EPA Region VIII
Mixing Zones and Dilution Policy
December, 1994
Updated September 1995
o^tDSr%
U.S. Environmental Protection Agency
Region VIII
Water Management Division (8WM)
999 18th Street, Suite 500
Denver, CO 80202-2466
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00
UNITED STATES ENVIRONMENTALfPROTECTION AGENCY
REGION VIII
999 18th STREET - SUITE 500
DENVER. COLORADO 80202-2466
DEC 2 9 I994
Subject: Transmittal of Mixing Zones & Dilution Policy
Dear Colleague,
I am pleased to enclose for your information and use a document titled EPA
Region VIII Mixing Zones and Dilution Policy. You may recall that two previous drafts of
this policy (dated August 13, 1993 and January 14, 1994) were distributed for comment.
The enclosed final policy responds to the comments received on the two drafts (see Appendix
B) and incorporates a number of changes that both alter and clarify the policy.
The overall objectives of the policy are to help States and Indian Tribes upgrade
current methods for deriving water quality-based permit limits, improve the technical
defensibility of NPDES permits, and reduce risks associated with mixing zone and dilution
practices. The basis for the policy is the Region's belief that the current approach of
presumptively providing the entire low flow for dilution often results in effluent plumes (with
elevated pollutant concentrations) extending far downstream of the discharge. This current
approach does not adequately control effluent plume size or quality and may pose
considerable risk to sensitive downstream uses.
Implementation of this policy will be a high priority for EPA during upcoming state
and tribal water quality standards triennial reviews. However, the Region recognizes that
there are other important tasks facing States and Tribes (e.g., antidegradation procedures,
wetland standards, biological criteria). The Region plans to work with each State and Tribe
individually to implement this policy in a timely manner along with all other priority water
quality standards initiatives. When completed, state and tribal implementation of the policy
will consist of the following: (1) adoption of necessary revisions to the state or tribal mixing
zone policy; (2) development of a detailed mixing zone and dilution implementation
procedure; and (3) clear resolution of each of the issues discussed in Chapter 2 of the policy.
In the past several years, Region VIII States have taken huge strides by adopting
numeric, chemical-specific criteria for toxic pollutants. Likewise, a number of Indian Tribes
will soon be establishing such criteria for reservation waters. It is my hope that this policy
will be useful in establishing improved procedures for the critical task of translating these
water quality criteria into effluent limitations for point source discharges.
Sincerely,
Max H. Dodson, Director
Water Management Division
Enclosure
o
Printed on Recycled Paper
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FORWARD
The Clean Water Act provides a clear mandate to the U.S. Environmental Protection
Agency (EPA), States and Indian Tribes to develop and implement water quality standards
for surface waters. Within the last five years, all States located within EPA Region vm
have significantly increased the number of specific substances that are the subject of numeric
water quality criteria. These efforts promise to greatly improve the level of water quality
protection afforded to surface waters. Although establishing appropriate water quality
criteria is a difficult and ongoing task that must respond to the latest scientific information,
equally important is the process of translating ambient criteria into technically-defensible total
maximum daily loads and water quality-based permit limits under the National Pollutant
Discharge Elimination System. Such standards implementation efforts must also be subjected
to periodic evaluation so that the procedures followed are consistent with the latest and best
available methods.
This document provides technical guidance and EPA Region VIII policy regarding one
particular aspect of water quality standards implementation: establishment of mixing zone
and dilution requirements. It was issued in support of EPA regulations. This document does
not establish or affect legal rights or obligations. It does not establish a binding norm and is
not finally determinative of the issues addressed. Agency decisions in any particular case
will be made by applying both federal and state/tribal requirements on the basis of specific
facts when permits are issued or water quality standards or other regulations are
promulgated.
This document may be revised in the future. Comments from users are welcome.
Send comments to the U.S. EPA Region Vm, Water Management Division. 999 18th Street,
Suite 500, Denver, CO 80202-2466.
This issue of the EPA Region VD1 Mixing Zones and Dilution Policy has been updated
to reflect some changes in Chapter 4 and Appendix A. In particular, changes have been
made in EPA Region Vin's mixing zone model STREAMIX I based on comments
received over the last year. Accordingly, the description and mathematical derivation for
STREAMIX I found in the Policy has been updated. The changes incorporated in the
STREAMIX I (version 2) code include changes to the calculation of extreme (e.g.
maximum) concentrations within the mixing zone. This change will have very little
effect, however, in situations where the effluent flow (Qeff) is much smaller than the
receiving water flow (Qup).
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EPA Region VHI ' r^,(
**' /a
Mixing Zones and Dilution Policy ~V ,
TABLE OF CONTENTS
EXECITITVE SUMMARY 1
CHAPTER 1. INTRODUCTION 4
CHAPTER 2. MIXING ZONE AND DILUTION ISSUES 8
CHAPTER 3. MODEL POLICY 17
CHAPTER 4. MODEL IMPLEMENTATION PROCEDURE 21
CHAPTER 5. CASE EXAMPLES 36
APPENDICES
APPENDIX A - EPA REGION Vffl
SIMPLIFIED MIXING ZONE MODEL A-l
APPENDIX B - EPA REGION VIE RESPONSES TO MAJOR
COMMENTS AND QUESTIONS B-l
APPENDIX C - STATE, TRIBAL AND EPA MIXING ZONE AND
DILUTION IMPLEMENTATION METHODS C-l
APPENDIX D - ALTERNATIVE PROCEDURES FOR CHEMICAL-
SPECIFIC ACUTE CRITERIA IN
INCOMPLETELY-MIXED SITUATIONS D-l
APPENDIX E- RECOMMENDED CRITICAL LOW FLOWS E-l
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EPA Region Vm
Mixing Zones and Dilution Policy
EXECUTIVE SUMMARY
Under EPA Clean Water Act (CWA) regulations, States
and Indian Tribes may designate a mixing zone or provide a
dilution allowance when setting water quality-based permit limits
for point source discharges, provided that an appropriate
authorizing policy is included in the state or tribal water quality
standards. In either case, relief is provided to the permittee by
allowing the discharge to mix with the receiving waterbody
before attainment with water quality criteria is required. Within
a mixing zone, for example, certain water quality criteria
otherwise applicable to the waterbody may be exceeded. Where
a discharge mixes with the receiving waterbody very rapidly, a
mixing zone analysis need not be completed, and a dilution
allowance based on the critical low flow of the receiving water
may be provided.
Although most EPA Region Vm States have a mixing
zone policy included in their water quality standards regulation,
a true mixing zone approach is generally not followed in developing water quality-based
permit limits. Instead, EPA Region VIE States typically follow a simplified mass balance
approach that effectively provides the entire critical low flow as a dilution allowance in
calculating the permit limit, regardless of the rate of mixing. Some States have recently
adapted this practice by applying chronic as well as acute critical low flows to accommodate
the two-tiered water quality standards scheme of chronic and acute standards.
Providing such dilution allowances, without considering how quickly the discharge
actually mixes with the receiving waterbody, can result in discharge-receiving water mixtures
that are considerably in excess of criteria far downstream of the discharge. High pollutant
concentrations in such effluent plumes represent a threat to designated and existing uses
(e.g., drinking water intakes, recreational areas, and aquatic life spawning and nursery
areas). For discharges with average physical characteristics and a typical outlet structure
design, EPA Region VHI believes that such effluent plumes normally are present, as such
discharges simply do not mix rapidly with receiving waters. Where there are multiple
discharges to a waterbody, overlapping effluent plumes pose especially significant ecological
and human health risks. However, properly implementing a mixing zone approach controls
the size and quality of effluent plumes, consistent with state or tribal water quality standards
requirements, and reduces risks to aquatic life and human health.
States and
Indian Tribes
may designate
a mixing zone
or provide a
dilution
allowance
when setting
water quality-
based permit
limits.
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This policy statement on mixing zones and dilution is intended to help EPA Region
Vm States and Tribes upgrade current methods for deriving water quality-based permit
limits, improve the technical defensibility of NPDES permits, and limit the environmental
risks posed by effluent plumes. Chapter 2 identifies the specific mixing zone and dilution
implementation issues that most concern EPA Region VEL For these issues, States and
Tribes will need to consider the range of technically-defensible approaches and then select
and document the approach that will be used in deriving water quality-based permit limits.
This will entail:
• establishing methods to determine if a discharge mixes completely or incompletely;
• developing criteria to limit the size of mixing zones;
• selecting methods to derive effluent limits to achieve mixing zone size restrictions;
• adopting minimum in-zone quality requirements for mixing zones;
• implementing the "no acute lethality" requirement for mixing zones;
• identifying dilution allowances to be granted where complete mixing occurs;
• specifying situations where a mixing zone or dilution allowance may be denied;
• explaining any differences in implementation of chemical-specific and whole effluent
toxicity requirements; and
• affirming state/tribal authority to re-evaluate mixing zone and dilution decisions based
on new information.
The remainder of the policy statement provides
guidance and recommendations on implementation of these
key issues. Chapter 3 provides a model mixing zone-
dilution policy that includes recommended policy language
on each of the key issues. Chapter 4 provides a specific
model implementation procedure that is consistent with, and
could be used to implement, the model policy statement in
Chapter 3. Note that on each of the key implementation
issues, the Region's model approach represents one option
in a range of acceptable options. Chapter 5 includes
discussion of particular case examples that examine how use
of the Region's model implementation procedure would
affect existing permit limits. Finally, the appendices
provide additional guidance and background information on
the basis for the Region's recommended policies and
procedures, including Regional responses to the major comments and questions that were
received on various drafts of this policy statement.
Implementation of this policy statement will be a high priority for EPA in working
with States and Tribes during upcoming water quality standards triennial reviews. However,
the Region recognizes that most States face a variety of high-priority needs with regard to
revising water quality standards and establishing implementation procedures. These priorities
include, for example, establishing antidegradation implementation procedures, developing
Implementation of
this policy
statement will be a
high priority for
EPA in working
with States and
Tribes during
upcoming water
quality standards
triennial reviews.
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water quality standards for wetlands, updating chemical-specific numeric criteria for toxic
pollutants, developing biological criteria, and ensuring water quality protection for threatened
and endangered species. Accordingly, the Region plans to continue to work with each State
and Tribe individually to develop and adopt all needed water quality standards revisions in a
timely manner and in an appropriate sequence. Because States and Tribes have varying
needs, implementation of this policy may not occur at the same time throughout the Region.
Thus, because not all States and Tribes will be implementing this policy immediately, where
a State or Tribe has not established a procedure, or has not documented a clear approach to a
particular issue, the model implementation procedure in Chapter 4 of this policy statement
will be considered in the interim as the Region's preferred method of making mixing zone
and dilution decisions. This policy may therefore influence individual NPDES permits issued
by the Region or certain State-issued NPDES permits. The model policy and procedure will
also be a likely starting point for the Region in developing federal replacement requirements,
should disapproval of state or tribal water quality standards become necessary.
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EPA Region VIII Mixing Zones and Dilution Policy
CHAPTER 1.
INTRODUCTION
A mixing zone is an area surrounding or downstream
of a point source discharge where the effluent plume is
progressively diluted by the receiving water and certain
numerical criteria otherwise applicable to the segment may be
exceeded. A mixing zone analysis is typically incorporated
into the derivation of total maximum daily loads (TMDLs)
and water quality-based National Pollutant Discharge
Elimination System (NPDES) permit limits where point
source discharges mix in an incomplete manner with
receiving waters. The mixing zone analysis is developed to
ensure compliance with mixing zone requirements (such as
size and in-zone quality requirements) that are included in the
applicable state or tribal standards. A mixing zone provides
relief to the permittee in that compliance with certain criteria
is not required within the zone. However, a mixing zone is
granted on a parameter-by-parameter basis, and for some
parameters or criteria, compliance may be required at the end-
of-pipe. Where a discharge mixes with the receiving water in a rapid and complete
manner, a mixing zone analysis is not needed and a dilution allowance based on critical low-
flow conditions may be incorporated into the derivation of TMDLs and NPDES permits.
A mixing zone
analysis is
developed to
ensure
compliance with
mixing zone
requirements that
are included in
the applicable
state or tribal
standards.
COMMON MIXING ZONE AND DILUTION PRACTICES
Situation
Slow or "Incomplete" Mixing
Rapid and "Complete" Mixing
No Dilation Available
(Low Flow = Zero)
Approach
Mixing zone analysis is required to
determine the allowable dilution.
Dilution up to critical low flow may
be allowed.
Achieve criteria at end-of-pipe.
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EPA Region VIII Mixing Zones and Dilution Policy
State/tribal mixing zone-dilution policies should establish, in language readily
understood by the public, the situations in which a mixing zone or dilution allowance may be
authorized. To facilitate implementation, such policies should also define the situations in
which a mixing zone or dilution allowance may be limited or denied. State/tribal mixing
zone-dilution implementation procedures should establish the specific methods, guidelines,
and approaches that will be followed in implementing the state/tribal mixing zone-dilution
policy. The procedures should be explained with a sufficient level of detail to ensure
consistency when used to derive point source discharge permit limits. Mixing zone-dilution
procedures should clearly identify the issues and decisions that are left to the discretion and
best professional judgment of the permit writer. However, to the extent appropriate, the
procedure should specify a particular approach to promote consistency.
Unfortunately, it is not possible to establish a wholly
deterministic procedure (i.e., a "black box") with which to make
all mixing zone-dilution decisions. Nor is it advisable to make
all mixing zone-dilution decisions based on a simplistic approach
which overlooks the mixing characteristics and waterbody uses
(e.g., fish spawning, drinking water supply) particular to the
site. To appropriately address site-specific environmental
concerns and the mixing characteristics of individual discharges,
mixing zone-dilution decisions should be made on a case-by-case
basis using the best factual information available at the time of
the decision. However, as a practical matter, sufficient
information and resources are not always available to fully
characterize mixing and localized waterbody uses. Accordingly,
mixing zone-dilution policies, and particularly the procedures
used to implement such policies, should clearly set forth the
considerations, guidelines, and default assumptions that will be
utilized in making such case-by-case decisions.
Under EPA's water quality standards regulation, States and Indian Tribes may adopt
policies authorizing the use of mixing zones in setting TMDLs and water quality-based
permit limits (see 40 CFR 131.13). Pursuant to federal regulation, the decision regarding
whether to allow mixing zones is made by the individual States/Tribes (i.e., States/Tribes
may elect to allow or to prohibit mixing zones for purposes of water quality based permit
limits). Where a State/Tribe elects to allow mixing zones, the State/Tribe must include an
; 'Mrizing policy in their water quality standards regulation. Such muting zone policies are
xt to EPA review and approval. States and Tribes must also establish procedures to be
ii wowed in implementing their mixing zone policies. Such mixing zone procedures also are
subject to EPA review and approval and should be incorporated into the narrative toxics
criterion implementation procedure required by the Federal water quality standards regulation
Mixing zone
and dilution
decisions
should be made
on a case-by-
case basis
using the best
information
available at the
time of the
decision.
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EPA Region VIII Mixing Zones and Dilution Policy
(see 40 CFR 131.11(a)(2)). State/tribal decisions regarding mixing zones for individual
discharges are subject to EPA review through the NPDES permits process.
To assist States and Tribes in establishing appropriate mixing zone and dilution
policies and procedures, EPA-Headquarters has periodically issued technical guidance on this
topic. National EPA guidance can be found in the Technical Support Document for Water
Quality Based Toxics Control (1991), the Water Quality Standards Handbook (1983 and
1993), and Quality Criteria for Water ("the Red Book", 1976). Other sources of
information and guidance include Water Quality Criteria 1972 ("the Blue Book", National
Academy of Sciences).
Purpose and Objectives
A primary purpose of this policy statement is to assist
the States and Indian Tribes in EPA Region VIII to establish
technically-defensible (and thus approvable) mixing zone and
dilution policies and implementation practices. The policy
statement identifies the particular mixing zone and dilution
issues that will most directly influence EPA Region VIE
approval decisions (see Chapter 2). The policy statement also
includes a model policy and procedure that States and Tribes
can adopt as their own, with or without modification (see
Chapters 3 and 4). The model policy and procedure are
included as separate items because, although both are required
elements of state/tribal water quality programs, incoiporating
the entire text of the mixing zone-dilution procedure into the
state/tribal water quality standards regulation is optional.
States/Tribes may, instead, include such detailed procedures
in their standards by reference in order to allow for periodic updates and improvements to
such procedures without going through a rulemaking action (i.e., where allowed under
state/tribal rulemaking requirements). Where such procedures are adopted by reference, they
must be included in any triennial review package submitted to EPA Region Vm for review
and approval.
The Region's intent is that this policy will result in two basic types of improvements
to existing mixing zone and dilution practices. First, it is the Region's intent to reduce the
environmental risks posed by mixing zones. This objective is embodied in the recommended
policy of requiring all point source discharges to comply with acute aquatic life criteria at the
end-of-pipe. Such an approach would result in rapid and Region-wide reduction in the risks
posed by mixing zones. Second, it is the Region's intent to eliminate unacceptable
environmental risks in specific cases. This goal resulted, for example, in the recommended
A primary
purpose of this
policy statement
is to assist States
and Tribes to
establish
technically-
defensible mixing
zone and dilution
practices.
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EPA Region VIII Mixing Zones and Dilution Policy
policy of requiring end-of-pipe compliance with applicable criteria where site-specific factors
support such an approach (e.g., where, in a specific case, there is a drinking water intake a
short distance downstream of a discharge). Thus, the Region's policy is intended to promote
more judicious use of mixing zone authority generally and to encourage elimination of
existing mixing zones where there is sufficient evidence that such mixing zones pose
unacceptable environmental risks.
As noted above, state and tribal implementation of this policy statement will serve the
additional purpose of satisfying, in part, the requirement in Section 131.11 of the Federal
water quality standards regulation for a narrative toxics criterion implementation procedure.
The recommended components of such implementation procedures have been discussed in
prior guidance issued by the Region (see January 17, 1990 Regional letter sent to each state
Water Division Director, Compliance with the Toxics Requirements of Section 303(c)(2)(B)
of the Clean Water Act and the Water Quality Standards Regulation (40 CFR 131.11),
copies available from the Region) and Chapter Two of the recent update to the Technical
Support Document for Water Quality Based Toxics Control (TSD) (see pp. 31-32).
The specific objectives of this policy statement on mixing zones and dilution are to:
(1) Supplement existing EPA policy and guidance in this area. Although extensive
EPA guidance has been issued in the past, that guidance has typically described a
range of acceptable approaches without clearly delineating a recommended approach.
This policy statement is intended to fill that gap.
(2) Identify the particular mixing zone and dilution issues that will most directly
influence EPA approval/disapproval decisions. In order to minimize the potential
for EPA disapproval of water quality standards, this policy provides advance notice of
the particular issues for which States and Tribes will need to document a clear
approach in order for EPA Region vm to consider the federal requirements satisfied.
(3) Establish a common framework to promote consistency among Region Vm States
and Tribes on mixing zone-dilution issues. Although the Region intends to allow
each State and Tribe the flexibility to customize their approach to mixing zones and
dilution, this policy statement is also intended to promote consistency on key issues
and to ensure that certain minimum elements are addressed by all States and Tribes.
(4; Promote implementation of policies and procedures that will appropriately
minimize the size and impacts of mixing zones in surface waters. Although EPA
regulations allow the use of mixing zones, it is important to remember that mixing
zones are basically allocated impact zones. As such, mixing zones should be
carefully limited or eliminated to ensure protection of aquatic life and human health.
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EPA Region VIII Mixing Zones and Dilution Policy
CHAPTER 2. MIXING ZONE AND DILUTION ISSUES
To assist States and Indian Tribes in establishing
mixing zone and dilution practices that are approvable under
CWA Section 303(c) and EPA's water quality standards
regulation (40 CFR 131), this chapter of the policy statement
identifies and discusses the particular issues that States and
Tribes will need to address and clearly resolve in their mixing
zone and dilution policies and implementation procedures.
For each of the issues, there is a range of approaches that
may be used. The particular approach that is recommended
by the Region is discussed briefly below and incorporated into
the model policy and implementation procedure presented in
Chapters 3 and 4, respectively. However, States and Tribes
may deviate from the Region's recommended approach as
long as each of the issues is clearly resolved and the approach
selected is documented and technically defensible.
Implementation of this policy statement will be a high
priority for EPA in working with States and Tribes during upcoming water quality standards
triennial reviews. However, the Region recognizes that most States face a variety of high-
priority needs with regard to revising water quality standards and establishing implementation
procedures. These priorities include, for example, establishing antidegradation
implementation procedures, developing water quality standards for wetlands, updating
chemical-specific numeric criteria for toxic pollutants, developing biological criteria, and
ensuring water quality protection for threatened and endangered species. Accordingly, the
Region plans to continue to work with each State and Tribe individually to develop and adopt
all needed water quality standards revisions in a timely manner and in an appropriate
sequence. Because States and Tribes have varying needs, implementation of this policy may
not occur at the same time throughout the Region. Thus, because not all States and Tribes
will be implementing this policy immediately, where a State or Tribe has not established a
procedure, or has not documented a clear approach to a particular issue, the model
implementation procedure in Chapter 4 of this policy statement will be considered in the
interim as the Region's preferred method of making mixing zone and dilution decisions.
This policy may therefore influence individual NPDES permits issued by the Region or
certain State-issued NPDES permits. The model policy and procedure will also be a likely
starting point for the Region in developing federal replacement requirements, should
disapproval of state or tribal water quality standards become necessary.
In some cases, state or tribal implementation of this policy statement may involve
adding considerable detail to previously adopted state or tribal water quality standards. In
States and Tribes
may deviate from
the Region's
recommended
approach as long
as each of the
issues is clearly
resolved and the
approach selected
is technically
defensible.
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EPA Region Vm Mixing Zones and Dilution Policy
keeping with past practice (e.g., antidegradation policies and procedures), EPA will approve
adoption of detailed implementation procedures or guidance which are not formally part of
the State or Tribe's water quality standard regulation as long as the results of such
procedures are legally enforceable under state or tribal law. Thus, one way to implement
this policy statement would be to develop a separate mixing zones-dilution implementation
procedure that is included in the water quality standards by reference.
The mixing zone and dilution issues for which a clear state or tribal approach will
need to be documented include the following:
Issue # 1: With regard to complete mix/incomplete mix decisions, the
policy/implementation method must satisfy the following minimum requirements:
(a) The policy and procedure must require, prior to concluding that a discharge
mixes in a near instantaneous and complete fashion, that a sound factual basis be
documented in the NPDES permit.
(b) Specific guidelines to be used in making such complete mix/incomplete mix
decisions must be established.
(c) A method or process must be described by which a permittee may demonstrate
that near instantaneous and complete mixing is achieved at critical conditions.
At a minimum, the policy and procedure must establish an operational definition
of "near instantaneous and complete mixing" and require a permittee to
coordinate with the State/Tribe and EPA on the development and execution of a
rate of mixing study plan.
Discussion: One of the primary problems with the existing approach followed by Region
vm States is that the entire low flow is assumed to dilute the effluent, regardless of the local
rate of ambient mixing. Where the rate of ambient mixing is slow (i.e., incomplete mixing
is occurring), assuming the entire low flow as dilution is likely to result in a lengthy
downstream effluent plume with water quality characteristics that are considerably in excess
of applicable criteria and toxicity objectives. Such effluent plumes can pose considerable
risks to human health and aquatic life and should be limited consistent with a state or tribal
mixing zone policy. Because ambient mixing is often slow, EPA Region Vm believes that
the current approach does not adequately address the potential risks of effluent plumes or
adequately control the size of mixing zones. To address this problem, the Region will expect
States and Tribes to establish a procedure for making complete mix/incomplete mix
determinations. Once this initial determination is made, States and Tribes will be expected to
apply a defensible mixing zone (in the case of incomplete mixing) or dilution approach (in
the case of complete mixing), as appropriate. The Regional model approach to this issue
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EPA Region Mil Mixing Zones and Dilution Policy
calls for determining complete or incomplete mixing based on
best professional judgment (BPJ). The procedure assumes
near instantaneous and complete mix where: (1) there is an
effluent diffuser that covers the entire stream or river width at
low flow, or (2) the mean daily flow of the discharge exceeds
the chronic low flow of receiving water. The permittee may
also show "near instantaneous and complete mixing"1
consistent with a study plan developed in consultation with the
State and EPA. Otherwise, incomplete mixing is assumed
and a mixing zone approach is implemented. The Region
intends to allow States and Indian Tribes the flexibility to use
this recommended approach or to follow alternative
approaches. For example, States and Tribes may elect to use
different guidelines for determining when near instantaneous
and complete mixing exists, or use a different operational
definition of "near instantaneous and complete mixing" for
purposes of field mixing studies. However, in issuing and
reviewing NPDES permits, the Region will not support any
assumptions of complete mixing unless a reasonable, factual
basis has been documented in the permit.
Issue if 2: Where mixing is incomplete, the policy/procedure must explain how mixing
zones will be sized for aquatic life and human health protection. At a minimum,
absolute maximum size restrictions (e.g., by waterbody type) and factors to be
considered in establishing site-specific mixing zone dimensions must be established.
Discussion: In developing a mixing zone approach, it is important to set absolute maximum
size restrictions. Such maximum size restrictions promote consistency and facilitate
development of water quality-based permits. The Region's recommended approach calls for
the size and shape of mixing zones, where allowed, to be determined case-by-case.
However, the following maximum size restrictions are specified. For streams and rivers,
mixing zones must not exceed one-half of the cross-sectional area or a length 10 times the
stream width at critical low flow, whichever is more limiting. For lakes, mixing zones must
not exceed 5% of the lake surface area or 200 feet in radius, whichever is more limiting.
Site-specific factors that may be the basis for down-sizing individual mixing zones include
1 "Near instantaneous and complete mixing" is defined in the model procedure as no
more than a 10% difference in bank-to-bank concentrations within a longitudinal
distance not greater than 2 stream/river widths. In addition, the phrases "near
instantaneous and complete mixing" and "complete mixing" are used interchangeably
in this policy statement.
One of the
primary problems
with the existing
approach
followed by
Region VIII
States is that the
entire low flow is
assumed to dilute
the effluent,
regardless of the
local rate of
ambient mixing.
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EPA Region VIII Mixing Zones and Dilution Policy
existing bioaccumulation problems in fish tissue or sediment,
biologically-important areas, low acute to chronic ratio, potential
human exposure from drinking water or recreation, attraction of
aquatic life to the effluent plume, toxicity/persistence of the
substance, zone of passage for migrating fish (including access to
tributaries), and cumulative effects of multiple discharges and
multiple mixing zones. EPA Region VIE intends to allow States
and Indian Tribes the flexibility to follow the Region's model
approach, to incorporate reasonable modifications, or to pursue
other protective alternatives. For example, States and Tribes
may elect to use somewhat smaller or somewhat larger maximum
size restrictions or to include more specific guidelines for
adjusting mixing zone size and shape in specific cases.
However, the Region will carefully review approaches that
deviate from the recommended size limits in its review of adopted
zone policies and procedures.
Maximum size
restrictions
ensure
consistency
statewide and
facilitate
development of
water quality-
based permits.
state and tribal mixing
Issue if 3: The policy/procedure must describe methods by which effluent limits will be
derived to achieve mixing zone size and shape requirements (such as particular
mathematical models).
Several different
modeling
methods are
recommended,
ranging from
simple to more
data4ntensive.
Discussion: Once a State or Tribe has established the
allowable size of a mixing zone in a particular case, methods
are needed to derive permit limits that will achieve the size
restrictions. These methods should be fairly specific, but
flexible enough to address both data-rich and data-poor
situations. The Region's recommended approach is to use one
of three progressively more sophisticated methods. The default
method may be used where data necessary to implement a more
sophisticated approach are lacking or where a conservative
approach is warranted based on site-specific environmental
concerns. For streams, the default method requires that no
more than 10% of the critical low flow be provided as dilution.
For lakes, the default method requires that no more than 4:1 dilution be allowed (20%
effluent). The default method is very easy to implement, but because it is not based on the
ambient mixing rate, the resulting dilution allowance is conservative (i.e., it is based on
worst-case mixing assumptions). The modeling method may also be used to ensure that
mixing zone size restrictions are achieved. Several different modeling methods, ranging
from simple (ambient diffusion only) to more data-intensive (discharge-induced and ambient
diffusion) are recommended. Because the modeling method is driven by site data, it yields
more dilution where mixing is relatively rapid and less dilution where mixing is relatively
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EPA Region VIII Mixing Zones and Dilution Policy
slow. In addition, the modeling method generally yields more dilution than the default
method. The field study method requires use of field data quantifying the actual ambient
mixing rate. As such, the field study method yields the most accurate estimate of the
dilution that will achieve mixing zone size restrictions, but it also requires the most site data.
EPA Region Vm intends to allow States and Indian Tribes the flexibility to follow the
Region's recommended protocol or to develop their own (technically-defensible) methods.
States and Tribes may elect to rely exclusively on a single method, or use a combination of
methods (such as those in the EPA model procedure). Regardless of the method(s) selected
by the State or Tribe, EPA will require the implementation document to include a level of
detail that is sufficient to ensure reasonable consistency when used to derive water quality-
based permit limits.
Issue # 4: Minimum in-zone quality requirements must be clearly established for all
mixing zones including, at a minimum, the narrative "free from" criteria (i.e., including
a prohibition of acute lethality to aquatic life).
Discussion: EPA policy is that state and tribal mixing zone policies must include minimum
in-zone quality requirements that apply within mixing zones. Such in-zone quality
requirements can serve to provide protection, for example, to organisms residing within or
passing through the mixing zone. The Region recommends that States and Tribes address
this issue by clarifying that their narrative "free from" water quality criteria apply within
mixing zones. Such narrative criteria typically require that surface waters shall be "free
from substances that settle to form objectionable deposits, float as debris, scum, oil, or other
matter, produce objectionable color, odor, taste, or turbidity, are acutely toxic, and produce
undesirable or nuisance aquatic life." Regardless of the exact language that is adopted by a
State or Tribe, a minimum narrative requirement that must apply within mixing zones is that
mixing zones must not result in lethality to aquatic life caused by passage through the mixing
zone by migrating fish, or by less mobile forms drifting through a plume. States and Tribes
may also specify that mixing zones may not result in toxicity to sessile organisms that reside
within mixing zones. See comment ti 11 in Appendix B for additional discussion.
Issue # 5: Where mixing is incomplete, the policy/procedure must clearly explain how
chemical-specific acute criteria are to be implemented to comply with the "no acute
lethality" requirement that applies within mixing zones. The policy/procedure must also
require compliance with acute whole effluent toxicity limitations at the end-of-pipe,
without an allowance for dilution (i.e., where such acute WET limits are included in a
permit).
Discussion: In developing water quality-based permit limits, a difficult issue is setting daily
maximum (acute) permit limits for individual substances that will achieve the no acute
lethality requirement that applies within mixing zones. This issue is difficult because a
12
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EPA Region VID Mixing Zones and Dilution Policy
discharge at concentrations in excess of acute criteria may be
lethal to organisms occupying or passing through the mixing
zone (i.e., depending upon the duration and magnitude of the
exposure). The Region's recommendation for acute chemical-
specific criteria is to require compliance with such criteria at
the end-of-pipe, without an allowance for dilution. The
Region believes that this approach is the best means of
ensuring that the no acute lethality requirement that applies
within mixing zones will be achieved. However, the Region
is aware that other approaches are currently in use that allow
a small area for mixing (often referred to as a zone of initial
dilution) where chemical-specific acute criteria need not be
achieved. Because these approaches are described in EPA
guidance and used by some States, the Region will approve
use of such methods if the method is clearly described and
appropriately protective (see Regional guidance on this topic in Appendix D). For acute
whole effluent toxicity limitations, the Region will continue to require end-of-pipe
compliance, with no allowance for dilution (i.e., except where mixing is found to be near
instantaneous and complete). See comments 8, 13, and 17 in Appendix B.
Issue # 6: Guidelines must be included regarding the amount of dilution to be provided
where near instantaneous and complete mixing is determined to occur (i.e., "critical"
low flows for human health as well as aquatic life criteria) and factors (e.g., drinking
water intakes, presence of biologically-important areas, etc.) that may be the basis for
site-specific reduction of the dilution allowance.
Discussion: In cases where complete mixing is occurring, it is important to describe how the
dilution allowance will be established. Establishing a clear approach to this issue ensures
consistency and provides a basis to address site-specific environmental concerns. To best
define dilution allowances for implementing water quality standards, the Region feels that it
is most appropriate to define both ambient critical flows and effluent critical flows. In
particular, a distinction should be made between the ambient and effluent flows to be used
for standards of longer duration (e.g. chronic aquatic life standards) and those to be used for
shorter duration standards (e.g. acute aquatic life standards). Under the model procedure,
the flows shown below are applied as a maximum dilution allowance. However, dilution
may be further limited in individual cases to a portion of the critical low flow based on site-
specific environmental concerns. The Region believes that the duration and frequency of the
flows used should match the duration and frequency criteria provisions found in the state or
tribal water quality standards. In other words, the duration (e.g., 1 hour, 4 day, 30 day) and
excursion frequency (e.g., 3 years) associated with each standard, whether it is for aquatic
life uses, recreational uses, drinking water, agricultural uses, wildlife protection or other
In incomplete mix
situations, acute
whole effluent
toxicity
limitations must
be achieved at the
end-of-pipe,
without an
allowance for
dilution.
13
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EPA Region VIII Mixing Zones and Dilution Policy
uses, should match the duration and frequency of the ambient dilutions flows used to
implement those standards. In addition, the Region is recommending the use of the
"biologically-based" method for calculating critical ambient flows. The Region has found
that there are significant advantages statistically and functionally to this distribution-free
r thod, including a thorough analysis of historical data and the ability to determine
seasonal/monthly flows without exceeding frequency provisions in state standards. The
biologically-based method contrasts with the more traditional "extreme" methods that utilize
much less data and return less accurate results. (See Appendix E.) Although the Region
recommends the flows shown below, the Region intends to allow States and Tribes the
flexibility to determine the critical low flows that are appropriate. States and Tribes may
also want to establish more specific guidelines for restricting dilution allowances in individual
cases (e.g., States and Tribes may want to further restrict dilution allowances for human
health criteria where a discharge is within 2 miles of a drinking water intake). The critical
flows1 recommended by the Region are as follows:
Stream Flows
Aquatic life, chronic
Aquatic life, acute
Human health (carcinogens)
Human health (non-carcinogens)
Effluent Flows
Aquatic life, chronic
Aquatic life, acute
Human Health (all)
4-day, 3-year flow (biologically based)
1-day, 3-year flow (biologically-based)
harmonic mean flow
4-day, 3-year flow (biologically-based) or
1-day, 3-year flow (biologically-based)
Mean daily flow
Maximum daily flow
Mean daily flow
1 These flows are recommended by EPA Region Vm. The actual duration (e.g. 4 day)
and frequency (e.g. 3-year) of the flows used should match the duration and
frequency provisions of the aquatic life, human health, and other standards found in
state water quality standards. For human health non-carcinogens, Region VIII is
making a distinction between parameters that typically have an effect after prolonged
exposures (e.g. copper) and those that have more of an immediate effect (e.g. nitrite).
For simplicity, Region VIII is recommending use of the chronic aquatic life flow for
the longer-acting parameters and the acute aquatic life flow for the shorter-acting
parameters. For information on how to calculate these flows, see EPA's Stream
Design Flow for Steady-State Modeling; Technical Guidance Manual for
Performing Wasteload Allocation; Book 6; Design Conditions (1986).
14
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EPA Region VIII Mixing Zones and Dilution Policy
Issue # 7: The policy/procedure must identify situations in which a mixing zone or an
allowance for dilution will/may be denied (at a minimum, such situations include
absence of available dilution at critical low flow conditions).
Discussion: One problem sometimes encountered in
implementing water quality controls is that dischargers may
believe they are automatically entitled to a dilution allowance or
a mixing zone. In fact, the Clean Water Act provides no express
entitlement even to discharge, let alone to discharge at
concentrations that exceed applicable water quality criteria.
Although discharges may be (and usually are) permitted under
the NPDES program, the Clean Water Act requires that such
permits include effluent limits that will fully protect designated
and existing uses. In some cases, protecting uses may require
that a discharge not be granted a dilution allowance or mixing
zone. Thus, it is important that mixing zone implementation
documents clearly communicate to dischargers the situations in
which a mixing zone or a dilution allowance may be denied.
For example, the Region's model approach calls for prohibiting
dilution or a mixing zone for any discharge to a wetland (see
question # 5 in Appendix B for additional discussion). In
addition, any of the factors for limiting or denying a mixing zone (e.g., presence of drinking
water intakes or biologically-important areas) may also be the basis for limiting or denying
an allowance for dilution. Finally, a mixing zone or dilution allowance may not be provided
where the critical low flow is zero (at a minimum, States and Tribes will be expected to
implement this requirement). States and Indian Tribes may follow this model approach or
develop their own approach, which may be more or less comprehensive than the Region's
model. For example, States and Indian Tribes may adopt additional guidelines for special
situations (e.g., South Dakota has prohibited mixing zones for all lake discharges).
Issue ft 8: The policy and procedure must address development of both chemical-
specific and whole effluent toxicity (WET) limits.
Discussion: It is important for State and Tribal mixing zone policies and procedures to
clearly address development of both toxicity and chemical-specific permit limits. Such
clarity facilitates permit issuance and avoids possible misunderstanding and potential disputes.
Although in general the dilution and mixing zone approach is likely to be the same in each
case, any differences should be explicit. For example, in incomplete mix situations, where a
State or Tribe elects to provide a limited zone of initial dilution for acute chemical-specific
criteria, the policy and procedure should: (1) include clear implementation guidelines, and
(2) clearly state that acute WET limits will be applied at the end-of-pipe, without an
The Clean
Water Act does
not grant
dischargers the
right to
discharge at
concentrations
that exceed
applicable
water quality
criteria.
15
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EPA Region VHI Mixing Zones and Dilution Policy
allowance for dilution (as discussed above under Issue # 5). In general, State and Tribal
mixing zone-dilution policies and procedures will be expected to cover both chemical-specific
and whole effluent toxicity limits and to clearly explain any policy or procedural differences
for the two types of permit limits.
Issue # 9: The mixing zone policy must clearly establish state/tribal authority to revisit
and adjust mixing zone analyses or dilution allowances as better information on the rate
of mixing and/or the impacts of the discharge becomes available.
Discussion: This issue may be addressed by inclusion of a simple provision in the state or
tribal mixing zone policy. For example, the Region's model policy includes the following:
"All mixing zone-dilution assumptions are subject to review and revision as information on
the nature and impacts of the discharge becomes available (e.g., chemical and/or biological
monitoring at the mixing zone boundary). At a minimum, mixing zone and dilution
decisions are subject to review and revision along with all other aspects of the discharge
permit upon expiration of the permit."
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EPA Region VHI Mixing Zones and Dilution Policy
CHAPTER 3. MODEL POLICY1
PURPOSE
(a) This policy addresses how mixing and dilution of point source discharges with
receiving waters will be addressed in developing chemical-specific and whole effluent
toxicity discharge limitations for point source discharges. Depending upon site-
specific mixing patterns and environmental concerns, some pollutants/criteria may be
allowed a mixing zone or dilution while others may not. In all cases, mixing zone
and dilution allowances shall be limited as necessary to protect the integrity of the
receiving water ecosystem and designated waterbody uses.
MIXING ZONES
(b) Where dilution is available at critical conditions and the discharge does not mix at a
near instantaneous and complete rate with the receiving water (incomplete mixing), an
appropriate mixing zone may be designated. However, mixing zones may be denied
on a parameter-by-parameter basis where practicable (e.g., many ambient criteria can
be achieved at the end-of-pipe without a mixing zone allowance). Where a mixing
zone is allowed, its size and shape will be determined on a case-by-case basis.
Mixing zones for streams and rivers shall not exceed one-half of the cross-sectional
area or a length 10 times the stream width at critical low flow, whichever is more
limiting. Mixing zones in lakes shall not exceed 5 % of lake surface area or 200 feet
in radius, whichever is more limiting. These limits are intended to establish the
maximum allowable size of mixing zones; however, individual mixing zones may be
further limited or denied in consideration of designated and existing uses or presence
of the following concerns in the area affected by the discharge:
(i) bioaccumulation in fish tissues or wildlife,
(ii) biologically important areas such as fish spawning/nursery areas,
(iii) low acute to chronic ratio,
(iv) potential human exposure to pollutants resulting from drinking water or
recreational activities,
(v) attraction of aquatic life to the effluent plume,
(vi) toxicity/persistence of the substance discharged,
(vii) zone of passage for migrating fish or other species (including access to
tributaries), and
(viii) cumulative effects of multiple discharges and mixing zones.
1 This model policy is included as an example of the type of mixing zone-dilution
policy that would satisfy federal requirements.
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EPA Region VIII Mixing Zones and Dilution Policy
(c) Within the mixing zone designated for a particular substance, certain numeric water
quality criteria for that substance may not apply. However, all mixing zones shall be
free from substances that:
(i) settle to form objectionable deposits,
(ii) float as debris, scum, oil, or other matter,
(iii) produce objectionable color, odor, taste, or turbidity,
(iv) are acutely toxic1, and
(v) produce undesirable or nuisance aquatic life.
(d) In incomplete mix situations, discharge limitations to implement acute aquatic life
(chemical-specific) criteria and narrative (no acute toxicity) criteria shall be based on
achieving such acute criteria at the end-of-pipe (i.e., without an allowance for
dilution). This approach is intended to implement the narrative requirement
prohibiting acutely toxic conditions in the mixing zone. In implementing this
requirement, the objective shall be to avoid acute toxicity to migrating fish, organisms
that are attracted to the effluent plume, less mobile organisms drifting through the
mixing zone, and sessile organisms that reside within the mixing zone. For chemical-
specific acute criteria, a limited exception to this rule is provided under paragraph (e)
of this policy (regarding certain minor POTWs).
DILUTION AT J ,fiW ANC1RS
(e) Where the discharge is to a river or stream, dilution is available at critical conditions,
and available information is sufficient to reasonably conclude that there is near
instantaneous and complete mixing of the discharge with the receiving water
(complete mixing), an appropriate dilution allowance may be provided in calculating
chemical-specific and WET discharge limitations. The basis for concluding that such
near instantaneous and complete mixing is occurring shall be documented in the
rationale for the NPDES permit. The dilution allowance for continuous dischargers
shall be based on the critical low flow (or some portion of the low flow). The
requirements and environmental concerns identified in paragraphs (b) and (c) above
1 Although EPA recommends that state and tribal narrative "free from" toxicity criteria
apply to acute and chronic toxicity to humans, animals and plants (see suggested
language on page 3-24 of the Water Quality Standards Handbook), within mixing
zones EPA recommends a narrative requirement prohibiting acutely toxic conditions.
The Region acknowledges that there are a variety of methods that can be used to
implement this requirement; the one recommended by the Region is described in
paragraph (d) above.
18
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EPA Region VIII Mixing Zones and Dilution Policy
may be considered in deciding the portion of the critical low flow to provide as
dilution. The following critical low flows1 shall be used for streams and effluents:
Stream Flows
Aquatic life, chronic
Aquatic life, acute
Human health (carcinogens)
Human health (non-carcinogens)
4-day, 3-year flow (biologically based)
1-day, 3-year flow (biologically-based)
harmonic mean flow
4-day, 3-year flow (biologically-based) or
1-day, 3-year flow (biologically-based)
Effluent Flows
Aquatic life, chronic
Aquatic life, acute
Human Health (till)
Mean daily flow
Maximum daily flow
Mean daily flow
For chemical-specific and chronic WET limits, an appropriate dilution allowance may
also be provided for certain minor POTWs where allowing such dilution will pose
insignificant environmental risks (i.e., regardless of whether mixing is complete or
incomplete). However, for acute WET limits, an allowance for dilution is authorized
only where dilution is available and mixing is complete.
For controlled discharges, such as lagoon facilities that discharge during high ambient
flows, the stream flow to be used in the mixing zone analysis should be the lowest
flow expected to occur during the period of discharge.
(f) Where a discharger has installed a diffuser in the receiving water, all or a portion of
the critical low stream flow may be provided as a dilution allowance. The
determination shall depend on the diffuser design and on the requirements and
potential environmental concerns identified in paragraphs (b) and (c) above. Where a
1 These flows are recommended by EPA Region Vm. The actual duration (e.g. 4 day)
and frequency (e.g. 3-year) of the flows used should match the duration and
frequency provisions of the aquatic life, human health, and other standards found in
state water quality standards. For human health non-carcinogens, Region Vm is
making a distinction between parameters that typically have an effect after prolonged
exposures (e.g. copper) and those that have more of an immediate effect (e.g.
nitrate). For simplicity, Region VIII is recommending use of the chronic aquatic life
flow for the longer-acting parameters and the acute aquatic life flow for the shorter-
acting parameters. For information on how to calculate these flows, see EPA's
Stream Design Flow for Steady-State Modeling; Technical Guidance Manual for
Performing Wasteload Allocation; Book 6; Design Conditions (1986).
19
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EPA Region VIII Mixing Zones and Dilution Policy
diffuser is installed across the entire river/stream width (at critical low flow), it will
generally be presumed that near instantaneous and complete mixing is achieved, and
that providing the entire critical low flow as dilution is appropriate.
OTHER CONSIDERATIONS
(g) Where dilution flow is not available at critical conditions (i.e., the waterbody is dry),
the discharge limits will be based on achieving applicable water quality criteria (i.e.,
narrative and numeric, chronic and acute) at the end-of-pipe, and neither a mixing
zone or an allowance for dilution will be provided.
(h) Discharge limitations for point sources to a wetland will be based on achieving all
applicable water quality criteria (i.e., narrative and numeric, chronic and acute) at the
end-of-pipe.
(i) All mixing zone-dilution assumptions are subject to review and revision as
information on the nature and impacts of the discharge becomes available (e.g.,
chemical or biological monitoring at the mixing zone boundary). At a minimum,
mixing zone and dilution decisions are subject to review and revision along with all
other aspects of the discharge permit upon expiration of the permit.
(j) For certain pollutants (e.g., ammonia, dissolved oxygen, metals) that may exhibit
increased toxicity or other effect on water quality after dilution and complete mixing
with receiving waters is achieved, the wasteload allocation shall address such toxicity
or other effect on water quality as necessary to fully protect designated and existing
uses (i.e., the point of compliance may be something other than the mixing zone
boundary or the point where complete mixing is achieved).
20
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EPA Region Vm Mixing Zones and Dilution Policj
CHAPTER 4. MODEL IMPLEMENTATION PROCEDURE1
This procedure describes how dilution and mixing of point source discharges with
receiving waters wil] be addressed in developing discharge limitations for point source
discharges. For purposes of this procedure, a mixing zone is defined as a designated area or
volume of water surrounding or downstream of a point source discharge in which the
discharge is progressively diluted by the receiving water and numerical water quality criteria
may not apply. Where justified based on site-specific considerations, such a mixing zone
may be designated in the context of an individual permit decision. Discharges may also be
provided an allowance for dilution where it is determined that the discharge mixes with the
receiving water in a near instantaneous and complete fashion. Such mixing zones and
allowances for dilution will be granted on a parameter-by-parameter and criterion-by-criterion
basis as necessary to fully protect existing and designated uses.
The procedure to be followed is composed of six individual elements, or steps. The
relationship of the six steps and an overview of the mixing zone-dilution procedure is
illustrated in Figure 1. Please note that Figure 1 is a simplification of this procedure and is
not intended to be used or interpreted without the accompanying explanatory text. Each of
the six individual steps is described below.
Step 1 - No Dilution Available During Critical Conditions
Where dilution flow is not available at critical low flow conditions, discharge
limitations will be based on achieving applicable narrative and numeric water quality criteria
at the end-of-pipe.
Step 2 - Dilution Categorically Prohibited for Wetland Discharges
Permit limitations for discharges to a wetland shall be based on achieving all
applicable water quality criteria (i.e., narrative and numeric, chronic and acute) at the end-
of-pipe2.
1 This model procedure is included as an example of the type of mixing zones-dilution
implementation procedure that would satisfy federal requirements.
2 States and Tribes may also categorically prohibit dilution, for example, for discharges
of certain substances or classes of substances or for discharges to certain types of
waterbodies.
21
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Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Figure 1
EPA Region VIII Model Mixing Zone/Dilution Procedure *
Is dilution available
at critical conditions?
End-of-pipe limits
(no dilution).
no
yes
Does the discharge affect
a wetland or do State WQS
otherwise prohibit dilution?
yes
Is the facility a minor POTW
where allowing dilution
would pose insignificant
environmental risks? (Assumed
where dilution ratio ^ 50:1)
Is the discharge
to a lake?
J yes
termine dilution
)w no more than
Allow full
critical stream
flow for
acute, chronic,
& human
health limits.
yes
De
all<
no
:ase-by-case:
19:1 dilution
Would allowing dilution or a mixing zone
pose unacceptable environmental risks?
(5% effluent).
yes
no
End-of-pipe
limits (no
dilution).
For discharges to rivers and
streams, do available data
reasonably support a conclusion
that there is near instantaneous
and complete mixing? Is there
use of a diffuser?
yes
no
Allow critical
stream flow or
some portion
for acute,
chronic &
human health
limits.
Calculate effluent limits based on one of the following methods:
¦ DEFAULT METHOD
Streams/rivers: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health limits - Use no more than 10% of
critical stream low flow.
Lakes/reservoirs: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health limits - No more than 4:1 dilution allowed
(20% effluent)
¦ MODELING METHOD
Streams/rivers: Acute Limits - End-of-pipe limits (no dilution)
Chronic/human health limits - Mixing Zone limited to no more
than 1/2 of cross sectional area of stream or
no more than 10 times stream width
Lakes/reservoirs: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health Limits - Mixing zone limited to no more
than 200 foot radius or 5 % of lake surface
¦ FIELD STUDY METHOD
Actual dilution in receiving water is determined by field study, with mixing
zone limited by size provisions described in above Modeling Method.
* This procedure is applied to both chemical-specific and WET limits. In the
case of complex discharges, the dilution or mining zone may vary parameter-by-parameter.
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EPA Region Vlfl Mixing Zones and Dilution Policy
Step 3 - Procedure for Certain Minor POTWs
This step establishes the dilution procedure that is to be followed for certain minor
POTWs where it is determined by the permit writer that applying the procedure, based on
available information, poses insignificant environmental risks. POTWs that are classified as
minor dischargers and discharge to a lake or discharge to a river/stream segment at a dilution
ratio greater than or equal to 50:1 shall be presumed to qualify for this procedure. Minor
POTWs with dilution ratios less than 50:1 may also qualify for this procedure, at the
discretion of the permit writer, where the permittee is able to adequately demonstrate that
applying this procedure poses insignificant environmental risks. For purposes of this
procedure, the river/stream dilution ratio is defined as the chronic low flow of the segment
upstream of the POTW discharge divided by the mean daily flow of the POTW. (An
exception to this applies to controlled discharge discharges, such as lagoon facilities that
discharge during high flows. In such cases, the river/stream dilution ratio is defined as the
lowest upstream flow expected during the period of discharge divided by the mean daily flow
of the discharge.)
In any case where the permit writer determines that applying this procedure could
pose unacceptable environmental risks, the minor POTW will not qualify for this procedure.
Factors that are to be considered in evaluating potential environmental risks are the same as
those described below in Step 4.
For minor POTWs that qualify for this procedure and discharge to lakes, the
allowance for dilution for chemical-specific and chronic WET limits will be determined on a
case-by-case basis (dilution for acute WET limits shall not be provided under this procedure).
As a general guideline, dilution up to 19:1 (5% effluent) may be provided. However, this
allowance may be adjusted downward (made more stringent) on a case-by-case basis
depending upon lake size during critical conditions, lake flushing potential, designated and
existing uses of the lake, uses of the lake portion affected by the discharge, and the factors
described in Step 4.
For minor POTWs that qualify for this procedure and discharge to a river/stream
segment, dilution up to the full chronic aquatic life, acute aquatic life and human health
critical flows may be provided as dilution (dilution for acute WTET limits shall not be
provided under this procedure). This maximum allowance may be adjusted downward (made
more stringent) based on factors described in Step 4.
Step 4 - Site-Specific Risk Considerations
Where allowing a mixing zone or a dilution allowance would pose unacceptable
environmental risks, the discharge limitations will be based on achieving applicable narrative
22
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EPA Region VIII Mixing Zones and Dilution Policy
and numeric water quality criteria at the end-of-pipe. The existence of environmental risks
may also be the basis for a site-specific mixing zone size restriction or dilution allowance.
Such risk determinations will be made on a case-by-case and parameter-by-parameter basis.
In general, this procedure does not establish any bright line tests with which to make such
risk determinations. Rather, such decisions are to be made in consideration of the designated
and existing uses and all relevant site-specific environmental concerns, including the
following:
Bioaccumulation in fish tissues or wildlife. Both potential and existing
bioaccumulation concerns should be evaluated. As a general guideline, discharge of
pollutants with bioconcentration factors (BCF) greater than 300 indicates a potential
risk of downstream bioaccumulation.
Biologically important areas such as fish spawning areas or shallow water
nursery areas. Information on either the existence of spawning areas within the
proposed zone of influence or a "shore hugging" effluent plume in an aquatic life
segment could support a conclusion that allowing dilution or a mixing zone would
pose significant risk to a biologically important area. Presence of a threatened or
endangered species downstream should also be considered in light of the duration and
magnitude of potential exposure of the species (i.e., to the effluent plume) and the
sensitivity of the particular species.
Low acute to chronic ratio. For substances with low acute to chronic ratios,
indicating that acute effects may occur at concentrations "close" to those that have
been demonstrated to result in chronic effects, restricting or denying a mixing zone or
dilution allowance may be appropriate in order to avoid acutely-toxic concentrations
within the mixing zone.
Potential human exposure to pollutants resulting from drinking water or
recreational activities. Existence of a drinking water intake or a recreational area
within or near the proposed zone of influence would strongly suggest that an
allowance for dilution or a mixing zone is not appropriate (i.e., particularly where
human health exposure concerns are limiting for the substance in question).
Attraction of aquatic life the effluent plume. Where available data support a
conclusion that fish or oth ^uatic life are attracted to the effluent plume, resulting
in adverse effects such as acuic or chronic toxicity, it may well be appropriate to set
discharge limitations based on achieving applicable narrative and numeric water
quality criteria at the end-of-pipe (i.e., for the substances believed to be causing the
toxic effects).
23
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EPA Region VIE Mixing Zones and Dilution Policy
Toxicity/persistence of the substance discharged. It may also be appropriate to
deny dilution or a mixing zone for particularly toxic or persistent substances. This
factor should be given added weight where the discharge is to a closed aquatic system
where the substance is expected to remain biologically available and a watershed-
based solution such as total maximum daily load implementation is unlikely in the
near-term.
Zone of passage for migrating fish or other species (including access to
tributaries). Where available data suggest that allowing dilution or a mixing zone
would inhibit migration of fish or other species, it may be appropriate to set discharge
limitations based on achieving applicable narrative and numeric water quality criteria
at the end-of-pipe. This factor includes consideration of whether the effluent plume
will block migration into tributary segments.
Cumulative effects of multiple discharges and mixing zones. In some cases,
existence of overlapping effluent plumes may necessitate limiting or denying dilution
or mixing zones for the discharging facilities. Any allowances for dilution should be
restricted as necessary to protect the integrity of the receiving water ecosystem and
designated waterbody uses. Although such concerns may be more appropriately
addressed in a watershed-based control program (such as a TMDL), it may be
appropriate to limit or deny dilution for all discharges to a watershed as an interim
measure until a long-term solution can be developed and implemented.
Step 5 - Complete Mix Procedure
For point source discharges to rivers/streams where available data are adequate to
support a conclusion that there is near instantaneous and complete mixing of the discharge
with the receiving water (complete mixing) the full critical low flow (or a portion thereof)
may be provided as dilution for chemical-specific and WET limitations. Such determinations
of complete mixing will be made on a case-by-case basis using best professional judgment.
Presence of an effluent diffuser that covers the entire stream/river width (at critical low flow)
will generally be assumed to provide complete mixing. Further, where the mean daily flow
of the discharge exceeds the chronic low stream flow of the receiving water, complete
mixing will generally be assumed. Conversely, where the mean daily flow of the discharge
is less than or equal to the chronic low flow of the receiving water, it will generally be
assumed that complete mixing does not occur unless otherwise demonstrated by the
permittee. Demonstrations of complete mixing by the permittee should be consistent with a
study plan that is developed in cooperation with the State/Tribe and EPA Region VIE. Refer
to EPA's Technical Support Document for Water Quality Based Toxics Control, 1991, for a
list of appropriate mixing study references. For purposes of such studies, "near
instantaneous and complete mixing" is operationally defined as no more than a 10%
24
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EPA Region VIII Mixing Zones and Dilution Policy
difference in bank-to-bank concentrations within a longitudinal distance not greater than 2
stream/river widths. For controlled discharges, such as lagoon facilities that discharge only
during high flow, the test of "near and instantaneous and complete mixing" will be made
using the expected rate of effluent discharge and the lowest upstream flow expected to occur
during the period of discharge.
The following critical low flows1 shall be applied for streams and effluents:
Stream Flows
Aquatic life, chronic
Aquatic life, acute
Human health (carcinogens)
Human health (non-carcinogens)
Effluent Flows
Aquatic life, chronic
Aquatic life, acute
Human Health (all)
4-day, 3-year flow (biologically based)
1-day, 3-year flow (biologically-based)
harmonic mean flow
4-day, 3-year flow (biologically-based) or
1-day, 3-year flow (biologically-based)
Mean daily flow
Maximum daily flow
Mean daily flow
Where complete mixing can be concluded, and environmental concerns identified in
Step 4 do not justify denying dilution but are nevertheless significant, some portion of the
critical low flows identified above may be provided as dilution. Such decisions will take
site-specific environmental concerns into account as necessary to ensure adequate protection
of designated and existing uses.
Step 6 - Incomplete Mix Procedure
This step addresses point source discharges that exhibit incomplete mixing. Because
the mixing zone policy requires compliance with narrative and numeric acute criteria at the
1 These flows are recommended by EPA Region VIII. The actual duration (e.g. 4 day) and
frequency (e.g. 3-year) of the flows used should match the duration and frequency
provisions of the aquatic life, human health, and other standards found in state water
quality standards. For human health non-carcinogens, Region VIII is making a distinction
between parameters that typically have an effect after prolonged exposures (e.g. copper)
and those that have more of an immediate effect (e.g. nitrate). For simplicity, Region
VIII is recommending use of the chronic aquatic life flow for the longer-acting parameters
and the acute aquatic life flow for the shorter-acting parameters. For information on how
to calculate these flows, see EPA's Stream Design Flow for Steady-State Modeling;
Technical Guidance Manual for Performing Wasteload Allocation; Book 6; Design
Conditions (1986).
25
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EPA Region VIII Mixing Zones and Dilution Policy
end-of-pipe in incomplete mix situations, this step focusses on chronic aquatic life (chemical-
specific and WET) and human health limits. The following provides guidelines for
determining the amount of dilution available for dischargers that exhibit incomplete mixing.
There are three methods described below, each with different levels of information needed to
perform a mixing zone analysis.
¦ Default Method
This method addresses situations in which necessary information to implement the
modeling method (e.g. channel slope, depths, widths, velocities, etc.) are not readily
available. This method may also be utilized where there are concerns about the
potential environmental impacts of allowing a mixing zone (i.e., there is information
to suggest that environmental impacts may result from allowing a mixing zone but the
information is not sufficiently conclusive to completely deny dilution as provided
under step 4). In these situations, the default method provides a conservative (i.e.,
minimal) dilution allowance. A conservative allowance for dilution is appropriate in
such cases in order to ensure adequate protection for designated and existing uses and
to ensure that chronic aquatic life and human health criteria are attained at the edge of
the mixing zone. Following this method may incidentally create an incentive for the
discharger to generate the site-specific information necessary to use the modeling
method described below. An exception to this method will be made where available
data (though limited) support a preliminary conclusion that dilution should not be
allowed. In such cases, effluent limits may be based on achieving criteria at the end-
of-pipe, consistent with step 4. In addition, there may be some parameters where no
mixing zone is necessary because achieving compliance with criteria at the end-of-
pipe is practicable.
In general, this method will be employed where either of the following two tests are
satisfied:
(1) Environmental Impacts Test
This test is satisfied where either:
(a) available data on potential environmental impacts of allowing a mixing
zone support a conclusion that a full (maximum) mixing zone should not be
allowed (e.g., there is a downstream concern regarding bioaccumulation or
toxicity of a particular substance), or
(b) available data on the receiving water and downstream uses are inadequate
to determine the appropriate (regulatory) mixing zone dimensions (width,
length) necessary to fully protect designated and existing uses (e.g., it is not
26
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EPA Region Vm Mixing Zones and Dilution Policy
clear where the physical extent of downstream fish spawning or human
recreational areas begin and end).
(2) Modeling Input Data Test
This test is satisfied where needed data to proceed with the modeling method
(e.g. channel slope, depths, widths, velocities, etc.) are not readily available.
Stream/River Dischargers: For discharges to streams or rivers where either of the
two tests described above are satisfied, dilution will be established on a case-by-case
basis. As a general guideline, dilution calculations which use up to 10% of the
critical low flow for chronic aquatic life limits (numeric and WET) or human health
limits may be used in developing effluent limitations. However, this allowance may
be adjusted downward on a case-by-case basis depending upon relevant site-specific
information, designated and existing uses of the segment, and especially the uses of
the segment portion affected by the discharge. No dilution may be provided for acute
aquatic life limits (i.e., either chemical-specific or WET). In addition, where
available data suggest that an allowance for dilution may pose unacceptable
environmental risk, it may be appropriate to deny an allowance for dilution for
chronic aquatic life or human health limits until sufficient data are available to support
a decision.
T .akp./Rfiservoir Dischargers: For dischargers to lakes or reservoirs where either of
the two tests described above are satisfied, dilution will be established on a case-by-
case basis. As a general guideline, dilution up to 4:1 (20% effluent) may be provided
for chronic aquatic life analyses (numeric and WET) or human health analyses.
However, this allowance may be adjusted downward on a case-by-case basis
depending upon discharge flow, lake size, lake flushing potential, designated and
existing uses of the lake, and uses of the lake portion affected by the discharge. No
dilution may be provided for acute aquatic life limits. In addition, where available
data suggest that an allowance for dilution may pose unacceptable environmental risk,
it may be appropriate to deny an allowance for dilution until sufficient data are
available to support a decision.
¦ Modeling Method
Stream/River Dischargers: For parameters where a mixing zone is allowed (e.g.,
based on a finding that compliance at the end-of-pipe is not practicable), the chronic
aquatic life (numeric and WET) and human health mixing zone should not exceed
one-half the cross-sectional area or a length 10 times the stream width, whichever is
27
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EPA Region VIII Mixing Zones and Dilution Policy
more restrictive. These restrictions apply to the stream or river at critical low flow
and the effluent discharge at critical flow (see flows described in Step 5). Individual
mixing zones may be further limited or denied due to site-specific considerations (as
described under Step 4). It may be necessary only to limit mixing zone size for
particular parameters or for particular criteria (i.e., aquatic life or human health).
For human health water quality parameters that have an effect over a short period of
exposure (e.g. N03), it may be more appropriate to require compliance with
applicable criteria at the end-of-pipe.
A calculation must first be performed to see if the discharge mixes within the one-half
area before or after the length limit (10 times the stream width). (See Example 4.1)
All calculations are at the critical stream and effluent design flows (see flows
described under Step 5). This calculation as well as other mixing zone calculations
can be performed using the simplified equations below or using more complex models
(e.g. CORMIX, PLUMES, etc.) In addition, EPA Region VIII has developed a
spreadsheet model (STREAMIX I) for mixing in rivers and streams based on the
equations presented below. Please see Appendix A for a discussion of STREAMIX I
and field validation examples.
Appendix A provides methods to determine both the average concentration in a
mixing zone as well as the maximum, or extreme, concentration in the mixing zone.
In doing a site specific analysis on mixing zones, it may become important to estimate
concentrations immediately along the shoreline rather than estimate concentrations that
are lateral averages within the mixing zone. In the course of calculating mixing zone
concentrations to determine discharge effluent limits, Region VIII recommends using
as a first course of action the average mixing zone concentrations. In that light, the
following discussion on methods for calculating mixing zone concentrations are based
on the average values rather than extreme values.
The equations below are for surface discharges (not submerged) to stream or rivers.
This approach can be adapted to address both bank discharges as well as surface
dischargers anywhere along the lateral width of the stream. In addition, the equations
below provide a conservative estimate of mixing because mixing due to momentum of
the discharge is not considered; only mixing due to ambient diffusion is considered.
When a discharge has a significant amount of momentum laterally across the stream,
the actual time and distance to achieve one-half mixing will most likely be shorter
than indicated by the equations below. Using more complex models or performing
field tests is recommended if mixing characteristics due to discharge-induced
momentum as well as other factors are significant.
28
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EPA Region VT1I Mixing Zones and Dilution Policy
An approximation for plume width (wmix) at distance X downstream from a discharge
at the bank of a river or stream can be made by using the following equation:
w
fin DyX/u
(1)
Solving for X, the equation is changed to:
(2)
2nDs
The distance for one-half width mixing for a bank discharge is then estimated by the
following equation:
<3)
B 2lt Dy
For a discharge near the center of flow in the river or stream, the following
equation can be used for one-half width mixing:
*1/2= (4)
1/2 8
where
X1/2 = distance downstream to achieve one-half width mixing (ft)
u = velocity of stream at critical low flow downstream of discharge (ft/sec)
W = width of the river at critical low flow downstream of discharge (ft)
Dy = lateral dispersion coefficient for critical low flow downstream of discharge
(ft2/sec)
29
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EPA Region VT3I Mixing Zones and Dilution Policy
where
Dy = cdu * (5)
c = channel irregularity factor (unitless)
c = 0.1 for straight, rectangular streams
c = 0.3 for channelized streams or irrigation canals
c = 0.6 for natural channels with moderate meandering
c = 1.0 for streams with significant meandering
c > 1.0 for streams with sharp 9(f and greater bends
d = water depth at critical low flow downstream of discharge (ft)
u* = shear velocity (ft/sec)
where
u* = \jgds ^
g = acceleration due to gravity (32.2 ft/sec2)
s = slope of the channel downstream of discharge (ft/ft)
Example 4.1: Calculation of distance to achieve one-half width mixing. An
industry has a surface bank discharge into a stream which, at critical low flow,
has a depth of 1 foot and a width of 62 feet. The velocity of the stream at chronic
low flow (4-day, 3-year flow) below the discharge is 1.5 feet per second. The
average slope of the stream is 0.0008 feet/feet. The stream below the discharge
exhibits moderate meandering. Ignoring any momentum from the discharge itself,
what is the distance below the discharge where the effluent plume will extend to
one half the stream width?
Solution: The distance to achieve one-half width mixing is given by the equation
Xi/2 = 0^/2)^/27rDr The shear velocity is calculated as u" = (gds)1'2 = (32.2
ft/sec2 x 1 ft x 0.0008)1/2 = 0.16 ft/sec. The dispersion coefficient is given as Dy
= cdu* = 0.6 x 1 ft x 0.16 ft/sec = 0.096 ft2/sec. The distance to achieve one-
half width mixing is estimated by:
X1/2 = ((62 ft/2)2 x 1.5 ft/sec)/(2 x ir x 0.096 ft2/sec) = 2400 feet.
30
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EPA Region VIII Mixing Zones and Dilution Policy
• If Distance to Attain One-Half Width Mixing (XI/2) < 10 Stream Widths
If the downstream distance to attain one-half width mixing is less than 10 times the
stream width, then the chronic/human health analysis uses one-half of the critical low
flow of the stream (chronic aquatic life or human health) as dilution. The normal
mass balance equation can be used to determine effluent limits in this situation:
c _C^(
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EPA Region VIII Mixing Zones and Dilution Policy
• If Distance to Attain One-Half Width Mixing (X1/2) > 10 Stream Widths
If the downstream distance to attain one-half width mixing is greater than 10 times the
stream width, then the effluent limit is given as:
Ceff = effluent concentration (mg/1 or /xg/1)
Qeff = effluent flow (ft3/sec)
Cup = upstream concentration (mg/1 or /xg/1)
Qup = upstream low flow at critical conditions (ft3/sec)
Cmix = average concentration in the mixing zone plume (chronic or human
health criterion) (mg/1 or /xg/1)
C
Cmix(Qeff+ Q Qup)~ (6 QUpCUp)
(8)
where
and
6 = percentage of upstream flow mixing with effluent flow
For bank dischargers:
6 -
(9)
32
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EPA Region VIII Mixing Zones and Dilution Policy
For center dischargers:
MtzDXIu)
(V J + (10)
e =
a
¦up
Dy = lateral dispersion coefficient for critical low flow (ftVsec) (See
page 31 for equation used to calculate Dy)
X = distance downstream from discharge = 10 times the stream width (ft)
W = stream width at critical stream flow downstream of discharge (ft)
u = stream velocity at critical stream flow downstream of discharge
(ft/sec)
An Example of how these equations may be applied is presented in Example 4.2.
Further discussion of the equations presented above is given in Appendix A. In
addition, Appendix A presents the derivation and validation of Region Vm's
spreadsheet model (STREAMIX I).
WET Limits
For chronic aquatic life WET limitations, the dilution for testing would be calculated
using only the fraction of the upstream critical low flow that has mixed with the
effluent flow. This fraction of upstream flow used in the dilution is defined by 6 as
defined in equation (9) and (10) (e.g. effluent dilution = Qeff/(Qeff + 0Qup)-
Lake Dischargers: The chronic aquatic life/human health mixing zone should not
exceed 5% of the lake surface area or 200 foot radius, whichever is more restrictive.
Individual mixing zones may be further limited or denied due to site specific
considerations. It may be necessary only to limit mixing zone size for particular
parameters or for particular criteria (i.e., aquatic life or human health). For human
33
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EPA Region VIII Mixing Zones and Dilution Policy
Example 4.2: Calculation of effluent limits for discharge. A municipal facility
has a surface bank discharge into a river which has a depth of 1 foot and a width of
80 feet. The velocity of the river at chronic low flow (4-day, 3-year flow) below the
discharge is 1.4 feet per second. The lateral dispersion coefficient (Dy) at critical
flow has been calculated as 0.32 fF/sec for the river below the discharge. The
municipal discharge has a mean daily flow of 17 cfs and a maximum daily flow of 25
cfs. The receiving water has a 4-day, 3-year low flow above the discharge of 90 cfs
and a 1-day, 3-year low flow of 74 cfs. The particular pollutant of concern is
aluminum which has ambient criteria of 87 ptg/1 (4-day chronic) and 750 /zg/1 (1-hour
acute). No aluminum has been detected in the upstream waters. It has been
determined that a mixing zone would be allowed in this situation. Determine the
effluent limits for the facility based on the appropriate mixing zone procedures.
Solution: Since the facility does not mix instantaneously with the receiving water, the
daily maximum effluent limitation would be the acute criteria of 750 /ig/1 aluminum
independent of what procedure is used to determine the chronic effluent limitation.
To determine how much of the receiving water can be used in dilution calculations for
the chronic limitation, the distance downstream to the point of half-width mixing must
be computed. Substituting the values into the equation for distance to half-width
mixing, we obtain Xin = (W/2)2u/27rDy = ((80/2)2 x 1.4)/(2 x i x 0.32) = 1114
feet. The distance downstream to achieve half-width mixing is greater than 10 times
the width (1114 feet > 800 feet), therefore the amount of flow from the receiving
water used in the dilution calculations will be less than half of the river flow. Rather,
the fraction of flow which mixes with the effluent at the downstream point 10 times
the width (800 feet) is used in the chronic dilution calculation. This fraction is given
with equation 9 on page 32 of the Policy text. Substituting into the equation, we
obtain 0 — {[(2 x ir x 0.32 ft2/sec x 800 ft/1.4 ft/sec)1/2/80] (17 cfs + 90 cfs) - 17}/
90 = 0.31. The chronic limits (4-day average) for the discharger would be based on
the mass balance equation using 31 % of the upstream chronic low flow for dilution:
Ccff = [Cmi^Qjff + OQup) - (0QupCup)]/QCfr
= [87 ng/\(ll ft3/sec + (31 %)90 tf/sec) - ((31%)90 tf/sec x 0 /ig/l)]/17 ffVsec
= 230 ttg/l (chronic limit)
Since this is a 4-day chronic limit, it should be converted into a 30 day limit by using
the appropriate conversion procedures (see EPA's Technical Support Document for
Water Quality-based Controls for methods of conversion).
34
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EPA Region Vm Mixing Zones and Dilution Policy
health water quality parameters that have an effect over a short period of exposure
(e.g. N03), it may be more appropriate to require compliance with applicable criteria
at the end-of-pipe.
Once a distance from the point of discharge to the edge of the mixing zone is
determined, the amount of dilution available and the required effluent limit is
determined by computer modeling or the simple relationship:
C,#.0.3(z/
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EPA Region Vm Mixing Zones and Dilution Polic\
CHAPTER 5. CASE EXAMPLES
This chapter provides some case examples to demonstrate the potential effect of
application of the Region vm mixing zone policy on NPDES permit effluent limits. The
simplified procedures as described in the previous chapter (4. Model Mixing Zone-Dilution
Implementation Procedure) were followed. The rationale for each example is documented
using the mixing zone flow chart found in Figure 1, also found in Chapter 4, as a template.
When the "Modeling Method" approach was used in the examples to calculate revised
effluent limits, the equations and mixing zone size limitations found in Section IV were used.
The results pertaining to effluent limits found in
the case examples are not intended to be
conclusive. Assumptions pertaining to discharge
and receiving water characteristics were made in
some instances to complete the example.
The case examples are
based on actual NPDES
dischargers in Region Vm. The
results pertaining to effluent
limits found in the case examples
are not intended to be conclusive.
Assumptions pertaining to
discharge and receiving water
characteristics were made in
some instances to complete the example. The case examples nonetheless serve to illustrate
how the mixing zone procedures could affect limitations in NPDES permits.
DEFAULT METHOD EXAMPLES
Bismarck. ND: Yankton. SD These are two examples where the default method was
used. The default method is intended primarily for those situations where necessary
information to perform modeling (e.g. channel slope, depths, widths, velocities, etc.)
are not readily available. In addition, the default method can be used, such as in
these examples, for pollutants where achieving the acute numeric criteria at the end-
of-pipe (without an allowance for dilution) will always be more limiting than the
(mixing zone-based) chronic effluent limitations (i.e., regardless of what mixing zone
method is selected). The default method may also be appropriate where there are
concerns about the potential environmental impacts of allowing a mixing zone and a
conservative amount of dilution is desired.
36
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==^^^====^=^=^^=== EPA Region Vm Mixing Zones and Dilution Policy
MODF.T TNG METHOD EXAMPLES
Tremonton. UT; Silverthome/Dillon. CO. Green River. WY. Steamboat Springs. CO
In these examples, enough information was available to perform simplified modeling
using the equations and approaches described in Chapter 4. The first calculation to be
made is to determine whether or not the mixing plume extends to the half stream
width prior to a distance 10 times the stream width downstream from the discharge.
In the case of Tremonton, Silverthome/Dillon, and Steamboat Springs the 1/2 stream
width was reached prior to the 10 X width. In the case of Green River, the 10 X
width longitudinal limit was reached prior to the 1/2 width limit. The daily maximum
limit for Green River is based on meeting acute standards at the end-of-pipe. This
limit (1 mg/l-N) is low because of the high pH (9.0 s.u.) exhibited in the effluent.
Take note that for parameters which have criteria that vary with pH, temperature, or
hardness, the most critical point in the stream may be further downstream beyond the
mixing zone (far field) where more ambient mixing occurs. In these far field
analyses, it may be seen that factors such as pH, temperature, or hardness are not as
favorable as within the mixing zone. Consider the example of un-ionized ammonia
toxicity which is particularly sensitive to pH conditions. The pH regime downstream
from the mixing zone may be higher than the pH within the mixing zone, possibly
making the point downstream where complete mix occurs more limiting to effluent
limits.
37
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Case Example: Bismarck, ND
Pollutant: Chlorine
Receiving Water: acute low flow (1 day, 3 year) = 10,000 cfs
Discharge: 9.3 cfs
Current
Effluent Limit: 4.5 mg/1 TRC
Mixing Zone-based
F.fflnp.nt T.imir 0.019 mg/1 TRC (daily maximum)
The mixing zone-based limit is based on meeting the acute
chlorine criteria (0.019 mg/1) at the end of the pipe (no dilution allowed).
Using the default method, the chronic limit would be based on using
10% of the upstream flow for dilution. The resultant chronic limit
would be greater than the acute limit, making the acute limit the
more restrictive of the two.
-------
Mixing Zone Flow Chart
Bismarck Case Example
(Shaded boxes indicate decision path for example.)
Is dilution available
at critical conditions?
yes
no
End-of-pipe limits
(no dilution).
Does ti&e discharge alfcct
a wetland or da State WQS
otherwise joohibit dilution?
yes
no
Is the facility a minor POdTW
where allowing dilution
would poseinsigntficant
environmental risks? (Assumed
where dilation ratio &
yes
Is the discharge
to a lake?
no
yes
no
Would allowing diliitioa or araring zone
pose unacceptable enviroruneotal risks?
Determine dilution case-by-case:
allow no more than 19:1 dilution
(5% effluent).
yes
no
For discharges to rivers ana
ajreams, do available
reasonably support a conclusion
that there is near instantaneous
andcomplete mixing? Is there
useof a^Uffuser?
1 no
yes
Allow hill
critical stream
flow for
acute, chronic,
& human
health limits.
End-of-pipe
limits (no
dilution).
Allow critical
stream flow or
some portion
for acute,
chronic &
human health
limits.
Calculate based on one of the following methods;
* DEFAULT METHOD
Streams/rivers: Acute limits - ^H^f-pi^limks {no dilution)
Ctmmic/human health limits- Use no more than 1056 of
critical stream low flow.
Lakss/reserviiirs: Acute limits - End-of-pipe limits (no dihitioii)
Gtoic/huiBanheal& Iiaafc -No more than 4:ld2ution allowed
C2096 efBnoaft
MODELING METHOD
Streams/rivers: Acute Limits - End-of-pipe limits (no dilution)
Chronic/human health limits - Mixing Zone limited to no more
than 1/2 of cross sectional area of stream or
no more than 10 times stream width
Lakes/reservoirs: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health Limits - Mixing zone limited to no more
than 200 foot radius or 5% of lake surface
FIELD STUDY METHOD
Actual dilution in receiving water is determined by field study, with mixing
zone limited by size provisions described in above Modeling Method.
-------
Case Example: Yankton, SD
Pollutants:
Receiving Water:
Discharge:
Current
Effluent Limits:
Mixing Zone-based
Effluent limits:
Yankton
POTW
Ammonia and Chlorine
approx. 9000 cfs low flow
2.5 cfs
no ammonia limits; 1.0 mg/1 TRC (summer)
3.7 mg/l-N ammonia; 0.02 mg/1 TRC
(both daily maximum limits)
The mixing zone-based limits are based on meeting acute standards at the end-
of-pipe (no dilution allowed). Hie acute standard for TRC is 0.02 mg/1 and the
chronic ammonia standard is 0.04 mg/l-N un-ionized ammonia. At an effluent pH
of 7.7 s.u. and a temperature of 20° C, the 0.04 mg/l-N is equivalent to
2.11 mg/l-N total ammonia. In South Dakota, the acute standard is determined by
multiplying the chronic standard by 1.75. The acute standard (as well as the
acute effluent limit) is 1.75 x 2.11 = 3.7 mg/l-N total ammonia. In calculating
the chronic limits using the default dilution of 10% of the upstream flow, it is
shown that the chronic limits are greater than the acute limits, making the
acute limits more restrictive.
-------
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Mixing Zone Flow Chart
Yankton Case Example
(Shaded boxes indicate decision path for example.)
Is dilution available
at critical conditions?
End-of-pipe limits
no
(no dilution).
1 yes
i
i
Does &e discharge affect
a wetland or do State WQS
o&emiaeprohibit dilution?
yes
Is the facility a minor POTW
where allowing dilution
would pose insignificant
environmental risks? (Assumed
where dilution ratio S 50:1)
Is the discharge
to a lake?
| yes
termine dilution
>w no more than
Allow full
critical stream
flow for
acute, chronic,
& human
health limits.
yes
De
allc
DO
^se-by-case:
19:1 dilution
Would aUowingdihition or a mixingzone
pose unacceptable environmental risks?
(5% effluent).
yes
no
For discharges to rivers and
streams, do available: data
reasonably support a conclusion
that there Is near instantaneous
and con^letemixing? Is there
nseof&difiuser?
End-of-pipe
limits (no
dilution).
yes
no
Allow critical
stream flow or
some portion
for acute,
chronic &
human health
limits
Calculate efiluentiimlts based on one ofthe following methods;
* DEFAULT METHOD
Streams/riven: Acute limits Bid*o£'pipe limits {no dilution)
Gironic/human health limits- Use no more than 1055 of
critical stream tow How.
Lakes/reservoirs: Acutelimks - Satof-pipe limits (nodilntiori}
Qbroitfc/buman health limits - No more than 4:1 dilution allowed
: G6 % efftneafl
MODELING METHOD
Streams/rivers: Acute Limits - End-of-pipe limits (no dilution)
Chronic/human health limits - Mixing Zone limited to no more
than 1/2 of cross sectional area of stream or
no more than 10 times stream width
Lakes/reservoirs: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health Limits - Mixing zone limited to no more
than 200 foot radius or 5% of lake surface
FIELD STUDY METHOD
Actual dilution in receiving water is determined by field study, with mi-ring
zone limited by size provisions described in above Modeling Method.
-------
Case Example: Tremonton, UT
Pollutant: Ammonia
Receiving Water: 30 cfs (7Q10 June-Sept)
Discharge: 2.32 cfs
Current
Effluent TJmif 14 mg/l-N 30 day; 25 mg/l-N daily max.
Mixing Zone-based
Effluent limit- 7.6 mg/l-N 30 day; 11 mg/l-N daily max.
Model assumptions: stream slope = 0.0003; width = 11 ft;
c = 0.8; depth = 2.9 ft; velocity = 1 ft/sec; Dy = 0.388 f?/sec
Background quality: 0.1 mg/l-N ammonia
Hie mixing zone analysis shows that the effluent plume reaches the 1/2 width
limit prior to the longitudinal limit of 10 X width. Thus, the mining zone-
based effluent limits are calculated using 1/2 the upstream critical low flow
for the chronic (30 day) limit and and using the end-of-pipe requirement for the
the acute (daily max.) limit. Tlie in-stream chronic standard is 1.1 mg/l-N
total ammonia. The effluent conditions were assumed to be 7.6 pH and 20° C.
At these conditions, die acute standard is 11 mg/l-N total ammonia.
-------
Mixing Zone Flow Chart
Tremonton Case Example
(Shaded boxes indicate decision path for example.)
-------
Case Example: Silverthorne/Dillon, CO
Pollutant:
Receiving Water:
Discharge:
Current
Effluent limit;
Mixing Zone-based
Hfflnftnt TJmit'
Model assumptions:
c = 0.6; depth =
Background quality:
Ammonia
24 cfs (30E3 October)
4.2 cfs
8.8 mg/l-N 30 day; no acute limit
3.1 mg/l-N 30 day; 22 mg/l-N daily max.
stream slope = 0.013; width = 20 ft;
1 ft; velocity = 1.4 ft/sec; Dy = 0.388 f?/sec
0 mg/l-N
The mixing zone analysis shows that the effluent plume reaches the 1/2 width
limit prior to the longitudinal limit of 10 X width. Thus, the mixing zone-
based effluent limits are based on using 1/2 the upstream critical low flow
for the chronic (30 day) limit and using the end-of-pipe requirement for the
acute (daily max.) limit. The in-strcam chronic standard is 0.8 mg/l-N total
ammonia. The effluent conditions were assumed to be 6.7 pH and 16° C temp.
At these conditions, the acute standard is 22 mg/l-N total ammonia.
-------
Mixing Zone Flow Chart
Silverthorne/Dillon Case Example
(Shaded boxes indicate decision path for example.)
Is dilution available
at critical conditions?
yes
no
End-of-pipe limits
(no dilution).
Doestbe discharge affect
a wetland or do State WQS
otherwise prohibit dilution?
yes
no
Is the facility a minor POTW
where allowing dilution
would pose insignificant yes
environmeotal risks? (Assumed
where dilution ratio -ife 5&1)
11 no
Would allowing dilution or a mixing zone
poseunaccq^leenvirotanental risks?
| no
^discharges to rivers and
streams, do available data
reasonably support a conclusion
that there is near
and complete mixing? Isthere
use of a diffuser?
no
Is the discharge
to a lake?
no
yes
Determine dilution case-by-case:
allow no more than 19:1 dilution
(5% effluent).
yes
Allow full
critical stream
flow for
acute, chronic,
& human
health limits.
End-of-pipe
limits (no
dilution).
yes
Allow critical
stream flow or
some portion
for acute,
chronic &
human health
limits.
Calculate effluent limits based on one of the following methods:
¦ DEFAULT METHOD
Streams/rivers: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health limits - Use no more than 10% of
critical stream low flow.
Lakes/reservoirs: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health limits - No more than 4: l dilution allowed
(20% effluent)
MODELING METHOD
Streams/rivers: Acwte lifflats - End-of-pipe limits (no dilutikjn)
Chrotoc/huiaan health limits - Mixing Zone limited to no more
tiian l/2 of cross sectional area of stream or
no more hum 10 tunesstream width
Lakes/reservoirs: Acute limits - End-of-pipe Hosts (no dilution)
Chronic/human health Limits - Mixing zone limited to no more
than 200 foot radius or 556 of lake surface
FIELD STUDY METHOD
Actual dilution in receiving water is determined by field study, with miTing
zone limited by size provisions described in above Modeling Method.
-------
Case Example: Green River, WY
Green River
POTW
Approx. 2 miles to
complete mix.
Pollutant: Ammonia
Beceivinp Water- 236 Cfs (7Q10 winter)
Discharge: 2.3 cfs
Current
Effluent T .imit- no limits
Mixing Zone-based
reffliiCTt t imif 15 mg/l-N 30 day; 1 mg/l-N daily max.
Model assumptions! stream slope = 0.0021; width = 115;
c = 0.6; depth = 1.6 ft; velocity = 1.3 ft/sec; Dy = 0.316 ft/sec
Background quality: 0.0 mg/l-N ammonia
Hie mixing analysis in the Green River indicates that the mixing plume
reaches the longitudinal limit of 10 X width before reaching the 1/2 width
limit. At this distance from the discharge (1150 feet), only 36% of the
Green River flow has mixed with the effluent and the plume width is 42 ft.
The chronic limit (30 day) was based on the 36% dilution. The in-stream
conditions were assumed to be pH 8.5 s.u. and 5° C. At these conditions, the
chronic standard is 0.39 mg/l-N of total ammonia. The acute limit (daily max.)
was based on end-of-pipe requirements. Effluent conditions were pH 9.0 s.u.
and 16° C. At these conditions, the acute standards is < 1 mg/l-N total ammonia
-------
Mixing Zone Flow Chart
Green River Case Example
(Shaded boxes indicate decision path for example.)
is dilution available
at critical conditions?
no
End-of-pipe limits
(no dilution).
yes
Does the discharge affect
a wetland or do State WQS
otherwise prohibit dilution?
yes
no
Is the iacility a minor POTW
where allowing dilution
would pose insignificant
environmental risks? (Assumed
where dilution ratio 50:1)
I
yes
no
no
Would allowing dilation or a mixing zone
pose unacceptable environmental risks?
I
Is the discharge
to a lake?
| y«
Determine dilution case-by-case:
allow no more than 19:1 dilution
(5% effluent).
Allow full
critical stream
flow for
acute, chronic,
& hnmnn
health limits.
yes
no
End-of-pipe
limits (no
dilution).
For dischatgesto tlver&and
streams, do available data
reasonably support a conclusion
that there is near instantaneous
and complete mixing? Is there
use of a diffuse*?
Allow critical
stream flow or
some portion
for acute,
chronic &
human health
limits.
yes
1 no
Calculate effluent limits based on one of the following methods:
¦ DEFAULT METHOD
Streams/rivers: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health limits - Use no more than 10% of
critical stream low flow.
Lakes/reservoirs: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health limits - No more than 4:1 dilution allowed
(20% effluent)
MODELING METHOD
Streams/rivets: AcutelJmits - End-of-pipe limits (no dilution)
Chronic/human health limits - Mixing Zone limited to no more
than 1/2 of cross sectional area of stream or
no moret&anlO times stream width
Lakes/reservoirs: Acntelimits - End-of-pipe limits (no dilution)
Chronic/human health Limits - Mixing zone limited to no more
than 200 foot radius or 5 % of lake surface
FIELD STUDY METHOD
Actual dilution in receiving water is determined by field study, with mixing
zone limited by size provisions described in above Modeling Method.
-------
Case Example: Steamboat Springs, CO
Pollutant:
Receiving Watpr-
Discharge:
Current
Effluent Limit:
Mixing Zone-based
Effluent T jmit:
Model assumptions:
Copper
56 cfs (30E3)
5.2 cfs
no current limit for copper; normal full-mix
limit would be 141 /zg/1 Cu
78 pg/1 (30 day); 18 pg/1 (daily max.)
stream slope = 0.011; width = 30 ft;
c = 0.7; depth = 1.5 ft; velocity = 1.3 ft/sec; Dy = 0.244 ft2/sec
Background quality: 0 /zg/1 Cu
The mixing zone analysis shows that the effluent plume reaches the 1/2 width
limit prior to the longitudinal limit of 10 X width. Thus, the mi-ring zone-
based effluent limits are based on using 1/2 the upstream critical low flow
for the chronic (30 day) limit and using the end-of-pipe requirement for the
acute (daily max.) limit. The in-stream chronic standard is 12 pg/1 copper and
an acute standard of 18 /ig/1 copper. The daily maximum limit provides the
greatest limitation for the discharger because it is less than the 30 day limit.
-------
Mixing Zone Flow Chart
Steamboat Springs Case Example
(Shaded boxes indicate decision path for example.)
Is dilution available
at critical conditions?
no
End-of-pipe limits
(no dilution).
yes
Does the discharge affect
a wetland or do State WQS
otherwise ptohlbitdilution?
| no
Is the facility a minarPOTW
where allowing dilution
would pose insignificant
environmental risks? (Assumed
where dilution ratio ^ 50:1)
yes
yes
no
no
Would allowing dilation or a mixing zone
pose unacceptable environmental risks?
Is the discharge
to a lake?
\ yes
Determine dilution case-by-case:
allow no more than 19:1 dilution
(5% effluent).
Allow full
critical stream
flow for
acute, chronic,
& human
health limits.
yes
no
End-of-pipe
limits (no
dilution).
For discharges to rivers and
streams, do available data
reasonably support a conclusion
that there Is near instantaneous
and complete mixing? Is there
use of a diffuser?
Allow critical
stream flow or
some portion
for acute,
chronic &
human health
limits.
yes
1 no
Calculate effluent limits based on one of the following methods:
¦ DEFAULT METHOD
Streams/rivers: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health limits - Use no more than 10% of
critical stream low flow.
Lakes/reservoirs: Acute limits - End-of-pipe limits (no dilution)
Chronic/human health limits - No more than 4:1 dilution allowed
(20% effluent) m
¦ MODELING METHOD
Streams/rivers: Acute limits - End-o£-pipe limits {no dilution)
Chronic/human health limits - Mixing Zone limited to no more
thanl/2 of cross sedionalarea of stream or
no more than 10 times stream width,
Laksea/reservoks: Acote Kraits - End-of-pipe limits (no dilation)
Chronic/human health Limits ~ Mixing zone limited to no more
than 200 foot radius or 5% of lake surface
¦ FIELD STUDY METHOD
Actual dilution in receiving water is determined by field study, with mixing
zone limited by size provisions described in above Modeling Method.
-------
EPA Region VTD Mixing Zones and Dilution Policj
APPENDIX A - REGION \TQ SIMPLIFIED MIXING ZONE MODEL
There are several equations that are presented in the EPA Region vm Mixing Zones
and Dilution Policy to aid in the calculation of mixing zone plume size and pollutant
concentration within the mixing zone. The following describes the derivation and validation
of the equations as well as the model that Region Vm recommends for use in performing
mixing zone analyses if more complex models are not used. The Region vm approach is
based on utilization of simplified mathematical relationships found in the literature.
The Region VIII model (STREAMIX I, version 2) can be used to estimate both
average and extreme pollutant concentrations within the mixing zone as well as the width of
the mixing zone plume for surface dischargers (not submerged) into a river or stream.
Upstream pollutant concentrations are taken into account in the model. The discharge can be
situated at the bank of the receiving water or at any point laterally across the waterbody.
The model is two dimensional with assumptions of vertically uniform concentration, constant
depth, and constant velocity. There is also an assumption that mixing occurs through
ambient diffusion with no contribution from discharge momentum. In addition, the model
calculates in-mixing zone concentrations for instances where the upstream water has a higher
concentration than the effluent.
Figure A-l provides a schematic of a discharge into a river. The basic mass balance
equation can be used to describe the pollutant concentration in the mixing zone:
+ = (A"1)
where
Cm,* = concentration of pollutant in the mixing zone at various distances
downstream of the discharge (mg/1)
Qup = upstream low flow at critical conditions (ft3/sec)
Cup = upstream concentration (mg/1)
Ceff = effluent concentration (mg/1)
Qeff = effluent flow (ft3/sec)
A-l
-------
Figure A-l
Schematic of Mixing Zones in Rivers
Bank Discharge
Qeff
Ceff
(top view)
Center Discharge
(top view)
(cross section)
-w.
w
total
Qmix = 0Qup + Qeff
(cross section)
¦f
w
total'
Qmix = 0Qup + Qeff
Cmix = (QeffCeff + 0QupCup)/(OQup + Qeff)
Cmix = mixing zone concentration
Ceff = effluent concentration
Qeff = effluent flow
Cup = upstream concentration
Qup = upstream flow
Wmi*
= width of mixing zone
W total = total width of river
0 = fraction of upstream flow
mixed with effluent flow
y = distance of discharge from
stream bank
A-2
-------
EPA Region VIII Mixing Zones and Dilution Policy
6 = percentage of upstream flow mixing with effluent flow at distance X from
the discharge (unitless)
An equation that describes the ratio of flow in the mixing zone plume (QmJ and the
total combined flow of the upstream waters and the effluent discharge (Q^i) can be shown to
equal the ratio of the width of the mixing zone plume and the total width of the stream
(assuming a rectangular cross section):
where
Solving for 6,
«?.
eff + )
•total
«?.
eff + Qup)
U'mu du
^totai ^
W
W.
(A-2)
total
wn
W
d
u
total
width of the mixing zone plume (ft)
width of the total stream (ft)
depth of the stream (ft)
velocity of the stream (ft/sec)
(
w
e
total
eff
(A-3)
-up
From the literature (Fischer, et al; 1979), an equation that describes the width of a
mixing plume (wmiJ associated with a centerline discharge in the middle of a river can be
approximated by the equation 4(2DyX/u)I/2 where Dy is the lateral dispersion coefficient and
X is the longitudinal distance downstream from the point of discharge. The Region VHI
model recommends the following similar formulation:
w
87ZD ^
' u
(A-4)
A-3
-------
EPA Region VLLI Mixing Zones and Dilution Policy
for centerline discharges and
w . =
mix A
2n Z) - (A"5)
y u
for bank discharges. Region Vm recommends these equations based on validation runs with
actual data. These two equations for mixing plume width can be combined into one
formulation which is a function of the distance off the nearest bank:
(2— + Y)1 2tz D — (A"6)
W u
where
wmix = width of mixing zone plume (ft)
W = width of stream downstream of discharger (ft)
y = the lateral distance from the stream bank to the discharge at some point in
the river where y is less than or equal to 1/2 the entire stream width (ft)
X = distance downstream of discharge (ft)
u = velocity of stream downstream of discharge (ft/sec)
Dy = lateral dispersion coefficient (ft2/sec)
where
Dy = cdu * (A-7)
A-4
-------
EPA Region Vm Mixing Zones and Dilution Policy
c = channel irregularity factor (unitless)
c — 0.1 for straight, rectangular streams
c = 0.3 for channelized streams or irrigation canals
c — 0.6 for natural channels with moderate meandering
c = 1.0 for streams with significant meandering
c > 1.0 for streams with sharp 90f and greater bends
d = water depth at. critical low flow downstream of discharge (ft)
u* = shear velocity (ft/sec)
where
u- = v£S (A"8)
g = acceleration due to gravity (32.2 ft/sec2)
d = water depth at critical low flow downstream of discharge (ft)
s = slope of the channel downstream of discharge (ft/ft)
c
The value for the mixing zone plume (w^J should be restricted such that (w^J^ = Wtoul.
Rearranging the mass balance equation (A-l) to solve for CmJ[, the equation becomes:
C_. =
_ Qeff^eff + ® Qup Cm (A-9)
To determine the laterally-averaged mixing zone concentration, we can substitute 6 as given
in equation (A-3) into this equation, we obtain:
«?„ -
0= ~ (A"1Q)
[-= (Qup + QeJf) ~ QJ
— «? ) + Q *
Q ^up' *
-------
EPA Region Vffl Mixing Zones and Dilution Policy
This equation is used in the STREAMIX I model to represent the laterally averaged
concentration in the mixing plume at distance X downstream from the discharge (Caveragc).
This further reduces to:
«?„ ~ -
(Qv + QJ
w
With a zero concentration of the pollutant in the upstream water (C^ = 0), the equation
reduces to:
(C ) = (A.12)
WIX^f ^up + Qeff)
or, substituting the formulation for w^ from equation (A-6) into this equation, we define
(CmJ.vcragc as a function of distance downstream of the discharge:
mix^ average
\
(A-13)
(2i + V22«DyZ^ + ^
yu -up
For approximating the extreme concentration within the mixing plume ((CmT)m, for
instances where Ceff > Cupstresm or (Cm„) for instance where Ceff < the same
formulation used to derive the average lateral concentration is used, except a slightly
different expression is used for w^. We will define this new w^ formulation w^'.
Instead of equation (A-6), wmu' will be defined as:
A-6
-------
EPA Region VIII Mixing Zones and Dilution Policy
w_
(2— + l)2 7T D -
W y u
(A-14)
The equation for wm)X' is used only to mathematically approximate the extreme concentration
and has no physical meaning. The full equation used in STREAMIX I to represent the
extreme concentration is then given as:
™mix
(CJ,
«?„ ^ -
This equation is the same equation presented in both the Technical Support Document for
Water Oualitv-based Toxics Control (US Environmental Protection Agency; 1991) and in
Fischer, et al (1979). In the Technical Support Document, the Qs is the same as
(Qup + Qefr)- 1° both the Technical Support Document and in Fischer, et al (1979), the
formulation given in equation (A-16) describes the maximum concentration within the mixing
zone associated with a bank discharge.
A-7
-------
EPA Region VIII Mixing Zones and Dilution Policy
In summary, Region VIII recommends the following mathematical relationships for use in
approximating plume width and mixing zone concentrations
¦ Plume width:
w . =
ma
(2— + l)22rcZ> -
W
u
(A-17)
Concentration in mixing zone:
Q JC ~ + 0 O C
Q _ ^-effeff i
up up
mix
(e
-------
EPA Region Vm Mixing Zones and Dilution Policy
velocity. The lateral diffusion coefficient can be changed from river segment to segment to
accommodate different channel configurations. In addition, an optional first order decay
term is included which can be used to influence both the maximum and average mixing zone
concentrations.
Since STREAMIX I calculates both the extreme and average pollutant concentrations
within the mixing zone, certain terms are defined in Figure A-2 to help describe the
nomenclature used in STREAMIX I. The "extreme" concentration in the mixing zone can
represent either a maximum concentration in the cross section of the river (for the instance
where the upstream water has a lower concentration than the effluent discharge) or a
minimum concentration (for the instance where the upstream water has a higher concentration
than the effluent discharge).
To validate STREAMIX I, comparisons were made between the modeled output and
data from several field mixing zone studies. Figures A-3 through A-7 provide the results of
these comparisons. In all cases, plume widths as well as average and shoreline/centerline
extreme concentrations were modeled. Where information on specific lateral diffusion
coefficients were given, the channel irregularity factor was adjusted to render the actual
value reported value for the coefficient. Coefficients were otherwise estimated. For
additional validation, STREAMIX I was used to calculate plume concentrations for a
hypothetical mixing problem given in Fischer, et al. Figure A-8 gives the result of that
comparison.
The input parameters needed to run the STREAMIX I spreadsheet model include:
. effluent flow (ft3/sec)
. effluent concentration (mg/1 or fig/1) The value of effluent concentrations can be either
a known or unknown value for the model. As an
unknown, iterative runs are performed using the
ambient criteria as the target concentration within
the mixing zone.
. upstream flow and concentration (mg/1 or ^g/1)
. stream slope (ft/ft)
. stream depth at critical low flow downstream from discharge (ft)
. stream width at critical low flow downstream from discharge (ft)
. stream velocity at critical low flow downstream from discharge (ft/sec)
1st order decay coefficient for pollutant (1/day) (optional; use only if needed)
Also, a general understanding of the stream or river shape is needed to estimate the channel
irregularity factor (see discussion above associated with equation A-7).
A-9
-------
Concentration
in Stream
(lateral profile)
Figure A-2
Cross Sections of Mixing Zones in River
Effluent Concentration > Upstream Concentration
C
extreme
fc
I average ,
mixing tone
plume
Bank Discharge
Cross Section
Concentration
in Stream
(lateral profile)
rxiz:
C
extreme
J \
[c
1 average ,
•mixing tMt
ptame
Center Discharge
Cross Section
Concentration
in Stream
(lateral profile)
Effluent Concentration < Upstream Concentration
/I
J
C 1—
average 1
mixing ztmc.
Bank Discharge
plume
Cross Section
Concentration
in Stream
(lateral profile)
I
\ /
viz
c ,—
average r
J&bctegzoffi
plume
Center Discharge
Cross Section
A-10
-------
60
50
"So 40
3
c
o
s 30
8
e
o
U
20
10
Figure A-3
Validation of STREAMIX Model
Atrisco Feeder Canal (NM)
(Fischer, 1967)
(Centerline Discharge)
Dye Concentration vs. Distance Downstream
Distance from Discharge (ft)
(Thousands)
Average Cone, (obs.)
^ Shoreline Cone, (obs.)
Plume Width vs. Distance Downstream
-0.1 0.1 I 0.3 I 0.5 I 0.7 I 0.9 I 1.1 I 1.3 I 1.5 I
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Distance from Discharge (ft)
(Thousands)
o Observed Width
A-ll
-------
Figure A-4
Validation of STREAMIX Model
Atrisco Feeder Canal (NM)
(Fischer, 1967)
(Bank Discharge)
00
3
a
.2
1
a
8
a
o
U
Distance from Discharge (ft)
(Thousands)
Average Cone, (obs.)
A Shoreline Cone, (obs.)
Plume Width vs. Distance Downstream
¦3
•o
-01 i 01o!2°-3o!40:5o!60-7o!80-9 i 11l!21-3l.l41-31.6
Distance from Discharge (ft)
(Thousands)
° Observed Width
A-12
-------
Figure A-5
Validation of STREAMIX Model
Arkansas River (CO)
(USGS, 1980)
(Bank Discharge)
Dye Concentration vs. Distance Downstream
(Thousands)
+ Average Cone, (obs.) ^ Extreme Cone, (obs.)
Plume Width vs. Distance Downstream
(Thousands)
o Observed Width
A-13
-------
60
3
a
o
c
8
a
o
U
26
24
22
20
18
16
14
12
10
8
6
4
2
0
Figure A-6
Validation of STREAMIX Model
South Platte River (CO)
(USGS, 1985)
(Bank Discharge)
Dye Concentration vs. Distance Downstream
\ \ Average Cone, (modeled)
\ Extreme Cone, (modeled)
A " —
+ t
A
-0.5
0.5 ] i. 5 1 2.5 1 3^5 1
1 2 3 4
Distance from Discharge (ft)
(Thousands)
+ Average Cone, (obs.)
^ Extreme Cone, (obs.)
Distance from Discharge (ft)
(Thousands)
° Observed Width
A-14
-------
Figure A-7
Validation of STREAMIX Model
Missouri River
(Yotsukura, 1970)
(Centerline Discharge)
(Thousands)
+ Average Cone, (obs.) ^ Extreme Cone, (obs.)
Plume Width vs. Distance Downstream
(Thousands)
A Observed Width (maximum)
« Observed Width (minimum)
A-15
-------
C = 100 mg/1
C = 0 mg/1
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Figure A-8
Validation of STREAMIX Model
Sample Mixing Zone Problem
(Fischer, et al, 1979)
Mixing of two streams of equal flow.
C = 50 mg/1
Mixing Zone Concentration
Concentration vs. Distance Downstream
(Thousands)
A-16
-------
EPA Region Vm Mixing Zones and Dilution Policy
For a copy of STREAMIX I, direct requests to:
US EPA; Region VHI
Monitoring and Standards Section
Water Management Division
999 18th Street; Suite 500
Denver, Colorado 80202-2466
Attention: Bruce Zander
References for Appendix A
Fischer, H.B. (1967). "Transverse Mixing in a Sand-Bed Channel"; USGS Professional
Paper 575-D, Pages D267-D272.
Fischer, H.B., List, E.J., Koh, R.C.Y., Imberger, J. and Brooks, N. (1979). Mixing in
Inland and Coastal Waters: Academic Press, Inc., NY
United States Geological Service (1980). Selected Hvdrologic Data. Arkansas River Basin.
Pueblo and Southeastern Fremont Counties. Colorado 1975-1980: USGS Open-File
Report 80-1185.
United States Geological Service (1985). Selected Hvdrologic Data for the South Platte
River Through Denver. Colorado: USGS Open-File Report 84-703.
Yotsukura, N., Fischer, H.B., Sayre, W. (1970). "Mixing Characteristics of the Missouri
River Between Sioux City, Iowa, and Plattsmouth, Nebraska"; USGS Water-Supply
Paper 1899-G.
A-17
-------
EPA Region VIII Mixing Zones and Dilution Policy
APPENDIX B - EPA REGION \TT1 RESPONSES TO MAJOR
COMMENTS AND QUESTIONS
Comments on First Draft Issued August 13. 1993
1. A number of commenters asserted that the policy statement does not establish a
sound technical basis for changing existing dilution approaches in EPA Region Vm
States, and that documented proof that existing approaches are causing an
environmental problem is lacking.
Response: The Region acknowledges that there is a lack of information on impacts to biota
and human health resulting from the generous dilution allowances currently incorporated into
water quality-based NPDES permits in the Region. The Region agrees that it would be
useful to characterize the ecological and human health risks of current dilution practices.
However, the Region also believes that, except in effluent-dominated situations, most point
sources do not mix rapidly with receiving waters. Where ambient mixing is incomplete,
generous dilution allowances result in effluent plumes with concentrations and toxicity
considerably in excess of ambient water quality objectives (i.e., regardless of stream flow
and assuming effluent quality is at or near permitted levels). Such plumes are of concern
because:
1) chemical-specific and toxicity objectives are not achieved in the plume,
2) effluent plumes can extend far downstream before complete mixing and criteria are
achieved,
3) effluent plumes are often located along the shore in shallow waters that are critical
nursery areas for aquatic organisms,
4) aquatic life can be attracted to effluent because of its temperature, and
5) studies have demonstrated correlation of criteria exceedences with in-stream impacts
to biota, particularly for toxicity (see TSD, 1991).
The Region believes that the current Region-wide approach of presumptively providing the
entire low flow as dilution ignores these site-specific mixing and dilution issues. The Region
is particularly concerned that, in incomplete mix situations, the same generous dilution is
always assumed regardless of the actual rate of mixing. The Regional position is that
exceedence of criteria in ambient waters is a serious matter, that mixing zones should be kept
small, and that mixing zone size limitations should be implemented in a realistic manner.
B-l
-------
EPA Region VIII Mixing Zones and Dilution Policy
Thus, the Region's basis for changing existing practice (in addition to the legal and
consistency issues discussed below) is the need to consider site-specific information such as
the rate of mixing in determining the level of dilution that is appropriate. Quite frankly, the
Region sees little or no technical merit in the mixing zone-dilution approach currently in use,
and believes that a more realistic approach should be employed to improve this aspect of the
water quality-based permitting process. The Region's recommended approach allows
considerable flexibility to take relevant site information into account, and use of the
recommended approach will improve the technical basis for water quality-based limits in
Region Vm.
2. Several commenters were concerned that the economic impacts of implementation of
the policy statement could be tremendous and have not been evaluated by the Region; a
related concern was that the proposed policy requiring compliance with acute criteria at
the end-of-pipe in incomplete mix situations will be costly for dischargers. Specific
concerns were expressed regarding small municipalities that have controlled-discharge
lagoon wastewater treatment facilities.
Response: The Region recognizes that implementing the Region's policy statement will
require an investment of state/tribal resources, and in some cases will result in more stringent
effluent limits. However, the Region emphasizes that the dilution allowances currently
provided are considerably more generous than those which are routinely applied in other
Regions, including other western Regions (see Appendix C). The Region believes that if the
costs of implementing a more protective approach were "tremendous," the problem would be
apparent in these other Regions.
In addition, the costs of implementing the Region's policy statement will depend upon the
specifics of the particular approach developed by the State or Tribe. The Region emphasizes
that its policy statement includes more recommendations than it does hard and fast
requirements. Although the Region generally will expect States and Tribes to do a better job
of making mixing zone and dilution decisions, the Region is not requiring any specific
approach. The policy statement does identify key issues that States and Tribes will need to
address and resolve; however, there is flexibility on these key issues to modify the Region's
recommended approach.
One example is the issue of determining when a discharge mixes with the receiving water in
a "near instantaneous and complete" manner. Where such "complete mixing" occurs (such
as when a diffuser has been installed), there will probably be little or no change to existing
permit limits for stream/river discharges as a result of the Region's policy statement, because
the full low flow will continue to be provided as dilution. The Region notes that, as long as
a reasonable technical basis is evident, States and Tribes may establish their own definitions,
guidelines, or procedures for identifying when "complete mixing" is occurring.
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EPA Region V III Mixing Zones and Dilution Policj
As a second example, although recommended low flows are included in the policy, the final
choice of appropriate low flows is another matter which has been left to the discretion of
each State and Tribe. In the case of low flows to implement human health criteria for
carcinogens, the Region has recommended use of the harmonic mean flow. For sixty
streams where the harmonic mean and 7Q10 flows were compared, EPA's TSD reports that
the harmonic mean was at least double the 7Q10 at all sixty sites. Thus, for carcinogens, the
Region notes that implementation of the recommended approach would likely result in less-
stringent treatment requirements.
A third example is the issue of mixing zone size restrictions. Although the policy statement
includes recommended maximum size restrictions, the policy also provides States and Tribes
with discretion to establish maximum size allowances somewhat smaller or somewhat larger
than the Regional recommendation. Although there are limits on what the Region can
accept, the survey of State mixing zone size restrictions in Appendix C shows that there are a
variety of different approaches to restricting mixing zone size.
Other notable issues where States and Tribes have flexibility include the approach to wetland
discharges (see comment # 5), the approach for small POTWs (see comment # 7), and the
choice of particular modeling or other methods for deriving effluent limits to achieve mixing
zone size restrictions.
With respect to the comments regarding implementation of acute chemical-specific criteria in
incomplete mix situations, the Region has changed its position on this issue. Although the
Region continues to recommend compliance with such criteria at the end-of-pipe, the policy
statement has been changed to allow each State and Tribe the flexibility to determine whether
or not to allow a limited mixing zone (i.e., a zone of initial dilution) on a case-by-case basis.
Further, Appendix D of the policy statement now contains Regional guidance on how such an
approach for acute chemical-specific criteria could be implemented.
Regarding small municipalities with controlled-discharge lagoon facilities, the Region notes
that step 3 of the Region's model procedure includes provisions allowing certain small
POTWs to qualify for more generous dilution allowances (see page 22). The Region
anticipates that some lagoon facilities would satisfy the requirements for coverage under
step 3. The Region also intends to allow States and Tribes the flexibility to develop their
own provisions establishing conditions under which small facilities may qualify for less-
restrictive dilution requirements; various formulations of such provisions may be appropriate,
provided that they are based on an evaluation of environmental risk, rather than treatment
costs (see comment # 7 below).
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EPA Region VIII Mixing Zones and Dilution Policy
In sum, although there will be some costs of implementing the Region's policy statement, the
Region believes these costs will not be "tremendous" and will depend upon how a host of
specific policy issues are addressed by the individual States and Tribes.
3. One commenter asserted that since federal regulations at 40 CFR 131.13 state that
development of mixing zone policies is discretionary, there is no clear regulatory
mandate for the Region's policy statement.
Response: The Region disagrees with this assessment and believes that its policy statement
has a strong legal basis. Although it is true that EPA's water quality standards regulation
provides for state/tribal discretion on mixing zone policies, the discretion concerns whether
to adopt such policies or alternatively to require criteria compliance at the end-of-pipe.
Where a State or Tribe elects to allow mixing zones, the water quality standards regulation
clearly establishes Agency authority to review and approve/disapprove the adopted mixing
zone policy. The Region's policy statement is intended to assist States and Tribes to
establish mixing zone approaches that are fully approvable.
EPA's water quality standards regulation in no way authorizes dilution approaches that
automatically and presumptively provide the entire low flow as dilution. To the contrary,
EPA's position has always been that where mixing is incomplete, mixing zone size should be
carefully limited in each individual case, and that the approach should take the site-specific
uses, mixing rate, and discharge location into account (see Red Book-1976, WQS Handbook-
1983, TSD-1991). In incomplete mix situations, the Region does not consider the existing
approach of Region Vm States to be a true "mixing zone approach;" such an approach
requires, at a minimum, some consideration of the rate of mixing and an effort to achieve
mixing zone size restrictions based on that rate of mixing. Rather, the Region considers the
existing approach of its States to be an approach that automatically and presumptively
provides generous dilution regardless of site-specific mixing or environmental risk factors.
The result is considerable uncertainty regarding effluent plume size, quality, and effects. As
such, the Region is not convinced that the existing practice of Region Vm States is
authorized by the EPA water quality standards regulation.
In a number of cases, the Region also has some doubt regarding whether existing practice is
consistent with State mixing zone authority as defined by the State water quality standards.
Such State mixing zone policies typically authorize mixing zones, but only if such mixing
zones will, for example, "provide a zone of passage," "be as small as practicable," and/or
"not intersect spawning or nursery areas, migratory routes, or municipal water intakes."
F \ Region vm believes that current approaches do not implement, or even consider, such
t e requirements and that a strong argument can be made in any case that current
approaches exceed the mixing zone authority provided under EPA's water quality standards
regulation.
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4. A comment was received that since the Region Vm policy statement is not a federal
law, State approaches will vary from Region to Region and will not be consistent
throughout the United States.
Response: The Region believes that consistency on mixing zone and dilution approaches is
already a serious problem, but that implementation of the Regional policy statement will
improve national consistency on this issue. The Region's evaluation of current approaches
nationally indicates that the existing practice in Region VHI is less protective, less
technically-sound, and less reflective of site-specific circumstances than what is done
elsewhere, particularly in incomplete mix situations. Even among States in neighboring
Regions, Region VIII has found that Arizona, Arkansas, California, Missouri, and Nebraska,
for example, routinely apply mixing zone practices that are more protective, and more
consistent with state and federal requirements, than those in Region Vm. Because many
States currently use approaches more technically and legally defensible than those in use by
Region VIH States, the Region believes that implementation of the recommended approach
would actually improve national consistency on this issue.
5. On the subject of mixing zones and dilution for discharges to wetlands, a commenter
pointed out that in some instances, sewage treatment plants were intentionally
constructed near wetlands to provide additional treatment prior to the wastewater
reaching a relatively pristine receiving stream. The commenter suggested that these
discharges may not have an overall detrimental effect on the wetland environment, and
often benefit wetlands during periods of drought. This commenter was concerned that
implementation of the proposed policy could result in dischargers bypassing wetlands
and increasing nutrient loads to receiving streams.
Response: Regarding discharges to wetlands, the Region notes that the issue of the water
quality standards and particularly the numeric criteria applicable to wetlands will probably
have a more significant effect on permit limits than will the approach to mixing zones and
dilution. In many wetlands, there is little or no surface water flow that could be the basis for
a dilution allowance. Further, where there is surface water flow in a wetland, the amount of
dilution that could be provided to a discharge may not be significant where the flow is small
compared to the discharge or where there is a slow rate of mixing of the effluent with the
receiving waters. As a result, even where a State or Tribe decided to allow mixing zones for
wetlands, any potential permit relief for a discharger would be small because the available
dilution will typically be small. The more important threshold issue for discharges to
wetlands will be the application of water quality standards in protecting wetlands (e.g.,
selection of criteria appropriate to protecting wetland uses could potentially have a much
more significant effect than any decision about whether or not to allow a mixing zone). The
Region also emphasizes that its mixing zones and dilution policy statement allows for State
and Tribal flexibility on this question. Although the Regional recommendation is to require
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compliance with applicable criteria at the end-of-pipe for wetland discharges, States and
Tribes are free to implement other defensible approaches. Wetlands are surface waters of
the United States subject to the same water quality standards requirements as any other
waterbody (e.g., criteria must be sufficient to protect uses, etc.), and under federal
regulations, surface waters (including wetlands) may not be used for waste assimilation. The
Region's recommendation for wetland discharges is based on its belief that, given the
sluggish flows, low mixing potential, and important values and functions of wetlands, a
requirement for wetland discharges to achieve applicable criteria at end-of-pipe is
appropriate. The Region generally tends to discount the validity of providing a "mixing
zone" in a wetland because of the lack of mixing in wetlands. The Region recognizes that
implementation of its recommended approach may create an incentive for dischargers to
route effluent directly to streams where greater mixing rates, greater dilution, and less-
restrictive effluent limits may be available. The Region notes that this incentive is not
exclusive to wetland discharges, but may occur wherever the available dilution at an existing
discharge location is less than at an alternative location.
6. Several commenters expressed the belief that the Region is rushing States to comply
with its mixing zone policy, and that moving ahead with the document is premature and
unnecessary, given the pending reauthorization of the Clean Water Act.
Response: The Region emphasizes that improvements to State mixing zone implementation
procedures has been a Regional priority for more than three years (as evidenced, for
example, by the Region's State-EPA Agreement (SEA) guidance). On the subject of
potential Clean Water Act (CWA) revisions, the Region acknowledges that revisions
concerning mixing zones are possible, and that if such revisions are adopted, changes to the
federal water quality standards regulations would be likely. However, the Region has no
basis to anticipate new requirements that would fundamentally change EPA's approach to
mixing zones. Any changes to the CWA and the federal water quality standards regulation
are likely to simply support and formalize EPA's position on mixing zones over the last
twenty years. The Region believes that EPA's approach to mixing zones has been
remarkably constant over the years, and that this policy statement is not a departure from
EPA's long-standing philosophy, but rather an identification of priority issues and a
clarification of one acceptable means of addressing those issues. Because this Region Vm
policy statement is consistent with previous EPA guidance, and would only bring current
Region Vm practices reasonably into line with that guidance, the Region sees no reason to
wait for CWA revisions. To the contrary, as discussed above, the Region believes there are
strong technical, equity, and legal reasons for improving existing State policies and
implementation procedures on mixing zones and dilution.
7. A comment was received that mixing zone/dilution requirements for small POTWs
should be consistent with those for all other dischargers. This commenter suggested
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that requirements should be based on the type of pollutant, the critical low flow
conditions and cumulative effects of all dischargers (point and nonpoint).
Response: The Region emphasizes that the policy statement allows flexibility on this issue.
The Region continues to recommend that less-stringent requirements (e.g., dilution equal to
the full low flow) should generally be provided to small POTWs discharging to large
waterbodies where the environmental risk of allowing such dilution is small. This provision
is included based primarily upon practical considerations, weighing potential risks against
both state/tribal resources and potential costs to small communities. However, States and
Tribes may elect to apply the same mixing zone-dilution requirements to all facilities
consistently. In addition, States and Tribes may elect to develop their own risk-based
approaches for identifying facilities that merit less stringent requirements. As the commenter
suggests, such approaches should consider the type of pollutant, the amount of available
dilution, and potential cumulative effects. In addition, the Region believes that such
approaches should consider existing pollutant accumulation in fish tissues or sediments and
other site-specific environmental risks. The Region cautions, however, that EPA approval of
any less-stringent state or tribal approach for small POTWs will require a showing that the
approach applies only in situations where the environmental risks are small. Although
treatment costs play a role in the water quality standards process (e.g., as a basis for issuing
a variance), it is not appropriate to apply less-stringent mixing zone or dilution requirements
based solely on cost considerations. State or tribal approaches to this issue that are not
founded on an evaluation of environmental risks will not be approved by the Region.
8. One commenter supported the Region's proposed policy to meet acute chemical-
specific criteria at the end-of-pipe at all times regardless of the presence of a mixing
zone, but suggested that chronic and acute WET limits should be based on critical
effluent and low flow stream conditions.
Response: The Region believes that in incomplete mix situations, compliance with acute
chemical-specific and acute WET limits should be required at the end-of-pipe without an
allowance for dilution. However, the Regional policy has been modified to provide
additional state/tribal flexibility to decide whether to allow limited dilution for acute
chemical-specific criteria (i.e., a zone of initial dilution). For acute whole-effluent toxicity
limits, the Region will continue to require compliance at the end-of-pipe without an
allowance for dilution. The basis for the decision on acute chemical-specific criteria is that
some (but not all) States in other Regions are allowing a zone of initial dilution. In addition,
EPA guidance (TSD, 1991) supports a range of approaches, including the zone of initial
dilution approach and the Region's recommended approach of requiring end-of-pipe
compliance. The basis for the decision on acute WET limitations is that such limits are
based on an LC50, with typically two species tested, while acute chemical-specific criteria
are designed to achieve a much higher level of protection, based on a much larger species
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database. Thus, the Region believes that acute WET limits are a somewhat less rigorous
measure of potential acute effects, and that dilution for acute WET limits is not justified in
any case because of the importance of avoiding acutely toxic exposures to organisms in
receiving waters.
9. One commenter was concerned that mixing zone monitoring procedures weren't
included in the policy statement and probably should be addressed in State/Tribal
mixing zone policies before being approved by EPA.
Response: The Region believes that any monitoring of mixing zones should be required of
the permittee, and that it is a good idea to include a permittee monitoring provision in
state/tribal mixing zone policies or implementation procedures. Such a provision would
serve to provide notice to permittees and a basis for including monitoring requirements in
NPDES permits. For example, it may be necessary in some cases to require ambient
monitoring in the NPDES permit to confirm that mixing zone size limitations are being
achieved. At a minimum, the Region recommends that mixing policies or implementation
procedures include a provision addressing how a permittee may evaluate whether near
complete mixing exists. Such a provision would be useful in situations where a permittee
chooses to determine the local mixing rate as a basis for requesting revised effluent
limitations.
Comments on Second Draft Issued January 14. 1994
10. Several commenters urged that greater flexibility be given to States to establish
mixing zone provisions that consider site-specific conditions and factors.
Response: The Region believes that more than adequate flexibility is provided by the policy
and that changes to the policy to address this comment are not necessary. The Region
believes that these commenters may have misunderstood the purpose of the Region's model
policy and procedure included in Chapters 3 and 4, respectively. The model policy and
procedure are intended to provide a recommended example of the type of policy and
procedure that States and Tribes may adopt. However, States and Tribes are not required to
follow the approaches contained within the Region's models. Although States and Tribes are
expected to resolve all key issues, there is a range of approaches that may be used on each
issue. The Region believes that this approach provides more than adequate flexibility to
States and Tribes to consider site-specific conditions and factors in developing mixing zone
and dilution provisions. The Region also notes that the purpose of step 4 in the model
procedure is to promote consideration of site-specific environmental risks in determining the
mixing zone or dilution allowance that is appropriate.
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11. Comments were received that the policy seems to be geared towards protecting
organisms which "reside" within or pass through the mixing zone, and that the purpose
of a mixing zone is to provide an "allocated impact zone," that does not expose
organisms swimming or drifting through the mixing zone to acute toxicity. These
commenters expressed concern that the term "reside" broadens the protection of
organisms to include unintended organisms, such as sessile plants, and urged Region
VIE to remove any ambiguities relating to the term "reside" prior to finalization.
Response: The Region intends to provide States and Tribes with the flexibility to determine,
as a matter of policy, whether or not to protect organisms that reside within the mixing zone.
Issue #4 discussed within Chapter 2 of the policy explains that States and Tribes will need to
establish minimum in-zone quality requirements that apply within mixing zones. In
discussing issue # 4, protection of sessile organisms is identified as an example of the type of
protection that can be provided by such requirements. However, although States and Tribes
may establish a policy of protecting sessile organisms residing within mixing zones, it is not
the intent of EPA to require such protection. In establishing minimum quality requirements,
the Region does recommend that States and Tribes clearly resolve whether or not protection
of sessile organisms is required. At a minimum, the Region will expect that state and tribal
policies will provide for mixing zones that will not result in lethality to aquatic life caused by
passage through the mixing zone by migrating fish, or by less mobile forms drifting through
the plume.
12. Comments were received that Region VIII, in establishing generic mixing zone
dimensions, ignores the need to configure mixing zones on the basis of site-specific
factors and actual risk of exposure. These commenters also asserted that the Region's
recommended mixing zone size limitations are not appropriate for ephemeral streams
located in arid regions.
Response: Although the Region intends to provide States and Tribes with flexibility to
address the size restriction issue, we continue to recommend that States and Tribes establish
maximum mixing zone size restrictions. An important consideration on this issue is that
specifying maximum size limits simplifies implementation by avoiding the need to conduct
detailed and resource-intensive studies in each case. The Region notes that a large number
of States, including those in arid Regions, now specify maximum allowable mixing zone
dimensions. In several cases. States in arid Regions have specified maximum mixing zone
size limitations that are more restrictive than those recommended by EPA Region Vm.
However, where States or Tribes arc interested in developing an approach to limiting mixing
zone size based on site-specific factors, we would consider all proposals and work with the
State or Tribe. We also note that the recommended approach provides for much greater
consideration of site-specific factors than does the current dilution approach now applied
within the Region. The Region is confused by the ephemeral streams comment since in most
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EPA Region VIIJ Mixing Zones and Dilution Policy
cases discharges to ephemeral streams will not be eligible for a mixing zone because of a
lack of flow at the critical condition.
13. Several commenters questioned EPA Region Vm's draft recommendation to
require compliance with chemical-specific acute aquatic life criteria at the end-of-pipe.
These commenters asserted that it is sufficient to demonstrate (consistent with EPA TSD
methods) that lethality to organisms, either swimming or drifting through the mixing
zone, will not occur.
Response: The Region intends to provide States and Tribes with the flexibility to determine
how best to ensure that no acute lethality occurs within mixing zones. The TSD method
referenced would be acceptable to the Region. For States and Tribes that elect to provide for
zones of initial dilution (i.e., zones where acute criteria may be exceeded), the Region would
also accept and prefers the method specified in Appendix D of this policy. In general,
however, the Region continues to recommend that a zone of initial dilution approach not be
followed and that chemical-specific acute aquatic life criteria be implemented at the end-of-
pipe as the most efficient and effective way of ensuring compliance with the "no acute
lethality" requirement.
14. Comments were received questioning EPA's recommended one-hour averaging
period for acute aquatic life criteria, particularly for metals. These commenters
recommended that EPA Region VIQ should ensure that the draft policy is consistent
with any recommendations coming out of the Aquatic Life Criteria Guidelines
Committee findings.
Response: EPA Region Vm is monitoring the progress of the Aquatic Life Criteria
Guidelines Committee and is aware that changes to the Agency's recommended § 304(a)
criteria may be issued at some point in the future. When and if such changes are finalized,
the Region will make appropriate revisions to this policy. The Region also notes that,
regardless of the averaging period specified by the criterion, a mixing zone or zone of initial
dilution is an area where the criterion magnitude may be continuously exceeded. Since most
State criteria (per EPA guidance) specify that exceedences may occur only once every three
years, and State standards generally require that mixing zones be kept as small as
practicable, the Region believes that there are strong reasons for keeping mixing zones and
zones of initial dilution small. This is one reason why the Region continues to prefer that
acute criteria be applied at the end-of-pipe, without a zone of initial dilution.
15. Comments were received asserting that the Region's draft policy presumes that an
exceedence of the acute criteria is indicative of actual acute in-stream toxicity. This
commenter recommended that the Regional policy integrate WET testing results into the
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EPA Region VTT1 Mixing Zones and Dilution Policy
mixing zone decision; where WET testing results show no actual acute toxicity,
extending mixing zones should be authorized.
Response: The presumption incorporated into the policy is that exceedence of the acute
criteria is predictive of acute toxicity and therefore that exceedence of the acute criteria
should be avoided. The Region also continues to believe, as advocated in EPA's TSD and
the Agency's policy of independent application, that both whole effluent toxicity and
chemical-specific methods should be fully implemented. This means that a finding of "no
toxicity" based on a WET test is not adequate grounds for suspending application of
chemical-specific requirements. Because they are areas where criteria are exceeded, mixing
zones should be kept small, regardless of the outcome of WET tests.
16. Several commenters asserted that the Region's approach should consider the actual
toxicological mode of action for the pollutant of concern (e.g., the selenium standard is
a bioaccumulation concern based on long exposures, not rapidly induced acute toxicity).
Response: Although this issue could be considered a mixing zone issue, the Region believes
that it is more appropriately considered when establishing criteria. It is our understanding
that this issue is being addressed by the Aquatic Life Criteria Guidelines Committee.
Developing guidance to address this concern is beyond the scope and intent of this Regional
policy statement.
17. Comments were received that the Region should account for dilution when
establishing acute WET limitations.
Response: As explained in the response to a similar comment on the first draft of this policy
(see comment ft 8, above), the Region believes that in incomplete mix situations, compliance
with acute chemical-specific and acute WET limits should be required at the end-of-pipe
without an allowance for dilution. The Region has decided to allow States and Tribes the
flexibility to consider dilution in establishing chemical-specific, but not WET, daily
maximum or "acute" permit limits. For acute WET limits, the Region will continue to
require compliance at the end-of-pipe without an allowance for dilution. The basis for this
requirement is the need, based on State requirements and long-standing EPA policy, to avoid
acute lethality within mixing zones. Because the endpoint for acute WET tests is 50%
lethality of test species, an allowance for dilution would result in a portion of the mixing
zone where greater than 50% lethality of aquatic organisms could potentially occur. The
likelihood for lethality within the mixing zone is increased due to the fact that only two
species are generally used in conducting WET tests. Thus, the Region believes that
implementation of acute WET limits at 100% effluent provides reasonable but not certain
assurance that acutely lethal conditions will be avoided in mixing zones; and that providing a
dilution allowance would create an unacceptable source of additional uncertainty.
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18. A comment was received inquiring about the use of diffusers including their
effectiveness and any operation and maintenance problems.
Response: In response to this comment, the Region conducted a survey of industries and
municipal facilities nationwide that utilize diffusers. The methods and results of that survey
are described in a report that is available from the Region. Generally, the survey indicated
that the facilities contacted have been extremely satisfied with the performance of the
diffusers, with 75 % of facilities reporting no minor or major operational problems since
installation. State officials expressed satisfaction with the enhanced mixing, as verified
through dye dilution tests.
19. One commenter noted that under step 3 of the model procedure, a small POTW
with a high dilution ratio may be allowed the full critical low flow as a dilution
allowance. For a pollutant such as chlorine, and assuming the dilution ratio is greater
than 50:1, that could lead to water quality-based effluent limits for total residual
chlorine of 1 mg/1 or more. What is the rationale for concluding that such POTW
discharges will not result in toxicity in the receiving water, when the ambient acute
criterion for chlorine is 19 ug/1?
Response: Step 3 of the model procedure does allow special considerations for small
POTWs with high dilution ratios (e.g., an allowance of the full critical flow as a dilution
allowance). For minor POTWs with dilution ratios greater than or equal to 50:1, the
procedure presumes that such special considerations are appropriate. Minor POTWs with
dilution ratios less than 50:1 may also qualify at the discretion of the permit writer.
However, the procedure does not require the permit writer to follow this approach for every
parameter. The procedure specifies that "in any case where the permit writer determines that
applying this procedure could pose unacceptable environmental risks, the minor POTW will
not qualify for this procedure." The intent is to allow the permit writer the discretion to
override, on a parameter-by-parameter basis, the presumption that is built into the model
procedure for minor POTWs with greater than or equal to 50:1 dilution. Using the example
of a discharge of chlorine at 1 mg/1 or more, a permit writer would have the flexibility to
determine that such a discharge poses an unacceptable risk of acute lethality to aquatic life in
the vicinity of the discharge. Such a determination would make sense in cases where aquatic
life is known to be attracted to the effluent plume. Permit writers also have discretion to
establish technology-based requirements where it is determined that limitations more stringent
than required by water quality standards are reasonably achievable. Finally, any new or
expanded discharge could be subjected to antidegradation requirements, which may result in
an analysis of alternatives and a similar finding that a more restrictive permit limit is a
reasonably achievable alternative.
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APPENDIX C. STATE, TRIBAL, AND EPA MIXING ZONE AND DILUTION
POLICIES AND IMPLEMENTATION METHODS
Introduction
This appendix presents information on current mixing zone and dilution policies and
implementation methods of various States, Indian Tribes, and EPA Regional Offices. The
information was submitted by the water quality standards coordinators and water quality
permit specialists in EPA's Regional offices in response to a written request from EPA
Region Vm dated August 26, 1993. The Region Vm request for information included
specific questions on how States, Indian Tribes, or EPA Regions currently address a number
of mixing zone and dilution issues when developing water quality-based permits. The
purpose of the survey was to allow a comparison of current State, Tribal, and Regional
approaches to each other and to the methods included in EPA's various guidance documents.
A main objective was to characterize the range of implementation approaches nationally on
key mixing zone and dilution topics.
The survey requested information on current approaches on a number of mixing zone
and dilution issues, including any information regarding:
• methods for determining whether a discharge exhibits complete or incomplete mixing,
• dilution flows typically provided in complete mix situations,
• whether any incomplete mix, mixing zone-based permits have been issued, and if so,
how such permits were developed,
• situations where dilution or mixing zones are or may be denied,
• policies specifically for wetland discharges,
• policies specifically for small facilities,
• the legal basis for providing mixing zones and dilution, and
• regulatory mixing zone size limitations.
Results
Seven responses were received from EPA Regions HI (Philadelphia), IV (Atlanta), V
(Chicago), VI (Dallas), VII (Kansas City), IX (San Francisco), and X (Seattle). Including
the six States in EPA Region VIE, the survey included 48 of the 60 jurisdictions currently
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EPA Region VIII Mixing Zones and Dilution Policy
administering a CWA § 303(c) water quality standards program. A summary of the
responses for each survey question is presented below. Included in the summary for each
question is an overview of both the current approach employed in EPA Region VIE and the
"model" or recommended approach proposed by the Region. In general, the survey
responses show that there is a range of approaches in use nationally, and that the approaches
recommended for use by EPA Region Vin are identical, or very similar, to those already in
use elsewhere.
Question 1(a). What guidelines, rules-of-thumb, initial presumptions, or other thresholds,
if any, are used by your States/by the Region to determine whether a discharge exhibits
complete or incomplete mixing?
Current Approach Used in EPA Region VIII: This issue is typically not addressed in Region
VHI when developing water quality-based permit limits (i.e., the decision regarding the
appropriate dilution allowance does not take the local rate of mixing into account).
"Model" Approach Proposed by the Region: The model procedure calls for determining
complete or incomplete mixing based on best professional judgment (BPJ). The procedure
presumes complete mix where there is an effluent diffuser that covers the entire stream/river
width at low flow or when the mean daily flow of discharge exceeds the chronic low flow of
receiving water (i.e., where effluent flow > stream flow). In other cases, the permittee may
show complete mixing consistent with a study plan developed in consultation with the State
and EPA. "Complete mixing" is defined as no more than a 10% difference in bank-to-bank
concentrations within a longitudinal distance not greater than 2 stream/river widths.
Overview of Survey Responses: The survey responses indicate that most Regions and States
do not have formal guidelines or "rules of thumb" to define where complete mix or
incomplete mix is occurring. In Region HI, if the effluent constitutes 50% or more of
instream flow, it is assumed that the stream is effluent-dominated and that complete mixing
occurs. The other Regions typically indicated that either "complete mix is generally
presumed" (Regions V and VI) or "incomplete mix is generally presumed" (Regions VII and
IX), and that the initial presumption can be overridden based on site-specific analysis. It is
important to note that some States that were reported as "assuming complete mix" also
provide dilution allowances which are less, and sometimes significantly less, than the critical
low flow (see Question 1(b)). Thus, many times where "complete mixing" is assumed, a
dilution allowance is ultimately provided that is consistent with, or more stringent than, what
would result from an "incomplete mix" approach. One example of a narrative approach to
this issue was reported for Illinois. In Illinois, for purposes of determining whether a zone
of initial dilution (ZID) should be allowed when implementing acute aquatic life criteria,
immediate dispersion is defined as: "an effluent's merging with receiving waters without
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EPA Region VIII Mixing Zones and Dilution Policy
delay in time after its discharge and within close proximity of the end of the discharge pipe,
so as to minimize the length of exposure time of aquatic life to undiluted effluent."
Question 1(b). What human health and aquatic life dilution flows are typically used by
your States/by the Region where complete mixing is occurring?
Current Approach Used in EPA Region VIII: As discussed above, water quality-based
permits in Region VHI are typically not affected by whether the discharge exhibits complete
mixing or incomplete mixing. Rather, in nearly all permits the following dilution flows are
provided:
Colorado
Montana
North Dakota
South Dakota
Utah
Wyoming
Aquatic Life
Chronic (30 day) - 30E3
Acute (1 day) - 1E3
7Q10
7Q10
7Q25 or 7Q51
7Q10
7Q10
Human Health
30 day standards - 30E3
1 day standards - 1E3
7Q10
7Q10
7Q25 or 7Q51
7Q10
7Q10
"Model" Approach Proposed by the Region: The flows shown below are used as a
maximum allowance under the model procedure. However, dilution may be limited to a
portion of the critical low flow based on site-specific environmental concerns. The
procedure specifies that the actual duration (e.g., 4 day) and frequency (e.g., 3-year) of the
flows used should match the duration and frequency provisions found in state water quality
standards.
Region Vm Recommended Critical Low Flows:
Aquatic life, chronic
Aquatic life, acute
Human health (carcinogens)
Human health (non-carcinogens)
4-day, 3-year flow (biologically based)
1-day, 3-year flow (biologically based)
harmonic mean flow
4-day, 3-year flow (biologically based) or
1-day, 3-year flow (biologically based)
1 7Q25 is provided for segments classified coldwater permanent, coldwater marginal or
warmwater permanent. 7Q5 is provided for segments classified warmwater
semipermanent or warmwater marginal.
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EPA Region VIII Mixing Zones and Dilution Policy
Overview of Survey Responses: The survey indicated a wide range of different dilution
flows in complete mix situations. By contrast, Region IX indicated that: (1) this question is
not relevant because complete mix assumptions are not made in Region IX, (2) their focus
has been on implementing mixing zone size requirements, and (3) complete mixing has not
typically been achieved even at the edge of the mixing zone. Responses from other Regions
indicated that, where complete mix is assumed, dilution may be based on all or a portion
(e.g., 1/6, 1/4, 1/3, 1/2) of the following flows: 1 cfs, 1Q10, 7Q10, 7Q2, 30Q10, 1Q20,
3Q20, or the biological flow for aquatic life, and the annual average, long-term average,
harmonic mean, 30Q5, 30Q2, or the 7Q10 flow for human health. In some States, acute
aquatic life criteria are required to be met at the end-of-pipe without an allowance for
dilution (e.g., Florida), even in complete mix situations. In other States, acute criteria are
implemented assuming full low flow as dilution (e.g., Georgia, where the 7Q10 is provided).
See examples in Table 1. The differences between the low flow statistics listed in Table 1
will vary. For example, the TSD reports that, for sixty streams selected at random, the
harmonic mean flow was at least 2 times the 7Q10 flow at all sixty sites, and was at least 3.5
times the 7Q10 flow at forty sites. The TSD also reports that for smaller rivers (i.e., low
flow of 50 cfs), the 30Q5 flow averaged 1.1 times the 7Q10 flow; while for larger rivers,
(i.e., low flow of 600 cfs) the 30Q5 flow averaged 1.4 times the 7Q10 flow.
Question 2(a). Have your States or has the Region issued mixing zone-based permits
following an assumption of incomplete mixing (please give any notable examples or some
indication of frequency)?
Current Approach Used in EPA Region VIII: No, with very few exceptions. Permits
generally are provided the full critical flow as dilution, with no mixing zone analysis or
requirements considered. Exceptions include a very small number of permits in Colorado
and Utah where a mixing zone-based permit limit was developed.
"Model" Approach Proposed by the Region: Under the Region's model procedure, mixing
zone-based permit limits would be developed wherever incomplete mixing is occurring. As
discussed under Question 1(a), incomplete mixing would generally be presumed unless: (1)
the low flow is zero, (2) there is an effluent diffuser that covers the entire stream/river width
at low flow, (3) the mean daily flow of discharge exceeds the chronic low flow of receiving
water, or (4) the permittee demonstrates complete mixing consistent with a study plan
developed in consultation with the State and EPA.
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EPA Region Vm Mixing Zones and Dilution Policy
TABLE 1
EXAMPLE REGION/STATE DILUTION FLOWS ALLOWED
IN COMPLETE MIX SITUATIONS
REGION/
STATE
AQUATIC LIFE
HUMAN HEALTH
NOTES
Region 3
Acute: 1Q10
Carcinogens: harmonic Mean
Chronic: 7Q10
Non-Carcinogens: 30Q5
Alabama
Acute: 1Q10
Carcinogens: annual average
Chronic: 7Q10
Non-Carcinogens: 7Q10
Florida
Acute: end-of-pipe
harmonic mean
Chronic: not specified
Georgia
7Q10
annual average
Kentucky
7Q10
Carcinogens: harmonic mean
Non-carcinogens: 7Q10
Tennessee
3Q20
30Q2
1Q20 for regulated streams
Illinois
1/4 7Q10
1/4 harmonic mean
Indiana
1/2 7Q10
1/2 7Q10
Ohio
Acute: 7Q10
harmonic mean
Chronic: 30Q10
Wisconsin
1/4 7Q10 (c)
1 /4 annual average
Arkansas
Acute: 1/8 7Q10 (d)
harmonic mean or
Chronic: 1/4 7Q10 (d)
long-term average
Oklahoma
1/4 7Q2 or 1 cfs
long-term average
Texas
Acute: 1/4 7Q2
harmonic mean
Chronic: 7Q2
Region 7
mostly 7Q10
mostly 7Q10
Region 9
N/A
N/A
Idaho
Acute: 1Q10
harmonic mean
Chronic: 7Q10
Table 1 Notes:
a For aquatic life, Florida is proposing 7Q10 for chemical-specific criteria, 1Q10 for
chronic WET.
b Ohio allows a varying portion of the low flow that is inversely related to stream flow
(the more flow in the stream, the less is assumed to mix completely with the
discharge) up to the entire low flow,
c One quarter of the biological flow is used for aquatic life if that flow is available,
d These restrictions are for toxics; for non-toxics, the 7Q10 flow is applied.
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— EPA Region VTII Mixing Zones and Dilution Policy
Overview of Survey Responses: Although some States were reported as providing the full
low flow regardless of the local mixing rate (e.g., New Mexico, Georgia), true "mixing
zone" based permits are routinely developed in Regions D3, VII, IX, and X (for estuarine
discharges). In addition, in a number of other States (including all States in Region V) the
amount of dilution provided is similar to, or more stringent than, what would result from
application of a mixing zone approach. Some examples include:
• Region EQ: Pennsylvania, West Virginia, Delaware, and Virginia implement a mixing
zone-based approach wherever incomplete mixing is occurring. The procedure
involves assessing each discharge to determine complete/incomplete mixing, and then
developing the wasteload allocations using their procedures (the incomplete mix
procedures all involve some sort of mixing zone approach). For example,
Pennsylvania has been modeling mixing zones using concepts similar to those
proposed by Region VIE for some time. The District of Columbia and Maryland are
starting to implement this approach.
• Region IV: In Florida, approximately 10 permits have been developed using a mixing
zone-based approach.
• Region VI: In Arkansas, for toxics, water quality-based permits are developed
assuming a mixing zone equal to one quarter of the 7Q10 flow. In Oklahoma, for
aquatic life, water quality based permits are developed assuming complete mixing
with 25% of the 7Q2 low flow, or 1 cfs, whichever is greater. For the three Indian
Tribes administering water quality standards programs, no mixing zone or dilution is
provided.
• Region VII: All medium-large river discharges have mixing zone-based permits, with
very few exceptions. For example, in Nebraska, the State models the critical low
flow to be awarded for dilution based on restricting the plume to the appropriate
length.
• Region IX: The focus has been on compliance with standards at the edge of the
mixing zone. Generally, complete mixing has not been found to occur.
• Region X: In Idaho, complete mix is generally assumed, with 25% of the low flow
provided as dilution. In Alaska, the Region is assessing marine mixing using near
and far-field models. Most major permits to marine and estuarine receiving waters
have mixing zone analyses based on modeling. Mixing zone is set at specified radius
from the discharge, not to exceed 10% of the width of a waterbody or 10% of the
area (whichever is less). For river discharges, a complete mix assumption is the
norm, but careful assessment of mixing may occur on a case-by-case basis.
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EPA Region VTTI Mixing Zones and Dilution Policy
Question 2(b). What method is used by the Region/by your States to quantify the level of
dilution provided in incomplete mix situations (e.g., modeling, a set % of critical low flow,
field study)?
Current Approach Used in EPA Region VIE: In one of the very few examples of a "mixing
zone" approach, a field study was conducted. The purpose of the study was primarily to
determine the point downstream where complete mixing was achieved. That point was
chosen as the end of the mixing zone and the point of compliance. In another case involving
a lake discharge, however, a modeling method was used to achieve chronic criteria within a
200 foot radial distance of the discharge.
"Model" Approach Proposed by the Region: Any of three progressively more sophisticated
methods may be utilized under the model procedure. The default method is to be used
where data necessary to implement a more sophisticated approach are lacking or where a
conservative approach is warranted based on site-specific environmental concerns. For
streams, the default method requires that no more than 10% of the critical low flow be
provided as dilution. For lakes, the default method requires that no more than 4:1 dilution
be allowed (20% effluent). The modeling method is used to ensure that regulatory mixing
zone size restrictions are achieved at low flow. Several different modeling methods, ranging
from simple (ambient diffusion only) to more complex (discharge-induced and ambient
diffusion) are recommended. The field study method requires use of field data quantifying
the actual ambient mixing rate to implement regulatory mixing zone size restrictions.
Overview of Survey Responses: Generally either a set percentage of the low flow or a
mixing zone model (such as CORMIX or PLUME) is used to implement mixing zone size
and shape requirements. Occasionally, a field study is conducted to characterize the actual
pattern and rate of mixing. In Region IX States, monitoring requirements are included in the
permit to verify that standards are achieved outside the mixing zone. Examples include the
following:
• Region HI: The Region and the States have used the Cornell Mixing Zone Model
(CORMIX), a % of the low flow, exposure analysis, and a few dye studies to
implement mixing zone requirements.
• Region IV: In Florida, for 3 paper mills and 5 POTWs, dye studies were completed.
In Kentucky, 1/3 of the 7Q10 flow is used to implement chronic mixing zone
requirements; for acute, the State routinely uses the models UMERGE AND
UDKHDEN to implement ZID requirements for small discharges to large rivers (such
as the Ohio).
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EPA Region VIII Mixing Zones and Dilution Policy
• Region VII: Generally, a set % of the low flow is provided as dilution to implement
mixing zone requirements (e.g., 1/4 low flow). In some cases, site-specific mixing
dynamics are characterized with modeling (e.g., in Nebraska).
• Region IX: In marine waters, the PLUME model or equivalent is used to determine a
dilution ratio based on momentum and density differences under critical conditions.
The dilution ratio is then used with numeric standards (chronic values) to determine
effluent limits. In fresh waters, a set percentage of the low flow (often a court
decreed minimum flow) is used to implement mixing zone requirements. Field
studies/ambient water quality monitoring are normally a requirement for marine and
fresh waters to verify compliance with mixing zone requirements.
• Region X: In Idaho, a set % of the low flow is used (25%). In Alaska, the models
UMERGE, UPLUME, CDIFF, RDIFF, WASP4, etc. are used by the Region for
marine discharges, sometimes with dye study for confirmation.
Question 2(c). What mixing zone or dilution, if any, is provided by the Region/by your
States for acute aquatic life criteria where incomplete mixing is occurring?
Current Approach Used in EPA Region VIII: Generally, an incomplete mixing assumption
is not made and the critical low flow is provided as dilution. In one of the few instances
where a mixing zone approach was implemented, the 1985 TSD method of requiring
compliance with acute chemical-specific criteria within a short distance of the outfall1 was
used. For acute whole effluent toxicity limitations, compliance is generally required at the
end-of-pipe without an allowance for dilution, with very few exceptions.
"Model" Approach Proposed by the Region: Under the model procedure, to avoid acutely
lethal concentrations within mixing zones, acute chemical-specific aquatic life criteria and
acute whole effluent toxicity limits must be achieved at the end-of-pipe without an allowance
for dilution2.
1 This method requires compliance with acute criteria within the most restrictive of the
following: (1) 10% of the distance from the outfall to the edge of the mixing zone,
(2) a distance of 50 times the discharge length scale in any spatial direction, or (3) a
distance of 5 times the local water depth in any horizontal direction from the
discharge outlet.
2 Although not recommended by the Region, States and Tribes may designate a small
portion of the chronic mixing zone within which acute chemical-specific criteria must
be achieved (i.e., a zone of initial dilution).
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EPA Region Vm Mixing Zones and Dilution Policy
Overview of Survey Responses: Generally, States are using a variety of methods to
implement acute criteria in incomplete mix situations. A number of States require
compliance with acute criteria or the FAV (twice the CMC) at the end-of-pipe. Other States
provide a set percentage of the chronic mixing zone or critical low flow as a zone of initial
dilution (ZID). As discussed above under Question 1(b), a number of States routinely
"assume complete mixing" and thus avoid this issue. Finally, some States provide fixed ZID
size limitations in their water quality standards. Examples include:
• Region HI: The Region recommends any of the methods for preventing lethality to
passing organisms outlined in the TSD. In Pennsylvania, the State uses the 15 minute
travel time guideline described in the TSD. In Virginia, half of the 1Q10 flow is
provided as dilution for acute criteria or compliance with the FAV is required at the
end-of-pipe for stream dominated situations.
• Region IV: In Florida, compliance with acute criteria is required at the end-of-pipe.
In Kentucky, the State is proposing that no new ZIDs be allowed in High Quality
waters, lakes, and reservoirs; for all other waters, a new ZED will only be allowed if
a submerged multi-port outfall structure is present, and ZID size will be restricted
based on the 3 TSD recommendations (square root of port cross-sectional area, etc.)
• Region V: Compliance with the FAV is typically required at the end-of-pipe.
• Region VI: In Arkansas, for toxics, the acute dilution allowance is assumed to be 1/8
of the 7Q10 flow. In Texas, for streams and rivers, the standards limit ZID size to
60 feet downstream and 20 feet upstream from a discharge point; however, the
dilution allowance for acute criteria is generally one quarter of the 7Q2 low flow.
For lakes and reservoirs, the standards limit ZID size to a radius of 25 feet from the
point of discharge. New Mexico requires compliance with acute criteria at the end-
of-pipe, without an allowance for dilution.
• Region VII: In Nebraska, for coldwater Class A and B and Warmwater Class B
streams, acute mixing zones must be <_ 125 feet in length or < 5% of chronic
mixing zone length, whichever is most restrictive. For warmwater Class A streams,
ZIDs must be <_ 250 feet or < 5% of chronic mixing zone length, whichever is less.
Also, all acute mixing zones must be <. 1/2 stream width. In Missouri, for Class C
streams with 7Q10 < 0.1 cfs, no ZID is allowed; for all other streams and for lakes,
the ZID equals 10% of the mixing zone.
• Region IX: Normally, acute aquatic life criteria are met at the end-of-pipe.
Exceptions have been granted for some thermal discharges and for some deep ocean
water discharges having very high initial dilution.
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EPA Region V III Mixing Zones and Dilution Policy
• Region X: In Idaho, 25% of 1Q10 flow is provided. In Alaska, for marine
discharges, compliance with acute criteria is required at the end-of-pipe.
Question 3. Does your Region/do your States have policies or cases where dilution or
mixing zones are prohibited either in general or for particular pollutants or types of
waterbodies (i.e., for a reason other than a zero critical flow)?
Current Approach Used in EPA Region VIII: In South Dakota, lake discharges are not
allowed a zone of mixing. In some of the other States, there is policy language that could be
used as a basis for denying a mixing zone in specific cases. For example, the Colorado
water quality standards state that "the ecological and human health effects of some pollutants
may be so adverse that a mixing zone for such pollutants will not be allowed."
"Model" Approach Proposed by the Region: The model procedure requires all discharges to
wetlands to achieve compliance with water quality criteria at the end-of-pipe, without an
allowance for dilution. Also, a mixing zone or dilution allowance may be denied or limited
where allowing such dilution would pose unacceptable environmental risks. Such risk
determinations are to be made on a case-by-case basis in consideration of relevant site-
specific concerns, including bioaccumulation in fish tissue or sediment, biologically-important
areas, potential human exposure from drinking water or recreation, attraction of aquatic life
to the effluent plume, toxicity/persistence of the substance, zone of passage for migrating fish
(including access to tributaries), and cumulative effects of multiple discharges and multiple
mixing zones.
Overview of Survey Responses: The responses indicate that mixing zones may be prohibited
based on a variety of factors, including presence of: drinking water intakes, critical habitat,
recreational areas, ceremonial use areas, shore fishing areas, or tributaries. Mixing zones
may also be denied for particular substances such as bioaccumulative or persistent toxics, or
for particular types of waterbodies such as lakes or reservoirs. Examples include:
• Region ED: Mixing zones are not appropriate where: (1) the mixing zone would
infringe on critical habitat areas such as spawning areas, areas with sensitive biota,
drinking water supplies, or public recreation areas, (2) the discharge plume attaches to
the stream bottom, or (3) the discharge contains bioaccumulative pollutants defined as
substances with a BCF of 100 or greater. In Virginia, for discharges to lakes, the
acute and chronic wasteload allocation should be set equal to the acute and chronic
standard, respectively.
Region IV: In Alabama, industrial facilities not discharging below the surface are not
provided a mixing zone. In Kentucky, the State is proposing that no new mixing zone
or ZID be allowed for existing priority pollutant discharges to lakes and reservoirs.
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EPA Region VIII Mixing Zones and Dilution Policy
• Region V: The Great Lakes Water Quality Guidance proposes to eliminate mixing
zones for certain bioaccumulative and persistent pollutants. In general, Region V
States restrict mixing zones in close proximity to bathing beaches, drinking water
intakes, shore fishing areas, and the mouths of tributaries.
• Region VI: In Arkansas, mixing zones are not allowed for bacteria, oil, grease, and
pH. Mixing zones shall also not include any domestic water supply intake. For the
Pueblos of Isleta, Sandia, and San Juan, mixing zones are prohibited to allow for
recreational and/or ceremonial use of all reservation waters.
• Region VII: In Nebraska, for public water supply criteria, mixing zones cannot
extend to within a 2 mile zone of influence from any public drinking water supply
intake.
• Region IX: In Guam, mixing zones are prohibited in high quality waters (M-l
marine and S-l inland).
• Region X: Under the current Alaska water quality standards, mixing zones are not
currently allowed for carcinogens, mutagens, teratogens, and pollutants that
bioaccumulate without a finding from the State that the effects within such a zone
would be insignificant.
Question 4. Does your Region/do your States have policies or cases with regard to dilution
or mixing zones for discharges to wetlands?
Current Approach Used in EPA Region VIE: Colorado recently clarified the water quality
standards applicable to wetlands, and that effort resulted in development of a draft
implementation document that described the mixing zone approach to be followed for wetland
discharges. This draft guidance calls for the size and shape of the mixing zone within a
wetland to be determined by the Division considering factors contained within the State water
quality standards. Generally, the guidance calls for viewing a discharge to a wetland in a
manner similar to a discharge to a standing water body.
"Model" Approach Proposed by the Region: The model procedure requires all discharges to
wetlands to achieve compliance with water quality criteria at the end-of-pipe, without an
allowance for dilution.
Overview of Survey Responses: Overall, the responses indicate few mixing zone-dilution
policies specific to wetland discharges. Those that exist generally require compliance with
criteria at the end-of-pipe. Examples include:
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EPA Region VIE Mixing Zones and Dilution Policy
• Region HI: In Virginia, for discharges to marshes or swamps, the acute and chronic
wasteload allocation should be set equal to the acute and chronic standard,
respectively.
• Region IV: In Florida, the water quality standards require that "substances in
concentrations which are chronically toxic to humans, animals, or plants, or provide
adverse physiological or behavioral response in humans or animals, shall not be
present" in a wetland. EPA implements this requirement by not giving credit for
dilution when calculating WET limits for discharges to wetlands.
• Region V: Generally, mixing is not allowed if a discharge is to a wetland.
• Region VI: In Louisiana, discharges to freshwater lakes, ponds, and wetlands must
achieve criteria within a 100 foot mixing zone and a 25 foot ZID.
• Region VII: In Iowa, for all pollutants, no mixing zone or ZID is allowed for waters
designated as lakes or wetlands.
Question 5. Does your Region/do your States have policies or cases where less-restrictive
mixing zone-dilution requirements are applied to small facilities (e.g., de minimis
discharges, exemptions for certain types of discharges, etc.) ?
Current Approach Used in EPA Region VIII: The full critical low flow is provided as
dilution in virtually all permits, regardless of mixing rate, facility size, stream size, or other
site-specific information.
"Model" Approach Proposed by the Region: For certain minor POTWs, the model
procedure would allow the full critical low flow as dilution, as long as allowing such dilution
poses insignificant environmental risks.
Overview of Survey Responses: No mixing zone or dilution policies specific to small
facilities were reported in the survey responses.
Question 6. In each of your States what written authority, if any, is the basis for
providing a mixing zone or dilution for water quality-based permits (e.g., mixing zone
policy, low-flow policy, other)?
Current Approach Used in EPA Region VIE: EPA Region VIE States generally have some
type of mixing zone or low flow policy, or both, in their adopted water quality standards. In
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EPA Region VQI Mixing Zones and Dilution Polic>
some cases, however, these policies are fairly general and lacking in specific requirements,
particularly size limitations.
"Model" Approach Proposed by the Region: The Region's model policy statement would
provide detailed and specific authority to establish a mixing zone or make an allowance for
dilution when implementing aquatic life and human health criteria. The policy covers
chemical-specific and whole effluent toxicity discharge limitations and allows for a site-
specific approach based on local mixing rates and environmental concerns. Maximum size
restrictions for mixing zones in incomplete mix situations are specified, as are critical low
flows that represent the maximum allowable dilution in complete mix situations.
Overview of Survey Responses: The responses indicated that all States have written
authority to provide mixing zones or dilution in their water quality standards. A number of
States have additional detail in a guidance, policy, or implementation document.
Question 7. What mixing zone size limitations, if any, are included in the water quality
standards in your States (e.g., width, length for streams/rivers, diameter for lakes, etc.)?
Current Approach Used in EPA Region VTH: In general, although Region Vm States have a
mixing zone and/or low flow policy in their water quality standards, State standards lack
specific limitations on mixing zone size. However, at a minimum, narrative requirements for
mixing zones are described. As mentioned above, these mixing zone requirements do not
normally influence actual permitting decisions.
Colorado: Specific size limitations are not included. The State standards
do include several narrative provisions potentially affecting
mixing zone size: "where necessary to protect aquatic life,
there shall be a zone of passage...", "mixing zones shall not
overlap so as to cause harmful effects in adjacent waters or to
interfere with zones of passage," and "biological communities
shall not be interfered with to a degree which is damaging to the
ecosystem..."
Montana: Specific size limitations are not included, but the mixing zone
policy requires the extent of the mixing zone to be minimized to
the extent practicable, and that a mixing zone not affect existing
or reasonably anticipated uses outside the mixing zone.
North Dakota: Mixing zone size is limited to no more than 25 % of the cross-
sectional area or volume of flow and no more than 50% of the
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EPA Region Yr[II Mixing Zones and Dilution Policy
width. Mixing zones must also be as small as possible and shall
not intersect spawning or nursery areas, migratory routes, or
municipal water intakes.
South Dakota: Size limitations are not included. The State standards do
specify, however, that discharges to flowing waters must meet
the applicable criteria at the edge of its zone of mixing. For
lakes, no zone of mixing is permitted.
Utah: Specific size limitations are not included. Narrative mixing
zone requirements in State standards include: "the zone shall
small in extent and must not form a barrier to migrating aquat
life."
Wyoming: Size limitations are not included. The State mixing zone polic
does require, however, a zone of passage around the mixing
zone which "shall not contain pollutant concentrations that
exceed the chronic aquatic life values."
"Model" Approach Proposed by the Region: The model procedure specifies that the size
and shape of mixing zones, where allowed, should be determined case-by-case. The
following maximum size restrictions apply. For streams and rivers, mixing zones should n
exceed one-half of the cross-sectional area or a length 10 times the stream width at critical
low flow, whichever is more limiting. For lakes, mixing zones should not exceed 5% of tl
lake surface area or 200 feet in radius, whichever is more limiting.
Overview of Survey Responses: In general, State approaches to limit mixing zone size for
streams specify a given fraction (e.g., 1/4, 1/3, 1/2, etc.) of the stream width, cross-
sectional area, or flow (i.e., at the critical low flow condition) beyond which the mixing zc
may not extend. Note that States with apparently identical width restrictions (e.g., 1/4 cro
sectional area) may in fact have different requirements in practice if their critical low flow
assumptions are different. Some States supplement the width restriction with a length
restriction. A few States provide only a length restriction. For lakes, the typical approach
to limit the radial extent of the mixing zone from the point of discharge; however, other
approaches are in use. See Table 1 for specific examples.
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EPA Region Vm Mixing Zones and Dilution Policy
TABLE 2
EXAMPLE MIXING ZONE SIZE RESTRICTIONS
REGION/
STATE
WIDTH
(STREAMS)
LENGTH
(STREAMS)
RESTRICTION
FOR LAKES
NOTES
Region 3
<
2/3 width
a
Florida
<
3/4 width (b)
< 800 meters
< 125,600 m2
c
Kentucky
<
<
1/3 width
1/2 area
< 1/10 width (b,d)
Illinois
<
1/4 area/flow
< 26 acres
Indiana
<
<
1/4 area/flow
1/2 width
no discharges
Ohio
<
<
<
1/2 width
1/5 width at mouth (e)
1/3 area
< 5 x width
f
Louisiana
<
1/3 low flow (g)
< 100 ft. radius (h)
Oklahoma
<
1/4 7Q2 (i)
< 13 x width
Texas
< 300' (dwnstrm) < 100 ft. radius
< 100' (upstrm)
Missouri
<
<
1/4 width, area
1/4 low flow
< 1/4 mile
< 1/4 width or
100 feet radius
f,j
Nebraska
< 2,500 feet
k
Arizona
<
1/2 area
< 500 meters
(1)
California
< 250 feet
< 25 ft. radius (m)
Guam
<
1/4 area
< 5 x width
Idaho
< 1/4 flow, width, or
300 m + diffuser length
< 10% area
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EPA Region Vm Mixing Zones and Dilution Policy
Table 2 Notes:
a Regional recommendation based on the Red Book,
b Proposed language.
c In addition, all mixing zones shall not exceed 10 % of total waterbody length or 10 %
of total waterbody area,
d Would be applied from point of discharge in any spatial direction,
e The mouth is defined as that area of the stream from the confluence upstream for a
distance five times the width of the stream at the confluence,
f For non-thermal discharges.
g For streams with 7Q10 flow > 100 cfs; where flow <_ 100 cfs, the full 7Q10 flow is
provided as dilution; for Mississippi and Atchafalaya Rivers other restrictions apply,
h For discharges to freshwater lakes, ponds, and wetlands,
i At a minimum, 1 cfs is provided as dilution.
j For streams with 7Q10 > 20 cfs; for smaller and Class C streams, other restrictions
apply.
k For coldwater Class A and B and warmwater Class B streams; for Warmwater Class
A streams, mixing zones shall be less than 5,000 feet.
1 Adjacent mixing zones in lakes shall be no closer than the greatest horizontal
dimension of any of the individual mixing zones,
m In addition, the sum of all lake mixing zones shall not exceed 5 % of lake volume.
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EPA Region VID Mixing Zones and Dilution Policy
APPENDIX D - ALTERNATIVE PROCEDURES FOR CHEMICAL-SPECIFIC
ACUTE CRITERIA IN INCOMPLETELY-MIXED SITUATIONS
Background
A common narrative provision found in state water quality standards is the prohibition
of acute lethality within a mixing zone1. However, applying this "no acute lethality" mixing
zone provision can be somewhat problematic in that a decision to allow an acute mixing zone
(zone of initial dilution) will typically lead to pollutant concentrations in the immediate
vicinity of the discharge that are in excess of the acute chemical-specific criterion.
Depending upon the magnitude of exceedence and an organism's duration of exposure,
allowing such zones of initial dilution could well result in conditions that are acutely lethal to
aquatic life. Not surprisingly, implementation of the "no acute lethality" requirement (i.e.,
when setting acute chemical-specific as well as WET permit limits) has been addressed in
different ways across the country (see summary to Question 2(c) in Appendix C). EPA has
acknowledged the difficulties of addressing the issue by presenting four options to avoid
acute lethality within a mixing zone in its Technical Support Document for Water Quality-
based Toxics Control (see Section 4.3.3 on page 71).
For both chemical-specific and WET limits, Region Vin recommends the TSD option
of meeting acute standards (and toxicity objectives) at the end-of-the-pipe with no
consideration for dilution. The Region believes that this is the simplest and most reliable
method of avoiding acute lethality within the mixing zone. This recommendation is based, in
large part, on the fact that discharges (especially in the winter) tend to attract aquatic life;
this phenomenon results in an actual period of exposure to the effluent plume that precludes a
simple "swim through" exposure time assumption. In contrast, the Region acknowledges that
allowing ZIDs is a fairly common existing practice which is recognized in the TSD, and that
certain "incomplete mix" discharges can nevertheless exhibit fairly rapid mixing with the
receiving water (especially where there is high discharge momentum). With this in mind, the
Region provides the following guidance which allows a zone of initial dilution under certain
conditions:
1 Note that this Region Vm policy statement establishes such a provision as a
minimally-required element of state and tribal mixing zone policies (see Issue # 4 in
Chapter 2).
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EPA Region VIII Mixing Zones and Dilution Policy
Policy
For acute chemical-specific standards in incomplete mix situations, although achieving
such standards at the end-of-pipe is recommended by the Region, EPA will also approve
mixing zone policies that allow a zone of initial dilution on a case-by-case basis where:
¦ there is evidence of rapid mixing between the discharge and receiving water based on
factors such as a high exit velocity of the discharge (e.g., > 10 feet per second), and
¦ the rationale for the discharge permit includes an evaluation of risks (such as those
described in Step 4 of the Region's model procedure) and a finding that allowing a
zone of initial dilution poses no unacceptable risks.
Where both of the above two conditions are met in a particular case, it is
recommended that the zone of initial dilution (ZID) for achieving acute standards be limited
as follows:
Lakes: The ZID volume may not exceed 10% of the volume of the chronic
mixing zone. This may be implemented by allowing a ZID radial
distance equal to 10% of the chronic mixing zone radial distance, or
through other appropriate methods (e.g., if the chronic mixing zone is
200 feet in radius, then the allowable acute ZID may not be more than
20 feet in radius).
Rivers and
Streams: The ZID volume must be small. This may be implemented by applying
the more stringent of the following two restrictions:
1) ZID volume or flow may not exceed 10% of the chronic mixing
zone volume or flow (e.g., if the chronic mixing zone allows a
dilution flow rate of 18 cfs, then the allowable acute ZID would
be no more than a flow rate of 1.8 cfs); or
2) ZID length may not exceed a maximum downstream length of
100 feet.
For acute whole effluent toxicity objectives, EPA Region VIII will continue to require
that such objectives be achieved at the end-of-pipe without an allowance for dilution (see
Issue ^ 5 in Chapter 2 and Question # 8 in Appendix B for additional discussion).
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EPA Region Vm Mixing Zones and Dilution Policy
APPENDIX E - RECOMMENDED CRITICAL LOW FLOWS
Background
There are numerous methods discussed in the scientific literature on how low flows
can be calculated for a stream or river. The two fundamental approaches to performing such
a calculation are the extreme methods (e.g., log Pearson Type HI analysis) and the empirical
methods (e.g., EPA's biologically-based method). In addition to deciding what method is
most appropriate for their waters, both States and Tribes must address the issue of what is
the appropriate return interval (frequency) and averaging period (duration) for the chronic
and acute numeric criteria they have adopted.
Region Vm has made recommendations pertaining to critical low flows in their
Mixing Zone and Dilution Policy Document while at the same time acknowledging the need
to tailor decisions concerning critical low flows. The following describes some of the
rationale for those recommendations.
Recommendations Related to Critical Low Flow
Method of Calculating Flows: Region vm believes calculation of critical low
flows is best performed by using the method developed by EPA. This method is referred to
as the "biologically-based" method, but is actually a non-parametric method for analyzing a
flow record. This method could be used on any combination of an X day, Y year low flow.
For example, it can be used to calculate a 7Q10, a 7Q25, a 30Q3, as well as a 4 day, 3 year
low flow. The following observations are made when comparing the biologically-based
method and the more traditional, extreme methods:
¦ The extreme methods typically rely on the existence of a numeric distribution (the
ability to fit a line through a certain plot of flow data). Because data typically does
not fit the distribution exactly (especially at the very low flows), there can be
considerable error associated with this "force-fitting" of the data to a distribution.
The EPA biologically-based method, on the other hand, does not require the flow data
to follow any numeric distribution. An empirical analysis is simply performed to
determine what X-day flow has occurred on the average every Y-years.
¦ The extreme methods typically use only one data point for every year of record. In
contrast, the EPA biologically-based method utilizes all the data and performs an
analysis of all flows and their long term trends through time. For example, for a
12 year data set, many of the extreme methods would make a determination using
only 12 data points. In contrast, the biologically-based method would use over
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EPA Region Vm Mixing Zones and Dilution Policy
4000 data points for that same period to determine critical low flows. A greater
number of data points makes the biologically-based method more statistically robust.
¦ The extreme methods only account for one low flow excursion value per year even
if there were multiple low flow excursions of the same magnitude within the year.
The biologically-based method considers all the low flow excursions for the full
period of record. Because of this, flows calculated using the biologically-based
method are typically lower than the same X-day Y-year flow calculated using an
extreme method.
An analysis was performed on daily flow data from several stream stations to
determine the actual frequency of a 7Q10 low flow calculated using the log Pearson III
extreme method. It was seen that 7Q10's calculated using the extreme method reoccur much
more frequency than once every 10 years. Typically, 7Q10's calculated through this method
reoccur every 3 years or more frequently. The results of this analysis as well as a
comparison between different critical low flows are given in Table E-l.
table E.i Comparison of Various Critical Low flows
Stream
Station
(USGS Gauge
Number)
7Q10 Flow
Extreme
Method
log Pearson HI
(cfs)
Actual
Recurrence
Interval for
7Q10
(years)
7Q10 Flow
Biologically-
based Method
(cfs)
4 Day, 3 Year
Flow
Biologically-
based Method
(cfs)
Harmonic
Mean Flow
(cfs)
Shoshone R.
(WY)
06285100
124
2.5
73
79
557
Waphpeton R.
(ND)
05051500
16.7
1.8
2.0
7.8
131
Virgin R.
(UT)
09408150
38
2.5
32
34
116
Wind R.
(WY)
06235500
67
1.8
54
56
246
Green R.
(CO)
09217000
32
2.7
27
31
85
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EPA Region VHI Mixing Zones and Dilution Policy
Duration/Frequency: Some of the fundamental aspects of numeric water quality
standards for surface waters are the duration (e.g., 4 day averaging period) and frequency
(e.g., exceedence allowed every 3 years). States and Tribes are encouraged to take these
aspects into consideration when defining "critical" low flows-those flows that are used in
establishing acceptable loads to achieve water quality standards. Region vm recommends
that the duration and frequency provisions found in the standards reflect the duration and
frequency of critical design flow. For example, if a particular aquatic life criteria has a
4 day, 3 year duration and frequency, the 4 day, 3 year low flow is recommended for use in
implementing that standard. It is important to note that a State or Tribe should use different
critical low flows for each of their chronic standards and acute standards, matching the
appropriate durations and frequencies of these criteria.
EPA has developed a PC-based program that can be used to analyze daily flow
records using either the log Pearson HI extreme method or the EPA biologically-based
method. This computer program (DFLOW) can calculate any combination of X-day and
Y-year for the period of record. In addition, the program can be used to calculate the
harmonic mean for use with certain human health criteria.
The following are two references available from EPA to further provide details on
flow analysis as well as describe the basis of the DFLOW model:
US EPA (1986). Technical Guidance Manual for Performing Waste Load Allocation:
Book VI: Design Conditions: Chapter 1. Stream Design Flow for Steady-State Modeling:
Office of Water.
US EPA (1990). DFLOW User's Manual: Dr. L. Rossman, ORD/RREL.
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