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NPDES Topics Alphabetical Index Glossary About NPDES

Whole Effluent Toxicity

Whole Effluent Toxicity (WET) is a term used to describe 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). WET tests replicate the total effect and actual 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 of the water quality standards implementation through the NPDES permitting process and supports meeting the goals of the Clean Water Act (Section 402), "..maintain the chemical, physical and biological integrity of the nation's waters."

Click here to view WET Guidance Documents.

WET Requirements

Picture of a Fathead Minnow EPA's promulgated WET test methods include two basic types of WET tests: an acute test and a chronic test. EPA has developed WET test protocols using both freshwater and marine and estuarine test species. EPA recommends running tests using an invertebrate, vertebrate and a plant to identify the most sensitive species for developing NPDES WET permit limits or testing requirements. Organisms used in WET tests (e.g., Ceriodaphnia dubia (freshwater flea) and Pimephales promelas (fathead minnow)) are indicators or surrogates for the aquatic community to be protected, and a measure of the real biological impact from exposure to the toxic pollutants. To protect water quality, EPA recommends that WET tests be used in NPDES permits together with requirements based on chemical-specific water quality criteria.

WET tests are designed to predict the impact and toxicity of effluents discharges from point sources into waters of the U.S. WET limits developed by permitting authorities are included in NPDES permits to ensure that the state or tribal water quality criteria for aquatic life protection (WET) are met. WET monitoring requirements that are representative of the discharge effluent (40 CFR Part 122.44(d)(1)(ii)) are included in NPDES permits to generate WET data used to determine whether reasonable potential for WET has been demonstrated. If reasonable potntial has been demonstrated then a WET limit must be included in the permit (40 CFR Part 122.44(d)(1)(iv) and (v)). WET test results are also used in determining compliance with NPDES WET permit limits.

Key References

WET Training Tools:

EPA Whole Effluent Toxicity (WET) DVDs (freshwater and saltwater series) - EPA has produced training DVDs on EPA WET test methods and culturing of test organisms and a supplemental training guide to use with the DVDs for each WET test method listed below. These DVD WET materials were produced to be used as an important training tool for EPA Regions, states and private laboratories to use as part of their NPDES WET program implementation. These WET training DVDs provide an overview of EPA's WET test methods promulgated at 40 CFR Part 136. The training DVDs were developed so that permit writers, their permittees and commercial laboratories can have a better understanding of the EPA WET test methods and how to properly conduct them. They are excellent for training new staff and for NPDES WET training courses. However, these training materials are not intended to replace a thorough understanding and reading of the EPA WET test method manuals.

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Last updated on September 24, 2013 9:04 AM