Chronic tests estimate the concentration of effluent that interferes with the growth, development, and reproductive potential of aquatic organisms. Acute tests last 48 to 96 hours. Chronic tests predict the concentrations that interfere with normal growth, development and reproductive potential of these aquatic organisms. During a chronic test, several life stages of the organism are continuously exposed to the test material at various concentrations. Tests last 4-8 days and responses such as growth, reproduction and survival are measured.
Tests encompass the entire life cycle or most sensitive life stage of the organisms.
The IC 25 is a statistical calculation used to estimate effects on growth or reproduction of an effluent in chronic toxicity tests. If the IC 25 is lower than the IWC, the effluent has the potential to inhibit aquatic organisms in the receiving water. In a TIE, effluent samples are manipulated to remove suspect chemicals and then re-tested to see if toxicity remains. If a specific effluent manipulation removes toxicity, then the researcher has a clue about the source of toxicity.