Assessing the toxicity of crude oil and spill treating agent mixtures to aquatic species

Spill treating agents (STAs) such as surface washing agents (SWA), surface collecting agents (or chemical herders) and solidifiers are designed to alter the behavior of crude oil by enhancing its removal from affected surfaces, aggregating it for easier recovery, or by transforming it into a more manageable solid form. They may be authorized for use under the United States National oil and Hazardous Substances Pollution Contingency Plan to support oil spill mitigation efforts. By the nature of their role in spill response, direct application to oil or around oil slicks, STA yields various mixtures of oil components and STA chemicals. This study evaluated the acute toxicity of mixtures of Alaska North Slope (ANS) crude oil and six STAs (three SWAs, two herders, and one solidifier) using standard freshwater and marine test species. A combination of traditional (frequentist) and Bayesian statistical approaches were used to analyze dose-response data. Results showed that SWAs significantly increased oil toxicity by enhancing the bioavailability of toxic hydrocarbons, where oil and agent mixtures were more toxic than oil alone in approximately 80% of expected cases for the four aquatic species. Meanwhile, herders and solidifiers generally caused minimal changes to oil toxicity. Bayesian credible intervals provided more nuanced differentiation between oil and STA-oil mixtures, revealing the potential for increased ecological risk from certain STA-oil combinations that may be indistinguishable using conventional methods. These findings support a more comprehensive understanding of the toxicity of oil spills treated with STAs and expands the knowledgebase for agents with only limited toxicity data.