&EPA
United States
Environmental Protection
Agency
 Off ice of Water

EPA 820-B-14-004

September 2014
Water Quality Standards Handbook
Chapter 5: General Policies

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Water Quality Standards Handbook
Chapter 5: General Policies

(40CFR131.13)

Table of Contents
  Introduction	1
  5.1 Mixing Zones	1
    5.1.1 Recommended Contents of State and Tribal Mixing Zone Policies	3
    5.1.2 Situations in Which Mixing Zones May Not Be Appropriate	9
    5.1.3 Mixing Zones forthe Discharge of Dredged or Fill Material	10
    5.1.4 Mixing Zones for Aquaculture Projects	11
  5.2 Critical Low Flows for Water Quality Criteria Implementation	11
  5.3 Variances from Water Quality Standards	15

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Introduction

As specified in 40 CFR 131.13. states and authorized tribes may, at their discretion, adopt certain
policies into their water quality standards (WQS) that generally affect how their WQS are applied or
implemented.1 Examples of such general policies include those affecting mixing zones, critical low
flows, and WQS variances. As the regulation indicates, states and tribes are not required to adopt
general policies.  However, if a state or tribe chooses to adopt a general policy, such policies are
subject to EPA review and approval or disapproval  under Section 303(c) of the Clean Water Act
(CWA) if they constitute new or revised WQS (see Chapter 1 of this  Handbook). This chapter
provides an overview of three types of general WQS policies. In  particular,  Section 5.1 of this
chapter discusses mixing zones, Section 5.2 discusses critical low flows, and Section 5.3 discusses
variances.
5.1  Mixing Zones

A mixing zone is a limited area or volume of water where initial dilution of a discharge takes place
and where certain numeric water quality criteria may be exceeded. The CWA does not require that
all criteria be met at the exact point where pollutants are discharged into a receiving water prior to
the mixing of such pollutants with the receiving water. Sometimes it is possible to expose aquatic
organisms to a pollutant concentration above a criterion for a short duration within a limited,
clearly defined area of a waterbody while still maintaining the designated use of the waterbody as
a whole. Where this is the case, a state or authorized tribe may find it appropriate to allow ambient
concentrations of a pollutant above the criterion in small areas near point-source outfalls (i.e.,
mixing zones).

Mixing zones do not constitute new state or tribal criteria or changes  to the state- or
tribe-adopted  and EPA-approved criteria. Therefore, the narrative and/or numeric criteria for  the
waterbody are still the applicable criteria within the boundaries of the mixing zone. A mixing zone
simply authorizes an applicable criterion to be  exceeded within a defined area  of the waterbody
while still protecting the designated use of the waterbody as a whole.  Since 1 983, the guidance in
this Handbook has described  mixing zones as areas where  criteria may be exceeded rather than
areas where criteria do not apply.
1  Throughout this document, the term "states" means the fifty states, the District of Columbia, the
Commonwealth of Puerto Rico, the Virgin Islands, Guam, American Samoa, and the Commonwealth
of the Northern Mariana Islands. The term "authorized tribe" or "tribe" means an Indian tribe
authorized for treatment in a manner similar to a state under CWA Section 51 8 for purposes of
Section 303(c) WQS.

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By authorizing a mixing zone, states and tribes allow some portion of the waterbody to mix with and
dilute particular wastewater discharges before evaluating whether the waterbody as a whole is
meeting its criteria. In addition to the WQS regulation at 40 CFR 131.13 described above, the use of
dilution is supported by the National Pollutant Discharge Elimination System (NPDES) permitting
regulation at 40 CFR 1 22.44(d)(1 )(ii). which requires the permitting authority to consider, where
appropriate, "the dilution of the effluent in the receiving water" when determining whether a
discharge causes, has the reasonable potential to cause, or contributes to an instream excursion
above a criterion. Depending on  the state or tribal WQS and implementation policies, a
consideration of dilution could be expressed in the form of a dilution allowance or a mixing zone.
A dilution allowance typically is expressed as the flow or portion of the flow of a river or stream and
is typically applied in flowing waters where rapid and complete mixing occurs. A mixing zone is
typically applied in any waterbody type in which incomplete mixing occurs. For more information,
see Chapter 6 of the NPDES Permit Writers' Manual'(201 0).

While mixing zones serve to dilute concentrations of pollutants in effluent discharges, they also allow
increases in the mass loading of the  pollutant to the waterbody (more so than would occur if no
mixing zone were allowed). Therefore, if not applied appropriately, a mixing zone could adversely
affect mobile species passing through the mixing zone as well as less mobile species (e.g., benthic
communities) in the immediate vicinity of the discharge. Because of these and other factors, mixing
zones should be applied carefully  so that they do not result in impairment of the designated use of the
waterbody as a whole or impede progress toward the CWA goals of restoring and maintaining the
physical, chemical, and biological integrity of the Nation's waters. Keeping this in mind,  a state  or
tribe has the  discretion to choose whether to authorize mixing zones and adopt a mixing zone
policy. However, as described below, if  a state ortribe chooses to adopt a mixing zone policy, such
a policy is generally considered a new or revised WQS that must be adopted into state or tribal law
and approved by the EPA before  it is effective for CWA purposes.

An important note is that "mixing zone" is used in multiple ways. A mixing zone  policy \s a legally
binding state  or tribal policy that is adopted into WQS and describes the general characteristics of and
requirements associated with mixing zones without taking into account site-specific information. The
EPA generally views such mixing zone polices as constituting new or revised WQS that require EPA
review and approval or disapproval under Section 303(c) of the CWA. Consistent  with the four-part
test described in What is a New or Revised Water Quality Standard Under  CWA  Section 303(c)?
Frequently Asked Questions (201 2)  and Chapter 1 of this Handbook, a state or tribal mixing zone
policy is a legally  binding provision  that is adopted into state or tribal law (part one), and it
addresses the criteria component of WQS (part two). Additionally,  a  mixing zone policy expresses a
desired condition in the waterbody to allow flexibility in meeting the applicable criteria within
certain areas  of the waterbody (part three), and if it is a new provision or revises an existing policy
(part four), it clearly  meets the requirements to be a new or revised WQS.

On the other hand, an individual, site-specific mixing zone is authorized fora particular point-source
discharge in accordance with a state  or tribal mixing zone policy and accounts for the site-specific
characteristics of a particular discharge and receiving water. An individual mixing  zone is defined and
implemented through the NPDES  permitting  process. The EPA does not view individual mixing zones

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as constituting new or revised WQS requiring EPA review under Section 303(c). Like a mixing zone
policy, an individual mixing zone is a legally binding provision that is established pursuant to state
or tribal law (part one), and it addresses the criteria component of WQS (part two). However, unlike
a mixing zone policy, an individual mixing zone does not express or establish a desired condition
in the waterbody (part three). Instead, the individual mixing zone is used to establish appropriate
water quality-based effluent limits (WQBELs) for a specific discharger's NPDES permit. An
individual mixing zone also does not establish a new provision or revise an existing provision (part
four). Rather, it implements a WQS (i.e., the state or tribal mixing zone policy) for a specific
discharger using site-specific information.

Additionally, any time an effluent is discharged into a receiving water, there will be a zone of actual
or physical mixing m which the discharge and receiving water naturally mix regardless of whether
a mixing zone, in the regulatory sense, has been authorized. Such actual mixing is described using
field studies and a water quality model and is  used in establishing an individual, site-specific
mixing zone for a particular discharge.

The authorization of mixing zones under incompletely mixed discharge and receiving water
situations pre-dates the CWA. The EPA's current mixing zone guidance, contained in this
Handbook,  the  Technical Support Document for Water Quality-based Toxics Control (JSP) (1 991).
and the NPDES Permit Writers' Manual'(201 0), evolved from previous guidance from the EPA and its
predecessor agencies on the use of mixing zones as a  regulatory tool to address the incomplete
mixing of wastewater discharges in receiving waters. This Handbook describes the  EPA's
recommendations for state and tribal mixing zone policies. The other two documents  listed above
describe the technical and  permitting aspects of defining individual, site-specific mixing zones for
point-source discharges during the NPDES permitting process. Additional information on mixing
zones can also be found in the EPA's Compilation of EPA Mixing Zone Documents (2006) and
Advanced Notice of Proposed Rulemaking for Water Quality Standards (1 998).
5.1.1  Recommended Contents of State and Tribal Mixing Zone Policies

The EPA recommends that states and authorized tribes adopt, at a minimum, a definitive statement
into their WQS specifying whether the state or tribe intends to authorize mixing zones. Consistent
with the discussion above, where a  mixing zone is authorized, water quality criteria are met at the
edge of the mixing zone during critical low-flow conditions (which are described in Section 5.2 of
this chapter) so that the designated use of the waterbody as a whole is protected. If a state or tribe
chooses to adopt a mixing zone policy, such a policy should ensure the following:

      Mixing zones do not impair the designated use of the waterbody as a whole.
      Pollutant concentrations within the mixing zone are not lethal to organisms passing
       through the mixing zone.2
2 Lethality is a function of the magnitude of a pollutant concentration and the duration an
organism is exposed to that concentration. Section 4.3.3 of the TSD (1 991)  describes various

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       Pollutant concentrations within the mixing zone do not cause significant human health
       risks considering likely pathways of exposure.
       Mixing zones do not endanger critical areas such as breeding or spawning grounds,
       habitat for threatened or endangered species, areas with sensitive biota, shellfish beds,
       fisheries, drinking water intakes and sources, or recreational areas.

Because pollutant concentrations may exceed numeric criteria within mixing zones, these elevated
concentrations could adversely affect the productivity of the waterbody and have unanticipated
ecological consequences. Therefore, the EPA recommends that the use of mixing zones in the
development of WQBELs in NPDES permits be carefully evaluated and appropriately limited on a
case-by-case basis in light of the  overarching requirement to protect the designated use of the
waterbody as a whole pursuant to 40 CFR 131.10.

Due to potential additive or synergistic effects of certain pollutants that could result in the
designated use of the waterbody as a whole not being protected, state and tribal mixing zone
policies should specify, and permitting authorities should ensure, that mixing zones do not
overlap. Additionally, the EPA recommends that permitting  authorities evaluate the cumulative
effects of multiple mixing zones within the same waterbody. The EPA has developed a holistic
approach to determine whether a  mixing zone is appropriate  based on such cumulative effects
considering all of the impacts to the designated uses of the waterbody (see Allocated Impact Zones
for Areas of Non-Compliance (1 995)). If the total area affected by elevated concentrations within all
mixing zones combined is small compared to the total area of the waterbody in which the mixing
zones are located, then mixing zones are likely to have little effect on the designated use of the
waterbody as a whole, provided that they do not impinge on  unique or critical habitats. As
understanding of pollutant impacts on ecological systems evolves, states and tribes may find
specific cases in which no mixing  zone is appropriate.

States and tribes that choose to adopt mixing zone policies should describe the general
procedures for defining and  implementing mixing zones in terms of location, maximum size,
shape, outfall design, and in-zone water quality, at a minimum. Such policies should be sufficiently
detailed to support regulatory actions, issuance of permits, and determination of best
management practices for nonpoint sources.

The EPA recommends that specific characteristics of an individual mixing zone for a specific
discharger be defined on a case-by-case basis  using the state or tribal  mixing zone policy. This
site-specific assessment would ideally take into consideration the physical, chemical, and
biological characteristics of the discharge (including the type of pollutant discharged) and
receiving waterbody; the life  history and behavior of organisms in the receiving waterbody; and the
designated uses of the waterbody.

Location
methods for preventing lethality to organisms passing through a mixing zone.

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States and authorized tribes should restrict the potential locations of mixing zones as a way to
protect stationary benthic organisms and human health from the potential adverse effects of
elevated pollutant levels. In addition, states and tribes should  prohibit mixing zones where they
may endanger biologically important and other critical areas that the state, tribe, or federal
government has identified. These include breeding and spawning grounds, habitat for threatened
or endangered species, areas with sensitive biota, shellfish beds, fisheries, drinking water intakes
and sources, and recreational areas.

Pollutant concentrations above the chronic aquatic life water quality criterion may prevent sensitive
taxa from  living and reproducing successfully within the mixing zone. In this regard,  benthic and
territorial organisms  may be of greatest concern  in protecting aquatic life within a mixing zone.
The higher the pollutant concentrations occurring within the mixing zone, the more taxa are  likely
to be adversely affected, thereby affecting the structure and function of the ecological community
and, potentially, the designated use of the waterbody as a whole.

For protection of human health, states and tribes should restrict mixing zones such that they do
not result  in significant human health risks when evaluated using reasonable assumptions about
exposure pathways. For example, where drinking water contaminants are a concern, the mixing
zones should not encroach on drinking water intakes and sources. Where fish tissue residues  are a
concern (either because of measured or predicted residues), mixing zones should not result  in
significant human health risks to average and sensitive subpopulations  of consumers of fish and
shellfish after considering exposure duration of the affected aquatic organisms in the mixing  zone
and the patterns of fisheries use in the area. Where waters are designated for primary contact
recreation, mixing zones for bacteria should not result in significant human health risks to people
recreating in such waters. In all cases, it is critical that the designated use of the waterbody as a
whole is protected.

Size

In order to protect the designated uses  of the waterbody as a whole, pollutant concentrations
within any mixing zone should not be lethal to mobile, migrating, and drifting organisms in  the
waterbody or cause significant human health risks considering likely pathways of exposure.  One
means of achieving these objectives is to limit the size of the mixing zone.

Most states and authorized tribes allow  mixing zones as a matter of policy but also specify general
spatial dimensions that limit their size.  States and tribes have  developed various methods of
defining the maximum allowable size of mixing zones for various types  of waters. State and tribal
policies dealing with  streams and rivers often limit mixing zone widths, cross-sectional areas,
and/or flow volumes and allow lengths  to be determined on a case-by-case basis. For lakes,
estuaries,  and coastal waters, dimensions are usually specified by surface area,  width,
cross-sectional area, and/orvolume. The EPA recommends that states and tribes use methods that
result in quantitative measures sufficient for permitting authorities to develop WQBELs in  a
transparent  and  straightforward manner.

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If a mixing zone is authorized for a specific discharge, the permitting authority then defines the
actual size of an individual, site-specific mixing zone for the specific discharge on a case-by-case
basis using the general size restrictions in the state or tribal mixing zone policy. The area or
volume of an individual mixing zone or group of mixing zones should be as  small as practicable so
that it does not interfere with the designated uses orwith the established community of aquatic life
in the segment for which the uses are designated.

In general, where a state or tribe has both acute and chronic aquatic life water quality criteria as
well as human health criteria for the same pollutant, states and tribes may  establish independent
mixing zone size specifications that apply to each criteria type. For aquatic life criteria, there may
be up to two types of mixing zones: one for the acute criterion and one for the  chronic criterion
(see Figure 5.1).

In the zone immediately surrounding the outfall,  both the acute and the chronic criteria may be
exceeded,  but the acute criterion is met at the edge of this zone,  which is often referred to as the
acute mixing zone or the zone of initial dilution. The acute mixing  zone is sized to prevent lethality
to  passing organisms in order to protect the designated  use of the waterbody as a whole.

In the next mixing zone, which  is often called the chronic mixing  zone, the chronic criterion may
be exceeded, but the acute criterion is met. The chronic criterion  is met at the edge of the chronic
mixing zone. The chronic mixing zone is sized to  protect the designated use of the waterbody as a
whole.

Where the state or tribe also has human health criteria for the pollutant of concern, the human
health mixing zone is sized to  prevent significant human risks in  order to protect the  designated
use of the waterbody as a whole.

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                                                Chronic Criterion Met
  Figure 5.1: Example Mixing Zones for Acute and Chronic Aquatic Life Criteria
For a particular pollutant found in a particular discharge, the magnitude, duration, frequency, and
any authorized mixing zone associated with each of the criteria types (i.e., human health and acute
and chronic aquatic life) will determine which criterion most limits the allowable discharge. In all
cases, the permitting authority should evaluate the size of the site-specific mixing zone to determine
its effect on the designated use of the waterbody as a whole. Section 2.2.2  of the TSD (1 991) contains
information for determining whether a mixing zone's size is appropriate.

State and tribal mixing zone policies should identify zones of passage within waterbodies that
contain migrating, free-swimming, or drifting organisms. Zones of passage are continuous water
routes  of such volume, area, and quality as to allow the passage of free-swimming and drifting
organisms without significant adverse effects on their populations. Many species migrate for
spawning and other purposes. Not only do migrating  species (e.g., anadromous and catadromous
species) need to be able to reach suitable spawning areas, their young (and  in  some cases the
adults) require a safe return route to their growing and  living areas. Elevated pollutant
concentrations within a mixing zone can create  barriers that hinder or prevent safe migration.
Therefore, mixing zones should be sized and located appropriately within the waterbody to
provide a continuous zone of passage that protects migrating, free-swimming, and drifting
organisms.
The waterbody type, outfall design, and characteristics of the discharge will determine the shape of
a mixing zone. The shape should be a simple configuration that is easy to locate in a waterbody
and that avoids impingement on biologically important areas. In lakes, a circle with a specified
radius is generally preferable, but other shapes may be appropriate in the case of unusual site

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requirements.

"Shore-hugging" plumes should be avoided in all waterbodies. Shore areas are often the most
biologically productive and sensitive areas of a waterbody, and they are often used for recreation.
Shore-hugging plumes generally do not mix as well with receiving waters and, thus, do not dilute
as well as mixing zones with other shapes that do not hug shorelines. Because shore-hugging
plumes tend to keep unmixed water over the benthic area or in the recreational area, they are more
likely to adversely affect the designated uses of the waterbody.

Outfall Design

Because outfall design affects the amount of initial mixing that occurs, state and tribal mixing zone
policies should instruct dischargers to utilize the best practicable engineering design of the outfall
to maximize initial mixing. Sometimes, modifying the design of the diffuser, the location of the
outfall, or other outfall design characteristics can reduce significant adverse  impacts to the
waterbody because  different design characteristics have different effects on  mixing. Many
different factors affect how well the outfall design allows the discharge to mix with the receiving
water including the  following:

       The height of the outfall with respect to the  surface and bottom of the waterbody.
       The distance of the end of the pipe to the nearest bank (i.e., whether the outfall  is  in the
       middle of the waterbody or close to one side).
       The angle of the discharge.
       The type of diffuser that is used (i.e., single-port or multi-port diffuser).

Section 4.4.1  of the TSD (1 991) describes recommendations for outfall design in more detail.

In-zone Water Quality

States and authorized tribes should ensure that a minimum level of water quality is maintained
within a mixing zone. Mixing zones should attain the "free from" narrative water quality criteria
that are applicable to all waters in a state or reservation. For example, the  EPA recommends that
mixing zones be free from the following:

       Materials in  concentrations that will cause acutely toxic conditions to aquatic life.3
       Materials in  concentrations that settle to form objectionable deposits.
       Floating debris, oil, scum, and other material  in concentrations that form nuisances.
       Substances in concentrations that produce objectionable color, odor, taste, or turbidity.
3 Acutely toxic conditions are those that are lethal to aquatic organisms that may pass through the
mixing zone. The underlying assumption for allowing a mixing zone is that pollutant
concentrations in excess of acute and chronic criteria, but below acutely toxic concentrations, may
exist in  small areas without causing adverse effects to the designated  use of the waterbody as a
whole.

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       Substances in concentrations that produce undesirable aquatic life or result in a dominance of
       nuisance species.


5.1.2 Situations in Which Mixing Zones May Not Be Appropriate

As discussed above, states and authorized tribes are not required to allow mixing zones. Even if a
state or tribe chooses to allow mixing zones generally, it may also choose to define in its policy
circumstances under which mixing zones  are prohibited (e.g., for particular pollutants and/or
waterbodies). Likewise, where the state or tribe generally allows mixing zones, the permitting
authority may decide that a mixing zone is not appropriate for a particular discharge  on a
site-specific basis.4 States and tribes should conclude that mixing zones are not appropriate in
the following situations:

       Where they may impair the designated use of the waterbody as a whole.
       Where they contain  pollutant concentrations that may be lethal to passing organisms.
       Where they contain pollutant concentrations that may cause significant human health risks
       considering likely pathways of exposure.
       Where they may endanger critical  areas such as breeding and spawning grounds, habitat
       for threatened or endangered species, areas with sensitive biota, shellfish beds, fisheries,
       drinking water intakes and sources,  and recreational areas.

Additionally, states and tribes should carefully consider whether mixing zones are appropriate
where a discharge contains bioaccumulative, pathogenic, persistent, carcinogenic, mutagenic, or
teratogenic pollutants or where a discharge  containing toxic pollutants may attract aquatic life.

Bioaccumlative pollutants are one example of a pollutant for which mixing zones may not be
appropriate because they may cause significant human health risks such that the designated use of
the waterbody as  a whole may not be protected.5  Therefore, the EPA recommends that state and
tribal mixing zone policies do not allow mixing zones for discharges of bioaccumulative
pollutants. The EPA adopted this  approach in 2000 when it amended its  1995 Final Water Quality
Guidance for the Great Lakes System at 40 CFR Part 1 32 to phase out mixing zones for existing
discharges of bioaccumulative pollutants within the Great Lakes Basin and ban such mixing zones
for  new discharges within the Basin.

Because fish tissue contamination tends to be  a far-field problem affecting entire or downstream
waterbodies rather than a near-field problem being confined to the area within a mixing zone, a
state or tribe may find it appropriate to restrict or eliminate mixing zones for bioaccumulative
4 The 1 996 memorandum EPA Guidance on Application of State Mixing Zone Policies in EPA-issued
NPDES Permits describes the circumstances under which the EPA may include a mixing zone in an
NPDES permit when the EPA is the permitting authority.
  However, note that some chemicals of relatively low toxicity such as zinc will bioconcentrate in
fish without harmful effects resulting from human consumption.

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pollutants in certain situations such as the following:

       Where mixing zones may encroach on areas often used for fish harvesting, particularly for
       stationary species such as shellfish.
       Where there are uncertainties in the protectiveness of the water quality criteria or the
       assimilative capacity of the waterbody.

Chapter 3 of this Handbook and Chapter 5 of Methodology for Deriving Ambient Water Quality
Criteria for the Protection of Human Health (2000)  provide additional information about
bioaccumulation, and Section 4.3.4 of the TSD (1 991) discusses preventing bioaccumulation
problems for human health in calculating WQBELs.

Another example of a pollutant for which a mixing  zone may  not be appropriate is bacteria.
Because bacteria mixing zones may cause significant human health risks and endanger critical
areas (e.g., recreational areas), the EPA recommends  that state and tribal mixing zone policies do
not allow mixing zones for bacteria in waters designated for primary contact recreation. The
presumption in a river or stream segment designated  for primary contact recreation is that primary
contact recreation can safely occur throughout the segment and, therefore, that bacteria levels will
not exceed criteria throughout the segment. Epidemiological studies have demonstrated that
illness rates are higher when the criteria are exceeded compared to when those criteria are not
exceeded (see Sections 3.2 and 3.3 of the EPA's Recreational Water Quality Criteria (201 2)).
Therefore, people recreating in or through a bacteria mixing zone (where bacteria levels  may be
elevated  above the criteria levels)  may be exposed  to greater risk of gastrointestinal illness than
would otherwise be allowed by the state or tribal criteria for protection of the recreation use. Given
this presumption, states and tribes should carefully evaluate whether authorizing a mixing zone
that results in elevated levels of bacteria in a river or stream designated for primary contact
recreation will adversely affect the designated  use.  If so, then states and tribes should not
authorize such mixing zones because they could result in a significant human health  risk.

A third example of a situation in which the EPA recommends that states and tribes prohibit  a mixing
zone is when an effluent is known to attract biota. In such cases, a continuous zone of passage
around the mixing area will not protect aquatic life. Although most toxic pollutants elicit a neutral or
avoidance response, there  are some situations in which aquatic life are attracted to a toxic discharge
and, therefore, can potentially incur significant exposure.  For example, temperature can be an
attractive force and may counter an avoidance response to a particular pollutant. Therefore, the
organisms would tend to stay in the mixing zone rather than passing through or around it.  Innate
behavior  such as migration may also counter an avoidance response and cause fish to incur
significant exposure.


5.1.3  Mixing Zones for the Discharge of Dredged or Fill Material

In  conjunction with the Department of the Army, the  EPA has developed guidelines at 40 CFR Part
230 for evaluating discharges of dredged or fill material into navigable waters, which  include
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provisions at 40 CFR 230.1 l(f) for determining the acceptability of mixing zones for such material.
Discharges of dredged or fill material are generally temporary and result in short-term disruption
to the waterbody rather than constituting a continuous discharge with long-term disruption
beyond the fill area. In authorizing and establishing mixing zones for dredge and fill activities, the
state or authorized tribe's primary consideration should be achieving and protecting the
designated uses of the waterbody pursuant to 40 CFR 131.10. As such, states and tribes should
evaluate the particular pollutants involved for their effects on the designated use. Technical
guidance for determining the potential for contaminant-related impacts associated with the
discharge of dredged material can be found in Evaluation of Dredged Material Proposed for
Discharge in Waters of the U.S. - Testing Manual: Inland Testing Manual'(1 998).
5.1.4 Mixing Zones for Aquaculture Projects

Under Section 31 8 of the CWA. permitting authorities may allow discharges of certain pollutants
associated with approved aquaculture projects. Consistent with 40 CFR 1 22.25. an aquaculture
project is a defined, managed water area into which certain pollutants are discharged for the
maintenance or production of harvestable freshwater, estuarine, or marine plants or animals. The
EPA's regulations at 40 CFR 125.11 provide that aquaculture project approval must not result in
the enlargement of a pre-existing mixing zone beyond the area designated for the original
discharge and that the designated project area (which is also defined at 40 CFR 1 22.25) must not
include a portion of a waterbody large enough to expose a substantial portion of the indigenous
biota to the conditions within the designated  project area.  For example, a designated project area
should not include the entire width of a stream because all of the indigenous organisms might be
exposed to pollutant discharges that would exceed WQS. The areas designated for approved
aquaculture projects should be treated in the same manner as other mixing zones.
5.2  Critical  Low Flows for Water Quality Criteria Implementation

Pursuant to 40 CFR 131.11 (a), states and authorized tribes must adopt those water quality criteria
that protect designated uses. To ensure that adopted criteria are protective of the designated uses,
states and tribes generally establish critical low-flow values to support implementation of the
applicable criteria through such programs as  NPDES permitting.

Critical low-flow conditions present special challenges to the  integrity of the aquatic community
and the protection of human health. Dilution  is one of the primary mechanisms by which the
concentrations of contaminants in effluent discharges are reduced following their introduction into
a receiving water. Low flows in the receiving water typically aggravate the effects of effluent
discharges because, during a low-flow event, there is less water available for dilution, resulting  in
higher instream concentrations of pollutants.  Therefore, the allowable dilution (which may be only
a portion of the critical low flow depending on the state or tribal WQS and implementation
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procedures) for purposes of determining the need for and establishing WQBELs in NPDES permits
should ensure protection of the applicable criteria at the calculated critical low-flow value.

The EPA has historically encouraged states and tribes to specify directly within their WQS which
calculated critical low-flow values should be used to determine the available dilution for the
purposes of determining the need for and establishing WQBELs. Such critical low-flow values  have
historically been reviewed and approved or disapproved by the EPA as new or revised WQS under
Section 303(c) of the CWA. Likewise, revisions to those critical low-flow values would generally
constitute new or revised WQS subject to EPA review and approval or disapproval (see Chapter 1  of
this Handbook and What is a New or Revised Water Quality Standard Under CWA Section 303(c)?
Frequently Asked Questions (201 2)).

Most states and tribes generally follow the guidance  in the TSD (1 991) when adopting  critical
low-flow values for criteria implementation. The EPA recommends that states and tribes adopt the
critical low-flow values for use in steady-state analyses so that criteria are implemented
appropriately. If criteria are implemented using inappropriate critical low-flow values (i.e.,
calculated values that are too high), the resulting control of toxic pollutants may not be fully
protective because the resulting ambient concentrations could exceed criteria when such low flows
occur. In the case of aquatic life, more frequent excursions than  are allowable (e.g., more than
once in three years) could result in unacceptable effects on aquatic organisms and designated  uses
if the appropriate value is not used in the calculations.

In addition to steady-state models, the TSD recommends the use of three dynamic models to
perform wasteload allocations. Because dynamic wasteload models do not generally use specific
steady-state critical low-flow values but accomplish the same effect by factoring in the probability
of occurrence of stream flows based on the historical flow record, this Handbook discusses only
steady-state conditions.

In Appendix D of the TSD and Technical Guidance Manual for Performing Wasteload Allocations,
Book VI: Design Conditions - Chapter /: Stream Design Flow for Steady-State Modeling (1 986). the
EPA describes and recommends two methods for calculating acceptable critical low-flow values:
the traditional hydrologically based method developed by the United States Geological Survey
(USGS) and a biologically based  method developed by the EPA.6 The hydrologically based critical
low-flow value is determined  statistically using probability and extreme values, while the
biologically based critical low flow is determined empirically using the specific duration and
frequency associated with the criterion.

Additionally, the two documents listed above describe the flow values that the EPA recommends
for implementing acute and chronic criteria using both methods. Table 5.1  below summarizes
these  recommendations.
6 In some EPA documents such as those cited, critical low flow is also called "design flow" or
"stream design flow." These terms are different from a facility or effluent design flow.
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Table 5.1 : EPA-recommended Critical Low Flows for Aquatic Life and Human Health Criteria
Criteria
Acute Aquatic Life
Chronic Aquatic Life
Human Health
Hydro logically Based Flow
1Q10
7Q10
Biologically Based Flow
IBS
4B3
Harmonic mean
Using the hydrologically based method, 1 Ql 0 represents the lowest one-day average flow event
expected to occur once every ten years, on average, and 7Q1 0 represents the lowest
seven-consecutive-day average flow event expected to occur once every ten years, on average.
Using the biologically based method, 1 B3  represents the lowest one-day average flow event
expected to occur once every three years, on average,  and 4B3 represents the lowest
four-consecutive-day average flow event expected to occur once every three years, on average.

States and tribes may designate other critical low-flow values to implement the applicable criteria,
provided they are scientifically justified. The EPA has  also recommended critical low-flow values
that differ from the above recommendations for specific pollutants such 30Q5,  30Q1 0, and  30B3
for implementing chronic criteria for ammonia.

The EPA does not view the fact that many  streams within a state or tribe have no flow at 7Q1 0 as
adequate justification for designating alternative flows. Note that, when a  criterion specifies a
four-day average concentration that should not be exceeded more than once every three years,
this condition should not be interpreted as implying that a 4Q3 low flow is appropriate for use as
the hydrologically based critical low-flow value for assessing impacts on the receiving water.

The EPA recommends the harmonic mean flow for implementing human health  criteria. The
concept of a harmonic mean  is a standard statistical data analysis technique. The EPA's model for
human health effects assumes that such effects occur because of a long-term exposure to low
concentrations of a toxic pollutant (e.g., two liters of  water per day for seventy years). The
harmonic mean flow allows for estimating the concentration of toxic pollutant contained in those
two liters of water per day when the daily variation in the flow rate is high. Therefore, the EPA
recommends use of the harmonic mean flow in computing critical low flows for human health
criteria rather than using other averaging techniques.

In addition to the documents listed above, see the EPA's Flow 101 webpage  and Advanced Notice of
Proposed Rulemaking for Water Quality Standards (1 998) for additional information on critical low
flows.

The EPA notes that the USGS  has documented that, in some areas of the United  States, there have
been changes to the critical low flows in freshwater rivers and streams or increased duration and
frequency  of low flow occurrence. The  source of the reductions may often  be anthropogenic in
origin such as over-pumping of groundwater, hydrologic alteration  including impoundments, or
surface water withdrawals. Some of these reductions may persist long  enough to cause changes to
the critical low-flow values. In addition, prolonged droughts have resulted in a reduction of  the
low-flow minimums released on regulated rivers or revisions to drought control manuals to  allow
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for further reductions of the low-flow values. During prolonged droughts, there may also be a
trend towards increased pumping of groundwater, which may,  in  turn, lead to a reduction of
surface water flows. New water intakes may also permanently change a waterbody's critical  low
flow. The following documents provide additional information on changing flow patterns:

      The USGS's National Water Census - Streamf/owwebpaQe.
      The USGS's Groundwater Depletion in the United States (1900-2008) (2 01 3).
      The USGS's Alteration of Streamflow Magnitudes and Potential Ecological Consequences: a
       Mult/regional Assessment (201 1).
      The EPA's Report on the Environment - Fresh Surface M/aferwebpage.

It may be prudent for states and tribes to review and revise, as  appropriate, their critical low-flow
values during the triennial  review process to account for changes to historical flow patterns. Also,
NPDES permitting authorities should be aware that these altered historical flow patterns in rivers
and streams may render historical flow records less  accurate in predicting current and future
critical flows. Where appropriate, permitting authorities should consider alternate approaches to
establishing critical low-flow conditions that account for these  climatic and anthropogenic
changes when conducting  reasonable potential analyses and in establishing protective WQBELs
(see NPDES Permit Writers' Manual: Inclusion of Climate Change Considerations).
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5.3  Variances from Water Quality Standards

A WQS variance is a time-limited designated use and water quality criterion for a specific
pollutant(s) or water quality parameter(s) that reflect the highest attainable condition during the
term of the WQS variance. A WQS variance may apply to an NPDES-permitted discharger or
waterbody/waterbody segment(s). The regulation at 40 CFR 131.13 provides that states and
authorized tribes may adopt into their WQS general variance policies that describe how they intend
to apply and implement variances. Although such variance policies require EPA review and
approval, states and tribes are not required to adopt variance policies in order to adopt individual
variances. Nevertheless, as opposed to individual mixing zones (discussed in Section 5.1 of this
chapter), the individual variances themselves must be adopted into WQS (or other legally binding
state or tribal requirements) and approved by the EPA before they can be effective for CWA
purposes.

Although the legal authority to adopt a WQS variance is the same as a revision to a designated use,
the purpose of a variance is different from that of a designated use revision (described in Chapter
2. of this Handbook). A variance  is  intended to serve as a mechanism to provide time for states,
tribes, and stakeholders to implement actions to  improve water quality over an  identified period of
time when and where the designated use currently in place is not being met. When utilizing a
variance, the state or tribe retains the designated use that is currently in place as a long-term goal.
As first articulated  in 1 977 in Decision of the General Counsel on Matters of Law Pursuant to 40
CFR Section 125.36(m). No. 58,  a state or tribe may adopt  a WQS variance if the state or tribe can
satisfy the same substantive and procedural  requirements as a designated use  removal, which are
described in 40 CFR 131.10(g).

A variance is also different from a  permit compliance schedule. While both tools can  provide time
to meet regulatory requirements, which tool is appropriate depends upon the circumstances.
Variances can be appropriate to address situations where it is known that the designated use and
criterion are unattainable today  (or for a limited period of time), but feasible progress could be
made toward attaining the designated use and criterion. A permit compliance schedule, on the
other hand, may be appropriate when the designated use is attainable, but the discharger needs
additional time to modify or upgrade treatment facilities in order to meet  its WQBEL such that a
schedule and  resulting milestones will lead to compliance "as soon as possible" with  the WQBEL
based on the currently applicable WQS. See CWA Section 502(1 7) for a definition of "schedules of
compliance" and 40 CFR 122.47.

A variance may be appropriate where a state or tribe determines that the designated use cannot be
attained for a period of time because the discharger cannot immediately meet  a WQBEL, which is
written to meet a particular WQS, or a waterbody/waterbody segment cannot immediately meet the
criteria to protect the designated use. Under such circumstances, the variance provides a targeted,
time-limited revision to the WQS that reflects the highest attainable condition. These new
time-limited WQS then serve as the basis for pollution control requirements during the term  of the
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variance. For WQS variances that apply to aquatic life, wildlife, and recreational uses (i.e., the
Section 1 01 (a)(2) uses), this means that attainment of the designated use is infeasible under at
least one of the six factors at 131.1 0(g) for at least the term of the variance.

The practical effect of the variance is an NPDES permit containing aWQBELthat complies with a less
stringent criterion than would otherwise be in effect in the absence of the variance. However, the
underlying designated  use and criteria remain in effect for Section 303(d) listing and total
maximum daily load development regardless of whether the variance is for a single discharger,
multiple dischargers, or a waterbody/waterbody segment. At the end of the variance term, the
discharger's WQBEL must  ensure compliance with the underlying designated use and criterion or
the state or tribe must obtain a new variance. To obtain a new variance, the state or tribe must
again demonstrate that the designated use is not attainable at the point of discharge and again
submit the variance to the EPA for review and approval  or disapproval.

In many cases, a WQS variance is an environmentally useful tool  because a variance exists only for
a defined term and  retains designated use protection for all pollutants and sources, with the sole
exception of those specified in the variance. Even the discharger with a variance for a particular
pollutant is required to meet applicable criteria for all other pollutants. Thus, a variance can result
in water quality improvements over time and, in some cases, full attainment of designated  uses by
maintaining existing water quality protections while allowing time for advances in  treatment
technologies, control practices, or other changes in circumstances.

States  and tribes typically adopt a WQS variance for an individual discharger for a specific pollutant
in a specific waterbody. However, where multiple dischargers have similar attainment challenges,  a
state or tribe may streamline its variance process by adopting a multiple-discharger WQS variance.
Such a variance applies to several dischargers but may be supported by a single technical rationale
justifying the need for  the variance. The EPA has previously published information on both
individual- and multiple-discharger variances at 40 CFR Part 1 32. For additional information on
variances, also see Discharger-Specific Variances on a Broader Scale: Developing Credible
Rationales for Variances that Apply to Multiple Dischargers (2013).
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