Compilationof C

EPAMixingZoneDocuments
United States Environmental Protection Agency

Office of Water

Washington, DC 20460

(4305T)

EPA 823-R-06-003

July 2006
Compilation of EPA 

Mixing Zone Documents 
Please Note 

The discussions in the documents collected in this Compilation are intended solely as guidance. The 

statutory provisions and EPA regulations described in this Compilation contain legally binding 

requirements. This document (or the documents referenced herein) are not a regulations, nor do they 

change or substitute for those statutory provisions and regulations. Thus, this document or the 

documents collected here do not impose legally binding requirements on EPA, states, authorized 

tribes, territories, or the regulated community. This Compilation does not confer legal rights or 

impose legal obligations upon any member of the public. 

While EPA has made every effort to ensure the accuracy of the discussion in this Compilation, the 

obligations of the regulated community are determined by statutes, regulations, or other legally 

binding requirements. In the event of a conflict between the discussion in this Compilation and any 

statute or regulation, this Compilation would not be controlling. 

The general description provided here may not apply to every circumstance. Interested parties are free 

to raise questions and objections about the substance of this guidance and the appropriateness of its 

application to a particular situation. EPA and other decision-makers retain the discretion to adopt 

approaches on a case-by-case basis that differ from those described in this guidance where 

appropriate. 

 

Mention of trade names or commercial products does not constitute endorsement or recommendation 

for their use. 

This is a living document and may be revised periodically without public notice. EPA welcomes 

public input on this document at any time. 
Compilation of EPA Mixing Zone Documents 

Table of Contents 

Purpose............................................................................................................................................ 1 

Organization of Information ........................................................................................................... 1 

Mixing Zone Definition .................................................................................................................. 1 

Types of Mixing Zones Covered .................................................................................................... 1 

 Allocated impact zone................................................................................................................ 1 

 Legal mixing zone...................................................................................................................... 2 

 Toxic dilution zone .................................................................................................................... 2 

 Zone of initial dilution ............................................................................................................... 2 

Types of Pollutants Addressed........................................................................................................ 2 

 Toxic pollutants ......................................................................................................................... 2 

 Conventional pollutants............................................................................................................. 2 

 Nonconventional pollutants....................................................................................................... 2 

Types of Concentrations Discussed................................................................................................ 2 

 CMC (criterion maximum concentration) ................................................................................ 2 

 CCC (criterion continuous concentration) ................................................................................ 2 

 RAC (reference ambient concentration ..................................................................................... 3 

Code of Federal Regulations (CFR) Language .............................................................................. 3 

Technical and Policy Guidance Documents ................................................................................... 6 

Technical Support Document for Water Quality-based Toxics Control (1991)..................... 6 

Water Quality Standards Handbook: Second Edition (1994)................................................. 6 

Memorandum: EPA Guidance on Application of State Mixing Zone Policies in 

EPA-Issued NPDES Permits (1996)................................................................................ 7 

Allocated Impact Zones for Areas of Non-Compliance (1995).............................................. 7 

Technical Guidance Manual for Performing Waste Load Allocations, Book III: Estuaries. 

Part 3: Use of Mixing Zone Models in Estuarine Waste Load Allocations (1992)......... 8 

U.S. EPA NPDES Permit Writer’s Manual (1996) ................................................................ 9 

Modeling Documents...................................................................................................................... 9 

Dilution Models for Effluent Discharges, 4th Edition (Visual Plumes) (2003) ...................... 9 

Dilution Models for Effluent Discharges, 3rd Edition (1994)............................................... 10 

CORMIX2: An Expert System for Hydrodynamic Mixing Zone Analysis of 

Conventional and Toxic Multiport Diffuser Discharges (1991).................................... 10 

User’s Manual for CORMIX: A Hydrodynamic Mixing Zone Model and Decision 

Support System for Pollutant Discharges into Surface Waters (1997).......................... 11 

Expert System for Hydrodynamic Mixing Zone Analysis of Conventional and 

Toxic Single Port Discharges (CORMIX1) (1990)....................................................... 12 

Compendium of Tools for Watershed Assessment and TMDL Development (1997) ......... 12 

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Initial Mixing Characteristics of Municipal Ocean Discharges: Volume 1 – Procedures 

and Applications (1985)................................................................................................. 13 

Great Lakes Rule........................................................................................................................... 13 

Final Rule to Amend the Final Water Quality Guidance for the Great Lakes System to 

Prohibit Mixing Zones for Bioaccumulative Chemicals of Concern (2000)................. 13 

Tables 

Comparison of Mixing Zone Documents from EPA Headquarters by Topic ................................ 4 

Major Topics in EPA Headquarters Mixing Zone Documents....................................................... 5 

Appendices 

Appendix A: EPA Regional Documents..................................................................................... A-1 

Appendix B: State Documents.................................................................................................... B-1 

Appendix C: Other Documents to Consider When Establishing Mixing Zones........................ C-1 

Other Resources 

Resources.......................................................................................................................Resources-1 

EPA 823-R-06-003 July 2006 ii
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Purpose

The purpose of this Compilation is two-fold: (1) to provide a source of information for states, 

authorized tribes, and territories to use when developing and refining their mixing zone policies 

and (2) to assist NPDES permit writers when implementing mixing zones. 

This Compilation presents a number of regulatory, guidance, and policy documents available 

from EPA on mixing zones. It also provides information and links to state and EPA regional 

information on this subject. 

Organization of Information

The Compilation is organized as follows: 

• Basic information 

• Technical and policy guidance documents (6 documents) 

• Modeling documents (7 documents) 

• Great Lakes Rule (1 document) 

• Appendices 

o Appendix A: EPA regional documents (3 documents) 

o Appendix B: Reference list of state documents (16 documents) 

o Appendix C: Reference list of other EPA and Federal documents (6 documents) 

• Resources 

A brief overview is included with each document except those in Appendices B and C. 

 

The Compilation is available electronically at 

http://www.epa.gov/waterscience/standards/mixingzone. 

Mixing Zone Definition

According to EPA’s Technical Support Document for Water Quality-based Toxics Control 

(TSD) (USEPA, 1991), “a mixing zone is an area where an effluent discharge undergoes initial 

dilution and is extended to cover the secondary mixing in the ambient waterbody. A mixing zone 

is an allocated impact zone where water quality criteria can be exceeded as long as acutely toxic 

conditions are prevented.” (Water quality criteria must be met at the edge of a mixing zone.) 

Types of Mixing Zones Covered

Allocated impact zone (AIZ): According to Allocated Impact Zones for Areas of NonCompliance (USEPA, 1995), an allocated impact zone is the same as a mixing zone. The term is 

also used in Water Quality Standards Regulation; Proposed Rule (USEPA, 1998). 

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Legal mixing zone (LMZ): Refers to a mixing zone in a regulatory sense (e.g., the dimensions of 

the zone as the state has defined them) as opposed to the mixing zone that naturally occurs in a 

stream. This term is used in Technical Guidance Manual for Performing Waste Load 

Allocations, Book III: Estuaries. Part 3: Use of Mixing Zone Models in Estuarine Waste Load 

Allocations (USEPA, 1992) and CORMIX2: An Expert System for Hydrodynamic Mixing Zone 

Analysis of Conventional and Toxic Multiport Diffuser Discharges (USEPA, 1991). 

Toxic dilution zone (TDZ): According to Technical Guidance Manual for Performing Waste 

Load Allocations, Book III: Estuaries. Part 3: Use of Mixing Zone Models in Estuarine Waste 

Load Allocations (USEPA, 1992), the toxic dilution zone, which is a short distance from the 

outfall or in the pipe itself, is an additional subregion within the usual mixing zone. The TDZ is 

usually more restrictive than the legal mixing zone for conventional and nonconventional 

pollutants. This term is also used in CORMIX2: An Expert System for Hydrodynamic Mixing 

Zone Analysis of Conventional and Toxic Multiport Diffuser Discharges (USEPA, 1991). 

Zone of initial dilution (ZID): According to Technical Guidance Manual for Performing Waste 

Load Allocations, Book III: Estuaries. Part 3: Use of Mixing Zone Models in Estuarine Waste 

Load Allocations (USEPA, 1992), the zone of initial dilution is a regularly shaped area (e.g., 

circular or rectangular) surrounding the discharge structure (e.g., submerged pipe or diffuser 

line) that encompasses the regions of high (exceeding standards) pollutant concentrations under 

design conditions. This term is also used in Initial Mixing Characteristics of Municipal Ocean 

Discharges: Volume 1⎯Procedures and Applications (USEPA, 1985). 

Types of Pollutants Addressed

Toxic pollutants: sometimes referred to as “priority pollutants.” EPA identified 126 pollutants 

from the 65 families of pollutants specified in Section 307(a) of the Clean Water Act. These 

pollutants are listed at 40 CFR Part 423, Appendix A. 

Conventional pollutants: the five pollutants as defined by Section 304(a)(4) of the Clean Water 

Act and listed at 40 CFR 401.16. Those are biochemical oxygen demand (BOD), total suspended 

solids (nonfilterable) (TSS), pH, fecal coliform, and oil and grease. 

Nonconventional pollutants: any pollutant not already defined as a toxic or conventional pollutant. 

Types of Concentrations Discussed

CMC (criterion maximum concentration): the EPA national water quality criteria 

recommendation for the highest instream concentration of a toxicant or an effluent to which 

organisms can be exposed for a brief period of time without causing an acute effect (USEPA, 

1991). The term applies to all pollutant types. 

CCC (criterion continuous concentration): the EPA national water quality criteria 

recommendation for the highest instream concentration of a toxicant or an effluent to which 

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EPA 823-R-06-003 3 July 2006 

organisms can be exposed indefinitely without causing unacceptable effect (USEPA, 1991). The 

term applies to all pollutant types. 

RAC (reference ambient concentration): the concentration of a chemical in water which will not 

cause adverse impacts to human health; RAC is expressed in units of mg/L (USEPA, 1991). 

Code of Federal Regulations (CFR) Language

Section 131.13 general policies. States may, at their discretion, include in their state standards, 

policies generally affecting their application and implementation, such as mixing zones, low 

flows and variances. Such policies are subject to EPA review and approval. 
Comparison of Mixing Zone Documents from EPA Headquarters by Topic

EPA 823-R-06-003 4 July 2006

Technical guidance 

Policy guidance 

Recommends states specify whether 

mixing zones are allowed 

Effluent characterization⎯WET 

Effluent characterization⎯specific 

pollutants 

Effluent characterization⎯

bioconcentratable pollutants 

Application of criteria in mixing zones 

Size and/or area considerations 

Technical and Policy Guidance DocumentsTechnical Support Document for Water Quality-based Toxics Control (USEPA, 1991) 

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Water Quality Standards Handbook: Second Edition (USEPA, 1994) 

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Memorandum: EPA Guidance on Application of State Mixing Zone Policies in EPA-Issued NPDES Permits (USEPA, 1996) 

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Allocated Impact Zones for Areas of Non-Compliance (USEPA, 1995) 

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Technical Guidance Manual for Performing Waste Load Allocations, Book III: Estuaries. Part 3: Use of Mixing Zone Models in Estuarine Waste Load Allocations (USEPA, 1992) 

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U.S. EPA NPDES Permit Writer’s Manual (USEPA, 1996) 

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Modeling Documents Dilution Models for Effluent Discharges, 4th ed. (Visual Plumes) (USEPA, 2003)

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Dilution Models for Effluent Discharges, 3rd ed. (USEPA, 1994)

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CORMIX2: An Expert System for Hydrodynamic Mixing Zone Analysis of Conventional and Toxic Multiport Diffuser Discharges (USEPA, 1991)

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User's Manual for CORMIX: A Hydrodynamic Mixing Zone Model and Decision Support System for Pollutant Discharges into Surface Waters (USEPA, 1997)

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Expert System for Hydrodynamic Mixing Zone Analysis of Conventional and Toxic Single Port Discharges (CORMIX1) (USEPA, 1990)

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Compendium of Tools for Watershed Assessment and TMDL Development (USEPA, 1997)

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Initial Mixing Characteristics of Municipal Ocean Discharges: Volume 1

⎯Procedures and 

Applications (USEPA, 1985)

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Great Lakes RuleFinal Rule to Amend the Final Water Quality Guidance for the Great Lakes System to Prohibit Mixing Zones for Bioaccumulative Chemicals of Concern (USEPA, 2000) 

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z

z The document describes in detail the associated topic; 

 The document provides very little detail concerning the associated topic.
Major Topics in EPA Headquarters Mixing Zone Documents

Technical and Policy Guidance Documents 

Technical Support Document for Water Quality-based Toxics Control 

(USEPA, 1991)

Water Quality Standards Handbook: Second Edition (USEPA, 1994)

Memorandum: EPA Guidance on Application of State Mixing Zone Policies 

in EPA-Issued NPDES Permits (USEPA, 1996)

Allocated Impact Zones for Areas of Non-Compliance (USEPA, 1995)

Technical Guidance Manual for Performing Waste Load Allocations, Book III: 

Estuaries. Part 3: Use of Mixing Zone Models in Estuarine Waste Load 

Allocations (USEPA, 1992)

U.S. EPA NPDES Permit Writer’s Manual (USEPA, 1996)

Modeling Documents 

Dilution Models for Effluent Discharges, 4th ed. (Visual Plumes), 

(USEPA, 2003)

Dilution Models for Effluent Discharges, 3rd ed. (USEPA, 1994) 

CORMIX2: An Expert System for Hydrodynamic Mixing Zone Analysis of 

Conventional and Toxic Multiport Diffuser Discharges (USEPA, 1991)

User's Manual for CORMIX: A Hydrodynamic Mixing Zone Model and 

Decision Support System for Pollutant Discharges into Surface Waters 

(USEPA, 1997)

Expert System for Hydrodynamic Mixing Zone Analysis of Conventional 

and Toxic Single Port Discharges (CORMIX1) (USEPA, 1990)

Application of criteria in mixing zones

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z

Cautions about mixing zones in certain situations (e.g., near fish harvesting)

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Determining WLAs

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Effluent characterization⎯ bioconcentratable pollutants

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Effluent characterization⎯specific pollutants

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Effluent characterization⎯WET

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Lethality to passing organisms

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Mixing zone analyses (e.g., models) 

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Recommends states specify whether mixing zones are allowed

z

z

z

z

Size and/or area considerations

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Special situations: lake, marine, estuarine

z

z The topic is discussed in detail in the associated document; 

 Very little detail about the topic is provided in the associated document

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Technical and Policy Guidance Documents

1) USEPA. 1991. Technical Support Document for Water Quality-based Toxics Control. 

EPA 505-290-001. U.S. Environmental Protection Agency, Office of Water, Washington, 

DC. http://www.epa.gov/npdes/pubs/owm0264.pdf. 

EPA’s 1991 Technical Support Document for Water Quality-based Toxics Control (TSD) 

provides guidance to states and EPA Regions for the water quality-based control of toxic 

pollutants. It includes recommendations for controlling point source discharges of toxic 

pollutants to waters of the United States. 

States and EPA Regions can use this document’s “standards to permits” approach to 

guide water quality protection from development of water quality standards through 

development of compliance monitoring. The standards to permits approach incorporates 

both human health and aquatic toxicity issues and uses an integrated approach to water 

quality-based toxics control. This includes whole effluent and chemical-specific 

approaches and the use of biological assessment to control toxic pollutants. The concept 

of mixing zones is introduced in Chapter 2 (Water Quality Criteria and Standards) of the 

TSD. More detailed guidance is provided in Chapter 3 (Effluent Characterization) and 

Chapter 4 (Exposure and Wasteload Allocation). 

The TSD recommends that mixing zones be designed to avoid lethality to aquatic 

organisms and to ensure that the designated use of the waterbody as a whole is protected. 

The TSD also recommends that states have a definitive statement in their standards on 

whether or not mixing zones are allowed and describe the procedures for defining mixing 

zones consistent with CWA goals. 

2) USEPA. 1994. Water Quality Standards Handbook: Second Edition. EPA 823-B-94-

005a. U.S. Environmental Protection Agency, Office of Water, Washington, DC. 

http://www.epa.gov/waterscience/standards/handbook. 

The second edition of the Water Quality Standards Handbook (the Handbook) is a 

compilation of EPA’s guidance on the water quality standards program and provides 

direction for states in reviewing, revising, and implementing water quality standards. This 

edition incorporates subsequent guidance issued since the 1983 handbook. The Handbook 

is subject to future revisions as the water quality standards program moves forward to 

reflect the needs and experiences of EPA and the states. 

The handbook’s overview of the water quality standards program provides a brief 

discussion of mixing zones, including: 

o How states have the discretion to use mixing zones in their water quality 

standards, subject to EPA approval. 

o How state water quality standards describe methods for determining location, 

size, shape, and other factors of mixing zones. 

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o How states should give careful consideration to the appropriateness of a mixing 

zone depending on the pollutants in the discharge (e.g., bioaccumulative, 

persistent). 

The Handbook provides information on the general provisions of 40 CFR Part 131 

Subpart A, designated uses, water quality criteria, antidegradation, procedures for review 

and revision of water quality standards, and water quality-based approaches to pollution 

control. According to the Handbook, both it and the Technical Support Document for 

Water Quality-based Toxics Control (TSD) (USEPA, 1991) evolved from and supersede 

the following resources: Water Quality Criteria 1968 (the “Green Book”), Water Quality 

Criteria 1972 (the “Blue Book”), Quality Criteria for Water 1976 (the “Red Book”), and 

the first edition of the Water Quality Standards Handbook (1983). 

3) USEPA. 1996. Memorandum: EPA Guidance on Application of State Mixing Zone 

Policies in EPA-Issued NPDES Permits. U.S. Environmental Protection Agency. Memo 

from Robert Perciasepe to Water Program Directors, dated August 6, 1996.

http://www.epa.gov/waterscience/library/wqstandards/mixingguide.pdf. 

EPA’s Guidance on Application of State Mixing Zone Policies in EPA-Issued NPDES 

Permits (the Guidance) is designed for EPA permit writers when EPA administers the 

National Discharge Elimination System (NPDES) for a state. It discusses the 

circumstances under which the EPA permit writer may include mixing zones in NPDES 

permits. Specifically, the Guidance presents policy guidelines for EPA permit writers for 

including mixing zones in EPA-issued permits for a state. The Guidance includes a 

summary of EPA’s water quality standards (WQS) Regulations, which allow states to 

adopt provisions authorizing mixing zones in their water quality standards. 

State WQS regulations addressing mixing zones generally fall into one of two categories. 

Some states have regulations that generically authorize mixing zones without specifying 

who may exercise that authority. Other states have regulations that specifically confer 

discretionary authority to allow mixing zones only on the state agency. The guidance 

details when it is appropriate to interpret the state law to authorize EPA to grant a mixing 

zone. 

4) USEPA. 1995. Allocated Impact Zones for Areas of Non-Compliance. EPA 823-R-95-

003. U.S. Environmental Protection Agency, Office of Water, Washington, DC. 

http://www.epa.gov/waterscience/library/modeling/zones.pdf. 

Allocated Impact Zones for Areas of Non-Compliance (the AIZ document) presents an 

impact allocation procedure. This procedure is an attempt to assess cumulative impacts 

and addresses the potential limitations of state water quality standards mixing zone 

policies. It organizes and manages discharges by including all point source discharges 

within the decision making process. Specifically, this procedure can supplement mixing 

zone policies that might be limited to the cross-sectional or surface area of streams and 

lakes or a uniform linear distance limitation in mixing zone size determinations. For 

example, prior to 1995, some state guidance did not consider multiple source impacts, 

sensitivity of aquatic resources, and socioeconomic factors. In contrast, the impact 

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allocation procedure addresses many of the socioeconomic and ecological factors that can 

be considered in waste management decisions. 

The procedure described in the AIZ document can be used to determine the 

environmentally acceptable size of mixing zones. It defines allocated impacted zones 

(AIZs) and provides a detailed discussion of the AIZ procedure. When using this 

procedure to perform analyses of mixing zones, the results are carefully evaluated for 

reasonableness using prior experience. In addition, the data requirements and 

socioeconomic decisions required to complete all levels of the AIZ procedure are 

extensive, and in most cases, not practically achievable. However, several of the initial 

steps provide a reasonable approach to help state water quality regulators meet designated 

use goals for a waterbody. A detailed discussion of the historical development of mixing 

zone guidance is presented in Appendix A of the document. It also includes several 

example allocation procedures. 

5) USEPA. 1992. Technical Guidance Manual for Performing Waste Load Allocations, 

Book III: Estuaries. Part 3: Use of Mixing Zone Models in Estuarine Waste Load 

Allocations. EPA 823-R-92-004. U.S. Environmental Protection Agency, Office of 

Water, Washington, DC.

http://www.epa.gov/waterscience/library/modeling/wlabook3part3.pdf. 

Technical Guidance Manual for Performing Waste Load Allocations, Book III: Estuaries. 

Part 3: Use of Mixing Zone Models in Estuarine Waste Load Allocations (Part 3) is the 

third of a series of manuals that provide information and guidance for preparing waste 

load allocations. Book I of the series provides general guidance for performing waste 

load allocations. Book II provides guidance specifically directed toward streams and 

rivers, while Book III provides guidance for preparing waste load allocations in estuaries. 

Book III is divided into four parts. Part 1 provides technical information and policy 

guidance for preparing estuarine waste load allocations. Part 2 provides a guide to 

monitoring and model calibration and testing, and a case study tutorial on simulation of 

waste load allocation problems in simplified estuarine systems. Part 3, summarized here, 

describes the initial mixing of wastewater in estuarine and coastal environments and 

mixing zone requirements. Part 3 also details the important physical processes that 

govern the hydrodynamic mixing of aqueous discharges as well as application of 

available models to four case study situations. Part 4 summarizes several historical case 

studies, with critical review by experts. 

Chapter 7 of Part 3, the first chapter of this document, describes initial mixing of 

wastewater in estuarine and coastal waters. It also describes mixing zone definitions and 

mixing zone recommendations, including special ones for toxic substances. Chapter 8 

provides an overview of the important physical processes that govern the hydrodynamic 

mixing of aqueous discharges. The chapter also reviews the mathematical background 

and formulations for different mixing zone models. Chapter 9 illustrates the application 

of jet integral models and of the expert system CORMIX. Four case studies are also 

presented to demonstrate the capabilities and limitations of individual models. 

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6) USEPA. 1996. U.S. EPA NPDES Permit Writer’s Manual. EPA 833-B-96-003. U.S. 

Environmental Protection Agency, Office of Water, Washington, DC. 

http://www.epa.gov/npdes/pubs/owm0243.pdf. 

U.S. EPA NPDES Permit Writer’s Manual (the Manual) provides guidance for writing 

and issuing legally defensible and enforceable National Pollutant Discharge Elimination 

System (NPDES) permits to dischargers, including technical and legal issues that should 

be considered in permitting decisions. The document outlines the minimum requirements 

that all state and regional NPDES permit programs share. The manual offers a variety of 

information, ranging from basic knowledge about what elements should be required in an 

NPDES permit to technical considerations related to the establishment of permit limits. 

As a policy related to water quality-based effluent limits, mixing zones may be 

considered during the permitting process. If a mixing zone is being considered, permit 

writers consider site-specific characteristics of a given discharge in addition to the 

condition of the receiving water to determine the dilution that will occur from the point 

source and to determine the impact that a discharge will have on the receiving water. 

This manual discusses a number of factors that should be considered to assess the fate 

and transport of pollutants and to determine how the mixing zone will affect water 

quality. It also discusses models for assessing mixing zones. 

Modeling Documents

7) USEPA. 2003. Dilution Models for Effluent Discharges, 4th Edition (Visual Plumes). 

EPA/600/R-03/025. U.S. Environmental Protection Agency, Ecosystems Research 

Division, Athens, Georgia. http://www.epa.gov/ceampubl/swater/vplume/VPManual.pdf. 

Dilution Models for Effluent Discharges, 4th Edition (Visual Plumes) describes a mixing 

zone modeling system. Visual Plumes (VP) is a Microsoft Windows-based suite of 

models that supersedes the DOS PLUMES mixing zone modeling system. VP allows 

users to simulate single and merging submerged plumes in arbitrarily stratified ambient 

flow and buoyant surface discharges. Among its additional features are: 

• Graphics 

• Time-series input files 

• User specified units 

• A conservative tidal background-pollutant buildup capability 

• A sensitivity analysis capability 

• A multistressor pathogen decay model that predicts coliform mortality using 

temperature, salinity, solar insolation (the amount of radiation hitting a surface or 

object), and water column light absorption 

VP includes several models intended to encourage the continued improvement of plume 

models. VP also allows modelers to access the superseded DOS PLUMES if the user 

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requires consistency in modeling applications. A time-series file-linking capability 

provides a way to simulate outfall performance over long periods of time. Most effluent 

and ambient variables can be input from files that store data that vary over time. This is 

the heart of the pollutant buildup capability, designed for one-dimensional tidal rivers or 

estuaries to estimate pollution from the source in question. The time-series file linking 

capability is served by summary graphics (i.e., graphics that focus on overall 

performance indicators, like mixing zone dilutions or concentrations). 

8) USEPA. 1994. Dilution Models for Effluent Discharges, 3rd Edition. EPA /600/R94/086. U.S. Environmental Protection Agency, Pacific Ecosystems Branch, ERL-N, 

Newport, Oregon. 

http://www.epa.gov/waterscience/standards/mixingzone/files/RSB_UM_PLUMES.pdf. 

The document describes two initial dilution plume models (RSB and UM) and a model 

interface and manager (PLUMES) for preparing common model input and running the 

models. Two farfield algorithms are automatically initiated beyond the zone of initial 

dilution. In addition, PLUMES incorporates the flow classification scheme of the Cornell 

Mixing Zone Expert System (CORMIX), with recommendations for model usage, 

thereby providing a linkage between two existing EPA systems. The PLUMES models 

are intended for use with plumes discharged to marine and fresh water. Both buoyant and 

dense plumes, as well as single source and multiple diffuser outfall configurations may be 

modeled. The PLUMES software accompanies the document. The use of the model 

interface is explained in detail, including a user’s guide and a detailed tutorial. Other 

examples of RSB and UM usage are also provided. This document contains information 

that is not duplicated in the Visual Plumes version, notably plume modeling theory. Also, 

the software can be used to calculate similarity parameters. 

9) USEPA. 1991. CORMIX2: An Expert System for Hydrodynamic Mixing Zone 

Analysis of Conventional and Toxic Multiport Diffuser Discharges. EPA 600-3-91-073. 

U.S. Environmental Protection Agency, Office of Research and Development, 

Washington, DC. 

http://www.epa.gov/waterscience/standards/mixingzone/files/1991_CORMIX2.pdf. 

CORMIX2: An Expert System for Hydrodynamic Mixing Zone Analysis of Conventional 

and Toxic Multiport Diffuser Discharges describes the Cornell Mixing Zone Expert 

System (CORMIX). CORMIX is a series of software systems that allows users to 

analyze, predict, and design aqueous toxic or conventional pollutant discharges into 

watercourses. It emphasizes the geometry and dilution characteristic of the initial mixing 

zone. CORMIX2 emphasizes rapid initial mixing and assumes no physical, chemical, or 

biological decay processes. 

CORMIX2 models submerged multiport discharges into flowing water environments 

such as rivers, lakes, estuaries, and coastal waters. It includes effects of ambient 

stratification, dynamic attachment of the plume to the bottom of the receiving water, and 

the limiting case of stagnant conditions. This report documents the development and 

implementation of an engineering tool for analyzing submerged multiport diffuser 

discharges into waterbodies with variable and complex conditions. 

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CORMIX2 requires relevant data for the ambient and discharge situations, computes the 

physical parameters, and classifies the given discharge into one of many possible 

hydrodynamic configurations. CORMIX2 then (1) simulates the corresponding 

hydrodynamic simulation for the flow, (2) interprets the results of the simulation relative 

to applicable requirements of a mixing zone, including toxic discharge criteria, and 

(3) suggests possible design alternatives and improvements concerning the mixing 

characteristics. 

The results of CORMIX’s hydrodynamic simulations have been validated and generally 

agree with available field and laboratory data. In particular, CORMIX2 correctly predicts 

highly complex discharge situations involving boundary interactions, internal layer 

formation, buoyant intrusions, and large-scale induced currents in shallow environments, 

all features that are beyond the predictive capabilities of other currently available initial 

mixing models for multiport diffusers. 

10) USEPA. 1997. User's Manual for CORMIX: A Hydrodynamic Mixing Zone Model 

and Decision Support System for Pollutant Discharges into Surface Waters. EPA 

823/B-97-006. U.S. Environmental Protection Agency, Office of Science and 

Technology, Washington, DC. (Originally printed in 1996.) 

http://www.epa.gov/waterscience/models/cormix.html. 

The user’s manual gives a comprehensive description of the CORMIX system and 

provides guidance for assembly and preparation of required input data for the three 

subsystems (CORMIX1, CORMIX2, and CORMIX3). It also delineates ranges of 

acceptability, provides guidance for interpretation and graphical display of system output, 

and illustrates practical system application through several case studies. 

The manual is designed for personnel in environmental management positions who want 

an overview of CORMIX systems capabilities and technical staff needing assistance in 

applications. Chapter II provides a summary of the physical processes of effluent mixing 

and an overview of the regulatory background and practice on mixing zone applications. 

Chapter III explains the general features of the CORMIX system, including summaries of 

(a) predictive capabilities and limitations, (b) overall system structure and method of 

processing information, (c) user interaction, and (d) individual computational elements. 

Detailed guidance on preparing and entering input data, as required by the three 

CORMIX subsystems, is provided in Chapter IV. Chapter V describes system output and 

contains descriptive, quantitative, and graphical information on the predicted effluent 

flow. Chapter VI describes the background and input and output features of both the 

CORJET jet integral model and the far-field plume locator program FFLOCATR. 

Finally, Chapter VII provides information on system availability and user support, as well 

as possible future developments and enhancements. 

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11) USEPA. 1990. Expert System for Hydrodynamic Mixing Zone Analysis of 

Conventional and Toxic Single Port Discharges (CORMIX1). Technical Report 

EPA/600/3-90/012. U.S. EPA Environmental Research Laboratory, Athens, GA. 

http://www.epa.gov/waterscience/standards/mixingzone/files/1990_CORMIX1.pdf. 

 

This document is a technical report for CORMIX1. It describes the development and 

implementation of an engineering tool (CORMIX1) for analysis of submerged single port 

discharges into a stratified or uniform density ambient environment with or without cross 

flow. Chapters of the document provide detailed information about hydrodynamic 

elements of mixing processes, hydrodynamic flow classification, an outline of the 

computer programs in CORMIX1, flow protocols and simulation modules for 

CORMIX1, system evaluation and verification, design case studies showing applications 

of CORMIX1, and conclusions and recommendations. Appendices provide further 

information such as online user advice for data input, flow classifications, and a case 

study. 

12) USEPA. 1997. Compendium of Tools for Watershed Assessment and TMDL 

Development. EPA 841-B-97-006. U.S. Environmental Protection Agency, Office of 

Water, Washington, DC. 

http://www.epa.gov/waterscience/standards/mixingzone/files/1997_Tool_

Compendium.pdf. 

Compendium of Tools for Watershed Assessment and TMDL Development (the 

Compendium of Tools) represents an update to and expansion of a previous EPA 

publication, Compendium of Watershed-scale Model for TMDL Development (EPA 841-

R-92-002). The revised Compendium of Tools broadens the review of models and 

techniques to include receiving water models and ecological assessment techniques and 

models in addition to watershed loading models. 

The Compendium of Tools summarizes available techniques and models that assess and 

predict physical, chemical, and biological conditions in waterbodies, including mixing 

zones for point source discharges. The compendium provides watershed managers and 

other users with helpful information for selecting models that are appropriate to their 

needs and resources. Specifically, this document includes information regarding: 

• A wide range of watershed-scale loading models 

• Field-scale pollutant loading models 

• Receiving water models, including eutrophication/water quality models, toxics 

models, and hydrodynamic models 

• Integrated modeling systems that can be used to link watershed-scale loading with 

receiving water processes 

• Ecological techniques and models that can be used to assess or predict the status 

of habitat, single species, or biological community 

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The Compendium of Tools contains short descriptions of near-field models developed for coastal 

areas, rivers, and streams, including CORMIX, PLUME, PC-VirGIS, GISPLS, WSTT, LWWM, 

and BASINS. 

13) USEPA. 1985. Initial Mixing Characteristics of Municipal Ocean Discharges: 

Volume 1⎯Procedures and Applications. EPA 600385073a. U.S. Environmental 

Protection Agency, Washington, DC. 

http://www.epa.gov/waterscience/standards/mixingzone/files/1985_Municipal_Ocean

_Discharges.pdf. 

Initial Mixing Characteristics of Municipal Ocean Discharges: Volume 1—Procedures 

and Application (the Ocean Discharges document) describes the behavior of plumes 

generated when municipal wastewater is discharged into the open ocean). Volume I 

contains analytical solutions and descriptions of five mathematical models that address a 

variety of discharge, diffuser, and receiving water characteristics. Model output includes 

rise height and initial dilution. The Ocean Discharges document provides guidance for 

the range of values within which analytical solutions provide acceptable estimates. The 

format of model input data is the same for all five computer programs. 

Great Lakes Rule

14) USEPA. 2000. Final Rule to Amend the Final Water Quality Guidance for the Great 

Lakes System to Prohibit Mixing Zones for Bioaccumulative Chemicals of Concern. U.S. 

Environmental Protection Agency. 40 CFR Part 132 Federal Register, November 13, 

2000, 65:67638. http://www.epa.gov/fedrgstr/EPA-WATER/2000/November/

Day-13/w28709.htm. 

Final Rule to Amend the Final Water Quality Guidance for the Great Lakes System to 

Prohibit Mixing Zones for Bioaccumulative Chemicals of Concern (the Final Rule) was 

written to revise EPA’s 1995 Final Water Quality Guidance for the Great Lakes System. 

The original 1995 rule was challenged in court and, although the court upheld most of the 

provisions contained in the 1995 rule, it remanded the provisions that would have 

eliminated mixing zones for bioaccumulative chemicals of concern (BCCs). The court 

held that EPA failed to address whether the measure was cost-justified and remanded the 

issue to EPA. 

In response, EPA reexamined the factual record and cost analyses and published a 

proposal to amend the 1995 rule to reinstate the provision to prohibit mixing zones for 

BCCs. This provision (Procedure 3C) is described in more detail in Appendix F of the 

rule. After reviewing and analyzing information in the rule-making record and public 

comments on the Proposal to Amend the Final Water Quality Guidance for the Great 

Lakes System to Prohibit Mixing Zones for Bioaccumulative Chemicals of Concern

(published in the Federal Register on October 4, 1999), EPA finalized the rule in 2000. 

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Appendix A: EPA Regional Documents 

Note: No guidance has been developed specifically by Regions 1, 2, 3, 5, 6, 7, 9, and 10. 

USEPA. 1995. EPA Region VIII Mixing Zones and Dilution Policy. U.S. Environmental 

Protection Agency, Region 8, Water Management Division, Denver, CO. 

http://www.epa.gov/waterscience/standards/mixingzone/files/1995_Reg_8_MZ_and

_Dilution_Policy.pdf. 

The objective of Region VIII Mixing Zones and Dilution Policy is to help states and 

Indian tribes upgrade methods for deriving water quality-based permit limits, improve the 

technical defensibility of National Pollutant Discharge Elimination System (NPDES) 

permits, and reduce risks associated with mixing zone and dilution practices. The basis 

for the policy was the desire of the region to clarify the approaches for using mixing 

zones in NPDES permits. Prior to the policy, some applications of mixing zones 

presumptively provided the entire stream flow for dilution, which often resulted in 

effluent plumes (with elevated pollutant concentrations) extending far downstream of the 

discharge. The clarifications sought to prevent applications of mixing zones that did not 

adequately control effluent plume size or quality, possibly posing considerable risk to 

sensitive downstream uses. 

USEPA. 1993. EPA Region VIII Interim Guidance: Water Quality Standards for Indian 

Tribes. U.S. Environmental Protection Agency, Region 8, Water Management Division, 

Denver, CO. http://epa.gov/region8/water/wqs/GUIDANCE.pdf. 

EPA Region VIII Interim Guidance: Water Quality Standards for Indian Tribes is 

intended to assist the Indian tribes in EPA Region 8 to qualify to administer water quality 

standards programs and to develop water quality standards pursuant to Clean Water Act 

(CWA) sections 518 and 303(c). The guidance includes recommendations on mixing 

zone policy development. 

USEPA. 1980. EPA Region IV Guidance on Mixing Zones. U.S. Environmental 

Protection Agency, Region 4, Atlanta, GA. 

http://www.epa.gov/waterscience/standards/mixingzone/files/1980_Reg_4_MZ_

Guidance.pdf. 

EPA Region IV Guidance on Mixing Zones provides a basis for Region 4 review and 

approval of state use of mixing zones in the development of effluent limitations. The 

guidance provides detailed descriptions of (1) appropriateness of assigning a mixing zone 

to a discharger, (2) the level of water quality that should be maintained in a mixing zone 

and surrounding waters, and (3) the factors governing the size and shape of a mixing 

zone. 

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Appendix B: State Documents 

California Environmental Protection Agency. 2005. Policy for Implementation of Toxics 

Standards for Inland Surface Waters, Enclosed Bays, and Estuaries of California. 

California Environmental Protection Agency, State Water Resources Control Board. 

http://www.waterboards.ca.gov/iswp/docs/final.pdf. Accessed July 2006. 

Colorado Department of Public Health and Environment. 2002. Colorado Mixing Zone 

Implementation Guidance. Colorado Department of Public Health and Environment, 

Water Quality Control Division. 

http://www.cdphe.state.co.us/op/wqcc/Guidance/comixzoneimplguid.pdf. Accessed 

July 2006. 

Massachusetts Department of Environmental Protection. 1993. Massachusetts Surface 

Water Quality Standards Implementation Policy for Mixing Zones. Massachusetts 

Department of Environmental Protection, Boston, MA. 

http://www.epa.gov/ost/wqs/ma/ma_1_mswqs.pdf. Accessed July 2006. 

New York State Department of Environmental Conservation. 1996. Total Maximum 

Daily Loads and Water Quality Based Effluent Limits. Memo dated July 8, 1996. 

Reissued February 26, 1998. 

http://www.dec.state.ny.us/website/dow/togs/togs131.pdf. Accessed July 2006. 

North Carolina Department of the Environment and Natural Resources. 1999. Mixing 

Zones in North Carolina. North Carolina Department of the Environment and Natural 

Resources, Division of Water Quality. Available online: 

http://h2o.enr.state.nc.us/NPDES/documents/mixing.pdf. Accessed July 2006. 

Oregon Department of Environmental Quality. 2000. A Mixing Zone Guidance 

Document Prepared for the Oregon Department of Environmental Quality. 

S. Schnurbusch. Portland State University. 

http://www.esr.pdx.edu/ESR/docs/mem/schnurbusch.pdf. Accessed July 2006. 

South Carolina Department of Health and Environmental Control. 1997. Groundwater 

Mixing Zone Application Guidance. South Carolina Department of Health and 

Environmental Control, Columbia, SC. 

http://www.scdhec.gov/water/pubs/mixingzone.pdf. Accessed July 2006. 

South Dakota Department of Environmental and Natural Resources. 1998. Mixing Zone 

and Dilution Implementation Procedures. South Dakota Department of Environmental 

and Natural Resources. 

http://www.epa.gov/ost/standards/wqslibrary/sd/sd_8_zone_dil.pdf. Accessed 

July 2006. 

EPA 823-R-06-003 B-1 July 2006 
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Utah Division of Water Quality. 2000. Mixing Zone Implementation Procedure. Utah 

Division of Water Quality. 

http://www.epa.gov/waterscience/standards/wqslibrary/ut/ut_8_wq.pdf. Accessed 

July 2006. 

Virginia Department of Environmental Quality. 2000. Guidance Memo No. 00-2011; 

Guidance on Preparing VPDES Permit Limits. Memo from Larry G. Lawson to Regional 

Directors, dated August 24, 2000. Virginia Department of Environmental Quality, 

Richmond, VA. http://www.deq.state.va.us/waterguidance/pdf/002011.pdf. Accessed 

July 2006. 

Virginia Department of Environmental Quality. 2004. Guidance Memorandum 

No. 04-2021; Guidance for Exceptional State Waters Designations in Antidegradation 

Policy Section of Virginia Water Quality Standards Regulation (9 VAC 25-260-30.A.3). 

Memo from Ellen Gilinsky to Regional Directors, dated November 15, 2004. Virginia 

Department of Environmental Quality, Richmond, VA. 

http://www.townhall.state.va.us/Utils/DisplayContent.cfm?fileName=E%3A%5Ctownh

all%5Cdocroot%5CGuidanceDocs%5C440%5CGDoc%5FDEQ%5F2553%5Fv1%2Epdf. 

Accessed July 2006. 

Washington Department of Ecology. No date. Guidance for Conducting Mixing Zone 

Analyses. Washington Department of Ecology, Olympia, WA. 

http://www.ecy.wa.gov/programs/eap/mixzone/mixzone.doc. Accessed July 2006. 

Washington Department of Ecology. 2005. Water Quality Program Permit Writer’s 

Manual. Washington Department of Ecology, Olympia, WA. 

http://www.ecy.wa.gov/pubs/92109.pdf. Accessed July 2006. 

West Virginia Division of Environmental Protection. 1997. Water Quality 

Standards/Mixing Zones Implementation Guidance. West Virginia Division of 

Environmental Protection Office of Water Resources. 

http://www.epa.gov/ost/wqs/wv/wv_3_wqs.pdf. Accessed July 2006. 

Wisconsin Department of Natural Resources. 1992. Mixing Zone Guidance for Chronic 

Toxicity and Zones of Initial Dilution. Wisconsin Department of Natural Resources, 

Modeling and Analysis Unit, Bureau of Water Resources Management. 

http://www.epa.gov/ost/wqs/wi/wi_5_mixin_zone.pdf. Accessed July 2006. 

Wyoming Department of Environmental Quality. 2001. Wyoming Surface Water 

Quality Standards, Implementation Policies for Antidegradation, Mixing Zones, 

Turbidity, and Use Attainability Analysis. Wyoming Department of Environmental 

Quality. 

http://deq.state.wy.us/wqd/watershed/surfacestandards/Downloads/Standards/

11968-doc.pdf. Accessed July 2006.

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Appendix C: Other Documents to Consider When Establishing 

Mixing Zones* 

*Note: these documents are often referred to in mixing zone guidance, but they have been updated or 

superseded by newer guidance documents and rules. 

FWPCA. 1968. Water Quality Criteria (Green Book). Federal Water Pollution Control 

Administration. Report of the National Technical Advisory Committee to the Secretary 

of the Interior. U.S. Department of Interior, Washington DC. (Superseded by USEPA, 

1994.) 

NAS/NAE. 1973. Water Quality Criteria 1972 (Blue Book). A Report of the Committee 

on Water Quality Criteria. National Academy of Science and National Academy of 

Engineers, Washington, DC. NTIS-PB 236199. USGPO #5501-00520. (Superseded by 

EPA, 1994.) 

USEPA. 1976. Quality Criteria for Water (Red Book). U.S. Environmental Protection 

Agency, Washington, DC. 

USEPA. 1986. Quality Criteria for Water (Gold Book). EPA 440586001. U.S. 

Environmental Protection Agency, Office of Regulations and Standards, 

Washington, DC. 

USEPA. 1987. Permit Writers Guide to Water Quality-based Permitting for Toxic 

Pollutants. EPA 440487005. U.S. Environmental Protection Agency, Office of Water, 

Washington, DC. (Rescinded) 

USEPA. 1995. Final Water Quality Guidance for the Great Lakes System; Final Rule. 

U.S. Environmental Protection Agency. Federal Register, March 23, 1995, 60:15366. 

(Superseded by the 2000 Final Rule) 

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Resources 

Models

Cornell Mixing Zone Expert System (CORMIX) 

The Cornell Mixing Zone Expert System (CORMIX) is a mixing zone model and decision 

support system for environmental impact assessment of regulatory mixing zones 

resulting from continuous point source discharges. CORMIX emphasizes the role of 

boundary interaction to predict steady state mixing behavior and plume geometry. 

http://www.epa.gov/waterscience/models/cormix.html (EPA Office of Water) 

http://www.cormix.info (CORMIX Home Page) 

Visual Plumes (VP) 

Visual Plumes (VP) is a windows-based computer application that supersedes the DOSbased version, called simply PLUMES. VP simulates single and merging submerged 

aquatic plumes in arbitrarily stratified ambient flow and buoyant surface discharges. 

http://www.cee.odu.edu/model/visual_plume.php (VP Home Page) 

Training

Water Quality Standards Academy 

The “Water Quality Standards Academy” is best known for the “Basic Course,” which is an 

introductory course designed for those with fewer than six months experience with water 

quality standards and criteria programs. However, others may benefit from the course, 

including veterans of the water quality standards program who want a refresher course. 

http://www.epa.gov/ost/standards/academy.html

NPDES Permit Writers' Training Course

The objective of the NPDES Permit Writers' Training Course is to provide the basic 

regulatory framework and technical considerations that support the development of 

wastewater discharge permits as required under the National Pollutant Discharge 

Elimination System (NPDES) Permit Program. The course is designed for new permit 

writers, highlighting the process of developing, issuing and complying with NPDES 

permits. The format of the course is a combination of lecture, case examples, and 

practical exercises that are geared to acquaint participants with the tools and resources 

available to assist them in writing NPDES permits. 

http://cfpub.epa.gov/npdes/courses.cfm?program_id=0&outreach_id=1&o_type=1 

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Links to NPDES Permitting Information

Overview of the Water Quality Standards-to-Permits Process 

http://cfpub.epa.gov/npdes/wqbasedpermitting/wqoverview.cfm

Water Quality and Technology-Based Permitting 

http://cfpub.epa.gov/npdes/generalissues/watertechnology.cfm?program_id=45

EPA 823-R-06-003 Resources-2 July 2006