological and Health Risk Aspects1.3.1.2 Environment Boundaries and Scope1.3.1.3 Ground Water–Surface Water Connections1.3.1.4 Stream–Subsurface Exchange Processes1.3.1.5 Implications for Controlled and Uncontrolled

Contaminant Discharges1.3.2 Enabling Technologies Perspective — Simulation Models

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1.3.2.1 Introduction and Policy Implications

of Technological Limits1.3.2.2 Modeling Stream–Subsurface Exchange Processes1.3.2.3 Tidal Exchanges and Oscillations

1.3.2.4 Mathematical Formulation — Tidally Inßuenced Case1.3.2.5 Exit Concentration

1.3.3 Emerging Technologies1.3.3.1 Mathematical Models1.3.3.2 New Laboratory Techniques1.3.3.3 New Field Techniques1.3.4 Alternative Approaches

1.3.4.1 InsigniÞcant Momentum-Induced Dilution1.3.4.2 Modeling Approach

1.3.4.3 Case Studies of Model Applications1.4 Acceptance of Methodology

1.5 Summary, Conclusions, and RecommendationsAcknowledgments

References

1.1 INTRODUCTION AND OVERVIEW

The interaction between surface water and ground water bodies traditionally hasbeen idealized as a simple unidirectional transport process More recent detailedexamination has shown that ßow systems can be complicated Complicated ßow andmixing patterns can have signiÞcant implications for physical, biogeochemical, andbiological 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 groundwater interaction on a spatial and temporal scale The regulatory framework ofmixing zones was reviewed, and the policy implications of mixing zones on groundwater and surface water interaction were discussed The panel focused on mathe-matical modeling of these processes and reviewed the state-of-the-art technology inaqueous 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 primaryexchange processes between the sediment and the overlying surface water to occurwithin the upper 2 in of sediment deposits.Important elements in estimating the groundwater contribution are distinguishing and characterizing the various inputs to the surfacewater–sediment system, which, in some cases, can be contaminated sediment.ThespeciÞc 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 consistedprimarily of hydrologic and physicochemical techniques (USEPA, 1998a).Promis-ing new research is focusing on the use of biological indicators (organisms thatL1667_book.fm Page 2 Tuesday, October 21, 2003 8:33 AM

spend all or part of their life cycle in contact with ground water) to characterizezones of ground water–surface water interaction, reviewed later in this chapter.This chapter attempts to present the best understanding of the underlyinghydrodynamic, chemical, and biological processes required to describe contami-nant transport between ground water and surface water and the limitations ofnumerical modeling

Figure 1.1 (Minsker et al., 1998) shows some of the interactions between groundwater and surface water bodies, including atmospheric exchange and exchangesbetween 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 modelingissues that deserve further discussion.Among the most salient technical issues thatneed resolution with regard to surface water–ground water interactions (i.e., themixing 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)

after Minsker et al., 1998.)

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• 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 ing issues that are required to resolve the issues listed above adequately:

model-• Formulate screening and tiered approaches using modeling A tieredapproach addresses two important technical issues First, simple analyticsolutions increasing in complexity allow for a greater understanding ofthe system Second, less complex sites require less complex models, andthe tiered approach allows the evaluation to stop at an appropriate level

• Apply veriÞcation procedures (including peer review), benchmarking,validation, and Þeld testing

• Use and develop scientiÞc process models to deÞne various types of mixingzones and transition zones in support of conceptual model development

• Obtain new data sets to develop and validate modeling approaches toaddress regulatory requirements

• Establish feedback mechanisms between data collection, modeling, andresource decisions

• Develop methods to account for uncertainty and heterogeneity

For complex surface water sites where an unacceptable risk to human healthand 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 ical and biological criteria to deÞne more quantitatively the mixing zoneregulatory boundaries and target goals (e.g., ecological impacts on alocalized scale, large-scale ecological or human health concerns)

chem-• Evaluate alternative control or management strategies to achieve soundrisk-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 modelL1667_book.fm Page 4 Tuesday, October 21, 2003 8:33 AM

for complex sites In addition, these models should have the capability to addresscurrent and potential regulatory deÞnitions of the various types of mixing zones andtransition zones Accounting for parameter uncertainty can permit key regulatorypolicies 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, asillustrated in Figure 1.2

For an improved understanding of the ecological relevance of the biologicalcommunity in the transition zone, the following needs were identiÞed:

• Compare site chemical data to the appropriate ecological benchmarkcriteria

• Perform basic research in community structure, life histories, faunal ture, functional structure

struc-• Improve sampling techniques

• Improve evaluation techniques, including community analysis and toxicityassessment

• Incorporate into the risk paradigm

• Evaluate risk presented by a discrete plume in terms of the risk posed tooverall ecologic and environmental health of the system

The policy and management issues below remain to be resolved by the regulatoryagencies Changes in policy can alter not only the regulatory landscape but also thetechnical analysis

• Should the geometry of regulatory mixing zones be based on the dynamic mixing zone?

hydro-• Under what conditions are mixing zones acceptable in terms of risk toecological receptors?

• Can mixing zones be integrated into total maximum daily loads (TMDLs)such as storm water discharges?

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

• 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 integralparts of ground water ßow systems, and ground water interacts with surface water innearly all landscapes — from small streams, lakes, and wetlands in headwater areas

to major river valleys and seacoasts On a relatively large scale, characterization ofwater mass transfer between ground water and surface water bodies is relatively wellunderstood For example, streams are either gaining or losing However, the ßowsystem on a smaller scale near the interface of the surface water column and thesediment 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 andprocesses such as seasonally high surface water levels, evaporation and transpiration

of ground water from around the perimeter of surface water bodies, rifße and pooldynamics 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 thefollowing zones (see Figure 1.1 [Minsker et al., 1998]):

• The surface water column (both near the discharge area and further outinto the larger part of the surface water body)

• The bank storage zone or the shallow sediment section near the sedimentbed, also referred to as the biologically active zone

• The zone of transition from ground water to surface water below thesediment–water interface but not into the aquifer proper

Within various surface water body environments, speciÞc processes can play a moresigniÞcant role than in other environments For instance, streams present a veryspecial 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 zonebased on biological environment The process of water and solute exchange in bothdirections across a streambed is usually termed the hyporheic exchange The direc-tion of seepage through the streambed is commonly related to abrupt changes inbed slope or to meanders in the stream channel.The dimensions of the hyporheiczone depend on the type of sediment in the streambed and banks, streambed slopeand variability, and hydraulic gradients The hyporheic zone is a potentially signif-icant zone of biological activity in aquatic systems Because of ground water andsurface water mixing within the hyporheic zone, the chemical and biological char-acteristics of water within the zone can differ considerably from those of adjacentsurface water and ground water systems

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The ecological and health risk factors associated with mixing zones, whether inthe surface water column or in the sediment bed and deeper, are reviewed in moredetail later in this chapter Examples of modeling the exchange of ground water andsurface 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 tononpoint sources has not been widely addressed in regulatory management Fur-thermore, the spatial deÞnition of a regulatory mixing zone and the actual physicalmixing zone resulting from a hydrodynamic sequence of events are not usuallythe same Thus, a distinction is made throughout this chapter between regulatorymixing zones and hydrodynamic mixing zones The regulatory deÞnition of themixing zone describes it as an allocated impact zone where numeric water qualitycriteria can be exceeded as long as acutely toxic conditions are prevented Cur-rently, the USEPA (2001) is conducting a review to consider a potential nationwidephase-out of mixing zones for the most persistent, toxic, and bioaccumulativechemicals 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 nonpointsource aspect of the problem (surface water–ground water interaction) that is ofprimary 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 nonpointsources For example, the handbook (USEPA, 1993) does not explicitly excludenonpoint sources in the mixing zone deÞnition Furthermore, the handbook indicatesthat the mixing zone can be deÞned in terms of volume and that the location andshape 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 someinterpretation for nonpoint source application A discussion of the nonpoint sourceregulatory 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, FederalRegister, 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 qualitycriteria can be exceeded as long as acutely toxic conditions are prevented A mixingzone can be thought of as a limited area or volume where the initial dilution of adischarge occurs Water quality standards apply at the boundary of the mixing zone,not within the mixing zone itself The USEPA has published numerous documentsproviding guidance for determining mixing zones (e.g., USEPA, 1991 and 1993;USEPA Region VIII, 1994)

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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 thesmallest 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 whileavoiding impingement of biologically important areas In general, a mixing zoneshould 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 dominantnuisance species

The USEPA rules for mixing zones recognize that the state has discretionwhether to adopt a mixing zone and to specify its dimensions The USEPA allowsthe use of a mixing zone in permit applications except where one is prohibited instate regulations State regulations addressing streams or rivers generally limit mix-ing zone widths or cross-sectional areas and allow lengths to be determined on acase-by-case basis According to a report prepared for the Chemical ManufacturersAssociation by The Advent Group (1994), 23 states have a narrative mixing zonelanguage and 27 states have speciÞc mixing zone area and/or volume deÞnitions intheir regulations SpeciÞc examples from the MDEQ Surface Water Quality Divisionadministrative rules on mixing zones are presented in the section after point sourceregulations, for it is clear that in those regulations some thought was given tononpoint source compliance as well

In the case of lakes, estuaries, and coastal waters, some states specify the surfacearea 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 mixingzone 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 municipalwastewater into the coastal ocean, as regulated under Section 301(h) of the CleanWater Act Frequently, these same deÞnitions are used for industrial and otherdischarges 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 ambientsaline water) occurs The USEPA requires that the ZID be a regularly shaped area(e.g., circular or rectangular) surrounding the discharge structure (e.g., submergedpipe or diffuser line) and encompassing the regions of high (exceeding standards)pollutant concentrations under design conditions In practice, limiting boundariesdeÞned by dimensions equal to the water depth measured horizontally from anyL1667_book.fm Page 8 Tuesday, October 21, 2003 8:33 AM

point of the discharge structure is accepted by the USEPA, provided no other mixingzone 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 Initialdilution 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 dilutesthe discharge Through the entrainment process, the plume density approaches theambient density (neutral buoyancy) and, depending on the location at which thisoccurs, the plume either reaches the surface or becomes trapped at some intermediatelevel Therefore, the spatial extent of the mixing region is sometimes referred to asthe ZID because it is where initial dilution is achieved Beyond the ZID, ambientmixing processes tend to control further dilution of the plume However, in a 1994USEPA 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 allowableconcentration of toxic substances: a criterion maximum concentration (CMC) toprotect 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 atthe 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 fromthe outfall If dilution of the toxic discharge in the ambient environment

is allowed, a TDZ that is usually more restrictive than the legal mixingzone for conventional and nonconventional pollutants can be used Therevised 1991 toxic technical support document (USEPA, 1991) recom-mends a minimum exit velocity for new discharges of 10 ft/s in order toallow sufÞciently rapid mixing that minimizes organism exposure time totoxic material The document does not set a requirement in this regard,recognizing that the restrictions listed in the following paragraph can inmany instances also be met by other designs, especially if the ambientvelocity is large

L1667_book.fm Page 9 Tuesday, October 21, 2003 8:33 AM

• 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 ofthe outfall structure to the edge of the regulatory mixing zone in anyspatial direction

• The CMC must be met within a distance of 50 times the dischargelength scale in any spatial direction The discharge length scale isdeÞned as the square root of the cross-sectional area of any dischargeoutlet This restriction is intended to ensure a dilution factor of at least

10 within this distance under all possible circumstances, includingsituations of severe bottom interaction and surface interaction

• The CMC must be met within a distance of Þve times the local waterdepth in any horizontal direction The local water depth is deÞned asthe natural water depth (existing prior to the installation of the dis-charge outlet) prevailing under mixing zone design condition (e.g., lowßow for rivers) This restriction prevents locating the discharge in veryshallow 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 beexposed 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 dischargerswith stringent efßuent limits When the efßuent concentration must meet the ambientwater quality standard, no mixing zone is allowed for the discharge On the otherhand, many states allow mixing zones in rivers and streams under certain conditions.QuantiÞcation of mixing zones to comply with regulations is an urgent topic facingmany regulatory staff, water quality engineers, and water quality management deci-sion makers More speciÞcally, determining how to combine the regulatory aspectswith technical issues in quantifying mixing zones is the key to this water qualityproblem in receiving waters

There are a number of water quality constituents related to mixing zones, rangingfrom 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 dividedinto two stages with distinct mixing characteristics.The initial momentum of thedischarge determines mixing and dilution in the Þrst stage The design of thedischarge outfall should provide ample momentum to dilute the concentrations inthe immediate contact area as quickly as possible.(It should be noted that manyexisting outfalls with small ßows do not have sufÞcient momentum for initialL1667_book.fm Page 10 Tuesday, October 21, 2003 8:33 AM

dilution.) The second stage of mixing covers a more extensive area in which theeffect of initial momentum is diminished and the waste is mixed primarily by residualplume buoyancy and ambient turbulence

1.2.2.2 Federal Guidelines

For toxic discharges, the USEPA maintains two water quality criteria for allowablemagnitude 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 overallmixing zone.In some states, this zone of initial dilution is referred to as the allocatedimpact zone In rivers or tidal rivers that have a persistent through-ßow in thedownstream direction and do not exhibit signiÞcant natural density stratiÞcation, the1-day, 10-year low ßow (1Q10) and 7-day, 10-year low ßow (7Q10) for the CMCand CCC, respectively, are recommended in steady-state mixing-zone modelinganalysis (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 toxicunit (TUa) for whole efßuent toxicity (USEPA, 1991).The acute toxicity unit isexpressed as TUa = 100/LC50, where LC50 is the percentage of efßuent that causes50% of the organisms to die through the exposure period.For example, an efßuentthat is found to have an LC50 of 5% is evaluated as 20 TUa

1.2.2.4 Dimensions of Regulatory Mixing Zones

The dimensions of a mixing zone in a river are usually determined by state tions.Doneker and Jirka (1990) provide a summary of state mixing zone regulations

regula-In general, regulatory mixing zones are speciÞed by a distance, area, or volumearound the discharge point For example, Virginia water quality standards state thatthe overall mixing zone shall not constitute more than one half of the width of thereceiving watercourse or one third of the area of any cross section of the receivingwatercourse (Lung, 1995).In addition, it shall not extend downstream for a distancemore 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 overallmixing 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% insmall- and medium-sized streams (Alley et al., 1999).Extrapolation of these numbers

to large rivers is more complicated; however, the ground water contribution to allL1667_book.fm Page 11 Tuesday, October 21, 2003 8:33 AM

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–surfacewater interaction zone is important from federal statutory, regulatory, and policyperspectives because 75% of Superfund and Resource Conservation and RecoveryAct (RCRA) sites are located within a half mile of a surface water body In 47% ofthese Superfund sites, there have been recorded impacts to surface water Althoughprogress has been achieved in controlling point sources within the past 25 yearsthrough 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 groundwater–surface water interactions, and good policy depends on good technical infor-mation Recently, greater attention has been placed by the USEPA (2000) on theseinteractions The goal of Superfund is to return usable ground water to beneÞcialuses (current and future) where practical When this is not practical, Superfundstrives to prevent further migration and exposure and to evaluate opportunities forfurther risk reduction Preliminary remedial goals are set at levels that protectresources, including surface waters that receive contaminated ground water, takinginto account Clean Water Act requirements or state standards, whichever are morestringent Final cleanup levels are attained throughout the plume and beyond theedge of any wastes left in place, where the point of compliance for a surface waterbody is where the release enters the surface water Alternate concentration limits(ACLs) can be considered where contaminated ground water discharges to surfacewater, where contaminated ground water does not lead to increased contaminants

in surface water, where enforceable measures are available to prevent exposure toground water, or where restoring ground water is “not practicable.”

Tomassoni (2000) further points out that RCRA has similar requirements toSuperfund with respect to the following: returning usable ground water to beneÞcialuses, points of compliance for ground water and surface water, protection of surfacewater from contaminated ground water, and provisions for ACLs and treatment ofprincipal threats Therefore, if current human exposures are under control and nofurther migration of contaminated ground water is expected, primary near-term goalsare established using two environmental indicators Thus, surface water becomesthe boundary if the discharge of contaminated ground water is within protectivelimits.It is estimated that the majority of contaminated sites have serious potential

to affect surface waters Although the federal framework allows for RBDM withrespect to ground water–surface water interaction, the expectation of restoringground water to beneÞcial use and ensuring that discharges of ground water tosurface water are protective must be achieved The following are among the keypolicy 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

• 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 zonedeÞnitions and regulation However, particularly for TDZs, some of the deÞnitionsand control strategies that apply to point sources are not relevant to nonpoint sources.Furthermore, it is difÞcult to generalize the actual practice of implementing themixing zone regulations given the large number and diverse types of jurisdictionsand permit-granting authorities involved By and large, however, current procedurefalls 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 allowsnonpoint 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 mixedwith the receiving water.” As a minimum restriction, the FAV for aquatic life shall not

be exceeded when determining awasteload allocation for acute aquatic life protectionunless it is determined by the MDEQ that a higher level is acceptable or it can bedemonstrated to the MDEQ that an acute mixing zone is acceptable consistent withsubrule (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

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

Research has not determined how rapidly hyporheic zone organisms spread

following the episodes of extensive surface water intrusion into ground water as a

result of periods of ßooding or how rapidly the extent of the zone varies seasonally

or in response to drought (USEPA, 1998a) However, the species richness and

community structure of these organisms have been shown to change with alterations

in ground water quality Thus, the organisms living within the shallow ground water

zone can serve as indicators of surface water–ground water interactions

1.3.1.1.1 Hyporheic Zone Biological Community

The structure and characteristics of the biological community that lives within the

hyporheic zone is not well deÞned This is due, in part, to the general difÞculties in

obtaining good samples from this zone Therefore, the zone has not been as well

investigated as other components of aquatic systems Traditionally, scientists have

considered only the upper few inches (typically only the upper 6 in or 15 cm) as

the biologically active zone in most aquatic systems, but investigations have found

a diverse community of organisms that inhabit substrate at depths greater than 6 in

or 15 cm (Williams and Hynes, 1974) Biological members of the hyporheic zone

include permanent members, which complete their entire life cycle within the zone,

and transient members, which spend only a portion of their life cycle within this

zone Permanent members include species of rotifers, oligocheates, copepods,

ostra-cods, cladocerans, and other crustaceans Transient members include species

typi-cally found as members of the streambed benthos that migrate as early instar larval

stages to avoid disturbance (e.g., high river ßows) (Williams, 1987) or stress (e.g.,

temperature extremes) (Harper and Hynes, 1970) In addition to serving as habitat

Water

table

Water table

Ground water

Hyporheic zone

Impermeable layer Permeable layer

L1667_book.fm Page 15 Tuesday, October 21, 2003 8:33 AM

and refuge for groups of organisms, the hyporheic zone can also be an area of

signiÞcant nutrient recycling within aquatic systems (Williams, 2000)

1.3.1.1.2 Hyporheic Community Assessments

Traditional water quality and sediment assessments have focused on those portions

of the aquatic community that are relatively well understood One only has to open

a general college textbook on ecology to obtain information about water column

and macrofaunal communities Testing procedures designed to assess the potential

impacts of contaminants on these communities have been developed and are

com-monly used in environmental studies This is in sharp contrast to both the

under-standing of the biological community that can be associated with the hyporheic zone

and the role this community plays in the functioning of aquatic ecosystems Studies

attempting to characterize the functional role of the hyporheic zone suggest that it

is an important site for the transformation and storage of nutrients (Triska et al.,

1994) and it mediates the availability of N and P (Storey et al., 1999) Research on

the hyporheic zone biological community has been hampered by the difÞculty of

obtaining quantitative biological samples (Williams, 2000) As new sampling

tech-niques are developed and old ones are improved, a better understanding of the role

hyporheic zones play in aquatic ecosystems will emerge The ecological risks

asso-ciated with the discharge of contaminated ground water to surface water currently

are evaluated only superÞcially in risk assessments (Burton and Greenberg, 2000)

Although ground water can serve as a signiÞcant pathway of exposure to aquatic

communities, the risks of such discharges are typically considered only for water

column receptors (e.g., Þsh and submerged plants) or benthic communities (e.g.,

macrofaunal members of the benthic community that live in the upper 15 cm of

sediment) The hyporheic community — whether one considers deeper dwelling

macrobenthic organisms smaller organisms that comprise the meiofaunal community

or the microbial community — is not considered in such assessments The

signiÞ-cance of omitting the hyporheic community from risk assessments is unknown The

National Oceanic and Atmospheric Administration (NOAA), in a recent review of

11 Superfund sites, found that 8 of the sites contained ground water with contaminant

concentrations exceeding screening level concentrations (Matta and Dillon, 2000)

Although the screening level concentrations used in the review were highly

conser-vative, the results suggest that the potential for impacts to those organisms occupying

the hyporheic zone may need to be considered at similar sites

At this time, there are severe limitations to assessing risk to the hyporheic zone

realistically These limitations include the following:

• Lack of understanding of the complex hydrogeologic structure and

mor-phology of the surface water body and other variables that deÞne the

existence and boundaries of the hyporheic zone

• Lack of understanding of the functional role of the biological community

within the hyporheic zone in aquatic ecosystems and under what

condi-tions the functional role is signiÞcant

• Lack of understanding of the biological effects upon the hyporheic zone

community of the hydrogeologic structure of an area

L1667_book.fm Page 16 Tuesday, October 21, 2003 8:33 AM

1.3.1.2 Environment Boundaries and Scope

The mixing behavior of a point source discharge is governed by the interplay of

ambient conditions in the receiving water body and by the discharge characteristics

The ambient conditions in the receiving water body (e.g., stream, river, lake,

reser-voir, estuary, coastal waters) are described by the water body’s geometric and

dynamic characteristics Important geometric parameters include plan shape, vertical

cross sections, and bathymetry, especially in the discharge vicinity Dynamic

char-acteristics are given by the velocity and density distribution in the water body, again

primarily in the discharge vicinity In many cases, these conditions can be taken as

steady state with little variation because the time scale for the mixing processes is

usually of the order of minutes up to perhaps 1 h In some cases, particularly tidally

inßuenced ßows, the ambient conditions can be highly transient and the assumption

of steady-state conditions can be inappropriate In this case, the effective dilution

of the discharge plume can be reduced relative to that under steady-state conditions

A speciÞc example of a tidally inßuenced case is presented below

The discharge conditions relate to the geometric and ßux characteristics of the

point source considered For a single port discharge, the port diameter, its elevation

above the bottom, and its orientation provide the geometry; for multiport diffuser

installations, the arrangement of the individual ports along the diffuser line, the

orientation of the diffuser line, and construction details represent additional

geomet-ric features; and for surface discharges, the cross section and orientation of the ßow

entering the ambient watercourse are important The ßux characteristics are given

by the efßuent discharge ßow rate, by its momentum ßux, and by its buoyancy ßux

The buoyancy ßux represents the effect of the relative density difference between

the efßuent discharge and the ambient conditions in combination with the

gravita-tional acceleration It is a measure of the tendency of the efßuent ßow to rise (i.e.,

positive buoyancy) or fall (i.e., negative buoyancy)

In surface water–ground water interactions, the source characterization of the

recharge area requires adequate description In particular, the area of the recharge

zone, the temperature (density) of the recharge water, the local bathymetry near

the recharge area, the downstream ambient water body cross section, and the

possible chemistry of the water–sediment interface must be described well for

mixing zone analyses

The hydrodynamics of an efßuent continuously discharging into a receiving

water body can be conceptualized as a mixing process occurring in two separate

regions In the Þrst region, the initial characteristics of discharge momentum ßux,

buoyancy ßux, and source geometry inßuence the jet trajectory and mixing This

region is referred to as the near-Þeld and encompasses any surface, bottom, or

terminal layer interaction As the turbulent plume travels farther from the source,

the source characteristics become less important Conditions existing in the ambient

L1667_book.fm Page 17 Tuesday, October 21, 2003 8:33 AM

environment control trajectory and dilution of the turbulent plume through buoyant

spreading motions and passive diffusion due to ambient turbulence This region is

referred to as the far-Þeld It is stressed that the distinction between near-Þeld and

far-Þeld is made purely on hydrodynamic grounds and is unrelated to any regulatory

mixing zone deÞnitions

Information about the density distribution in the ambient water body is very

important to correctly predict mixing zone characteristics Ambient density (not

temperature) is the controlling parameter in most mixing zone analyses If the

ambient water is fresh water and it is above 4ûC, the entering ambient temperature

can be used to specify ambient density If the ambient water body is below 4ûC or

is brackish, then density must be speciÞed directly As a practical guide, vertical

variation in density of less than 0.1 kg/m3 or in temperature of less than 1ûC can be

neglected for mixing zone analyses For uniform conditions, the average ambient

density or average temperature can be speciÞed When in doubt about the

speciÞ-cation of the ambient density values, it is reasonable to Þrst simplify as much as

possible The sensitivity of a given assumption can be explored in subsequent mixing

zone simulations

1.3.1.3 Ground Water–Surface Water Connections

Recently, the interface between surface water and ground water has received

increas-ing attention Traditional hydrologic methods, which operationally separate surface

water and ground water components, are focused only on relatively large-scale water

ßuxes The emphasis on contaminant transport and stream ecology requires a closer

examination of ßow paths and processes because local-scale biogeochemical

inter-actions often can control reactive substance transport The interface between surface

water and ground water is especially important in this regard because this region is

characterized by strong gradients in physical, chemical, and ecological parameters

Thus, additional emphasis must be placed on understanding both the underlying

heterogeneity of the system and the actual ßow paths that contaminants take through

this heterogeneous system These factors control the types of transformation

reac-tions and microbial processes that contaminants are exposed to when a subsurface

contaminant plume impinges either on a surface water body or downstream of a

surface water release

1.3.1.4 Stream–Subsurface Exchange Processes

The large-scale ground water ßow system, often the only one considered in ground

water modeling, provides the setting within which the stream ßows Indeed, it is

well known that ground water base ßow maintains perennial streams Even at this

scale, examining ground water ßow paths can show that stream–aquifer connections

can be more complicated than expected For example, upstream losing reaches can

be directly linked to downstream gaining reaches Seminal work by Hynes (1975,

1983) advanced the idea that large-scale stream–subsurface exchange is intimately

linked to the ground water ßow system by periodic bedrock controls on subsurface

ßow in steep mountain streams Hynes elegantly described this regional

stream–sub-surface exchange pattern as a set of “beads on a string,” evoking visualization of

L1667_book.fm Page 18 Tuesday, October 21, 2003 8:33 AM

the periodic large-scale penetration of stream water into the subsurface and

subse-quent return to the stream

The same sort of ßow pattern also can develop at smaller spatial scales

Geo-morphic evolution of stream channels produces a wide range of periodic

topograph-ical features High-gradient streams are often characterized by pool-rifße sequences,

which can produce stream–subsurface exchange ßows similar to those envisioned

by Hynes (Harvey and Bencala, 1993; Wondzell and Swanson, 1996)

Lower-gra-dient, alluvial rivers generally have large planform meanders which can generally

be expected to admit a stream–subsurface exchange ßow These meanders are subject

to a difference in hydraulic head from the upstream to the downstream side, which

yields an exchange ßow through the meander (Harvey and Bencala, 1993; Wroblicky

et al.,1998) Additional stream–subsurface exchange can be induced whenever the

stream changes orientation relative to the local ground water ßow system These

ßows can be inßuenced by some particular features of the local ground water ßow

system such as preferential subsurface ßows through relict stream channels

(Won-dzell and Swanson, 1996)

The interplay of hydrodynamics and sediment transport also produces

char-acteristic, periodic topographical patterns on the streambed These features, known

as bedforms, then induce vertical stream–subsurface exchange ßows underneath

the stream Depending on the type of sediment and the size of the stream,

dune-shaped bedforms in loose alluvial material can have a length ranging from

centi-meters up to hundreds of centi-meters (Vanoni, 1975; Raudkivi, 1998) Streamßow over

bedforms produces drag, which serves as frictional resistance to the streamßow

and induces a subsurface advective ßow through the bedform called pumping This

process has been examined in some detail in laboratory experiments and analyzed

using process-based models that explicitly calculate the induced subsurface ßows

for various stream and subsurface boundary conditions (Savant et al., 1987;

Thi-bodeaux and Boyle, 1987; Elliott and Brooks, 1997a, 1997b; Packman, 1999;

Packman et al., 2000b, 2000c) In addition, this pumping process can occur in

many other natural systems, potentially whenever there is a ßow over a porous

bed This form of exchange is clearly important on the sea ßoor (Huettel et al.,

1996, 1998), and wind pumping through snow packs also has been theorized

(Colbeck, 1989, 1997)

Bed sediment transport produces bedform migration, which also mixes stream

and subsurface water This exchange process has been termed turnover Turnover

is expected to be the dominant exchange process in streams with very Þne-grained

sediments (Rutherford et al., 1993, 1995; Elliott and Brooks, 1997a, 1997b) In

addition, turnover moderates the advective pumping process by causing the

bed-forms that induce advection to move downstream The interplay of pumping and

turnover has been analyzed by Elliott and Brooks (1997a, 1997b) and Packman

and Brooks (2001)

The near-surface structure of exchange ßows is expected to be particularly

important for contaminant transport because this region is both highly heterogeneous

and characterized by sharp gradients in biogeochemical parameters Figure 1.4 shows

Elliott’s observations of dye penetration from a stream into a streambed covered

with bedforms Under stable bed conditions (no bed sediment transport), there is

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clearly a periodic variation of penetration and release under each bedform Bedsediment transport tends to homogenize the interfacial region Contaminant releasefrom the bed would be a mirror-image of the inÞltration shown in Figure 1.4 Inthis case, the contaminant would be expected to be preferentially carried out of thebed at periodic release points on each bedform A photograph of upward dyetransport in a gravel bed can be found in Thibodeaux and Boyle (1987); Huettel et

al (1998) provide detailed microprobe measurements of metal transport out of anatural marine sediment, and numerical simulations of tracer release are given byFeng et al (2000) Anecdotal information supports the picture developed fromlaboratory experiments: contaminant releases from streambeds are often found tohave a high degree of spatial variability, and discharge of nutrient-rich ground water

to streams has been observed to produce periodic algae growth on the streambed.The smaller-scale processes described above are expected to be particularlyimportant for contaminant transport and ecologically relevant substances such asnutrients These processes deÞne the most active region of stream water and groundwater mixing in the sense of Triska and coworkers (1989) This relatively sharptransition between waters with surface water and ground water signatures (in thegeochemical sense) has profound implications for the behavior of reactive sub-stances In particular, generally there is a complex transition region near the streamchannel whose characteristics are determined by both the sediment heterogeneityand the various imposed exchange ßow patterns Contaminant transfer from ground

FIGURE 1.4 Observations of dye penetration fronts due to stream–subsurface with bedforms.

(From Elliott, 1990.)

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water to surface water is expected to be highly mediated by those processes ered unique to this transition region Downstream contaminant transport is expected

consid-to be similarly mediated by hyporheic processes due consid-to the active exchange of waterwith the shallow subsurface These factors will be examined in more detail in thenext section

Table 1.1 summarizes stream–subsurface exchange processes that can occur atvarious spatial scales Additional information on these processes can be found inreviews by Sharp (1988), Larkin and Sharp (1992), Winter (1995), and Winter et al.(1998) Additional analysis of stream–subsurface exchange processes and a directeddiscussion of the importance of these processes for stream ecology can be found inthe recent book edited by Jones and Mulholland (1999) and the review by Brunkeand Gonser (1997)

1.3.1.5 Implications for Controlled and Uncontrolled

Contaminant Discharges

Stream–subsurface exchange can be expected to be important for downstream taminant transport whenever streambed sediments are permeable enough to admit asigniÞcant exchange ßux Interactions with channel sediments are expected to mod-ify both accidental discharges such as spills and long-term contaminant discharges.Downstream transport of a contaminant pulse is modiÞed by the exchange of some

con-of the contaminant into the subsurface, where it is retained for some time and fromwhere it returns to the streamßow This process has been termed transient storage(Bencala and Walters, 1983) and is one of the processes responsible for the well-known tailing of solute pulses in streams (Fischer et al., 1979) The key implication

of this exchange for contaminant transport is that the contaminant is subject toconsiderably different biogeochemical processes in the subsurface For example,sorption of chromium to streambed sediments caused chromium to be retained inthe bed much longer than a conservative solute (Wörman, 1998; Wörman et al.,1998; Johannson et al., 2001) Thus, over the time scale of a contaminant pulse,

TABLE 1.1

Stream–Subsurface Exchange Processes That Can Occur at Various Spatial Scales

Relevant Spatial Scale Representative Exchange Processes

Bed sediment grain scale Turbulent stream–subsurface interactions

Advective ßows induced around small-scale bed features Bedform scale Advective pumping due to streamßow over bedforms

Turnover due to bed sediment transport Channel scale Subsurface ßow induced at meanders or pool rifße sequences

Exchange ßow due to orientation of stream relative to the local ground water table

Reach scale and larger Interaction with regional ground water ßow system

Periodic lateral subsurface ßow due to tidal variation in stream level

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stream–subsurface exchange often can represent a sink for the contaminant, withrelease coming a considerable time after the passage of the main pulse.

When the surface water contaminant release persists for a long period of time,

be it a permitted discharge or an uncontrolled release, stream–subsurface exchangecan cause the contaminant to be transported from the stream to the near-streamground water ßow system The net result often is the development of a subsurfacereservoir of the introduced contaminant This is particularly the case when thecontaminant is persistent and has a high afÞnity for sediments This effect has beenseen most clearly for mine-derived contaminants such as metals and arsenic, whichhave produced large-scale contamination of ßoodplain sediments and near-streamsurÞcial aquifers (Moore and Luoma, 1990; Broshears et al., 1996; Harvey andFuller, 1998) In these cases, stream–subsurface exchange can cause these subsurface

or off-channel sources to remain a source of stream contamination after the maincontaminant discharge ceases In other cases, stream–subsurface exchange couldpotentially be beneÞcial if it exposes contaminants to subsurface conditions thatreduce their toxicity However, these processes rarely have been examined, and soadditional study is necessary to characterize the range of interactions that can beimportant for various contaminants

Stream–subsurface exchange also inßuences the discharge of ground water taminants to streams by causing the development of a subsurface mixing region —the hyporheic zone — with water quality characteristics between those of the bulksurface water and ground water The hyporheic zone can inßuence ground watercontaminant discharges in two important ways First, the contaminant plume can besubject to enhanced dilution just before it enters the stream Second, the hyporheicregion can have unique conditions that facilitate contaminant transformation Forthe common case of a reduced, contaminated ground water discharging to an oxicsurface water, the hyporheic zone often is characterized by at least partially aerobicconditions In this sense, the hyporheic zone can be thought of as a relatively well-ßushed sedimentary environment, and, as such, apparently often represents a regionwhere there is the opportunity for relatively specialized microbial population growth(Baker et al., 1999; Findlay and Sobczak, 1999) Thus, the hyporheic zone cansubject contaminants to transformation processes that cannot occur elsewhere in thestream–aquifer system (Nagorski and Moore, 1999) In some cases, these hyporheicprocesses can be highly beneÞcial, for example, by offering the opportunity for

con-naturally enhanced in situ bioremediation of reduced, organic-rich plumes

discharg-ing to streams (Hynes, 1983; USEPA, 2000) When a reduced, organic-rich plumedischarges to a stream, the hyporheic zone can thus provide a region of naturalaerobic bioremediation

Knowledge of stream–subsurface exchange processes can be used to designimproved assessment and monitoring strategies for contamination in the near-streamenvironment Subsurface contaminant distributions, ßuxes from contaminated sed-iments, and ground water plume discharges are all expected to show a high degree

of spatial heterogeneity Further, streams are a dynamic sedimentary environment,and both streambed conditions and stream–subsurface exchange ßuxes are expected

to change over time (Packman et al., 2000a) In particular, any ßood that rearrangeschannel sediments could potentially alter the local hydrodynamic conditions andL1667_book.fm Page 22 Tuesday, October 21, 2003 8:33 AM

thus the resulting interfacial ßuxes Sampling strategies should be designed tomeasure spatial variability so as to produce reasonable estimates of the averagecontaminant ßux to the stream in a given area and may also need to account forlong-term variability in stream conditions Another successful approach is to com-bine conservative tracer injections to probe stream–subsurface interactions withextensive measurement of contaminant concentrations in the stream (Kimball, 1997;Harvey and Wagner, 1999) Sampling strategies that do not adequately considerstream–subsurface exchange ßuxes are likely to produce contaminant ßux estimateswith high uncertainty.

1.3.2 E NABLING T ECHNOLOGIES P ERSPECTIVE —

S IMULATION M ODELS

An issue that is always present is to what degree are models used, certiÞed, or

approved for speciÞc uses, including the regulatory process itself For example, the

CORMIX family of models has undergone extensive USEPA and scientiÞc journalpeer review for modeling a wide range of discharge sources in mixing zone analysis.The CORMIX system has been distributed worldwide since 1990 with over 1200existing user groups That is a form of certiÞcation, although not a formal testsubjected to known benchmarks of success

The CORMIX models are used by both regulators and applicants in the NPDESpermit program for point source discharges CORMIX models are used to assessmixing zone characteristics, optimize outfall design, and determine regulatory com-pliance and environmental impact The primary impediment to model use regardingmixing zones of ground water–surface water interaction is the need to characterizethe contamination sources Existing CORMIX models can represent the sourceconditions that would be typical for these discharge sources

Federal agencies such as the USEPA have an important role in simulation modelselection and utilization This role should include, at a minimum, providing guide-lines on acceptable models and measures of performance and validation Such adocument exists for ground water models (USEPA, 1988) General guidelinesaddress three factors: objectives criteria, technical criteria, and implementation cri-teria The role of federal agencies should also include providing leadership in modeldevelopment through sponsored research at the appropriate program ofÞces Medina

et al (1996) developed a ground water quality modeling advisory system for theU.S Air Force for use in investigating remediation alternatives for subsurface con-tamination cleanup The system is capable of accounting for uncertainty not only inthe prediction of solute transport but also in the optimization of the remediationscheme through chance constraints The system guides users in selecting the mostappropriate transport models through an algorithm independently tested withmachine learning codes (Reich et al., 1996)

Currently, most pollutant fate and transport models do not integrate well overmultiple space and time scales This is certainly the case for mixing zone models.There is an obvious and persistent need within mixing zone models to link near-Þeld effects with far-Þeld effects; however, no general procedures or guidelines foraccomplishing this linkage exist

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The criteria for technical implementation in mixing zone problems should, at aminimum, include the following:

• Correct simulation of mixing zone physical processes in groundwater–surface water interaction

• Validation of proposed physical process models with available data sets

• Model guidance for correct simulation method application

The corresponding application objectives should include the following:

• Appropriate space and time scales of physical process simulation forground water–surface water interaction

• Adequate ground water source characterization for mixing zone analysis

• Adequate availability of downstream site characteristics

1.3.2.1 Introduction and Policy Implications

of Technological Limits

The need to manage and understand uncertainties for both scientiÞc and regulatoryapplications in the subsurface environment has long been recognized by theNational Research Council (NRC, 1990) This awareness has extended to interna-tional circles as well The International Association of Hydrological Sciencespublished a guide on ground water contamination risk assessment in 1990 (Rei-chard et al., 1990) as a contribution of the U.S to the UNESCO InternationalHydrological Programme

Over the past several decades, many different models for surface and face contaminant transport (including sediment) under varying conditions andassumptions have been proposed and tested These studies range from modelsbased on very simple, one-dimensional analytical (closed form) solutions thatassume a completely homogeneous and isotropic ßuid (or porous medium in thecase of subsurface models) to complex, three-dimensional numerical codes thatallow for complete speciÞcation of ßow and contaminant characteristics through-out a three-dimensional grid Other examples besides CORMIX include thehydrodynamic-eutrophication model (HEM-3D) developed by Park et al (1995)and the hydrodynamic and sediment transport model EFDC (Hamrick, 1992,1996) In essence, HEM-3D is an integration of a water quality model with 21state variables and the EFDC Upon receiving the information of physical trans-port from EFDC, HEM-3D simulates the spatial and temporal distributions ofwater quality parameters (e.g., dissolved oxygen, suspended algae, carbon, nitro-gen, phosphorus and silica cycles, coliform bacteria) Upon receiving the partic-ulate organic matter deposited from the overlying water column, a sedimentprocess model (also with 21 state variables) simulates their diagenesis and theresulting ßuxes of inorganic substances and sediment oxygen demand back tothe water column It is doubtful that a model with 42 state variables can ever befully calibrated and veriÞed

subsur-For any complex set of models, two key questions are as follows:

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