The Deepwater Horizon blowout resulted in the release of millions of barrels of crude oil. As part of the Trustees' Natural Resource Damage Assessment, a testing program was implemented to evaluate the toxicity of Deepwater Horizon oil and oil/dispersant mixtures to aquatic organisms from the Gulf of Mexico. Because of the variety of exposures that likely occurred, the program included 4 Deepwater Horizon oils, which encompassed a range of weathering states, and 3 different oil-in-water mixing methods, for a total of 12 unique water accommodated fractions (WAFs). The present study reports on the chemical characteristics of these 4 Deepwater Horizon oils and 12 WAFs. In addition, to better understand exposure chemistry, an examination was conducted of the effects of WAF preparation parameters-including mixing energy, starting oil composition, and oil-to-water mixing ratios-on the chemical profiles and final concentrations of these 12 WAFs. The results showed that the more weathered the starting oil, the lower the concentrations of the oil constituents in the WAF, with a shift in composition to the less soluble compounds. In addition, higher mixing energies increased the presence of insoluble oil constituents. Finally, at low to mid oil-to-water mixing ratios, the concentration and composition of the WAFs changed with changing mixing ratios; this change was not observed at higher mixing ratios (i.e., >1 g oil/L). Ultimately, the present study provides a basic characterization of the oils and WAFs used in the testing program, which helps to support interpretation of the more than 500 Deepwater Horizon Natural Resource Damage Assessment toxicity testing results and to enable a comparison of these results with different tests and with the field. Environ Toxicol Chem 2017;36:1450-1459. © 2016 SETAC.

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Abstract-Behavioral avoidance of copper (Cu), cobalt (Co), and a Cu and Co mixture in soft water differed greatly between rainbow trout (Oncorhynchus mykiss) and chinook salmon (O. tshawytscha). Chinook salmon avoided at least 0.7 g Cu/L, 24 g Co/L, and the mixture of 1.0 g Cu/L and 0.9 g Co/L, whereas rainbow trout avoided at least 1.6 g Cu/L, 180 g Co/L, and the mixture of 2.6 g Cu/L and 2.4 g Co/L. Chinook salmon were also more sensitive to the toxic effects of Cu in that they failed to avoid Ն44 g Cu/L, whereas rainbow trout failed to avoid Ն180 g Cu/L. Furthermore, following acclimation to 2 g Cu/L, rainbow trout avoided 4 g Cu/L and preferred clean water, but chinook salmon failed to avoid any Cu concentrations and did not prefer clean water. The failure to avoid high concentrations of metals by both species suggests that the sensory mechanism responsible for avoidance responses was impaired. Exposure to Cu concentrations that were not avoided could result in lethality from prolonged Cu exposure or in impairment of sensory-dependent behaviors that are essential for survival and reproduction.

We cause of the relationship between water hardness and the toxicity of many metals, total hardness is used as a model parameter to calculate ambient water quality criteria for copper and other metals. However, the relative contribution of the Ca and Mg components of total hardness as modifiers of metals toxicity is not considered in the water quality criteria. Acute Cu toxicity was measured in rainbow trout (Oncorhynchus mykiss) and chinook salmon (O. tshawytscha) swim‐up fry in laboratory waters that were formulated to have similar total hardness and alkalinity but different Ca and Mg concentrations. Experiments were performed at nominal total hardness values of 40 and 90 mg/L (as CaCO3). In four paired toxicity tests, acute Cu toxicity was significantly lower, i.e., 96‐h LC50s were higher, in laboratory waters containing proportionately more Ca (Ca:Mg molar ratios of 1.5–5.2) than in waters containing less Ca (Ca:Mg molar ratios of 0.2–0.8). The relative increase in the 96‐h Cu LC50 at higher Ca concentrations, but similar total hardness concentrations, was between 28 and 86% when the low Ca treatment was similar to American Society for Testing and Materials laboratory water (Ca:Mg molar ratio = 0.7). Failure to account for differences in Ca when matching or adjusting for total hardness thus exerts an important influence on the prediction of metal toxicity. These differences must be addressed in water‐effect ratio testing in which paired tests with laboratory and site waters are conducted.

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