Comparison of Test Methods and Early Toxicity Characterization for Five Dispersants

The acute toxicities of a commercial dispersant (Corexit 9527) and four experimental dispersant formulations were evaluated using the 96-h mysid (Mysidopsis bahia) test and two rapid screening tests, Microtox and the Mysid IQ Toxicity Test. During 96-h toxicity tests, survival observations were recorded at 3, 6, 9, 12, and 24 h to document mortalities from short-term exposures more consistent with field exposure times and more approximate to exposure times used in Microtox and the Mysid IQ Toxicity Test. At nominal concentrations (6.25 and 12.5 mg/liter) and exposure times (3–24 h) near the upper range of predicted field conditions, mysid mortalities were ≤5% for all test materials. Microtox and Mysid IQ Toxicity Test were evaluated for their ability to differentiate test materials compared with that of the 96-h mysid test. Dispersant formulations were ranked by relative toxicities based on LC50or EC50values and ranks compared among test methods. Microtox ranked the test materials similar to the 96-h mysid test. Ranks from the Mysid IQ Toxicity Test were dissimilar to those of the other tests. Early mortality observations during 96-h tests did not provide a better basis for comparing results of the rapid screening tests.