 | Aquatic Consulting & Testing, Inc |
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Whole Effluent Toxicity
Whole Effluent Toxicity (WET) refers to the toxic effect on aquatic organisms caused by pollutants contained in a facility's wastewater (effluent). WET tests measure the effect a wastewater discharge has on the survival, growth, and/or reproduction of specific test organisms. The tests are often part of a facility’s State or Federal wastewater discharge permit (AZPDES or NPDES). WET test methods consist of exposing living aquatic organisms (plants, vertebrates and invertebrates) to various concentrations of a sample of wastewater, usually from a facility's effluent stream. Laboratory-prepared water or upstream receiving water is used as a control.The most common test orgamisms are early life-stages of the inveretbrate water flea, Ceriodaphnia dubia, the vertebrate fathead minnow (Pimephales promelas), and the aquatic plant, Raphidocelis subcapitata. Survival and reproduction (number of offspring produced) are measured each day, while growth (gain in weight) is measured at the end of the exposure period. Physical and chemical properties of the effluent and test concentrations are measured throughout the test period. The duration of the test can be one to four days (acute tests) or one or more weeks (chronic tests). Composite effluent samples are supplied to the laboratory on one or more days during the testing period. The additional samples are used to produce fresh mixtures of the various concentrations of waterwater on a daily basis. To determine if the effluent meets permit limits, the survival, growth, and reproduction data are statistically analyzed with comparison to the response of the test organisms exposed to the control water. The most common end point (or permit limit) is the TU (toxicity unit) value. The value is computed as 100/NOEC; where the NOEC is the highest effluent concentration at which no significant adverse affect was detected. |