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National Pollutant Discharge Elimination System (NPDES)

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.

WET Guidance

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