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"WET" stands for Whole Effluent Toxicity. WET is defined as the aggregate toxic effects of an effluent measured directly with a toxicity test (ie. a Bioassay) as specified by the Agency. Agency approved WET tests involve standardized methods using freshwater and estuarine vertebrates, invertebrates, and aquatic plants to directly measure acute or short-term chronic effects of effluents monitored under NPDES permits. Rationale for WET Tests WET tests are designed to replicate as closely as possible the total effect that an effluent has on aquatic life, without requiring identification of actual toxicants that may be present in the effluent. The aquatic organisms used in WET tests are considered to be good indicators of the overall biological effects of the effluent being tested. For this reason, the EPA considers WET tests to be a vital component of the NPDES permitting process. Two Basic Types of WET Tests There are two basic types of WET tests: (1) Acute tests and (2) Chronic tests. The EPA has developed protocols for acute and chronic WET tests, using specific vertebrate, invertebrate, and plant species. Whether the test is an acute test or a chronic test, WET tests typically involve testing the effluent over a series of effluent concentrations (usually 5 different concentrations) with at least 4 replicates for each concentration. This is termed a "multi-concentration" (or "definitive") test. Acute vs. Chronic WET tests Acute tests are short-term tests, usually lasting for 96 hours or less (often only 24 hours or 48 hours). Acute tests consider mortality occurring during the testing period in the effluent, as compared with any mortality in the controls. The purpose of an acute test is to estimate the degree to which an effluent is lethally toxic to the test organisms. The end-point for an acute test is estimation of the LC50 (concentration of the effluent in which 50% of the test organisms die within a specified period of time). Chronic tests usually last for 7 days, and consider growth or reproduction (as well as mortality). The objective of chronic aquatic toxicity tests is to estimate the highest concentration of the effluent which can be considered "safe" (ie. has no observable deleterious effects on the test organisms). For practical reasons, the responses observed are usually limited to growth or reproduction. Chronic tests are useful in that they yield information concerning sub-lethal toxic effects as well as lethal effects on aquatic organisms. The results of chronic tests are usually expressed in terms of the highest concentration of the effluent that has no statistically significant observed effects when compared with the controls. This concentration is termed the NOEC (No Observed Effect Concentration). Chronic tests last longer, and are more complicated than acute tests. For this reason, chronic tests are considerably more expensive than acute tests.
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