United States        Office of Wastewater          EPA 832-B-04-003
             Environmental Protection   Management               (Month/Year)
             Agency          (4203M)       http://vwvw.epa.gov/npdes/permitbasics
vvEPA      National
             Whole Effluent Toxicity (WET)
             Implementation Guidance
             Under the NPDES Program
                          DRAFT
                         November 2004
                       (Released on December 28, 2004)

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                                Notice and Disclaimer

     This Whole Effluent Toxicity (WET) Implementation Guidance provides recommendations for
implementing National Pollutant Discharge Elimination System (NPDES) WET programs in a manner
consistent with existing U.S. Environmental Protection Agency (EPA) guidance and regulations and
advances in water quality management under the Clean Water Act. This guidance applies to all NPDES
permitting authorities—Federal, State, or Tribal. This document has been developed to promote national
consistency in WET implementation programs to the extent practical, and to ensure that the information
and requirements for WET in all NPDES permits are clear, well documented, and provide for protection
against impacts to aquatic life consistent with State and Tribal water quality standards. This document
does not substitute for the Clean Water Act, a NPDES permit, or EPA, State, or Tribal regulations
applicable to permits or WET testing; nor is this document a permit or a regulation itself. This document
does not and cannot impose any legally binding requirements on EPA, States, Tribes, or NPDES
permittees. EPA, State, and Tribal officials retain discretion to adopt approaches on a case-by-case basis
that differ from this guidance based on an analysis of site-specific circumstances. This guidance may be
revised without public notice to reflect changes  in EPA policy.
November 2004

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                                  Acknowledgments
     This guidance was prepared through the cooperative efforts of the U.S. Environmental Protection
Agency's (EPA) Office of Water and other EPA offices: Office of Wastewater Management, Office of
Science and Technology, Office of General Counsel, Office of Enforcement and Compliance Assurance,
representatives from EPA Regions and the Office of Research and Development. An EPA Whole
Effluent Toxicity (WET) Implementation Workgroup was established with representatives from each of
the above offices who reviewed the document during its development. The contributions of all EPA WET
Workgroup members, as well as the comments received from EPA's 10 Regions, were instrumental in
producing this document. The primary authors and contributors to the development of this document are
listed below.
Primary Authors and Contributors
Headquarters

Fox, John

Gardner, Thomas
Greenwald, Kathryn

Kelly, Marion
Kopylev, Leonid
Phillips, Laura
Swietlik, William
Zobrist, Marcus

Regional Offices

Hyatt, Marshall

Pfeifer, David
Jennings, Phillip
Denton, Debra
Stuber, Robyn
Office of Research and Development - National Center for Environmental
Assessment
Office of Science and Technology - Standards and Health Protection Division
Office of Enforcement and Compliance Assurance, Office of Regulatory
Enforcement - Water Enforcement Division
Office of Science and Technology - Engineering and Analysis Division
Office of Science and Technology - Engineering and Analysis Division
Office of Wastewater Management - Water Permits Division
Office of Science and Technology - Health Effects Criteria Division
Office of Wastewater Management - Water Permits Division
Region 4 - Water Management Division; Permits, Grants, and Technical
Assistance Branch
Region 5 - Water Division, Water Quality Branch
Region 6 - Water Quality Protection Division, NPDES Permits Branch
Region 9 - Water Division, Monitoring and Assessment Office
Region 9 - Water Division, Clean Water Act Standards and Permit Office
November 2004

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                                   Contents

                                                                             page
Notice and Disclaimer	iii
Acknowledgments 	 v
Tables and Figures	ix
Executive Summary	xi
Acronyms and Abbreviations  	 xv
Glossary	 xvii

1 INTRODUCTION	 1
    1.1  BACKGROUND	 1
    1.2  PURPOSE OF THIS GUIDANCE	 2
    1.3  WHAT'S NEW IN THIS GUIDANCE	 3
    1.4  WHO SHOULD USE THIS DOCUMENT	 5
    1.5  DOCUMENT ORGANIZATION 	 5

2 SETTING APPROPRIATE WATER QUALITY GOALS FOR WET	 7
    2.1  EPA'S RECOMMENDED WATER QUALITY CRITERIA FOR WET	 7
        2.1.1   Mixing Zones  	 8
    2.2  DESIGNATED USES  	 9

3 STATE IMPLEMENTATION PROCEDURES TO ENSURE WET CRITERIA ARE MET  	 11
    3.1  GENERAL RECOMMENDATIONS	 11
        3.1.1   Recommendations for NPDES States	 11
        3.1.2   Recommendations for EPA Regions	 12
    3.2  EPA/NPDES-AUTHORIZED PROGRAM OVERSIGHT	 13

4 DETERMINING THE NEED FOR WET LIMITS IN NPDES PERMITS	 14
    4.1  DETERMINING THE NEED FOR PERMIT LIMITS WITH
        EFFLUENT MONITORING DATA	 15
        4.1.1   Characterizing Effluent Quality with Respect to WET	 15
        4.1.2   Determining Validity of WET Data 	 15
        4.1.3   WET Data Evaluation Checklist	 16
        4.1.4   Determining Representativeness of WET Data  	 16
        4.1.5   Accounting for Effluent Variability 	 18
    4.2  APPROACHES FOR DETERMINING THE NEED FOR A WET LIMIT
        AND SETTING WASTELOAD ALLOCATIONS FOR TOXICITY
        AND PERMIT LIMITS	 19
        4.2.1   Dynamic Modeling 	 20
        4.2.2   Steady-state Modeling	 21
    4.3  DETERMINING THE NEED FOR PERMIT LIMITS WITHOUT
        EFFLUENT MONITORING DATA	 26

5 DEVELOPMENT OF WET REQUIREMENTS IN NPDES PERMITS	 27
    5.1  PERMIT DOCUMENTATION RECOMMENDATIONS	 27
    5.2  DERIVATION AND EXPRESSION OF PERMIT LIMITS	 28
        5.2.1   Permit Limit Derivation Procedures	 29
        5.2.2   Mixing Zone Application in WET Permit Conditions 	 29
        5.2.3   Expression Of Permit Limits	 29
        5.2.4   EPA's Recommendation for Deriving Permit Limits for Low-Flow

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              Dilution Situations	  30
    5.3 PERMIT CONDITIONS FOR POSSIBLE OUTCOMES OF A
       REASONABLE POTENTIAL DETERMINATION  	  31
       5.3.1   Permit Conditions for an Excursion above the Applicable Toxicity
              Water Quality Standard or for the Reasonable Potential for an Excursion
              above the Applicable Toxicity Water Quality Standard	  31
       5.3.2   Permit Conditions When Reasonable Potential Is Demonstrated
              With Fewer than 10 Valid WET Data Points 	  32
       5.3.3   When Reasonable Potential for WET Is Not Demonstrated With
              Sufficient Data Points	  36
    5.4 MONITORING FREQUENCY RECOMMENDATIONS 	  37
       5.4.1   Monitoring Frequency for Accelerated Testing  	  38
    5.5 TOXICITY REDUCTION EVALUATION/IDENTIFICATION STUDIES 	  38

6 CONCLUSION - EPA RECOMMENDATIONS TO REGIONS AND NPDES STATES	  41
    6.1 GENERAL NPDES WET PROGRAM RECOMMENDATIONS
       AND REQUIREMENTS	  41
    6.2 SPECIFIC RECOMMENDATIONS ON WET EFFLUENT TEST DATA
       USED FOR DETERMINING RP 	  42
    6.3 GENERAL RECOMMENDATIONS FOR DEVELOPING VALID AND
       REPRESENTATIVE WET NPDES PERMIT LIMITS, NUMERIC
       MONITORING TRIGGERS AND/OR OTHER PERMIT CONDITIONS 	  42
    6.4 SPECIFIC RECOMMENDATIONS FOR ISSUING  PERMITS WITH WET
       EFFLUENT LIMITS, NUMERIC MONITORING TRIGGERS AND OTHER
       PERMIT CONDITIONS  	  43

APPENDIX A - EXAMPLE: PERMIT LIMIT DERIVATION PROCEDURES  	  A-l
APPENDIX B - EXAMPLE: DERIVING PERMIT LIMITS FOR LOW-FLOW
             DILUTION SITUATIONS	  B-l
APPENDIX C - 1994 EPA WET CONTROL POLICY 	  C-l
APPENDIX D - BIBLIOGRAPHY	  D-l
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                                Tables and Figures

Table 1.  Permit Requirements 	 12
Table 2.  WET Permit Requirements and Examples of Conditions	 28
Table 3.  Likelihood of Detecting at Least One Toxic Event Using n Observations,
        For Specified True Rates of Occurrence	 37
Figure 1. Overview  	 4
Figure 2. WET reasonable potential flowchart	 21
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                                  Executive Summary

Background

     The Clean Water Act (CWA) was enacted in 1972 with the objective of "restoring the chemical,
physical, and biological integrity of the Nation's waters." Among the U.S. Environmental Protection
Agency's (EPA's) efforts to achieve this objective is implementation of the National Pollutant Discharge
Elimination System (NPDES) program. This program is designed to control toxic discharges, implement
water quality standards, and restore and maintain "fishable and swimmable" designated beneficial uses in
waters of the United States. In the 32 years since the CWA was enacted, EPA and States authorized to
administer EPA's NPDES permitting program have made significant progress toward achieving these
goals. Point sources that discharge pollutants must comply with the terms and conditions of NPDES
permits. One approach that NPDES permits use to control the discharge of toxic pollutants is to control
whole effluent toxicity (WET).

History

     During the 1980s, NPDES permits for industries and publicly owned treatment plants (POTWs)
included technology-based treatment requirements. In addition, permits for POTWs included
pretreatment programs to control the passthrough of and interference by pollutants from industrial users
of the POTW. Even with those technological solutions, many discharges remained toxic. WET data
gathered in the early 1980s indicated that some NPDES facilities had discharged effluents with sufficient
toxicity to cause water quality problems. Further reductions were necessary in order to achieve
compliance with State water quality standards that prohibited the discharge of toxic pollutants in toxic
amounts, or otherwise provided for the maintenance and propagation of a balanced population of aquatic
life.

      Therefore, EPA developed a policy to reduce or eliminate toxic discharges. The "Policy for the
Development of Water Quality-based Permit Limitations for Toxic Pollutants" (49 FR 9016, March 9,
1984) introduced EPA's integrated toxics control program under NPDES. This program consists of both
chemical-specific and biological methods for the assessment and reduction of toxic discharges. In 1989,
EPA promulgated regulations specifying procedures for determining when water quality-based effluent
limitations are required in NPDES permits. This regulation also established that, where necessary, the
level of water quality to be achieved by such limits must be  derived from and comply with water quality
standards. In support of these regulations, EPA developed the Technical Support Document for Water
Quality-based Toxics Control (TSD)  in 1985 and revised it in 1991. The TSD provides guidance on
water quality implementation issues, including chemical and biological approaches.

     On July 7, 1994, EPA issued a national policy on  effluent limitations in NPDES permits to control
WET for the protection of aquatic life (USEPA 1994d) (see Appendix C of this document). Comprising
eight policy statements, the document reaffirmed EPA's strong continuing commitment to the existing
CWA provisions and water quality permitting regulations at 40 CFR 122.44(d)(l). This guidance
reaffirms those eight policy statements and builds upon that foundation to provide further  clarification
for NPDES authorities (Regions, States, and Tribes). Based on existing regulations, NPDES authorities
must determine whether a discharge causes, has the reasonable potential to cause, or contributes to an in-
stream excursion above a numeric  criterion or a narrative criterion within an applicable State water
quality standard and, where appropriate, establish permit limits on WET, for lethal and sub-lethal effects.
On October 26, 1995, EPA promulgated WET test methods  (USEPA 1993a,b,c; 1994a,b) and added them
to the list of EPA methods approved under Section 304(h) of the CWA (40 CFR 136) for use in the


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NPDES program. The methods were subsequently challenged, and under a settlement agreement, EPA
conducted a round-robin study, which evaluated 12 of those test methods (USEPA 2001b,c) EPA also
prepared a WET test method variability guidance document (Understanding and Accounting for Method
Variability in Whole Effluent Toxicity Applications Under the National Pollutant Discharge Elimination
System, USEPA 2000b) and a WET test methods guidance document (USEPA 2000c). In November
2002, EPA promulgated the revised WET test methods (USEPA 2002 a,b,c) [see 67 FR 69951-69972,
November 19, 2002].

Why WET Testing

     Whole effluent toxicity is the aggregate toxic effect of an aqueous sample (e.g., a reference
toxicant, an effluent, or a receiving water) measured directly by an aquatic toxicity test. Aquatic toxicity
tests are laboratory experiments that measure biological effects (e.g., growth, survival, and reproduction)
of a test sample (effluents or receiving waters) on aquatic organisms. In aquatic toxicity tests, organisms
of a particular species and age are held in test chambers and exposed to different concentrations of an
aqueous sample. Observations are then made at predetermined exposure periods. At the end of the test,
the responses of test organisms are used to estimate the effects of the test sample, either in absolute terms
or relative terms (i.e., the effects of an effluent in receiving waters).

     Whole effluent toxicity test results are an integral tool in the assessment of water quality. For the
protection of aquatic life, EPA's integrated strategy includes the use of three control approaches: the
chemical-specific control approach, the WET control approach, and the biological criteria/bioassessment
approach. The two primary advantages of using WET controls over individual, chemical-specific controls
are (1) WET tests evaluate the integrated effects of all chemical(s) in the aqueous sample; and (2) while
EPA has established aquatic life criteria for a relatively small number (126) of chemical-specific
pollutants, WET tests can measure toxicity caused by other compounds for which EPA does not have
chemical-specific numeric criteria for the protection of aquatic life or approved parameter-specific
analytical test methods. Another advantage to using WET testing is that it enables prediction and
avoidance of a toxic impact before the detrimental impact might occur (i.e., after the aquatic population
in the receiving water has experienced prolonged exposure to such toxicity). Reliance solely on
chemical-specific numeric criteria or bioassessments could result in a considerably less effective toxics
control program. These toxicity tests therefore should be fully implemented in a NPDES authority's
water quality control program. EPA policy and guidance recommend that States and Tribes use  chemical-
specific, toxicity, and biological measurements and criteria to monitor and protect designated uses. In
1991, EPA established its policy on independent application [Transmittal memorandum of Final Policy
on Biological Assessment and Criteria from Tudor Davies to Regions, June 19, 1991(USEPA 1991c)].
EPA's independent  application policy addresses how assessments based on these three kinds of criteria
are to be used to make water quality management decisions related to protection of aquatic life and
aquatic life uses. This National WET Implementation Guidance presents EPA's  approaches to addressing
some issues concerning WET implementation through the three overall goals outlined after this  section.

Effect of This Guidance

     EPA Headquarters expects permits issued by EPA Regions to be consistent with this guidance. EPA
strongly encourages States1 authorized to administer the NPDES program to implement the
recommendations in this guidance so that national consistency in implementing the WET program can be
1    State means any of the 50 States, the District of Columbia, Guam, the Commonwealth of Puerto Rico, the Virgin
    Islands, American Samoa, the Commonwealth of the Northern Mariana Islands, the Trust Territory of the Pacific
    Islands, or an Indian Tribe as defined in 40 CFR 122 which meets the requirements of § 123.3.

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achieved. State permitting authorities may need to revise their current procedures (water quality
standards or NPDES, or both) to fully implement the national recommendations presented in this
document.

     This document clarifies several issues regarding WET implementation and reaffirms EPA guidance
in the TSD (USEPA 199la) and the Agency's 1994 WET Control Policy. This document provides
NPDES regulatory authorities and all stakeholders, including permittees, with guidance and
recommendations on how to implement WET in NPDES permits. EPA's recommendations and
conclusions are detailed in Chapter 6. A summary of the recommendations and a re-emphasis of existing
regulatory requirements referenced in this document include the following:

     •   Under the CWA, States are required to have statewide WET implementation procedures (see
         Section 3) for their numeric or narrative WET standards.

     •   Pursuant to EPA regulations at 40 CFR 122.44 (d)(l), during permit development, the permit
         writer must determine whether the discharge causes, has the reasonable potential to cause or
         contribute to non-attainment of WQS for WET [reasonable potential (RP) determination]. The
         RP determination may be based on toxicity data, if available, or without specific WET data (if
         there is none). Based on the RP outcome, permit conditions are expressed as either WET limits
         or WET numeric monitoring triggers (see Section 4).

     •   Permit limits  for WET should be expressed as both a monthly average (or a monthly median
         limit, for zero dilution situations) and a daily maximum2 (see Section 5.2).

     •   Permit limits  should be specific regarding the WET testing requirements [i.e., specify the test
         type (acute and/or chronic), statistical endpoint to be derived and reported, test species, and test
         method to be  used.] (See Section 3.1, Table 1, and Section 5.4, Table 2).

     •   Permits  with WET limits or monitoring requirements should include EPA's recommended
         "step-wise approach" to trigger remedial follow-up actions when the WET limits or numeric
         monitoring triggers are exceeded (see Section 5).

     •   Permits  with WET limits or monitoring requirements should establish an appropriate frequency
         for toxicity testing sufficient to appropriately characterize the discharge and assess compliance
         (see  Section 5.5).

     •   Permittees are responsible  for maintaining compliance with permit requirements and for the
         quality of their  self-monitoring data.

                                   GOALS OF THIS DOCUMENT

     This document addresses three  goals concerning WET implementation:

     1.   National Consistency: provide the framework for a WET program that is nationally consistent
         across NPDES  States, Tribes, and EPA Regions.
    This document provides examples using toxic units (TUs); however, EPA recognizes that permitting authorities also
    use other means to quantify and express WET values [(e.g., effluent concentrations related to a receiving water
    concentration (RWC)]. In cases where effluent concentrations are used, the permit limits would be expressed as
    minimums rather than maxim urns. For example, where a limit might be a maximum of two toxic units (therefore the
    permit limit expressed as < 2.0 TU), a similar limit expressed as an effluent concentration would be expressed as
    a minimum NOEC value (NOEC > 50% effluent).

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     2.   NPDES Regulatory Compliance: reinforce compliance with existing NPDES regulations,
         including the RP determination regulations at 40 CFR122.44(d) by reiterating the technical and
         regulatory basis for toxicity requirements in NPDES permits, while allowing flexibility
         consistent with the regulations where appropriate (e.g., allowing time to collect additional data
         for RP determination to supplement limited data sets).

     3.   Emphasis on Existing Guidance, Policy, and Regulations: restate and clarify, where
         necessary, existing guidance, policy, and regulations on WET testing in the NPDES program,
         by referring the reader to portions of the TSD, WET test methods (USEPA 2002a,b,c), and
         various WET guidance documents.
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                           Acronyms and Abbreviations3

 1Q10                lowest one-day stream flow during any 10-year period
 7Q10                lowest consecutive seven-day average stream flow during any 10-year period
 ACR                acute-to-chronic ratio
 AML                average monthly limit
 AWL                average weekly permit limit
 CCC                criterion continuous concentration/chronic criterion
 Ce                  concentration of effluent
 CFR                Code of Federal Regulations
 CFS                 cubic feet per second
 ChV                chronic value
 CMC                criterion maximum concentration/acute criterion
 CV                  coefficient of variation
 CWA                Clean Water Act
 DMR                discharge monitoring report
 EPA                U.S. Environmental Protection Agency (also, the Agency)
 FR                  Federal Register
 GLI                 Great Lakes Initiative (Water Quality Guidance for the Great Lakes System)
 IC25                 inhibition concentration, 25 percent
 LC50                lethal concentration, 50  percent
 LOEC               lowest observed effect concentration
 LTA                long-term average (LTAa = acute LTA; LTAC = chronic LTA;
                      LTAa c = acute-to-chronic LTA, expressed in chronic units)
 MDL                maximum daily limit
 MGD                million gallons per day
 MML                monthly median limit
 NOEC               no observed effect concentration
 NPDES              National Pollutant Discharge Elimination System
 PCS                 Permit Compliance System
 PMSD               percent minimum  significant difference
 POTW               publicly owned treatment works
 QA                  quality assurance
 QC                  quality control
3    These acronyms and abbreviations may have other meanings in other EPA programs or documents.
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 Qe
 Qs
 RP
 RPMF
 RWC

 STORET
 TAC
 TIE
 TMDL
 TRE
 TSD

 TU
 UAA
 WET
 WLA
 WQBEL
 WQS
effluent flow
receiving water flow
reasonable potential
reasonable potential multiplier factor
receiving water concentration (sometimes referred to as in-stream waste
concentration, IWC)
EPA Storage and Retrieval Database
test acceptability criterion/criteria
toxicity identification evaluation
total maximum daily load
toxicity reduction evaluation
EPA's Technical Support Document for Water Quality-based Toxics Control
(USEPA, 199 la)
toxic unit (TUa = acute toxicity; TUC = chronic toxicity)
use attainability analysis
whole effluent toxicity
wasteload allocation
water quality-based effluent limit
water quality standard
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                                          Glossary4


Acute-to-Chronic Ratio (ACR) is the ratio of the acute toxicity of an effluent or a toxic to its chronic
toxicity. It is used as a factor for estimating chronic toxicity on the basis of acute toxicity data, or for
estimating acute toxicity on the basis of chronic toxicity data.

Acute Toxicity Test is a test to  determine the concentration of effluent or ambient waters that causes an
adverse effect (usually death) on a group of test organisms during a short-term exposure (e.g., 24, 48, or
96 hours). Acute toxicity is measured using statistical procedures (e.g., point estimate techniques or a
hypothesis test).

Ambient Toxicity is measured by a toxicity test performed using solely receiving water.

Average Monthly Limit (AML) is the highest allowable value for the average of daily discharges
obtained over a calendar month.

Chronic Toxicity Test is a short-term test, usually 96 hours or longer in duration, in which sublethal
effects (e.g., significantly reduced growth or reproduction) are usually measured in addition to lethality.
Chronic toxicity is defined as TUC = 100/NOEC or TUC = 100/ECp or ICp.

ChV is the chronic value, the geometric mean of the NOEC and LOEC.

Coefficient of Variation (CV) is a standard statistical measure of the relative variation in a distribution
or set of data, defined as the standard deviation divided by the mean. It is also called the relative standard
deviation (RSD). The CV can be used as a measure of within-laboratory and between-laboratory
precision or as a measure of precision among replicates for each treatment concentration.

Criterion Continuous  Concentration (CCC) is the  highest in-stream concentration of a toxic or an
effluent to which organisms can be exposed indefinitely without causing unacceptable effects such as the
exceedence of a chronic water quality criterion.

Criterion Maximum Concentration (CMC) is the highest in-stream concentration of a toxic or an
effluent to which organisms can be exposed for a brief period of time without causing an acute effect
such as the exceedence of an acute water quality criterion.

Discharge Monitoring Report (DMR) is EPA's standardized reporting form for the reporting of self-
monitoring results by permittees. DMRs must be used by "NPDES-approved States," as well as by EPA.
The EPA standardized forms may be modified to substitute the State agency's name, address, logo, and
other similar information, as appropriate, in place of EPA's.

Effect Concentration is a point estimate of the toxic concentration that would cause an observable
adverse effect (e.g., death,  immobilization, or serious incapacitation) in a given percent of the test
organisms. EC25 is a point estimate of the toxic concentration that would cause an observable adverse
effect in 25 percent of the test organisms.

Effluent Flow (Qe) is the flow (in cubic feet per second or million gallons per day) of a wastewater
discharge from a NPDES-regulated facility expressed in standard NPDES formulas by permit writers as
"Qe" to calculate water quality based effluent limits.
    These terms may have other meanings in other EPA programs or documents, including other programs under the
    CWA.

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Endpoint is a biological measurement used to quantify the results obtained from analytical methods such
as whole effluent toxicity testing [e.g., lethal concentration (LC50); inhibition concentration (IC25); and no
observed effect concentration (NOEC)]. Such endpoints are quantitative measurements of the responses
of test organisms (e.g., survival, growth, mobility, reproduction, and weight gain or loss) in response to
exposure to a serial dilution of effluent.

Hypothesis Testing is a statistical technique (e.g., Dunnett's test) for determining whether a tested
concentration results in a statistically different response from that observed in the control. For the multi-
concentration tests, the reported values determined by hypothesis testing are the "no observed effect
concentration (NOEC)" and "lowest observed effect concentration (LOEC)."

Inhibition Concentration is a point estimate of the toxic concentration that would cause a given percent
reduction in a non-lethal biological measurement (e.g., reproduction or growth). IC25 is a point estimate
of the toxic concentration that would cause a 25-percent reduction in a biological measurement of a test
organism (i.e., an endpoint).

In-stream Waste Concentration is the concentration of a toxic in the receiving water after mixing, if
allowed by State water quality standards (WQS). The in-stream waste concentration is the inverse of the
dilution factor. It is also referred to as the receiving water concentration (RWC).

Lethal Concentration, 50 Percent (LC50) is the toxic or effluent concentration that would cause death
in 50 percent of the test organisms over a specified period of time.

Long-term Average (LTA) of pollutant concentration or effluent toxicity is calculated from a wasteload
allocation (WLA),  typically assuming that the WLA is a 99th percentile value (or another upper bound
value) based on the lognormal distribution. One LTA is calculated for each WLA (typically an acute
LTA and a chronic LTA for aquatic life protection). The LTA represents expected long-term average
performance from the permitted facility required to achieve the associated WLA.

Lowest Observed Effect Concentration (LOEC) is the lowest concentration of an effluent or test
sample with an effect different from the control effect according to the statistical test used for analysis of
toxicity that results in adverse effects on the test organisms (i.e., where the values for the  observed
endpoints statistically differ from the control).

Maximum Daily Limit (MDL) is the highest allowable discharge measured during a  calendar day or 24-
hour period representing a calendar day.

Minimum Significant Difference  (MSD) is a measure of test sensitivity that establishes  the minimum
difference required between a control and a test treatment in order for that difference to be considered
statistically significant.

Mixing Zone is an area where an effluent discharge undergoes initial dilution with water from upstream
and is extended to cover the secondary mixing in the ambient water body; an allocated impact zone in
which water quality criteria can be  exceeded provided that acutely toxic conditions are prevented. States
determine whether mixing zones are allowed.

National Pollutant Discharge Elimination System (NPDES) is the EPA program that regulates
discharges to the nation's waters. Discharge permits issued under the NPDES program are required by
EPA regulation to contain, where necessary, effluent limits based on water quality criteria for the
protection of aquatic life and human health.
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No Observed Effect Concentration (NOEC) is the highest tested concentration of an effluent or test
sample whose effect is not different from the control effect, according to the statistical test used (see
LOEC). The NOEC is usually the highest tested concentration of an effluent or toxic that causes no
observable adverse effect on the test organisms (i.e., the highest concentration of toxicity at which the
values for the observed responses do not statistically differ from the controls).

Percent Minimum Significant Difference (PMSD) is the minimum significant difference divided by
the control mean, expressed as a percent (see minimum significant difference).

Point Estimate is a statistical inference that estimates the true value of a parameter by computing a
single value of a statistic from a set of sample data.

Publicly Owned Treatment Works (POTWs) are facilities, operated by local communities or States or
their contractors, that treat domestic wastewater or wastewater from indirect dischargers (e.g., industrial
facilities).

Quality Assurance (QA) is  a practice in toxicity testing that addresses all activities affecting the quality
of the final effluent toxicity data. QA includes evaluation of effluent sampling and handling, source and
condition of test organisms, equipment condition, test conditions, instrument calibration, replication, use
of reference toxics, record keeping, data, and other aspects of the test and testing procedures.

Quality Control (QC) is the set of focused, routine, day-to-day activities carried out as part of an overall
QA program.

Reasonable Potential (RP)  is the likelihood that an effluent will cause or contribute to an excursion
above a water quality standard based on a number of factors, including the use of data (e.g., whole
effluent toxicity test data). In the context of this document, references to RP and WET limits include both
lethal and sub-lethal effects.

Reasonable Potential Multiplier Factor (RPMF) is a numerical value that multiplies the maximum
observed effluent value in an effluent data set.

Receiving Water Concentration (RWC) is the concentration of a toxic in the receiving water after
mixing, sometimes referred to as the in-stream waste concentration (IWC).

Receiving Water Flow (Qs) is the flow of the water receiving the discharge expressed in cubic feet per
second or millions of gallons per day.

Reference Toxicant Test is  a check of the sensitivity of the test organisms and the suitability of the test
methodology in a toxicity test. Reference toxicant data are part of a routine QA/QC program to evaluate
the performance of laboratory personnel and the robustness and sensitivity of the test organisms.

Significant Difference is defined as a statistically significant difference (e.g., 95 percent confidence
level) in the means of two distributions of sampling results.

Standard Deviation is a measure of the variability of a  set of data, calculated as the square root of the
variance.

Statistic is a computed or estimated quantity, such as the mean, standard deviation, or coefficient of
variation.

Test Acceptability Criteria (TAC) are specific criteria for determining whether toxicity test results are
acceptable pursuant to EPA's WET test methods in 40 CFR  136 (additional TACs may be established by

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a State permitting authority). The effluent and reference toxicant must meet specific criteria as defined in
the test method (e.g., for the Ceriodaphnia dubia survival and reproduction test, the criteria are: 80% or
greater survival of all control organisms and an average of 15 or more young per surviving female in the
control solution. Of the surviving control females, 60% must produce three broods.)

Total Maximum Daily Load (TMDL) is the allocation of the pollutant load to each source, which is
calculated by estimating the maximum amount of a pollutant that a water body can receive and still meet
water quality standards.

Toxicity Test is a procedure using living organisms to determine whether a chemical or an effluent is
toxic. A toxicity test measures the degree of the effect of a specific chemical or effluent on exposed test
organisms.

Toxic Unit (TU) is a measure of toxicity in an effluent as determined by the acute toxicity units (TUa) or
chronic toxicity units (TUC) measured. The larger the TU, the greater the toxicity.

Toxic Unit-Acute (TUJ is the reciprocal of the effluent concentration that causes 50 percent of the
organisms to die in an acute toxicity test (TUa = 100/LC50) (see LC50).

Toxic Unit-Chronic (TUC) is the reciprocal of the effluent concentration that causes no observable
effect on the test organisms in a chronic toxicity test (TUC = 100/NOEC or 100/EC25) (see NOEC).

Toxicity Identification Evaluation (TIE) is a set of site-specific procedures used to identify the specific
chemical(s)  causing effluent toxicity.

Toxicity Reduction Evaluation (TRE) is a site-specific study conducted in a step-wise process to
identify the causative agents of effluent toxicity, isolate the source of toxicity, evaluate the effectiveness
of toxicity control options, and then confirm the reduction in effluent toxicity after the control measures
are put in place.

Variance is a measure of the dispersion in a set of values, defined as the sum of the squared deviations
from the  mean divided by the total number of values in the set.

Wasteload Allocation (WLA) is the portion of a receiving water's TMDL that is allocated to one of its
existing or future point sources of pollution.

Water Quality Criterion. Numeric criteria are scientifically derived ambient concentrations  developed
by EPA or States for various pollutants of concern to protect human health and aquatic life. Narrative
criteria typically are statements that describe the desired water quality goal.

Water Quality-based Effluent Limit (WQBEL) is a NPDES permit limit established by either an EPA
or a State permit writer that is developed to assure protection of aquatic life or human health consistent
with applicable State or Tribal water quality standards, including the designated uses for a particular
water body,  the established criteria, and measured analytical data (e.g., chemical, WET or biosurvey), in
accordance with the recommendations provided in EPA's 1991 Technical Support Document (TSD).

Water Quality Standard (WQS). Water quality standards are provisions of State or Federal law which
consist of a designated use or uses for the waters of the United States and water quality criteria for such
waters based upon such uses. Water quality standards are to protect the public health or welfare, enhance
the quality of water, and serve the purposes of the Act.  States and authorized Tribes are required to
develop and adopt a statewide antidegradation policy and identify the methods for implementing the
policy.


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Whole Effluent Toxicity (WET) is the total toxic effect of an effluent measured directly with a toxicity
test.
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                                      1 INTRODUCTION

1.1  BACKGROUND

     The Clean Water Act (CWA) was enacted in 1972 with the objective of "restoring the chemical,
physical, and biological integrity of the Nation's waters." Several goals and policies were established in
the Act, including the following:

     •   Eliminate the discharge of pollutants into navigable waters by 1985;

     •   Achieve, wherever attainable, an interim goal of water quality that provides for the protection
         and propagation of fish, shellfish, and wildlife, and for recreation in and on the water by
         November 1,  1983; and

     •   Prohibit the discharge of toxic pollutants in toxic amounts.

     In the 32 years since the CWA was enacted, the U.S. Environmental Protection Agency (EPA) and
States authorized to administer EPA's National Pollutant Discharge Elimination System (NPDES)
permitting program have made  significant progress toward achieving these goals. The NPDES program is
designed to control toxic discharges, implement water quality standards, and restore and maintain
"fishable and swimmable" designated beneficial uses in waters of the United States. Point sources that
discharge pollutants must comply with the terms and conditions of a NPDES permit. In setting these
terms and conditions, EPA and the States have integrated their control of toxic pollutants through the
combined use of three approaches, as described in the Technical Support Document for Water Quality-
based Toxics Control (USEPA  199la, referred to as the TSD):

     •   Chemical-specific controls,

     •   Whole effluent toxicity (WET) controls, and

     •   Biological criteria/bioassessments.

The WET approach to protection of water quality is the subject of this document.

     In 1989, EPA defined WET as "the aggregate toxic effect of an effluent measured directly by an
aquatic toxicity test" [54 Federal Register (FR) 23868 at 23895, June 2, 1989]. Aquatic toxicity tests are
laboratory experiments  that measure the biological effect (e.g., growth, survival, and reproduction) of
effluents or receiving waters on aquatic organisms. In aquatic toxicity tests, groups of organisms of a
particular species are held in test chambers and exposed to different concentrations of an aqueous test
sample (e.g., effluent or effluent combined with receiving water). Observations are made at
predetermined exposure periods. At the end of the test, the responses of test organisms are used to
estimate the effects of the test sample, either in absolute terms or relative terms (i.e., the effects of an
effluent in receiving waters).

     In the early 1980s, EPA published WET test methods for estimating the short-term acute and
chronic toxicity of effluents and receiving waters to freshwater and marine organisms (USEPA 1985a,
1988, 1989e). WET data gathered in the 1980s indicated that some NPDES facilities had discharged
effluents with sufficient toxicity to cause water quality problems. Therefore, further reductions in the
toxicity of wastewater discharges were necessary to achieve compliance with State water quality


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standards that prohibit the discharge of toxic pollutants in toxic amounts, or otherwise provided for the
maintenance and propagation of a balanced population of aquatic life.

     Therefore, EPA developed a policy to reduce or eliminate toxic discharges. The Policy for the
Development of Water Quality-based Permit Limitations for Toxic Pollutants (49 FR 9016, March 9,
1984) introduced EPA's integrated toxics control program under the NPDES program. In 1989, EPA
promulgated regulations specifying procedures for determining when water quality-based effluent
limitations are required in NPDES permits. This regulation also established that, where necessary,  the
level of water quality to be achieved by such limits must be derived from and comply with water quality
standards. To support these regulations, EPA developed the TSD (USEPA 1991a).  The TSD provides
guidance to regulators on the implementation of WET through NPDES permit requirements and limits.

     On October 26, 1995, EPA promulgated WET test methods (USEPA 1993a,b,c; 1994a,b) and added
them to the list of EPA methods approved under Section 304(h) of the CWA (40 CFR 136) for use in the
NPDES program. The methods were subsequently challenged, and under a settlement agreement, EPA
conducted a round-robin study, which evaulated 12 of those test methods (USEPA  2001a,b). EPA also
prepared a WET test method variability guidance document (Understanding and Accounting for Method
Variability in Whole Effluent Toxicity Applications Under the National Pollutant Discharge Elimination
System, USEPA 2000b) and a WET test methods guidance document (USEPA 2000c). In November
2002, EPA promulgated the revised WET test methods (USEPA 2002 a,b,c) [see 67 FR 69951-69972,
November 19, 2002].

     EPA's existing NPDES enforcement guidance and policy regarding WET is included in three major
documents: (1) the January 25, 1989 memorandum entitled Whole Effluent Toxicity Basic Permitting
Principles and Enforcement Strategy, (2) the 1989 Enforcement Management System, National Pollutant
Discharge Elimination System (Clean Water Act)', and (3) the August 14, 1995 memorandum entitled
National Policy Regarding Whole Effluent Toxicity Enforcement.

1.2  PURPOSE OF THIS  GUIDANCE

     EPA Headquarters expects permits issued by EPA Regions to be consistent with this guidance. EPA
strongly encourages States5 authorized to administer the NPDES program to implement the
recommendations in this guidance so that national consistency in implementing the WET program  can be
achieved. State permitting authorities may need to revise their current procedures (water quality
standards or NPDES, or both) to fully implement the national recommendations presented in this
document.

     Existing regulations and EPA's 1994 WET  Control Policy require that regulatory authorities
(including States, Tribes, and EPA Regions) must perform a reasonable potential analysis and, where
appropriate, establish permit limits for WET when an effluent discharge causes, has the reasonable
potential to cause, or contributes to an in-stream excursion above a numeric or narrative criterion within
an applicable State water quality standard. Establishing appropriate permit conditions and limits when
necessary to control WET is a crucial step toward achieving the CWA objective of identifying,
characterizing, and eliminating toxic effects of discharges to waters of the United States.
5   State means any of the 50 States, the District of Columbia, Guam, the Commonwealth of Puerto Rico, the Virgin
    Islands, American Samoa, the Commonwealth of the Northern Mariana Islands, the Trust Territory of the Pacific
    Islands, or an Indian Tribe as defined in 40 CFR 122 which meets the requirements of § 123.3.


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     This document is intended to promote national consistency in the implementation of WET
programs, make certain that the information and requirements for WET relevant to NPDES permits are
clear and well documented, and ensure that the appropriate WET requirements are included in permits to
protect against impacts to aquatic life. This document has three major goals:

     1.   National Consistency: to provide the framework for a WET program that is nationally
         consistent across NPDES States, Tribes, and EPA Regions.

     2.   NPDES Regulatory Compliance: to reinforce compliance with existing NPDES regulations,
         including the reasonable potential (RP) determination regulations at 40 CFR 122.44(d) by
         reiterating the technical and regulatory basis for toxicity requirements in NPDES permits,
         while allowing flexibility consistent with the regulations, where appropriate (e.g., allowing
         time to collect additional data for RP determination to supplement limited data sets).

     3.   Emphasis on Existing Guidance, Policy, and Regulations: to restate and clarify, where
         necessary, existing guidance, policy, and regulations on WET testing in the NPDES program,
         by referring the reader to portions of the TSD, WET  test methods (USEPA 2002a,b,c), and
         various WET guidance documents.

     EPA's WET testing program is key to identifying, characterizing, and eliminating toxic effects of
point source discharges to waters of the United States. WET test methods are scientifically defensible
and provide reliable results when performed by qualified staff (see USEPA 2000c). Permitting authorities
should establish WET monitoring requirements with appropriate test methods at a testing frequency
designed to achieve the program objective and must include NPDES WET limits when reasonable
potential has been demonstrated. NPDES limits and other permit conditions are used to ensure that, when
toxic effects are predicted or demonstrated on aquatic organisms through the use of WET tests, the
permittee will be required to identify and reduce or eliminate the cause(s) so that aquatic life is protected.

     This guidance addresses how to appropriately incorporate WET limit or monitoring requirements
where necessary into individual (e.g., facility-specific) NPDES permits and presents a range of responses
for addressing WET violations. In addition, the TSD, test methods, and various WET guidance
documents are referred to throughout this document to help regulatory authorities easily consult the
background materials upon which this document is based.

1.3  WHAT'S NEW IN THIS GUIDANCE

     Significantly, this guidance recommends EPA's "step-wise approach" to permitting that would
standardize the practice of collecting high-quality WET data before or during the NPDES permit
development process (see Chapters 3 and 5). This step-wise approach addresses reasonable potential
(RP) determinations based on small data sets, which can result in conservative projections  of effluent
quality and unnecessary permit limits. EPA's step-wise approach in which RP is addressed in the overall
regulatory scheme of the "WQS to NPDES Permits Process" is highlighted in Figure 1 of this document.

     This guidance also recommends EPA's step-wise approach for establishing appropriate effluent
limits and other permit conditions. If the permitting authority determines that a facility's discharge is or
may be discharged at a level that causes, has reasonable potential to cause, or contributes to an
excursion above a narrative or numeric water quality criterion for toxicity, the permitting authority must
establish a water quality based effluent limit (WQBEL). This WQBEL must be for the parameter WET if
the State has numeric toxicity criteria [40 CFR 122.44(d)(l)(iv)]. If the State has narrative toxicity
criteria and the permitting authority can demonstrate, in the fact sheet, that chemical-specific limits are

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sufficient to attain and maintain applicable standards, appropriate chemical-specific limits can be
established in lieu of WET limits [40 CFR 122.44(d)(l)(v)]. For example, where the State has a narrative
criterion for aquatic life protection, and WET RP has been demonstrated, a WET limit is required.
However, if the permittee can through using EPA's toxicity reduction evaluation/toxicity identification
evaluation (TRE/TIE) procedures identify and confirm that the chemical(s) responsible for an effluent's
toxicity [40  CFR 122.44(d)(l)(v)] then chemical(s) limits (for the identified toxicant) in lieu of WET is
appropriate. In addition, WET testing would be required in the permit with monitoring triggers. A TRE is
a stepwise site-specific study to identify toxics and sources, and to narrow the search for effective control
measures for effluent toxicity. TREs are designed to identify the causative agents of WET, evaluate the
effectiveness of the toxicity control options, and confirm the reduction in effluent toxicity. If a TIE
analysis identifies the toxicant, and the State has a narrative criterion, the regulations at 40
CFR 122.44(d)(l)(v) allow for inclusion of a chemical-specific limit for the identified toxicant(s) in lieu
of a WET limit to meet that narrative criterion.
                                         StateWQSfor
                                      aquatic life protection
                                        approved by EPA
                                      (narrative or numeric)
     Permittee collects WET
     data and other info and
        submits NPDES
     application to permitting
           authority
                                             State
                                        establishes WQS
                                         implementation
                                          procedures
NPDES permit writer conducts
  RP determination on WET
       data submitted
   [See WETRP Flow Chart
  (F igure 2) in Section 4.2.2]
     Permit issued with
   appropriate WET limit
monitoring trigger and permit
    conditions including
      representative
   monitoring (TRErTIE,
  accelerated testing, etc.)
 Figure 1. Overview.
     Where limits for WET are necessary for continuous discharge, EPA regulations at 40
CFR 122.45(d)(l) stipulate that such limits be expressed as a Maximum Daily Limit (MDL) and an
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Average Monthly Limit (AML) unless impracticable for all dischargers other than POTWs. Where limits
for WET are necessary for POTWs, EPA regulations at 40 CFR 122.45(d)(2) stipulate that such limits
they be expressed as a Average Weekly Limit (AWL) and an Average Monthly Limit (AML) unless
impracticable. This document provides examples using toxic units (TUs); however, EPA recognizes that
permitting authorities also use other means to quantify and express WET values [e.g., effluent
concentrations related to a receiving water concentration (RWC)]. In cases where effluent concentrations
are used, the permit limits would be expressed as minimums rather than maximums. For example, a limit
might be a maximum of two toxic units (therefore the permit limit expressed as < 2.0 TU). A similar limit
expressed as an effluent concentration would be expressed as a minimum NOEC or point estimate value
(e.g., NOEC > 50% effluent).

     Of particular concern is establishing permit limits for WET for discharges where the effluent flow
comprises a high percentage of the available stream flow during critical conditions or otherwise lacks
assimilative capacity. In such waters, effluent toxicity may be a source of measurable ambient toxicity
(i.e., cause) excursions of numeric or narrative water quality criteria. In this guidance, EPA revises
national guidance for establishing appropriate effluent limitations for WET for receiving waters with no
assimilative capacity or where dilution is not available. For such situations, the limit derivation
procedures  described in Chapter 5 of this guidance would result in a maximum daily limit of 1.6 TUC and
an AML of 1.0 TUC. The 1.0 TUC AML should be expressed as a monthly median limit instead of an
average monthly limit.

     EPA's step-wise approach to permitting also includes a requirement for accelerated monitoring
when an effluent limit or numeric monitoring trigger value is exceeded. If toxicity re-occurs, the permit
should require the permittee to initiate additional measures to investigate the causes of, and identify
corrective actions to reduce, effluent toxicity that may cause or contribute to an exceedence of WQ
criteria applicable to WET. These measures could include a TRE study that requires toxicity reduction.

1.4  WHO SHOULD USE THIS DOCUMENT

     This document provides guidance to EPA Regions, NPDES State permitting authorities, NPDES
permittees,  WET testing laboratories and consultants, the public and its representatives, and any other
stakeholders interested in NPDES WET programs. This document also specifies that permit decisions
must take into account applicable Federal and State laws, regulations, guidance, and standards.

1.5  DOCUMENT ORGANIZATION

     This document contains six chapters:

     •    Chapter 1   -   Introduction
     •    Chapter 2  -   Setting Appropriate Water Quality Goals for WET
     •    Chapter 3  -   State Implementation Procedures to Ensure WET Criteria are Met
     •    Chapter 4  -   Determining the Need for WET Limits in NPDES Permits

     •    Chapter 5  -   Development of WET Requirements in NPDES Permits
     •    Chapter 6  -   Conclusion - EPA Recommendations to Regions and NPDES States
     The Introduction provides the general background and purpose of this document. Chapter 2 presents
EPA's recommended water quality criteria for WET and discusses mixing zones, designated uses, and
considerations for implementing a narrative toxicity criterion. In addition to general recommendations

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regarding narrative criteria, Chapter 3 provides guidance to States for implementing WET programs and
outlines considerations for State/Regional implementation. Chapter 4 provides information on
determining the need for effluent limits with facility-specific data, determining reasonable potential, and
determining the need for effluent limits without facility-specific data. Chapter 5 describes EPA's step-
wise approach to WQBELs, providing recommendations on permit documentation, monitoring, and
developing WET permit requirements; establishing limits under various scenarios; expression of limits;
and TRE/TIE studies. Chapter 6 summarizes the document's guidance and recommendations to
regulatory authorities and permittees. Additional information on the topics discussed in the six chapters
is provided in four appendices. Appendix A provides details on general permit derivation procedures,
including an example calculation. Appendix B provides further details on deriving permit limits for low-
flow dilution situations and presents an example calculation. EPA's 1994 WET Control Policy (Policy
for the Development of Effluent Limitations in National Pollutant Discharge Elimination System Permits
to Control Whole Effluent Toxicityfor the Protection of Aquatic Life) is reproduced in Appendix C.
Appendix D lists the major EPA publications on topics related to WET testing. Appendix D also lists
pertinent EPA Web sites where further details of WET and WET testing can be accessed.
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             2 SETTING APPROPRIATE WATER QUALITY GOALS FOR WET

     State water quality standards (WQS) provide the foundation for water quality-based pollution
control programs. The purpose of WET limits and monitoring requirements is to ensure compliance with
State numeric or narrative water quality criteria established to protect the designated uses of the water
body. This chapter provides a brief description of the WQS that are the basis for WET water quality-
based effluent limits (WQBELs) in NPDES permits.

2.1  ERA'S RECOMMENDED WATER QUALITY CRITERIA FOR WET

     Water quality standards are provisions of State  (or, in certain instances, Federal) law which define
the water quality goals of a water body, or portion thereof, by designating the use or uses to be made of
the water body and by setting criteria necessary to protect the uses. States adopt water quality standards to
protect public health or welfare, enhance the quality of water, and serve the purposes of the Clean Water
Act. Such standards serve the dual purposes of establishing the water quality goals for a specific water
body and serve as the regulatory basis for the establishment of water quality-based treatment controls and
strategies beyond the technology-based levels of treatment required by sections 301(b) and 306 of the
CWA(40CFR131.2).

     Water quality criteria are elements of State WQS, expressed as constituent concentrations, levels, or
narrative statements representing a quality of water that supports a particular use. When criteria are met,
water quality will generally protect the designated use (40 CFR 131.3). While states have adopted a
variety of criteria expressed as constituent concentration levels (or numeric criteria) for various pollutants
for the protection of aquatic life, all states have adopted criteria expressed as narrative statements (or
narrative criteria). These narrative criteria, often referred to as "free-from" (or, in the case of toxicity, "no
toxics in toxic amounts") criteria, are an effective  tool for controlling the discharge of pollutants where
numeric criteria are not available. Narrative criteria are a basis for establishing WET controls in the
NPDES permitting regulations at 40 CFR 122.44(d)(l).

     Section 304(a) criteria are developed by EPA under authority of section 304(a) of the Act based on
the latest scientific information on the relationship that a constituent concentration has on a particular
aquatic species and/or human health. This information is issued periodically to the States as guidance for
use in developing criteria. In adopting criteria to protect their designated uses, States may establish
criteria based  on (1) section 304(a) guidance; (2) section 304(a) guidance modified to reflect site-specific
conditions; or (3) other scientifically defensible methods.

     Although EPA has not published numeric water quality criteria under section 304(a) for  whole
effluent toxicity, EPA has provided general guidance on appropriate WET limits. The TSD (USEPA
1991a) recommends 0.3 acute toxic units (TUa) as an acute criterion and 1.0 chronic toxic units (TUC) as a
chronic criterion, for most water bodies. While effective for CWA purposes only in the Great Lakes
Basin, the final Water Quality Guidance for the Great Lakes System in 40 CFR 132 (also known as the
Great Lakes Initiative) also establishes 0.3 TUa and 1.0  TUC, either as numeric criteria or as equivalent
numeric interpretations of narrative criteria, for receiving waters of the  Great Lakes [40 CFR 132,
Appendix F, Items A.I. and A.2].

     Federal regulations  at 40 CFR 122.44(d)(l)(i) establish different approaches for implementing a
water quality criterion for toxicity in NPDES  permits, depending on whether the criterion is expressed  in
a numeric or narrative form. If the State has not adopted a numeric criterion for WET, EPA expects the
permitting authority to interpretthe State narrative criterion so that the  appropriate effluent limits,
including any necessary toxicity numeric limits, can be established. States should identify the method
they intend to use in regulating toxics based on narrative criteria and describe how their toxics control

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program will protect aquatic life and attain the narrative criterion [40 CFR 131.1 l(a)(2)]. The procedure
to implement the narrative criteria using a whole effluent approach should specify, at a minimum, the
testing procedure, the duration of the tests (acute or chronic), the test species, the frequency of testing
required, and the numeric benchmarks that are used to interpret the narrative criteria.6

     As indicated in Section 2.1, EPA's TSD provides a basis for establishing criteria for aquatic toxicity.
Criteria consist of three components: (1) magnitude, (2) duration, and (3) frequency.

     Magnitude refers to the concentration of the pollutant. EPA's recommended magnitudes for WET
are as follows:

     •   For acute protection, a criterion maximum concentration (CMC) is specified to protect against
         acute (short-term) effects. The CMC should be set at 0.3 TUa to the most sensitive of at least
         three test species for acute tests (e.g., one fish and two invertebrate, or two fish and one
         invertebrate).

     •   For chronic protection, a criterion continuous concentration (CCC) is specified to protect
         against chronic (long-term) effects. The CCC should be set at 1.0 TUC to the most sensitive of at
         least three test species for chronic tests (fish, invertebrate, plant).

     Duration is the period of time (averaging period) over which the in-stream concentration is averaged
for comparison with criteria concentrations. This specification limits the length of time that in-stream
concentrations may exceed the criteria concentrations. EPA's recommended duration for aquatic life
criteria, including toxicity are:

     •   For acute criteria, EPA recommends an averaging period of one hour to be representative of
         fast-acting toxicants. The 1-hour acute averaging period was derived primarily from data on
         response time for toxicity to ammonia, a fast-acting toxicant.  The 1-hour average exposure
         should not exceed the  CMC. Scientifically justifiable alternative (site-specific) averaging
         periods can be derived (see TSD, page 35, Section 2.3.4).

         For chronic criteria, EPA recommends an averaging period of four days. That is, the 4-day
         average exposure should not exceed the CCC. Different chronic averaging periods could be
         derived, depending on the nature of the pollutant and the toxic endpoint of concern (e.g., the
         rate of uptake and accumulation and the mode of action). EPA selected the 4-day averaging
         period based on the  shortest duration in which chronic effects are sometimes observed for
         certain species and toxics, and thus should be fully protective even for the fastest-acting toxics
         (see TSD, page 35, Section 2.3.4).

     Frequency is how often the criteria can be exceeded and still be protective to allow the aquatic
community sufficient time to recover from excursions of aquatic life criteria. For frequency, neither acute
nor chronic criteria should be exceeded for the above durations more than once per three years on the
average. Based on site-specific considerations, a State may allow for a different frequency (see TSD, page
36, Section 2.3.5).

2.1.1    Mixing Zones

     Federal regulation 40 CFR 131.13 states that "States may, at their discretion, include in their State
standards, policies generally affecting their application and implementation, such as mixing zones, low
    See text box in Section 3.1.1 of this guidance.
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flows and variances. Such policies are subject to EPA review and approval." EPA has addressed mixing
zones as an area near the discharge where certain numeric water quality criteria may be exceeded, as long
as there is no lethality to organisms passing through the mixing zone, no significant risk to human health,
and the designated uses of the water body are not impaired (see 63 FR 36788). Permitting authorities may
use mixing zones for WET provided use of mixing zones is authorized in State water quality standards. If
mixing zones are not allowed by State regulations for acute or chronic toxicity, then the criteria must be
applied at the end of the pipe. In determining whether an in-stream excursion of a water quality criterion
for WET exists, the effect of effluent dilution in the receiving water may be considered when appropriate
and allowed by the permitting authority's mixing zone policy consistent with the applicable water quality
standards. This determination is made at permit issuance. Where State water quality standards and policy
allow mixing zones, the permit must ensure that effluent limits derive from and comply with the
applicable State WQS.

2.2  DESIGNATED USES

     A water body's designated use is a fundamental articulation of its role in the aquatic and human
environments. All of the water quality protections established by the CWA follow from the water body's
designated use. As designated uses are critical in determining the water quality criteria that apply to a
given water body, determining the appropriate designated use is of paramount importance is establishing
criteria that are appropriately protective  of that designated use.

     It is in designating uses that States and Tribes establish the environmental goals for their water
resources, and it is in designating uses that States and Tribes are allowed to evaluate the attainability of
those goals. Water quality standards perform the dual function of establishing water quality goals and
ultimately serving as the regulatory basis for water quality-based treatment controls and strategies,
typically, although not exclusively, via water quality criteria protecting those uses. Thus, a State or Tribe
often weighs the environmental, social, and economic consequences of its decisions in designating uses.
The regulation allows the State or Tribe some flexibility in weighing these considerations and adjusting
these goals over time. Reaching a conclusion on the uses that appropriately reflect the potential for a
water body, determining the attainability of those goals, and appropriately evaluating the consequences of
a designation, however, can be a difficult and controversial task. Appropriate application of this process
involves a balancing of environmental, scientific, technical, and economic and social considerations, as
well as public opinion.

     Section 131.10 of the regulation describes States' and authorized Tribes' responsibilities for
designating and protecting uses. The regulation

         requires that States and Tribes specify the water uses to be achieved and protected,

         requires protection of downstream uses,

     •   allows for sub-category and seasonal uses,

     •   sets out minimum attainability criteria,

         lists six factors of which at least one must be satisfied to justify removal of designated uses that
         are not existing uses,

     •   prohibits removal of existing uses,

         establishes a mandatory upgrading of uses that are existing but not designated, and

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     •   establishes conditions and requirements for conducting use attainability analyses.

In addition, the regulations effectively establish a "rebuttable presumption" that the uses of protection and
propagation offish, shellfish and wildlife, and recreation in and on the water are attainable and should
apply to a water body, unless it has been affirmatively demonstrated that such uses are not attainable.

     40 CFR 131.10(a) require that States specify appropriate water uses to be achieved and protected.
The classification of the waters of the State must take into consideration the use and value of water for
public water supplies, protection and propagation of fish, shellfish, and wildlife, recreation in and on the
water, and agricultural, industrial, and other purposes, including navigation. Changing designated uses for
a specific water body requires a change in the water quality standards. Like all new and revised State and
Tribal water quality standards, these changes are subject to EPA review and approval.

     Where appropriate, a State may subcategorize or refine the aquatic life use designations for the
receiving water. States may adopt subcategories of a use and set the appropriate criteria to reflect varying
needs of such subcategories of uses, for instance, to differentiate between cold water and warm water
fisheries [see 40 CFR 131.10(c)]. States may also adopt seasonal uses [40 CFR 131.10(f)]. If seasonal
uses are adopted, water quality criteria should be adopted to reflect the seasonal uses; however, such
criteria shall not preclude the attainment and maintenance of a more protective use in another season.

     For example,  aquatic toxicity criteria have been modified in the State of Ohio based on refined
designated uses. Ohio's WQS regulations specify that a chronic toxicity level of 1.0 TUC shall apply
outside the mixing  zone to warm water,  exceptional warm water, cold water, seasonal salmonid, and
modified warm water habitat use designations. The chronic toxicity level, however, does not apply to
waters designated as "limited resource waters." (See Ohio Administrative Code Chapter 3745-2-09.)

     The Ohio example illustrates how a highly refined, biologically based designated use system can be
implemented through refined water quality criteria. It is not an example of a general use system typical to
many states. States with a general aquatic life protection designated use must adopt criteria and
implementation procedures to apply the  protection needed to all waters to which the use applies. Tiered
criteria and implementation procedures are only possible where there are tiered uses.

     EPA does not believe that tiered aquatic life uses per se can be established solely through  an
implementation procedure. Any new or revised WQS, including changes to designated uses, must meet
the requirements of the regulations at 40 CFR  131.

     In the Strategy for Water Quality Standards and Criteria (EPA-823-R-03-010), EPA is committed
to issuing methods on the use of bioassessments to help states and tribes refine designated aquatic life
uses. Further discussion of tiering and subcategorizing uses can be found in Guidance:  Coordinating CSO
Long-Term Planning With Water Quality Standards Reviews (EPA-833-R-01-002).
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                 3 STATE IMPLEMENTATION PROCEDURES TO ENSURE
                                  WET CRITERIA ARE MET

     The WET approach to toxics control should specify the applicable water quality criterion and the
method of interpreting the narrative criterion (see Chapter 2). The approach should also discuss how to
select appropriate testing procedures, the duration of the tests (acute or chronic), the test species, the
frequency of testing required, and biological and statistical endpoints,  as discussed below. The WET
approach to toxics control should also specify in writing the procedure for determining when an effluent
limit is necessary and how to establish appropriate limits and other permit conditions. These procedures,
which should be  consistent with 40 CFR 122.44(d)(l), are discussed in Chapters 4 and 5, respectively.

     To ensure that the narrative criteria for toxicants are attained, States should develop implementation
procedures. Such implementation procedures describe how the State's toxics control program adequately
protects the aquatic life and attains the narrative criterion.

3.1  GENERAL RECOMMENDATIONS
~ .. ..    -*            ...     ,   .._~_~ ~.  .             Recommended Elements for WET
3.1.1    Recommendations for NPDES States            implementation Procedures
     States should develop implementation
procedures that provide clear direction to permit
writers regarding NPDES permit development so the
permittee and all stakeholders, including the public,
can clearly understand the requirements and
expectations contained in the NPDES permit. The
implementation procedures should establish the
  ^,   ,  .  ,      , r-    , •       -c  ,   • •    •        •   RP procedures
methods to be used for making specific decisions in
                                                    States should specify the following elements when
                                                    implementing narrative criteria:

                                                    •   Method of interpreting the narrative criterion,
                                                       with numeric benchmarks
                                                    •   Test methods, species selection, sampling
                                                       type, monitoring frequency, etc.
                                                      WET limit expression
                                                      Steps to address toxicity
permits. A decision-tree approach allows permit
writers to address each permit in a unique manner,
matching the discharge situation to the appropriate
implementation procedure scenario, and enabling them
to choose the most appropriate permit condition while maintaining program consistency. When
establishing permit WET limits and conditions, providing detailed requirements in the permit is essential.
The permit should specify the types of requirements listed in Table 1 in this section. Examples of these
requirements are  provided in Chapter 5, Section 5.1 and Table 2.

     Although Federal regulations require the permit to include limits for WET after RP determination,
other decisions are made based on the State's WQSs and implementation procedures. Examples of such
decisions are how critical low flow and effluent dilution series are determined; receiving stream aquatic
life use designations; frequency of monitoring; use of marine species or freshwater species for testing;
sample type, test  type, and biological and statistical endpoints; conditions under which acute or chronic
testing is required; and the decisional basis for requiring WET limits. Note that states may consider how
these above factors would be applied for assessing storm water toxicity or a State's ambient toxicity
testing program. A State may recommend different applications of various factors, such as frequency of
monitoring, stepwise approach to  address toxicity, or different methods of collecting sample renewals for
routine versus continuous discharge sources. States should develop written implementation guidance that
explains these types  of decisions and their rationale, as appropriate. These decisions should be explained
clearly in the permit fact sheet to inform the permittee and the public of the basis for decisions about
permitting requirements for WET (see Chapter 5, Section 5.4). Permitting authorities should maintain
staff with the level of expertise necessary for thorough WET data review when performing RP
determinations, compliance monitoring review, and TREs for WET.

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Table 1. Permit Requirements
WET Testing Methods
Test Statistical Endpoint (LC50, NOEC, etc.)
Sample Type
Specify Receiving Water Concentration (RWC)
Test Acceptability Criteria
Testing Frequency
Reporting Requirements
WET Limit or WET Numeric Monitoring Trigger
Test Type
Test Species
Test Conditions (e.g., dilution water, temperature, duration)
Effluent Dilution Series
Criteria for Test Validity
Accelerated Testing Requirements
Toxicity Reduction Evaluation Requirements
Steps to Address Toxicity
     Ensuring that samples are collected and tested in a manner that is consistent with EPA sampling and
testing procedures is critical. Permitting authorities should encourage and work with permittees to ensure
they select a laboratory that has qualified staff who follow EPA's WET test methodology (40 CFR 136)
and that produces quality test results in a timely and consistent manner. EPA has developed guidance for
laboratories, permittees, and regulatory authorities on considering variability in WET testing, including
both analytic and effluent variability (USEPA 2000b). Three critical areas are identified to minimize
WET test method variability: (1) obtaining a representative effluent sample, (2) conducting the toxicity
tests properly to generate biological endpoints, and (3) calculating the appropriate statistical endpoints to
enable confidence in the effect concentration. EPA's variability guidance also presents specific guidance
for the testing laboratories (see Section 7.2.1 in USEPA 2000b).

     As part of the Test Review section of the toxicity test manuals, the test review should be conducted
on each test by both the testing laboratory and the regulatory authority. The WET methods require data
quality objectives and a quality assurance plan (USEPA 2002b,c). In addition, the WET methods require
that each multi-concentration test reported under the NPDES program be reviewed to ensure that
calculated test results are interpreted appropriately (USEPA 2000b; 2002a,b,c). The promulgated WET
methods specifically require that when NPDES permits specify hypothesis testing endpoints [i.e., NOEC,
LOEC using sublethal Methods 1000.0, 1002.0,  1003.0,  1006.0, or 1007.0] the within-test variability
must be reviewed and variability criteria must be applied [see Section 10.2.8.2 of WET promulgated
methods (USEPA 2002a,b,c)].

3.1.2    Recommendations for EPA Regions

     Where the EPA Region is the NPDES  permitting authority (e.g., due to non-delegated State NPDES
status), the EPA Region shall have written Regional Guidance to implement the Region's WET program
for issuing NPDES permits (which are consistent with established EPA guidance, regulations, and
policies). The EPA Regions 9 & 10 Guidance for Implementing Whole Effluent Toxicity Testing Programs
document (USEPA 1996e) is an example. Regions shall implement oversight procedures for approved
NPDES States, as well as the following:

         should establish and maintain their expertise through ongoing training of EPA and State permit
         writers;

     •    routinely inspect toxicity testing laboratories, or should have a lab certification program similar
         to those in California or North Carolina;

     •    review WET test data and TRE/TIE reports;

         review WQSs and implementation procedures for WET;
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     •    review NPDES permits with WET requirements;

         develop Regional WET guidance that is consistent with national EPA regulations, policy, and
         guidance; and

         review responses to WET violations.

3.2  EPA/NPDES-AUTHORIZED PROGRAM OVERSIGHT

     EPA encourages all NPDES permitting authorities to develop WET implementation policies and
procedures that are consistent with applicable regulations [see 40 CFR 122.44(d)(l)(ii); 40
CFR 123.22(c); 40 CFR 123.63(5); and 40 CFR 130.5(b)(l)] or amend existing policies and procedures if
necessary. Once the permitting authority has developed its WET implementation procedures (including
RP determination procedures), the NPDES authority—if it is an authorized State—should submit the
procedures to the appropriate EPA Regional Office for review.  This submittal may be included as part of
an existing Regional NPDES program  review or State WQS.

     EPA often reviews State determinations on WET when EPA reviews individual State-issued NPDES
permits. EPA is authorized to object to permits it finds inconsistent with the requirements of the CWA
and is authorized to assume authority for permit issuance in such circumstances.
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            4 DETERMINING THE NEED FOR WET LIMITS IN NPDES PERMITS

     NPDES regulations at 40 CFR 122.44(d)(l)(i) require the establishment of an effluent limit for any
pollutant that 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 (RP determination)." In determining the need for an effluent limit, the permit writer must
consider existing controls on other point and non-point sources, the variability of the pollutant or
pollutant parameter in the discharge, the sensitivity of the test species (for WET) and, where appropriate
and allowed in State WQSs or regulations, the dilution of the discharge in the receiving water [40
CFR 122.44(d)(ii)]. This section presents EPA's interpretation and application of 40 CFR 122.44(d)(l)
with respect to WET RP determination. When the RP determination is demonstrated based on a numeric
toxicity criterion, the permit must contain effluent limits for WET [40 CFR 122.44(d)(l)(iv)]. If a State
designates a water body for the protection of aquatic life, the permit must include appropriately derived
limits to protect that use, regardless of the attainment status of the water body if warranted after the RP
determination.

     EPA's TSD provides  guidance on how to apply RP requirements. This section reaffirms EPA's TSD
approach to determining RP and suggests additional flexibility when the WET RP determination is based
on sparse data sets. Facility-specific effluent monitoring data should be used, where available, to project
receiving water concentrations (RWC), which are then compared to applicable water quality criteria for
aquatic life. This comparison consists of (1) calculating the RWC threshold in the receiving water after
considering dilution (if appropriate and allowed by applicable water quality standards), the contributions
of other point and nonpoint sources, and the uncertainty associated with sparse data sets and effluent
variability to justify projecting higher effluent concentrations than have actually been measured; and (2)
comparing this calculation to the applicable water quality criterion or aquatic life (hereafter referred to in
this document to as "toxicity criterion"). Both the TSD and this guidance identify the flexibility to
determine  the appropriate approach for the RP determination.  For example, the NPDES authority may opt
to use a stochastic dilution model that incorporates both ambient dilution and effluent variability rather
than using a steady-state dilution model with a statistically defined maximum effluent concentration.
Also,  a permitting authority may determine the need for a WQBEL for WET even in the absence of
facility-specific effluent monitoring data. Regardless of which approach the NPDES authority selects, its
RP determinations must satisfy all requirements of 40 CFR 122.44(d)(l)(ii) summarized above (see
Section 4.3). One of the three outcomes listed in the text box below will be reached when using the TSD
procedures:
                      Three Possible Outcomes of the RP Determination for WET

              Facility has demonstrated it causes or contributes to an excursion above the water
              quality criterion for WET.

              Facility discharge has reasonable potential to cause an excursion above a water
              quality criterion for WET.

              Facility discharge does not have reasonable potential to cause an excursion above a
              water quality criterion for WET.
     Whether the permit contains WET limits or WET numeric monitoring triggers (both contain
monitoring requirements), the permit conditions should include these two permit requirements when the
toxicity that occurs above a limit or numeric monitoring trigger. For example, a test result which is
greater than the WET permit limit expressed in toxic units or the numeric monitoring trigger may result in
an increase in the WET testing frequency. Any unacceptable test result measured during the period of
increased testing frequency may then trigger a requirement to perform a TRE.

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4.1  DETERMINING THE NEED FOR PERMIT LIMITS WITH EFFLUENT MONITORING
     DATA

     The need for a WET limit should be determined using a sequential, tiered process (see TSD, Chapter
3 and 40 CFR 132, App. F, Procedure 6). In the first step, historical effluent data for WET and
appropriate statistics derived from those data are used to statistically estimate the maximum effluent
concentration, which for WET is expressed as acute or chronic toxicity units (unless expressed as an
effluent concentration; see Section 1.3 of this document). The uncertainty multiplier multiplies the
maximum observed effluent concentration to account for the fact that effluent toxicity monitoring is
discontinuous and values greater than those observed could have occurred during the times when the
effluent was not being monitored. The uncertainty associated with small samples decreases with
increasing sample size. In the second step, this higher concentration (the maximum observed value times
the multiplier value) is then used in the mass balance equation to project the maximum resultant in-stream
concentration for WET. If the projected in-stream concentration is less than the applicable ambient
toxicity criterion, the permit writer must then exercise judgement as to whether RP exists; however, if the
estimated in-stream concentration is greater than the applicable criterion, a limit is required.

4.1.1    Characterizing Effluent Quality with  Respect to WET

     In all RP determinations based on effluent data, only valid and representative data should be used to
evaluate the need for a permit limit. An important premise in the TSD and in this guidance is that the
effluent pollutant data used to project maximum effluent quality are valid data that are representative of
the effluent. Permittees should ensure that their permit applications report valid, representative data [see
40 CFR 122.41(j)(l)]. Where the permittee believes certain effluent measurements are not representative
of the effluent, the permittee should notify the permitting authority. If representative facility-specific
effluent monitoring data for WET are available from a point source to the waters of the United States, the
permitting authority should apply the procedures specified in this guidance or similar State procedure.

4.1.2    Determining Validity of WET Data

     A valid WET test is one performed consistently with the procedures specified in 40 CFR 136, that
meets all applicable test acceptability criteria (TAC) specified in those procedures, and meets any
additional QA/QC and testing requirements established in the permit.

     The permittee must submit all WET test results to the permitting authority [40 CFR 122.41(l)(4)(ii)].
The regulations for NPDES permit applications also contain requirements concerning submission of WET
data. The regulations at 40 CFR 122.21(g)(l 1) are for industrial applicants and the regulations at
§ 122.21(j)(5) are for municipal applicants. Valid, representative effluent data should not be ignored. The
permitting authority may require additional information, for example, results of tests determined to be
invalid for any reason (e.g., too many control organisms in the test die). If the permittee is concerned
about any WET test data or suspects that any data are not valid, the permittee should notify the permitting
authority early in the permitting process and should indicate why the permittee considers the data to be
invalid. Those data should be provided immediately to the permitting authority for review.

     WET tests are deemed invalid when the laboratory does not meet the TAC for the specific method.
The permit toxicity language should include a statement requiring that if a test result is invalid, then the
permittee must collect another effluent sample and re-test the newly collected effluent sample within a
reasonable time period (e.g., less than two weeks). The permittee should schedule tests early enough in
the reporting period to allow a re-test when a scheduled test does not meet the TAC.
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4.1.3    WET Data Evaluation Checklist

     The text box on the next page entitled Steps for Evaluating WET Data provides a guide for the
permit writer to evaluate and review WET test results. This information should be used as a checklist for
individual tests and does not cover the full range of quality control practices necessary for successful
completion of test data analysis. EPA also recognizes that individual State programs or EPA Regions may
have detailed guidance that is beneficial to ensuring a successful WET program.

4.1.4    Determining Representativeness of WET Data

     Effluent data used as the basis for effluent characterization should be representative of the monitored
activity (i.e., the discharge under current conditions with current treatment and management practices at
the plant) [40 CFR 122.41(j)(l)]. Representative WET data are those WET test results that appropriately
characterize the permittee's effluent with respect to ongoing facility operations, including the periodic
discharge of toxic pollutants (e.g., seasonal use of biocides). The permitting authority must determine
whether available effluent data are representative of the current operating conditions at the facility. For
example, data obtained prior to significant treatment, pretreatment,  or pollution prevention modifications
may no longer be representative of the discharge. Where such are data no longer representative of the
current discharge, the permitting authority may choose to exclude such data in the RP determination (i.e.,
because the data pre-date current operating conditions and treatment at the facility). In this case, the RP
determination should be based on data produced subsequent to the modification(s), assuming a sufficient
amount of recent representative data has been generated with which to make the RP determination.

     Ensuring that data are representative also includes consideration of whether effluent samples are
collected, preserved, shipped, and stored in a manner that is consistent with the permit and test method.
Samples collected and used for WET testing should approximate, as nearly as possible, effluent effects in
the receiving stream. Collection considerations include the use of appropriate collection equipment and
sampling location. For example, POTW effluent samples used for WET testing should be collected after
the final treatment and before discharge, not prior to chlorination/dechlorination processes. EPA test
methodology requires that effluent chlorine residual be determined  within 15  minutes of sampling.
Samples for WET testing  are generally 24-hour composite samples, thus there would be considerable loss
of chlorine during sampling and shipping. Therefore, where the  effluent contains residual chlorine,
samples used for WET testing would generally not be reflective  of actual in-stream exposure and test
results would not be representative of the effluent's actual toxic  potential in-stream. If an effluent is
known to contain residual chlorine at levels which may result in unacceptable toxicity in-stream, in situ
testing is recommended.

     Care should also be taken when determining if data are representative where ammonia is controlled
or removed from samples  used for WET testing. While this practice may be appropriate in some
circumstances, it does not apply under all conditions. When ammonia stripping or a CO2 cap is used to
adjust samples for ammonia or pH drift, consideration should be given to the  receiving stream conditions,
seasonal variations in temperature and pH, and whether test solutions are renewed on a daily basis.
Additional EPA guidance regarding representative WET samples and toxicity related to chlorine and
ammonia is discussed in Summary Statement No. 7 in EPA's 1994  WET Control Policy (Appendix C of
this document) and item number one  (pH and ammonia control) in EPA's Policy Memorandum—
Clarifications Regarding Flexibility in 40 CFR Part 136 Whole Effluent Toxicity (WET) Test Methods
(USEPA 1996d). Also see USEPA 2002a, section 9.5.9.
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                                     Steps for Evaluating WET Data

  Permit Review:
  1.   Examine the test results to verify that the laboratory is using the test method and dilution series as required
      in the NPDES permit. The dilution series being tested should always include the receiving water
      concentration (RWC). Note: This may need to be performed only after a permit has been first issued.

  2.   Evaluate the test results against the permit requirements for WET to assess whether the limit or numeric
      monitoring trigger is being achieved. For example, where a WET limit or numeric monitoring trigger is
      expressed in terms of TUs, then the value is expressed as a value "not to be exceeded." Where a WET
      limit or numeric monitoring trigger is expressed in terms of"% effluent at the RWC," the value is expressed
      as a value that the % effluent must be at or above.

  Test Review:
  Test review is an important part of the overall quality assurance program and is necessary for ensuring that all
  test results are reported accurately. Test review should be conducted on each test by both the testing
  laboratory and the regulatory authority. Note, see the chapter on Test Review of the specified toxicity test
  manual.

  1.   Examine the results to verify the sample was maintained at the proper temperature from time of collection
      to arrival at the testing laboratory. Also, does the sample meet the test initiation and  renewal holding time
      requirements?

  2   Evaluate the test results for the effluent to verify that the laboratory met the TAG as specified in the test
      method. See the individual "Summary of Test Conditions and TAG" section for each test method. All invalid
      tests must be repeated with a newly collected sample, as specified in permit.

  3.   Examine the "Summary of Test Conditions and TAG" section for the specific method to determine whether
      the required and recommended test conditions were met. Below is a single example for a required test
      condition and a recommended test condition that would be specific to the particular toxicity test method
      listed  in the permit.

      a. Did the laboratory conduct the test using the  required test conditions? Some of the test conditions listed
         are specified as "required" and therefore the  condition must be met. For example, did the test use the
         required minimum number of replicates, number of test organisms, test type, etc.? All required test
         conditions must be met or the test is considered invalid and must be repeated with a newly collected
         sample.
      b. Did the laboratory conduct the test using the  recommended test conditions? Some of the test conditions
         listed are specified  as "recommended" and therefore the range should be obtained. For example, when
         the test method specifies number of test organisms per test chamber, the test condition will provide a
         recommended number of test (e.g., 10 larvae per test chamber), as recommended. A testing laboratory
         can use more than the recommended number of test organisms per chamber as  long as the loading
         capacity is maintained.

  4.   Examine the statistical results to verify the  recommended flowcharts for statistical analysis were followed.
      Any deviation from the recommended flowcharts for selection of statistical  methods should  be noted in the
      data report.

  5.   Examine the concentration-response relationships as these must be reviewed to ensure that calculated test
      results are interpreted appropriately. All WET test results (from multi-concentration tests) reported under
      the NPDES program should be reviewed and reported according to USEPA guidance on the evaluation of
      concentration-response relationship (see USEPA 2000c).

  6.   Test review of a given effluent or receiving  water test should include review of the associated reference
      toxicant test and current control chart. Were out-of-control reference toxicant test results evaluated to
      determine appropriate corrective action?

  7.   The within-test variability of individual tests should be reviewed. When NPDES permits require sublethal
      hypothesis testing endpoints from Methods 1000.0,  1002.0, 1003.0, 1006.0, and 1007.0 (e.g., growth or
      reproduction NOECs and LOECs), within-test variability must be reviewed  and variability criteria must be
      applied as described in the Method Manuals Section on Test Review.
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      Effluent toxicity monitoring data should be generated before permit limit development for the
following reasons:

     •   The presence or absence of effluent toxicity can be more clearly determined.

     •   Effluent variability can be more clearly defined.

         A TRE can be initiated by the permittee to discover and control toxicity, thus possibly
         eliminating the need for a WET limit by identifying a specific chemical that may be limited
         instead, if the State has a narrative criterion for WET. 40 CFR 122.44(d)(l)(v).

     •   Required to complete the NPDES permit application package.

     When a permittee has a limited amount of valid WET data in advance of the permit application or
modification, permittees are encouraged to gather additional WET data from the permittee. The use of
larger data sets of 10 data points or more decreases the uncertainty associated with small sample sizes and
removes reliance on the default assumptions about variability associated with reliance on fewer than  10
data points. As discussed in more detail below in Section 4.1.5, the permitting authority may opt to use an
alternative approach which allows up to 18 months to gather and analyze the recommended minimum of
10 valid WET data points for the most sensitive species (where authorized by State WQSs or permitting
regulations).

4.1.5    Accounting for Effluent Variability

     RP determinations based on very few data points, using the statistical procedure described in this
guidance (and essentially similar to the procedures described in the TSD), can result in conservative
projections about possible effluent toxicity and thus may result in unnecessary permit limits. The
procedures in this guidance are designed to estimate a projected effluent value at a specified percentile
and confidence level based upon whatever representative effluent data are available. For example, when a
small data set is being used to make this projection, or in the extreme case, where only one data point is
being used, the projected effluent quality using EPA's recommended procedure will be 6.2 times the
observed value [assuming a coefficient of variation (CV) of 0.6 and a 95th percentile confidence level].
This statistical procedure remains valid even for very small data sets. EPA also recognizes that the more
data there are to characterize a discharge, the more accurate the projected effluent pollutant distribution
will be.

     Where dischargers are concerned that the results of the statistical analysis using a small data set
(e.g., a single valid and representative data point) result in a conservative decision, the discharger can
remedy the situation by collecting more effluent samples and reporting additional WET test results to the
permitting authority prior to permit issuance or re-issuance. EPA encourages this practice. Effluent data  in
the vast majority of cases become available to the permitting authority via reporting by the discharger. In
other words, the discharger almost always has the same effluent data that the permitting authority has and
the permittee is in the best position to remedy any perceived inadequacy in the data set presented to the
permitting authority.

     To further reinforce the goal that RP determinations be based on the most accurate projected effluent
distribution possible, where the statistical RP procedure projects the need for a new or more stringent
WET limit based on a small data set, EPA suggests that the permit specify a period of time (not to exceed
18 months) for the permittee to collect the additional WET effluent data before the  limit becomes
effective (when allowed by the applicable State water quality standards). If sufficient additional data are

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collected prior to the WET limit becoming effective, the larger data set will project a more accurate
estimate of effluent quality and therefore, the initial (more conservative) RP determination based on the
small data set and a high factor for uncertainty will no longer be necessary. Where the discharger reports
only a single valid and representative data point, however, such data must not be ignored.

     Limited amounts of test data result in greater uncertainty about the effluent's potential to impair
receiving waters. EPA's statistical approach accounts for effluent variability and the uncertainty
associated with small data sets to project an estimated upper confidence bound on a 95th or 99th percentile
estimated maximum concentration for the effluent. The estimated maximum concentration is calculated as
the upper bound of the expected lognormal distribution of effluent concentrations at a high confidence
level (e.g., 95 percent or 99 percent). The projected effluent concentration after consideration of dilution,
where allowed and appropriate, can then be compared to an appropriate water quality criterion to
determine the need for a WET limit (the RP determination).

     For n < 10 (thus when a CV of 0.6 is used), the RPMF will decrease with increasing sample size, n.
When n > 10, the facility-specific CV will determine the RPMF. For n > 10, the RPMF will tend to
decrease as n increases. RPMF will not necessarily decrease as n is increased because the CV, mean, and
standard deviation are random variables that can change as observations are added. However, these
quantities will be more precisely determined from more data, and the RPMF will tend to decrease as n
increases above 10. A minimum of 10 data points should be used to determine the variability of effluent,
because this reduces the uncertainty (of CV and variance) associated with small data sets. The fact sheet
should emphasize that if more  data are gathered, the RPMF will decrease predictably for n < 10 (and CV
= 0.6) and will tend to decrease for n >  10 (using a site-specific CV), making it less likely that the RP
procedure will indicate a need  for a WET limit, but the fact sheet should then also note that the sample
maximum may increase with n.

     Municipalities are required to submit at least four WET tests with their NPDES application [40
CFR 122.21(j)(5)]; however, permittees may be able to reduce the RPMF further by providing additional
test data (for n <  10), either by performing additional tests prior to submitting their permit application for
renewal, or by taking advantage of the optional WET data gathering schedule under a new permit, as
presented in this document. Permittees should assess the amount of WET data on hand 24 months prior to
their permit expiration date which are not older than five years. Permittees should ensure that they have at
least 10 valid test results for the most sensitive species by performing voluntary testing to make up the
difference. The additional tests should be performed with at least 30 days between test initiation dates.
The results of all voluntary tests performed must be submitted for review [see 40 CFR 122.41(j)].

     Determining whether a WET limit is needed in a permit requires an understanding of how the
quality of the effluent varies overtime.  EPA's guidance document on WET variability (USEPA 2000b;
see specifically Sections 6 and 7) recommends several methods for minimizing variability in WET test
used to characterize effluent discharges. In addition, test review is an important part of the overall quality
assurance program, (see text box Steps for Evaluating WET Data).

4.2  APPROACHES FOR DETERMINING THE  NEED FOR A WET LIMIT AND SETTING
     WASTELOAD ALLOCATIONS FOR TOXICITY AND PERMIT LIMITS

     The permitting authority has the flexibility to determine the appropriate approach for assessing the
need for a WET limit. This is best accomplished through a decision-making process, based on availability
of valid WET data, for determining whether the discharge has the reasonable potential to cause or
contribute to excursions above the narrative toxicity criteria. Figure 2 of this document illustrates the
decision-making process outlined in this document in Chapters 4 and 5.
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     The permit limit derivation procedure used by the permitting authority should result in limits that
adequately account for effluent variability, consider available receiving water dilution (where allowed and
appropriate), protect against acute and chronic toxicity, account for compliance monitoring sampling
frequency, and achieve the wasteload allocation (WLA) and, ultimately, protect the applicable water
quality standards. The objective of EPA's statistical procedure for the WET RP determination is to
provide an upper confidence bound for a percentile. To accomplish these objectives, permitting
authorities should use the statistical permit limit derivation procedure discussed below with the outputs
from either steady-state or dynamic WLA modeling. For lognormally or normally distributed data, Hahn
and Meeker (1991) provide a parametric procedure for the upper confidence bound of a percentile.7 This
procedure would be an acceptable alternative to the TSD procedure when the number of observations is
greater than one.

     EPA emphasizes that determining appropriate exposure assumptions, and providing the best
estimate possible of the effect of the discharge on the receiving water during the worst expected exposure
conditions, is a critical step in determining the need for effluent limits and establishing appropriately
protective WET limits.  Two major types of water quality models are used to develop WLAs: steady-state
and dynamic. Traditional single- or two-value steady-state WLA models calculate aquatic life WLAs at
critical conditions, using worst-case assumptions for flow, effluent, and environmental effects.

4.2.1    Dynamic  Modeling

     Dynamic or probabilistic models are a potentially useful tool for implementing water quality criteria,
especially for those criteria protecting aquatic life.  EPA considers dynamic models to be a more accurate
approach to implementing water quality criteria and continues to recommend their use. Dynamic models
make the best use of the specified magnitude, duration, and frequency of water quality criteria and,
therefore, provide a more accurate representation of the probability that a water quality standard
exceedence will occur. These models provide a way to incorporate site-specific data. Dynamic modeling
does require a commitment of resources to develop appropriate data. (See Chapter 4 of the TSD for
details on the use of dynamic models.)

     Dynamic modeling should be used to determine the need for a WET limit when adequate
information on receiving water flow, effluent flow, and toxicity is available to estimate frequency
distributions with a high degree of confidence. (See Box A in Figure 2.) Dynamic modeling techniques
incorporate the effects of variable receiving stream and effluent flow and toxicity regimes to calculate the
frequency and duration  of exposure at various levels. To determine the need for a WET limit
appropriately, dynamic modeling requires substantial amounts of high-quality data, collected over a long
period of time. If such data are not available, the water quality projections calculated by dynamic models
may be inaccurate and steady-state models should be used.

     Dynamic models simulate the entire distribution of RWCs (expressed as  a probability distribution
over time or a time series), rather than a single "worst case" based on assumed critical conditions. Water
quality criteria for aquatic life generally are expressed as concentrations not to be exceeded for more than
1 hour average in 3 years for acute criteria or 4 days average in 3 years for chronic criteria. If a dynamic
model simulation concludes that a toxicity concentration will be exceeded in the receiving water for
longer than allowed by the applicable water quality criterion, reasonable potential to cause an exceedence
exists. Consequently, a permit limit is necessary. (See Box B in Figure 2.)

     Dynamic models have received increased attention as a result of the widespread belief that steady-
state modeling is over-conservative due to conservative dilution assumptions. This belief has led to the
7   Statistical Intervals: A Guide for Practitioners. 1991. Gerald J. Hahn and William Q. Meeker. New York: Wiley.

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misconception that dynamic models will always lead to less stringent NPDES effluent limits than steady-
state models, which is not true in every application of dynamic models. Steady-state models frequently
apply a number of simplifying, worst-case assumptions that makes them less complex but also potentially
less accurate than dynamic models. EPA recognizes that the amount of data required to make use of
dynamic models will likely greatly limit their use for most WET RP determinations. Thus, steady-state
models continue to be used for the great majority of WET RP determinations. RP determinations and
WET limits should not be delayed while data required for dynamic models are collected.

4.2.2     Steady-state Modeling

     Steady-state WLA analyses should be used by permitting authorities, especially where few or no
WET measurements are available, or where daily receiving water flow and effluent flow records are not
available.

     Permit limits derived from  a steady-state model are designed to be protective of water quality
standards at critical  conditions and all environmental conditions less than critical. Steady-state models
calculate the effects of toxicity by assuming that the effluent discharge is constant and is rapidly and
completely mixed with the receiving water. This type of model requires single, constant inputs for
effluent flow,  effluent concentration, background RWC, and receiving water flow. The frequency and
duration of ambient concentrations predicted with a  steady-state model must be assumed to equal the
frequency and duration of the critical RWC used in the model. Steady-state models should be used when
the amount of valid WET test results is limited or when a daily flow record is not available. (See Box A
in Figure 2.)
                 Model Type
         Sufficient effluent and stream flow
           data to use dynamic model?
                                  Yes
                                               Dynamic Model Path
                                                RP for toxicity?
                                                                   Yes
                 Noj
No
            Steady-state Model Path
            Is any test result > toxicity
           criterion? (e.g., Is maximum
            RWC > toxicity criterion?)1
                                            Yes
                 No!
  No
                 Sample Size
                 Is n > 10?
                Yes
                                                Yes
               Determine RPMF
           Use facility-specific variance
                  and n.
                                                                                        WET Limit
                    1.  Issue permit that requires WET monitoring and
                      WET limit or chemical limit (if narrative criterion
                      and toxicant identified and confirmed).
                    2. WETtesting/TRE based on excursion above
                      WET limit or WET trigger value(s).
                                            Projected Maximum RWC
                                                                .No,
                                              > toxicity criterion?1
I
Determine RPMF
Use estimated CV of 0.6 and n.


                                         J  Projected Maximum RWC
                                              > toxicity criterion?1
                    H
                                                                                       No WET Limit
                    1. Issue permit that requires WET monitoring and
                      permit reopenerfor WET.3
                    2. Additionally, recommend accelerated WET
                      testing/TRE based on excursion above WET
                      trigger value(s).
           Np_

             Yes
                                                                          K
                              Delayed WET Limit
1. Issue permit with WET monitoring and a
  delayed WET limit2 and
  Interim WET monitoring to collect n >_ 10; and
  When n > 10, statistically estimate RP value
  using effluent-specific CV and equations from
  TSD, pg. 52.
  If statistically estimated RP value <_ toxicity
  criterion, delayed WET limit deactivated and
  permitting authority notifies permittee in writing.
2. Additionally, recommend accelerated WET
  testing/TRE requirements based on excursion
  above delayed WET  limit, or target.
    1 RWC expressed as TU or percent effluent dilution (e.g., toxicity criterion = 0.3 TU for acute and 1.0 TU
     for chronic) or low flow dilution expressed as a percent effluent.
    2 Compliance schedule may be included, where State and Tribal regulations and 40 CFR § 122.47 allow.
    ^ Permit reopenerfor WET always included in permit, unless permit already includes WET limit.
 Figure 2. WET reasonable potential flowchart.
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     Below is an example of a simple steady-state model that takes into consideration the effects of
dilution in the receiving water.

                                                   QeCe
                                      RWC =	
                                              Qe + (pmf)Qs

     where:
              RWC    =   Receiving water concentration, the resultant magnitude of toxicity in the
                           receiving water after effluent discharge in TUs (also known as the in-stream
                           waste concentration)

                 Qe    =   Effluent flow in MOD or cfs

                 Ce    =   Effluent concentration; magnitude of WET expressed in TUs (e.g., highest
                           TU value observed in the effluent) (Note: When toxicity test results are
                           expressed as NOEC % effluent, Ce = 100/NOEC, where NOEC is the lowest
                           NOEC result observed in units of % effluent.)

               pmf    =   Partial mix factor, percent of Qs allowed for mixing by State authority

                 Qs    =   Receiving water flow in MGD or cfs (the 1Q10 for acute and the 7Q10 for
                           chronic should be used when using hydrologically based flow and the 1B3
                           and the 4B3 when using biologically based flow.8 (See text box for EPA's
                           recommendation for determining biologically based flow.)


               EPA's Recommendations for Determining Appropriate Stream Design Flow
                                      in Steady-state Modeling

    In the document,  Technical Guidance Manual for Performing Wasteload Allocation. Book IV:  Design
    Conditions, Chapter 1, EPA discusses and recommends two methods for determining design flows, the
    hydrologically based method and the biologically based method, and the flows that should  be used for both
    the CCC and CMC. For wasteload allocation studies in which the hydrologically based method is used, EPA
    recommends the  use of the 1Q10 flow as the design flow for the CMC and the 7Q10 as the design flow for
    the CCC. The biologically based method makes exact use of whatever duration and frequency are specified
    in the CMC and CCC.  For wasteload allocations using the biologically based flows, EPA recommends the
    use of the 1B3 for CMC and 4B3 for the CCC or site-specific durations and frequencies.

    With respect to determining the most appropriate stream design flow, EPA recommends using DFLOW.
    DFLOW 3.0 is  a Windows-based tool developed to estimate user selected design stream flows for low-flow
    analysis and water quality standards. This latest version can be readily used by permit writers in State and
    EPA Regional offices,  modelers, or anyone else interested in calculating design flow statistics. DFLOW
    inputs daily stream flow records (such as those records from the systematic data collection program
    maintained by the USGS) and  calculates user-specified biologically based design flows, hydrologically based
    design flows, harmonic flows, and percentile flows. DFLOW can simultaneously determine and compare
    various design flow statistics for numerous water bodies. DFLOW is available at
    http://epa.gov/waterscience/dflow/.
  V	
8    The 1Q10 and 7Q10 are both hydrologically based design flows. The 1Q10 is the lowest 1-day average flow that
    occurs (on average) once every 10 years. The 7Q10 is the lowest 7-day average flow that occurs (on average) once
    every 10 years.

    The 4B3 is a biologically based 4-day average flow event that occurs (on average) once every 3 years. The 1B3 is
    a biologically based 1-day flow event that occurs (on average) once every three years.


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     EPA has developed the following three-step approach to using a steady-state model and facility-
specific WET data to determine whether a discharge causes, has reasonable potential to cause, or
contributes to an excursion above a numeric or narrative toxicity criterion. This approach assumes a
lognormal distribution of effluent data. (The basis for assuming lognormality of WET data is included in
EPA's TSD, Appendix E, pages E-6 through E-10 and Tables E-l through E-3.)

     Step 1. Determine whether a discharge causes an excursion above numeric or narrative water
quality criteria for aquatic life (i.e., toxicity). When assessing the need for a WET limit using a steady-
state model, the equation for determining the resultant RWC for WET after effluent discharge is:

                                                QeCe
                                   RWC =
                                           Qe + (pmf)Qs
     The value used for Ce is the maximum observed TU value. Once RWC is calculated, this magnitude
is compared to the toxicity criterion. If the calculated magnitude of the RWC is greater than the toxicity
criterion (e.g., RWC > 0.3 TUa for acute toxicity or RWC > 1.0 TUC for chronic toxicity), the discharge
causes an excursion above the toxicity criterion (see Box C in Figure 2), and a WET limit is necessary
(see Box D in Figure  2).

     Regulatory authorities should calculate the facility-specific CV using point estimate techniques to
determine the need for and derive a permit limit, even if the permit compliance monitoring test results
will be determined using hypothesis test procedures (USEPA 2000c, see sections 3.4.1 and 6.2). Point
estimates make the best use of the WET test data for purposes of estimating the CV, LTA, and RP factor
and calculating the permit limit.

     Alternatively, if the calculated magnitude of RWC is less than or equal to the toxicity criterion (e.g.,
RWC <  0.3 TUa or RWC < 1.0 TUC), the discharge does not cause, but may still have reasonable potential
to cause, an excursion above the toxicity criterion. The permitting authority then evaluates whether the
discharge has reasonable potential to cause an excursion above the applicable toxicity criterion based on
the sample  size [i.e., the number of available WET data points (n), see Box E in Figure 2].

     Step 2. Determine whether a discharge has reasonable potential to  cause an excursion above
numeric or narrative water quality criteria for aquatic life (i.e., toxicity). EPA's statistical approach
for determining whether a discharge will have reasonable potential to cause  an excursion above a water
quality criterion is outlined below and described in  Section 3.2.2 of the TSD. This approach accounts for
effluent variability and uncertainty associated with small effluent data sets by calculating a projected
effluent value associated with the upper 95-percent or 99-percent confidence bound representing the 95th
or 99th percentile of the lognormal distribution. Censored data may be encountered in WET test results.
These are data reported as "greater than" or "less than," for example "> 100% effluent" or "< 6.25%
effluent." Such data would be expressed in toxic units for reasonable potential calculations ("< 1.0 TUC"
or "> 16.0 TUC"). It is not appropriate to ignore such data or assign an arbitrary value such as detection
limit or one-half the detection limit, and to then proceed with calculations as if such values were
uncensored data. Two reasonable methods are available for accommodating censored data in calculations
used to determine reasonable potential and permit limitations (see text box Treatment of Censored Data
Sets).

     Steps for determining RP using a reasonable potential multiplying factor (RPMF) are described
below. Permitting authorities also can calculate RP multipliers directly using the equations in Appendix E

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of EPA's TSD. The RPMF is a function of the number of WET tests (n); the TSD recommends use of a
default value of CV = 0.6 if n is less than  10. The process is described below (NOTE: Use point estimates
rather than NO EC for these calculations):

     a.   If 10 or more valid, facility-specific WET data points for the most sensitive species are
         available (n > 10), calculate the facility-specific variance of log(TU) or log(concentration) using
         the equations in EPA's TSD (Appendix E, page E-8, page E-15) (see Box F in Figure 2). [Do
         not calculate variance from the CV as shown in Box 5-2 (page 100).]

                                                 -or-

     b.   If fewer than 10 valid, facility-specific WET data points are available (« < 10), the permitting
         authority should use the recommended default CV of 0.6. This CV is an empirical estimate of
         effluent variability  in relation to the mean developed using WET data presented in EPA's TSD,
         Appendix A (see Box I in Figure 2).


                                  Treatment of Censored Data Points


    1. The delta-lognormal method described  in the TSD (USEPA 1991a) may be applied, provided there is only
     one censoring level, for example, 100% (1 TUC). The delta-lognormal is expected to provide reasonable
     estimates for the mean and variance, even if the distribution is a mixture of a lognormal and a point mass
     at the censoring level. For the case of one or more censoring levels, one should consider the ROS method
     (below) or maximum likelihood methods (Cohen 1991).

    2. The regression-on-order-statistics (ROS) methodology is expected to provide reasonable estimates for the
     mean, variance, and possibly for high percentiles  (Helsel and Cohn 1988,  Hirsch and Stedinger 1987,
     Kroll and Stedinger 1996, Shumway et al. 2002). These authors evaluated the method for estimating the
     90th percentile with small (n = 10) samples. For higher percentiles, we are  not aware of any similar
     evaluations,  but one must expect greater variability than reported for the 90th percentile. The ROS method
     accommodates multiple detection limits. Be warned that the articles by Helsel and Cohn (1988) and Hirsch
     and Stedinger (1987) contained an inaccurate formula for Cj,  and the articles did not address ties in
     uncensored observations or the occurrence of uncensored observations below the lowest detection limit.
     The formula for Cj is: Cj = Bj - (Aj-1 + Bj-1), where
         AO = BO = 0.

    3. If the frequency of data at a detection limit is greater than expected from the censoring of a single parent
     distribution, a mixture model may be  needed. The ROS method is not intended to deal with this situation.
     The delta-lognormal method (USEPA 1991 a) and the approach described by Taylor et al. (2001) are  two
     ways of handling this situation

    Cohen, A.C. 1991. Truncated and Censored Samples. New York: Marcel Dekker.

    Helsel, D.R., and T.A. Cohn. 1988. Estimation of Descriptive Statistics for Multiply Censored Water  Quality
    Data. Water Resour. Res. 24:1997-2004.

    Hirsch, R.M., and J.R. Stedinger. 1987. Plotting Positions for Historical Floods and Their Precision. Wat.
    Resour. Res. 23:715-727.

    Kroll, C.N., and J.R. Stedinger. 1996. Estimation of Moments and Quantiles Using Censored Data. Wat.
    Resour. Res. 32:1005-1012.

    Shumway, R.H., R.S. Azari, and M. Kayhanian. 2002. Statistical Approaches to Estimating Mean Water
    Quality Concentrations with Detection Limits. Environ. Sci. Technol. 36:3345-3353.

    Taylor, D.J., L.L. Kupper, S.M. Rappaport, and R.H. Lyles. 2001. A Mixture Model for Occupational
    Exposure Mean Testing with a Limit of Detection. Biometrics 57:681-688.
  v	J

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     The next step is to use Chapter 3 of the TSD to obtain an RPMF for the 99th percentile (see Table
3-1 of the TSD) or the 95th percentile (see Table 3-2 of the TSD), using the number of WET data points
and the CV determined by (a) or (b) above. These values can then be used to calculate the RWC with the
following equation:
                                              Qe(rpmj)Ce
                                    RWC =
                                             Qe + (pmf)Qs
In this case, Ce will be the projected 95th or 99th percentile and is the product of the RPMF and the
observed maximum TU value. Once RWC is calculated, this magnitude of toxicity is compared to the
appropriate toxicity criterion.

     Ifn > 10 and

         the calculated toxic magnitude of RWC is greater than the toxicity criterion (e.g., RWC > 0.3
         TUa for acute toxicity or RWC > 1.0 TUC for chronic toxicity), the discharge has reasonable
         potential to cause an excursion above the toxicity criterion (see Box G in Figure 2). A WET
         limit is necessary (see Box D in Figure 2).

     •   the calculated toxic magnitude of RWC is less than or equal to the toxicity criterion (e.g., RWC
         < 0.3 TUa or RWC < 1.0 TUC), the discharge does not have reasonable potential to cause an
         excursion above the toxicity criterion. A WET limit need not be established (see Box H in
         Figure 2), and the permit should contain a monitoring-only requirement on WET, with
         numerical WET triggers for conducting accelerated testing and/or a TRE.

     Ifn < 10 and

     •   the calculated toxic magnitude of RWC is greater than the toxicity criterion (e.g., RWC > 0.3
         TUa or RWC > 1.0 TUC), the discharge has reasonable potential to cause an excursion above the
         toxicity criterion (see Box J in Figure 2). A WET limit must be established. This limit may be a
         delayed WET limit, as described in Section 5.3.2 of this guidance (see Box K in Figure 2).

     •   the calculated toxic magnitude of RWC is less than or equal to the toxicity criterion (e.g., RWC
         < 0.3 TUa or RWC < 1.0 TUC), the discharge does not have reasonable potential to cause an
         excursion above the toxicity criterion. A WET limit is not necessary (see Box H in Figure 2).
         The permit should contain a monitoring-only WET requirement.

     Step 3. Determine whether a discharge contributes to an excursion above numeric or narrative
water quality criteria for aquatic life (i.e., toxicity). In two situations, the permitting authority may
assess the facility's contribution to excursions above toxicity criteria:

     1.   If a facility discharges to a receiving water that is known or believed to have toxicity from other
         sources (as discussed in Section 3.3.4 of the TSD); or

     2.   If the permitting authority has no knowledge of or data on the presence or absence of toxicity in
         the stream.

     Because each effluent discharge may contain various toxic substances, a mixture of the effluents in a
receiving water produces a mixture of these individual pollutants. The cumulative toxic effect of the

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discharge on levels of toxicity in the receiving water can be measured by a WET test that uses upstream
receiving water as dilution water for the test. The aggregate toxic effect of the discharge and receiving
water levels of toxicity may be additive, synergistic, or antagonistic. WET testing that uses receiving
water as dilution water (where controls are valid) accounts for toxicity present in the receiving water, as
well as the toxic assimilative capacity or mitigating effects of the receiving water. To determine the need
for a WET limit for these types of facilities, see Steps 1 and 2 above.

     When conducting an  RP determination with facility-specific effluent monitoring data, the permitting
authority should provide in the permit fact sheet an adequate basis of how the requirements of 40
CFR 122.44(d)(l)(ii) were satisfied, particularly how the need for a WET limit was determined using the
available valid and representative WET data.

4.3  DETERMINING THE NEED FOR PERMIT LIMITS WITHOUT EFFLUENT MONITORING
     DATA

     If a regulatory authority chooses, or if the situation warrants it (i.e., past compliance history or
documented information on the facility's operation that is known to cause toxicity), the permitting
authority may decide to develop and impose  a limit for WET without facility-specific toxicity effluent
monitoring data, or prior to the generation of effluent data. In doing so, the regulatory authority must
satisfy all the requirements of 40 CFR 122.44(d)(l)(ii). Should the permit writers choose to impose
permit limits using this approach, they should present a clear rationale in the permit fact sheet for the
approach.

     When determining whether or not a discharge causes, has the reasonable potential to cause, or
contributes to an excursion of a narrative or numeric water quality criterion for individual toxics or
toxicity, the permitting authority can use a variety of factors and information where facility-specific
effluent monitoring data are unavailable. These factors should also be considered with available effluent
monitoring data. These factors include the following factors, which are described in detail in the TSD
(Section 3.2):

     •   Dilution

         Type of industry

     •   Type of publicly owned treatment  works

         Existing data on toxic pollutants

     •   History of compliance problems and toxic impact

         Type of receiving water and designated use


     The presence of a combination of the factors described above, such as low available dilution, high-
quality receiving waters, poor compliance record, and clustered industrial and municipal discharges,
could constitute a high priority for effluent limits for WET (i.e., based on a reasonable potential for
toxicity). If the permitting  authority chooses  to impose an effluent limit without facility-specific effluent
monitoring data, it will need to provide adequate justification for the  limit in the permit fact sheet.
However, the permitting authority should obtain facility-specific WET monitoring data before permit re-
issuance. The permitting authority may obtain these data through the information gathering authority
under CWA Section 308, or similar State authority.
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            5 DEVELOPMENT OF WET REQUIREMENTS IN NPDES PERMITS

     This chapter describes the elements that should be covered in a NPDES permit for WET, such as the
derivation and expression of the permit limit, the expression of a monitoring trigger, application of the
mixing zone if applicable, testing frequency, and steps to address toxicity such as accelerated testing and
TRE triggers. Based on the outcome of the RP determination (Chapter 4), the only difference in the
outcome is whether a WET limit or numeric monitoring trigger is based on State WQS. The derivation of
the WET limit or numeric monitoring trigger procedure is the same as described in the TSD.

5.1  PERMIT DOCUMENTATION RECOMMENDATIONS

     Recommendations for NPDES permits are made to ensure that the permit provisions and
requirements avoid ambiguity, provide clear instructions or guidance to the permittee on the intended
expectations, and ensure enforceability. permits should be issued with the following requirements stated
in the permit (see Table  2). The examples provided are not necessarily recommendations. They are
intended to represent the specific type of condition a permitting authority might require to meet the permit
requirement. Below are a few of the types of conditions to be considered for inclusion in a permit:

         Test species/methods. Toxicity testing species and methods should be accurately referenced in
         the permit.

         Frequency. States should require permittees to monitor regularly for toxicity and ensure that
         the frequency  of monitoring appropriately and adequately characterizes and represents their
         effluent, taking into consideration all pertinent features of the facility's entire operation and
         production.

         Number of Species. A minimum of three species should be tested (invertebrate, vertebrate,
         plant), at least through the screening phase. A minimum of three  screening tests is
         recommended in order to determine the most sensitive species for continued monitoring. See
         the TSD (Section 1.3.4, page 16) for further details.

     •   Quality Assurance/Quality Control (QA/QC). The permit should specify that if the TAC or
         required test conditions of the WET test manual are not achieved, then the permittee must
         collect an additional effluent sample and conduct a new test within at least two weeks.

         Toxicity Reduction Evaluation/Toxicity Identification Evaluation (TRE/TIE) Language.
         The permit should reference the appropriate TRE/TIE documents and numerical TRE triggers,
         including significant dates. Limits should be written with numerical TRE triggers. Note:
         Regardless as to whether the permit contains a WET limit or a WET numeric monitoring trigger
         both need to discuss steps to address toxicity including accelerated testing and TRE triggers. A
         meeting with the permitting authority should be held near the beginning of the TRE (EPA
         200 la). TREs that fail to identify and reduce or eliminate the source of toxicity such as TREs
         that do not meet established targets (e.g., an action level for the permit when the permit does not
         have a WET limit) are incomplete and typically should continue until all goals are met. Also,
         inconclusive TIE results should lead investigators to seek assistance from others with additional
         experience,  such as experts at EPA Headquarters or Regional offices or State or local experts.
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Table 2. WET Permit Requirements and Examples of Conditions
Permit Requirement
WET Testing
Test Type
Test Statistical Endpoint
Test Species
Sample Type
Dilution Water
Critical Low-flow Dilution
Effluent Dilution Series
Test Acceptability Criteria
Test Validity
Testing Frequency
Accelerated Testing
Requirements
Reporting Requirements
Toxicity Reduction
Evaluation Requirements
Example Condition
WET numeric monitoring triggers or limits (MDL/AML/MML) with
accelerated testing triggers
Acute/chronic and marine/freshwater with reference to specific test
and appropriate WET method manual
LC50, NOEC, IC25
Invertebrate, vertebrate, plant
Grab/composite (duration and minimum number of sub-samples for
composites)
Uncontaminated receiving water or lab synthetic of similar pH and
hardness
< 1 percent - 1 00 percent
0%, 12.5%, 25%, 50%, 75%, 100% or
or 0%, 4%, 8%, 16%, 32%, 64%
All criteria required by the test methodology, plus any additional
requirements established by the permitting authority
A valid WET test must be submitted for each species during each
reporting period for that species
See EPA's 1991 Technical Support Document (U.S. EPA 1991 a)
6 additional tests at 14-day intervals for the affected species, required
after exceeding a WET permit limit or WET numeric monitoring trigger
Permit Compliance System (PCS) reporting codes associated with
test endpoints
Mandatory initiation of TRE study and reporting for failure of any
additional tests in the accelerated testing mode
     •    WET Limits. The limit established in the permit should be based on a statistical derivation
         procedure that adequately accounts for effluent variability. The permit should include a
         maximum daily limit and an average monthly limit (AML) or monthly median limit (MML) for
         a low-flow receiving water body.

     •    Triggers. Whether the permit contains WET limits or WET numeric monitoring triggers (both
         contain monitoring requirements), the permit conditions should include these two requirements
         when toxicity occurs above a limit or monitoring trigger. For example, a test result which is
         greater than the WET permit limit expressed in toxic units or the monitoring trigger may result
         in an increase in the WET testing frequency. Any unacceptable test result measured during the
         period of increased testing frequency may then trigger a requirement to perform a TRE.

5.2  DERIVATION AND EXPRESSION  OF PERMIT LIMITS

     When deriving water quality based effluent limits (WQBELs) for NPDES permits, the regulatory
authority needs to examine the State's applicable water quality criterion for aquatic life, the State's
mixing zone policy, and the permit limit derivation procedures to  express the appropriate permit limits or
numeric monitoring triggers.
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5.2.1    Permit Limit Derivation Procedures

     WQBELs are based on maintaining effluent quality at a level that will comply with applicable State
water quality criteria for aquatic toxicity, even during critical low flow conditions in the receiving water.
These effluent limits are based on the allowable effluent loading concentration or wasteload allocation
(WLA). Pollutant WLAs can be adjusted for effluent variability using statistics calculated from historical
effluent data. These adjusted WLAs define the desired levels of performance, or targeted LTA discharge
conditions for specific applicable criterion effect levels (i.e., acute or chronic). Permit limits for WET are
calculated using statistics derived from historical effluent data and the most limiting target LTA to protect
both acute and chronic toxicity criteria. NPDES regulations require both an AML and a maximum daily
limit for WET, unless  otherwise specified by State WQS. Section 5.4.1 of the TSD provides general
guidance for permitting authorities developing and implementing two-value,  steady-state water quality
based effluent limits for acute or chronic toxicity. These procedures are described in Appendix A of this
document.

5.2.2    Mixing Zone Application in WET Permit Conditions

     When deriving permit limits, mixing zones are considered based on available assimilative capacity
based on dilution, and where appropriate and allowed, under State water quality standards and
regulations. If mixing zones are not allowed by State regulations for acute or chronic toxicity, then the
appropriate criterion (acute or chronic) must be applied at the end of the pipe. Even though a State's
WQSs may allow the consideration of mixing zones, they may not be appropriate under certain
circumstances. Such circumstances include discharges to ephemeral streams (when the receiving water
provides no dilution) or to a receiving water that has no capacity to assimilate additional toxic loads
(i.e., the ambient toxicity of the water upstream of the discharge point is equal to or exceeds the WQS for
toxicity). If considering the effects of a mixing zone at a discharge location is not appropriate, the
discharge must comply with the water quality criterion for toxicity at the end of the pipe.

     When allowed to be considered by State procedures or water quality standards, the effect of mixing
can be determined by either dynamic or steady-state modeling. Permitting authorities should consider all
available relevant data and information, including biological information from the receiving stream,
where appropriate, when establishing the exposure assumptions used to determine whether the discharge
will cause, have reasonable potential to cause, or contribute to an excursion above the applicable water
quality standard, including State narrative criteria for water quality.

5.2.3    Expression Of Permit Limits

     WQBELs for WET must be consistent with State water quality standards, expressed as the monthly
or daily values developed using the steady-state statistical approach, or other methods as previously
discussed. The NPDES regulations at 40 CFR  122.45(d) require that all permit limits be expressed, unless
impracticable, as both  an AML and a maximum daily limit for all discharges other than those  for POTWs
and as an average weekly limit (AWL) and average monthly limits for POTWs. The use of an AWL is not
appropriate for WET. The maximum daily limit is the highest allowable discharge measured during a
calendar day or 24-hour period representing a calendar day. The permit should contain a notation
indicating that the maximum daily limit is interpreted as signifying the maximum test result for that
month unless otherwise specified by the State's WQS. The AML is the highest allowable value for the
average of daily discharges obtained over a calendar month. For WET this translates into the average of
the toxicity test result(s) over a 30-day average. As discussed more fully below, in cases where little
dilution is available, or where State standards do not allow mixing zones, the average monthly limit
should be expressed as a monthly median.
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5.2.4    EPA's Recommendation for Deriving Permit Limits for Low-Flow Dilution
         Situations

     Under some discharge scenarios [e.g., low-flow receiving water body, absence of a mixing
zone/dilution policy approved by EPA under CWA Section 303(c)], mixing zones/dilution credits may
not be authorized by the permitting authority and State water quality criteria are applied to NPDES
discharges without allowing for mixing/dilution with the receiving water body. Currently, States have
different approaches for expressing WQBELs for chronic toxicity under these discharge scenarios. In this
policy, EPA continues to recommend that numeric WQBELs for chemicals and WET be established using
statistical procedures outlined in Section 5.4.1 and Tables 5-1 and 5-2 of the TSD. In addition, this policy
expands upon existing TSD guidance  (Sections 5.4.2 and 5.7.4) related to implementation considerations
unique to WET when a mixing zone/dilution allowance is not authorized for a NPDES discharge.

     Where EPA's recommended 1.0 TUC for chronic toxicity and 0.3 TUa acute toxicity are applied to a
NPDES discharge without a mixing zone/dilution allowance, this policy continues to recommend that
maximum daily limits be set in accordance with statistical procedures for calculating maximum daily
limits (see TSD Section 5.4.1 and Tables 5-1 and 5-2). For example, when using the 99th percentile to
calculate the long term average and an effluent CV of 0.6, the maximum daily limit for chronic WET is
1.6 TUC and the AML is to 1.0 TUC (see Appendix B of this document).

     Toxicity  limits are to be expressed as maximum daily limits and monthly average limits (AML).
For the WET AML, The calculation of the AML should be specified in the permit using the median
instead of the mean. The median is the middle of the distribution, above and below which lies an equal
number of values. Federal regulations at 40 CFR 122.41(l)(iv) specify that the calculation for all
limitations that require averaging of measurements shall use an arithmetic mean (i.e., average), unless
otherwise specified by the Director in the permit.

     EPA expands upon its earlier guidance regarding the use of statistics for calculating an AML when
applied in the absence of a mixing zone/dilution allowance because quantifying 1.0 as an average is not
mathematically feasible when the first toxicity test is greater than 1.0. Setting a median monthly limit
rather than an average monthly limit for chronic toxicity allows for a compliance determination based on
multiple toxicity test results from effluent samples  collected during a one-month period, while continuing
to protect against short-term excursions above the chronic criterion of 1.0 TUC. This recommendation is to
be considered only under these conditions:

     1.   Establish the monthly effluent limit magnitude at 1.0 TUC, rather than a statistically calculated
         magnitude, which is less than 1.0 (see Appendix B).

     2.   Apply the  monthly  median to the magnitude of 1.0 TUC, rather than the traditionally
         recommended monthly average.

     3.   Use in conjunction  with a maximum daily limit statistically calculated for chronic WET using
         TSD procedures.

     4.   Rely upon the basis that the permittee conducted three-species screening to ensure that the most
         sensitive species is  used for routine toxicity testing.

To ensure that  a monthly median limit of 1.0 TUC is protective of the water quality criterion for chronic
toxicity, discharge permits issued under this recommendation should include all four components.
Further, EPA emphasizes that applying a monthly median of 1.0 TUC without also applying a maximum
daily limit for chronic toxicity that is statistically calculated using TSD procedures is lexicologically

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unprotective. Finally, EPA notes that States may continue to establish a monthly average effluent limit for
chronic toxicity of 1.0 TUC, in lieu of a monthly median limit. As described in the TSD (Section 5.7.4), an
effluent with a NOEC > 100% contains < 1 TUC and would meet EPA's recommended 4-day average
CCC of 1.0 TUC for chronic toxicity, if no dilution were available.

5.3  PERMIT CONDITIONS FOR POSSIBLE OUTCOMES OF A REASONABLE POTENTIAL
     DETERMINATION

     This section discusses the possible outcomes of an RP determination for WET and the appropriate
permit conditions to be used in NPDES permits based on the amount of WET data made available to the
permitting authority before and during permit development.

     The three possible outcomes of an RP determination for WET are:

     •   Facility has demonstrated it causes or contributes to an excursion above the water quality
         criterion for WET,

     •   Facility discharge has reasonable potential to cause an excursion above a water quality criterion
         for WET, or

     •   Facility discharge does not have reasonable potential to cause an excursion above a water
         quality criterion for WET.

     Two possible scenarios  exist with respect to WET data when determining RP as described above in
the second bullet. These two  scenarios are:

         Reasonable potential is demonstrated based on a data set of at least 10 valid and representative
         toxicity data points, or

         Reasonable potential is demonstrated based on fewer than 10 valid and representative WET
         data points.

     Note, regardless of the three possible outcomes of an RP determination, each will require some level
of toxicity testing with either limits or monitoring triggers and steps to address toxicity if the limits or
triggers are exceeded. NPDES permits should include WET limits or numeric monitoring triggers to
accelerate the WET monitoring frequency for a specified time period following the exceedence of either
the limit or monitoring trigger. NPDES permits should also include a TRE trigger to address the
continued toxicity as demonstrated in the accelerated testing mode based on a specific number of toxicity
test failures within a specific  period of time. Monitoring and reporting requirements, as well as the term
of the TRE, should be established within the permit (USEPA 200 la).

5.3.1     Permit Conditions for an Excursion above the Applicable Toxicity Water Quality
         Standard or for the Reasonable Potential for an Excursion  above the Applicable
         Toxicity Water Quality Standard

     If the permitting authority determines that the WET of a facility's discharge is or will be discharged
at a level that causes or contributes to an excursion above a narrative or numeric water quality criterion
for aquatic life (i.e., toxicity), it must establish a WQBEL in the permit [40 CFR 122.44 (d)(l)(i)]. Or if
the regulatory authority has demonstrated that reasonable potential exists where  an effluent, in
conjunction with other sources of a pollutant, is statistically projected to cause an excursion above the
narrative  or numeric water quality criterion for aquatic life (i.e., toxicity) using data sets of at least 10 data

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points, it must establish a WQBEL in the permit. This projection is based upon an analysis of available
data that accounts for limited sample size and effluent variability, as well as consideration of such other
factors as the type of facility; existing data on toxic pollutants; type of receiving water and designated
uses; and/or compliance history of the facility, as noted in the TSD. Under either scenario—an excursion
above the water quality criterion or the reasonable potential to cause an excursion above a water quality
criterion—a WQBEL must be established. The WQBEL must be a WET limit, if the State has numeric
toxicity criteria [40 CFR 122.44(d)(l)(iv)]. If the State has narrative toxicity criteria and the permitting
authority can demonstrate in the fact sheet that a chemical-specific limit(s) is sufficient to attain and
maintain applicable water quality standards, appropriate chemical-specific limits can be established in
lieu of a WET limit [40 CFR 122.44(d)(l)(v)]. This provision would apply when the permittee has
identified and confirmed the chemical(s) resulting in an excursion above the applicable WQS. In addition
to the limit, EPA recommends continued WET monitoring and testing triggers in case of a re-occurrence
of an unacceptable level of toxicity. For this situation, the permit should be issued with the following:

     •   A WET limit or a chemical limit with WET numeric monitoring triggers (if the toxic has been
         identified and confirmed) and compliance monitoring requirements.

     •   A compliance schedule (if appropriate and where schedules are allowed under applicable WQSs
         or permitting regulations).

     •   Accelerated WET testing based on an exceedence of the WET limit or WET numeric
         monitoring trigger. EPA's "step-wise approach" to permitting also includes a requirement for
         accelerated monitoring as a first response to an exceedence of an effluent limit or numeric
         monitoring trigger. If a first test result is higher than the maximum daily limit or the multiple
         test results are above the AML, accelerated monitoring for six additional tests bi-weekly over a
         twelve-week period should occur, beginning within two weeks of the first exceedence of a
         WET limit or a numeric monitoring trigger.

     •   TRE triggers are based on an exceedence of the WET limit or numeric monitoring trigger and
         any toxicity observed in the accelerated tests. In response to continued toxicity, the permit
         should require that, if the results of any one  of the six additional toxicity tests exceed the limit
         or numeric monitoring trigger, the permittee implement corrective actions identified in a TRE.
         The purpose of a TRE is to investigate the causes of, and identify corrective actions to reduce,
         effluent toxicity. A TRE is a site-specific study conducted in a stepwise process to identify
         source(s) of toxicity and the effective control measures for effluent toxicity. TREs are designed
         to identify the causative agents of WET, evaluate the effectiveness of the toxicity control
         options, and confirm the reduction in effluent toxicity. When the source of toxicity is identified
         and controlled or eliminated, WET RP can be reassessed. A period of up to 24  months  should
         be reasonable to complete a TRE and a shorter period may also be appropriate on a case-by-
         case basis.

     •   Requirements for a WET trigger, WET monitoring, and a permit reopener for WET limits if a
         chemical-specific limit is used in lieu of a WET limit. A permit reopener is a permit clause that
         stipulates that the permit may be reopened to modify or add permit conditions (in this case,
         WET permit conditions).

5.3.2    Permit Conditions When Reasonable Potential Is Demonstrated with Fewer than
         10 Valid WET Data Points

     When a permittee has submitted fewer than 10 valid and representative WET test results, if RP
exists after application of the appropriate statistical procedures referenced above, Federal regulations

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require a WET limit in the permit. In such cases, where State water quality standards authorize a
compliance schedule for toxicity, permitting authorities may consider incorporating a delayed effective
date that allows time for the permittee to gather up to 10 samples to be used to validate the finding of the
need for a permit limit based on a facility-specific coefficient of variation (as opposed to reliance on the
assumed default value of 0.6 otherwise recommended for when there are fewer than 10 data points).

     First, allowing the permittee more time to gather additional  data enabling an estimate of effluent-
specific variability is reasonable. Because the optional data collection opportunity would delay
effectiveness of the WET permit limit, the opportunity itself probably constitutes a "schedule of
compliance" as defined in the NPDES regulations in that the gathering of facility-specific data could be
considered "remedial" (at least from the perspective of a permittee seeking to avail itself of the
opportunity) [40 CFR 122.2]. The schedule for data gathering will lead to "compliance," either because
any result exceeding the delayed limit would result in the limit becoming effective, or because, if the
facility-specific data demonstrate that a WET limit is not necessary, the permittee would have
demonstrated that the limit was unnecessary [40 CFR 122.47(a)]. Second, the compliance schedule
regulation requires that any schedule of compliance be "as soon as possible" but not later than the
applicable deadline, in this case, in a state water quality standard  or implementing regulations for toxicity.
Any such data gathering schedule will be necessarily limited by the duration of time necessary to gather
up to  10 valid and representative data points; more than 10 data points is unnecessary to determine a
facility-specific coefficient of variation. See Section 4.1.5 of this  document and the TSD, Appendix E, pg.
E-5.

     Allowing the permittee up to 18 months would be reasonable, depending on how many additional
WET tests were necessary to obtain up to 10 valid and representative WET test results. A schedule
allowing up to three months per additional data point necessary to characterize an effluent-specific
coefficient of variation could reasonably be shown to be "as soon as possible" within the meaning of 40
CFR Part  122.47(a)(l) to collect the additional data. A reasonable period of time to collect additional data
based on the expected sampling interval and the number of additional samples to be collected should be
established in the permit. Eighteen months is a reasonable maximum amount of time to collect the
additional data and is also consistent with existing sample collection requirements for WET reasonable
potential determinations. The NPDES permit application regulations at 40 CFR Part 122.21(j)(5)(iv)(A)
require POTWs to provide the results of a minimum of four quarterly WET tests for a year as part of their
permit application.9 The 18-month period of time provides for an additional 6 WET tests to be conducted
at the same quarterly monitoring frequency. Anything greater than 18 months would probably be
unreasonable, because permittees are often expected to collect WET data at a much shorter sampling
interval.10

     With a more comprehensive set of valid and representative data to characterize the effluent-specific
coefficient of variability, the permittee could then conduct a re-assessment of calculations used to
determine the need for the WET limit, and thus, determine whether the WET limit goes into effect.  All
testing and data analysis would need to be completed prior to the WET limit effective date. The WET
limit would become effective on its effective date, however, if the re-analysis (using the 10 or more data
points) supports the original finding  of the need for a WET limit, or if any test demonstrated toxicity at a
level that would result in an excursion above the State water quality criterion for aquatic life protection.
9   Alternatively, the permittee may submit 4 tests taken over a longer period of time, if certain conditions apply, see
    40 122.21(j)(5)(iv)(B).
10   For example, the recommended monitoring frequency for accelerated testing, as described in Section 5.4.1 of this
    guidance is 6 additional tests to be performed in 14-day intervals.

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This delayed WET limit option should be available in the context of a compliance schedule (for a water
quality-based effluent limit in a NPDES permit) but only when each of the following conditions is met:

         without accounting for effluent variability (see Section 4.2.2 Step 1), the discharge would not
         cause or contribute to an excursion of the applicable water quality criterion for toxicity, because
         the  collection of more data to better characterize effluent variability will not change the
         outcome of the determination that a WET limit is necessary;

         the  permittee has generated fewer than 10 valid WET test results for the most sensitive species,
         because EPA recommends the minimum of 10 data points to determine effluent variability
         using a facility-specific coefficient of variation (for more explanation see Section 4.1.5 of this
         document and the TSD, Appendix E, pg. E-5);

         State water quality standard or implementing regulations do not otherwise preclude a delayed
         effective date for the WQBEL, (i.e., where State water quality standards authorize a compliance
         schedule for toxicity);

     •   the  permittee is an existing discharger, because compliance schedules are generally not
         available for new  sources, new dischargers, and recommencing dischargers per 40 CFR Part
         122.47; and

     •   the  WET limit being imposed in the permit is a new limit or a revised limit that is more
         stringent than the  previously effective limit.

     A WET test, unlike a chemical-specific test, measures the combined toxic effect (additivity,
antagonism, and synergism) of an effluent with a complex mixture of constituent parameters. Thus, the
toxicity measured by WET may be the result of multiple causative agents that each individually vary over
time. Those causative agent(s), however, may be identified (e.g., through a TRE/TIE) and expressly and
specifically limited in the  permit, in which case, the NPDES permit could impose a chemical-specific
limit (or limits) to control that toxicity and that permit need not include a WET limit to achieve the
applicable narrative water quality criterion [40 CFR 122.44(d)(v)]. Such a delayed conditional limit is
appropriate for the parameter WET because it provides a means to collect more data to provide a better
estimate of effluent variability. The use of the delayed conditional limit only for WET and not for other
chemical-specific limits is recommended: (1) because WET is  a function of multiple chemicals in the
effluent that each individually vary over time, more WET data will result in a better estimate of effluent
toxicity over time; and (2) the alternative opportunity to control toxicity via chemical-specific limits
exists when the causative  toxicant can be identified and controlled.

     If the reasonable potential determination conducted at the end of the time period demonstrated that
the discharge  did not cause, have reasonable potential to cause, or contribute to excursions above water
quality criteria for aquatic life (i.e., toxicity), the permitting authority could then remove the effluent limit
through a permit modification or include a provision in the permit that would provide for automatically
deactivating the effluent limit. In this case, monitoring only would be required for WET, with a trigger for
accelerated WET testing and/or a TRE in the event of a future  demonstration of an unacceptable level of
toxicity. When using the automatic deactivation option, the permitting authority would need to be very
clear in the permit language and fact sheet when describing precisely the criteria for deactivating the
WET effluent limit. For example, the permit might state that

     "Each WET test result will be reported on the DMR the month after the month in which the
     test was performed. In its discretion, the permittee may elect to collect additional samples for
     WET measurement at a frequency higher than otherwise required by [insert monitoring

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     frequency provision.] If the permittee collects a minimum of [insert # representing 10 minus #
     of data points upon which  WET limit decision was based] valid WET test results by [insert
     date not to exceed 18 months after permit issuance], then the additional analysis may be
     conducted enabling re-evaluation of the extent to which the discharge will cause, has the
     reasonable potential to cause, or contribute to an excursion above the [insert "numeric " or
     "narrative "] water quality criterion for aquatic life. If the permittee obtains more than [insert
     # representing 10 minus #  of data points  upon which WET limit decision was based] data
     points during this period, all valid data points must be used in the re-calculation. The re-
     calculation must be performed according  to the procedure specified at [insert section and item
     number] of this permit. If the re-calculation confirms that the discharge has reasonable
     potential to cause an excursion of [insert  applicable WQ aquatic life criterion for WET], the
     WET effluent limitation will become effective on the date specified in [specify part of permit
     that includes the limit and  its effective date]. If the re-calculation shows that reasonable
     potential does not exist, the effluent limitation for WET does not become effective. The
     permittee shall promptly notify the [permitting authority] of the results of the re-calculation.
     The permitting authority may review the re-calculation of reasonable potential for WET,
     including upon request by the permittee. Any conclusions regarding the  re-calculation by the
     permitting authority, however, do not represent final agency action. The re-calculation
     opportunity is not available if the permittee does not collect the additional samples for
     measurement as described  above, or if any of the samples, in fact, causes an excursion above
     the  [insert "numeric" or "narrative"] water quality aquatic life criterion for WET."

     In either case, the permitting authority should acknowledge the reassessment and indicate in writing
whether the reassessment changes the status of the WET limit. Note that, if the effluent limitation has
never gone into effect, an anti-backsliding analysis is not necessary.

     To track the status of a delayed WET limit, the WET limit should be entered into the NPDES
national database, Permit Compliance System (PCS), like any other effluent limit. Every effluent limit in
PCS has  a "modification number." For a delayed WET limit, the initial limit entered is automatically
given a modification number of  zero or a "mod 0." Only key information and  monitoring requirements for
the permittee will be initially tracked. The delayed WET limit should be entered as a "mod 1" limit with
the mod limit start date being up to 18 months  after the effective date of the permit and the mod limit end
date  being equal to the "mod 0"  end date, usually the permit expiration date. If monitoring results indicate
the WET limit is unnecessary, the permitting authority should delete the delayed WET limit ("mod 1"
limit).

     Where State water quality standards preclude delaying the effective date of a permit limit (i.e.,
because State WQS  do not authorize a compliance schedule for toxicity) and the data available at the time
of permit issuance support the need for a WET limit, the permit must include  a WET limit effective upon
the effective date of the permit.

     When fewer than 10 valid WET data points exist and RP has been  demonstrated (based on the
statistical methodology of the TSD, or an equivalent approved methodology) to exceed the narrative or
numeric toxicity criteria within a State's WQSs, The permit should contain the following (see Box K in
Figure  2):

     •    A WET limit effective up to  18 months after permit issuance to allow the permittee to obtain 10
         valid test results. Provisions should include the requirement to discontinue the  18-month
         allowance if, during the WET data collection, a WET test failure occurs or if the permittee fails
         to provide the 10 valid WET data points. The WET limit remains effective  if the 10 valid WET
         data points confirm that the initial RP determination is correct.

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         A condition that the WET limit may be re-evaluated if a data set of at least 10 valid WET data
         points for the most sensitive species is submitted by the date stated in the permit or to determine
         whether to modify the permit to remove the WET limit.

     •   Permit language specifying that the permittee will be notified if the permitting authority
         deactivates the WET limit because the results of the WET RP re-evaluation based on a larger
         data set result in an RP determination that warrants removal of the WET limit.

         A permit re-opener clause to remove a WET limit and/or add a chemical-specific limit (see
         Footnote 3  in Figure 2) that is adequately outlined in the permit for the permittee and the public
         (to satisfy public notice requirements).

     •   The permittee may still collect additional data and seek a permit modification to remove the
         WET limit if a reasonable potential re-calculation based on  the larger data set (described above)
         demonstrates that  reasonable potential no longer exists.

     In addition, a permitting authority may use the approach of accelerated WET testing and TRE as
permit requirements triggered by or based on exceedence of the WET limit or may require accelerated
WET testing and a TRE/TIE through a CWA Section 308 information request or an enforcement order. A
permittee may elect to conduct TRE/TIE activities whenever toxicity is demonstrated without direction
from the permitting authority.

5.3.3    When Reasonable Potential for WET Is Not Demonstrated With Sufficient Data
         Sets

     If the permitting authority determines that a discharge from a facility has no reasonable potential
(with data sets of 10 or greater) to cause an excursion above a narrative or numeric water quality criterion
for aquatic toxicity, then a WQBEL for WET is not necessary. Even where reasonable potential to cause
an exceedence of water quality criteria does not exist, regular WET monitoring  should be performed for
the life of the permit. Regular monitoring is especially important if the facility is a POTW that may have
unforeseen toxicity related to new industrial users, or an industrial facility adding processes or chemicals
not previously introduced into their system (see Section 5.4).

     The permitting authority should establish appropriate monitoring requirements and a re-opener
clause in the permit (see TSD, Chapter 3.3.3). A re-opener clause authorizes "re-opening" the permit and
establishing additional permit conditions based on monitoring results  or other new factors that indicate
that the effluent may cause,  have the reasonable potential to cause, or contribute to an excursion above
applicable water quality standards. When permits are "re-opened" in this manner, permitting authorities
typically impose WQBELs for WET and/or require a permittee to perform a TRE.

     When RP is not demonstrated, the permit should include requirements for the following (see Box H
in Figure 2):

         Routine WET monitoring that adequately characterizes the effluent discharge to ensure that the
         WLA is not exceeded and to support an RP determination at permit re-issuance.

     •   Recommendations for accelerated WET testing and a TRE based on the exceedence of the
         permit-specified WET numeric monitoring trigger.

     •   A permit re-opener clause to allow the permitting authority to open the permit and modify it to
         include WET limits, should unacceptable levels of toxicity be demonstrated in future testing.

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5.4  MONITORING FREQUENCY RECOMMENDATIONS

     Once the need for a WET limit or monitoring requirement has been determined, the frequency of
WET testing must be determined. The frequency for monitoring pollutants or pollutant parameters such as
WET should be determined on a case-by-case basis, and decisions for setting the monitoring frequency
should be set forth in the permit fact sheet. Some States have their own recommended sampling
guidelines that can help a permit writer determine an appropriate monitoring frequency. The intent is to
establish a frequency of monitoring that will detect most events of noncompliance without requiring
needless or burdensome monitoring (see Table 3).

     As discussed in the EPA Permit Writer's Manual (U.S. EPA 1996a)12 to establish a monitoring
frequency, the permit writer should estimate the variability of WET. A highly variable discharge should
require more frequent monitoring than a discharge that is relatively consistent over time (particularly in
terms of flow or observed toxicity). In addition to the estimated variability, other factors that should be
considered when establishing appropriate monitoring frequencies include type of treatment process;
environmental significance and nature of the pollutant or pollutant parameter; past compliance
record/history; cost of monitoring relative to dischargers capabilities; number of monthly samples used in
developing the permit limit; and for intermittent dischargers, the frequency of the discharge (see also
TSD, page 113, Section 5.7.5).
Table 3. Likelihood of Detecting at Least One Toxic Event Using n
Observations, For Specified True Rates of Occurrence
Number of
Observations, n

1
2
3
4
5
6
8
10
12
16
20
True Probability of Occurrence3
10%
0.10
0.19
0.27
0.34
0.41
0.47
0.57
0.65
0.72
0.81
0.88
20%
0.20
0.36
0.49
0.59
0.67
0.75
0.83
0.89
0.93
0.97
0.99
30%
0.30
0.51
0.66
0.76
0.83
0.88
0.94
0.97
0.99
0.99
0.99
                 Assumes (i) negligible serial correlation among observations, and (ii) true rate
                 of occurrence remains the same over time. Probability of occurrence is stated
                 as a percentage of the possible independent sampling events.
12   See Section 7.1.2, pp.119-122.
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  Regulatory authorities should also consider the risks associated with infrequent WET monitoring,
whether the testing is routine or accelerated. Table 3 provides information on the likelihood of detecting
at least one toxic event based on a specified true rate of occurrence. The true probability of occurrence
refers to the probability that toxicity will be observed at the relevant permit limit or numeric trigger
expressed in TUs, using a particular WET method.

  For example, suppose the (unknown) probability is  0.20 (e.g., probability of occurrence is 20%) that
the NOEC for a chronic Ceriodaphnia test will occur at or above the permitted TU value. Then, if testing
is performed once per quarter (n = 4), the a probability that, in the course of one year, at least one of the
four tests will demonstrate a toxicity at or above the permitted TU value is 0.59. The same would apply to
monitoring once per year for four years (n = 4). As another example of the same true probability of
occurrence (20%), quarterly monitoring for three years (n = 12) would be expected to exhibit at least
results exceeding the permitted TU value with high probability (0.93).

  A permit writer may also establish a tiered monitoring schedule that reduces or increases monitoring
frequency during a permit cycle. Tiered monitoring, which reduces monitoring over time, may be
appropriate for discharges where the initial monitoring shows compliance with effluent limits. The
permit should establish a monitoring frequency with a permit clause that would allow a decrease in the
testing frequency after at least 20 observations (e.g., 20 independent toxicity test results) are measured
and are not deemed toxic (i.e., below the effluent limit or numeric monitoring triggers as specified in the
permit) within a four-year period (see Table 3). If a facility expects any changes in inputs or facility
treatment, the facility discharge should be assessed for the continued assumption of no toxicity at the
higher level of testing frequency to obtain a data set (n = 20) based on these new treatment changes before
consideration of reduced frequency. Another factor to consider before reducing the frequency of testing is
whether the facility has a consistent effluent stream (i.e., no changes in facility inputs or facility treatment
changes). Whether the facility is a POTW or an industrial facility should be considered. By the nature of
user input to the system, POTWs often produce varying concentration of toxicants and varying types of
toxicants can change readily. Therefore, some level of frequency is always necessary to assess the
continued potential for a toxic effluent exposure. Factors in favor for minimal level of testing frequency
for POTWs are those with no industrial inputs and those with a small discharge volume relative to the
volume of the receiving water body. As discussed in Section 5.4.1 below, permits should establish
triggers to increase the monitoring frequency of WET testing after a demonstration of an unacceptable
level of toxicity (i.e., above the permit limit or numeric monitoring trigger expressed in TUs or as an
RWC).

5.4.1           Monitoring Frequency for Accelerated Testing

  As  previously noted, permits should require numerical triggers to accelerate WET testing for the
affected species after a demonstration of an unacceptable level of toxicity (i.e., above the permit limit or
numeric monitoring trigger expressed in TUs or RWC). The number of tests and duration of testing
should be adequate to establish the presence of continued toxicity in order to perform a TRE. Following
notification that a test failure has occurred (and that the failed test met all test acceptability criteria), the
permit should trigger at least 6 additional tests, to be performed at 14-day intervals. The recommendation
of a minimum of 6 additional tests is based on the probability of encountering at least one exceedence
assuming a true, but unknown level of occurrence, as shown in Table 3 above.

5.5 TOXICITY REDUCTION EVALUATION/IDENTIFICATION  STUDIES

  The purpose of a TRE is to investigate the causes of, and identify corrective actions to reduce, effluent
toxicity that may cause or contribute to an excursion above a State water quality criterion for aquatic
toxicity. A TRE is a site-specific study conducted in a stepwise process to identify toxics and sources, and

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to narrow the search for effective control measures for effluent toxicity. TREs are designed to identify the
causative agents of WET, evaluate the effectiveness of the toxicity control options, and confirm the
reduction in effluent toxicity. TREs can vary widely in complexity, ranging from simply changing
housekeeping procedures to conducting TIEs. The basic numerical TRE trigger, requirements, and
objectives should be specified in the permit.

  Where toxicity is demonstrated at an unacceptable level (i.e., above the permit limit or numeric
monitoring trigger expressed in TUs or RWC) and in any subsequent test performed within accelerated
testing mode (the period of triggered increased testing frequency), a TRE should be required. Unless
revised in writing by the (permitting authority) the following apply when a TRE is triggered:

  • Notice of TRE implementation should be submitted to (agency contact) within 10 days of activation
    of this TRE trigger.

  • A TRE Schedule and TRE Action Plan should be submitted to (agency contact) within 60 days of the
    initiation of the TRE.

  • The term of the TRE is up to 24 months as follows: The "TRE initiation date" should be the initiation
    date of the test that confirms toxicity and the "TRE termination date" should be the date up to 24
    months from the TRE initiation date.

  • A Quarterly TRE Progress  Report should be submitted with the DMR to (agency contact) at the end
    of each quarter, based on the TRE initiation date. The progress report  should list all activities and
    findings related to resolution of toxicity, including all WET and chemical test data. The data
    summaries of the TRE should also be provided in tabulated format with explanations of procedures
    and findings. Any exceedence of a WET monitoring or permit limit should be reported within 5
    working days to the permitting authority. A Final TRE Report should be submitted to (agency
    contact) within 45 days of the TRE termination date. The Final TRE Report should summarize the
    TRE activities and findings, propose the corrective action(s) to be taken, and propose a schedule to
    complete any identified corrective action.

  • During the TRE, the minimum monitoring frequency for the (affected species) is (once per X).
    However, the permitting authority recommends additional testing, which may include streamlined
    tests using a single dilution and control, to  find toxic samples to work with.

  • All samples used for WET testing during the TRE should be analyzed for any toxicant which is
    identified as being suspect. If later testing dismisses the toxicant as probable, the analysis may be
    discontinued if the findings are documented in the  Quarterly TRE Progress Report. The objective of
    this testing is to ascertain whether the same level of toxicity occurs when the suspect toxicant level
    varies. This information may lead to finding additional toxicants or confirming or eliminating the
    suspect toxicant.

  • Where toxicity is intermittent (i.e., occurs at infrequent intervals) the (permitting authority) may
    include these requirements based  on best professional judgement.

  • This TRE trigger and the actions are the initial responses to the confirmation of a demonstrated
    toxicity above the WET limit or WET numeric monitoring trigger. These actions do not constitute a
    compliance order nor do they preclude enforcement action.

  EPA has published several guidance documents for conducting TREs and TIEs, which may be
performed as part of a TRE. These documents should be reviewed before committing resources to a

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particular course of action in the TRE and, as necessary, when performing the TRE. Additional guidance
may be available through toxicity specialists in EPA Regions or some NPDES State agencies.

  Generally, a TRE may translate into an additional permit control—(a WET limit, a chemical-specific
limit, or a direct requirement to reduce or eliminate toxicity). For more information on TREs, see relevant
published EPA guidance and other guidance available at http://www.epa.gov/owm and
http://www.epa.gov/waterscienceAVET.

  When the source of toxicity is identified and controlled or eliminated through a TRE/TIE, WET RP can
be reassessed. As discussed in Section 5.3.2 above, to ensure data are representative of the discharge, the
reasonable potential reassessment requires at least 10 new data points for the most sensitive species
obtained after the TRE is successfully completed. If these new data indicate that the toxicity is eliminated,
the permittee may request that the WET limit be removed from the permit at permit renewal or as
described in Section 5.3.2 of this document.
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   6 CONCLUSION - EPA RECOMMENDATIONS TO REGIONS AND NPDES STATES

  This document was prepared to address implementation of whole effluent toxicity (WET) tests in the
NPDES permits program. The document has three major goals as outlined in Chapter 1 of this document
and discussed in further detail in subsequent chapters: (1) National Consistency (framework for a
nationally consistent WET program); (2) NPDES Regulatory Compliance (to re-emphasize compliance
with existing NPDES regulations, especially with respect to determining the need for a WET limit while
allowing flexibility that is still consistent with regulations); and (3) Emphasis on Existing Guidance,
Policy, and Regulations (restate and clarify existing guidance, policy, and regulations). The guidance also
outlines EPA's step-wise approach and clarifies what this approach is (and what it is not) in order to
avoid potential misinterpretations of what is appropriate (and what is not).

6.1 GENERAL NPDES WET PROGRAM  RECOMMENDATIONS AND REQUIREMENTS

  • Water Quality Standards — States should develop (and seek EPA approval of) WQS and WQS
    implementation procedures in order to appropriately develop NPDES permits. State WQS, including
    designated uses, should be appropriate and well developed for each receiving water. States should
    consider incorporating schedules of compliance as part of their WQS and implementing WET
    regulations to fully take advantage of the recommendations in this guidance (i.e., delayed WET limit
    to allow for permittee to collect additional WET data when there are less than 10 valid and
    representative WET data points are available).

  • Monitoring — States should require permittees to monitor regularly for toxicity and ensure that the
    frequency of monitoring appropriately and adequately characterizes and represents their effluent,
    taking into consideration all pertinent features of the facility's entire operation and production.

  • Representative and Valid WET Effluent Data — States should encourage permittees to collect
    sufficient effluent samples at an appropriate frequency before permit development so that permit
    writers are provided a sufficient number of representative and valid WET data points to conduct an
    RP determination for the need of a WET limit (i.e., ideally 10 or more data points per effluent
    discharge point before permit issuance) (see USEPA 1995a). WET data should be generated such that
    the number of data points is 10 and that the data have been accumulated over a year or more to fully
    characterize effluent variability over time. The sampling dates and times should span a sufficient
    duration to represent the full range of effluent variability, and the samples must be tested according to
    EPA's WET test methods in 40 CFR 136.

  • EPA's Technical Support Document for Water Quality-based Toxics Control — EPA's TSD
    recommendations should be implemented. The TSD's statistical approach is appropriately protective,
    regarding both effluent and analytical variability, provided that the criteria and WLA are derived
    correctly. The TSD encourages permittees to have 10 or more WET data points but maintains that
    even with little or no WET data, other circumstances, when considered collectively, may warrant
    inclusion of a WET limit.

  • NPDES WET Program Implementation — States should strive to incorporate EPA guidance into
    their NPDES WET programs and at a minimum ensure that their State regulations and law are
    consistent with the CWA and with EPA's NPDES regulations.

  • WET Training — States should provide adequate training or seek out available training (i.e., other
    Federal, State, private, or non-profit resources) for their new and existing permit writers and
    laboratory inspectors on WET to ensure that permits are appropriately developed and WET

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    laboratory are operating properly in order to generate valid WET data. EPA's Web site at
    http://www.epa.gov/npdes provides links to the EPA's NPDES basic permit writer's course, which
    includes a module on WET.

  • EPA Guidance, Policy, and Publications — States should access current documents available from
    EPA or through EPA's Web site at http://www.epa.gov/npdes/pubs to ensure that both the permit
    writers and permittees have the same information from which to develop permits and can benefit
    from the recommendations provided in EPA guidance, policy, and other WET technical publications.

  • Communication — States and their permittees should communicate regularly and proactively,
    especially when the permit writer or the permittee are concerned about permit requirement(s) during
    the permit development process or if the permittee is concerned about the representativeness or
    validity of their effluent WET data to be submitted (or already submitted).

6.2 SPECIFIC RECOMMENDATIONS ON WET EFFLUENT TEST DATA USED FOR
    DETERMINING RP

  • Laboratory Selection — Permittees should choose laboratories such that the data analysis is done by
    one qualified toxicity testing laboratory for determining reasonable potential, deriving permit limits,
    and generating self-monitoring test results. All effluent testing must be conducted consistently with
    the EPA test manuals. In addition, the report must include reference toxicant tests in accordance with
    EPA test method manuals.

  • Laboratory Performance — States should inspect and audit WET testing laboratories' SOPs and
    QA/QC control charts.

  • WET Test Data Review — Permitting authorities and permittees should review WET test data
    submitted to ensure that they meet specified effluent holding times, temperature,  laboratory control
    limits, and test acceptability criteria (TAC), and review test requirements.

6.3 GENERAL RECOMMENDATIONS FOR DEVELOPING VALID AND REPRESENTATIVE
    WET NPDES PERMIT LIMITS, NUMERIC MONITORING TRIGGERS AND/OR OTHER
    PERMIT CONDITIONS

  • RP Determination — States and permittees should ensure that the reasonable potential
    determination of the need for a WET limit is supported by the analysis of the valid and representative
    WET monitoring data collected and that the monitoring was conducted using the appropriate WET
    test type and most sensitive species.

  • Permit Documentation — Permit writers should provide adequate documentation in the permit and
    the fact sheet to support the analysis of the need for WET and the WET limit itself, monitoring
    requirements, or other permit conditions included in a permit.

  • Permit Oversight — Permit writers should include in their permit clear documentation on how
    permit requirements were derived. The permitting authority should routinely and on schedule monitor
    the compliance of the permit requirements, especially those requirements or conditions with interim
    or final schedules (i.e., routine monitoring, accelerated monitoring triggers, TRE/TIE plans).

6.4 SPECIFIC RECOMMENDATIONS FOR ISSUING PERMITS WITH WET EFFLUENT
    LIMITS, NUMERIC MONITORING TRIGGERS AND OTHER PERMIT CONDITIONS
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    EPA's Step-Wise Approach — EPA's step-wise approach is one in which all parties can take an
    active role in the development of the permit requirements and compliance with those requirements.
    This approach accommodates both small and large WET data sets but provides an incentive to
    permittees to strive in their WET data generation for large data sets of 10 or more data points that are
    valid and representative of their discharge(s). The approach incorporates an opportunity (delayed
    WET limit) to address small data sets concerns by providing eligible permittees with more time to
    provide additional WET data for the WET RP determination. It also has features that address toxic
    events such as accelerated monitoring triggers and TIEs/TREs to identify and reduce or eliminate the
    source of toxicity and achieve State WQS and WET permit limits.

    NPDES Permit Limit Decisions — When the WET data RP determination results in a determination
    that an effluent causes, has reasonable potential to cause, or contributes to an excursion above a
    narrative or numeric water quality criterion for toxicity, the permitting authority must establish a
    water quality based effluent limit (WQBEL). This WQBEL must be for the parameter WET if the
    State has numeric toxicity criteria. If the State has narrative toxicity criteria and the permitting
    authority can demonstrate, in the fact sheet, that chemical-specific limits are sufficient to attain and
    maintain applicable standards, appropriate chemical-specific limits can be established in lieu of WET
    limits.

    Delayed WET Limit — Where applicable WQS allow a compliance schedule for WET, this feature
    of the EPA's step-wise approach is an enhancement of the NPDES WET program's current guidance
    recommendations, which enables NPDES agencies to provide an alternative to permittees with
    concerns about WET limits based on small data sets (fewer than 10 data points). This permit feature
    allows an eligible permittee, as part of their permit requirements, up to  18 months to collect more
    effluent samples for WET testing to generate additional valid and representative WET data to a
    permit writer for use in the WET RP determination.

    Monthly Median WET Limit — Permit writers may opt to develop WET permit limits that address
    discharges in which the effluent flow comprises a high percentage of the available stream flow during
    critical conditions  or otherwise lacks assimilative capacity. EPA has provided revised national
    guidance for establishing appropriate WET effluent limitations for receiving waters with no
    assimilative capacity For these situations only, using the limit derivation procedures described in this
    guidance (Chapter 5), a maximum daily limit of 1.6 TUC and an AML of 1.0 TUC can be developed.
    The 1.0 TUC AML should be expressed as a monthly median instead of a monthly average, in
    conjunction with a maximum daily limit.
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                 APPENDIX A
EXAMPLE: PERMIT LIMIT DERIVATION PROCEDURES

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       APPENDIX A- EXAMPLE: PERMIT LIMIT DERIVATION PROCEDURES

  This appendix presents an example of how to derive a permit limit.

  The mean and standard deviation of log(TU) or log(chemical concentration are calculated for each
pollutant using historical effluent data). Where historical data regarding effluent variability are
insufficient (e.g., n < 10), the default CV should be 0.6 (see TSD, Appendix E, pg. E-3). In that case only,
the variance of log(TU) or log(chemical concentration) is calculated from the CV using formulas in Box
5-2 of the TSD (page 100). Statistical derivation procedures for the AML for WET should assume that at
least four samples  (n) will be taken per month.

  The WLA required to protect against both acute and chronic effects under critical conditions may be
calculated using either steady-state or dynamic models. However, for derivation of the WLA, the equation
is rearranged to solve for the effluent concentration (Cj), or WLA, necessary to achieve the appropriate
applicable criterion. For compliance purposes, the water quality criterion for aquatic life (toxicity
criterion) is set equal to Cr, where  Cr is the applicable criterion:

    WLA = Cd = [Cr(Qd + %QS)] - [(Cs)(%Qs)]/Qd

  where:
           Qd  =  waste discharge flow in cubic feet per second (cfs) or MGD

           Cd  =  waste discharge pollutant concentration in TUs for WET (TUa or TUC)

           Qs  =  background in-stream flow in cfs or MGD above point of discharge

           Cs  =  background in-stream pollutant concentration in TUs for WET (TUa or TUC); setting
                 Cs = 0 is recommended for WET

         %QS  =  percent of upstream flow allowed by mixing zone standard, if applicable

           Qr  =  resultant in-stream flow after discharge in cfs or MGD: %QS + Qd

           Cr  =  applicable toxicity criterion = resultant in-stream pollutant concentration in TUs  for
                 WET (TUa  or TUC), 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
the critical stream  flow conditions, for example, 1Q10 and 7Q10 respectively (see TSD, Section 4.2, pg.
68). Ambient flow data from the U.S. Geological Survey are available on STORET.

  When calculating the WLA,  it should be noted that, if applicable State water quality standards and
plans do not explicitly  allow the application of mixing zones, the appropriate applicable criterion must be
met at the end-of-pipe  (i.e., applicable criterion = Cr = Cd = WLA). Where mixing zones are allowed,
appropriate State procedures should be applied.

  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, pg. 97). The steady-state mass balance
equation, however, when coupled  with the recommended conservative assumptions, should be adequately


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protective of receiving water beneficial uses, but may be more protective than necessary under some
circumstances.

  WLAs determined using State water quality criteria for WET may be converted to maximum daily
limits and AMLs. For low-flow situations, maximum daily limits and monthly medians should be used.
The following methodology (see TSD, Box 5-2, pg.  100; Figure 5-4, pg. 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-percent confidence level  for maximum daily limits and the  95-percent
confidence level for AMLs.

  1. Using the mass-balanced equation to solve for the allowable effluent concentration (Cd), or WLA, for
    WET:

    a.         Set Cr equal to acute, chronic criteria.

    b.         Background receiving water (Qs), discharge (Qd) flows, and background pollutant
               concentration (Cs) should represent critical conditions.

    c.         Solve for acute (WLAa) and chronic (WLAC) wasteload allocations.

  2. Convert the acute WLA to chronic toxic units (WLAa c), using the acute-to-chronic ratio (ACR). (see
    TSD, Section 1.3.4, pg. 17)

              WLAa c (in TUC) = WLAa (in TUa) x ACR.

  3.To calculate the standard deviation or CV:

    a.         Use the effluent data set of "k" observations (k is > 10) to calculate the mean (|i) and
               standard deviation (a) of log(TU) or log(chemical concentration) (see  TSD, Appendix E).

    b.        Where the effluent data set is small (k < 10), the conservative value of 0.6 is
               recommended (see TSD, Appendix  E, page E-3) to estimate the CV, from which the
              variance is then calculated using formulas in Box 5-2 of the TSD (page 100). Numerical
              values for the case when CV = 0.6 are provided in the TSD (Tables 5-1 and 5-2, pages
               102- 103).

  4.To determine LTA discharge conditions:

    Use the following equations to calculate acute and chronic LTA discharge conditions (LTAa c and
    LTAC) that will satisfy the acute and chronic wasteload allocation (WLAac and WLAC). The CV
    calculated above is used to estimate both acute and  chronic WLA multipliers (see  TSD, Table 5-1,
    pg. 102).

              LTAa,c  = WLAa,cxe[°-5o2-zo]

               LTAC  = WLAC x e[°-5a42- za4]

where:
         e[0.5a2-za]  =

       e[0.5a4 - za4]  = chrOnic WLA multiplier

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                   z  = 2.326 for the 99th percentile occurrence probability for the LTA is
                        recommended
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  5.Determine the lower (more limiting) LTA discharge condition

    LTA = minimum (LTAa c or LTAC)

  6.Calculate the maximum daily limit and average monthly limit using the lower (more limiting long-
    term average) discharge condition.

    Use the following equations to calculate the maximum daily limit and average monthly limit. The CV
    calculated above is used to estimate both acute and chronic long-term average multipliers (see TSD,
    Table 5-2, pg. 103).

               MDL  =  LTA  x e[za - °-5cj2]

         e[za - 0.5a2]  =  MDL LTA miutipiier

                   z  =  2.326 for the 99th percentile occurrence probability for the MDL is
                         recommended

               AML  =  LTA  x e[zan-  °-5an2]
  where:

       e [zan - 0.5 an2]  =  AML LTA miutipiier

                   z  =  1.645 for the 95th percentile occurrence probability for the AML is
                         recommended

                   n  =  number of samples/month

Following these procedures, the maximum daily limit and average monthly limit may then be
incorporated into the permit as justifiable WQBELs.
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                 APPENDIX B
EXAMPLE: DERIVING PERMIT LIMITS FOR LOW-FLOW
             DILUTION SITUATIONS

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          APPENDIX B - EXAMPLE: DERIVING PERMIT LIMITS FOR LOW-FLOW
                                   DILUTION SITUATIONS

  This appendix presents an example of how to derive permit limits for low-flow dilution situations.

  Many facilities across the country discharge to streams where the facility flow comprises a high
percentage of the available stream flow during critical conditions or for large parts of the year so an
alternative approach for deriving permit limits is needed. Receiving waters where little dilution is
available still often have a full aquatic life use designation and are afforded all protections based on
narrative and numeric State water quality criteria for aquatic life. Due to the low margin of safety in such
waters, effluent toxicity may cause ambient impacts and lead to exceedence of numeric or narrative water
quality criteria for aquatic life. Where such discharges cause or have the reasonable potential to cause or
contribute to excursions of such criteria, WET limits are required by 40 CFR 122.44(d)(l).

  Due to the inherent small margin of safety, only limited regulatory flexibility is available from chronic
WET limit requirements for discharges to waters where limited or no dilution is available. For low-flow
situations, the permit limit derivation procedures calculate 1.6 TUC as the maximum daily limit.

  Following is an example of a POTW discharging to a receiving water for which no assimilative
capacity is available (i.e., no dilution). The example shows the steps that a permitting authority would
take to establish a WQBEL for WET.

  General site description and information. This facility discharges up to 5.8 MGD. Based on the
available information, the ACR is 10. The CV, based on available data, is 0.6; the water quality criterion
for chronic toxicity is 1 .0 TUC and the acute criterion for acute toxicity is 0.3 TUa. The State water quality
standards allow an assumption of complete mixing.

  Determine waste load allocation (WLA). The WLA is used to determine the level of effluent
concentration that will comply with water quality standards in receiving waters.  Using the information
available for dilution, WLAs were calculated for WET using the complete mix equation:

              WLA (C,) = ([Cr(Qd+ Qs)] - [(Cs)(Qs)])/Qd

Because this is an effluent-dominated situation, and background concentration Cs is set to zero, the
equation simplifies to:
          = Cr[(Qd+Qs)/Qd]

    WLAa = 0.3 x 1 = 0.3 TUa

    WLAac - WLAa x ACR = 0.3 x 10 = 3.0 TUa,c

  Calculate Long-term Averages (LTAs). The process for calculating LTAs for toxicity is the same as
that for chemical-specific pollutants, except for the additional step of needing to express the WLA for
acute toxicity in equivalent chronic TUs.

    LTAac = WLAac x e[

    LTA  =3 x 0.321
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  where:

    0.321 is the acute WLA multiplier for CV = 0.6 at the 99th percentile (from Table 5-1, pg. 102 of the
    TSD)

    LTAac = 0.963 TUC

    LTAC = WLAC x e[°-5a42 - za4]

    LTAC = 1 x 0.527

  where:

    0.527 is the chronic WLA multiplier at the 99th percentile (from Table 5-1, pg. 102 of the TSD)

    LTAC = 0.527 TUC.

  Select the minimum LTA. The LTA based on the chronic WLA is more limiting and will be used to
develop permit limits.

  Calculate the maximum daily limit (MDL). The maximum daily limit is calculated as:

               MDL  = LTA x e[za - °-5cj2]

    where

         e[za-0.5a2]  =  MDL LTA multiplier

                   z  =  2.326 for the 99th percentile occurrence probability for the MDL is
                         recommended

               MDL  =  0.527 x 3.11 (from the LTA multiplier in Table 5-2, on pg. 102 of the TSD)

               MDL  =  1.6TU0.

        Calculate the average monthly limit (AML). Using the 95th percentile and monthly sampling,
the AML is calculated as:

               AML   =  LTA x e[zan - °-5an2]

    where:

        e [zan - 0.5 an2]  =  AML LTA miutipiier

                   z  =  1.645 for the 95th percentile occurrence probability for the AML is
                         recommended

                   n  =  number of samples/month (the TSD recommends that a minimum n of 4 be
                         used, even if monitoring is less frequent)


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               AML  =   0.527 x 1.55,




   where:




       1.55 is the LTA multiplier from Table 5-2 on pg. 103 of the TSD.




       AML = 0.8 TIL.
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          APPENDIX C
 1994 EPA WET CONTROL POLICY

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United States
Environmental Protection    Office of Water                  EPA 833-B-94-002
Agency              (4203)                            July 1994
Whole Effluent Toxicity
(WET) Control Policy
POLICY FOR THE DEVELOPMENT OF EFFLUENT LIMITATIONS IN NATIONAL
POLLUTANT DISCHARGE ELIMINATION SYSTEM PERMITS TO CONTROL WHOLE
EFFLUENT TOXICITY FOR THE PROTECTION OF AQUATIC LIFE

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                                                         July 1994

POLICY FOR THE DEVELOPMENT OF EFFLUENT LIMITATIONS  IN NATIONAL
POLLUTANT DISCHARGE ELIMINATION SYSTEM  (NPDES)  PERMITS TO CONTROL
WHOLE EFFLUENT TOXICITY  (WET) FOR THE PROTECTION  OF AQUATIC LIFE

INTRODUCTION

     EPA is today publishing a national policy  on the control of
WET in NPDES permits. This policy is intended  (i) to  promote
uniform, nationwide compliance with statutory and regulatory
requirements for the control of WET, and  (ii) to  assist permit
writers in implementing these requirements.

     This policy reflects EPA's experience  in implementing the
regulations at 40 C.F.R. § 122.44(d)(1), which  were originally
published at 54 Fed. Reg. 23868  (June 2, 1989)  (also  referred to
as "the water quality permitting regulations"). In  part,  this
policy restates those regulations and reaffirms EPA's strong,
continuing commitment to their prompt and complete
implementation. The water quality permitting regulations,  as well
as the statutory provisions restated in this policy document,  are
fully binding on EPA Regions as well as States  authorized to
administer the NPDES program.

     This policy also provides guidance to  permit writers on
implementation of the statutory and regulatory  requirements for
the control of WET.1 EPA permit writers are expected  to follow
the portions of this policy that provide such guidance,  although
decisions on individual permit provisions should  be made  on a
case-by-case basis, applying the law and regulations  to specific
facts and justifying decisions in the record for  the  permit.
Similarly, EPA Regions will consider this policy  in determining
whether State-issued NPDES permits adequately implement the
statutory and regulatory requirements for the control of  WET.

     This policy addresses some specific areas  where  questions
have arisen regarding the implementation of statutory and
regulatory requirements. It does not address all  areas where the
regulatory agency will exercise judgment in the implementation of
statutory and regulatory requirements. For  the  questions
addressed, it provides EPA's view as to the best  course of action
in most instances. This policy does not establish or  affect legal
rights or obligations and is not finally determinative of the
issues addressed. Most importantly, nothing in  this policy should
be interpreted as providing any relief from the statutory and
regulatory requirement that permits include conditions as
necessary to assure attainment of water quality standards.
1  A brief summary of existing Agency policy and guidance addressing WET issues
  is contained in Appendix One to today's policy.

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SUMMARY OF STATEMENTS  OF  POLICY

     Eight statements  of  policy appear below. Explanations of
each statement  follow.

1.    Basis for  WET  Controls

     The permitting authority should evaluate WET water quality
criteria attainment for acute WET at the edge of the acute mixing
zone and for chronic WET  at  the edge of the chronic mixing zone
except where the  State has different requirements for evaluating
WET criteria.2 The permitting  authority will  develop WET effluent
limitations based upon the more stringent of the acute or chronic
criterion applied at the  edge of the respective mixing zone, or,
alternatively,  on both.

2.    Evaluation of  Dischargers for Reasonable Potential

     At a minimum,  the permitting authority should review all
major dischargers for  reasonable potential to cause or contribute
to exceedance of  WET water quality criteria.

3.    Evaluating Reasonable Potential

     The permitting authority will consider available WET testing
data and other  information in evaluating whether a discharger has
reasonable potential to cause or contribute to exceedance of WET
water quality criteria.

4.    Consequences of Establishing Reasonable Potential

     Upon finding reasonable potential to cause or contribute to
exceedance of WET water quality criteria, the permitting
authority will  impose  effluent limitations to control WET.

5.    Whole Effluent Toxicitv Monitoring

     Where appropriate, the  permitting authority should impose
WET monitoring  conditions upon dischargers that do not have
effluent limitations to control WET.

6.    Compliance Schedules in NPDES Permits

     Where allowed  under  State and federal law, NPDES permits may
contain schedules for  compliance with WET effluent limitations.
2   Throughout this policy,  the term "WET water quality criteria" refers to State
   numeric  water quality criteria for  WET and State narrative water  quality
   criteria for toxicity such as  "no toxics in toxics amounts" in State water
   quality  standards.

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1.    Whole Effluent Toxicitv  Controls and the Pollutants Ammonia
and Chlorine

     The requirements  of  the  water quality permitting regulations
apply without regard to the pollutant(s)  that may be causing
toxicity, including ammonia and chlorine.

8.    Whole Effluent Toxicitv  Controls and Publicly Owned
Treatment Works  (POTWs)

     The requirements  of  the  water quality permitting regulations
apply to all dischargers,  including POTWs.
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EXPLANATION OF STATEMENTS OF POLICY3

1.    Basis for WET  Controls

     The  permitting authority  should evaluate  WET water  quality
     criteria attainment for acute WET at the  edge of the  acute
     mixing zone and for chronic  WET at the  edge of the  chronic
     mixing zone except where  the State has  different
     requirements for evaluating  WET criteria.  The permitting
     authority will develop WET effluent limitations based upon
     the  more stringent of the acute or chronic criterion  applied
     at  the edge of the respective mixing zone,  or,
     alternatively,  on both.

     This policy statement describes the procedure permitting
authorities should  use to evaluate WET water quality criteria
attainment and to develop effluent limitations  to control  WET.  In
the absence of more specific State requirements,  EPA believes
this approach most  appropriately  assures compliance with State
water  quality standards.4'5 The permitting authority must evaluate
WET water quality criteria attainment according to the applicable
state  water quality standard(s).  Permitting  authorities  should
assess WET concentrations as diluted in the  receiving water at
the edge  of the acute and chronic mixing zones  and apply the more
stringent of the acute criterion  at the edge of the acute  mixing
zone or  the chronic criterion  at  the edge of the chronic mixing
zone in  developing  WET effluent limitations. If there is
uncertainty as to which of the two criterion so applied  is more
stringent for the discharge, however, the permitting authority
will apply both.

     The  statement  reflects the specific requirement of  40 C.F.R.
§ 122.44(d)(1)(ii)  that "where appropriate,  [the permitting
3   To aid the reader in using this policy, Appendix Two contains some background
   materials on WET testing, the State water quality standards process and WET,
   and federal  statutory and regulatory requirements for development of water
   quality- based permit limitations  for WET.
4   State water  quality standards or implementation procedures may (1)  specify
   whether and how it is appropriate to account for dilution in establishing WET
   controls; (2) require the applicable criteria to  apply at the  outfall point
   of discharge  or may  contain  a specific  policy approved by EPA allowing or
   prohibiting mixing zones; as well as (3)  specify exposure factors  for WET
   which are relevant  to the  application  of this policy statement,  such as
   critical flow requirements  for  the receiving water,  appropriate modeling
   techniques for determining the fate of the pollutant or pollutant parameter
   in stream, or required techniques for evaluating the mixing of  the pollutant
   or pollutant parameter in the stream.
5   NPDES permitting authorities traditionally measure  compliance  with effluent
   limitations  at the  outfall  point of discharge.  By  issuing  this  policy
   statement, EPA does not  intend to disturb this well-established permitting
   practice. Permitting authorities  are familiar with procedures for determining
   the concentration of toxicity in-stream and establishing end-of-pipe effluent
   limitations on the  basis of the  information.

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authority  will consider]  the dilution of  the effluent  in  the
receiving  water" in determining whether a discharge causes,  has
the reasonable potential  to cause, or contributes to exceedance
of WET water quality criteria.  This statement should assist
permitting authorities  in establishing WET controls which meet
the requirements of sections 301(b)(1)(C)  and 402 of the  Clean
Water Act  (CWA)  and 40  C.F.R.  § 122.44(d)(1).

     40  C.F.R. § 122.44(d)(1)(iv) and  (v)  require the  permitting
authority  to impose effluent limitations  to control WET where it
determines that a discharge causes, has the reasonable potential
to cause,  or contributes  to exceedance of WET water quality
criteria.  40 C.F.R. § 122.44(d)(1)(vii) also requires  permitting
authorities to establish  effluent limitations on point sources
which are  consistent with the  requirements of applicable  State
water quality standards.  This  is a basic  premise of this  policy
statement.  Where the applicable State water quality standard or
implementation procedure  requires a different basis for
establishing WET controls,  the  permitting authority must  follow
applicable State requirements.6

     The second component of the policy statement also reflects
the principle of section  301(b)(1)(C) of  the CWA that  effluent
limitations must assure compliance with all State water quality
standards.  Here, the permitting authority will establish  WET
controls for the particular discharge based upon the more
stringent  of the acute  or chronic criterion (or both)  applied at
the edge of their respective mixing zones in order to  achieve
both criteria.

     Consistent with this policy statement,  the permitting
authority  will establish  two independent  zones for controlling
acute and  chronic WET.7 The  first zone,  the  acute mixing  zone,
immediately surrounds the discharge outfall. The acute mixing
zone is  normally sized  to prevent lethality (sometimes also
described  as "acute effects")  to passing  organisms. The permit
   For example, some State water quality standards  or implementation procedures
   do not allow mixing zones  at all or restrict  mixing zone use for certain
   dischargers. Where  this is  the case, the  permitting  authority will not use
   the procedure provided in policy statement one concerning the application of
   mixing zones.  The  permitting  authority must still  ensure that the permit
   includes  WET  limitations  as  necessary to  achieve  the applicable  State
   requirements .
7   This policy does not address what is acute or chronic WET. 40 C.F.R. § 122.2
   defines "whole effluent toxicity." Appendix Two, which provides an overview
   of the water quality standards process and WET, describes traditional acute
   and chronic  toxicity tests and EPA's recommended magnitudes for acute and
   chronic WET.   States  may  interpret narrative  water quality criteria for
   toxicity  in  State  implementation  procedures. In  the  absence  of such
   implementation procedures, EPA's recommended magnitudes for WET are  0.3 acute
   toxic unit and 1.0  chronic  toxic unit at the edge of  the appropriate mixing
   zone.  Technical Support Document for Water Quality Based Toxics Control, as
   revised in March 1991  (EPA/505/2-90-001),  at 2.3.3 and 2.3.4  (the TSD).

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must include  effluent limitations  as  necessary to meet numeric or
narrative water quality criteria for  acute toxicity at the  edge
of the acute  mixing zone. The second  zone,  the chronic mixing
zone, is typically a larger zone which surrounds the acute  mixing
zone. The chronic mixing zone is normally sized to protect  the
ecology of  the  water body as a whole  from all point-source
related stresses including WET. The permit must include effluent
limitations as  necessary to meet numeric or narrative water
quality criteria for chronic toxicity at the edge of the  chronic
mixing zone.8

     Once it  is determined what the appropriate mixing zones are,
the permitting  authority will take several additional steps
consistent  with this policy statement.  The permitting authority
will  (1) evaluate the receiving water concentration of acute WET
at the edge of  the acute mixing zone  and of chronic WET at  the
edge of the chronic mixing zone for the particular discharge,  (2)
determine which of the acute criterion or the chronic criterion
applied at  the  edge of the appropriate mixing zone is the more
stringent of  the two for the particular discharge, and  (3)
establish effluent limitations to  assure attainment of the  more
stringent criterion (or both where it is unclear which is more
stringent). The Technical Support  Document for Water Quality-
based Toxics  Control,  as revised in March 1991  (EPA/505/2-90-
001) (the TSD) at 3.3 and 5.4, illustrates how to apply this
procedure to  evaluate a particular discharge for reasonable
potential and to develop effluent  limitations.

2.   Evaluation of Dischargers for Reasonable Potential

     At a minimum,  the permitting  authority should review all
major dischargers for reasonable potential to cause or
contribute  to exceedance of WET water quality criteria.

     40 C.F.R.  §§ 122.44(d)(1)(iv) and (v)  require permitting
authorities to  impose effluent limitations to control WET
whenever a  discharge causes,  has the  reasonable potential to
cause, or contributes to an instream  excursion of applicable
water quality criteria.9 This policy  statement identifies which
   The  implementation  of  this  policy  requires  permitting  authorities  to
   establish mixing zones unless State standards or implementation procedures
   direct otherwise;  however,  the  specific  size of a particular mixing zone
   depends on  a  variety of  factors which can also be specified in the  State
   water quality standard or implementation procedure. See generally the  Water
   Quality Standards Handbook at 2-7  (1983); the TSD at 2.2.2,  for discussions
   of this issue.
9   Throughout this policy, any reference to "reasonable potential" includes both
   reasonable potential to cause and reasonable potential to contribute  to an
   excursion  of  numeric water  quality criteria for WET or narrative  water
   quality criteria.

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dischargers  the  permitting authority should,  as  a first priority,
assess for reasonable potential.10

     The group of  dischargers which the permitting authority
should evaluate  first for reasonable potential  are "major"
facilities.  EPA  defines a major POTW as a  POTW  having a design
flow of one  million gallons per day or greater,  a service
population of 10,000 or greater, or a significant impact on water
quality. EPA identifies a major industrial  discharger on the
basis of a combination of factors, including  size,  toxic
pollutant potential,  and stream flow volume.11 EPA believes that
these facilities (either POTWs or industrial  facilities)  have the
greatest opportunity for impacting surface  water quality and
therefore should be evaluated for "reasonable potential" to
exceed an applicable State water quality standard.

     Permitting  authorities should continue to  evaluate other
dischargers  of concern for reasonable potential  to exceed WET
water quality criteria. Factors which permitting authorities may
consider in  deciding whether a particular  discharge is "of
concern" obviously would include those factors  which are
described in Chapter 3 of the TSD as factors  for assessing
reasonable potential (including WET data,  chemical-specific data,
instream survey  data,  industry or publicly  owned treatment work
type, compliance history, receiving water  type,
designated/existing uses, and dilution calculations).  Under
§ 122.44(d)(1)(iv)  and (v), permitting authorities must impose
effluent limitations to control WET where  reasonable potential is
established. In  addition, the permitting authority should
consider WET controls,  where appropriate,  in  issuing general
permits.

3.    Evaluating  Reasonable Potential

     The permitting authority will consider available WET
testing data and other information in evaluating whether a
discharger has reasonable potential to cause  or  contribute to
exceedance of WET  water quality criteria.

     This policy statement describes what  information is
considered in evaluating whether a specific discharger has the
reasonable potential to cause or contribute to  excursion of WET
water quality criteria. The permitting authority first determines
10  This policy  statement continues to reflect EPA's  position  on  this matter
   articulated in the January 25, 1989, memorandum of Rebecca W. Hanmer, Acting
   Assistant Administrator for Water, to Regional Administrators  entitled "Whole
   Effluent Toxicity Basic Permitting Principles and Enforcement Strategy."
11  See the June 27, 1990, memorandum "New NPDES Non-Municipal Permit Rating
   System" from James R. Elder, Director of the Office of Water  Enforcement and
   Permits, to Regional Water Management Division Directors,  which is Appendix
   Three to today's policy.

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whether valid  WET testing data is available  that is relevant to
the particular discharge.12 Whole  effluent toxicity data may be
available  from previous monitoring. Additionally,  under 40 C.F.R.
§ 122.21(j), certain POTWs and required  to  submit WET testing as
part of the permit application. The permitting authority may also
decide to  require the permittee to generate  WET data prior to
permit issuance or as a condition of  the permit. See policy
statement  five below. If valid WET testing  data is available that
is relevant to the particular discharge,  the permitting authority
uses this  data to determine if the discharge exhibits reasonable
potential  under §§ 122.44(d) (1) (iv) or  (v).13 Where such WET data
exist and  demonstrate reasonable potential,  the permitting
authority  does not need to gather or  generate other information
to verify  or support the WET results. EPA believes it is
appropriate to assess reasonable potential  on the basis of WET
testing. Whole effluent toxicity testing is  comparable in
precision  to chemical analytical measurements in wide use. See
discussions of these questions in 55  Fed. Reg. 30082, 30112-30115
(July 24,  1990);  54 Fed. Reg.  23868,  23874  (June 2, 1989); the
TSD at 1.3 and 3.3.

     The permitting authority should  also consider whether other
factors establish reasonable potential for  the discharge. The TSD
at 3.2 offers  a discussion of factors other  than facility-
specific WET monitoring data which a  permitting authority may
consider in making a reasonable potential determination for a
particular discharge. These factors include  1) industry type
(primary,  secondary,  raw materials used,  products produced, best
management practices, control equipment,  treatment efficiencies,
etc.), 2)  publicly owned treatment work  type  (pretreatment,
industrial loadings,  number of taps,  unit processes,  treatment
efficiencies,  chlorination/ammonia problems,  etc.), 3)  compliance
history, 4) existing chemical data from  discharge monitoring
reports and applications, 5)  available instream survey data, 6)
receiving  water type and designated/existing uses, 7) available
dilution,  etc.  For each individual permit,  the permitting
authority  must include a clear explanation  in the statement of
basis or fact  sheet accompanying the  permit  of the specific
factors considered in evaluating reasonable  potential for the
particular discharge.
12  The permitting authority  determines whether available WET testing  is valid
   and addresses  concerns relative to toxicity for the particular discharge. For
   example,  where a facility discharges  to  a  low flow stream, submission of
   acute WET testing data showing no toxicity is insufficient (absent conversion
   of the acute results to chronic results using an acute-to-chronic ratio, as
   explained in the  TSD)  to  assess reasonable potential for chronic toxicity.
13  If additional factors also demonstrate reasonable potential  (see main text
   discussion below), the permitting authority should also discuss these factors
   in the fact sheet or statement of basis accompanying the permit.

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     EPA believes  this  approach to assessing  reasonable potential
implements the  requirements of sections 301(b)(1)(C)  and 402 of
the CWA and  40  C.F.R.  § 122.44(d)(1). 40 C.F.R.  §  122.44(d)(1)
(ii),  (iv),  and (v)  require the permitting authority to use valid
procedures which account for at least the following four factors
in establishing whether a discharge causes, has  the reasonable
potential to  cause,  or  contributes to an exceedance of WET water
quality criteria:  (1)  existing controls on point and nonpoint
sources of pollution,  (2)  the variability of  the pollutant or
pollutant parameter  in  the effluent,   (3) the  sensitivity of the
test species  when  evaluating WET,  and  (4) the  dilution of the
effluent in  the receiving water where appropriate.  40 C.F.R.
§ 122.44(d)(1)(v)  also  explicitly provides that  the permitting
authority must  establish an effluent limitation  to  control WET
where it determines,  using "toxicity testing  data,  or other
information," that the  discharge causes, has  the reasonable
potential to  cause,  or  contributes to an exceedance of a
narrative water quality criterion.

4.    Consequences  of Establishing Reasonable  Potential

     Upon finding  reasonable potential to cause  or  contribute to
     exceedance of WET  water quality criteria, the  permitting
     authority  will  impose effluent limitations  to  control WET.

     This policy statement reiterates the requirements of
sections 301(b)(1)(C)  and 402 of the CWA as well as 40 C.F.R.
§§ 122.44(d)(1)(iv)  and (v).  40 C.F.R. §§ 122.44(d)(1)(iv)  and
(v)  require  the permitting authority to establish  effluent
limitations  in  a permit to control WET where  it  determines that a
discharge has the  reasonable potential to cause  or  contribute to
an instream  excursion above a numeric criterion  for WET or a
narrative criterion.14

     The permitting  authority can either modify  the permit or
reissue the  permit upon expiration, as appropriate,  to
incorporate  effluent limitations to control WET. In no instance
will the permitting  authority reissue the permit without
including appropriate effluent limitations to  control WET.  In
appropriate  cases, the  permitting authority may  also require the
discharger to conduct a toxicity identification
evaluation/toxicity  reduction evaluation to identify and
eliminate the cause  of  the toxicity as part of a compliance
schedule to  comply with effluent limitations  to  control WET.
   Paragraph (v) provides that where the permitting authority determines that a
   discharge causes, has the reasonable potential to cause, or contributes to an
   instream excursion above a narrative water quality criterion,  the permit must
   contain (1)  a WET effluent limitation or (2) a pollutant-specific limitation,
   where the permitting authority demonstrates  that  a pollutant- specific
   limitation  is  sufficient to  attain and maintain applicable numeric  and
   narrative water quality standards.

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5.    Whole  Effluent Toxicitv Monitoring

     Where  appropriate, the permitting authority should  impose
     WET monitoring conditions upon  dischargers that do  not  have
     effluent  limitations to control WET.

     Where  appropriate, the permitting authority should  impose
WET monitoring conditions upon those dischargers for which  it did
not determine  reasonable potential and did not impose effluent
limitations to control WET. Where the permitting authority
concludes that a continued monitoring requirement is warranted
based upon  the particular circumstances of the discharger,  the
permitting  authority should require  WET monitoring for a
reasonable  period of time and evaluate the monitoring results at
the conclusion of this period.15

     EPA and authorized NPDES States have  broad authority under
the CWA to  require continued monitoring to assure attainment of
water quality  criteria. Under Sections 308 and 402 of the CWA EPA
or a State  with an authorized NPDES  program can require
permittees  to  provide WET testing data to  assure State water
quality standards will be attained and maintained. The permitting
authority can  impose a requirement on the  discharger to  collect
monitoring  data through conditions in the  NPDES permit or through
CWA Section 308 letters. Under sections 301(b)(1)(C) and 402 of
the CWA as  well as 40 C.F.R. §§  122.44(d)(1)(iv)  and  (v), EPA or
a State with an authorized NPDES program must impose effluent
limitations to control WET where continued monitoring results in
a determination of reasonable potential to exceed WET water
quality criteria.

6.    Compliance Schedules in NPDES Permits

     Where  allowed under State and federal law, NPDES permits may
     contain schedules for compliance with WET effluent
     limitations.

     This policy statement reflects  the principles for allowing
compliance  schedules in NPDES permits which were articulated in
In re Star-Kist Caribe, Inc., NPDES  Appeal No. 88-5  (May 26,
15  40 C.F.R.  §  122.21(j) requires many new and existing POTWs to collect WET
   data for submission  to the permitting  authority  at time of application or
   reapplication for an NPDES permit. Whereappropriate, the permitting authority
   may, in its  discretion, require  more frequent WET  monitoring of  POTWs or
   industrial dischargers.  For example,  it  may  be appropriate  to  impose  a
   continued  WET monitoring  obligation upon a major industrial discharger for
   which WET testing data is not available. Similarly, it may be appropriate to
   impose a monitoring obligation upon a discharger for which available WET data
   is  limited  or  for  which  later  information  raises  the  possibility  of
   reasonable potential.

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1992)  (order denying modification request).16 Section
301(b)(1)(C) of the CWA establishes a deadline of no later than
July 1,  1977, for compliance  with effluent limitations developed
to meet  State water quality standards.  In light of this CWA
provision, EPA has determined that all  permits must reflect this
deadline, unless the following requirements are met.17 NPDES
permits may contain schedules of compliance beyond July 1, 1977,
to meet water quality-based effluent limitations if two
requirements are met.  The  two requirements are: 1)  the permit
effluent limitation must be based either on a post-July 1, 1977
State water quality standard or a new or revised interpretation
of a pre-July 1, 1977  State water quality standard; and  (2) the
applicable State water quality standard or implementing
regulations must explicitly authorize schedules of compliance.

     40  C.F.R. § 122.47 also  governs compliance schedules in
NPDES permits. The regulation authorizes, where appropriate,
schedules requiring compliance with effluent limitations as soon
as possible and no later than the applicable CWA statutory
deadline. The regulation imposes certain restrictions on allowing
schedules of compliance for new sources, new dischargers, and
recommencing dischargers.  The regulation establishes requirements
for interim dates for  certain schedules of compliance and for
permittee reporting. Any compliance schedules developed for WET
limitations must also  satisfy § 122.47, if applicable.

     Thus, to decide whether  to allow a compliance schedule in an
NPDES permit for effluent  limitations to control WET, the
permitting authority must  answer these  questions:

     1.   Was the applicable  State water quality criterion
     promulgated or interpreted after July 1, 1977?

          At this time,  most  permitting authorities establish
     effluent limitations  to  control WET on the basis of State
     narrative water quality criteria.  Most  State narrative
     water quality criteria for toxicity were adopted before July
     1,  1977. Where this is the case, the permitting authority
     can only allow a  schedule of compliance in the NPDES permit
     where the State has made a new or  revised interpretation of
     the applicable narrative water quality criterion after July
     1,  1977. Where the permitting authority establishes an
     effluent limitation to control WET on the basis of a State
16  40  C.F.R.  § 122.2 defines a  "schedule of compliance" as  a "schedule  of
   remedial measures included in a ''permit' , including an  enforceable sequence
   of  interim  requirements...   leading  to   compliance  with  the  CWA  and
   regulations."
17  This entire  discussion  does  not apply to permit  limitations which  are
   governed by section 304(1)  of the CWA.

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     numeric water quality criterion for WET, it is more likely
     that the criterion is a post-July 1, 1977 criterion.

     2.Do the State water quality standards or implementing
     regulations explicitly authorize schedules of compliance?

          The State must include an explicit statement
     authorizing compliance schedules in the State water quality
     standard or implementing regulations. If the State water
     quality standard or implementing regulations are silent on
     whether schedules of compliance are authorized for NPDES
     permits, the permitting authority cannot place a schedule of
     compliance in the NPDES permit. Permit writers may find the
     express authorization in the State statute or water quality
     standards, water quality planning, or NPDES regulations.

     3.    Do other relevant provisions of State or federal law or
          policy allow the schedule of compliance?

          Here, for example, the permitting authority should
     consider whether allowing a schedule of compliance for the
     specific discharge meets the requirements of 40 C.F.R.
     § 122.47,  if applicable, or any other requirements of State
     law.

Where the permitting authority answers yes to each of these
questions, it may allow a schedule of compliance in the NPDES
permit.

     The permitting authority, however, is not compelled to
establish a schedule of compliance in the NPDES permit where so
authorized. The permitting authority should impose a schedule of
compliance only where appropriate under the specific conditions
of discharge. It has long been EPA's policy that EPA and
authorized NPDES States should require compliance with State
water quality standards as soon as possible in order to further
the goals of the CWA.

7.    Whole Effluent Toxicitv Controls and the Pollutants Ammonia
and Chlorine

     The requirements of the water quality permitting regulations
     apply without regard to the pollutant(s) that may be causing
     toxicity,  including ammonia and chlorine.

     This policy statement is designed to address several
questions which permitting authorities have encountered in
establishing WET controls where ammonia or chlorine is the
primary cause of toxicity. The questions typically arise on the
following set of facts: A permittee discharges to a stream for

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which the  State has not established  numeric water quality
criteria for  ammonia or chlorine;  the  State has adopted  a
narrative  water quality criterion  for  toxicity which is
applicable to the stream; the permittee conducts WET monitoring;
and the permittee exceeds the narrative criterion due to ammonia
or chlorine.  The permitting authority  must answer several
questions  in  permitting this discharge:  What effluent limitations
must it establish in the permit? Will  the permitting authority
require the permittee to control ammonia or chlorine, so that  the
permittee  does not exceed the narrative criterion due to ammonia
or chlorine?  Is there a basis to treat ammonia or chlorine
differently from other pollutants  in applying § 122.44  (d)  (1)  to
these facts?

     The requirements of § 122.44(d)(1)  apply to all pollutants
and pollutant parameters, including  ammonia and chlorine. The
Agency sees no basis upon which to treat WET due to ammonia  or
chlorine differently from WET due  to other pollutants in applying
the requirements of 40 C.F.R. § 122.44(d)(1)  or other provisions
of federal law.18 Several provisions  of 40 C.F.R. §  122.44 (d) (i)
provide the answers to the questions posed in this policy
statement.

     40 C.F.R. § 122.44(d)(1)(v) requires the permitting
authority  to  establish effluent limitations to control WET which
causes, has the reasonable potential to cause, or contributes  to
an excursion  above a State narrative water quality criterion.
Under § 122.44(d)(1)(v),  the permitting authority must establish
either a WET  effluent limitation designed to meet the narrative
criterion  or  a pollutant-specific  effluent limitation where  the
permitting authority is satisfied  it will assure compliance  with
applicable narrative and numeric criteria to control the WET due
to ammonia or chlorine. 40 C.F.R.  §  122.44(d)(1)(vi) allows  the
permitting authority to use one of three options for developing
effluent limitations for a pollutant determined to cause an
exceedance of a State narrative criterion where the State has
developed  no  numeric water quality criterion for the specific
pollutant.  These options are  (1) establish an effluent limitation
using a calculated numeric water quality criterion which will
attain and maintain the applicable narrative criteria and fully
protect the designated use;   (2) establish an effluent limitation
18  This policy  statement does not mean that  permitting authorities disregard
   technical  factors which are unique  to ammonia and chlorine in implementing
   regulatory requirements. For example,  holding pH constant in the laboratory
   during a WET test may be  necessary to assure a representative WET sample,
   where WET is due to ammonia. The permitting authority may also use judgment
   in interpreting testing results  and setting  limits  where temperature has a
   significant impact upon WET, which may be the case  for ammonia discharges
   during winter.  In addition, the permitting  authority may require WET testing
   prior to chlorination if a  facility is  under a schedule to dechlorinate. Once
   dechlorination  is implemented,  then WET testing should be  conducted on the
   final effluent.

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on a case-by-case basis using EPA criteria supplemented by other
relevant information where necessary; and  (3) establish an
effluent limitation for an indicator parameter provided certain
factors are established.

8.    Whole Effluent Toxicitv Controls and POTWs

     The requirements of the water quality permitting
regulations apply to all dischargers, including POTWs.

     This policy statement reaffirms EPA's longstanding policy of
treating all dischargers on an equal basis in imposing and
enforcing effluent limitations to control all pollutants and
pollutant parameters. Under sections 301 (b) (1) (C) and 402 of the
CWA as well as 40 C.F.R. § 122.44(d)(1), all dischargers must
meet effluent limitations designed to attain and maintain
applicable State water quality standards. Under the current NPDES
program, EPA exercises enforcement discretion where appropriate
for particular violations of effluent limitations designed to
meet State water quality standards, and provides technical
guidance and support to dischargers in seeking solutions to water
quality-permitting and compliance problems. See Attachment 2 to
the January 25, 1989, memorandum from Rebecca W. Hanmer, Acting
Assistant Administrator for Water, to EPA Regional Administrators
entitled "Whole Effluent Toxicity Basic permitting Principles and
Enforcement Strategy." EPA's Enforcement Management System for
the National Pollutant Discharge Elimination System  (1989)also
discusses how EPA exercises such enforcement discretion.

     Authorized NPDES States and EPA will work with
municipalities (as well as other permittees) to solve water
quality-related problems, including those resulting from WET. In
1991, EPA published the revised TSD, which provides comprehensive
technical guidance for assessing and regulating the discharge of
toxic substances to the waters of the United States. In 1989, the
Agency published a specific toxicity reduction manual for the
municipal discharger, the Toxicitv Reduction Evaluation Protocol
for Municipal Wastewater Treatment Plants  (EPA 600/2-88/062 April
1989). Since 1985, EPA's National Effluent Toxicity Assessment
Center in Duluth, Minnesota has provided assistance to regulatory
authorities in connection with dischargers that have complex
toxicity problems. EPA has published the Toxicitv Identification
Evaluation: Characterization of Chronically Toxic Effluents.
Phase I EPA-600/6-91/005F)(May 1992), a guidance document
designed to assist dischargers and their consultant laboratories
in conducting chronic aquatic toxicity identification
evaluations.

CONCLUSION
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     The fundamental premises of  today's  policy are not new.
Because of the importance of WET  controls,  however,  EPA is taking
this opportunity to reiterate key principles  associated with
implementation of existing statutory  and  regulatory requirements
for WET. The publication of this  policy is  designed to foster
consistent nationwide compliance  with existing statutory and
regulatory provisions for the control of  WET,  to facilitate the
development of appropriate controls for WET in NPDES permits, and
to help assure attainment of water quality  standards throughout
the nation.
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APPENDIX ONE

HISTORY OF FEDERAL REGULATION OF WHOLE EFFLUENT TOXICITY  (WET)

     Since 1984, the Environmental Protection Agency, the States,
and the regulated community have employed an integrated strategy
consisting of both biological and chemical means to control toxic
effects upon water quality beyond Clean Water Act  (CWA)
technology-based requirements in order to achieve  and maintain
State water quality standards. One method for measuring the
biological effects of toxic effluents upon aquatic life is WET
testing. EPA and the States have used the data derived from WET
testing to assess compliance with State water quality standards
and to establish National Pollutant Discharge Elimination System
(NPDES) permit effluent limitations necessary to attain and
maintain those standards.

     In the past ten years, the Agency has published regulations,
policy statements, and guidance documents which address a variety
of issues associated with WET controls in NPDES permits. In 1984,
EPA issued the "Policy for the Development of Water Quality-Based
Permit Limitations for Toxic Pollutants"  (published at 49 Fed.
Reg. 9016 (March 9, 1984)). This policy addresses  the use of
biological and chemical methods to assure that toxic whole
effluent discharges are regulated consistent with  federal and
State requirements. The document discusses such specific issues
as integration of chemical and biological approaches; chemical,
physical, and biological testing requirements  (WET requirements);
use of data; setting of effluent limitations; and monitoring.

     On January 25, 1989, Rebecca W. Hanmer, Acting Assistant
Administrator for Water, sent to EPA Regional Administrators the
memorandum entitled "Whole Effluent Toxicity Basic Permitting
Principles and Enforcement Strategy." A group of Regional and
State representatives developed the strategy, which discussed the
minimum acceptable national requirements for WET permitting.

     In 1989, the Agency revised existing 40 C.F.R.
§ 122.44(d)(1), which previously required NPDES permits to
contain any more stringent requirements necessary  to achieve
State water quality standards. See 54 Fed. Reg. 23868  (June 2,
1989). The revised regulation described in greater detail
requirements for NPDES permitting authorities to follow in
developing NPDES effluent limitations to assure compliance with
State water quality standards. On August 14, 1992, Michael B.
Cook,  Director, Office of Wastewater Enforcement and Compliance,
and Robert H. Wayland, III, Director, Office of Wetlands, Oceans
and Watersheds, transmitted a memorandum to the Water Management
Division Directors, Regions I-X, entitled "Clarifications
Regarding Certain Aspects of EPA's Surface Water Toxics Control

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Regulations."  (See Appendix Four.) In 1990,EPA also published new
40 C.F.R. § 122.21(j), which established a requirement  for
certain publicly owned treatment works  (POTWs) to provide the
results of valid WET testing with applications for NPDES permits.
See 55 Fed. Reg. 30082  (July 24, 1990). The preambles to these
regulatory revisions and the 1984 policy provide a detailed
explanation of legal and policy support for WET testing and
effluent limitations.

     Since 1984, the Agency has published various guidance
documents which address the subject of water  quality-based toxics
control, including WET control. See, in particular, the Technical
Support Document for Water Quality-based Toxics Control, which
was originally published in September and revised in March 1991
(EPA/505/2-90-001)   (the TSD). The revised TSD provides  an
explanation of the technical support for WET  testing and gives
detailed guidance on development of water quality-based permit
limitations for WET and toxic pollutants.

     On June 19, 1991, EPA issued the "Policy on the Use of
Biological Assessments and Criteria in the Water Quality
Program." In discussing integration of Various assessment
methods, the 1991 policy reflects a position  of "independent
application." In essence, "independent application" means that
appropriate regulatory action should be taken when any  one of
biosurvey, pollutant-specific, or WET testing methods indicates
that an applicable water quality standard is  not attained.
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APPENDIX TWO

BACKGROUND MATERIALS ON WHOLE EFFLUENT TOXICITY  (WET)TESTING. THE
STATE WATER QUALITY STANDARDS PROCESS AND WET FEDERAL STATUTORY
AND REGULATORY REQUIREMENTS FOR WET, AND WET REDUCTION GUIDANCE

Overview of WET Testing for
Aquatic Life Protection

     The WET approach to toxics control for the protection of
aquatic life involves the use of acute and chronic WET testing to
measure the toxicity of wastewaters. Whole effluent  toxicity
tests typically use standardized, surrogate freshwater or marine
plants, vertebrates, or invertebrates to measure the aggregate
toxic effect of an effluent. An acute WET test is typically a
test of 96-hours or less in duration in which lethality is the
measured endpoint. A chronic WET test is typically a longer-term
test in which sublethal effects, such as fertilization, growth,
and reproduction can be measured in addition to lethality. On
December 4, 1989, EPA published proposed Part 136 methods for
conducting short-term acute and chronic WET testing  for marine
and freshwater species. See 54 Fed. Reg. 50216. Once these
methods are final, they will constitute approved Part 136 test
methods for the NPDES program.

Overview of the State Water Quality Standards Process and WET

     Section 303 of the Clean Water Act (CWA) establishes the
statutory basis for the current State water quality  standards
program. Under this provision of the CWA,  States bear primary
responsibility for adopting water quality standards. State water
quality standards represent the means by which EPA and authorized
NPDES States control point source discharges when technology-
based controls for point source discharges are inadequate.

     A water quality standard defines the water quality of a
water body by designating the uses to be made of the water, by
setting criteria necessary to protect the uses, and  by
establishing antidegradation policies and implementation
procedures that serve to maintain and protect water  quality. See
section 303(c)  of the CWA and 40 C.F.R. Part 131. States adopt
water quality standards to protect public health or  welfare,
enhance the quality of the water, and serve the purposes of the
CWA. Among other requirements, State water quality criteria must
protect aquatic life.

     Under section 303 of the CWA and 40 C.F.R. Part 131, EPA
must approve State water quality standards or disapprove State
water quality standards and overpromulgate with federal water
quality standards. State water quality standards are effective

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until EPA overpromulgates with federal standards. Once
promulgated, the federal standards are the applicable water
quality standards for the State.

     State water quality criteria may be expressed as constituent
numeric concentrations of pollutants or pollutant parameters or
as narrative statements representing a quality of water that
supports a particular use. This is true of the pollutant
parameter WET. Several States have adopted numeric criteria for
WET. Most evaluation and control of WET at this time, however, is
based upon maintenance of the State's designated uses for the
water body through basic narrative water quality criteria for
toxicity. All States have narrative criteria for toxicity which
are statements of a desired water quality goal, such as "all
State waters must, at all times and flows, be free from
substances that are toxic to humans or aquatic life." EPA
considers narrative criteria to apply to all designated uses at
all flows unless specified otherwise in the State's water quality
standards. EPA regards narrative criteria for toxicity to cover
both short-term and long-term WET effects (acute and chronic
effects, respectively).

     Section 303(d)  of the CWA establishes requirements for
determining the specific pollutant reductions necessary to attain
water quality standards. Under section 303(d), the State  (or,
upon the failure of the State, EPA) must establish a Total
Maximum Daily Load  (TMDL) for water quality-limited waters. A
TMDL is an estimate of the total loading of a pollutant or
pollutant stressor that may be allowed within a receiving water
and an allocation of the total loading between the sources. See
40 C.F.R. Part 130.  A TMDL consists of Wasteload Allocations for
point sources, Load Allocations for other sources, and a Margin
of Safety to account for uncertainty in the relationship between
loadings and water quality. EPA has issued program guidance and
revised regulations pursuant to section 303(d)  (see Guidance for
Water Quality-based Decisions: The TMDL Process  (EPA 440/4-91-
001 April 1991); 57 Fed. Reg. 33040-33050 (July 24, 1992; and
April 13, 1992, memorandum from Geoffrey H.  Grubbs, Director,
Assessment and Watershed Protection Division, Office of Wetlands,
Oceans and Watersheds, entitled "Supplemental Guidance on Section
303 (d)  Implementation"). TMDLs may be developed for a single
source to support the issuance of an NPDES permit or for a larger
water body when multiple sources need to be considered together.

     State implementation procedures may further explain how the
State implements the applicable narrative criterion to establish
effluent limitations to control WET. Many effluent limitations to
control WET, however, result from the permitting authority's
case-by-case determination of what concentration of WET meets the
narrative criterion.

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     State WET criteria or the procedures implementing the State
criteria typically describe the magnitude, duration, and return
frequency for WET. The duration and frequency of the discharge
may also be defined by the design stream flow appropriate to the
criterion. Magnitude is the maximum allowable concentration of
WET, which is typically expressed as a concentration of toxicity
instream; duration is the period of time over which the instream
concentration is averaged for comparison with criteria
concentrations, in order to limit the durations of concentrations
above the criterion; and return frequency is a designation of how
often the criterion may be exceeded without impacting the
organisms in the water body. This information is needed because
ambient water quality typically varies in response to changes in
effluent quality, stream flow, and other factors. Accordingly,
organisms in the receiving water typically experience fluctuating
exposure to pollutants, including some periods of exposure to
high pollutant concentrations, which may have adverse effects.
For this reason, criteria indicate a time period over which
exposure is to be averaged, as well as a maximum concentration,
thereby limiting the duration of exposure to elevated
concentrations. In addition, to predict or ascertain the
attainment of criteria it is necessary to specify the allowable
frequency for exceeding the WET criteria. The permitting
authority uses the magnitude, duration, and return frequency
provisions of WET criteria to develop wasteload allocations and
effluent limitations to control the WET of the discharge.

     Whole effluent toxicity criteria as adopted by the State or
as implemented by the permitting authority on a case-by-case
basis typically consist of two expressions of magnitude of
toxicity to surrogate test species, one to protect against acute
toxicity effects and one to protect against chronic toxicity
effects, along with a duration and return frequency for each. As
of the publication date of this policy, EPA has not developed a
recommended section 304(a)  criterion for WET. The Technical
Support Document for Water Quality-based Toxics Control, which
was originally published in September 1985 and was revised in
March 1991 (EPA/505/2-90-001)  (the TSD), contains recommended
magnitudes, durations, and return frequencies for assessing acute
WET  (0.3 acute toxic unit to the most sensitive of at least three
species, with an averaging period of one hour and a once in
three-year return frequency) and chronic WET  (1.0 chronic toxic
unit to the most sensitive of at least three species, with an
averaging period of 4 days and a once in three-year return
frequency). See the TSD at 2.3.3 to 2.3.5.

     At their discretion, States may adopt certain policies for
implementation of water quality standards, such as critical low
flow and mixing zone policies. EPA has the authority to review
and approve or disapprove such policies. See 40 C.F.R. § 131.13.

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Additionally, EPA and States may establish a Technical Agreement
describing procedures that will be used in developing TMDLs and
wasteload allocations. TMDLS, wasteload allocations, and permit
limitations developed consistent with Technical Agreements are
subject to a lesser degree of EPA review. State water quality
standards protect water quality for designated uses in critical
low flow situations. Under § 131.13, States may designate
critical low flows below which numeric water quality criteria do
not apply. Mixing zones are small areas in the receiving water
near certain discharge outfalls where ambient concentrations
above the otherwise applicable State Water quality criteria are
allowed. See generally the Water Quality Standards Handbook at 2-
7  (1983); the TSD at 2.2.2. Some States prohibit mixing zones
entirely for all pollutants or pollutant parameters. Others may
allow mixing zones in general, but provide spatial dimensions to
limit the areal extent of the mixing zones. Permitting
authorities may allow mixing zones on a case-by-case basis for
individual discharges. EPA strongly recommends that States have a
definitive statement in their water quality standards on whether
or not mixing zones are allowed and clearly explain the
procedures for defining mixing zones where allowed.

Overview of Federal Statutory and Regulatory Requirements for
Development of Water Qualitv-based Permit Limitations for WET

     Section 101(a)  of the CWA establishes a national policy of
restoring and maintaining the chemical, physical, and biological
integrity of the Nation's waters. In addition, section 101(a)(3)
of the CWA states the national policy that the discharge of toxic
pollutants in toxic amounts is prohibited. Under sections 301 and
402 of the CWA, point source dischargers must obtain an NPDES
permit before discharging into waters of the United States. Under
sections 301(b)(1)(C) and 402 of the CWA, dischargers with NPDES
permits must meet all of the technology-based requirements of the
CWA as well as any more stringent requirements necessary to
achieve State water quality standards established under section
303 of the CWA.

     In 40 C.F.R.  § 122.2, EPA defines "whole effluent toxicity"
as "the aggregate toxic effect of an effluent measured directly
by a toxicity test." EPA has published 40 C.F.R. § 122.44(d)(1),
which describes requirements for NPDES permitting authorities to
follow in developing water quality-based effluent limitations,
including those necessary to control WET. The regulation consists
of seven subparagraphs:

     1.   Section 122.44(d)(1)(i)

          Limitations must control all pollutants or pollutant
          parameters  (either conventional, nonconventional, or

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          toxic pollutants) which the Director determines  are  or
          may be discharged at a level which will cause, have  the
          reasonable potential to cause, or contribute  to  an
          excursion above any State water quality standard,
          including State narrative criteria for water  quality.
     2.   Section 122.44(d)(1)(ii)

          When determining whether a discharge causes, has  the
          reasonable potential to cause, or contributes  to  an in-
          stream excursion above a narrative or numeric  criteria
          within a State water quality  standard, the permitting
          authority shall use procedures which account for
          existing controls on point and nonpoint  sources of
          pollution, the variability of the pollutant or
          pollutant parameter in the effluent, the  sensitivity of
          the species to toxicity testing  (when evaluating  whole
          effluent toxicity), and where appropriate, the dilution
          of the effluent in the receiving water."

     3.   Section 122.44(d)(1)(iii)

          When the permitting authority determines, using the
          procedures in paragraph  (d)(1)(ii) of this section,
          that a discharges causes, has the reasonable potential
          to cause, or contributes to an in-stream  excursion
          above the allowable ambient concentration of a State
          numeric criteria within a State water quality  standard
          for an individual pollutant,  the permit must contain
          effluent limits for that pollutant."

     4. Section 122.44(d)(1)(iv)

          When the permitting authority determines, using the
          procedures in paragraph  (d)(1)(ii) of this section,
          that a discharges causes, has the reasonable potential
          to cause, or contributes to an in-stream  excursion
          above the numeric criterion for whole effluent
          toxicity, the permit must contain effluent limits for
          whole effluent toxicity."

     5.   Section 122.44(d)(1)(v)

          Except as provided in this subparagraph,  when  the
          permitting authority determines, using the procedures
          in paragraph  (d)(1)(ii) of this section,  toxicity
          testing data,  or other information, that  a discharge
          causes, has the reasonable potential to  cause, or
          contributes to an in-stream excursion above a  narrative

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          criterion within an applicable State water quality
          standard, the permit must contain effluent limits for
          whole effluent toxicity. Limits on whole effluent
          toxicity are not necessary where the permitting
          authority demonstrates in the fact sheet or statement
          of basis of the NPDES permit, using the procedures in
          paragraph (d)(1)(ii) of this section, that chemical-
          specific limits for the effluent are sufficient to
          attain and maintain applicable numeric and narrative
          State water quality standards."

     6. Section 122.44(d)(1)(vi)

          "Where a State has not established a water quality
          criterion for a specific chemical pollutant that is
          present in an effluent at a concentration that causes,
          has the reasonable potential to cause, or contributes
          to an excursion above a narrative criterion within an
          applicable State water quality standard, the permitting
          authority must establish effluent limits using one, or
          more of the following options:

          (A) Establish effluent limits using a calculated
     numeric water quality criterion for the pollutant which the
     permitting authority demonstrates will attain and maintain
     applicable narrative water quality criteria and will fully
     protect the designated use. Such a criterion may be derived
     using a proposed State criterion, or an explicit State
     policy or regulation interpreting its narrative water
     quality criterion,  supplemented with other relevant
     information which may include: EPA's Water Quality Standards
     Handbook, October 1983,  risk assessment data, exposure data,
     information about the pollutant from the Food and Drug
     Administration, and current EPA criteria documents; or

     (B)   Establish effluent limits on a case-by-case basis,
     using EPA's water quality criteria, published under section
     304(a)  of the CWA,  supplemented where necessary by other
     relevant information;  or

     (C)   Establish effluent limitations on an indicator
     parameter for the pollutant of concern, provided:

          (1) The permit identifies which pollutants are intended
              to be controlled by the use of the effluent
              limitation;

          (2) The fact sheet  required by § 124.56 sets forth the
              basis for  the limit,  including a finding that
              compliance with the effluent limit on the indicator

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              parameter will result in controls on the pollutant
              of concern which are sufficient to attain and
              maintain applicable water quality standards;

           (3) The permit requires all effluent and ambient
              monitoring necessary to show that during the term
              of the permit the limit on the indicator parameter
              continues to attain and maintain applicable water
              quality standards; and

           (4) The permit contains a reopener clause allowing the
              permitting authority to modify or revoke and
              reissue the permit if the limits on the indicator
              parameter no longer attain and maintain applicable
              water quality standards."

     7.   Section 122.44(d)(1)(vii)

          "When developing water quality-based effluent limits
          under this paragraph  the permitting authority shall
          ensure that:

           (A) The level of water quality to be achieved by limits
     on point sources established under this paragraph is derived
     from, and complies with all applicable water quality
     standards; and

           (B)  Effluent limits  developed to protect a narrative
          water quality criterion, a numeric water quality
          criterion, or both, are consistent with the assumptions
          and requirements of any available wasteload allocation
          for the discharge prepared by the State and approved by
          EPA pursuant to 40 CFR 130.7."

Overview of Toxicitv Reduction  Guidance

     The purpose of a toxicity  reduction evaluation  (TRE),
including a toxicity identification evaluation  (TIE), is  to
investigate the causes and determine corrective actions for WET
problems. The permitting authority may require the permittee to
conduct these evaluations in specific cases. Section 5.8  of the
contains a detailed discussion  of EPA's recommended approach for
conducting TIES and TREs, including a list of guidance documents
EPA has developed describing methods and procedures for
conducting TIES and TREs.
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           APPENDIX D
          BIBLIOGRAPHY

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                                       BIBLIOGRAPHY

Cohen, A.C. 1991. Truncated and Censored Samples. New York: Marcel Dekker.

Hahn, G.J., and W.Q. Meeker. 1991. Statistical intervals: A Guide for Practitioners. New York: Wiley.

Helsel, D.R., and T.A. Cohn.  1988. Estimation of Descriptive Statistics for Multiply Censored Water
Quality Data. Water. Resour. Res. 24:1997-2004.

Hirsch, R.M., and J.R. Stedinger. 1987. Plotting Positions for Historical Floods and Their Precision. Wat.
Resour. Res. 23:715-727.

Kroll, C.N., and J.R. Stedinger. 1996. Estimation of Moments and Quantiles Using Censored Data. Wat.
Resour. Res. 32:1005-1012.

North Carolina Department of Environment and Natural Resources. 1998. North Carolina Biological
Laboratory Certification/Criteria Procedures Document.  Division of Water Quality, Water Quality
Section. Raleigh, NC.

Shumway, R.H., R.S. Azari, and M. Kayhanian. 2002. Statistical Approaches to Estimating Mean Water
Quality Concentrations with Detection Limits. Environ. Sci. Technol. 36:3345-3353.

Taylor, D.J., L.L. Kupper, S.M. Rappaport, and R.H. Lyles. 2001. A Mixture Model for Occupational
Exposure Mean Testing with a Limit of Detection. Biometrics 57:681-688.

USEPA. 1984. Ambient Water Quality Criteria for Chlorine -1984. Office of Water Regulations and
Standards, Criteria and Standards Division, Washington, D.C.

USEPA. 1985a. Methods for Measuring the Acute Toxicity of Effluents to Freshwater and Marine
Organisms (3rd Edition). W. Peltier and C.I. Weber, eds. Environmental Monitoring Systems Laboratory.
Cincinnati, OH. EPA/600/4-85/013.

USEPA. 1985b. Ambient Water Quality Criteria for Ammonia - 1985. Office of Water Regulations and
Standards, Criteria and Standards Division, Washington, D.C.

USEPA. 1987. Permit Writer's Guide to Water Quality-Based Permitting for Toxic Pollutants. Office of
Water Enforcement and Permits, Washington, D.C.

USEPA. 1988. Short-term Methods for Estimating the Chronic Toxicity of Effects of Receiving Water to
Marine/Estuarine Organisms (1st Edition). C.I. Weber, W.B. Horning, D.J. Klemm, T.W. Neiheisel, P.A.
Lewis, E.L. Robinson, J.R. Menkedick, F.A. Kessler, eds. Office of Research and Development.
Cincinnati, OH. EPA/600/4-87/028.

USEPA. 1988. Methods for Aquatic Toxicity Identification Evaluations: Phase I Toxicity
Characterization Procedures. EPA/600/3-88/034. National Effluent Toxicity Assessment Center, Duluth,
Minnesota.

USEPA. 1989a. Toxicity Reduction Evaluation Protocol for Municipal Wastewater Treatment Plants.
EPA-600-2-88-062. Office of Research and Development, Risk Reduction Engineering Laboratory,
Cincinnati, OH.


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USEPA. 1989b. Generalized Methodology for Conducting Industrial Toxi city Reduction Evaluations.
EPA-600-2-88-070. Office of Research and Development, Cincinnati, OH.

USEPA. 1989c. Methods for Aquatic Toxicity Identification Evaluations: Phase II, Toxicity Identification
Procedures. EPA-600-3-88-035. Office of Research and Development, Duluth, MN.

USEPA. 1989d. Methods for Aquatic Toxicity Identification Evaluations: Phase III, Toxicity
Confirmation Procedures. EPA-600-3-88-036. Office of Research and Development, Duluth, MN.

USEPA. 1989e. Short-term Methods for Estimating the Chronic Toxicity of Effluents and Receiving
Waters to Freshwater Organisms (2nd Edition). C.I. Weber, W.H. Peltier, T.J. Norberg-King, W.B.
Homing, II, F.A. Kessler, J.R. Menkedick, T.W. Neiheisel, P.A. Lewis, D.J. Klemm, Q.H. Pickering,
E.L. Robinson, J.M. Lazorchak, L. J. Wymer, R.W. Freyberg, eds. Office of Research and Development.
Cincinnati, OH. EPA/600/4-89/001.

USEPA. 1989f. The Enforcement Management System. National Pollutant Discharge Elimination System
(Clean Water Act). Office of Water.

USEPA. 1989g. Policy Memorandum - Whole Effluent Toxicity Basic Permitting Principles and
Enforcement Strategy. January 25, 1989. R.W. Hanmer (Office of Water) to Regional Administrators.

USEPA. 1989h. Whole Effluent Toxicity Basic Permitting Principles and Enforcement Strategy (January
25, 1989) Attachment Two: Compliance Monitoring and Enforcement Strategy for Toxics Control. Office
of Water, Washington, DC.

USEPA. 1990. Seminars-Wastewater Treatment Plant Toxicity Evaluation, Reduction and Control:
Presentations. Vol.1. CERI-89-243. Center for Environmental Research Information, Cincinnati, OH. 441
pp.

USEPA. 1991a. Technical Support Document for Water Quality-based Toxics Control. Office of Water.
Washington, DC. EPA/505/2-90-001.

USEPA. 1991b. Methods for Aquatic Toxicity Identification Evaluations: Phase I, Toxicity
Characterization Procedures. 2nd Edition. EPA-600-6-91-003. National Effluent Toxicity Assessment
Center, Duluth, MN.

USEPA. 1991c. Transmittal Memorandum: Final Policy on Biological Assessment and Criteria. June 19,
1991. T. Davies to Regions. Office of Science and Technology, Washington, DC.

USEPA. 1992a. Toxicity Identification Evaluation: Characterization of Chronically Toxic Effluents,
Phase I. EPA-600-6-91-005F. National Effluent Toxicity Assessment Center, Duluth, MN.

USEPA. 1992b. Policy Memorandum — Clarifications Regarding Certain Aspects of EPA 's Surface
Water Toxics Control Regulations. August 14, 1992. M.B. Cook and R.H. Wayland to Water
Management Division Directors, Regions I-X.

USEPA. 1993a. Methods for Aquatic Toxicity Identification Evaluations: Phase II Toxicity Identification
Procedures for Samples Exhibiting Acute and Chronic Toxicity. EPA-600-R-92-080. National Effluent
Toxicitv Assessment Center, Duluth, MN.
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USEPA. 1993b. Methods for Aquatic Toxicity Identification Evaluations: Phase III Toxicity Confirmation
Procedures for Samples Exhibiting Acute and Chronic Toxicity. EPA-600-R-92-081. National Effluent
Toxicity Assessment Center, Duluth, MN.

USEPA. \99^>c. Methods for Measuring the Acute Toxicity of Effluents and Receiving Waters to
Freshwater and Marine Organisms (4th Edition). C.I. Weber, ed. Office of Research and Development.
Cincinnati, OH. EPA/600/4-90/027F.

USEPA. 1994a. Short-term Methods for Estimating the Chronic Toxicity of Effluents and Receiving
Waters to Marine and Estuarine Organisms (2nd Edition). D.J. Klemm, G.E. Morrison, T.J. Norberg-
King, W.H. Peltier, and M.A. Heber, eds. Office of Research and Development. Cincinnati, OH.
EPA/600/4-91/003.

USEPA. 1994b. Short-term Methods for Estimating the Chronic Toxicity of Effluents and Receiving
Waters to Freshwater Organisms (3rd Edition). P.A. Lewis, D.J. Klemm, J.M. Lazorchak, T.J. Norberg-
King, W.H. Peltier, and M.A. Heber, eds. Office of Research and Development. Cincinnati, OH.
EPA/600/4-91/002.

USEPA. 1994c. Short-term Methods for Estimating the Sediment Toxicity of Effluents and Receiving
Waters to Freshwater and Estuarine Organisms. Office of Research and Development. Duluth, MN.
EPA/600/R-94-001.

USEPA. 1994d. Policy for the Development of Effluent Limitations in National Pollutant Discharge
Elimination System (NPDES) Permits to Control Whole Effluent Toxicity for the Protection of Aquatic
Life. EPA833-B-94-002. Office of Water, Washington, DC.

USEPA. 1994e. Methods for Determination of Metals in Environment Samples - Supplement I.EP A
600/R-94/111. Office of Research and Development, Washington, D.C.

USEPA. 1995a. Short-term Methods for Estimating the Chronic Toxicity of Effluents and Receiving
Waters to West Coast Marine and Estuarine Organisms. G.A. Chapman, D.L. Denton, and J.M.
Lazorchak, eds. Office of Research and Development. Cincinnati, OH. EPA/600/R-95-136.

USEPA. 1995b. Policy Memorandum — National Policy Regarding Whole Effluent Toxic Enforcement.
August 14, 1995. R. Van Heuvelen and M. Cook to Water Management Division Directors, Regions I-X;
Regional Counsels, Regions I-X; and State NPDES Directors. Office of Wastewater Management,
Washington, DC.

USEPA. 1995c. Whole Effluent Toxicity: Guidelines Establishing Test Procedures for the Analysis of
Pollutants (FR Vol. 60 Number 199, pages 53529-53544), 10/16/95. Office of Science and Technology,
BAD.

USEPA. 1996a. NPDES Permit Writer's Manual. Office of Water. Washington, DC. EPA/833/B-96-003.

USEPA 1996b. Guide to Method Flexibility and Approval of EPA Water Methods. EPA/82 l/D-96-004.

USEPA. 1996c. Marine Toxicity Identification Evaluation (TIE): Phase I Guidance Document.
EPA/600/R-96-054. National Health and Environmental Effects Laboratory, Narragansett, RI.
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USEPA. 1996d. Policy Memorandum — Clarifications Regarding Flexibility in 40 CFR Part 136 Whole
Effluent Toxicity (WET) Test Methods, April 10, 1996. T. Davies to: Water Management Division
Directions, Regions I-X and Environmental Services Division Directors, Regions I-X. Office of Science
and Technology, Washington, DC.

USEPA. 1996e. Regions 9 and 10 Guidance for Implementing Whole Effluent Toxicity Testing Programs.
U. S. Environmental Protection Agency, Regions 9 Water Division, San Francisco, CA and Region 10
Office of Water, Seattle, WA.

USEPA. 1997. Policy Memorandum — Clarifications Regarding Whole Effluent Toxicity Test Methods
Recently Published at 40 CFR Part 136 and Guidance on Implementation of Whole Effluent Toxicity in
Permits. July 21, 1997. T.T. Davies and M.B. Cook to: Water Management Division Directors, Regions I-
X and Environmental Services Division Directors, Regions I-X. Office of Science and Technology and
Office of Wastewater Management, Washington, DC.

USEPA. 1999b. Toxicity Reduction Evaluation Guidance for Municipal Wastewater Treatment Plants
(2nd Edition). Office of Water. Washington, DC. EPA/833/B-99-002.

USEPA. 1999c. Errata for Effluent and Receiving Water Toxicity Test Manuals: Acute Toxicity of
Effluents and Receiving Waters to Freshwater and Marine Organisms; Short-Term Methods for
Estimating the Chronic Toxicity of Effluents and Receiving Waters to Freshwater Organisms; and Short-
Term Methods for Estimating the Chronic Toxicity of Effluents and Receiving Waters to Marine and
Estuarine Organisms. EPA-600-R-98-182. Office of Research and Development, Duluth, MN.

USEPA. 2000a. Methods for Measuring the Toxicity and Bioaccumulation of Sediment-Associated
Contaminants with Freshwater Invertebrates (2nd Edition). Office of Research and Development. Duluth,
MN. EPA/600/R-99/064.

USEPA. 2000b. Understanding and Accounting for Method Variability in Whole Effluent Toxicity
Applications Under the National Pollutant Discharge Elimination System Program. Eds. Denton, D.L.,
Fox, J., Fulk, F.A., Greenwald, K., Narvaez, M., Norberg-King, T.J., Phillips, L.  EPA/833/R-00-003.
Office of Water, Washington, DC.

USEPA. 2000c. Method Guidance and Recommendations for Whole Effluent Toxicity (WET) Testing (40
CFR Part 136). Office of Water, Office of Science and Technology. Washington, DC. EPA/821/B-00-
004.

USEPA. 2001a. Policy Memorandum: Clarifications Regarding Toxicity Reduction and Identification
Evaluations in the National Pollutant Discharge Elimination System Program, 3/27/01. OWM,
Washington, D.C.

USEPA 2001b. Final Report: Interlaboratory Variability Study of EPA Short-term Chronic and acute
Whole Effluent Toxicity Test Methods, Vol. 1. EPA 821-B-01-004. Office of Science and Technology.

USEPA 200 Ic. Final Report: Interlaboratory Variability Study of EPA Short-term Chronic and Acute
Whole Effluent Toxicity Test Methods, Vol. 2: Appendix. EPA 821-B-01-004. Office of Science and
Technology.

USEPA. 2002a. Methods for Measuring the Acute Toxicity of Effluents and Receiving Waters to
Freshwater and Marine Organisms, 5th edition. EPA 821/R-02/012. Office of Science and Technology.
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USEPA. 2002b. Short-term Methods for Estimating the Chronic Toxicity of Effluents and Receiving
Waters to Freshwater Organisms, 4th edition. EPA 821/R-02-13. Office of Science and Technology.

USEPA. 2002c. Short-term Methods for Estimating the Chronic Toxicity of Effluents and Receiving
Waters to Marine and Estuarine Organisms, 3rd edition. EPA 821/R-02-14). Office of Science and
Technology.
                                         WEB SITES

http ://www. epa. gov/waterscience/

   Main EPA site for information on WET analytical methods. Includes updated reference manuals and
   the text of the 2002 WET methods promulgated rule.

http://cfpub2.epa.gov/npdes/

   Main EPA site for the National Pollution Discharge Elimination System (NPDES) program. Includes
   a link to search for publications.

http://www.epa.gov/clariton/index.htm

   National Environmental Publications Information System, providing many manuals as .pdf files.

http ://www.epa. gov/nscep/

   National Service Center for Environmental Publications, reader may request a hard copy of any
   manual cited in this document.

http://www.gpoaccess.gov/nara/index.html

   Office of the Federal Register, providing online access to CFR and FR.
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