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WATER QUALITY STANDARDS CRITERIA DIGEST
A COMPILATION OF FEDERAL/STATE CRITERIA ON
-MIXING ZONES-
ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C.
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INTRODUCTION
This digest was compiled in order to provide general information to the
public as well as to Federal, State, and local officials. It contains excerpts
from the individual Federal-State water quality standards establishing criteria
for mixing zones for interstate waters. The water quality standards program is
directed by the Environmental Protection Agency, an independent regulatory
agency which has responsibility for approving State-adopted standards for inter-
state waters, evaluating adherence to the standards, and overseeing enforcement
of standards compliance.
Standards, the first nationwide strategy for water quality management,
contain four major elements: the use (recreation, drinking water, fish and
wildlife propagation, industrial, or agricultural) to be made of the interstate
water; criteria to protect those uses; implementation plans (for needed indus-
trial-municipal waste treatment improvements, among others) and enforcement
plans; and an antidegradation statement to protect existing high quality waters.
Minimum water quality criteria, or numerical specifications of physical,
chemical, temperature, and biological levels, are stated in the National
Technical Advisory Committee report to the Secretary of the Interior, Water
Quality Criteria, dated April 1, 1968, and published by the Government Printing
Office, Washington, D.C. Unavailability of the NTAC report before June 30,
1967--the date set by the Water Quality Act of 1965 for formal adoption of
State standards--resulted in significant variations between the state-adopted
and the NTAC minimum criteria. Some standards were adopted and approved before
the NTAC report became available. Also, the Water Quality Criteria report is
subject to updating in light of new scientific and technical information.
Mixing zones are areas which are unavoidably and harmfully polluted
and which are allowed for mixing of the discharged waters with the receiving
waters. They have defined and identifiable limits, and the waters out-
side of the zones must meet the standards for that particular body of water.
The Water Quality Criteria report recommends when several mixing zones are
located close together that they lie on the same side of the stream to allow
a continuous passageway for aquatic organisms on the opposite side. The NTAC
report specifies that mixing zones be as small as possible and provided only
for mixing in order to preserve the "welfare of the aquatic life resource."
This is because mixing zones constitute barriers which can harmfully block
the spawning migration of anadromous and catadromous species and damage the
plankton organisms and aquatic invertebrates in the water flow. Adequate
zones of passage (at least 75% of the cross-sectional stream area, according
to NTAC) must be maintained at all times for the fish, and adequate provision
must be made for the survival of the drift organisms. Mixing zones cannot be
considered a substitute for or an extension of a waste treatment facility. The
EPA supports the NTAC recommendations.
Since water quality standards experience revisions and upgrading from
time to time, following procedures set forth in the Federal Water Pollution
Control Act, individual entries in this digest may be superseded. As these
revisions are accomplished, this digest will be updated and reissued. Because
this publication is not intended for use other than as a general information
resource, for the latest information, and for special purposes and applications,
refer to the existing approved water quality standards which can be obtained
from the State water pollution control agencies or EPA Washington, D.C. or
regional offices.
Individual state-adopted standards follow.
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MIXING ZONES
Alabama
Alaska
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
District of Columbia
With respect to cooling water discharges only, the
ambient temperature of receiving waters shall not be
increased by more than 10 degrees Fahrenheit by the
discharge of such cooling waters, after reasonable
mixing; nor shall the discharge of such cooling waters,
after reasonable mixing, cause the temperature of the
receiving waters to exceed 93 degrees Fahrenheit.
No reference to mixing zones.
No reference to mixing zones.
No reference to mixing zones.
No reference to mixing zones.
In temperature measurement, allowance shall be made
for a mixing zone. Provisions shall be made for
adequate mixing and no thermal barrier to migration
and free movement of aquatic biota shall be permitted
in any waters of the state.
In the discharge of waste treatment plant effluent
and cooling waters to the receiving waters, cognizance
shall be given both in time and distance to allow for
mixing of effluent"and streams. Such distances required
for complete mixing shall not affect the water usage
Class adopted but shall be defined and controlled by
the Water Resources Commission.
The standards proposed are based upon the ability of
the Air and Water Resources Commission to measure and
to determine compliance. All measurements will be made
at selected sites after determining "representativeness"
of the sample obtained and the nature of the mixing at
the station. Also, short transition zones will exist
between adjacent zones of varying water quality.
Criteria shall apply to an entire stretch of the stream.
However, reasonable allowance shall be made for the
mixing and dispersion of approved discharges. Sampling
frequency shall provide a sound basis for computations.
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Within the limits of field conditions,sampling point
locations will be selected to permit the collection of
representative samples. . . .
No increase in natural water temperature caused by
artificial heat inputs shall exceed 5 degrees F.
after reasonable allowance for mixing.
Florida
Georgia
Guam
Hawaii
The criteria of water quality hereinafter provided
will be applied only after reasonable opportunity for
mixture of waste with receiving waters has been afforded;
the reasonableness of the opportunity for mixture of
waste and receiving water shall be determined on the
basis of the physical characteristics of the receiving
waters,and the methods in which the discharge is
physically made shall be approved by the regulatory
agency.
Effluents released to streams or impounded waters
shall be fully and homogeneously dispersed and mixed
insofar as practical with the main flow or water body
by appropriate methods at the discharge point. Use of
a reasonable and limited mixing zone may be permitted
on receipt of satisfactory evidence that such a zone is
necessary and that it will not create an objectionable
or damaging pollution condition.
No reference to mixing zones.
Upon the application of any person requesting that a
portion of the water areas meeting the basic standards
applicable to all waters be zoned for the assimilation
of agricultural, municipal and industrial discharges,
if the Director of Health shall determine that such uses
will not unreasonably interfere with any actual use of the
water areas for which it is classified, he shall then
designate such portions as a zone of irixing.
The boundaries of each zone of mixing shall be fixed
by the Director, taking into account protected uses of
the body of water, existing natural conditions of the
receiving water (i.e., depth, currents, location, etc.),
character of the effluent, and the adequacy of the
design of the outfall and diffuser system to achieve a
maximum dispersion and assimilation of the treated or
controlled waste with a minimum of undesirable or
noticeable effect on the receiving water.
The application shall be made on forms furnished by the
Director and shall contain the information required
therein.
The establishment of a zone of mixing and the boundaries
thereof shall be made only after hearing held by the
Director on the island where the area is situated in
accordance with the Hawaii Administrative Procedure Act
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and the Rules of Practice and Procedure of the
Department of Health.
Idaho
Illinois
The Director, on his own motion, or on the application
of any person, shall terminate the designation of a
water area as a zone of mixing, if after a hearing, he
shall determine that such water area meeting the basic
standards applicable to all coastal waters will un-
reasonably interfere with any actual use of the water
area. Such termination shall be made only after a
hearing held by the Director on the island where the
area is situated in accordance with the Hawaii Admin-
istrative Procedure Act and the Rules of Practice and
Procedure of the Department of Health. Upon such
termination, the standards of water quality applicable
thereto shall be those established for the water as other-
wise classified.
For purposes of enforcement of these standards, sampling
will be done at a point where these standards can be
evaluated, except for areas immediately adjacent to out-
falls. Cognizance will be given to the opportunity for
admixture of waste effluents with receiving waters.
In the application of any of the rules and regulations in
this Chapter, whenever a water quality standard is more
restrictive than its corresponding effluent standards then
an opportunity shall be allowed for the mixture of an
effluent with its receiving waters. Water quality
standards must be met at every point outside of the mixing
zone. The size of the mixing zone cannot be uniformly
prescribed. The governing principle is that the propor-
tion of any body of water or segment thereof within mix-
ing zones must be quite small if the water quality stan-
dards are to have any meaning. This principle shall be
applied on a case-by-case basis to ensure that neither
any individual source nor the aggregate of sources shall
cause excessive zones to exceed the standards. The water
quality standards must be met in the bulk of the body of
water, and no body of water may be used totally as a
mixing zone for a single outfall or combination of outfalls,
Moreover, except as otherwise provided in this Chapter,
no single mixing zone shall exceed the area of a circle
with a radius of 600 feet. Single sources of effluents
which have more than one outfall shall be limited to a
total mixing area no larger than that allowable if a
single outfall were used.
In determing the size of the mixing zone for any discharge,
the following must be considered:
1. The character of the body of water,
2. the present and anticipated future use of body
of water,
3. the present and anticipated water quality of the
body of water,
4. the effects of the discharge on the present and
anticipated future water quality,
5. the dilution ratio, and
6. the nature of the contaminant.
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Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
In addition to the above, for waters designated for aquatic
life (General Standards), the mixing zone shall be so
designed as to assure a reasonable zone of passage for
aquatic life in which the water quality standards are met.
The mixing zone shall not intersect any area of any such
waters in such a manner that the maintenance of aquatic
life in the body of water as a whole would be adversely
affected.
No reference to mixing zones.
Sampling to determine conformance to these criteria
shall be done at sufficient distances downstream from
waste discharge points to permit adequate mixing. . . .
The measurement system to be used in each case should
provide for temperature measurement at the outfall and
with the maximum temperature allowed at the outfall re-
flecting a reasonable mixing zone in the receiving waters
so that the 5°F or 3°F rise specified is not violated in
the contiguous waters. Any barrier to migration and the
free movement of aquatic biota is prohibited.
Areas immediately adjacent to outfalls shall be as small
as possible, be provided for mixing only, and shall not
prevent the free passage of fish and drift organisms.
No reference to mixing zones.
After adoption of any classification by the legislature
for surface waters or tidal flats or sections thereof,
it shall be unlawful for any person, corporation, munici-
pality, or other legal entity to dispose of any sewage,
industrial or other waste, either alone or in conjunction
with another or others, in such manner as will, after
due consideration for seasonal, climatic, tidal and
natural variations and after reasonable opportunity for
dilution, diffusion, mixture or heat transfer to the
atmosphere, within mixing zones reasonably established
by the Environmental Improvement Commission in the
manner provided by this section, lower the quality of
said waters, outside such zones, below the minimum re-
quirements of such classification and notwithstanding
any licenses which may have been granted or issued under
section 413 to 415.
The commission may establish a mixing zone with respect
to.any discharge at the time application for license
for such discharge is made pursuant to section 414, and
when so established shall be a condition of and form a
part of the license issued. The commission may, after
30 days' notice to and a hearing with the affected party,
establish by order a mixing zone with respect to any
discharge for which a license has heretofore been issued
pursuant to section 414, or for which no license is
required by virtue of the last sentence of section 413.
Prior to the issuance of any order, or commencement of
any enforcement action to abate a classification vio-
lation, the commission shall establish, in the manner
above provided, a mixing zone with respect to the dis-
charge sought to be thereby affected.
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In determining the extent of any mixing zone to be by
it established under this section, the commission shall
solicit and receive testimony concerning the nature and
rate of the discharge; the nature and rate of existing
discharges to the waterway and their effect upon the
ability of the waterway to achieve its classification
standards; the size of the waterway and the rate of flow
therein; any seasonal, climatic, tidal and natural varia-
tion in such size, flow, nature and rate and the effect
of such variation upon the ability of the waterway to
achieve its classification standards; the uses of the
waterways in the vicinity of the discharge, and such other
and further evidence as in the commission's judgment will
enable it to establish a reasonable mixing zone for such
discharge. An order establishing a mixing zone may pro-
vide that the extent thereof shall vary in order to take
into account seasonal, climatic, tidal and natural varia-
tions in the size and flow of, and the nature and rate
of discharges to, the waterway.
Where no mixing zones have been established by the
commission, it shall be unlawful for any person,
corporation, municipality or other legal entity to
dispose of any sewage, industrial or other waste, either
alone or in conjunction with another or others, into
any classified surface waters, tidal flats or sections
thereof, in such manner as will, after reasonable
opportunity for dilution, diffusion, mixture or heat
transfer t.o the atmosphere, lower the quality of any
significant segment of said waters, tidal flats or sections
thereof, affected by such discharge, below the minimum
requirements of such classification, and notwithstanding
any licenses which may have been granted or issued under
section 413 to 415.
Maryland For all water use categories other than IV, there must
be no temperature change that adversely affects fish,
other aquatic life, or spawning success. There must be
no thermal barriers to the passage of fish or other
aquatic life. Maximum temperature must not exceed 100
degrees F beyond 50 ft. from any point of discharge.
For NONTIDAL WATERS: For the propagation of fish and
other aquatic life (Water Use Category IV) in all other
nontidal waters, temperature must not exceed 93 degrees F.
beyond such distance from any point of discharge as
specified by the Department of Water Resources as necessary
for the protection of the water use.
For TIDAL WATERS: For the propagation of fish and
other aquatic life (Water Use Category IV), temperature
must not exceed 90°F. beyond such distance from any
point of discharge as specified by the Department as
necessary for the protection of the water use.
GENERAL: It is possible that relatively small regions
for effluent mixing and assimilation may be permitted td '
exist without detriment to the water uses of the zone.
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Massachusetts
Michigan
When an effluent is permitted to be discharged to the
receiving waters, cognizance shall be given both in
time and distance to allow for mixing of effluent and
stream. Such distances required for complete mixing
shall not affect the water usage class adopted.
In areas adjacent to outfalls the standards for the
designated water use or uses shall apply after admixture
of waste effluents with the public waters but in no
instance shall the mixing zone act as a barrier to fish
migration or interfere unreasonably with the designated
water use or uses for the area. The Water Resources
Commission must have discretion in determining the extent
of the mixing zone. In general, the Water Resources
Commission encourages the use of outfall structures which
minimize the extent of the mixing zone.
Minnesota
Mississippi
Missouri
Montana
Nebraska
Nevada
New Hampshire
Reasonable allowance will be made for dilution of
the effluents in relation to the uses of the inter-
state waters into which they are discharged or other
interstate waters which may be affected.
Temperature shall not be increased more than ten de-
grees F. (10°F) above the natural prevailing back-
ground temperatures, nor exceed a maximum of 93°F.
after reasonable mixing. In cognizance of the fact that
certain waters of the state may not fall within desired
or prescribed limitations as outlined, the Air § Water
Pollution Control Commission may authorize exceptions to
these limits upon presentation of good and sufficient
evidence of intent to comply to the extent practical or
technically feasible.
No reference to mixing zones.
No reference to mixing zones.
No reference to mixing zones.
No reference to mixing zones.
A heated discharge to a lake shall not raise the temoerature
more than 3 degrees F.at the surface immediately outside
a designated mixing zone. New Hampshire had also adopted
verbatim the entire criteria pertaining to temperature
and zbnes of passage contained in Section 3 of the National
Technical Advisory Committee Report on Water Quality
Criteria dated April 1, 1968. This report makes recom-
mendations regarding mixing zones and zones of passage
on page 31.
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New Jersey Localized areas of surface waters, as may be designated
by the Departmenti of Environmental Protection, into which
wastewater effluents, including heat, may be discharged
for the purpose of mixing, dispersing or dissipating such
wastewater without creating nuisances or hazardous
conditions,
Trout Maintenance Streams: No heat may be added which
would cause temperatures to exceed 2°F over the natural
temperatures at any time or which would cause temperatures
in excess of 68°F. Reductions in temperatures may be
permitted where it can be shown that trout will benefit
without detriment to other designated water uses. The
rate of temperature change in designated mixing zones shall
not cause mortality of the biota.
Non-Trout Waters: No thermal alterations, except in
designated mixing zones, which would cause temperatures
to deviate more than 5°F at any time from natural stream
temperatures or more than 3°F in the epilimnion of lakes
and other standing waters. No heat may be added, except
in designated mixing zones, which would cause temperatures
to exceed 82°F for smallmouth bass or yellow perch waters
or 86°F for other non-trout waters. The rate of tempera-
ture change in designated mixing zones shall not cause
mortality of the biota.
New Mexico Samples taken for the regulation and enforcement
of these standards are to be collected at
the mid-point of the stream flow at locations a sufficient
distance downstream from the point of introduction of
waste-water inflow to provide for reasonable mixing of
the stream and the inflowing water. Sampling in reservoirs
and lakes for the purposes of the general standards may be
at any point in the body of the water, but not closer
than 250 feet from the point of introduction of a water
contaminant. A reservoir or lake is considered to in-
clude all of the area flooded when the water in the basin
is at the spill-way level.
New York Trout Maintenance Streams: No heat may be added which
would cause temperature to exceed 2°F over the natural
temperatures at any time or which would cause temperatures
in excess of 68°F. Reductions in temperatures may be
permitted, where it can be shown that trout will benefit
without detriment to other designated water uses. The
rate of temperature change in designated mixing zones
shall not cause mortality of the biota.
Non-Trout Waters: (1) Mixing Zone: The mixing zone will
be separately determined for each discharge so as to
minimize detrimental effects. Fish and other aquatic
life shall be protected from thermal blocks by providing
for a minimum fifty percent stream or estuarine cross-
section and/or volumetric passageway, or establishing
artificial fishways where considered necessary. Generally,
the surface water temperature shall not exceed 90°F.
within the mixing zone. Consideration will be given to
effects of each discharge based on hydrodynamics and
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North Carolina
North Dakota
Ohio
Oklahoma
other factors of receiving waters. (2) Outside Mixing
Zone: Stream temperature in excess of 80°F will not be
permitted after mixing. Further, no permanent change
in excess of 5°F will be permitted from naturally occurring
background temperatures. In multiple discharge situations
stream capacity to meet such criteria will be apportioned
among the discharges. (3) Outside Mixing Zone: Fresh
Surface Water Classes: Temperature change rate shall be
limited to 2°F per hour, not to exceed 9°F in any 24-hour
period, further limited in that for any seven day neriod
the average change will meet the 5°F change of
background criteria stated in item 2 above. (4)
Outside Mixing Zone: Tidal Salt Water Classes:
Discharges shall not raise monthly means of maximum
daily temperatures more than 4°F from September
through May, nor more than 1.59F during June, July,
and August. Temperature change shall not be more
than IT? per hour, not to exceed 7°F in any 24-hour
period at maximum, except when natural phenomena cause
these limits to be exceeded.
In making tests for analytical determinations of
classified waters to determine conformity or non-
conformity with the established standards, samples
shall be collected outside the limits of prescribed
mixing zones in such manner and at such times and
locations as to be representative of the receiving
waters after-treasonable opportunity for dilution
and mixture with the waste discharged thereto. The
limits of mixing zones will be defined by the Depart-
ment of Water and Air Resources on a case by case basis
after consideration of the magnitude and character of
the waste discharged and the si?e and character of the
receiving waters. Such zones shall be restricted to as
small an area and length as possible, and shall not pre-
vent free passage of fish or cause fish mortality.
The distance of river flow to allow for a reasonable
opportunity to mix and dilute wastes shall be at the
discretion of the State Department of Health and will
be based upon stream flow conditions at the time of
sampling, except where such discharges may adversely
affect a beneficial water use immediately downstream
or in close proximity to the waste point. In such
instances, a change in the method of waste discharge
or other control measures may be required.
For areas designated for aquatic life or recreation
uses cognizance will be given to opportunity for the
admixture of effluents with stream water.
All tributary streams and all waste effluents shall
be in such condition that when discharged to the stream^
reaches as defined, and interstate tributaries, they
shall not create conditions which will adversely affect
public health, or use of the water for beneficial pur-
poses .
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Oregon
Pennsylvania
Puerto Rico
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
No reference to mixing zones.
No reference to mixing zones.
No reference to mixing zones.
In the discharge of waste treatment plant effluents
to the receiving waters, cognizance shall be given
both in time and distance to allow for mixing of
effluent and stream. Such distances required for
complete mixing shall not affect the water usage Class
adopted but shall be defined and controlled by the
regulatory authority.
Lakes and Reservoirs: The size of the mixing zone will
be determined on an individual project basis and will be
based on normal engineering considerations, and the area
affected shall be kept at a minimum. The mixing zone
shall not prevent free passage of fish or cause fish
casualty.
Streams: That the zone of mixing shall be limited to
not more than 25 percent of the cross-sectional area
and/or volume of the flow of the stream and shall not
include more than one-third of the surface area measured
from shore to shore.
In making tests or analytical determinations of surface
waters to determine conformity or non-conformity with
the established criteria, samples shall be collected in
such manner and at such locations, times and frequencies
as approved by the Committee on Water Pollution. Every
effort should be made to make the samples representative
of the receiving waters after reasonable opportunity for
dilution and mixture with the polluting material dis-
charged thereto.
Mixing zone refers to that section of flowing stream or
impounded waters necessary for effluents to become dis-
persed. The mixing zone necessary to each particular
case shall be defined by the Tennessee Stream Pollution
Control Board.
Sampling will be in accordance with fully recognized
procedures. Samples must be representative of the
receiving waters allowing time and distance for mixing.
No reference to mixing zones.
In assigning classifications to the waters of the State
the Department of Water Resources may designate certain
lengths or areas of such waters as mixing zones provided
that any such mixing zones shall be only for the dispersal
and dilution of wastes which have been treated in a manner
approved by the department, shall be of no greater length
or area than is required for such purposes and may only
be allowed if such wastes conform substantially with the
technical and other requirements established for the
receiving waters. Such a mixing zone shall not constitute
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Virgin Islands
Virginia
Washington
West Virginia
Wisconsin
Wyoming
a barrier to the passage or migration of fish or produce
adverse effects on any fishery or other forms of wild
or aquatic life." The state interprets this to mean that
they will not authorize any mixing zone which will reduce
the passageway to less than 75% of the cross sectional
area of volume of flow of the stream.
For waters classified for use as harbors and docking
facilities the following criteria are-applicable at any
point in the harbor except for areas immediately adjacent
to outfalls or drainage ditches. In such areas recognition
will be given to opportunities for the admixture of waste
effluents with harbor waters.
There shall be no sudden temperature changes that may
affect aquatic life. There shall be no thermal barriers
to the passage of fish. Essential spawning areas shall
not be affected.
No reference to mixing zones.
No reference to mixing zones.
The Department of Resources Development may require
management of waste admixture zones depending on such
factors as effluent quality and quantity,available
dilution, temperature, current, and restrictions to the
movement of fish.
Bacteriological standards which protect water supply
and full body contact will be determined at water supply
intakes and designated full body contact recreation areas.
In regard to water quality parameters it shall be the
goal to promote facilities which will, in time, allow
these standards to be applied in the zone of mixing,i.e.,
diffusion or rapid mixing.
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