Permit Limits-Whole Effluent Toxicity (WET)

Whole Effluent Toxicity (WET) describes the aggregate toxic effect of an aqueous sample (e.g., whole effluent wastewater discharge) as measured by an organism's response upon exposure to the sample (e.g., lethality, impaired growth, or reproduction). EPA’s WET tests replicate the total effect of environmental exposure of aquatic life to toxic pollutants in an effluent without requiring the identification of the specific pollutants.

WET testing is a vital component to implementing water quality standards under the NPDES permits program in accordance with the CWA Section 402. It supports meeting the goals of the CWA Section 101(a) and (2), especially with respect to restoring and maintaining "the chemical, physical, and biological integrity of the Nation’s waters and “…the protection and propagation of fish, shellfish, and…”

• WET Requirements
• WET Guidance
• WET Training Tools

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WET Requirements

WET tests are used for water quality assessment, permit limit development, and compliance assessment. To protect water quality, EPA recommends using WET tests in NPDES permits together with requirements based on chemical-specific water quality criteria.

WET limits are included in permits to ensure that the state or tribal water quality criteria for aquatic life protection are met. WET monitoring requirements are included in permits to generate valid WET data used to determine whether reasonable potential for WET has been demonstrated (e.g., toxicity that would result in an excursion of state or tribal WQS). If reasonable potential has been demonstrated, then a WET limit must be included in the permit. WET test results are used in determining compliance with NPDES WET permit limits.

• Conditions applicable to all permits (40 CFR Part 122.41(j)(1))
• Establishing limitations, standards, and other permit conditions (40 CFR Part 122.44(d)(1)(ii),(iv), and (v))

WET test methods include two basic types of WET tests, acute and chronic (including sublethal endpoints). WET test methods include procedures for freshwater, marine, and estuarine test species. EPA recommends running tests using an invertebrate, vertebrate, and a plant to identify the most sensitive species for use with the NPDES permits program. Ceriodaphnia dubia (freshwater flea) and Pimephales promelas (fathead minnow)) are examples of EPA approved indicators or surrogates used in the protection of freshwater aquatic community organisms.

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WET Guidance

Methodology

• Clarifications Regarding Toxicity Reduction and Identification Evaluations in the National Pollutant Discharge Elimination System Program -- The document supplements existing EPA guidance and policies on WET and TREs/TIEs but does not change current policy or legal authority.
  • Clarification Document
  • Transmittal 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 -- Supplemental memorandum to "Clarifications Regarding Flexibility in 40 CFR Part 136 WET Test Methods," guidance provided in the Technical Support Document for Water Quality-Based Toxics Control, WET Control Policy .

• Errata for the Effluent and Receiving Water Toxicity Testing Manuals

• Methods for Aquatic Toxicity Identification Evaluations: Phase I Toxicity Characterization Procedures Second Edition -- Aid in conducting aquatic organism Toxicity Identification Evaluations. Part one of a three part set.

• Methods for Aquatic Toxicity Identification Evaluations: Phase II Toxicity Identification Procedures for Samples Exhibiting Acute and Chronic Toxicity -- Technical guidance on how to identify the cause of whole effluent toxicity. Part two of a three part set.

• Methods for Aquatic Toxicity Identification Evaluations: Phase III Toxicity Confirmation Procedures for Samples Exhibiting Acute and Chronic Toxicity - Technical guidance on how to identify the cause of whole effluent toxicity. Part three of a three part set.

• Toxicity Identification Evaluation: Characterization of Chronically Toxic Effluents, Phase I -- Methods to characterize the chemical/physical nature of the constituents in effluents which cause their chronic toxicity.

Programmatic

• Whole Effluent Toxicity (WET) NPDES Spreadsheet -- This spreadsheet can be used to analyze valid acute or chronic WET test data using statistical approaches. The results generated by the spreadsheet can be used by NPDES permit writers for reasonable potential (RP) determinations in accordance with EPA’s TSD (see pages 53-57, Chapter 5) and for NPDES WET compliance determinations (see TSD pages 112-113, Chapter 6). NPDES permittees and WET testing laboratories may also find the spreadsheet helpful when analyzing valid WET test data.
  • Whole Effluent Toxicity (WET) Analysis Spreadsheet (Excel) (xlsm)
  • Disclaimer: Please note that neither the EPA nor any of its employees assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information disclosed. Furthermore, the NPDES WET Spreadsheet is supplied “as-is” without guarantee or warranty, expressed or implied, including without limitation, any warranty of merchantability or fitness for a specific purpose.
  • Factsheet on the Whole Effluent Toxicity (WET) Analysis Spreadsheet

• Clarifications Regarding Flexibility in 40 CFR Part 136 Whole Effluent Toxicity (WET)

• Clarifications Regarding Toxicity Reduction and Identification Evaluations in the National Pollutant Discharge Elimination System Program (03/27/2001) -- Provides additional clarification on conducting toxicity reduction evaluations and toxicity identification evaluations with respect to WET testing. The document supplements existing EPA guidance and policies on WET and TREs/TIEs, but does not change current policy or legal authority.
  • Clarification Document
  • Transmittal Memorandum
     
• EPA Whole Effluent Toxicity (WET) Control Policy (07/01/1994) -- Policy for the development of effluent limitations in NPDES permits to control WET for the protection of aquatic life.
   
• Generalized Methodology for Conducting Industrial Toxicity Reduction Evaluations (TREs) (April 1989) -- This document presents a generalized methodology for designing and conducting a TRE and 10 supporting case studies which illustrate various approaches that have been used in the performance of TREs to date.
   
• Memorandum: Certification of "Accuracy" of Information Submissions of Test Results Measuring Whole Effluent Toxicity (03/03/2000) -- Clarifies that EPA intends that a certification of "accuracy" occurs in information submissions of testing data for WET, but as the layperson uses the term, not "accurate" as the term is used to describe the quantifiable performance of a measurement system.

• Method Guidance and Recommendations for Whole Effluent Toxicity (WET) Testing (07/01/2000) -- This document provides guidance and recommendations on the conduct of the approved WET test methods and interpretation of WET test results reported under the NPDES program. This guidance partially fulfills the obligations of a legal settlement agreement that resolves a judicial challenge to the WET final rule. The document provides guidance on the following issues: nominal error rate adjustments, confidence intervals, concentration-response relationships, dilution series, and dilution waters.

• National Policy Regarding Whole Effluent Toxicity (WET) Enforcement (08/14/1995) -- Memorandum clarifying national enforcement policy related to single exceedances of WET limits and response when toxicity reduction evaluations are inconclusive.

• National Pollutant Discharge Elimination System Test of Significant Toxicity Implementation Document (06/01/2010) -- The transmittal memo and implementation document provides the basis for implementing the Test of Significant Toxicity (TST) approach under NPDES for permitting authorities (states and regions) and persons interested in analyzing WET test data using the traditional hypothesis testing approach as part of the NPDES Program.
  • Transmittal Memorandum
  • Implementation Document

• National Pollutant Discharge Elimination System Test of Significant Toxicity Technical Document (06/01/2010) -- This document provides the technical basis for the TST approach under NPDES for permitting authorities (states and regions) and persons interested in analyzing valid WET test data using the traditional hypothesis testing approach as part of the NPDES Program.
   
• National Pollutant Discharge Elimination System (NPDES) Whole Effluent Toxicity (WET) Permit Writers’ Manual (July 2024, EPA 833-B-24-001) -- This document provides the NPDES permit writer with information and resources that support writing NPDES permit WET requirements. This manual was designed for the NPDES permit writer, whether new to or experienced with the NPDES program, or anyone else interested in learning about the regulatory, programmatic, and technical aspects of developing and implementing WET provisions in an NPDES permit. It summarizes recommendations and requirements for developing NPDES WET permit requirements based on EPA guidance, policy, and regulations, as well as the Clean Water Act (CWA).
  • NPDES WET Permit Writers' Manual (pdf) (3.22 MB)
  • NPDES WET Permit Writers' Manual - Fact Sheet (pdf) (256.14 KB)
     
• Toxicity Reduction Evaluation Guidance for Municipal Wastewater Treatment Plants (August 1999) -- Provides a general framework for conducting TREs at publicly owned treatment works (POTWs) and describes the available methods and procedures. 
   
• Toxicity Reduction Evaluation Protocol for Municipal Wastewater Treatment Plants (April 1989) -- Provides a systematic framework for conducting a toxicity reduction evaluation (tre) and a description of the available methods and procedures.
   
• Understanding and Accounting for Method Variability in Whole Effluent Toxicity Applications Under the NPDES Program (06/01/2000) -- This document was developed to address questions raised by stakeholders on the issue of analytical variability that might be associated with WET testing and how it relates to the NPDES permit program.
   
• Whole Effluent Toxicity (WET) Control Policy (07/01/1994) -- This policy is intended to promote nationwide compliance with statutory and regulatory requirements for the control of WET.

Case Studies

• Effluent and Ambient Toxicity Testing and Instream Community Response on the Ottawa River, Lima, Ohio -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system
• Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact on Five Mile Creek, Birmingham, Alabama -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
• Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Back River, Baltimore Harbor, Maryland -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
• Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Kanawha River, Charleston, West Virginia -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
• Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Naugatuck River, Waterbury, Connecticut -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
• Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Ohio River, Near Wheeling, West Virginia -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
• Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Scippo Creek, Circleville, Ohio -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
• Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Skeleton Creek, Enic, Oklahoma -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.

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WET Training Tools

EPA WET DVDs (freshwater and saltwater series) – EPA developed these DVDs as an important training tool for regional and state NPDES permit writers and laboratories to use as part of their program implementation. These training materials do not replace a thorough understanding and reading of the EPA WET test method manuals.

• EPA Freshwater WET Test Method Video Factsheet
• EPA Saltwater WET Test Method Video Factsheet

If you would like to receive a hard copy of EPA’s Whole Effluent Toxicity (WET) test method video training packages (electronic copies of supplemental guides and the DVDs), please email Laura Phillips (phillips.laura@epa.gov) or Jackie Clark (clark.jackie@epa.gov), Water Permits Division/Office of Wastewater Management.

To watch the WET freshwater and saltwater series, visit the NPDES Trainings page