Whole Effluent Toxicity (WET) Testing



Image of spotted fish swimming by rocks, from Massimo Lorenzoni.
Courtesy of: Massimo Lorenzoni.
Università di Perugia (University of Perugia). Italy.
(http://www.bio.unipg.it/staff/massimo.html) . Further use requires his permission.


Whole effluent toxicity (WET) is the toxicity of an effluent (i.e. wastewater) sample measured directly with a toxicity test in order to assess the total toxic effect of all pollutants. WET testing is the only method for assessing the toxic interaction of all pollutants in a wastewater discharge. The approach is called “whole effluent toxicity” to contrast it with measuring the individual concentrations of the multiple toxic chemicals in an effluent for a one-by-one comparison to a water quality criterion if there happens to be one. Because WET testing often discovers effects caused by unknown toxicants, the toxic effects cannot be reduced until the toxicants involved have been identified.

Monitoring effluents for toxicity is a necessary activity. We need a regulatory strategy for detecting, identifying, and eliminating toxic substances or combinations of substances in effluents that would otherwise be missed. Effluents thoroughly characterized chemically and considered safe can still be toxic due to unknown constituents. Low flows will eventually occur, and even if control of effluent toxicity has been adequate for the last few years, such controls must anticipate dry weather that will occur on average only once per decade or so. WET testing is done to discover effluent toxicity at levels of concern for future low flow events and to identify unknown toxicants. WET testing is not performed to match receiving water conditions at the time of discharge. WET tests are conducted under standard conditions set to optimize test organism performance and reliability.

Authority for WET Testing

Point of Compliance for WET

WET Test Review and Reporting


Rainbow trout (Oncorhynchus mykiss) © 1999 Aaron Nadig


Wet Testing Documents

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The Canary Book and instructions for finding an accredited lab

The Whole Effluent Toxicity Program Evaluation

Ambient Monitoring Reports

Herring Reports

Useful EPA Links


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Last updated January 2017





Quick Links

Submitting Results Online (WQWebPortal)

EPA Vessel General Permit




WET Testing Program Coordinator

Randall Marshall
Department of Ecology
Water Quality Program
Box 47600
Olympia, WA 98504-7600
E-Mail: randall.marshall@ecy.wa.gov

Randall is also the state's contact for the EPA Vessel General Permit.