TD223.U6
M59
1980
SEPA
United States
Environmental Protection
Agency
Office of Water
Regulations and Standards
Washington. D.C. 20460
July 1980
Water
TD223U 6M59
Mixing Zones
Water Quality Standards
Criteria Digest
A Compilation
of State/Federal Criteria
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MIXING ZONES
Water Quality Standards
Criteria Summaries
A Compilation of State/Federal Criteria
September 1980
U. S. Environmental Protection Agency
Office of Water Regulations and Standards
Washington, D. C. 20460
pf0toclion Agency
230 fouth Leri^orn Street
Chicaco, Illinois £0604
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INTRODUCTION
This digest is compiled 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 pollutant specific criteria for interstate surface
waters. The water quality standards program is implemented by the
U.S. Environmental Protection Agency where responsibility for
providing water quality recommendations, approving State-adopted
standards for interstate waters, evaluating adherence to the
standards, and overseeing enforcement of standards compliance, has
been mandated by Congress.
Standards, a nationwide strategy for surface water quality
management, contain three major elements: the use (recreation,
drinking water, fish and wildlife propagation, industrial, or
agricultural) to be made of the navigable water; criteria to protect
these uses; and an antidegradation statement to protect existing high
quality waters, from degradation by the addition of pollutants.
Water quality criteria (numerical or narrative specifications) for
physical, chemical, temperature, and biological constituents are
stated in the July 1976 U.S. Environmental Protection Agency
publication Quality Criteria for Water (QCW), available from the
Government Printing Office, Washington, D.C. The 1976 QCW, commonly
referred to as the "Red Rook," is the most current compilation of
scientific information used by the Agency as a basis for assessing
water quality. This publication is subject to periodic updating and
revisions in light of new scientific and technical information.
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Mixing zones in State water quality standards, which are the subject
of this digest, enable a State to achieve aquatic protection through
a less stringent stream management approach. The mixing zone is a
designated area or location of a receiving water where wastes waters
and receiving waters mix and the ambient water quality criteria do
not need to be met. Although this mechanism permits a zone of
somewhat less desirable water quality than required by the State in
ambient waters, it does provide a diluting function which aids in the
achievement of the standards. If no such zone is recognized by a
State, then the waters must meet the criteria at the point of
discharge.
The primary purpose in designating mixing zones is to limit areas of
degradation and to not require excessive wastewater treatment.
Furthermore, the in zone quality should be at a level to support the
most sensitive aquatic life form indigenous to the receiving water
body. The 1976 QCW recommends the following consideration to be
included in State water quality standards mixing zone policy:
In essence, the positioning of mixing zones should be accomplished in
a manner that will provide the greatest protection to aquatic life
and for the various uses of water. Generally, shoreline and surface
areas for waste admixture should be discouraged in preference to deep
water, offshore designations. The relative social and ecological
values of the aquatic life that may inhabit a particular waterway
area should be given due consideration in zone definition. The
designation of particular mixing zones is a task that should follow
the biological, physical, and chemical appraisal of the receiving
waterway.
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Since water quality standards experience revisions and upgrading from
time to time, following procedures set forth in the Clean Water Act,
individual entries in this digest may be superseded. As these
revisions are accomplished and allowing for the States to revise
their standards accordingly, this digest will be updated and
reissued. Because this publication is not intended for use other
than as a general information resource, to obtain the latest
information and for special purposes and applications, the reader
needs to refer to the current approved water quality standards.
These can be obtained from the State water pollution control agencies
or the EPA or Regional Offices.
Individual State-adopted criteria follow:
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ALABAMA
The reasonableness of the opportunity for the mixture of wastes
and receiving waters shall be judged on the basis of the physical
characteristics of the stream and approval by the Commission. Mixing
zones shall not preclude passage of free-swimming and drifting
aquatic organisms to the extent that their populations are
significantly affected.
ALASKA
(a) In applying the water quality criteria of sec. 20 of this
chapter to surface waters, the department will, in its
discretion, prescribe in wastewater disposal permits a
volume of dilution for the effluent within the receiving
water. Water quality standards shall be met at every point
outside its boundaries. The department will disallow mixing
zones in instances where the substance discharged is
bioaccumulative in food chains, concentrates in sediments,
is persistent, carcinogenic, mutagenic or teratogenic, or if
the potential ecological or human health effects are so
potentially adverse that a mixing zone is not appropriate.
(b) The department will, in its discretion, establish effluent
limitation requirements in its wastewater disposal permits
in lieu of or in addition to a defined mixing zone.
(c) No individual mixing zone or combination of mixing zones
will be permitted to form a barrier to the migratory routes
of aquatic species.
(d) In determining the size of the mixing zones for any surface
water receiving a wastewater discharge, the department will
consider the following:
(1) the physical, biological and chemical
characteristics of the receiving water;
(2) the effects of the discharge on the present and
anticipated protected water uses and quality of the
receiving water;
(3) the mixing characteristics of the receiving water,
and
(4) the characteristics of the effluent, including flow
rate and composition.
(e) Unless it is demonstrated to the satisfaction of the
department, in accordance with (f) of this section, that the
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size limitations can be increased, mixing zones will be as
small as practicable and will comply with the following size
limitations:
(1) the cumulative linear width of the mixing zone(s)
intersected on any given cross section of a river
or stream will not exceed one third of the total
width of that cross section;
(2) the total horizontal area allocated to all mixing
zones on a lake will not exceed 10 percent of the
lake's surface area,
(3) the cumulative linear length of the mixing zone(s)
intersected on any given cross section of an
estuary, inlet, cove, channel, or other marine water
measured at mean lower low water may not exceed 10
percent of the total length of that cross section,
nor may the total horizontal area allocated to
mixing zones in these waters exceed 10 percent of
the surface area measured at mean lower low water.
(f) A person conducting an operation for which a mixing zone is
sought or required by the department shall submit to the
department the permit application under 18 AAC 15.010(a)(9)
or 130 or 180, all information necessary for assignment of a
mixing zone, including:
(1) type of operation being conducted,
(2) the characteristics of the effluent, including flow
rate and composition;
(3) the characteristics of the receiving water at the
location of the proposed discharge or activity
including but not limited to, where appropriate,
water quality, flow rate, current patterns, depth
and width, and seasonal changes;
(4) a description of the extent to which the operation
may impact the physical, biological and chemical
characteristics of the receiving water, and
(5) a proposed design for outfall and diffuser
structures.
ARIZONA
No reference to mixing zones.
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ARKANSAS
(b) Mixing Zones - The effects of wastes on the receiving stream
shall be determined after the wastes have been thoroughly mixed with
the stream water, but consideration will also be given to the quality
of the waste effluent in determining the adequacy of treatment.
Outfall structures should be designed to minimize the extent of
mixing zones and in the larger streams the zone of mixing shall not
exceed 1/4 of the cross sectional area and/or volume of the stream
flow. The remaining 3/4 of the stream shall be maintained as a zone
of passage for swimming and drifting organisms, and shall remain of
such quality that stream ecosystems are not significantly affected.
In the smaller streams, because of varying local physical and
chemical conditions and biological phenomena, no single-value
recommendation can be made on the percentage of river width necessary
to allow passage of critical free-swimming and drifting organisms so
that negligible or no effects are produced on their populations. As
a guideline no more than 2/3 the width of smaller streams should be
devoted to mixing zones thus leaving at least 1/3 free as a zone of
passage.
In lakes and reservoirs the size of mixing zones shall be
defined by the Department of Pollution Control and Ecology on an
individual basis, and the area shall be kept at a minimum. Mixing
zones shall not prevent free passage of fish or significantly affect
aquatic ecosystems.
CALIFORNIA
Ocean Waters:
Initial dilution is the process which results in the rapid and
irreversible turbulent mixing of wastewater with ocean water around
the point of discharge.
For a submerged buoyant discharge, characteristic of most
municipal and industrial wastes that are released from the submarine
outfalls, the momentum of the discharge and its buoyancy act together
to produce turbulent mixing. Initial dilution in this case is
completed when the diluting wastewater ceases to rise in the water
column and first begins to spread horizontally.
For shallow water submerged discharges, surface discharges, and
nonbuoyant discharges, characteristic of cooling water wastes and
some individual discharges, turbulent mixing results primarily from
the momentum of discharge. Initial dilution, in these cases, is
considered to be completed when the momentum induced velocity of the
discharge ceases to produce significant mixing of the waste, or the
diluting plume reaches a fixed distance from the discharge to be
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specified by the Regional Board, whichever results in the lower
estimate for initial dilution.
For the purpose of this plan, minimum initial dilution is the
lowest average initial dilution within any single month of the year.
Dilution estimates shall be based on observed waste flow
characteristics, observed receiving water density structure and the
assumption that no currents of sufficient strength to influence the
initial dilution process flow across the discharge structure.
The Executive Director shall issue guidelines to be used by the
State and Regional Boards for determining the initial dilution
achieved by each ocean discharge.
COLORADO
(a) The mixing zone is that area of a water body designated on a
case-by-case basis by the Division which is contiguous to a
point source and in which the standards may not apply. The
mixing zone is intended to serve as a zone of initial
dilution in the immediate area of a discharge; however, the
ecological and human health effects of some pollutants may
be so adverse that a mixing zone for such pollutants will
not be allowed.
(b) The size and shape of the mixing zone will be determined by
the Division considering the following factors:
(i) Where necessary to protect aquatic life, there
shall be a zone of passage around the mixing zone
which allows sufficient passage of aquatic life so
as not to have a detrimental effect on their
population.
(ii) Biological communities or populations of imported
species shall not be interfered with to a degree
which is damaging to the ecosystem in adjacent
waters; nor shall there be detrimental effects to
other beneficial uses.
(iii) There shall be no mixing zones for certain harmful
substances such as those identified pursuant to
307(a) of the Federal Act.
(iv) Mixing zones shall not overlap so as to cause
harmful effects in adjacent waters or to interfere
with zones of passage.
(v) Concentrations of harmful substances in the mixing
zone shall not exceed the 96-hour LC50
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concentrations for biota significant to the
aquatic community.
(vi) The conditions of the mixing zone shall be
controlled so as to comply with items l(a), (b)
and (f) of the Basic Standards, Section 3.1.11.
(vii) In establishing a mixing zone, potential
groundwater aquifer contamination shall be
considered.
(viii) The Division will also be guided by other concerns
such as the mixing zone discussion in EPA,
Guidelines for State and Areawide Water Quality
Management Program Development, published November
1976, or similar documents.
CONNECTICUT
Cognizance may be given to reasonable time and distance to allow
mixing of effluent and receiving waters. Such instances shall not
affect the water usage class adopted but shall be defined and
controlled as appropriate by the Commissioner.
(a) Wherever mixing zones are allowed, zones of passage, e.g.,
continuous water routes of the volume, area and quality
necessary to allow passage of free-swimming and drifting
organisms with no significant effects produced on their
populations, shall be provided.
(b) Because of varying local, physical and chemical conditions
and biological phenomena, no single value can be given on
the percentage of river width necessary to allow passage of
critical free-swimming and drifting organisms so that
negligible or no effects are produced on their populations.
(c) As a guideline, mixing zones should be limited to no more
than 1/4 of the cross-sectional area and/or volume of flow,
leaving at least 3/4 free as a zone passage.
(d) Total area and/or volume assigned to mixing zones shall be
limited to that which will: (1) not interfere with
biological communities or populations of important species
to a degree which is damaging to the ecosystems; (2) not
diminish other beneficial uses disproportionately.
DELAWARE
Section 4 - Conditions/Exceptions
Where water quality standards are found to be unattainable,
exceptions may apply:
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1. In the discretion of the Department, water quality standards
may not be required to be achieved in transition zones which
may exist between adjacent zones of water quality.
2. In the discretion of the Department, water quality standards
may not be required to be achieved in mixing zones.
3. Water quality standards for segments may be unattainable
because of naturally occuring phenomena. In such cases,
standards will be evaluated and modified by the Department as
it deems appropriate.
Section 5 - Requirements for Mixing Zones
The following requirements shall apply to mixing zones.
1. Location: Mixing zones shall not be located in biologically
important areas, including but not limited to nursery areas
for aquatic life and water fowl. The stream or river channel
is not to be included in any mixing zone in order to allow
for the passage of anadromous fish.
2. Size: Any mixing zone shall not utilize more than
approximately one-third of the receiving stream's width, and
10% of the area if the receiving water is a lake or estuarine
stream.
3. Shape: "Shore hugging" plumes shall be designed to provide
maximum protection to humans, aquatic life and wildlife.
4. Outfall Design: Outfalls shall be designed to provide maximum
protection to humans, aquatic life and wildlife.
5. In Zone Quality: Waters in the mixing zone shall be free of
the following:
(a) materials in concentrations that exceed 96-hour LC50
for biota significant to the indigenous (and
anadromous) aquatic and marine community,
(b) Materials in concentrations that settle to form
objectionable deposits, or smother or otherwise harm
bottom dwelling aquatic and marine life or their
habitats downstream from the mixing zone proper;
(c) Floating debris, oil, scum, and other matter in
concentrations that constitute a nuisance;
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(d) Substances in concentrations that produce objectionable
color, ordor, taste or turbidity and,
(e) Substances in concentrations which produce undesirable
aquatic or marine life or more nuisance species.
6. Heat dissipation areas shall not be longer than 3,500 feet,
or twenty times the average width of the stream, whichever is
less, measured from where the waste discharge enters the
stream. In tidal streams, except for the Delaware River and
Bay, the most restrictive length as determined herein shall
be applied both upstream and downstream as measured from the
point of discharge.
7. Within any one heat dissipation area, only one shore shall be
used in determining the limits of the area. The
determination in estuarine waters shall take into special
consideration the extent and nature of such water so as to
meet the extent and purpose of the criteria and standards in
order to provide for the passage of free-swimming and
drifting organisms so that negligible or no effects are
produced on their populations. Except for the Delaware River
and Bay, at least 66% of the stream width, as measured at the
extreme width of the heat dissipation area, shall remain free
as a zone of passage for aquatic biota.
DISTRICT OF COLUMBIA
1. The positioning of mixing zones should be done in a manner
that provides the greatest protection to aquatic life and for
the various uses of water,
2. Within an estuary, the minimal dimension of the mixing area
should not exceed 10% of the cross-sectional area of the
waterway.
In addition to the inclusion of these mixing zone guidelines,
the District of Columbia Water Quality Standards should specify that
mixing zones are to be established for major discharges of pollutants
which immediately threaten the nearby aquatic community or other
expected water uses.
It is recommended that the District of Columbia adopt the
guidelines for determining mixing zones, contained in the EPA
Continuing Planning Process Guidelines, Section 5.5. These
guidelines are as follows:
1. Permissible size of the zone is dependent on acceptable
amount of damage and size of receiving water body;
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2. Zone should be free from effluent substances that will settle
to form objectionable color, odor or turbidity,
3. Zone should protect aquatic life in shallow areas which serve
as nursery areas;
4. Number of mixing zones within a stream reach or impoundment
should be limited to maintain a mixing zone to water body
ratio;
5. Mixing zones should not form barriers to migratory aquatic
life;
6. As a guideline, the quality for life within a mixing zone
should be such that the 96-hour LC50 for biota significant to
the area's aquatic life community is not exceeded.
FLORIDA
Mixing Zones: Surface Waters
(1) Zones of mixing for non-thermal components of discharges.
(a) The Department may allow the water quality adjacent to
a point of discharge to be degraded to the extent that
only the minimum conditions described in Section
17-3.051(1) apply within a limited, defined region
known as the mixing zone. Under the circumstances
defined elsewhere in this section, a mixing zone may be
allowed so as to provide an opportunity for mixing and
thus to reduce the costs of treatment. However, no
mixing zone or combination of mixing zones shall be
allowed to significantly impair any of the designated
uses of the receiving body of water.
(b) A zone of mixing shall be determined based on
consideration of the following:
1. The condition of the receiving body of water
including present and future flow conditions
and present and future sources of pollutants;
2. The nature, volume and frequency of the
proposed discharge of water including any
possible synergistic effects with other
pollutants or nuisances which may be present in
the receiving body of water,
3. The cumulative effect of the proposed mixing
zone and other mixing zones in the vicinity.
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(c) Except for the thermal component of discharges and
nitrogen and phosphorus acting as nutrients, to which
this paragraph is inapplicable, mixing zones which do
not adhere to all of provisions (l)(d) through (l)(i)
below shall be presumed to constitute a significant
impairment of the designated uses of surface water: of
Classes I, II, and III. However, an applicant for a
specified mixing zone who affirmatively demonstrates in
a public hearing, pursuant to Chapter 120, Florida
Statutes, and after public notice in the Florida
Administrative Weekly and in a newspaper of general
circulation in the area where the mixing zone is
proposed, that a proposed mixing zone which does not
comply with one or more of the provisions of paragraphs
l(d) through l(i) will not produce a significant
adverse effect on the established community of
organisms in the receiving body of water or otherwise
significantly impair any of the designated uses of the
receiving body of water, shall be exempt from these
requirements. The Secretary shall authorize that
mixing zone for which the applicant makes an
affirmative demonstration by the preponderance of
competent substantial evidence that the applicable
requirements of this section have been met.
(d) A mixing zone shall not include an existing drinking
water supply intake nor include any other existing
water supply intake if such mixing zone would
significantly impair the purposes for which the supply
is utilized.
(e) A mixing zone shall not include a nursery area,
indigenous aquatic life nor include any area approved
by the Department of Natural Resources for shellfish
harvesting.
(f) In canals, rivers, streams, and other similar water
bodies, the length of a zone of mixing shall be 800
meters unless a shorter length is necessary to prevent
significant impairment of a designated use. In no case
shall a mixing zone be longer than that necessary to
meet water quality standards.
(g) In lakes, estuaries, bays, lagoons, bayous and sounds,
the area of a mixing zone shall be 125,600 square
meters unless a lesser area is necessary to prevent
significant impairment of a designated use. In no case
shall a mixing zone be larger than that necessary to
meet water quality standards.
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(h) The Mixing zones in a given water body shall not
cumulatively exceed the limits described below:
(1) In rivers, canals, and other similar water
bodies: 10% of the total length,
(2) In lakes, estuaries, bays, lagoons, bayous,
and sounds: 10% of the total area.
(i) Additional standards which apply within mixing zones in
Class I-A, II, and III waters are as follows:
(1) The dissolved oxygen within a mixing zone
shall not average less than 4.0 milligrams per
litre (mg/1) in the mixing zone volumes:
(2) The turbidity within the mixing zone shall not
average greater than 75 Jackson Units in the
mixing zone volume above natural background as
related to a standards candle turbidimeter.
(j) Mixing zones in Class IV and V-A waters are subject
only to provisions of (d) above and of Section 17-3.051
and shall not significantly impair the designated uses
of the receiving body of water.
2. Until such time as a permit is issued, modified, or removed,
discharges in existence prior to the effective date of this
rule shall continue to meet such mixing zone restrictions
(for each component or characteristic of a discharge):
(a) As respecified by permits; or
(b) Which were applied to the discharge in the Department's
permitting process prior to the effective date of this
rule.
3. Except for discharges covered by (2) above, after the
adoption of this rule there shall be no zone of mixing for
any component of any discharge unless a Department permit
containing a description of its boundaries has been issued
for that component of the discharge.
4. Waters within mixing zones shall not be degraded below the
applicable minimum standards prescribed for all waters at all
times in Section 17-3.051. In determining compliances with
the provisions of 17-3.051(1), the average concentration of
the waters in the mixing zone shall be measured or computed
using scientific techniques approved by the Department;
provided that, in no event shall the maximum concentration of
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wastes in the mixing zone exceed the amount lethal to 50% of
the test organisms in 36 hours (36 hr. LC50) for a species
significant to the indigenous aquatic community. The
dissolved oxygen value within a mixing zone shall not be less
than 1.5 milligrams (mg/1) at any time or place.
5. Except for the minimum conditions of waters as specified in
Section 17-3.051 and the provisions of Section 17-4.244, no
other water quality criteria apply within a mixing zone.
6. Mixing zones for dredge and fill permits shall not be subject
to provisions (1) (c) through (l)(j), (2), (3), (4), or (5)
of this section, provided that applicable water quality
standards are met at the boundary and outside the mixing
zone.
(a) The dimensions of dredge and fill mixing zones shall
be proposed by the applicant and approved, modified or
denied by the Department.
(b) Criteria for departmental evaluation of a proposed
mixing zone shall include site-specific biological and
hydrographic considerations.
(c) In no case, however, shall the boundary of a dredge and
fill mixing zone be more than 150 meters downstream in
flowing streams or 150 meters in radius in other bodies
of water, where these distances are measured from the
cutterhead, return flow discharge, or other points of
generation of turbidity or other pollutants.
7. Additional relief from mixing zone restrictions necessary to
prevent significant impairment of a designated use is
through:
(a) Reelassification of the water body pursuant to Section
17-3.081, Florida Administrative Codei
(b) Variance granted for any one of the following reasons:
1. There is no practicable means known or
available for the adequate control of the
pollution involved.
2. Compliance with the particular requirement or
requirements from which a variance is sought
will necessitate the taking of measures, which,
because of their extent or cost must be spread
over a considerable period of time. A variance
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granted for this reason shall prescribe a
timetable for the taking of measures required.
3. To relieve or prevent hardship of a kind other
than these provided for in paragraphs 1 or 2.
Variances and renewals thereof granted upon
authority of this sub-paragraph shall each be
limited to a period of 24 months except that
variances granted pursuant to the Florida
Electrical Power Plant Siting Act may extend
for the life of the permit or certification.
(c) Modification of the requirements of this section for
specific criteria by the Secretary upon compliance with
the notice and hearing requirements for mixing zones
set forth in (l)(c) above and upon affirmative
demonstration by an applicant by the preponderance of
competent substantial evidence that:
1. The applicant's discharge from a source
existing on the effective date of this rule
complies with best technology economically
achievable, best management practices, or other
requirements set forth in Chapter 17-6, FAC and
there is no reasonable relationship between the
economic, social, and environmental costs and
the economic, social and environmental benefits
to be obtained by imposing more stringent
discharge limitations necessary to comply with
the mixing zone requirements of subsection
17-4.244(1) and the provisions relating to
dissolved oxygen in subsection 17-4.244(4).
2. No discharger may be issued more than one
permit or permit modification or renewal which
allows a modification pursuant to this
subsection unless the applicant affirmatively
demonstrates that it has undertaken a
continuing program, approved by the Department,
designed to consider water quality conditions
and review or develop any reasonable means of
achieving compliance with the water quality
criteria from which relief has been granted
pursuant to this subsection.
3. With respect to paragraph 17-4.244(l)(c) and
17-4.244(7)(c) the applicant must affirmatively
demonstrate the minimum area of the water body
necessary to achieve compliance with either
subsection. Within a minimum area determined
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by the Secretary to be necessary to achieve
compliance, the discharger shall be exempt from
the criterion for which a demonstration has
been made.
GEORGIA
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.
HAHAII
Zones of mixing for the assimilation of municipal, agricultural, and
industrial discharges which have received the best degree of
treatment or control are recognized as necessary.
It is the objective of this limited zone to provide for a
current realistic means of control over such discharges and at the
same time, achieve the highest attainable level of water quality.
Establishment, Renewal, and Termination:
Every application for a zone of mixing shall be made on forms
furnished by the director of Health and shall be accompanied by a
complete and detailed description of present conditions, how present
conditions do not conform to standards, and such other information as
the Director of Health may prescribe.
Each application for a zone of mixing shall be reviewed in light
of the descriptions, statements, plans, histories, and other
supporting information as may be submitted upon the request of the
Director of Health, and in light of the effect or probable effect
upon the water quality standards established pursuant to this
Chapter.
Whenever an application is approved, the Director of Health
shall establish the zone of mixing taking into account protected uses
of the body of water, existing natural conditions of the receiving
water, character of the effluent, and the adequacy of the design of
the outfall and diffuser system to achieve maximum dispersion and
assimilation of the treated or controlled waste with a minimum of
undesirable or noticeable effect on the receiving water.
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Approval of a zone of mixing shall be made only after a public
hearing is held by the Director of Health in the county where the
source is situated in accordance with the Hawaii Administrative
Procedure Act and the Rules of Practice and Procedure of the
Department of Health.
No zone of mixing shall be granted by the Director of Health
unless the application and the supporting information clearly show
that:
(1) The continuation of the function or operation involved in
the discharge by the granting of the zone of mixing is in
the public interest, and
(2) The discharge occurring or proposed to occur does not
substantially endanger human health or safety; and
(3) Compliance with the existing water quality standards from
which a zone of mixing is sought would produce serious
hardships without equal or greater benefits to the public;
and
(4) The discharge occurring or proposed to occur will not
unreasonably interfere with any actual or probable use of
the water areas for which it is classified, and has
received, or, in the case of a proposed discharge, will
recei ve the best degree of treatment or control.
Any zone of mixing or renewal thereof shall be granted within
the requirements of this section and for time periods under
conditions consistent with the reasons therefore and within the
following limitations:
(1) If the zone of mixing is granted on the ground that there is
no practicable means known or available for the adequate
prevention, control, or abatement of the discharge involved,
it shall be only until the necessary means for prevention,
control, or abatement become practicable and subject to the
taking of any substitute or alternate measures that the
Director of Health may prescibe. No renewal of a zone of
mixing granted under this subsection shall be allowed
without a thorough review of known and available means of
preventing, controlling, or abating the discharge involved.
(2) The Director of Health may issue a zone of mixing for a
period not exceeding five years.
(3) Every zone of mixing granted under this section shall
include, but not be limited to, conditions requiring the
grantee to perform effluent and receiving water sampling and
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report the results of each sampling to the Director of
Health, and a program of research to develop practicable
alternatives to the methods of treatment or control in use
by the grantee may be required if such research is deemed
prudent by the Director of Health.
Any zone of mixing granted pursuant to this section may be
renewed from time to time on terms and conditions and for periods not
exceeding five years which would be appropriate on initial granting
of a zone of mixing, provided that the applicant for renewal had met
all of the conditions specified in the immediately preceding zone of
mixing, and provided further, that the renewal, and the zone of
mixing established in pursuance thereof, shall provide for discharge
not greater in quantity of mass emissions than that attained pursuant
to the terms of the immediately preceding zone of mixing at its
expiration. No renewal shall be granted except on application.
Therefore, any such application shall be made at least sixty days
prior to the expiration of the zone of mixing.
No zone of mixing granted pursuant to this part shall be
construed to prevent or limit the application of any emergency
provisions and procedures provided by law.
The establishment of any zone of mixing shall be subject to the
concurrence of the U.S. Environmental Protection Agency.
The Director of Health, on his own motion, or upon the
application of any person, shall terminate a zone of mixing, if after
a hearing, he determines that the water area does not meet the basic
criteria applicable to all water areas, or that the zone of mixing
granted will unreasonably interfere with any actual or probable use
of the water area that the discharge does not receive, or, in the
case of a new discharge, will not receive, the best degree of
treatment or control. Such termination shall be made only after a
hearing held by the Director of Health on the island where the area
is situated in accordance with the Hawaii Administrative 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
otherwise classified.
Upon expiration of the period stated in the designation, the
zone of mixing shall automatically terminate and no rights shall
become vested in the designee.
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IDAHO
After a biological, chemical, and physical appraisal of the
receiving water and the proposed discharge and after consultation
with the person(s) responsible for the wastewater discharge, the
Department will determine the applicability of a mixing zone and, if
applicable, its size, configuration, and location. In defining a
mixing zone, the Department will consider the following principles:
(a) The mixing zone may receive wastewater through a submerged
pipe, conduit or diffusor.
(b) The mixing zone is to be located so it does not cause
unreasonable interference with or danger to existing
beneficial uses.
(c) When two (2) or more individual mixing zones are needed for
a single activity, the sum of the areas and volumes of the
several mixing zones is not to exceed the area and volume
which would be allowed for a single zone.
(d) Multiple mixing zones can be established for a single
discharge, each being specific for one (1) or more
pollutants contained within the discharged wastewater.
(e) Mixing zones in flowing receiving waters are to be limited
to the following:
(1) The cumulative width of adjacent mixing zones when
measured across the receiving water is not to exceed
fifty percent (50%) of the total width of water at
that pointi
(2) The width of a mixing zone is not to exceed
twenty-five percent (25%) of the stream width or
three hundred (300) meters plus the horizontal
length of the diffuser as measured perpendiculary to
the stream flow, whichever is less,
(3) The mixing zone is to be no closer to the ten (10)
year, seven (7) day low-flow shoreline than fifteen
percent (15%) of the stream width,
(4) The mixing zone is not to include more than
twenty-five percent (25%) of the volume of the
stream flow.
(f) Mixing zones in reservoirs and lakes are to be limited to
the following:
(1) The total horizontal area allocated to mixing zones
is not to exceed ten percent (10%) of the surface
area of the lake,
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(2) Adjacent mixing zones are to be no closer than the
greatest horizontal dimension of any of the
individual zones.
(g) The water quality within a mixing zone is subject to General
Water Quality Standards contained in Manual Sections
1-2200.3, "Radioactive Materials", 1-2200.04, "Floating and
Submerged Matter", and 1-2200.05 "Excess Nutrients", and can
be exempt from the standards contained in Manual Sections
1-2200.01 "Hazardous Materials" and 1-2200.02 "deleterious
Material", as well as from Manual Section 1-2250, as
determined appropriate, provided that the receiving water's
existing quality is not in violation of that standard or
provision.
(h) Concentrations of hazardous materials within the mixing zone
must not exceed the ninty-six (96) hour LC50 for biota
significant to the receiving water's aquatic community.
ILLINOIS
201 Mixing Zones
(a) 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 standard 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 proportion of any body of
water or segment thereof within mixing zones must be quite
small if the water quality standards 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 determining the size of the mixing zone for any
discharge, the following must be considered:
1. The character of the body of water,
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2. The present and anticipated future use of the body of
water,
3. The present and anticipated water quality of the body
of water,
4. The effect of the discharge on the present and
anticipated future water quality,
5. The dilution ratio; and
6. The nature of the contaminant.
(b) In addition to the above, 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, nor shall any mixing zone contain more than 25%
of the cross-sectional area or volume of flow of a stream
except for those streams where the dilution ratio is less
than 3:1.
Temperature standards contain additional requirements for
heated discharges.
INDIANA
Sec. 4
(a) All water quality standards in this Regulation, except
those provided in subsection 6 (a) below, are to be applied at a
point outside of the mixing zone to allow for a reasonable admixture
of water effluents with the receiving water.
(b) Due to varying physical, chemical, and biological
conditions, no universal mixing zone may be prescribed. The Board
shall determine the mixing zone upon application by the discharger.
The applicability of the guideline set forth in Section 4(c) will be
on a case-by-case basis and any application to the Board should
contain the following information:
(1) The dilution ratio,
(2) The physical, chemical, and biological characteristics of the
receiving body of water,
(3) The physical, chemical, and biological characteristics of the
waste effluent,
(4) The present and anticipated uses of the receiving body of
water;
(5) The measured or anticipated effect of the discharge on the
quality of the receiving body of water,
(6) The existence of an impact upon any spawning or nursery areas of
any indigenous aquatic species,
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(7) Any obstruction of migratory routes of any indigenous aquatic
species, and
(8) The synergistic effects of overlapping mixing zones or the
aggregate effects of adjacent mixing zones.
(c) Where possible, the general guideline is to be that the
mixing zone should be limited to no more than 1/4 (25 percent) of the
cross-sectional area and/or volume of flow of the stream, leaving at
least 3/4 (75 percent) free as a zone of passage for aquatic biota,
nor should it extend over 1/2 (50 percent) of the width of the
stream.
Section 6. Water Quality Standards
(Minimum Water Quality Conditions) All waters at all times and
at all places, including the mixing zone, shall meet the minimum
conditions of being free from substances, materials, floating debris,
oil or scum attributable to municipal, industrial, agricultural, and
other land use practices or other discharges:
(1) That will settle to form putrescent or otherwise
objectionable deposits,
(2) That are in amount sufficient to be unsightly or
deleterious,
(3) That produce color, odor or other conditions in such degree
as to create a nuisance,
(4) Which are in amount sufficient to injure, be toxic to or
produce adverse physiological responses in humans, animals, aquatic
life or plants. As a guideline, toxic substances should be limited
to the 96-hour median lethal concentration (LC50) for biota
significant to the idigenous aquatic community. This subsection
shall not apply to the chemical control of aquatic plants or animals
when that control is subject to approval by the Indiana Department of
Natural Resource as provided by the Fish and Wildlife Act (1C 1971,
14-2-1); and
(5) Which are in concentrations or combinations that will cause
or contribute to the growth of aquatic plants or algae in such a
degree as to create a nuisance, be unsightly or deleterious or be
harmful to human, animal, plant, or aquatic life or otherwise impair
the designated uses.
Lake Michigan and Contiguous Harbor Areas (temperature standards
contain additional requirements for thermal plumes)
Section 5. (Mixing Zones) The mixing zone shall be considered a
place where waste and receiving waters mix and not as a place where
effluents are treated. All mixing zones will be determined on a
case-by-case basis by the Indiana Stream Pollution Control Board
after consideration of the following:
(a) The dilution ratio,
(b) The physical, chemical, and biological characteristics of
the receiving body of water,
(c) The physical, chemical, and biological characteristics of
the waste effluent,
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(d) The present and anticipated uses of the receiving body of
water,
(e) The existence of and impact upon any spawning nursery areas
of any indigenous aquatic species, and
(f) The synergistic effects of overlapping mixing zones or the
aggregate effects of adjacent mixing zones.
Section 4. (Water Quality Standards)
(Minimum Water Quality Conditions) All waters of Lake Michigan
and the contiguous harbor areas at all times and at all places,
including the mixing zone, shall meet the minimum conditions of being
free from substances, materials, floating debris, oil or scum
attributable to municipal, industrial, agricultural, and other land
use practices, or other discharges:
(1) That will settle to form putrescent or otherwise objectionable
deposits,
(2) That are in amount sufficient to be unsightly or deleterious,
(3) That produce color, odor, or other conditions in such degree as
to crease a nuisance,
(4) Which are in amounts sufficient to injure, be toxic to or
produce adverse physiological responses in humans, animals,
aquatic life, or plants. As a guideline, toxic substances
should be limited to the 96-hour median lethal concentration
(LC50) for biota significant to the indigenous aquatic
community. This subsection shall not apply to the chemical
control of aquatic plants or animals when that control is
subject to the approval of the Indiana Department of Natural
Resources as provided by the Fish and Wildlife Act (1C
14-2-1),and
(5) Which are in concentrations or combinations that will cause or
contribute to the growth of aquatic plants or algae in such a
degree to create a nusiance, be unsightly or deleterious or
be harmful to humans, animals, plant or aquatic life or
otherwise impair the designated uses.
Grand Calumet River and Indiana Harbor Ship Canal
Section 4. (Mixing Zones)
(a) All water quality standards in this Regulation, except those
provided in subsection 5(a) below, are to be applied at a point
outside of the mixing zone to allow for a reasonable admixture
of waste effluents with the receiving waters.
(b) Due to varying physical, chemical, and biological conditions, no
universal mixing zone may be prescribed. The Board shall
determine the mixing zone upon application by the discharger.
The applicability of the guideline set forth in Section 4(c)
will be on a case-by-case basis and any application to the Board
should contain the following information:
(1) The dilution ratio,
(2) The physical, chemical, and biological characteristics of
the receiving body of water;
(3) The physical, chemical, and biological characteristics of
the waste effluent;
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(4) The present and anticipated uses or the receiving body of
water,
(5) The measured or anticipated effect of the discharge on the
quality of the receiving body of water,
(6) The existence of and impact upon any spawning or nursery
areas of any indigenous aquatic species,
(7) Any obstruction of migratory routes of any indigenous
aquatic species; and
(8) The synergistic effects of overlapping mixing zones of the
aggregate effects of adjacent mixing zones.
(c) Where possible, the general guideline is to be that the mixing
zone should be limited to no more than 1/4 (25 percent) of the
cross-sectional area and/or volume of flow of the stream,
leaving at least 3/4 (75 percent) free as a zone of passage for
aquatic biota, nor shall it extend over 1/2 (50 percent) of the
width of the stream.
Section 5. (Water Quality Standards)
(Minimum Water Quality Conditions) All waters at all times and
at all places, including the mixing zone, shall meet the minimum
conditions of being free from substances, materials, floating debris,
oil or scum attributable to municipal, industrial, agricultural, and
other land use practices or other discharges:
(1) That will settle to form putrescent or otherwise
objectionable deposits;
(2) That are in amounts sufficient to be unsightly or
deleterious,
(3) That produce color, odor, or other conditions in such
degree as to create a nuisance;
(4) Which are in amount that will be toxic or harmful to human,
animal, plant or aquatic life, and
(5) Which are in concentrations or combinations that will cause
or contribute to the growth of aquatic plants or algae in
such a degree as to create a nuisance, be unsightly or
deleterious or be harmful to human, animal, plant, or
aquatic life or otherwise impair the designated uses.
Wolf Lake
Section 5. (Mixing Zone) The mixing zone shall be considered a
place where waste and receiving waters mix and not as a place where
effluents are treated. All mixing zones will be determined on a
case-by-case basis by the Indiana Stream Pollution Control Board
after consideration of the following:
(a) The dilution ratio,
(b) The physical, chemical, and biological characteristics of
the receiving body of water;
(c) The physical, chemical, and biological characteristics of
the waste effluent,
(d) The present and anticipated uses of the receiving body of
water;
(e) The existence of and impact upon any spawning or nursery
areas of any indigenous aquatic species, and
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(f) The synergistic effects of overlapping mixing zones or the
aggregate effects of adjacent mixing zones.
Section 4. Water Quality Standards
(Minimum Water Quality Conditions) All waters at all time and
at all places, including the mixing zone, shall meet the minimum
conditions of being free from substances, materials, floating debris,
oil or scum attributable to municipal, industrial, agricultural, and
other land use practices or other discharges:
(1) That will settle to form putrescent or otherwise
objectionable deposits;
(2) That are in amounts sufficient to be unsightly or
deleterious;
(3) That produce color, odor, or other conditions in such
degree as to create a nuisance,
(4) Which are in amounts sufficient to injure, be toxic to or
produce adverse physiological responses in humans, animals,
aquatic life or plants. As a guideline, toxic substances
should be limited to the 96-hour median lethal
concentration (LC50) for biota significant to the
indigenous aquatic community. This subsection shall not
apply to the chemical control of aquatic plants or animals
when that control is subject to approval by the Indiana
Department of Natural Resource as provided by the Fish and
Wildlife Act (1C 14-2-1), and
(5) Which are in concentrations or combinations that will cause
or contribute to the growth of aquatic plants or algae in
such a degree as to create a nuisance, be unsightly or
deleterious or be harmful to human, animal, plant, or
aquatic life or otherwise impair the designated uses.
(6) Which are in concentrations or combinations that will cause
or contribute to the growth of aquatic plants or algae in
such a degree as to create a nuisance, be unsightly or
deleterious or be harmful to human, animal, plant, or
aquatic life or otherwise impair the designated uses.
IOWA
Mixing zone in the receiving water. The area of diffusion of an
effluent in the receiving water is a mixing zone and the water
quality standards shall be applied beyond the mixing zone.
The mixing zone shall be a specified linear distance, volume, or
area which is determined on a case-by-case basis using the following
criteria:
a. The mixing zone shall be as small as practicable and shall
not be of such size or shape as to cause or contribute to the
impairment of water uses.
b. The mixing zone shall contain not more than 25 percent of
the cross sectional area or volume of flow in the receiving body of
water.
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c. The mixing zone shall be designed to allow an adequate
passageway at all times for the movement or drift of aquatic life.
d. Where there are two or more mixing zones in close proximity,
they shall be so defined that a continuous passageway for aquatic
life is available.
e. The mixing zone shall not intersect any area of any waters
in such a manner that the maintenance of aquatic life in the body of
water as a whole would be adversely affected.
In determining the size and location of the mixing zone for any
discharge on a case-by-case basis, the following shall be
considered:
f. The size of the receiving water, the volume of discharge,
the stream bank configuration, the mixing velocities, and other
hydrologic or physiographic characteristics;
g. The present and anticipated future use of the body of
water;
h. The present and anticipated future water quality of the body
of water,
i. The ratio of the volume of waste being discharged to the
seven-day, ten-year flow of the receiving stream; and
j. The mixing zone shall be free from unsightly floating
materials and wastewater consitutuents in concentrations which are
toxic or harmful to human, animal or plant life, which will settle to
form sludge deposits, or which will produce aesthetically
objectionable color or odor.
KANSAS
The water quality criteria listed herein shall apply beyond the
mixing zone for each individual discharge, except that concentrations
within the mixing zone area shall be maintained below acute toxicity
levels for any parameter or combination of parameters. The total
area and/or volume of a receiving stream assigned to mixing zones
shall be limited to that which will: 1) not interfere with
biological communities or populations of important species to a
degree which is damaging to the ecosystem, and 2) not diminish other
beneficial uses disproportionately.
(Zones of Passage)
Zones of passage shall be provided in streams, reservoirs, or
lakes wherever mixing zones are allowed, and such zones shall be
continuous water routes of the volume, area, and quality necessary to
allow passage of free swimming and drifting organisms with no
significant effects on their populations. Because of varying local
physical and chemical conditions and biological phenomena, no single
value can be given on the percentage of the receiving water area
and/or volume necessary to allow a sufficient zone of passage.
Ordinarily, mixing zones shall be limited to no more than 1/4 of the
cross-sectional area and/or volume of flow of a stream or reservoir,
leaving at least 3/4 free as a zone of passage.
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KENTUCKY
Mixing zone means a domain of a water body contiguous to a
treated or untreated wastewater discharge of quality characteristics
different from those of the receiving water. The discharge is in
transit and progressively diluted from the source to the receiving
system. The mixing zone is the domain where wastewater and receiving
water mix.
Section 1. Mixing Zones. The following guidelines are applicable in
determining all mixing zones:
(1) The department shall, on a case-by-case basis, specify
definable, geometric limits for mixing zones. Applicable limits
shall include but may not be limited to the linear distances from the
point of discharge, surface area involvement, volume of receiving
water, and taking into account other nearby mixing zones.
(2) The mixing zone shall be free from pollutants which are in
excess of 0.44 times the 96 hour LCsg for a representative
indigenous aquatic organism.
(3) The location of a mixing zone shall not interfere with
spawning areas, nursery areas, fish migration routes, public water
supply intakes, bathing areas, nor preclude the free passage of fish
or other aquatic life.
(4) Whenever possible the mixing zone shall not exceed
one-third (1/3) of the width or cross-sectional area of the receiving
stream, and in no case shall exceed one-half (1/2) of^ this volume.
(5) In lakes and other surface impoundments, the volume of a
mixing zone shall not affect in excess of ten (10) percent of the
volume of that portin of the receiving waters available.
(6) In all cases, a mixing zone must be limited to an area or
volume which will not adversely alter the legitimate uses of the
receiving water, nor shall a mixing zone be so large as to adversely
affect an established community of aquatic organisms.
LOUISIANA
Mixing Zones
The total area and/or volume of a stream assigned to mixing
zones will be limited to that which will: (1) not interfere with
biological communities or populations of important species to a
degree which is damaging to the ecosystem, (2) not diminish other
beneficial uses disproportionately.
Zones of Passage
In rivers, streams, reservoirs, lakes, estuaries and coastal
waters, zones of passage are continuous water routes of the volume,
area and quality necessary to allow passage of free-swimming and
drifting organisms with no significant effects produced on their
populations. These zones must be provided wherever mixing zones are
allowed.
Because of varying local physical and chemical conditions and
biological phenomena, no single value can be given on the percentage
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of river (or stream) width necessary to allow passage of critical
free-swimming and drifting organisms so that negligible or no effects
are produced on their populations. As a guideline and except when
otherwise specified by the Louisiana Strem Control Commission in a
valid waste discharge permit the mixing zone will be limited to no
more than 1/4 of the cross-sectional area and/or volume of flow of
stream or estuary, leaving at least 3/4 free as a zone of passage.
MAINE
Zone of Passage:
All discharges of pollutants shall, at a minimum, provide for a
zone of passage for free-swimming and drifting organisms. Such zone
of passage shall not be less than 3/4 of the cross-sectional area at
any point in the receiving body of water. Such zone of passage may
be reduced whenever the applicant for a discharge can demonstrate
that (a) because of physical phenomena in the receiving body of water
such minimum zone cannot be maintained and (b) such minimum zone of
passage is not necessary to protet organisms in the receiving body.
MARYLAND
A. Mixing zones are areas which are allowed for mixing of
effluent waters with the receiving water. They have defined
and identifiable limits, and the waters outsde of the zones
must meet the standards for that particular body of water.
B. The Administrtion will establish the limits of a mixing
zone, in terms of the Total area and/or volume of the
receiving waters, so that: (a) there is no interference
with biological communities or populations of important
species to a degree which is damaging to the aquatic life or
ecosystem; and (b) there is no diminishing of other
legitimate beneficial uses.
MASSACHUSETTS
Regulation 2.2 Mixing Zones.
In aplying these standards, the Division may recognize, where
appropriate a limited mixing zone or zone of initial dilution on a
case-by-case basis. The location, size and shape of these zones
shall provide for the maximum protection of aquatic resources. At a
minimum, mixing zones must:
a) Meet the criteria for aesthetics,
b) Be limited to an area or volume that will minimize
interference with the designated uses or established
community of aquatic life in the segment;
c) Allow an appropriate zone of passage for migrating fish and
other organisms, and
d) Not result in substances accumulating in sediments, aquatic
life or food chains to exceed known or predicted safe
exposure levels for the health of humans or aquatic life.
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MICHIGAN
R 323.1082. Mixing Zones.
Rule 1082. (1) A mixing zone to achieve a mixture of a point
source discharge with the receiving waters shall be considered a
region in which organism reponse to water quality characeristies is
time-dependent. Exposure in mixing zones shall not cause an
irreversible response which results in deleterious effects to
populations of important aquatic life and wildlife. As a minimum
restriction the toxic substance 96 hour TLm for important species
of fish or fishfood organisms shall not exceed in the mixing zone at
any point inhabitable by these organisms, unless it can be
demonstrated to the commission that a higher concentration is
acceptable. The mixing zone at any transect of a stream shall
contain not more than 25% of the cross-section area or volume of flow
of the stream or both unless it can be demonstrated to the commission
that designation of a greater area or volume of streamflow will allow
passage of fish and fishfood organisms so that effects on their
immediate and future populations are negligible or not measurable.
Watercourses or portions thereof which, without one of more point
source discharges, would have no flow except during periods of
surface runoff may be considered as a mixing zone for a point source
discharge. For Lake Michigan, mixing zones shall not exceed a
defined area equivalent to that of a circle of radius of 1,000 feet
unless the discharger can demonstrate to the commission that the
defined area for a thermal discharge is more stringent than necessary
to assure the protection and propagation of a balanced indigenous
population of aquatic life and wildlife in the receiving water.
MINNESOTA
Means for expediting mixing and dispersion of sewage, industrial
waste, or other waste effluents in the receiving interstate waters
are to be provided so far as practicable when deemed necessry by the
Agency to maintain the quality of the receiving interstate waters in
accordance with applicable standards. Mixing zones can be
established by the Agency on an individual basis, with primary
consideration being given to the following guidelines: (a) mixing
zones in rivers shall permit an acceptable passageway for the
movement of fish, (b) the total mixing zone or zones at any transect
of the stream should contain no more than 25% of the cross-sectional
area and/or volume of flow of the stream, and should not extend over
more than 50% of the width, (c) mixing zone characteristics shall not
be lethal to aquatic organisms, (d) for contaminants other than heat,
the 96 hour median tolerance limit for indigenous fish and fish food
organisms should not be exceeded at any point in the mixing zone, (e)
mixing zones should be as small as possible, and not intersect
spawning or nursery area, migratory routes, water intakes, nor mouths
of rivers, and (f) overlapping of mixing zones should be minimized
and measures taken to prevent diverse synergistic effects.
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MISSISSIPPI
It is recognized that limited areas of mixing are unavoidable,
however, mixing zones will not be used for, or considered as a
substitute for waste treatment. Mixing zone limits will be defined
by the Commission on a case-by-case basis. In all cases mixing zones
will be restricted to as small an area as feasible. Adequate zones
of passage for migration and free movement of fish and other aquatic
biota will be maintained. Where there are several mixing zones close
together, they will all be on the same side so that the passageway is
continuous.
Also, mixing zones will not interfere with the established
community of aquatic organisms nor adversely affect recreational uses
of an area. Any mixing zone shall be free from materials in
concentrations that exceed the 96-hour LC50 for biota significant to
the indigenous aquatic community and shall comply with the general
provisions of Section I and II of these standards.
MISSOURI
1. The mixing zones shall be exempted from the specific
criteria for those substances that are rendered non-toxic by
dilution, dissipation, or rapid transformation. Bioaccumulative and
persistent toxic substances are not exempted in mixing zones. The
mixing zone shall not overlap another mixing zone in such a manner
that the maintenance of aquatic life in the body of water as a whole
would be adversely affected.
2. As a guideline for design of outfalls, no more than 25% of
the cross-sectional area or volume of a stream shall be allowed as a
mixing zone. However, plumes that do not allow for rapid mixing are
not acceptable.
3. In determining the size and location of the mixing zone for
any discharge, the following characteristics must be considered:
A. The size of the river, the volume of discharge, and
stream bank configuration, the mixing velocities, and
other hydrologic or physiographic characteristics;
B. The present and anticipated future uses of the water,
including type of aquatic life supported, and
C. The dilution ratio, that is, the ratio of the 7-day,
once-in-10-year low flow of the receiving stream to
the average dry weather flow of the discharge.
4. Zones of passage must be provided wherever mixing zones are
allowed, where and when necessary to protect aquatic life. Because
of varying local physical and chemical conditions and biological
phenomena, no single value can be given on the percentage of river
width necessary to allow passage of free-swimming and driting
organisms, so that negligible effects are produced on their
populations. As a guideline, at least three-quarters of the
cross-sectional area or volume of flow of a stream should'be left
free as as zone of passage.
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MONTANA
Existing discharges to state waters will be entitled a mixing
zone as determined by the department. "Mixing zone" means that
volume of state water wherein any pollutant may exceed allowable
water quality standards.
NEBRASKA
The Water Quality Standards shall apply at and beyond the mixing
zone boundaries. The mixing zone exception does not apply to fecal
col iform criteria in waters designated as full body contact. The
boundary limits of the mixing zone shall be a specified linear
distance, volume, or area, and should meet the conditions listed
below unless the physical characteristics of the receiving waters in
such a manner that will not affect the assigned beneficial uses.
(i) The mixing zone should be kept as small as possible and
shall not be of a size or shape that would impair or
contribute to the impairment of water use.
(ii) The mixing zone should allow for a continuous zone of
passage.
(iii) The mixing zone shall not overlap with any other
mixing zones in such a manner that the maintenance of
beneficial uses in the body of water, as a whole, would be
adversely affected.
NEW JERSEY
The total area and/or volume of a body of water assigned to
non-thermal mixing areas shall be limited to that which will not
interfere with biological communities or populations of important
species to a degree which is damaging to the ecosystem; and not
diminish other beneficial uses disproportionately. Water quality
criteria are intended to apply outside of designated non-thermal
mixing areas.
In river systems, reservoirs, lakes, estuaries and coastal
waters, zones of passage are considered to be continuous water routes
of the volume, area and quality necessary to allow passage of
free-swimming and drifting organisms with no significant effects
produced on their populations. These zones of passage must be
provided wherever non-thermal mixing areas are allowed.
Heat Dissipation Area Determinations - The determination of heat
dissipation areas shall take into special consideration the
extent and nature of the receiving waters so as to meet the
intent and purpose of the criteria and standards including
prevision for the passage of free-swimming and drifting
organisms so that negligible or no effects are produced on their
populations.
As a guideline, heat dissipation areas shall be limited to no
more than 1/4 of the cross-sectional area and/or volume of flow
of the stream, leaving at least 3/4 free as a zone of passage
28
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including a minimum of 1/3 the surface measured from shore to
shore at any flow.
Adjacent Heat Dissipation Areas - Where waste discharges would
result in heat dissipation areas in close proximity to each
other as to impair protected uses, additional limitations may be
prescribed to avoid such impairment.
NEW HAMPSHIRE
A heated discharge to a lake shall not raise the temperature
more than 3 degrees F at the surface immediately outside a designated
mixing zone.
The water temperature at the surface of an estuary shall not be
raised to more than 90°F at any point, provided further, at least 50
percent of the cross-sectional area and/or volume of the flow of the
estuary including a minimum of 1/3 of the surface as measured from
water edge to water edge at any stage of tide, shall not be raised to
more than 4°F over the temperature that existed before the addition
of heat of artificial origin or a maximum of 83°F, whichever is
less.
However, during July through September, if the water temperature
at the surface of an estuary before the addition of heat of
artificial origin is more than 83°F, an increase in temperature not
to exceed 1.5°F, at any point of the estuarine passageway as
delineated above, may be permitted.
New Hampshire had also adopted verbatim the entire criteria
pertaining to temperature and zones of passage contained in Section 3
of the National Technical Advisory Report on Water Quality Criteria,
dated April 1, 1968. This report makes recommendations regarding
mixing zones and zones of passage on page 31.
The New Hampshire Water Pollution Commission may consider mixing
zones, except as otherwise provided in these water quality standards
or by statutes, and where mixing zones are allowed they shall conform
to the latest requirements of the Environmental Protection Agency or
to the requirements of the Commission which shall be no less rigorous
than existing federal requirements.
NEW MEXICO
Mixing Zones and Zones of Passage - In any waters receiving a
waste discharge, a continuous zone must be maintained in the stream
or reservoir where the water is of adequate quality to allow the
migration of all desirable aquatic life presently common in New
Mexico waters with no significant effect on their populations.
Wastewater mixing zones, in which the standards may be exceeded,
shall generally be less than 1/4 of the cross-sectional area of the
stream or reservoir, allowing at least 3/4 of the stream or reservoir
as a zone of passage.
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NEW YORK
Collection of samples. In making any tests of analytical
determinations to determine compliance or non-compliance of sewage,
industrial wastes or other waste discharges with established
standards, samples shall be collected in such manner and at such
locations as are approved by the commissioner. In approving such
locations, the commissioner shall be guided by the fact that:
(a) There must be prompt mixing of the discharge with the
receiving waters,
(b) That the mixing will not interfere with biological
communities to a degree which is damaging to the
ecosystems; and
(c) That the mixing will not diminish other beneficial uses
disproportionately.
Mixing zone criteria. The following criteria shall not apply to all
waters of the State receiving thermal discharges, except as provided
in section 704.6 of this Part.
(a) The department shall specify definable, numerical limits
for all mixing zones (e.g. linear distances from the point
of discharge, surface area involvement, or volume or
receiving water entrained in the thermal plume).
(b) Conditions in the mixing zone shall not be lethal in
contravention of water quality standards to aquatic biota
which may enter the zone.
(c) The location of mixing zones for thermal discharges shall
not interfere with spawning areas, nursery areas and fish
migration routes.
Criteria Governing Thermal Discharges. At least 50 percent of the
cross-sectional area and/or volume of the flow of water including a
minimum of one-third of the surface as measured from water edge to
water edge at any stage of tide (if tidal) shall not exceed
designated temperature standards.
NEVADA
1. Zones of mixing for the assimilation of municipal,
agricultural, and industrial discharges from point sources which have
received the best degree of treatment or control practicable under
existing technology are recognized as necessary. It is the objective
of this limited zone to provide for a current realistic means of
control over such discharges and at the same time achieve the highest
attainable level of water quality.
2. Every application for a zone of mixing shall be made to
the Director and shall be accompanied by a complete and detailed
description of the present physical, chemical, biological, and
radiological conditions of the receiving waters and of the proposed
zone of mixing, and a demonstration that no appreciable harm to
beneficial uses, either designated or actual, will result from the
proposed zone of mixing, and such other information as the Director
may prescribe. Applications shall identify, by discharge, the
individual water quality parameters for which the zone of mixing is
requested.
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The applications for a zone of mixing shall be submitted along
with an application for a discharge permit or a request for
modification of a discharge permit.
3. Each application for a zone of mixing shall be reviewed
in light of the descriptions, statements, plans, histories, and other
supporting information as may be submitted upon request of the
Director. The review shall result in a determination by the Director
concerning the appropriateness of a zone of mixing for each water
quality parameter, by discharge identified in the application.
4. The zone of mixing shall be established by the Director
such that the water quality standards for individual parameters
determined to be appropriate pursuant to Article 4.1.2.h.3 for the
receiving water, but in no case including aesthetic and acute
toxicity values, may be relaxed within the zone of mixing. In
determining the size of a zone of mixing, each application will be
reviewed on a case-by-case basis taking into consideration the
quality of the effluent or wastewater discharged and the nature and
condition of the receiving water including the effects of the
effluent or wastewater on the designated or actual beneficial uses of
the receiving water.
5. Zones of mixing shall not be granted by the Director:
a. unless the applicant and supporting information clearly
demonstrates that the discharge occuring or proposed to occur does
not substantially endanger human health or safety; and
b. unless the applicant and supporting information clearly
demonstrate that the discharge occuring or proposed to occur will
assure the protection and propagation of a balanced, indigenous
population of shellfish, fish, and wildlife in and on that body of
water; and
c. Unless the Regional Administrator concurs in writing with
the proposed zone of mixing.
6. Streams-mixing zones in which the water quality standards
may be exceeded, shall be a maximum of 1/3 of the cross-sectional
area of the receiving water, allowing a minimum of a continuous 2/3
of the receiving water as a zone of passage. The allowable
stream-mixing zone shall be oriented in the stream in a manner which
maximizes the effectiveness of the zone of passage.
7. Any zone of mixing granted pursuant to this section may
be granted or renewed for periods not exceeding five years.
Applications for renewal shall be made prior to the expiration of the
zone of mixing. Applications for renewal of a zone of mixing may be
granted by the Director provided that the applicant for renewal had
met all of the conditions specified in the immediately preceeding
zone of mixing, granted pursuant to this Article; and provided
further, that the renewal, and the zone of mixing established in
pursuance thereof, shall provide for discharge not greater in
quantity of mass emission than that attained pursuant to the terms of
the immediately preceding zone of mixing at its expiration.
8. Unless an application for renewal of a zone of mixing
has been made, the zone of mixing shall automatically terminate at
the expiration of the period in the designation and no rights shall
become vested in the designee.
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9. A separate permit will not be issued for mixing zones. All
requirements and authorizations shall be part of a discharge permit
issued or reissued pursuant to Article 2. The fact sheet required by
section 4.3 of the Procedural Regulations for Administration of
Discharge Permits shall contain a summary of the data from which the
zone of mixing was determined.
10. The Director shall periodically review all zones of
mixing and may terminate any such zones for which the conditions of
approval of the zone mixing have changed.
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NORTH CAROLINA
A mixing zone may be established in the area of a discharge in
order to provide reasonable opportunity for the mixture of the
wastewater with the receiving waters. The limits of such mixing
zones will be defined by the division on a case-by-case basis after
consideration of the magnitude and character of the waste discharge
and the size and character of the receiving waters. Such zones shall
not:
(1) prevent free passage of fish around or cause fish mortality
within the mixing zone,
(2) result in offensive conditions,
(3) produce undesirable aquatic life or result in a dominance
of nuisance species outside of the assigned mixing zone,
(4) endanger the public health or welfare.
In addition, a mixing zone shall not be assigned for fecal
coliform organisms in waters classified "A-II," "B," "SB," or "SA."
For the discharge of heated wastewater, compliance with federal rules
and regulations pursuant to Section 316(a) of the Federal Water
Pollution Control Act, as amended, shall constitute compliance with
this Subsection (1).
NORTH DAKOTA
The size and configurations of a mixing zone cannot be uniformly
prescribed for all streams due to the particular characteristics of
each stream. However, the following considerations are to be taken
into account when mixing zones are determined:
(1) The Water Quality Standards must be met at every point
outside of the mixing zone. The Department may require a
means of expediting mixing and dispersion of wastes, if
found necessary.
(2) The total mixing zone (or zones) at any cross-sectional
area of the stream should not be larger than 25 percent of
the cross-sectional area or volume of flow and shall not
extend more than 50 percent of the width. Mixing zones
shall provide an acceptable passageway for movement of fish
and other aquatic organisms.
(3) The 96-hour TLm for indigenous and/or resident fish and
fish food organisms shall not be exceeded at any other
point in the mixing zone.
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NORTH DAKOTA (Cont'd)
(4) Mixing zones shall be as small as possible and shall not
intersect spawning or nursery areas, migratory routes, or
municipal water intakes. Overlapping of mixing zones
should be avoided or minimized to prevent adverse
synergistic effects.
OHIO
The following general water quality standards shall apply to all
surface waters of the state including mixing zones. To every extent
practical and possible as determined by the Director, these waters
shall be:
(A) Free from suspended solids or other substances that enter the
waters as a result of human activity and that will settle to form
putrescent or otherwise objectionable sludge deposits, or that will
adversely affect aquatic life.
(B) Free from floating debris, oil, scum and other floating materials
entering the waters as a result of human activity in amounts
sufficient to be unsightly or cause degradation,
(C) Free from materials entering the waters as a result of human
activity producing color, odor or other conditions in such a degree
as to create a nuisance;
(D) Free from substances entering the waters as a result of human
activity in concentrations that are toxic and harmful to human,
animal or aquatic life and/or are rapidly lethal in the mixing zone;
(E) Free from nutrients entering the waters as a result of human
activity in concentrations that create nuisance growths of aquatic
weeds and algae.
(A) NON-THERMAL
(1) Except as subsequent provisions of Division (A) of this
Rule establish different limits, no mixing zone shall:
(a) Interdict the migratory routes or interfere with
natural movements, survival, reproduction, growth, or increase
the vulnerability to predation of any representative aquatic
species;
(b) Include spawning or nursery areas of any
representative aquatic species,
(c) Include a public water supply intake,
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(d) Include any bathing area where bath houses and/or
lifeguards are provided,
(e) Constitute more than one-half of the width of the
receiving watercourse nor constitute more than one-third of the
area of any cross-section of the receiving watercourse;
(f) Constitute more than one-fifth of the area of any
cross-section of the mouth of a receiving watercourse (the mouth
constituting that area of the stream from the confluence
upstream for a distance five times the width of the stream at
the confluence);
(g) Extend downstream at any time a distance more than
five times the width of the receiving watercourse at the point
discharge.
(2) The Director may waive the requirements of Sections (A)
(l)(e)» (f) and (g) of this rule whenever a discharger provides:
(a) information defining the actual boundaries (where the
Water Quality Standards are met) of the mixing zone in question,
and
(b) information and data providing no violation of
Sections (A)(l)(a), (b), (c) and (d) of this rule by the mixing
zone in question.
MIXING ZONES
(3) For watercourses classified as Coldwater Habitat and
Exceptional Warmwater habitat in Rule 3745-1-08 of the Ohio
Administrative Code the standards set forth in Sections
(A)(l)(a) through (d) of this rule shall apply, and in addition
no mixing zone shall:
(a) constitute more than one-third of the width of the
receiving watercourse nor constitute more than one-fifth of the
area of any cross-section of the receiving watercourse; or
(b) extend downstream at any time a distance more than
three times the width of the receiving watercourse at the point
discharge.
(4) At least 90 percent of the volume of the mixing zone shall
not exceed at any time the 24 to 96 hour median tolerance limit
(TLM) or LCso for any representative aquatic species, as
determined by static bioassays for persistent toxicants and
dynamic bioassays for non-persistent toxicants in accordance
with methods described in "Standard Methods for the Examination
of Water and Wastewater," 14th Edition, 1975, published by the
American Water Works Association and the Water Pollution Control
Federation.
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OHIO (Cont'd)
(5) For lakes and reservoirs except Lake Erie classified as
State and National Resource Water in Rule 3745-1-05 of the Ohio
Administrative Code, no mixing zone shall be permitted.
(6) For all streams classified as Seasonal Warmwater Habitat in
Rule 3745-1-08 of the Ohio Administrative Code, the mixing zone
criteria in Division (A)(l) through (5) of this rule will not
apply.
(B) THERMAL
(1) A thermal mixing zone to permit dilution and cooling of a
waste heat discharge shall be considered a region in which
organism response to temperature is time-dependent. Exposure to
temperatures in a thermal mixing zone shall not cause an
irreversible response which results in deleterious effects to
the wildlife and aquatic life representative of the receiving
waters. The daily average temperature in a thermal mixing zone
at a point nearest to the discharge that is accessible to the
resident aquatic organisms shall not exceed the temperature in
Table 1 at the corresponding ambient temperature. At ambient
temperatures of 59F(15.0C) and above, the daily average
temperature in a thermal mixing zone will be determined on a
case-by-case basis.
(2) Thermal mixing zone size limitations shall be established
by the Director pursuant to Section (B)(l) of this rule on a
case-by-case basis for all point source discharges subject to
permit.
(3) Except as Section (B)(l) and (B)(2) of this rule establish
different limitations, no thermal mixing zone shall:
(a) interdict to migratory routes or interfere with
natural movements, survival, reproduction, growth, or increase
the vulnerability to predation of any representative aquatic
species.
(b) interfere with or prevent the recovery of an aquatic
community or species population that could reasonably be
expected as previously limiting water quality conditions
improve,
(c) include a public water supply intake, or;
(d) include any bathing area where bath houses and/or life
guards are provided.
(4) For all watercourses classified as Coldwater Habitat and
Exceptional Warmwater Habitat in Rule 3745-1-08 of the Ohio
Administrative Code, thermal mixing zones will not be permitted.
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OHIO (Cont'd)
(5) For lakes and reservoirs, except Lake Erie, classified as
State and national Resource Water in Rule 3745-1-05 of the Ohio
Administrative Code, no thermal mixing zone shall be permitted.
(6) For all streams classified as Seasonal Warmwater Habitat in
Rule 3745-1097 of the Ohio Administrative Code, the mixing zone
criteria in Division (B)(l) through (3) of this rule will not
apply.
(7) Discharges of closed-cycle cooling blowdown with a flow of
less than five percent of the seven-day once-in-ten-year
low-flow of the receiving water body will be exempt from
Division (B)(l) through (3) of this rule.
Table 1:
Daily average temperatures of thermal mixing zones at
corresponding ambient temperatures as required in Section
(B)(l) of this rule. Shown as degrees Fahrenheit and
Celsius.
Ambient-
Daily Average
Temperature-°F(°C)
32(0)
33(0.6)
34(1
35(1
36(2
37(2
38(3.3)
39(3.9)
40(4
41(5
42(5
1)
7)
2)
8)
•4)
.0)
.6)
45(7.2)
46(7.8)
47(8.3)
50(10.0)
50(10.0)
50(10
51(10
52(11
54(12
55(12
57(13
58(14.4)
60(15.6)
62(16.7)
.0)
.6)
• 1)
.2)
.8)
.9)
43(6.1) 63(17.2)
44(6.7) 65(18.3)
66(18.9)
68(20.0)
70(21.1)
Ambient-
48(8.9)
49(9.4)
50(10.o;
51(10.6]
52(11.1)
53(11.7)
54(12.2)
55(12.8)
56(13.3)
57(13.9)
58(14.4)
59(15) and
Daily Average
Temperature-°F(°C)
71(21.7)
73(22.8)
75(23.9)
76(24.4)
78(25.6)
79(26.1)
81(27.2)
83(28.3)
85(29.4)
86(30.0)
88(31.1)
above - daily
average limit will be
determined on a case-by-
case basis pursuant to Rule
3745-1-06(8)0) and (2).
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OHIO (Cont'd)
LAKE ERIE STANDARDS
(B) MIXING ZONES
(1) Non-Thermal
For Lake Erie, outside of the excepted areas established in
Division (C) of this rule, the following criteria will
apply:
(a) Except as subsequent provisions of this section
provide different limits, no mixing zone shall:
(1) interdict the mouth of a stream, thereby blocking
any portion of it; or
(2) interdict the migratory routes or interfere with
natural movements, survival, reproduction,
growth, or increase the vulnerability to
predation of any representative aquatic species;
or
(3) include spawning or nursery areas of any
representative aquatic species, or
(4) include a public water supply intake, or
(5) include any bathing area where bath houses and/or
lifeguards are provided,
(6) contact the shoreline, whenever such contact can
be avoided.
(b) At least 90 percent of the volume of the mixing zone
shall not exceed at any time the 24 to 96 LCsQ for
any representative aquatic species, as determined by
static bioassays for persistent toxicants and dynamic
bioassays for non-persistent toxicants in accordance
with methods described in "Standard Methods for the
Examination of Water and Wastewater," 15th Edition,
1975, published by the American Public Health
Association, American Water Works Association and the
Water Pollution Control Federation.
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OHIO(Cont'd)
(2) Thermal
(a) A thermal mixing zone to permit dilution and cooling
of a waste heat discharge shall be considered a region
in which organism response to temperature is
time-dependent. Exposure to temperatures in a thermal
mixing zone shall not cause an irreversible response
which results in deleterious effects to the wildlife
and aquatic life representative of the receiving
waters. The daily average temperature in a thermal
mixing zone at the point nearest to the discharge that
is accessible to the resident aquatic organisms shall
not exceed the temperatures in Table 7d at the
corresponding ambient temperature. At ambient
temperatures of 59°F (15°C) and above the daily
average temperature in a thermal mixing zone will be
determined on a case-by-case basis.
(b) Thermal mixing zone size limitations shall be
established by the Director pursuant to Section
(B)(2)(a) of this rule on a case-by-case basis for all
point source discharges subject to permit.
(c) Except as Division (B)(2)(a) and (B)(2)(b) of this
rule establish different limitations, no thermal
mixing zone shall:
(1) interdict the migratory routes or interfere with
natural movements, survival, reproduction,
growth, or increase the vulnerability to
predation of any representative aquatic species;
(2) interfere with or prevent the recovery of an
aquatic community or species population that
could reasonably be expected as previously
limiting water quality conditions improve;
(3) include a public water supply intake; or
(4) include any bathing area where bath houses and/or
lifeguards are provided.
(d) Closed-cycle cooling blowdown discharge will be exempt
from Divisions (B)(2)(a) and (b) of this rule.
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OHIQ(Cont'd)
LAKE ERIE STANDARDS
Table 7d:
Daily average temperatures of thermal mixing zones at
corresponding ambient temperatures. Shown as degrees
Fahrenheit and Celsius.
Ambient
Daily Average
Temperature
Ambient
Daily Average
Temperature
32(0)
33(0.6)
34(1
35(1
36(2
37(2
38(3
39(3
40(4
41(5
42(5
43(6
44(6.7)
45(7.2)
•1)
.7)
.2)
.8)
.3)
•9)
.4)
.0)
41(5.0)
41(5.0)
43(6.
45(7,
46(7,
48(8.9)
50(10.0)
52(11
53(11
55(12,
57(13,
59(15,
61(16.
• 1)
•2)
.8)
•1)
.7)
.8)
.9)
.0)
.1)
62(16.7)
46(7.8)
47(8.3)
48(8.9)
49(9.4)
50(10.0)
51(10.6)
52(11.1)
53(11.7)
54(12.2)
55(12.8)
56(13.3)
57(13.9)
58(14.4)
59(15) and
65(18.3)
66(18.9)
68(20.0)
70(21.1)
71(21.7)
73(22.8)
75(23.9)
77(25.0)
78(25.6)
80(26.7)
82(27.8)
84(28.9)
86(30.0)
above-daily
average limit will be
determi ned
case basis,
on a case-by-
»
Lake Erie within excepted areas shall comply with the requirements in
Rule 3745-1-06.
OHIO RIVER STANDARDS
(M) Mixing Zone:
A mixing zone is an area contiguous to a discharge where
receiving water quality may neither meet all quality
criteria nor requirements otherwise applicable to the
receiving water. It is obvious that any time an effluent
is added to a receiving waterway, where the effluent is
poorer in quality, there will be a zone of mixing. The
mixing zone should be considered as a place where wastes
and water mix and not as a place where effluents are
treated. As appropriate, the regulatory agency will define
the mixing zone in accordance with the following
guidelines, provided, however, that no mixing zone shall be
recognized for any wastewaters discharged without a valid
permit.
(1) The regulatory agency shall specify definable,
numerical limits for mixing zones on a case-by-case
basis, considering the linear distances from the point
of discharge, surface area involvement, or volume of
receiving water within the defined zone.
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OHIO(Cont'd)
(2) Conditions within the mixing zone shall not be lethal
to aquatic biota which may enter the zone.
(3) The location of a mixing zone shall not interfere with
spawning areas, nursery areas, fish migration routes,
potable water supply intakes, or bathing areas.
THE MAHONING RIVER BASIN
Upper Basin
Mixing zones shall be determined on a case-by-case basis with
the requirement that each mixing zone shall be limited to the
greatest practical extent and where possible not to overlap another
one. In addition, a reasonable zone of passage will be preserved for
the movement of fish and other aquatic biota.
Lower Basin
(a) A mixing zone is a portion of a watercourse where effluent mixes
with waters in the receiving watercourse, and the zone is
managed to preserve the major region of the watercourse for its
designated use(s), particularly in the cases of the highest use
classification (Alg, PWS, ALsw and Rpc), thereby protecting a
region for the passage of fish and for proper habitat of aquatic
biota, and to preserve unhindered regions for public water
supply and livestock watering intake and bathing.
(b) Waters within a mixing zone will not be rapidly lethal to native
aquatic biota and will have a temperature such that the water
temperature outside the mixing zone will not be increased or
decreased more than 5°F.
(c) Water quality standards for the designated use(s) will be met
outside of the mixing zone.
OKLAHOMA
SECTION 8.1 (a)
When a liquid of different quality than the receiving water is
discharged to an aquatic system, a mixing zone is formed. The
concept of mixing zone is recognized as a necessary element of
Oklahoma's Water Quality Standards. In the case of perennial
streams, the mixing zone extends downstream a distance equivalent to
thirteen (13) times the width of the water at the point of effluent
discharge. The dissolved oxygen concentration in a mixing zone shall
be not less than 4.0 mg/1, and the concentration of toxic substances
shall not exceed the 96 hr. LCso for tne ms^ sensitive
indigenous species. Mixing zones in lakes and intermittent streams
shall be designated on a case-by-case basis. It is recognized that
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OKLAHOMA (Cont'd)
the water quality in the mixing zone may be unsuitable for certain
beneficial uses. Where the overlapping mixing zones due to multiple
outfalls occur, the total length of the mixing zone will be equal to
the sum of the lengths of the individual mixing zones.
SECTION 82. ZONES OF PASSAGE
All discharges shall be regulated to insure that at the outfall
and throughout the mixing zone, a zone of passage shall be maintained
with the stream that shall be no less than seventy-five percent (75%)
of the cross-sectional area or flow volume, whichever is more
beneficial to the free-swimming and drifting organisms.
OREGON
4. a. The Department may suspend the applicability of all or part
of the water quality standards set forth in this section, except
those standards relating to aesthetic conditions, within a
defined immediate mixing zone of specified and appropriately
limited size adjacent to or surrounding the point of waste
discharge.
b. The sole method of establishing such mixing zone shall be by
the Department defining same in a waste discharge permit.
c. In establishing a mixing zone in a waste discharge permit the
Department:
1) May define the limits of the mixing zone in terms of
distance from the point of the waste water discharge or
the area or volume of the receiving water or any
combination thereof,
2) May set other less restrictive water quality standards to
be applicable in the mixing zone in lieu of the suspended
standards, and
3) Shall limit the mixing zone to that which in all
probability, will
a) Not interfere with any biological community or
population of any important species to a degree which
is damaging to the ecosystem; and
b) Not adversely affect any other beneficial use
disproportionately.
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PENNSYLVANIA
No reference to mixing zones.
RHODE ISLAND
Thermal Mixing Zones - In the case of thermal discharges into
tidal rivers or estuaries, or fresh water streams or estuaries, where
thermal mixing zones are allowed by the director, the mixing zone
will be limited to no more than 1/4 of the cross sectional area
and/or volume of flow of river, stream or estuary, leaving at least
3/4 free as a zone of passage. In wide estuaries and oceans, the
limits of mixing zones will be established by the director.
SOUTH CAROLINA
5. Mixing Zone - as used in Section III, Number 11, shall mean a
designated area within which specified water quality standards are
not applicable. The boundary of this zone shall be determined by the
Department of Health and Environmental Control on an individual
project basis after consideration of the waste discharge and the
receiving waters. A mixing zone shall not prevent free passage of
fish and shall not interfere with the designated use outside its
established boundary.
Fresh Waters - The water temperature at the inside boundary or
the mixing zone shall not be more than 10 degrees C (18 degrees F)
greater than that of water unaffected by the heated discharge. The
appropriate temperature criteria or the size of the mixing zone will
be determined on an individual project basis and will be based on
biological, chemical, engineering and physical considerations. Any
such determination shall assure the protection and propagation of a
balanced, indigenous population of shellfish, fish and wildlife in
and on a body of water to which the heated discharge is made and
shall allow passage of aquatic organisms.
Tidal Salt Waters - The size of the mixing zone will be
determined on an individual project basis and will be based on
biological, chemical, engineering and physical considerations. Any
such determination shall assure the protection and propagation of a
balanced, indigenous population of shellfish, fish and wildlife in
and on a body of water to which a heated discharge is made. Further,
the mixing zones shall be kept at a minimum and shall allow the
passage of aquatic organisms.
Lakes and Reservoirs - The water temperature at the inside
boundary of the mixing zone shall not be more than 10 degrees C (18
degrees F) greater than that of water unaffected by the heated
discharge. The appropriate temperature criteria of the size of the
mixing zone will be determined on an individual project basis and
will be based on biological, chemical, engineering and physical
considerations. Any such determination shall assure the protection
and propagation of a balanced, indigenous population of shellfish,
fish and wildlife in and on a body of water to which the heated
discharge is made and shall allow passage of aquatic organisms.
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SOUTH DAKOTA
Each discharge to a flowing water shall be entitled to a mixing
zone at the edge of which the criterion established for the
beneficial uses of the receiving water shall be met. Mixing zones in
streams must permit an acceptable passageway for movement of aquatic
organisms. The total mixing zone or zones, at any transect of a
stream shall not contain more than seventy-five percent of the
cross-sectional area of the stream, shall not extend over more than
seventy-five prcent of the width of the stream or one hundred yards,
whichever is the least, and the dimensions parallel to the stream
flow shall not exceed one-half mile. Mixing zone characteristics
must not be lethal to aquatic organisms. The median tolerance limit
for indigenous fish or fish food organisms, whichever is more
stringent, shall not be exceeded at any point in the mixing zone.
Mixing zones shall not intersect spawning or nursery areas, migratory
routes, water intakes, or mouths of rivers. Mixing zones should not
overlap, but where they do, measures shall be taken to prevent
adverse synergistic effects.
Lakes not allowed a mixing zone. Discharges to lakes are not
entitled to a mixing zone. These effluents shall meet the water
quality standards at the point of discharge. No discharge of
pollutants shall be allowed which reaches a lake classified for the
beneficial use of fish life propagation and causes impairment of any
assigned beneficial use.
TENNESSEE
Mixing zone refers to that section of a flowing stream or
impounded waters necessary for an effluent to become dispersed and
mixed insofar as practical with the main flow or water body by
appropriate methods at the discharge point. Such zones shall be
restricted to as small an area and length as possible and shall not
(i) prevent the free passage of fish or cause aquatic life mortality
in the receiving waters; (ii) contain materials, that adequately
represent the defined zone, in concentrations that exceed the 96-hour
LC50 for biota significant to the aquatic community in the receiving
waters; (iii) result in offensive conditions; (iv) produce
undesirable aquatic life or result in dominance of nuisance species;
(v) endanger the public health or welfare; or (vi) adversely affect
the reasonable and necessary uses of the area. The mixing zone
necessary in each particular case may be designated in the discharge
permit as required by the Tennessee Water Quality Control Act
(T.C.A., Section 70-324 through Section 70-342).
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TEXAS
2. Mixing Zones
Where mixing zones are specifically defined in a valid waste
control order issued by the Texas Water Quality Board or a
National Pollutant Discharge Elimination System permit, the
defined zone shall apply.
Where the mixing zone is not so defined, a reasonable zone shall
be allowed. Because of varying local physical, chemical, and
biological conditions, no single criterion is applicable in all
cases. In no case, however, where fishery resources are
considered significant, shall the mixing zone allowed preclude
the passage of free-swimming and drifting aquatic organisms to
the extent of significantly affecting their populations.
Normally, mixing zones should be limited to no more than 25
percent of the cross-sectional area and/or volume of flow of the
stream or estuary, leaving at least 75 percent free as a zone of
passage unless otherwise defined by a specific Board Order or
permit.
3. Buffer Zones in Bay and Gulf Waters
For all bay and gulf waters, exclusive of those contained in
river or coastal basins as defined in Section IV, a buffer zone
of 1,000 feet measured from the shoreline at ordinary high tide
is hereby established. In this zone, the bacteriological
requirements enumerated in other sections of these standards
shall not apply. In these zones, the logarithmic mean (geometric
mean) density of fecal coliform organisms shall not exced 200/100
ml, nor shall more than 10% of the total samples exceed 400/100
ml. The foregoing percentages are applicable when examining data
from not less than 5 samples collected over no more than 30 days.
For routine observation and evaluation of water quality, lesser
numbers of samples collected over longer periods will be used.
UTAH
A mixing zone is a limited portion of a body of water, contiguous
to a discharge, where dilution is in progress but has not yet
resulted in concentration which will meet standards for all
pollutants. Mixing zones may be delineated for the purpose of
guiding sample collection procedures. The zone shall be small in
extent and must not form a barrier to migrating aquatic life.
Domestic wastewater effluents discharged to mixing zones shall meet
effluent requirements specified in Section 1.3 of the Utah
regulations.
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VERMONT
RULE 14: Thermal Mixing Zones
As a requirement of any permit for the discharge of heated wastes
to the waters of the State, the Secretary may designate a specific
portion of the receiving waters as a thermal mixing zone. Thermal
mixing zones shall be allowed only where the wastes otherwise conform
with the technical and other requirements established for the
receiving waters and shall be utilized solely for the dispersal and
dilution of heated wastes which have been adequately treated in the
judgment of the Secretary.
Thermal mixing zones shall be designated so as to not constitute
a barrier to the passage or migration of fish or produce significant
adverse effects on any fishery or other forms of wild or aquatic
life. As a guideline, thermal mixing zones should be limited to no
more than 25 percent of the cross-sectional area and/or volume of the
receiving water.
VIRGINIA
Zones for mixing wastes with receiving water shall be determined
on a case-by-case basis, shall be kept as small as practical, shall
not be used for, or considered as, a substitute for waste treatment,
and shall be implemented, to the greatest extent practicable, in
accordance with the provisions of subsections A and D hereof. Mixing
within these zones shall be as quick as practical and may require the
installation and use of devices which insure that waste is mixed with
the allocated receiving waters in the smallest practical area. The
need for such devices will be determined on a case-by-case basis.
The boundaries of these zones of admixture shall also be such as to
provide a suitable passageway for fish and other aquatic organisms.
In an area where more than one discharge occurs and several mixing
zones are close together, these mixing zones shall be so situated
that this passageway is continuous.
WASHINGTON
The total area and/or volume of a receiving water assigned to a
dilution zone shall be as described in a valid discharge permit as
needed and be limited to that which will:
(a) not cause acute mortalities of sport, food, or commercial fish
and shellfish species of established biological communities
within populations or important species to a degree which
damages the ecosystem.
(b) not diminish aesthetic values or other beneficial uses
disproportionately.
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WEST VIRGINIA
(A) Definition - a mixing zone is an area contigious to a
discharge where receiving water quality may neither meet all quality
criteria nor requirements otherwise applicable to the receiving
water. The mixing zone is considered as a place of mixing and not as
a place where effluents are treated.
(B) Management - management over those waters which receive
waste discharges must be addressed to the task of maintaining the
mixing areas in as small a volume and area as practicable. The
geographic limits of a mixing zone is complicated by both the
wastewater discharge and receiving water characteristics and will not
be determined except on a case-by-case basis or where it is necessary
to exercise more stringent controls over the receiving waters in
question.
WISCONSIN
Water quality standards must be met at every point outside of a
mixing zone. The size of the mixing zones cannot be uniformly
prescribed, but shall be based on such factors as effluent quality
and quantity, available dilution, temperature, current, type of
outfall, channel configuration and restrictions to fish movement. As
a guide to the delineation of a mixing zone, the following shall be
taken into consideration:
(a) Limiting mixing zones to as small an area as practicable,
and conforming to the time exposure responses of aquatic
life.
(b) Providing passageways in rivers for fish and other mobile
aquatic organisms.
(c) Where possible, mixing zones being no larger than 25% of the
cross-sectional area or volume of flow of the stream and not
extending more than 50% of the width.
(d) For contaminants other than heat, the 96-hour TLm to
indigenous fish and fish food organsms not being exceeded at
any point in the mixing zone.
(e) Mixing zones not exceeding 10% of a lake's total surface
area.
(f) Mixing zones not interfering with spawning or nursery areas,
migratory routes, nor mouths of tributary streams.
(g) Mixing zones not overlapping, but where they do, taking
measures to prevent adverse synergistic effects.
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WISCONSIN (Cont'd)
(5) Exemptions. The thermal mixing zone provisions of this chapter
are not applicable to municipal waste and water treatment plants, to
vessels, or to discharge to enclosed harbors.
Categories of standards. (1) GENERAL. To preserve and enhance
the quality of waters, standards are established to govern water
management decision. Practices attributable to municipal,
industrial, commercial, domestic, agricultural, land development or
other activities shall be controlled so that all waters including the
mixing zone and the effluent channel meet the following conditions at
all times and under all flow conditions.
(a) Substances that will cause objectionable deposits on the
shore or in the bed of a body of water, shall not be present in such
amounts as to interfere with public rights in waters of the state.
(b) Floating or submerged debris, oil, scum or other material
shall not be present in such amounts as to interfere with public
rights in waters of the state.
(c) Materials producing color, odor, taste, or unsightliness
shall not be present in such amounts as to interfere with public
rights in waters of the state.
(d) Substances in concentrations or combinations which are toxic
or harmful to humans shall not be present in amounts found to be of
public health significance, nor shall substances be present in
amounts which are acutely harmful to animal, plant or aquatic life.
Lake Michigan and Lake Superior thermal standards. For Lake
Michigan and Lake Superior the following thermal standards are
established so as to minimize effects on the aquatic biota in the
receiving waters.
(l)(a) Thermal discharges shall not raise the receiving water
temperature more than 3°F above the existing natural temperature at
the boundary of mixing zones established in paragraphs (b) and (c).
(b) 1. The mixing zone for a shoreline thermal discharge
shall be the area included within the perimeter of a rectangular
figure extending 1,250 feet in both directions along the shoreline
from the outfall and 1,250 feet into the lake.
2. The mixing zone for an offshore thermal discharge
shall be the area within a 1,000-foot radius circle with its center
at the point of discharge.
(c) The department may, upon request from the owner of a
source of thermal discharge, adjust the boundaries of the mixing
zones established in paragraph (b) for that source. In no case may
any mixing zone so established include an area greater than 72 acres
nor may it include more than 2,800 feet of shoreline.
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WISCONSIN (Cont'd)
Review of thermal standards.
(1) Whenever the owner of any source of thermal discharges
that existed on or before July 31, 1975, in compliance with
department guidelines and after opportunity for public hearing, can
demonstrate to the satisfaction of the department that the mixing
zone established pursuant to this chapter is more stringent than
necessary to assure the protection and propagation of a balanced
indiegenous population of shellfish, fish and wildlife in and on the
receiving water, the department may:
(a) Impose a mixing zone with respect to such thermal
discharge that will assure the protection and propagation of such a
population, or
(b) Exempt such thermal discharge from the thermal
requirements of this chapter provided this exemption will not
endanger the propagation of such a population.
(2) Any owner desiring a review pursuant to NR 102.07 (1)
shall submit a demonstration to the department no later than June 30,
1976. The department shall reach a decision no later than December
31, 1976.
(3) In the event the owner fails to make a satisfactory
demonstration pursuant to NR 102.07 (1), the department shall
establish a compliance date for the thermal component to be achieved
no later than July 1, 1979.
(4) Whenever the owner of any source of thermal discharges
that commenced on or after August 1, 1975, in compliance with
department guidelines and after opportunity for public hearing, can
demonstrate to the satisfaction of the department that the mixing
zone established pursuant to this chapter is more stringent than
necessary to assure the protection and propagation of a balanced,
indigenous population of shellfish, fish and wildlife in and on the
receiving water, the department may:
(a) Impose a mixing zone with respect to such thermal
discharge that will assure the protection and propagation of such a
population, or
(b) Exempt such thermal discharge from the thermal
requirements of this chapter provided this exemption will not
endanger the propagation of such a population.
(5) In the event an owner fails to make a satisfactory
demonstration pursuant to NR 102.07 (4), the discharge shall be in
compliance with the thermal requirements of this chapter upon
commencement of the discharge.
(6) The department may require the reduction of thermal
discharges or the size and configuration of a mixing zone if it finds
that environmental damage is imminent or existent.
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WYOMING
From Section 10. Sampling Points. Except for Sections 15, 16,
17 and 18 of these regulations, compliance with water quality
standards shall be determined after allowing reasonable time for
mixing. Size of the mixing zone shall be determined after
consideration of the effect of the discharge on the biological
community, water uses and aesthetic conditions, as well as
consideration of the flow conditions and physical nature of the
receiving water. The portion of a surface water body designated as a
mixing zone shall be limited to that which will not interfere with
biological communities or populations of important species to a
degree which is damaging to the ecosystem and which will not cause
substantial damage to other beneficial uses. In addition, there
shall be a zone of passage through the mixing zone sufficient to
allow passage of free swimming and drifting organisms in a manner
producing no significant effects on their populations, except during
periods when stream flows are less than the average of the minimum
seven consecutive day flow which has the probability of occurring
once in ten years.
Mixing zone is defined by the State as that portion of a surface
water body within which an effluent becomes thoroughly mixed with the
water body.
AMERICAN SAMOA
In accordance with the granting of permission to discharge into
the receiving waters of American Samoa, it shall be recognized that
this permission does not authorize pollution. Approval of discharges
will be made only upon evidence of a discharge meeting the
requirements of the water quality standards of the receiving waters
or the approval by the Environmental Quality Commission of an
acceptable abatement schedule which is to be developed by the
applicant if the waste discharge does not meet the requirement. The
Commission, at its discretion, may define a dispersion zone for each
discharge - outside of which the water quality standards will be
strictly enforced. This dispersion zone may be defined for specific
discharge parameters only; and shall not be construed as permission
for unregulated discharge.
A dispersion zone shall be defined as an inverted forty-five
degree conoidal shape cone ending six feet below the water surface.
The apex of the cone shall begin at the point of discharge. This
cone is to be the only dispersion zone under all normal tidal and
current conditions. Water quality standards will be strictly
enforced in the waters adjacent to the cone. A dispersion zone may
be granted on a parameter by parameter basis (pH, temperature, etc.,)
or for several parameters.
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GUAM
The water quality criteria in Section II shall apply within a
mixing zone unless specific alternate criteria are approved by the
Administrator for specified parameters. The mixing zone shall be
defined by specified linear distance, volume or area, discharge
location, maximum flow, and maximum concentrations of important
constituents which are determined on a case-by-case basis using the
following criteria:
1. Mixing zones shall be as small as practicable and shall not be of
such size or shape as to cause or contribute to the impairment of
water uses. In determining the size and location of mixing zones
for any discharge, the following shall be considered:
a) size of receiving water, volume of discharge, stream bank or
shoreline configuration, the mixing velocities, and other
hydrologic and physiographic characteristics;
b) present and anticipated future use of the body of water;
c) present and anticipated future quality of the body of water;
and
d) the ratio of the maximum flow rate of waste being discharged
to the lowest recorded flow rate of the receiving waters.
2. An adequate zone of passage shall exist at all times for the
movement or drift of aquatic life.
3. Where two or more mixing zones are in close proximity, they shall
be so defined that a continuous zone of passage for aquatic life
is available.
4. Mixing zones shall not intersect any area of any waters in such a
manner that the maintenance of aquatic life in the body of water
as whole would be adversely affected.
5. The discharge shall not violate the basic standards applicable to
all waters nor shall it unreasonably interfere with any actual or
probable use of the waters within the mixing zone.
PUERTO RICO
General Provisions
No person shall cause or permit a discharge for which an IMZ
(Initial Mixing Zone) or an FMZ (Final Mixing Zone), or both has not
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PUERTO RICO (Cont'd)
been defined and authorized by the Board. For defining and
authorizing any mixing zone, the Board shall take into consideration
the following general conditions:
A. These zones shall be provided solely for mixing. Mixing must be
accomplished as quickly as possible through the use of means
which insure that the waste is mixed with the dilution water in
the smallest possible space.
B. Suspended solids shall not settle to form observable deposits in
the mixing zone as a result of the discharge.
C. The concentration of waste discharge at the edge of the IMZ as
defined in 5.2.1. (A) shall not exceed the TLm 96 hours.
D. Nuisance growths of organisms shall not result in the mixing zone
as a result of the discharge.
E. Maximum vertical dispersion of wastewater discharge stream shall
be provided for in the mixing zone.
F. The mixing zone shall be located so as to allow, at all times,
passageways for the movement or drift of the biota. In closed
bodies of water, estuaries or rivers, any mixing zones shall be
located close to the bank itself, in such manner that the
passageway continues lengthwise for a considerable distance which
permits the adequate and safe flow of free floating or drifting
organisms. The passageway shall contain at least 75 percent of
the cross-sectional area and/or volume of flow in the case of
estuaries.
G. A mixing zone shall not overlap with an adjacent mixing zone.
Notwithstanding, overlapping may be allowed when demonstrated to
the satisfaction of the Board that no adverse synergistic effect
will result from the interaction of the overlapping mixing zones.
H. A mixing zone shall be allowed only if the applicant demonstrates
to the satisfaction of the Board that the best control technology
economically feasible is being used or proposed.
Standards for Granting Initial Mixing Zone (IMZ)
No Initial Mixing Zone (IMZ) shall be defined and authorized
unless the applicant proposed an IMZ that meets all of the following
requirements:
A. The largest dimension of the geometric figure that describes the
IMZ shall not exceed four hundred feet (400 ft.) in all waters
except ocean waters. For discharges to ocean waters, the IMZ
shall be the volume of the mixture of wastewater and ocean water
ascending from the outfall until the mixture becomes equal to the
surrounding ocean water.
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PUERTO RICO (Cont'd)
B. The IMZ is mainly caused by the turbulence due to the momentum of
the discharge and to the density gradient.
C. Satisfies the provisions of Section 5.1 of this Regulation.
D. At the boundary of the IMZ the water shall comply with all water
quality standards set forth for the corresponding
classification, except as provided in subsection 5.2.2.
Notwithstanding requirement (D) of the Subsection 5.2.1 the Board
may define and authorize an IMZ only for those pollutants which
comply with the water quality standards as the boundary of the
proposed zone. In this case, the remaining pollutants not in
compliance with water quality standards at the boundary of the
proposed IMZ shall be subject to the provisions of Section 5.3.
In the proposal, the applicant shall take into consideration such
aspects as the physico-chemical characteristics of the discharge and
of the receiving body of water, and use the best currently available
simulation or determination technique to describe the proposed IMZ.
Within 60 days of the submittal of an approvable application, the
Board shall define and authorize an IMZ.
Final Mixing Zones (FMZ)
Condition Precedent
A. When deemed absolutely necessary, the Board may allow a final
mixing zone (FMZ) with respect to a discharge. The Board shall
consider a proposed FMZ only if the applicant demonstrates to the
satisfaction of the Board that requirement (D) of Subsection
5.2.1 cannot be met, and all water quality standards set forth in
Article II can be met at the edge of the FMZ.
B. The Board shall notify the applicant of its determination with
respect to the need of a FMZ.
C. Upon receiving an affirmative decision by the Board, the
applicant may proceed to submit a proposed FMZ.
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PUERTO RICO (Cont'd)
Standard for Granting Final Mixing Zone
A. The Board shall define and authorize a FMZ only if the following
conditions are met:
1. The proposed FMZ complies with the provisions of Section 5.1.
2. The largest dimension of the geometric figure that describes
the proposed FMZ does not exceed four thousand feet (4,000
ft.).
3. The most advanced mathematical modeling method approved by
the Board for the dispersion of effluents in a body of water
was used in developing the propsoed FMZ.
4. The proposed FMZ is not located in spawning areas, shellfish
growing areas, or any other areas which the Board may deem as
inappropriate for the establishment of an FMZ.
5. In developing the proposed FMZ, the size, nature and
classification of the receiving body of water shall be taken
into consideration.
6. The proposed FMZ shall explicitly describe and define any
IMZ's included therein.
VIRGIN ISLANDS
The need, location, size and depth of the mixing zones in surface
waters and estuaries shall be established according to the following
mixing zone criteria and boundaries.
Mixing Zone Criteria
A. Mixing zones shall be provided solely for mixing. Mixing must be
accomplished as quickly as possible through the use of devices
which insure that the waste is mixed with the allocated dilution
water in the smallest practicable area.
B. For the protection of aquatic life resources, the mixing zones
must not be used for, or be considered as, a substitute for waste
treatment facilities.
C. At the boundary of the mixing zone the water should comply with
all the water quality standards set forth for its classification.
If, after complete mixing with the available dilution water,
these requirements are not met, the effluent must be adequately
pre-treated until the standards are met.
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VIRGIN ISLANDS (Cont'd)
D. No conditions shall be permitted to exist within the mixing zone:
(1) that are rapidly lethal (i.e. exceed the 96 hour median
tolerance limit) to locally important and desirable indigenous
aquatic life; (2) that prohibit planktonic organisms from being
carried through the mixing zone. These organisms will be exposed
to its conditions only for the period of time required to drift
through the mixing zone and will survive without undue damage or
stress while they are passing through.
E. Maximum vertical dispersion of wastewater discharge flow shall be
provided for in the mixing zone.
F. Mixing zones shall not intersect spawning or nursery areas,
migratory routes, water intake nor mouths of rivers.
G. Suspended solids in wastewaters being discharged shall not settle
in measurable amounts in the mixing zones.
Mixing Zone Boundaries
A. The mixing zone must be located in such manner as to allow at all
times, passageways for the movement or drift of the biota
(pelagic or invertebrate organisms). The width of the mixing
zone and the volume of flow in it shall depend on and will be
determined by the nature of the water current and/or the estuary.
The area, depth, and volume of the flow must be sufficient to
provide a usable and desirable passageway for fish and other
aquatic organisms.
B. The passageway must contain 75 percent of the cross-sectional
area and/or volume of flow of the estuary, and should extend to
at least 50% of the width.
C. A mixing zone shall not overlap with an adjacent mixing zone.
(Thermal Mixing Zone can be found in Temperature Digest)
TRUST TERRITORIES - No reference to zones of mixing.
55 o U. S. GOVERNMENT PRINTING OFFICE 19SO 720-016/5965
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