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National Pollutant Discharge Elimination System (NPDES)

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 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.

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


  • 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.


  • 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.
  • 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.
  • Draft National Whole Effluent Toxicity (WET) Implementation Guidance under the NPDES Program (12/28/2004) -- This draft document gathers together previous EPA guidance, policy, and other information and organizes it in one comprehensive document. The document also addresses a number of important issues that have been raised by states, regulated industry, and interested stakeholders on the implementation of WET limits in NPDES permits.
  • 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 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.

Case Studies

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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.

If you would like to order WET test method video training packages (electronic copies of supplemental guides and the DVDs), please contact the National Service Center for Environmental Publications (NSCEP) at (800) 490-9198 or Make sure to include the appropriate WET publication number in your request.

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

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