GUIDANCE FOR NPDES PERMIT ISSUANCE
FEBRUARY 1994
PREPARED BY:
U.S. ENVIRONMENTAL PROTECTION AGENCY
REGION IX
WATER MANAGEMENT DIVISION
PERMITS AND COMPLIANCE BRANCH
PERMITS ISSUANCE SECTION
WITH THE ASSISTANCE AND COOPERATION OF
CALIFORNIA STATE WATER RESOURCES CONTROL BOARD
AND
CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARDS

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TABLE OF CONTENTS
I.	Introduction 		1
II.	Permit Application 		3
III.	Drafting the Permit 		3
A.	Water Quality-Based Effluent Limitations 		4
1.	Basis for Implementing Water Quality-
Based Effluent Limitations 	 4
2.	Reasonable Potential/Selection of
Pollutants 	 5
3.	Establishing Water Quality-Based
Effluent Limitations 	 7
4.	Interim Permit Limitations 	 9
B.	Technology-Based Effluent Limitations
for Industrial Sources 	 10
1.	Effluent Limitations Guidelines 	 11
2.	Effluent Limitations Based on Best
Professional Judgement 	 11
C.	Technology-Based Effluent Limitations for POTWs .... 12
IV.	Special Considerations 	 12
A.	Water Quality-Based Effluent Limitations
for Metals 	 12
1.	Expression of Aquatic Life Criteria
for Metals 	 12
2.	Translating Dissolved Criteria into Total
Recoverable Permit Limitations 			 13
3.	Including Translator Requirements in
NPDES Permits 	 13
B.	Whole Effluent Toxicity Testing in NPDES Permits ... 14
1. Basis for Whole Effluent Toxicity 	 14
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2.	Toxicity Requirements 	 14
3.	Types of Toxicity Testing 	 16
Glossary 	 19
Appendix A: Establishing Reasonable Potential 	 A-l
Appendix B: Determining Water Quality-Based Effluent
Limitations 	 B-l
Appendix C: Determining Dilution for Oceans and Estuaries ... c-1
Appendix D: Case Examples 	 D-l
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GUIDANCE FOR NPDES PERMIT ISSUANCE
I. INTRODUCTION
This guidance was prepared at the request of the California
State Water Resources Control Board (SWB) and the Regional Water
Quality Control Boards (RWB) in anticipation of a State Superior
Court Judgement invalidating the Inland Surface Water Plan (ISWP)
and the Enclosed Bays and Estuaries Plan (EBEP). The primary aim
is to provide guidance for issuing NPDES permits in the absence of
State numeric water quality objectives for toxics.
The process for preparing NPDES permits, regardless of the
availability of State promulgated numeric water quality objectives,
is fundamentally the same. The difference lies in the documents
upon which the permit writer relies when making judgements
regarding the appropriate bases for permit requirements and the
supporting documentation (i.e., statement of basis or fact sheet)
necessary to defend such requirements. Where water quality
standards are available, the numeric objectives therein are applied
using available permitting methods. The bases for these permit
requirements are not subject to challenge at the permitting stage,
although the method of translating the objective to a permit
requirement may be appealed. In the absence of standards
containing numeric water quality objectives, the same permitting
methods are used to derive permit requirements. However, the bases
for such permit requirements are no longer the water quality
objectives, but available federal criteria and other scientific
information that may be (or have been) used to develop
state-specific numeric water quality objectives. The rationale for
selecting a numeric criterion as basis for the permit requirement,
in addition to methods used to translate a criterion to a permit
requirement, are now subject to challenge. Therefore, the
rationale must be thoroughly discussed in the statement of basis,
fact sheet, or findings that accompany a permit.
Consequently, this guidance focuses on the methods available
for preparing NPDES permits. The proper and consistent use of
these methods throughout the State will strengthen the permit
process, thereby, making it easier to issue permits in the absence
of adopted State numeric water quality objectives. This guidance
is equally applicable whether or not the ISWP and EBEP are in
effect. However, without the ISWP and EBEP, "reasonable potential"
(see III.A.2 and Appendix A) is used to establish which pollutants
should be limited in the permit, although both state-wide plans
have implementation provisions which specify when effluent
limitations should be established for a pollutant. The "standards-
to-permit" process set forth in this guidance is otherwise
applicable and consistent with the Clean Water Act (CWA) and
implementing NPDES regulations. Other approaches may also be

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acceptable provided they are consistent with the CWA and NPDES
regulations.
It should be noted that if the ISWP and EBEP are invalidated,
the federal criteria promulgated for California through the
National Toxics Rule (NTR) will apply. The NTR specifies numeric
criteria for 40 toxic pollutants. These criteria must be used as
the basis for effluent limitations. Effluent limitations for other
toxic pollutants will have to be developed using other references
(e.g., federal criteria and other scientific information as
discussed in this guidance).
II. PERMIT APPLICATION
Persons who intend to discharge to waters of the State are
required to provide the information necessary to develop
appropriate provisions of a permit. The discharger should be made
aware of the information needed to prepare a permit at the earliest
possible date. If necessary, the State should exercise its legal
authority to formally request information. The intent is to have
all information provided in, or along with, the permit application
that must be submitted at least 180 days prior to the expiration of
an existing permit, or commencement of a discharge [see 40 CFR
122.21(c) and (d)]. Failure to submit the required application at
least 180 days prior to the expiration date results in termination
of the permit. Thereafter, any discharge is unauthorized. A
permit may be administratively extended only if a complete and
timely application is submitted.
If, for valid reasons, a discharger is unable to provide all
of the necessary monitoring data prior to permit issuance, the
issuance of the permit need not be delayed. The State may issue
the permit with a provision that the discharger conduct the
necessary monitoring under the permit. A reopener clause must also
be included in the permit so that appropriate conditions can be
added where monitoring data indicate that the discharge has
reasonable potential to cause or contribute to the exceedance of an
ambient water quality standard or objective. In general, a
reopener clause may be included in a permit to address new
information and regulations. The latter includes new water quality
standards and effluent limitations guidelines (ELGs).
III. DRAFTING THE PERMIT
Effluent limitations are defined by NPDES regulations (see 40
CFR 122.2) as any restriction imposed by a State or EPA on
quantities, discharge rates and concentrations of pollutants which
are discharged from point sources into waters of the United States.
Effluent limitations are either technology-based or water
quality-based. In practice, technology-based requirements will
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define achievable treatment levels for a particular pollutant or
class of pollutants (e.g., lime precipitation would be the basis
for technology-based requirements for metals. These technology-
based requirements are compared with water quality-based
requirements for each pollutant. The more stringent requirement is
included as an effluent limitation in the permit.
A. WATER QUALITY-BASED EFFLUENT LIMITATIONS
The regulatory basis for establishing water quality-based
effluent limitations (WQBELs) is set forth in 40 CFR 122.44(d) of
the NPDES regulations. This regulation requires that NPDES permits
contain requirements in addition to, or more stringent than,
promulgated effluent limitations guidelines or standards under
sections 301, 304, 306, 307, 318 and 405 of the CWA necessary to
achieve water quality standards established under section 303 of
the CWA, including narrative criteria for water quality. Effluent
limitations must be established for pollutants (either
conventional, nonconventional or toxic) that are, or may be
discharged at levels which cause, have the reasonable potential to
cause, or contribute to an excursion above any State water quality
standard, including narrative objectives for water quality [see 40
CFR 122.44 (d) (1) (i) ] .
The requirement to impose water quality-based effluent
limitations applies regardless of whether pollutant specific
numeric water quality objectives have been established in State
water quality standards. In such instances a narrative objective,
such as the statement "all waters shall not contain toxics in toxic
amounts," would be sufficient basis to develop permit limitations
for toxic pollutants to protect beneficial uses designated for the
receiving water body. Depending on the subsection(s) of 40 CFR
122.44 which apply to the particular discharge under consideration,
effluent limitations may be pollutant specific, based on whole
effluent toxicity (WET), or a combination of both.
1. BASES FOR IMPLEMENTING WATER QUALITY-BASED EFFLUENT
LIMITATIONS
State water quality standards containing numeric water quality
objectives are the primary basis for establishing water
quality-based effluent limitations. In the absence of State
numeric water quality objectives, the permit writer must rely on
available information to identify the receiving water body
beneficial uses and the ambient water quality, including numeric
protective levels (NPLs), necessary to attain such uses. The
permit writer must then rely on available methods to convert the
NPLs to effluent limitations, taking into consideration factors
enumerated in the regulations [see 40 CFR 122.44(d)(ii)].
Available information includes State water quality plans and/or
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documentation supporting the applicability of objectives to a water
body, technical literature, and federal numeric ambient water
quality criteria for the protection of aquatic life and human
health. In California, State and federal documents that may be
used as bases for developing and supporting water quality-based
effluent limitations include:
•	40 CFR 122.44 - Establishing limitations, standards, and
other permit conditions applicable to State NPDES
programs.
•	National Toxics Rule (57 FR 60848, 22 December 1992; NTR)
which specifies numeric criteria for 40 toxic pollutants.
In absence of ISWP and EBEP objectives, NTR numeric
criteria must be used to establish effluent limitations.
•	Water Quality Control Plans (Regional Basin Plans) which
specify beneficial uses, and narrative and numeric
objectives for Regional water bodies.
•	Supporting technical documentation for -he California
Inland Surface Waters Plan (91-12 WQ, 1991, and subsequent
amendments); the California Enclosed Bavs and Estuaries
Plan (91-13 WQ, 1991, and subsequent amendments); and the
California Ocean Plan (SWRCB, 1990, and subsequent
amendments; Ocean Plan).
•	Individual federal water quality criteria documents;
criteria are summarized in Quality Criteria for Water (EPA
440/5-86-001, 1986; Gold Book) and 40 CFR 131.
•	Technical Support Document for Water Oualitv-based Toxics
Control (EPA/505/2-90-001, March 1991; TSD).
NPDES regulations provide the basis for establishing the
permit limitations. The NTR, State water quality plans and
supporting technical documents, and federal criteria documents,
specify ambient numeric objectives or criteria that are used to
establish NPLs. These NPLs should be achieved in the receiving
water body to protect beneficial uses identified in the Regional
Basin Plans. Finally, the TSD presents the method for converting
effluent and ambient data, and ambient federal criteria into
effluent limitations. These documents along with other relevant
information must be used to develop and justify water quality-based
effluent limitations included in the permit.
2. REASONABLE POTENTIAL/SELECTION OF POLLUTANTS
When considering water quality-based effluent limitations, the
permit writer should first select the pollutants for which effluent
limitations must be established. The permit writer can choose to
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establish limitations for all pollutants covered by State water
quality plans or federal criteria. However, a more reasonable and
defensible approach is to selectively establish effluent
limitations for those pollutants which may create or contribute to
ambient water quality problems. This latter approach should reduce
obstacles for issuing permits, especially in the absence of
specific State numeric water quality objectives. Other approaches,
such as those set forth in the ISWP and EBEP are acceptable. These
plans provide that where the State is satisfied that any effluent
pollutant does not occur, or is not likely to occur in a discharged
effluent, the State may elect not to establish effluent
limitations.
NPDES regulations at 40 CFR 122.44(d)(1)(i) require the
establishment of an effluent limitation for any pollutant which is
or may be discharged at a level that "will cause, have a reasonable
potential to cause, or contribute to an excursion above any State
water quality standard, including State narrative criteria for
water quality." In determining the need for an effluent
limitation, the permit writer must also consider existing controls
on other point and nonpoint sources, the variability of the
pollutant or pollutant parameter in the discharge, the sensitivity
of the test species (for WET) and, where appropriate, the mixing of
the discharge in the receiving water [see 40 CFR 122.44(d)(ii)].
Effluent limitations must be included, as appropriate, for specific
pollutants and/or WET. No effluent limitation is required for any
pollutant which is not present in the discharge, or will not cause,
have the reasonable potential to cause, or contribute to an
excursion above water quality objectives.
Reasonable potential is determined using a sequential (i.e.,
tiered) process (see Appendix A and TSD, Chapter 3). In the first
step, the steady-state mass balance equation is used to project the
maximum resultant in-stream concentration for a pollutant after
complete mixing under critical flow conditions. If the projected
in-stream concentration is greater than the applicable NPL (i.e.,
the objective, criteria, or standard necessary to attain the
designated beneficial uses), then effluent limitations must be
established for that pollutant. If the projected in-stream
concentration is less than the applicable NPL, the permit writer
must then exercise judgement as to whether reasonable potential
exists.
In the second step, historical effluent data for the pollutant
of concern and appropriate statistics derived from those data are
used to statistically estimate the maximum effluent concentration.
In practice, these statistics are used to calculate an uncertainty
multiplier that adjusts the maximum observed effluent concentration
to a probability-based maximum concentration (see TSD, Chapter
3.3.2, p. 52). This higher concentration is then used in the mass
balance equation to project the maximum resultant in-stream
concentration for the pollutant after complete mixing (see Appendix
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A). Reasonable potential is established if the projected in-stream
concentration exceeds the NPL. If reasonable potential is
established for a pollutant, water quality-based effluent
limitations must be included in the permit. Where reasonable
potential is not demonstrated, water quality-based effluent
limitations need not be included in the permit.
When effluent data are not available or are insufficient,
reasonable potential determinations may still be made and effluent
limitations included in the permit. In such instances, reasonable
potential determinations should consider the type of discharger,
existing knowledge regarding the use, generation, or presence of a
pollutant at the facility or contributing facility (in the case of
POTWs), and risks posed by the discharge. Permits should also
require additional monitoring and a reopener clause in cases where
insufficient or no information are available upon which to
adequately evaluate reasonable potential (see TSD, Chapter 3.2, pp.
50-51).
The tiered methodology used to evaluate reasonable potential
with and without facility-specific effluent and receiving water
quality data is outlined in Appendix A.
For ocean discharges, sufficient dilution is necessary to
assure compliance with numeric objectives for toxic pollutants set
forth in the Ocean Plan. Under the Ocean Plan, factors to consider
that influence the initial dilution achievable for ocean outfalls
include: 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. These factors are input parameters
for standard dilution models used to calculate the minimum initial
dilution (i.e., lowest average initial dilution within any single
month of the year) for ocean outfalls. The calculated minimum
initial dilution and maximum effluent pollutant concentrations
(either statistically unadjusted or adjusted for uncertainty) may
then be used to determine whether any pollutant which is or may be
discharged will cause, has the.reasonable potential to cause, or
contributes to excursions above Ocean Plan objectives. This process
is outlined in Appendix C.
Due to the complex circulation patterns observed in enclosed bays
and estuaries, tracer or dye studies conducted during conditions
that approach critical flows are recommended to determine the areal
extent of mixing in a water body, the boundary where the effluent
has completely mixed with the ambient water, and the dilution that
results from the mixing (see Appendix C and TSD, Chapter 4).
3. ESTABLISHING WATER QUALITY-BASED EFFLUENT LIMITATIONS
After the appropriate water quality NPL is determined (see
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III.A.1) and reasonable potential is established for a pollutant
(see III.A.2), the NPL must be converted to a permit limitation
using the usual permitting methods and appropriate receiving water
conditions. Where mixing zones are allowed, appropriate dilution
factors may be used to calculate the effluent limitation. However,
if mixing zones are not explicitly allowed by State water quality
standards, the NPL is applied directly as an end-of-pipe effluent
limitation.
In converting NPLs to effluent limitations, averaging times
and the use of mass and/or concentration limits should be
reconciled. NPDES regulations require that all numeric effluent
limitations be expressed, unless impracticable, as both daily
maximum and monthly average values (for all discharges other than
POTWs), or as weekly average and monthly average values (for POTWs)
[see 40 CFR 122.45(d)]. For data tracking purposes, it is
important that daily limits be expressed as "daily maximum," rather
than "daily average." The regulations further require that
pollutants must have mass-based limits except where such limits are
impractical or inappropriate, as set forth at 40 CFR 122.45(f).
Where effluent dilution is less than 100:1, both mass and
concentration effluent limits should be specified (see TSD, pp.
110-111). This section of the permit guidance outlines the final
step in the "standards-to-permit" process, where 1-hour average
(acute) and 4-day average (chronic) NPLs (e.g., federal criteria)
are converted to daily maximum and monthly average effluent
limitations. This methodology first calculates waste load
allocations (WLAs) for those pollutants where reasonable potential
has been established. Maximum daily limitations (MDL) and average
monthly limitations (AML) required to meet the most limiting WLA
are then calculated using statistical procedures outlined in
Appendix B (also, see TSD, Chapter 5).
As used herein, WLA is the maximum allowable effluent
pollutant concentration or load that will comply with the
applicable NPL. The WLA is calculated using the steady-state mass
balance equation [QdCd + QsCs = QrCr, where Q is flow; C is
pollutant concentration; and subscripts refer to upstream (s),
discharge (d), and downstream after complete mixing (r)]. Thus,
where mixing zones are not allowed, the WLA becomes the appropriate
NPL applied at the end-of-pipe. The WLA is not premised upon the
existence of a total maximum daily load (TMDL); however, where a
TMDL exists, the WLA is that portion of the allowable load assigned
to a particular discharge on the basis of procedures specified in
the State water quality standards.
WLA calculations are always made assuming critical conditions
[i.e., l-day low flow with a 10 year recurrence interval (1Q10) for
calculating acute WLAs; consecutive 7-day low flow with a 10 year
recurrence interval (7Q10) for calculating chronic WLAs; and the
harmonic mean flow for calculating human health WLAs]. Where data
are available, background concentrations should also reflect
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critical flow conditions.
In this guidance, the steady-state mass balance equation is
used to determine WLAs. This approach, coupled with conservative
assumptions, should be protective of water body beneficial uses.
The permit should also require both effluent and ambient monitoring
as an on-going assessment of the impact of the discharge on the
receiving water. Where circumstances dictate, alternative models
(e.g., dynamic) that estimate dilution or fate of effluent
pollutants are available (see TSD, Chapter 4). The use of dynamic
models may be a more rigorous method for calculating WLAs.
However, they require large amounts of quality data. If these data
are not available, then dynamic model calculate inaccurate
projections. Under such conditions, the steady-state mass balance
equation is recommended.
Where reasonable potential analyses project an excursion above
an applicable NPL, WLAs for that pollutant are determined at each
effect level (i.e., acute, chronic and human health) (see Appendix
B).
Once a WLA is determined for each effect level, the long term
average (LTA) discharge conditions required to meet these WLAs at
a specified confidence level (i.e., 99%) are calculated using
statistics derived from appropriate effluent data. The LTA is a
discharge performance level that should be achieved to ensure that
effect level WLAs will not be exceeded at least 99% of the time
(see Appendix B).
Finally, using the lowest (most limiting) effect level LTA,
a maximum daily limitation (MDL) and an average monthly limitation
(AML) are calculated using statistics derived from appropriate
effluent data.
This methodology is detailed in Appendix B, and summarized in
the TSD (see Chapter 5).
4. INTERIM PERMIT LIMITATIONS
The CWA at section 301(b)(1)(C) requires that water quality
standards be met by July 1, 1977. Consequently, subsequent changes
to State water quality standards must be met at the date of permit
issuance unless authorization (i.e, a compliance schedule) is
included in either State-wide water quality plans or basin plans.
A permit cannot contain a compliance schedule unless this condition
is met. Without this condition, the permit writer may use an
appropriate enforcement mechanism to address the noncompliance or
anticipated noncompliance (e.g., the issuance of an administrative
order concurrent with the permit).
Where compliance schedules are authorized, interim effluent
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limitations should be included in. the permit or administrative
order, as appropriate, unless, for valid reasons, data are
insufficient. Interim permit limitations may be developed as
follows:
•	Where the permittee is in compliance with existing
effluent limitations, these values may be specified as
interim limitations in the reissued permit. Otherwise,
they are the least stringent limitations that can appear
in subsequent permits.
•	Where effluent data are available, daily maximum and
monthly average interim limitations based on facility
performance should be implemented in the reissued permit.
The appropriate statistical methodology should be used to
calculate performance-based effluent limitations (see TSD,
Chapter 5, Table 5.2, p.103, and Appendix E). However,
performance-based interim limitations can not be less
stringent than limitations specified in the previous
permit, unless antibacksliding requirements are met. If
effluent concentrations exceeded previous permit
limitations, the permittee would not have been in
compliance with the previous permit. Hence, before the
permit is reissued, the noncompliance under the previous
permit must be addressed through appropriate enforcement
action. If the permittee made good faith effort to comply
(e.g., installed and properly operated appropriate
treatment), previous effluent limitations may be relaxed,
provided that applicable NPLs are met in the receiving
water body (see Great Lakes Initiative, 57 FR 20803, 16
April 1993).
Where data are lacking and reasonable potential cannot be
evaluated, effluent monitoring may be required as a condition of
the reissued permit. In this situation, since reasonable potential
has not been established, final effluent limitations need not be
specified in the permit at the time of issuance. A permit reopener
clause [see 40 CFR 122.44(c)] allowing for the implementation of
water quality-based effluent limitations, if effluent monitoring
data establishes that the discharge shows reasonable potential to
exceed the NPL, should be included in the permit (see TSD, Chapter
3, p. 51).
B. TECHNOLOGY-BASED EFFLUENT LIMITATIONS FOR INDUSTRIAL
SOURCES
As noted previously, the CWA requires compliance with the more
stringent of technology-based or water quality-based requirements.
Consequently, the permit writer should evaluate both requirements
when developing effluent limitations.
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For industrial sources, section 301(b)(2) of the CWA requires
by March 31, 1989, the application of Best Available Treatment
Economically Achievable (BAT) for toxic and nonconventional
pollutants, and Best Conventional Pollutant Control Technology
(BCT) for conventional pollutants. BAT and BCT replace Best
Practical Control Technology (BPT) which was to have been achieved
by July 1, 1977. Toxic pollutants (see 40 CFR 401.15) include 126
pollutants (primarily metals and organics) of particular concern.
Conventional pollutants are defined at 40 CFR 401.16 and include
pH, BODc, oil and grease, suspended solids, and fecal coliform.
All other pollutants (e.g., nutrients and WET) are classified as
nonconventional pollutants. Since the deadline has passed for
compliance with BAT/BCT effluent limitations, all newly issued
permits must require immediate compliance with appropriate BAT/BCT
limitations.
Section 306 of the CWA requires compliance with New Source
Performance Standards (NSPS) for industrial facilities classified
as new sources (see 40 CFR 122.2). NSPS are another category of
technology-based effluent limits which may be more stringent than
BCT/BAT to reflect the greater opportunities for incorporating
pollution control technologies into new facilities.
1. EFFLUENT LIMITATIONS GUIDELINES
As provided by section 304(b) of the CWA, EPA has promulgated
effluent limitations guidelines for 51 categories of industrial
dischargers. These guidelines are based on analyses conducted by
EPA of the technological and financial capacity of these industries
to control pollutants in their discharges.
2. EFFLUENT LIMITATIONS BASED ON BEST PROFESSIONAL JUDGEMENT
Effluent limitations guidelines have not been promulgated by
EPA for all categories of industries. In addition, promulgated
guidelines may not address all sources of wastewater from a
particular facility. In such cases, the permit writer must develop
appropriate technology-based limits based on best professional
judgment (BPJ), as authorized by section 402(a)(1) of the CWA and
NPDES regulations at 40 CFR 122.43-44 and 125.3.
When developing BPJ limits, the same factors must be
considered by the permit writer as are considered by EPA in the
formal promulgation of effluent limitations guidelines. These
factors are set forth in section 304(b)(2)(B) of the CWA and
include: the age of equipment and facilities involved; the process
employed; the engineering aspects of the application of various
types of control techniques; process changes; the cost of achieving
such effluent reduction; non-water quality environmental impacts
(including energy requirements); and other such factors as are
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deemed appropriate. Various BPJ permitting methods are available
to assist in the development of permit limitations (see Training
Manual for NPDES Permit Writers (EPA 833-B-93-003, March 1993,
Chapter 4).
Additional technology-based limitations based on BPJ may be
included in NPDES permits in the form of best management practices
[BMPs; see 40 CFR 122.44 (k) and 125.100-102]. BMPs generally refer
to operating or maintenance procedures designed to reduce
pollutants in discharges (see Best Management Practices Guidance
Document. June 1981).
C. TECHNOLOGY-BASED EFFLUENT LIMITATIONS FOR POTWs
Publicly-owned treatment works (POTWs) are also required to
comply with technology-based effluent limitations which are
referred to as "secondary treatment." These effluent limitations
are set forth at 40 CFR 133 and include limits for B0D5, suspended
solids and pH. The regulations also provide that under certain
circumstances (e.g., waste treatment ponds), permits for POTWs may
be written with less stringent "equivalent to secondary" effluent
limitations or alternate State requirements.
IV. SPECIAL CONSIDERATIONS
A. WATER QUALITY-BASED EFFLUENT LIMITATIONS FOR METALS
1. EXPRESSION OF AQUATIC LIFE CRITERIA FOR METALS
After reviewing the available information on metals toxicity,
EPA recently recommended implementing for metals the dissolved
measurement as the basis to set and measure compliance with water
quality standards; however, EPA also recognizes that the total
recoverable measurement may be used by States to satisfy
appropriate risk management decisions [see Policy Guidance for
Aquatic Life Metals Criteria, EPA Office of Water, 1 October 1993
(Metals Policy)]. In the absence of adopted State water quality
objectives for metals, federal metals criteria or other rigorously
developed criteria should be used to develop NPLs.
Federal metals criteria were developed using total recoverable
measurements. If a State chooses to use the dissolved measurement
for determining NPLs and effluent limitations, the appropriate
correction factor must be used to convert from total recoverable to
dissolved. EPA's "Guidance document on dissolved criteria,
expression of Aquatic Life Criteria, October 1993" (see Metals
Policy attachment) presents correction factors for converting
metals criteria from total recoverable to dissolved. These
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correction factors are the simple ratios of the two metals
measurements and were derived from laboratory data used to
establish federal metals criteria.
2. TRANSLATING DISSOLVED CRITERIA INTO TOTAL RECOVERABLE
PERMIT LIMITATIONS
NPLs for metals should be expressed as dissolved, provided
that the State has accepted the use of dissolved in lieu of total
recoverable. However, NPDES regulations require that limitations
for metals in permits be expressed as total recoverable, except
under those conditions set forth at 40 CFR 122.45(c). These
include circumstances where technology-based effluent limitations
specify dissolved, or where the analytical method employed only
measures dissolved. Thus, where NPLs for metals are expressed as
dissolved metals, it may be necessary to develop translators to
convert dissolved NPLs to total recoverable effluent limitations.
The translator is based on the relationship between the two metal
measurements in the receiving water body. Consequently, these
translators should be developed using site-specific ambient data.
When developing total recoverable effluent limitations for
metals, the permit writer should assume that the relationship
between total recoverable and dissolved is 1:1 (i.e., translator =
1). If the applicant requests the opportunity to develop
site-specific translators, the NPDES permit should include
conditions specifying required outcomes and a schedule for
completion of the translator study. During the translator study,
the permit writer may apply interim metals limits.
National data are available for developing translators;
however, this approach should be used only for establishing interim
effluent limitations. Final effluent limitations should be based
on translators that reflect partition coefficients developed using
site-specific ambient water chemistry. Where ambient data are
insufficient, a monitoring program must be undertaken by the
permittee to acquire data necessary to develop the translator (see
Guidance Document on Dynamic Modeling and Translators. August 1993;
and Technical Guidance Manual for Performing Waste Load
Allocations. Book II: Streams and Rivers, Chapter 3 - Toxic
Substances, EPA-440/4-84-022, June 1984).
3. INCLUDING TRANSLATOR REQUIREMENTS IN NPDES PERMITS
A period of up to 2 years is recommended for a translator
study. Minimum data requirements necessary for the study are
outlined in the Metals Policy. A study plan should be completed
within the first 3 months of permit issuance, followed by 12 months
of monitoring and data collection. Up to 3 months can be allowed
for data analysis and submission of a report that includes
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recommendations for site-specific translators and where
appropriate, a request for permit modification. The final report
should also describe: 1) whether compliance with translated metals
limits can be achieved; and if not 2) what is required to achieve
compliance; and 3) where applicable, how changes resulting from the
study would satisfy State antidegradation requirements. Where
appropriate, a permit modification should be issued within 6 months
following the modification request.
The permit must clearly specify the form of metals limitations
(i.e., total, total recoverable, or disolved).
B. WHOLE EFFLUENT TOXICITY TESTING IN NPDES PERMITS
1. BASIS FOR WHOLE EFFLUENT TOXICITY
The whole effluent approach to water quality-based toxics
control for the protection of aquatic life involves the use of
acute and chronic toxicity tests to measure the toxicity of
effluents or ambient water. The Whole Effluent Toxicity (WET)
approach is important because specific NPLs for all pollutants have
not been developed, and complex mixtures of effluent pollutants may
have toxic effects that specific NPLs do not address. The WET
approach allows the permit writer to require achievement of the
narrative standard, "no toxics in toxic amounts" [see CWA section
101(a)(3)], that is applicable to all waters of the United States
(see 40 CFR 122.2). The CWA clearly authorizes the use of toxicity
testing and WET limitations in NPDES permits (see TSD, Appendix
B-l) .
When determining the need for a WET permit limit, the permit
writer must consider those conditions specified under 40 CFR Part
122.44(d)(1)(ii). WET permit limits are required when a discharge
exceeds, has the reasonable potential to exceed, or contributes to
excursions above narrative and/pr numeric WET criteria [see 40 CFR
Part 122.44(d)(1)(iv) and (v)]. However, when a permittee has
identified the toxicant(s) causing toxicity and numeric limitations
have been established for the pollutant(s) , numeric limitations for
WET need not be placed in the permit. Although, continued
monitoring for toxicity will still be required. Reasonable
potential determinations are made as discussed in III.A.2.
2. TOXICITY REQUIREMENTS
In the absence of specific numeric water quality objectives
for acute and chronic toxicity, the narrative criterion "no toxics
in toxic amounts" applies. Achievement of the narrative criterion,
as applied herein, means that ambient waters shall not demonstrate
for acute toxicity: 1) less than 90% survival, 50% of the time,
14

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based on any monthly median, or 2) less than 70% survival, 10% of
the time, based on any monthly median. For chronic toxicity,
ambient waters shall not demonstrate a test result of greater than
1 TUc.
The chronic toxicity limitation is to be expressed as 1 TUc
as a daily average. Any one test that shows greater than 1 TUc
would be considered a violation. Immediately upon exceedance of
the limitation the permittee shall conduct a Toxicity Reduction
Evaluation (TRE) and where appropriate, a Toxicity Identification
Evaluation (TIE) to identify the cause(s) of toxicity. The permit
shall also require the submission of a TRE workplan to the State
within 60 days of permit (re)issuance.
At the discretion of the permit writer the limitation can be
expressed as 1 TUc as a monthly median. This discretion is
dependent on consideration of the permittees ability and
willingness to conduct multiple testing during a month. The
expression 1 TUc as a monthly median addresses the dischargers
concern of one sample being a violation. This expression allows
the permittee the option to conduct additional tests within a
month, and have those additional tests be used to determine
compliance.
Effluent limitations shall be calculated to achieve these NPLs
using the methods discussed in III.A.3. Where mixing zones are
allowed, it may be used for calculating limitations for chronic
toxicity in effluent dominated waters. The chronic toxicity NPL of
1 TUc may be met at the end of the mixing zone to allow for
dissipation of the effects, due to volatization, or chemical or
physical changes. (The behavior of chlorine and ammonia best
illustrate the applicability of mixing zones to effluent dominated
waters). The dilution will have to be determined empirically by
the discharger through a monitoring program designed to determine
the dissipation that occurs in the receiving waterbody. The
dilution credit would then be applied to the 1 TUc NPL to determine
the effluent limitations.
Published EPA TRE and Toxicity Identification Evaluation (TIE is
component of the TRE) guidance manuals include:
•	Generalized Methodology for Conducting Industrial Toxicity
Reduction Evaluations fTREs) (EPA/600/2-88/070, 1989);
•	Toxicity Reduction Evaluation Protocol for Municipal
Wastewater Treatment Plants (EPA/600/2-88/062, 1989);
•	Methods for Aquatic Toxicity Identification Evaluations;
Phase	I	Toxicity	Characterization	Procedures
(EPA/600/6-91/003, 1991);
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•	Toxicity Identification Evaluation: Characterization of
Chronically Toxic Effluents. Phase I (EPA/600/6-91/005,
1991);
•	Methods for Aquatic Toxicity Identification Evaluations:
Phase II Toxicity Identification Procedures
(EPA/600/R-92/080, 1992); and
•	Methods for Aquatic Toxicity Identification Evaluations:
Phase III Toxicity Confirmation Procedures (EPA/R-92/081,
1992).
3. TYPES OF TOXICITY TESTING
The two types of toxicity tests are acute and chronic. An
acute toxicity test is defined as a static-renewal, static non-
renewal or flow-through test of 96-hours or less in duration with
lethality as the measured endpoint. Acute toxicity can be reported
as a lethal concentration (LC), or a no observable adverse effect
concentration (NOAEC).	NOAEC is the highest effluent
concentration at which survival is not significantly different from
the control. LC is the toxicant concentration that would cause
mortality to a certain percentage of the test organisms (e.g. LC
10) .
Traditionally, chronic toxicity tests are full life-cycle or
shortened tests of about 3 0 days. However, the duration of most
chronic toxicity tests has been shortened to 7 days by focusing on
the most sensitive life-cycle stages (e.g., juveniles instead of
adult fish). The measured endpoint can be reduced fertilization,
reproduction, growth and/or mortality. Chronic toxicity endpoints
can be recorded as the no observed effect concentration (NOEC), the
lowest observed effect concentration (LOEC), or the effect
concentration (EC). The NOEC is the highest concentration of
toxicant, in terms of percent effluent, to which the test organisms
are exposed that causes no observable adverse effect. The LOEC is
the lowest concentration of toxicant to which the test organisms
are exposed that causes an observed effect. The EC is the toxicant
concentration that would cause an adverse effect on a certain
percentage of the test organisms (e.g., EC10 or EC50).
WET permit limits should be expressed as toxic units (TU).
Chronic toxicity is expressed as TUc = 100/NOEC.
EPA has published WET guidance and recommended toxicity test
protocols in four manuals:
• Technical Support Document for Water Oualitv-based Toxics
Control (EPA/505/2-90/001, 1991);
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•	Methods for Measuring Acute Toxicity of Effluents and
Receiving Waters to Freshwater and Marine Organisms
(EPA/600/4-90/027, 1991);
•	Short-Term Methods for Estimating the Chronic Toxicity of
Effluents and Receiving Waters to Freshwater Organisms
(EPA/600/4-91/002, 1992); and
•	Short-Term Methods for Estimating the Chronic Toxicity of
Effluents and Receiving Waters to Marine and Estuarine
Organisms (EPA/600/4-91/003, 1992).
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18

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GLOSSARY

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GLOSSARY
Antidegradation refers to policies which are part of each State's
water quality standards which are designed to protect water
quality and provide a method for assessing activities that may
impact the integrity of a water body.
Average monthly limitation (AML) is the highest allowable average
of daily discharges over a calendar month, calculated as the
sum of all daily discharges measured during that month divided
by the number of daily discharges measured during that month.
Best management practices (BMPs) are schedules of activities,
prohibitions of practices, maintenance procedures, and other
management practices to prevent or reduce the pollution of
waters of the United States. BMPs also include but are not
limited to treatment requirements, operating procedures, and
practices to control plant site runoff, spillage or leaks,
sludge or waste disposal or drainage from raw material storage.
Best professional judgment (BPJ) is the highest technical opinion
developed by a permit writer after consideration of all
reasonably available and pertinent data or information which
.forms the basis for the terms and conditions of a permit.
Daily Discharge is the discharge of a pollutant measured during a
calendar day or any 24-hour period that reasonably represents
the calendar day for purposes of sampling. For pollutants with
limitations expressed in units of mass, the daily discharge is
calculated as the total mass of the pollutant discharges over
the day. For pollutants with limitations expressed in other
units (e.g., concentration), daily discharge is calculated as
the average measurement of the pollutant over the day.
Effluent limitation is any restriction imposed by a permitting
authority on quantities, discharge rates, and concentrations of
pollutants which are discharged from point sources into waters
of the United States.
Effluent limitation guideline (ELG) is a regulation published by
the Administrator of EPA under section 304(b) of the Clean
Water Act to adopt or revise an effluent limitation.
Harmonic mean flow (HMF) is the number of daily flow measurements
of a stream divided by the sum of the reciprocals of the flows.
This is the design flow which is used for estimating human
health impacts of a discharge.
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Load allocations (LAs) are the portion of a receiving water's
total daily maximum load that are attributed either to one of
its existing or future nonpoint sources of pollution or to
natural background sources.
Long term average (LTA) is performance level that should be
achieved in a discharge to ensure that the WLA will not be
exceeded at least 99% of the time.
Maximum daily limitation (MDL) is the highest allowable daily
discharge of a pollutant.
Numeric protective level (NPL) is a numeric water quality value
determined to be appropriate for a given receiving water body
based on a review of the beneficial uses of the water body and
the water quality necessary for protection of such uses.
1Q10 is the lowest 1-day flow of a receiving water to be expected
over a 1 year period. This flow is used for estimating acute
effects of a discharge.
7Q10 is the lowest 7-day flow of a receiving water to be expected
over a 10 year period. This flow is used for estimating chronic
effects of a discharge.
Toxicity reduction evaluation (TRE) is a site-specific study
conducted in a step-wise process designed to identify the
causative agents of effluent toxicity, isolate the sources of
toxicity, evaluate the effectiveness of toxicity control
options, and then confirm the reduction in effluent toxicity.
Total maximum daily load (TMDL) is the sum of the individual
waste load allocations and load allocations. A margin of
safety is included with the two types of allocations so that
any additional loading, regardless of source, would not produce
a violation of water quality standards.
Waste load allocation (WLA) is the maximum allowable effluent
pollutant concentration or load which is allocated to existing
or future point sources of pollution which, considering the
total maximum daily load to the receiving water, will ensure
compliance with the NPL.
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appendix a

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APPENDIX A
ESTABLISHING REASONABLE POTENTIAL
I. BASIS FOR ESTABLISHING REASONABLE POTENTIAL
NPDES regulations at 40 CFR 122.44(d) require the permit
writer to establish effluent limitations for pollutants which show
reasonable potential to cause, or contribute to an excursion above
State water quality standards, including State narrative objectives
for water quality.
As required under 40 CFR 122.44(d) (ii)i , the permit writer must
consider a number of factors in establishing reasonable potential
including existing pollution controls, pollutant variability in the
effluent, sensitivity of toxicity test species, and dilution in the
receiving water. The following discussions outline the tiered
methodology followed when conducting a reasonable potential
evaluation. Regulations supporting reasonable potential
determinations are discussed in the TSD (see Chapter 3).
Justification for imposing water quality-based effluent
limitations based on reasonable potential is required in the
statement of basis, or fact sheet [see 40 CFR 122.44(d)(vi)(C)].
II. ESTABLISHING REASONABLE POTENTIAL WITH FACILITY-SPECIFIC
DATA
Where facility-specific effluent data are available,
reasonable potential is evaluated in a sequential (i.e., tiered)
process. The first-tier analysis may be performed by using a
simple steady-state mass balance equation. The mass balance
equation relates the mass of pollutants upstream of a point source
discharge, to the mass of pollutants downstream after mixing of the
discharge in the receiving water is. complete. The general mass
balance equation for the recommended steady-state model (see
Training Manual for NPDES Permit Writers. EPA 833-B-93-003, March
1993, pp. 6-10) is:
QdCd + QsCs = QrCr , where
Qd = waste discharge flow in million gallons per day
(MGD), or cubic feet per second (cfs)
Cd = waste discharge pollutant concentration in milligrams
per liter (mg/L)
Qs = background in-stream flow in MGD or cfs above point
of discharge during critical flow conditions
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Cs = background in-stream pollutant concentration in mg/L
Qr = resultant in-stream flow after discharge in MGD or
cfs (i.e., Qs + Qd)
Cr = resultant in-stream pollutant concentration in mg/L
in the stream reach (after complete mixing)
For reasonable potential determinations, this equation is
rearranged to solve for the resultant in-stream concentration (Cr)
at the edge of the mixing zone:
Cr = fOdl (Cd) + (Os) fCs)
Qr
Using the mass balance equation, Cr should be calculated using
conservative (i.e., critical) assumptions for background in-stream
receiving water flow (Qs), background in-stream receiving water
pollutant concentration (Cs), waste discharge flow (Qd) and waste
discharge pollutant concentration (Cd). Critical waste discharge
conditions should be represented by the highest observed pollutant
concentration and waste discharge flow. Critical background in-
stream receiving water flows are: 1) the 1Q10 flow (1-day low flow
over a 10-year recurrence interval) for calculating acute effects;
2) the 7Q10 flow (consecutive 7-day low flow over a 10-year
recurrence interval) for calculating chronic effects; and 3) the
harmonic mean flow for calculating human health effects. Where
possible, background in-stream pollutant concentrations should
correlate with critical background in-stream flows, as critical
pollutant concentrations occur during low flows. If site-specific
ambient pollutant concentration data are lacking, then other
appropriate ambient data, accessible through STORET, may be used.
Ambient low flow data, developed by the U.S. Geological Survey, are
also available through STORET.
Once the projected maximum in-stream pollutant concentration
(Cr) is calculated, this value can be compared to the appropriate
numeric protective level (NPL). Where Cr is greater than the NPL,
reasonable potential is established for that pollutant at the
specified effect level (i.e., acute, chronic or human health).
When reasonable potential is demonstrated, water quality-based
effluent limitations must then be developed for those individual
pollutants and/or WET.
If the projected maximum resultant in-stream pollutant
concentration (Cr) is less than the NPL, the permit writer must
then exercise judgement to determine whether reasonable potential
exists. This judgement depends on how large the difference is
between Cr and the applicable NPL, the uncertainty of maximum
effluent concentrations, type of discharger, and the sensitivity of
the receiving water. To assist in making this judgement, a second-
tier assessment may be performed that statistically addresses the
A-2

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uncertainty of maximum effluent concentrations for individual
pollutants. The second-tier analysis is a six step process (see
TSD, Box 3-2, p. 53) and is conducted for an effluent pollutant
data set as follows:
1.	Determine the total number of samples in the data set (k)
and the highest observed effluent concentration.
2.	Calculate the coefficient of variation (CV), where the CV
is the standard deviation over the mean (ct/m) (see TSD,
Appendix E) . For sample sizes less than 10 (k < 10) a
default CV of 0.6 can be used (see TSD, Box 3-2, p. 53).
3.	Choose uncertainty multiplier from Table 3-1 or 3-2 (see
TSD, p. 54) using k and the CV. The 99% confidence level
and 99% probability basis is recommended.
4.	Calculate the adjusted maximum effluent concentration by
multiplying the uncertainty multiplier times the highest
observed effluent concentration (Cd).
5.	Re-calculate the maximum resultant in-stream pollutant
concentration (Cr) using the adjusted maximum effluent
concentration (Cd) and the mass balance equation.
6.	Compare Cr with the applicable NPL. Reasonable potential
is established when Cr exceeds the NPL.
When reasonable potential is established by either first-
and/or second-tier analyses, a water quality-based effluent
limitation must be included in the permit for that particular
pollutant.
III. ESTABLISHING REASONABLE POTENTIAL WITHOUT FACILITY-SPECIFIC
EFFLUENT DATA
Where facility-specific effluent data are lacking, the permit
writer may still conduct a reasonable potential evaluation.
Establishing reasonable potential under such circumstances requires
a systematic consideration of all applicable factors in 40 CFR
122.44 (d) (1) (ii) (see TSD, pp. 50-51 and Box 3-1, p. 49) including:
•	Existing ambient water quality data;
•	Available dilution in the receiving water;
•	Type of receiving water and designated uses;
•	Industry/POTW type and nature of the discharges;
•	Compliance history and historical toxic impacts; and
A-3

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• Information from permit application or DMRs.
If a review of ambient monitoring data shows in-stream
exceedances or near exceedances of a NPL and that pollutant is
present in the discharge, reasonable potential is clearly
established and effluent limitations for that pollutant should be
included in the permit. The in-stream exceedance of a NPL
indicates that the receiving water body cannot assimilate any
additional load of that pollutant. Consequently, compliance with
the NPL must be met at the end-of-pipe (i.e., no dilution).
When effluent data are lacking the permit writer may choose
to require periodic monitoring for pollutants that may be present
in a discharge, or periodic effluent scans for all 126 priority
pollutants. The type of monitoring should be determined by the
nature of the discharge and receiving water, and the amount of
available dilution (see TSD, pp. 57-59, Figure 3-2). Under these
conditions, the permit should be issued with a reopener clause
allowing for modification of the permit to include effluent
limitations where monitoring data show reasonable potential for
in-stream excursions above ambient NPLs.
IV. FINDING NO REASONABLE POTENTIAL
Where existing effluent monitoring data show no reasonable
potential for excursions above ambient NPLs, the permit need not
contain water quality-based effluent limitations. However, the
permit writer may include monitoring requirements in the permit to
continue to re-affirm initial reasonable potential determinations
and to monitor for effluent changes (see TSD, pp. 59, 64).
A-4

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Figure 2
REASONABLE POTENTIAL
FLOW CHART FOR PERMITTING

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A-6

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APPENDIX B

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APPENDIX B
DETERMINING WATER QUALITY-BASED EFFLUENT LIMITATIONS
Water quality-based effluent limitations (WQBELs) are based
on maintaining effluent quality at a level that will comply with
appropriate NPLs, even during critical conditions in the receiving
water. These effluent limitations are based on the allowable
effluent loading concentration, or waste load allocation (WLA)
(i.e., Cd). Pollutant WLAs can be adjusted for uncertainty using
statistics calculated from historical effluent data; these adjusted
WLAs define the desired levels of performance, or targeted
long-term average discharge conditions (LTAs) for specific NPL
effect levels (i.e., acute, chronic, or human health). Permit
limits are calculated using statistics derived from historical
effluent data and the most limiting target LTA for a specific NPL.
The coefficient of variation (CV) is the critical statistic
calculated for each pollutant using historical effluent data.
Where historical data are insufficient (i.e., k < 10), the CV may
be estimated by 0.6 (see TSD, Appendix E, p. E-3). Statistical
derivation procedures for the average monthly limit (AML) should
assume that at least four samples (n) will be taken per month (see
TSD, pp. 107, 110).
The WLA required to protect against both acute and chronic
effects under critical conditions may be calculated using either
steady-state or dynamic models. In many cases, a WLA for a
pollutant of concern is not apportioned under a total maximum daily
load (TMDL) for the receiving water. In such cases, the allowable
effluent loading concentration (Cd) based on steady-state
assumptions may be substituted for the more rigorously determined
WLA. The steady-state model is the mass balance formula, QdCd +
QsCs = QrCr, used in reasonable potential evaluations. However,
the equation is rearranged to solve for the effluent concentration
(Cd), or WLA, necessary to achieve the appropriate NPL, which for
compliance purposes is set equal to Cr:
Cd = rcr rod + osn - r (Csl (Os) 1 , where
Qd
Qd = waste discharge flow in million gallons per day
(MGD), or cubic feet per second (cfs)
Cd = waste discharge pollutant concentration in milligrams
per liter (mg/L)
Qs = background in-stream flow in MGD or cfs above point
of discharge
Cs = background in-stream pollutant concentration in mg/L
B-l

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Qr = resultant in-stream flow after discharge in MGD or
cfs (i.e., Qs + Qd)
Cr = Numeric Protective Level (NPL) = resultant in-stream
pollutant concentration in mg/L in the stream reach
(after complete mixing)
In most cases, this steady-state model should be used to
calculate the WLA (i.e., allowable effluent concentration) that
will meet acute and chronic water quality criteria for the
protection of aquatic life at 1Q10 and 7Q10 design flows,
respectively, and chronic water quality criteria for the protection
of human health at the harmonic mean flow (see TSD, p. 68).
Ambient low flow data from the U.S. Geological Survey are available
on STORET.
Background pollutant concentrations (Cs) used in the mass
balance equation should reflect critical flow conditions. However,
if site-specific pollutant data are not available, other
appropriate data (e.g., STORET data) should be used to calculate
Cs. In such cases, the permit should require both effluent and
ambient monitoring.
When calculating the WLA, it should be noted that if State
water quality standards and plans do not explicitly allow the
application of mixing zones, the appropriate NPL must be met at the
end-of-pipe (i.e., NPL = Cr = Cd = WLA). Where mixing zones are
allowed, there should be no acute effects within the mixing zone;
chronic NPLs must be met at the edge of the mixing zone (see TSD,
p. 58) .
If adequate receiving water flow and effluent concentration
data are available to estimate frequency distributions, dynamic
modeling techniques can be used to calculate allowable effluent
loadings that will more precisely maintain water quality standards
(see TSD, p. 97). However, the steady-state mass balance equation,
when coupled with the recommended conservative assumptions, should
be adequately protective of receiving water beneficial uses.
Most WLAs calculated using federal water quality criteria for
the protection of aquatic life have both acute and chronic
requirements, whereas WLAs determined using federal water quality
criteria for the protection of human health have only chronic
requirements. For permit implementation, acute and chronic WLAs
need to be converted to maximum daily limits (MDLs) and average
monthly limits (AMLs). The following methodology (see TSD, Box
5-2, p. 100; Figure 5-4, p. 101; and Tables 5-1, 5-2 and 5-3, pp.
102-103, 106) is designed to derive permit limits for specific
pollutants and WET to achieve calculated WLAs at the 99% confidence
level for MDLs and the 95% confidence level for AMLs.
B-2

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Using the mass balance equation to solve for the allowable
effluent concentration (Cd), or WLA, for a pollutant of
concern:
a.	Set Cr equal to applicable acute, chronic, and human
health NPLs.
b.	Background receiving water (Qs) and discharge (Qd)
flows, and background pollutant concentration (Cs)
should represent critical conditions.
c.	Solve for Cd, or acute (WLAa), chronic (WLAc) and
human health (WLAh) waste load allocations,
respectively.
To calculate the coefficient of variation (CV):
a.	Use effluent data set of 'k' observations to
calculate the mean (n) and standard deviation (a)
(see TSD, Appendix E).
b.	Calculate the coefficient of variation (CV), where
CV = a/n.
c.	Where the effluent data set is small (k < 10) , the
conservative value of 0.6 is recommended to estimate
the CV (see TSD, Appendix E, p. E-3).
To determine long-term averaged discharge conditions
(LTAs):
a.	Use the following equations to calculate acute and
chronic long-term average discharge conditions (LTAa
and LTAc) that will satisfy the acute and chronic
waste load allocation (WLAa and WLAc) . The CV
calculated above is used to estimate both acute and
chronic WLA multipliers (see TSD, Table 5-1, p. 102) .
LTAa = WLAa • e I0-5 °a " 2
LTAc = WLAc • e f0-5 c4a ~ z °4l , where
e [0-5 oa - z o] _ acute WLA multiplier
e t°*5 °42 2 °4] = chronic WLA multiplier
z = 2.326 for the 99th percentile occurrence
probability for the LTA is recommended
b.	Set the long-term average discharge condition for
human health (LTAh) equal to the waste load
allocation for human health (WLAh).
LTAh = WLAh
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4.	Determine the lowest (most limiting) long-term average
discharge condition (LTA).
LTA = minimum (LTAa, LTAc, or LTAh)
5.	Calculate the maximum daily permit limit (MDL) and average
monthly permit limit (AML) using the lowest (most
limiting) long-term average discharge condition.
a.	Use the following equations to calculate the MDL and
AML when the most limiting long-term average
discharge condition is either acute (LTAa) or chronic
(LTAc). The CV calculated above is used to estimate
both acute and chronic LTA multipliers (see TSD,
Table 5-2, p. 103).
MDL = LTA • e [z o - 0.5 oJ] ^ where
e I* o - 0.5 o»] = MDL LTA multiplier
z = 2.326 for the 99th percentile occurrence
probability for the MDL is recommended
AML = LTA • e t2 °n - °-5	, where
e cn " 0,5 anaJ = AML LTA multiplier
z = 1.645 for the 95th percentile occurrence
probability for the AML is recommended
n = number of samples/month
b.	Use the following equations to calculate the MDL and
AML when the most limiting long-term average
discharge condition is human health (LTAh).
AML = LTAh
MDL = AML * [MDL/AML] , Where
[MDL/AML] is taken from the TSD (Table 5-3, p.
106) , using the CV calculated above and the
number of samples/month (n).
Following these procedures, the maximum daily limit (MDL) and
average monthly limit (AML) may be then incorporated into the
permit as justifiable water quality-based effluent limitations.
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INPUTS
RwfiwMm Wrfnr fBackaoLrnll
Crttal Law Flow (0^
PaLtart Corcertndon (C«)
PbetmroB
Madrrun Daly Flow (QcO
Cont*TBd (Duwi tliBMrt
Cuitimi Flow (Qr)
Cr o NLfTBrfc
Protectee Lsveb (NPL)
Ante, Chronic, Hunan Heath
CALCULATE WASTE LOAD ALLOCATION (WLA)
Cd = Allowable Effluent
Concentration
= fCr(Qd+Qs)-CsQsl
Qd
Waste Load Allocation (WLA)
acute, chronic, human health
WLA = Cd
i
CALCULATE LONG TERM AVERAGE (LTA)
Sr^e Poiuart
EffluertDtta
k = # Total
Samples
Using Effluent Data Calculate:
Mean, Standard Deviation, Coefficient of Variation (CV)
T
ACUTE. CHRONIC
Long Term Average (LTA)
WLA Multipler
acute,
. chronic
Choose WLA Multipler from
TSD, Table 5-1
[LTA 1
=
[WLA 1
acute,

acute,
chronic

chronic.
HUMAN HEALTH
LTA = WLA
CALCULATE PERMIT LIMITS, USE THE LOWER OF THE ACUTE. CHRONIC
OR HUMAN HEALTH LTA TO CALCULATE AVERAGE MONTHLY LIMIT
(AML) & DAILY MAXIMUM LIMIT (DML) (PERMITS LIMITS)
Figure 3
Calculatinci Average Monthly Omits (AML) &
Maximum Daily Limits (MDL)
Using Steady-State Assumptions
B-5

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B-6

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APPENDIX C

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APPENDIX C
DETERMINING DILUTION FOR OCEANS AND ESTUARIES
I. CALCULATING MINIMUM INITIAL DILUTIONS FOR OCEAN DISCHARGES
For ocean outfalls, dilution, transport and dispersion of
discharged effluent are important variables to consider when
evaluating potential environmental impacts on marine communities.
Non-saline (lower-density) effluent discharged through a submerged
ocean outfall generally rises rapidly toward the surface in a
buoyant plume, entraining significant amounts of ambient saline
water. As the plume rises and entrains ambient water, its density
increases and its momentum and buoyancy decrease accordingly. If
a sufficient ambient vertical density gradient is present (e.g.,
pycnocline or thermocline), the plume will spread horizontally at
the level of neutral buoyancy (i.e., plume density equals ambient
water density). If a sufficient density gradient is not present,
the diluted effluent will reach the water surface and flow
horizontally. The dilution achieved at the completion of this
process is called the "initial dilution" and occurs within minutes
of discharge. With proper location and design, ocean outfalls can
achieve initial dilution values of about 100:1.
Sufficient initial dilution is necessary to assure compliance
with State water quality standards. Factors influencing the
initial dilution achievable for a particular outfall include:
•	Effluent density and historic flow rate;
•	Discharge depth;
•	Diffuser characteristics (i.e., port sizes, spacing and
orientation);
•	Receiving water density profiles; and
•	Ambient current speed and direction [i.e., zero (0) under
Ocean Plan.
These factors are input parameters for several standard
dilution models that calculate the initial dilutions expected under
different oceanographic and diffuser conditions. Older plume
models are summarized in Revised Section 301(h) Technical Support
Document (EPA 430/9-82-011); this document also references other
methods and mathematical models that may be adapted for estimating
initial dilution. More recently, PLUMES, a plume model interface
and manager available from EPA, offers users two initial dilution
models, RSB and UM. PLUMES is explained in the user guide,
Dilution Models for Effluent Discharges (EPA/600/R-93/139, July
1993), and includes a user-friendly tutorial providing examples of
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RSB and UM. The PLUMES models are intended for use with plumes
discharged to marine and freshwater bodies. Buoyant and dense
plumes, single sources, and many diffuser outfall configurations
may be modeled.
While effluent density and flows, discharge depth, and
diffuser characteristics are readily available for model input,
site-specific receiving water density profile data may be lacking.
Since initial dilution calculations can be strongly dependent on
ambient density profiles relative to effluent density, a
substantial amount of data from the discharge site and/or nearby
sites having similar environmental conditions should be evaluated
before selecting a worst-case ambient density profile for
calculating a minimum initial dilution.
Where site- or nearby site-specific density profiles are
larking, two options are available to the permit writer:
•	Ambient monitoring may be required during critical periods
to determine the worst-case density profile upon which to
model the minimum initial dilution;
•	Existing worst-case density profiles measured for several
ocean outfalls in California are available from EPA and
may be used to develop an interim minimum initial dilution
and interim numeric effluent limitations, while ambient
monitoring is conducted to determine the worst-case site-
specific density profile upon which to model the minimum
initial dilution.
Once the appropriate minimum initial dilution has been
calculated, the permit writer may either conduct a reasonable
potential evaluation to determine those pollutants requiring water
quality-based effluent limitations, or may choose to develop
effluent limitations for all toxic pollutants limited under the
Ocean Plan.
Water quality-based effluent limitations for Table B
pollutants (except radioactivity) are calculated using appropriate
background seawater concentrations (Cs) and the following equation
(see Ocean Plan, p. 10):
Ce = Co + Dm (Co - Cs) , where
Ce = effluent concentration limit
Co = concentration to be met at the completion of initial
dilution
Cs = background seawater concentration (see Ocean Plan, p.
10)
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Dm = minimum probable initial dilution expressed as parts
seawater per part wastewater
Table B marine aquatic life objectives are limited as
instantaneous maximum, daily maximum, and 6-month median
concentrations, while Table B human health objectives are limited
as 30-day average concentrations.
II. CALCULATING MINIMUM INITIAL DILUTIONS FOR ESTUARINE DISCHARGES
In estuaries and coastal bays, determining the nature and
extent of a discharge plume is complicated by conditions such as
differences in tides, riverine input, wind intensity and direction,
and thermal and saline stratification. For example, tidal
frequency and amplitude vary significantly in different coastal
regions of the United States. Furthermore, tidal influences at any
specific location have daily and monthly cycles. As a result,
discharge dilutions cannot be reliably estimated using ratios of
conservative discharge and receiving water flows. It is
recommended that dilutions for these water bodies be determined
empirically, by employing dye or tracer studies under critical
conditions.
In estuaries without stratification, the critical dilution
condition includes a combination of low-water slack at spring tide
for the estuary and design low flow for riverine inflow. In
estuaries with stratification, a site-specific analysis to
determine a period of minimum stratification and a period of
maximum stratification, both at low-water slack, should be made to
evaluate which one results in the lowest dilution. In general,
minimum stratification is associated with low river inflows and
large tidal ranges (spring tide), whereas maximum stratification is
associated with high river inflows and low tidal ranges (neap
tide).
After either stratified or unstratified estuaries are
evaluated at critical design conditions, an off-design condition
should be checked. The off-design condition (e.g., higher flow or
lower stratification) recommended for both cases is the period of
maximum velocity during a tidal cycle. This off-design condition
results in greater dilution than the design condition, but it
causes the maximal extension of the plume. Extension of the plume
into critical resource areas may cause more water quality problems
that the high-concentration, low-dilution situation.
Recommendations for a critical design for coastal bays are the
same as for stratified estuaries. The period of maximum
stratification must be compared with the period of minimum
stratification in order to select the worst case. The off-design
condition of maximum tidal velocity should also be evaluated to
predict the worst-case extent of the plume.
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APPENDIX D

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APPENDIX D
CASE EXAMPLES
I. INTRODUCTION
This appendix presents examples of the development of water
quality-based discharge limits to illustrate the integration of the
guidance detailed in sections III.A.2 and III.A.3, and Appendices
A and B (also, see TSD, Chapter 7). There are three examples: an
industrial discharge with ample dilution, a publicly owned treat-
ment works (POTW) with moderate dilution, and the combination of an
industrial facility and a POTW discharge to the same reach. For
each example, reasonable potential is determined for pollutants of
concern, and acute (1-day average), chronic (4-day average), and
human health NPLs are translated into daily maximum and monthly
average permit limits.
II. CASE 1: INDUSTRIAL DISCHARGE
The first example is the Jaybird Corporation, a metal
finishing firm. The NPDES permit for the facility is about to
expire, and the corporation has submitted an application for a new
permit. The example shows the steps that a permit writer would
take to determine if a water quality-based effluent limit is
necessary and then to establish such a limit. The example also
illustrates when best available technology (BAT) limits are applied
instead of water quality-based limits, the use of human health
NPLs, and the variations in the limits derived by different waste
load allocation (WLA) methods.
A. GENERAL SITE DESCRIPTION AND INFORMATION
The Jaybird Corporation facility discharges into the Locapunct
River. The river is approximately 60 miles long and its banks are
occupied by small towns separated by woodland and farmland. The
river is classified by the State in the water quality standards as
having designated uses of a fish habitat, primary contact recre-
ation, and a drinking water supply. For these uses, the State has
adopted the federal water quality criteria into the water quality
standards to protect aquatic life and human health. The State
standards also include a narrative objective of "no toxics in toxic
amounts" for other toxic materials.
Water quality monitoring indicates some infrequent excursions
above water quality objectives for copper and nickel. These
pollutants have been found in measurable quantities in the
effluents of several facilities.
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The Jaybird Corporation is a metal finishing facility that
specializes in copper plating of lead shells for a nearby military
installation. As a metal finisher, the Jaybird Corporation is
relatively-small with a discharge of 0.034 cfs (0.022 MGD). The
effluent at the Jaybird Corporation is treated by precipitation and
settles before discharge through a multiport diffuser. The
corporation is subject to BAT and best practicable technology (BPT)
effluent limits for the metal finishing industry.
B. EFFLUENT CHARACTERIZATION FOR SPECIFIC CHEMICALS
The permit writer has adopted a procedure in which pollutant
concentrations in each facility are evaluated for the potential to
cause, have the reasonable potential to cause, or contribute to an
excursion of the water quality standards. The permit writer used
the effluent characterization process for specific chemicals de-
scribed in section III.A.2 and Appendix A. In general, the
procedures are designed to determine which pollutants are of
concern and which require effluent limits.
STEP 1: Identify Pollutants of Concern
Data were obtained from a number of sources to identify and
quantify the pollutants of concern in the Jaybird Corporation
effluent:
•	Effluent chemical concentrations were taken from the Per-
mit Application Form 2C, Discharge Monitoring Reports
(DMRs), EPA's Permit Compliance System (PCS), and permit
files.
•	EPA's STORET data base was used to obtain U.S. Geological
Survey flow data and ambient monitoring data for the
river.
•	BAT limits for the metal finishing industry were obtained
from 40 CM 433 Subpart A.
The permit writer noticed in review of these data that the
information in Form 2C replicated the information in the DMRs, and
therefore decided to use the DMR data as the primary basis for
characterizing the effluent. These data for toxicants in the DMRs
are shown in Table D-l. For those parameters currently not covered
by the permit, Form 2C data indicated that pollutant concentrations
were below detection limits. The permit writer requested
information from the facility showing the detection levels used;
these levels were consistent with the detection levels listed in
the NPDES regulations at 40 CFR 136.
The effluent from the Jaybird Corporation is regulated by the
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Metal Finishing Point Source Category effluent guidelines at 40 CFR
433 Subpart A. These guidelines regulate the following toxic
pollutants: cadmium, chromium, copper, cyanide, lead, nickel,
silver, zinc, and total toxic organics.
Although these parameters were regulated at the Jaybird Corpo-
ration, the only toxic pollutants evident in the discharge were
lead, copper, and nickel. The facility's treatment system reduced
concentrations of other pollutants to below detection.
STEP 2: Determine the Acute, Chronic, and Human Health NPLs
for Pollutants of Concern
The State has adopted numeric acute (1-day average) and
chronic (4-day average) aquatic life objectives, and human health
objectives. The water quality standards present the acute and
chronic objectives as equations based on ambient hardness
concentrations. The standards require that the 85th percentile
lowest hardness be used. This value is 100 mg/1 as CaC03 for the
Locapunct River.
Table D-l. Effluent Data for the Jaybird Corporation

Copper
Lead
Nickel^
Toxicity
n
ug/1
ug/1
ug/1
TUc
1
1,317
187
223
5
2
1,092
230
261
10
3
1,073
258
464
5
4
1,059
423
341
20
5
1,072
227
369

6
1,677
275
1,058

7
2,664
364
199

8
1,058
170
259

9
3,439
259
437

10
6,596
264
773

11
1,211
267
300

12
1, 082
175
356

Mean
1,945
258
420
10
SD
1,650
74
252
7.1
CV
0.8
0.3
0.6
0.7
Max
6,596
423
1,058
20
Min
1,058
170
199
5
N
12
12
12
4
Notes:
Metals reported as total recoverable metals; toxicity reported in
chronic toxic units (100/NOEC). The permittee did not use a
geometric dilution series for the toxicity tests. The results are
the highest toxic units for any of the test organisms used.
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The acute and chronic aquatic life objectives for metals in
the standards are expressed as the acid soluble form of the metal.
The State has adopted a ratio to express the acid soluble form of
metals as the total recoverable form for the purposes of developing
NPDES permit limits. This ratio is based on historical data that
the State has collected for rivers in the basin where the Locapunct
lies. The values of the ratio are 0.35 for lead, 0.70 for copper,
and 0.85 for nickel. The standards consider the objectives for
human health protection to be in the total recoverable form of the
metal.
Based on the hardness and acid soluble-to-total recoverable
ratios, the applicable State water quality objectives are the
following:
Chronic	Acute Human Health
Pollutant	fug/1)	fug/1)	fug/1)
Lead 9.1	235 50
Copper 17.1	25.7 NA
Nickel 188	1,647 13.4
STEP 3: Determine Dilution for Aquatic Life and Human Health
Impacts
The State water quality standards require that compliance with
water quality NPLs be achieved at the edge of the mixing zone. The
standards specify the minimum dilution at which the NPLs apply.
These are the 7Q10 flow for chronic, the 1Q10 flow for acute, and
the harmonic mean flow for human health NPLs. The U.S. Geological
Survey operates a gaging station on the river; the flow statistics
were calculated using the data from this station:
•	7Q10 flow =13.0 cfs
•	1Q10 flow = 10.1 cfs
•	Harmonic mean flow = 38.0 cfs
The facility provided a study of the outfall that showed that
the multiport diffuser quickly achieved complete mixing across the
width of the river. Dilution at the edge of the mixing zone could,
therefore, be characterized by the mass balance equation:
Cr = (CdQd + CsQs)/(Qd + Qs) , where
Cr = the receiving water concentration
Cd = the maximum effluent concentration
Qd = the effluent flow
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Cs = the receiving water background concentration
Qs = the appropriate receiving water flow
Qr = Qd+Qs
STEP 4: Determine Reasonable Potential for Excursions
To determine if the facility discharge was expected to cause
or have the reasonable potential to cause acute, chronic, or human
health objective to be exceeded in the receiving water, the maximum
receiving water concentration of each pollutant was first compared
to the appropriate receiving water objective. If the objectives
were exceeded, then this was considered evidence that a water
quality based limitation must be developed.
Maximum expected concentrations were calculated using the
average effluent flow, maximum effluent concentrations, background
receiving water concentrations, and the relevant receiving water
flow: the 1Q10 for acute, the 7Q10 for chronic, or the harmonic
mean for human health objectives. The background receiving water
concentrations for total recoverable metals were obtained from
STORET data:
Lead	1.6 ug/1
Copper	4.8 ug/1
Nickel	13.2 ug/1
The maximum effluent concentration was estimated using the
statistical approach outlined in section III.A.2 and Appendix A.
There were 12 concentrations of each metal reported in the DMRs.
For lead, these concentrations had a maximum value of 423 ug/1, an
arithmetic mean of 258 ug/1, an arithmetic standard deviation of
74, and an arithmetic coefficient of variation of 74/258, or 0.3.
This coefficient of variation (CV) and the number of observations
determined which uncertainty multiplier was selected from TSD,
Table 3-1. In this case, the multiplier value for 12 observations
and a CV of 0.3 was interpolated from the values for 12
observations and CVs of 0.2 and 0.4. The 99th percentile
multiplier was estimated to be 1.7. Similar calculations were
conducted for copper (multiplier of 2.8) and nickel (multiplier of
3.7) .
The receiving water concentration for lead for comparison with
the chronic objective was calculated using data from Table D-l:
Cr = [(1.7 x 423 ua/1 x 0.034 cfs^ + (1.6 ua/1 x 13 cfsl]
(0.034 Cfs + 13 cfs)
= 3.5 ug/1 , where
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13 cfs = the background receiving water flow at 7Q10
0.034 cfs = the mean effluent flow
423 ug/1 = the maximum effluent concentration
1.7	= the statistical effluent multiplier to
estimate the 99th percentile concentration
1.6 ug/1 = the background receiving water concentration
The value of the calculated receiving water concentration, 3.5
ug/1, was less than the chronic water quality objective of 9.1 ug/1
for lead, and therefore there is no reasonable potential for the
chronic objective to be exceeded.
Using the effluent data presented in Table D-l, the receiving
water concentration is compared to the acute objective as:
Cr = [fl.7 x 423 ug/1 x 0.034 cfs)+fl.6 ug/1 x 10.1 cfs)]
(0.034 cfs + 10.1 cfs)
=4.0 ug/1
where 10.1 cfs is the receiving water 1Q10 flow and the other
values are identical to those for the chronic comparison. The
resulting concentration of 4.0 ug/1 was less than the acute
objective of 234 ug/1 for lead. There is no reasonable potential
for the acute objective to be exceeded.
The receiving water concentration for comparison to the human
health objective was calculated as:
Cr = [fl.7 x 423 ug/1 x 0.034 cfs) + (1.6 ug/1 x 38 cfs)]
(0.034 cfs + 38 cfs)
=2.2 ug/1
where 38 cfs is the harmonic mean flow and other values are the
same as above. This value was less than the human heath objective
of 50 ug/1 for lead, so there is no reasonable potential for the
human health objective to be exceeded.
Similar calculations were done for copper and nickel:
Objective Receiving Water
	fug/1)	Concentration fug/1)
Copper
Chronic	17.1	22.0
Acute	25.7	26.9
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Objective Receiving Water
		fua/11	Concentration fua/11
Nickel
Chronic	188	15.9
Acute	1,647	16.6
Human Health	13.4	14.1
The effluent characterization showed the reasonable potential
for excursions above the chronic objective for copper and above the
human health objective for nickel. Therefore, permit limits are
necessary for these two pollutants.
C. EFFLUENT CHARACTERIZATION FOR WHOLE EFFLUENT TOXICITY
Whole effluent toxicity also was evaluated since there was a
potential for excursions above the narrative water quality
objective due to the combination of effluent toxicants with other
toxicants in the receiving water and in the effluent, but below the
detection level. The procedures used below follow those outlined
in section III.A.2 and Appendix B, and presented schematically in
the TSD (see Chapter 3, Box 3-2).
STEP l: Dilution Determination
The initial dilution determination was used to establish the
types of toxicity tests that are conducted to characterize the
effluent. The dilution at the low-flow characteristics for the
facility is the following:
At the 7Q10, dilution = (0.034 cfs + 13 cfs)/0.034 cfs
= 383
At the 1Q10, dilution = (0.034 cfs + 10.1 cfs)/0.034 cfs
= 298
STEP 2: Conduct Toxicity Testing
EPA recommends that a discharger having a dilution between
100:1 and 1,000:1 be required to conduct either chronic or acute
toxicity testing. The permit writer decided to require chronic
testing but required the permittee to report the test results at
the 48-hour endpoint so that acute toxicity could be measured. One
year before the permit was due to expire, the permit writer
requested, under the authority of CWA section 308, that the
permittee test his effluent for toxicity to provide effluent
information in order to write the next NPDES permit. In this case,
the permit writer specified that the discharger submit quarterly
chronic toxicity data for 1 year using the EPA toxicity tests for
Selenastrum. Cerlodaphnia. and Pimephales. The permit writer also
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specified that upstream ambient water be used as the diluent in the
tests so as to allow the tests to measure additive effects from
ambient toxics. In response to the section 308 request, the
discharger submitted the whole effluent toxicity data shown in
Table D-l.
STEP 3: Determine Reasonable Potential for Excursions
The State interprets its narrative objective for whole
effluent toxicity to require that the TSD recommendations of 0.3
TUa, and 1.0 TUc be used as numeric values for acute and chronic
toxicity, respectively. In accordance with the State standards,
the acute objective applies under the 1Q10 flow and the chronic
objective applies under the 7Q10 flow.
The determination of exceedance of the acute or the chronic
objective was simplified by the way in which the tests were
conducted. Since the upstream ambient water was used as a diluent,
the test results already include an assessment of contributions
from background toxicity. Therefore, the upstream receiving water
concentration was set to zero.
The maximum effluent concentration was again estimated by
using the statistical approach in section III.A.2 and Appendix A.
As shown in Table D-l, there were four observations of whole
effluent toxicity. Based on the guidance in Appendix A (also, see
TSD, Box 3-2), these are insufficient to determine the CV
accurately; therefore, the default CV of 0.6 was used. The
effluent multiplier of 4.7 was obtained from TSD, Table 3-1 using
the number of observations, the CV, and the 99-percent probability
basis.
The receiving water concentration for chronic toxicity for
comparison with the chronic objective was calculated using data
from Table D-l:
Cr = [ (4.7 x 20 TUc x 0.034 cfs) + (0 TUc x 13 cfsl]
(0.034 cfs + 13 cfs)
= 0.25 TUc , where
13 cfs = the background receiving water flow at 7Q10
0.034 cfs = the mean effluent flow
4.7	= the statistical effluent multiplier
20 TUc = the maximum effluent concentration
The value of the calculated receiving water concentration,
0.25 TUc, was less than the chronic objective of 1.0 TUc, and
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therefore there is no reasonable potential for the chronic
objective to be exceeded.
To calculate the receiving water concentration for acute
toxicity, the permit writer first converted the chronic toxicity
data into equivalent acute toxicity units by applying the acute-to-
chronic ratio (ACR) of 5 obtained from the monitoring data. The
receiving water concentration for acute toxicity was then calcu-
lated:
Cr = [ M.7 X 20 TUc / 5 ACR X 0.034 CfS^ + (0 TUc X 10.1 cfsn
(0.034 cfs + 10.1 cfs)
= 0.06 TUa
where 10.1 cfs is the receiving water flow at 1Q10, 5 i . the acute
to chronic ratio, and the other values are the same as above. The
calculated value of 0.06 TUa is below the objective of 0.3 TUa;
therefore, there is no reasonable potential for the acute objective
to be exceeded. Since there was no reasonable potential for
exceedances above either acute or chronic objectives, permit limits
were not developed for whole effluent toxicity.
D. DETERMINE WASTE LOAD ALLOCATION
The waste load allocation (WLA) was used to determine the
level of effluent concentration that would comply with water
quality standards in the receiving waters. A VILA will only be
determined for those parameters that have a reasonable potential to
cause exceedances of water quality standards. Therefore, WLAs were
determined for copper and nickel. Since there was no reasonable
potential for excursions above the acute or chronic objectives for
nickel, only the WLA for human health was calculated.
To determine WLAs, the numeric objectives in the water quality
standards and background concentrations were used to calculate
effluent concentrations that would result in compliance with those
standards. The calculation of WLAs used receiving water flows that
were appropriate to each standard: chronic WLAs were calculated
using the 7Q10 flow, acute WLAs were calculated using the 1Q10
flow, and human health WLAs were calculated using the harmonic mean
flow. Since the effluent was mixed rapidly by the multiport
diffuser, the mass balance equation was used:
WLA = [NPL x (Qd + Qs) - QsCs]/Qd , where
Qd = the effluent flow
Qs = the receiving water flow
Cs = the background receiving water concentration
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NPL = the Numeric Protective Level
The chronic and acute WLAs for copper were calculated at the
7Q10 and 1Q10 flows, respectively:
WLAc = [17.1 ug/1 x (0.034 cfs + 13 cfs) - 13 cfs x
4.8 ug/1] / 0.034 cfs
= 4,720 ug/1
WLAa = [25.7 ug/1 x (0.034 cfs + 10.1 cfs) - 10.1 cfs x
4.8 ug/1] / 0.034 cfs
= 6,234 ug/1
The human health WLA for nickel was calculated at the harmonic
mean flow:
WLAh = [13.4 ug/1 x (0.034 cfs + 38 cfs) - 38 cfs x
13.2 ug/1 / 0.034 cfs
= 237 ug/1
E. DEVELOP PERMIT LIMITS
Permit limits were developed using the steady-state mass
balance model as described in section III.A.3 and Appendix B (also,
see TSD, Chapter 5) . Values for constants were obtained from
Tables 5-1, 5-2, and 5-3 in the TSD.
STEP 1: Calculate LTA
The chronic long-term average (LTA) for copper was calculated
using the following formula:
LTAC = WLAc • e t0*5 °4J " 2 °4l
= 4,720 ug/1 x 0.440
= 2,077 ug/1
where values of e f0,5 °4a ~ z °4l are presented in Table 5-1 (see
TSD, Chapter 5). The CV of 0.8, and the z value for the 99th
occurrence probability were used.
The acute LTA for copper was calculated, again using the 99th
percentile occurrence probability values from Table 5-1 as the
multiplier:
LTAa = WLAa • e t°*5 °2 " 2 °1
= 6,234 ug/1 x 0.249
= 1,552 ug/1
The human health LTA for nickel is considered to be the same
as the WLA because the 70-year averaging period is used for human
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health evaluations (see TSD, Chapter 5.4.4). The LTA is calculated
as:
LTAh = WLAh
= 237 ug/1
STEP 2: Determine the Most Limiting LTA
The limiting LTA for each pollutant is the minimum of the
chronic, acute and human health LTAs. The limiting LTA value.is
used in the next step to calculate maximum daily limits and average
monthly limits. The limiting LTA for copper was found to be the
acute LTA (1,552 ug/1) and the limiting LTA for nickel was found to
be the human health LTA (237 ug/1).
STEP 3: Calculate Maximum Daily and Average Monthly Limits
The maximum daily limit (MDL) for copper was calculated using
the expression:
MDL = LTA • e I* ° - °-5 o»]
= 1,552 ug/1 x 4.01
= 6,224 ug/1
where the appropriate value for e " " 0,5 was taken from Table
5-2 using the row with the CV for copper (0.8) and the column for
the 99th percentile probability basis.
The average monthly limit (AML) for copper was calculated
using the expression:
AML = LTA • e °n ' °-5 "n'l
= 1,552 ug/1 X 1.75
= 2,716 ug/1
where the value for e lz °n " 0,5 °naJ was taken from Table 5-2 and,
for this case, the number of samples per month was four. The z
value for the 95th percentile probability basis was used.
The effluent limits for nickel were determined by using the
recommendations in Appendix B and the TSD (see Chapter 5.4.4) . The
AML was considered to be identical to the WLAh whereas the MDL was
calculated from the AML by using the appropriate multiplier factor
in Table 5-3. With a CV of 0.6, four samples per month for
sampling, and a 99th percentile used for the MDL, the factor is
1.64:
MDL = AML X 1.64
= 237 ug/1 X 1.64
= 389 ug/1
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F. DETERMINING AND EXPRESSING THE CONTROLLING EFFLUENT LIMITS
The NPDES regulations specify that effluent limits require
treatment characteristic of the appropriate treatment technology
and also achieve water quality standards. If water quality-based
limits are more stringent than BAT limits, then the water
quality-based limits become the basis for the effluent limits.
Conversely, if the treatment technology (BAT) limits are more
stringent, then they become the basis for the limits.
A comparison between the water quality-based and technol-
ogy-based effluent limits are shown below. For nickel, water qual-
ity-based limits are more stringent, whereas for copper, BAT limits
are the more stringent.


CoDDer
Nickel
Water Quality
MDL
6,224
389

AML
2,716
237
BAT
MDL
3,380
3,980

AML
2,070
2,380
Limit to use
MDL
3,380
389

AML
2,070
237
In accordance with NPDES regulations, the effluent limits are
expressed in the permit as mass (pounds per day) by multiplying the
concentrations above by the effluent flow of 0.034 cfs and the
conversion factor of 5.394:
Copper	Nickel
	flb/d)	(lb/d)
MDL	0.62	0.071
AML	0.38	0.043
III. CASE 2: POTW DISCHARGE
The second example is of a fictitious POTW that discharges to
the same reach as the Jaybird Corporation. The NPDES permit for
this facility also is up for reissuance. The example highlights
the use of background receiving water concentrations, and
demonstrates the differences between industrial and POTW permit
limits. In developing permit limits for the POTW in this example,
potential impacts from the Jaybird Corporation discharge were
considered in the use of background receiving water concentrations.
The interrelationships between the two facilities are discussed
explicitly in Case 3.
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A. GENERAL SITE DESCRIPTION AND INFORMATION
The Locapunct River receives discharges from a POTW serving
Auburn, a small city of about 10,000 people. The POTW treats a
mixture of household and industrial waste with an activated sludge
process. The mean effluent flow from the POTW is 1.23 cfs. The
POTW has no pretreatment program, but the municipality is aware of
the small industries that are indirect dischargers because of
research conducted by a local university. Generally, the plant is
well operated.
B. EFFLUENT CHARACTERIZATION FOR SPECIFIC CHEMICALS
The approach for determining which pollutants cause, have the
reasonable potential to cause, or contribute to excursions above
water quality standards applies to POTWs as well as industries.
The permit writer used the procedures described for the Jaybird
Corporation in the evaluation of the Auburn POTW.
STEP 1: Identify Pollutants of Concern
At the time of the last permit issuance, there was evidence of
a number of toxic pollutants in the POTW's effluent, including
copper, chlorine, and ammonia. These pollutants had monitoring
requirements in the previous permit. Because there were metals in
the effluent and, due to the industries discharging into the POTW
sewer system, the permit writer requested the POTW to conduct a
complete priority pollutant scan of the effluent. The data
received following the section 308 letter request indicated that
the concentrations of all priority pollutants except copper were
below detection limits. The POTW's primary toxic pollutants of
concern were copper, chlorine, and ammonia (see Table D-2).
STEP 2: Determine Acute, Chronic and Human Health NPLs for
Pollutants of Concern
As described in the example of the industrial discharge, the
water quality standards include numeric objectives for copper. The
State also has adopted a numeric objective for ammonia that is a
function of the river 85th percentile pH and temperature; these
values are 8.25 and 25°C, respectively. Finally, the State inter-
prets its narrative objective of "no toxics in toxic amounts" to
require use of the federal water quality criteria in the absence of
a numeric State objective. As a result, the permit writer uses
federal criteria for chlorine. The applicable water quality NPLs
for the river are as follows:
D—13

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n
l
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Me
SD
CV
Ma:
Mi
:e:
is
Chronic	Acute
	fug/1)	fuq/i)
Copper	17.1	25.7
Chlorine	11	19
Ammonia	540	4,000
Table D-2. Effluent Data for the Auburn POTW
Copper
ug/1
Chlorine
ug/1
Ammonia
ug/1
Toxicity
TUc
268
115
228
59
53
213
68
200
262
519
53
474
115
259
404
57
101
187
103
76
198
265
60
112
185
133
0.7
519
52.6
185
301
881
372
245
244
123
343
153
448
1,022
347
130
128
271
451
701
582
178
436
347
475
153
268
366
235
0.6
1,022
123
11
13
12
24
9
15
21
3
22
7
11
8
4
9
6
6
37
14
16
28
12
11
3
4
13
8
37
3
009
025
201
548
700
645
358
976
307
427
834
430
382
330
137
448
772
307
848
205
119
778
109
474
182
491
0.6
772
109
2
1
1
2
1.5
0.6
0.4
2
1
DMR data for chemicals; 306 request for whole effluent toxicity.
Metals as total recoverable, toxicity in toxic units (100/NOEC). The
results are the highest toxic units for any of the test organisms
used.
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STEP 3: Determine Dilution for Aquatic Life and Human Health
Impacts
The State water quality standards require that compliance with
water quality NPLs be achieved at the edge of the mixing zone. The
standards specify the minimum dilution at which the NPLs apply.
These are the 7Q10 flow for chronic NPLs, the 1Q10 flow for the
acute NPLs, and the harmonic mean flow for human health NPLs. The
U.S. Geological Survey operates a gaging station on the river. The
flow statistics were calculated using the data from this station:
•	7Q10 flow = 13.0 cfs
•	1Q10 flow = 10.1 cfs
•	Harmonic mean flow = 38.0 cfs
The POTW is located at a	bend of the river where mixing is
rapid. Therefore, the permit	writer used the steady-state mass
balance equation to calculate the receiving water concentrations.
This is the same equation used for the industrial example.
STEP 4: Determine Reasonable Potential for Excursions
The determination of possible exceedances of acute or chronic
NPLs were based on a calculation of the maximum receiving water
concentration of each pollutant, followed by a comparison to the
appropriate receiving water NPL. The calculation of the maximum
receiving water concentrations were made using the statistical
estimate of the 99th percentile concentration of each pollutant in
the effluent, the same flow used in the industrial example, and
background receiving water concentrations of:
Maximum effluent concentrations were estimated using the
statistical approach outlined in section III.A.2 and Appendix A.
There were 24 concentrations for each chemical reported in the
DMRs. For copper, these concentrations had a maximum value of 519
ug/1, an arithmetic mean of 185 ug/1, an arithmetic standard
deviation of 133, and an arithmetic coefficient of variation of
133/185, or 0.7. The multiplier was calculated to be 2.4 based on
the CV of 0.7, 24 observations, and a 99-percent confidence level
(see TSD, Table 3-1). Similar calculations were conducted for
chlorine (multiplier of 2.2) and ammonia (multiplier of 2.2).
The receiving water concentrations for each pollutant were
calculated. An example calculation for the comparison of copper to
the chronic objective is as follows:
Copper
Chlorine
Ammonia
4.8 ug/1
0 ug/1
120 ug/1
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Cr = [f2.4 x 519 uq/1 x 1.23 cfsl + (4.8 uq/1 x 13 cfs)]
(1.23 cfs + 13 cfs)
= 112 ug/1 , where
519 ug/1 = the maximum measured effluent concentration
2.4	= the statistical multiplier
1.23 cfs = the average effluent flow
4.8 ug/1 = the upstream receiving water concentration
13 cfs = the 7Q10 flow
The maximum receiving water concentrations for comparison to
applicable standards for all pollutants were calculated to be:
NPL
(uq/11
Receiving Water
Concentration
	fug/1)	
Copper
Chronic
Acute
Chlorine
Chronic
Acute
Ammonia
Chronic
Acute
17.1
25.7
11
19
540
4,000
112
140
194
244
7,292
9,128
The effluent characterization showed the reasonable potential
for excursions above the chronic and acute NPLs for copper,
chlorine, and ammonia. Therefore, permit limits were developed for
these pollutants.
C. EFFLUENT CHARACTERIZATION FOR WHOLE EFFLUENT TOXICITY
STEP 1: Dilution Determination
The initial dilution determination was used to establish the
types of toxicity tests that must be conducted to characterize the
effluent. The dilution at the low flow characteristics for the
facility is the following:
At the 7Q10, dilution = (1.23 cfs + 13 cfs)/l.23 cfs
= 11.6
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At the 1Q10, dilution = (1.23 cfs + 10.1 cfs)/i.23 cfs
= 9.2
STEP 2: Conduct Toxicity Testing
EPA recommends that a discharger having a dilution less than
100 be required to conduct chronic toxicity testing. The permit
writer requested through a section 308 letter that the POTW provide
quarterly chronic toxicity data for the year prior to permit
reissuance. Tests using Selenastrum. Ceriodaphnia. and Pimephales
were required. The permit writer also required the permittee to
report the test results at the 48-hour endpoint so that acute
toxicity also could be measured. Table D-2 summarizes the results
of the whole effluent toxicity testing.
STEP 3: Determine Reasonable Potential for Excursions
As explained in the industrial example, the State interprets
its narrative objective for whole effluent toxicity to require that
the TSD recommendations of 0.3 TUa and 1.0 TUc be used as numeric
objective for acute and chronic toxicity, respectively. In
accordance with the State standards, the acute objective applies
under the 1Q10 flow and the chronic objective applies under the
7Q10 flow.
The reasonable potential determination of exceedance of the
acute or the chronic objective was conducted in the same way as
described in the industrial example. Upstream ambient water was
used as a diluent to assess contributions directly from background
toxicity; therefore, the upstream receiving water concentration was
set to zero. The maximum effluent concentration was again
estimated by using the statistical approach in section III.A.2 and
Appendix A. For the same reasons as were expressed in the
industrial example, a multiplier of 4.7 was used.
The receiving water concentration for chronic toxicity for
comparison with the chronic objective was calculated using data
from Table D-2:
Cr = [ (4.7 x 2 TUc x 1.23 cfs^ + (0 TUc X 13 cfs^]
(1.23 cfs + 13 cfs)
= 0.8 TUc , where
13 cfs = the background receiving water flow at 7Q10
1.23 cfs = the mean effluent flow
4.7	= the statistical effluent multiplier
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4 TUc = the maximum effluent concentration
The value of the calculated receiving water concentration, 0.8
TUc, is less than the chronic water quality objective of l.o TUc,
and, therefore, there is no reasonable potential for the chronic
objective to be exceeded.
To calculate the receiving water concentration for acute
toxicity, the permit writer first converted the chronic toxicity
data into equivalent acute toxicity units by applying the ACR of 2
obtained from the monitoring data. The receiving water concen-
tration for acute toxicity was then calculated:
Cr = [f 4.7 x 2 TUc / 2 ACR x 1.23 cfs)+(0 TUc x 10.1 cfs)]
(1.23 cfs + 10.1 cfs)
=0.5 TUa
where 10.1 cfs is the receiving water flow at 1Q10, 2 is the acute
to chronic ratio, and other values are the same as above. The
calculated value of 0.5 TUa is greater than the objective of 0.3
TUa. Therefore, there is reasonable potential for the acute
objective to be exceeded and permit limits were developed for whole
effluent toxicity.
D. DETERMINE WASTE LOAD ALLOCATIONS
WLAs for chemicals and whole effluent toxicity were determined
using information on the available dilution at the edge of the
mixing zone. The calculation of WLAs using the steady-state model
was described previously in Case 1 (also, see Appendix B). Using
this equation, the WLAs for the POTW are:
Toxicity	Copper Chlorine	Ammonia
	fTU)	fug/1)	fuq/1)	fuq/1)
WLAa	2.8	197	175	35,860
WLAC	11.6	.147	127	4,979
E. DEVELOP PERMIT LIMITS
The permit limit development process described in Appendix B
was applied to all pollutants. This process is identical to that
explained previously in Case 1 except that: 1) the WLA for acute
toxicity needs to be expressed in equivalent chronic toxic units by
multiplying by the ACR of 2; and 2) daily sampling of chlorine is
required in the permit.
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The calculated LTA and permit limits are:
Toxicity Copper Chlorine Ammonia
fTUal ruq/11 fua/1)	(uq/1)
LTAa	1.8	55.4	56.2	11,511
LTAc	6.1	70.7	66.9	2,625
MDL	5.6	197	175	8,162
AML	2.8	91	87	4,067
F. DETERMINING AND EXPRESSING THE CONTROLLING EFFLUENT
LIMITS
The treatment technology for POTWs is secondary treatment and
is characterized by effluent limits for biochemical oxygen demand,
total suspended solids, and pH. There are no BAT limits for toxics
for POTWs, so there was no need to compare these water quality-
based limits with other limits to determine which were more
stringent.
The permit writer decided to use acute toxicity tests rather
than chronic tests to measure compliance with the toxicity effluent
limits. The appropriate effluent limits in terms of TUa were
calculated by dividing the above calculation for TUc by the ACR of
2 that was obtained from effluent monitoring.
In accordance with NPDES regulations, the effluent limits for
chemicals were expressed in the permit as mass (pounds per day) by
multiplying the concentrations above by the effluent flow of 1.23
cfs and the conversion factor of 5.394. Because there is no
equivalent mass based unit for toxicity, toxicity mass limits are
impractical under the regulation.
Toxicity	Copper	Chlorine	Ammonia
	(TUa)	(lb/dl	flb/d)	flb/d)
MDL	2.8	1.31	1.16	54.2
AML	1.4	0.64	0.58	27.07
IV. CASE 3: MULTIPLE DISCHARGES INTO THE SAME REACH
Permit development for water quality-based toxics control has
been illustrated for two single dischargers. This process
increases in complexity in cases of multiple dischargers into a
reach. The development of permit limits for multiple dischargers
is based on the degradation in water quality resulting from the
combined discharges, the development of total maximum daily loads
(TMDLs) for the river reach before generating WLAs, and the
allocation of discharges to each discharger. The following example
describes the permit development process when two dischargers
release effluent into the same reach of a river. The dischargers
are the Jaybird manufacturing plant described in Case 1 and the
D-19

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Auburn POTW described in Case 2. These facilities discharge into
the Locapunct River, whose flow characteristics were described
previously.
A. EFFLUENT CHARACTERIZATION
The major differences in the effluent characterization for one
facility and for multiple facilities is to identify those
pollutants that are common to more than one facility, and to
determine whether the combined discharges cause or are likely to
cause water quality standards excursions.
STEP 1: Identify Pollutants of Concern
Based on the data in Form 2C, the DMRs from the Jaybird
Corporation and the data in the DMRs and section 308 request from
the Auburn POTW, the permit writer found two contaminants common to
both discharges: copper and whole effluent toxicity. Lead and
nickel were found to be a problem at the Jaybird Corporation, but
since there were no complicating discharges from the POTW, it was
dealt with as a pollutant only at the metal finishing facility.
Similarly, chlorine and ammonia were discharged solely by the POTW,
so it was not necessary to provide effluent limits for the metal
finishing facility for these chemicals.
STEP 2: Determine the Acute and Chronic NPLs for Pollutants of
Concern
The numerical standards adopted by the State already have been
presented. The relevant values for copper and whole effluent
toxicity are:
	Chronic	Acute
Copper	17.1 ug/1	25.7 ug/1
Toxicity	1.0 TUc	0.3 TUa
STEP 3: Determine Dilution for Aquatic Life and Human Health
Impacts
Since this example is concerned with potential excursions
above standards resulting from the collective discharge of two
dischargers, the calculation of dilution includes the combined
effluent flow from both facilities. The combined dilution can be
characterized by the complete mixing equation:
Cr = (Cd, Qd, + Cd2 Qd2 + CsQs) / (Qd1 + Qd2 + Qs) , where
Qd, and Qd2 = the flows of the two facilities
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Cd1 and Cd2
= the effluent concentrations of the two
facilities
Cs = the upstream receiving water concentration
Qs = the receiving water flow
STEP 4: Determine Reasonable Potential for Excursions
To determine if acute or chronic objectives were exceeded as
a result of the combined discharges into the river, the receiving
water concentration of each pollutant was calculated and compared
to the appropriate objective. The receiving water concentration
calculation was based on the maximum value of the effluent
concentrations (obtained from effluent data and multiplied by the
appropriate statistical factor), average effluent flows, background
receiving water concentrations, and appropriate river flows. All
this information has been presented previously in the separate
examples. The following results were obtained:
Receiving Water
Objectives	Concentration
	fuq/1)	fug/1)	
Copper
Chronic	17.1	156
Acute	25.7	194
Toxicity
Chronic	l.o	0.57
Acute	0.3	0.45
These calculations demonstrated exceedances of the copper
chronic and acute objectives and the toxicity acute objective.
Permit limits were required.
B. TMDLs AND WLAs
WLAs were calculated to develop permit limits. WLAs for each
discharger and chemical were based on calculated TMDLs, the total
load to the Locapunct River that would not result in water quality
standards exceedances. TMDLs are comprised of a load allocation
for nonpoint sources, WLAs for point sources, and, if required by
the State, a reserve capacity. TMDLs are further described in the
TSD (see Chapter 4).
STEP 1: Calculate TMDL
The first step in developing individual WLAs for the two
dischargers was to develop TMDLs for each pollutant of concern.
D-21

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TMDLs were developed in the same way as an individual WLA with the
total load of a pollutant from the two dischargers being considered
as a single discharge.
The calculation of TMDLs used the following formula:
TMDL = NPL x (Qt + Qs) , where
NPL = the numeric protective level
Qt = the combined flow of both effluent
Qs = the appropriate receiving water flow
The acute objective copper TMDL was calculated by using the
data presented in the previous two examples as:
TMDL = 25.7 ug/1 X (0.034 cfs + 1.23 cfs + 10.1 cfs)
= 292 ug-cfs/1 , where
25.7 ug/1	= the acute criterion
0.034 cfs and 1.23 cfs = the average effluent flows
10.1 cfs	= the 1Q10
Similar calculations were made for chronic copper and acute
toxicity. A TMDL was not calculated for chronic toxicity because
chronic toxicity does not demonstrate additivity (see TSD, Chapter
1). The results are summarized below.
Total Maximum Daily Loads
	Chronic	Acute	
Copper (ug-cfs/1)	244	292
Toxicity (TUa-cfs/1)	NA	3.4
STEP 2: Develop WLAs
The State had adopted an approach into the water quality plan
that described how WLAs were to be calculated. The approach
required that existing upstream concentrations be used to determine
the load allocation part of the TMDL and that 10 percent of the
TMDL had to be reserved and unavailable for allocation. The
remainder of the TMDL could be apportioned to point sources in the
WLA.
The permit writer decided to allocate the waste loads based on
the proportion of the existing load of each parameter that was
attributed to each of the existing discharges. Based on the
information shown in Tables D-l and D-2 and the average effluent
D-22

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flows, the pollutant loads from each facility are shown below.
Auburn POTW	Jaybird	Corporation
Parameter	Load Proportion	Load	Proportion
Copper
(ug-cfs/1)	227.6	0.77	66.1	0.23
Toxicity
(TUa-cfs/1)	1.23	0.90	0.14	0.10
Individual WLAs were then determined using the following
equation:
WLA = (TMDL - LA - 10% TMDL) x proportion / Qd
where the chronic TMDL was used to determine the chronic WLA, and
the acute TMDL was used to determine the acute WLA for each
facility. The WLAs for each pollutant and for each facility are
presented as follows:
Acute WLA	Chronic WLA
Parameter	POTW	Jaybird	POTW	Jaybird
Copper (ug/1)	134	1,450	98.4	1,063
Toxicity (TUa)	2.2	9.0	NA	NA
C. PERMIT LIMIT DEVELOPMENT
Once the WLAs had been determined, permit limit development
proceeded as in the previous examples. LTAs were calculated from
the WLAs, and the limiting LTA was selected for calculating permit
limits. For the metal finisher, where BAT limits were more
restrictive than the water quality-based limits, the BAT limits
applied. For the POTW, permit limits for toxic materials were
required only to prevent exceedances of water quality standards.
This process is summarized below.
STEP 1: Calculate LTAs
The LTA was calculated for each discharger and pollutant as
described in section III.A.3 and Appendix B; the LTAs are shown
below.
Acute LTA	Chronic LTA
Parameter	POTW	Jaybird	POTW	Javbird
Copper (ug/1)	37.7	361	47.3	468
Toxicity (TUa)	0.71	2.9	NA	NA
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STEP 2: Determine the Most Limiting LTA
The minimum LTA was used to calculate MDLs and AMLs. The
acute LTA was the lower LTA for both pollutants.
STEP 3: Calculate the Maximum Daily and Average Monthly Limits
The MDL and AML were calculated as described in section
III.A.3 and Appendix B.
Average Monthly Limit Maximum Daily Limit
Parameter	POTW	Javbird	POTW	Jaybird
Copper (ug/1) 62	632	134	1,448
Toxicity (TUa) 1.1	4.5	2.2	9.0
STEP 4: Express the Limits
The final step is to compare the water quality-based limits to
the BAT limits to ensure that the more restrictive of the two are
used, and to express the copper limits in terms of. mass. The
copper water quality-based requirements for Jaybird Corporation are
more limiting than BAT requirements (see Case 1) . Therefore, water
quality based limits are required by the permit. In addition, the
limits are lower than those calculated when only one of the
facilities were considered. The final permit limits are listed
below.
Average Monthly Limit Maximum Daily Limit
Parameter	POTW	Javbird	POTW	Javbird
Copper (ug/1) 0.41	0.12	0.89	0.27
Toxicity (TUa) l.l	4.5	2.2	9.0
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