The Use of Dispersants in Marine Oil Spill Response (2020) / Chapter Skim
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SUMMARY
SUMMARY
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In addition, each spill may be governed by policy guidelines, such as those set forth in the National Response Plan, Regional Response Plans, or Area Contingency Plans. To respond effectively to the specific conditions presented during an oil spill, spill responders have used a variety of response options, including mechanical recovery of oil using skimmers and booms, in situ burning of oil, monitored natural attenuation of oil,1 and dispersion of oil by chemical dispersants.
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After human safety, the next priority is development of a response strategy that most effectively reduces environmental consequences, offers the greatest protection, or promotes the fastest recovery. Determining whether the use of dispersants is appropriate for a given oil spill scenario requires decision-making tools for assessing the relative benefits of the various response options.
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It then covers the human health considerations that are critical concerns for decision makers. Based on this information, the report discusses the trade-offs associated with dispersant use versus other response options under various spill conditions and explains how these trade-offs are weighed using the NEBA approaches described above.
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As the plume rises, gas bubbles and soluble oil components dissolve into the entrained seawater, decreasing the buoyancy of the plume. A lateral intrusion layer forms, enriched in hydrocarbons, where the dissolved components and microdroplets4 encounter currents and the ambient density stratification of the water column (see Figure S.1)
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. Experiments can test how well models perform at different scales and can examine the effects of various oil types, proportions of methane, dispersant formulations, and DORs.
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However, the action of dispersants in a surface spill increases the amount of oil in the water column, both as dissolved oil constituents and as small droplets, where fish and other species may be exposed through absorption or ingestion. Concerns over the substantial use of dispersants during the DWH spill triggered an expansion of research on the toxicity of oil, dispersed oil, and dispersants.
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As underscored in the previous NRC reports, the concern with dispersant use is whether dispersed oil is more toxic than untreated oil is, not the toxicity of current dispersant formulations. Dispersed Oil Toxicity To determine the relative toxicity of dispersed oil, many laboratory studies have compared solutions of oil equilibrated with seawater to oil and dispersant mixtures equilibrated with seawater.
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However, without correction for the actual dissolved oil concentrations, a direct comparison of WAF and CEWAF toxicity will not produce meaningful results. Recommendation: Funding agencies, research consortia, and other sponsoring groups should require that research teams use standardized toxicity testing methods, such as those developed by the Chemical Response to Oil Spills: Ecological Effects Research Forum (CROSERF)
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Recommendation: The use of toxic units should be integrated into revised oil toxicity testing standards, evaluation criteria for models, and response option risk analysis. This represents a paradigm shift away from developing toxicity tests that attempt to reproduce field exposure conditions and toward developing a consistent means of using toxicity metrics such as HC5 and LC50 for toxicity models used with fate and transport models to compare the exposure and toxicity of various response options, including dispersants.
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Epidemiological Studies Two studies of DWH spill responders have attempted to disentangle the direct effects of dispersants from other worker health risks. While these studies noted similar adverse effects associated with dispersant exposures, both have limitations in their ability to validate exposure to dispersants based on self-reporting by workers.
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The primary response options considered in this report include surface dispersant operations, subsea dispersant injection, at-sea mechanical recovery, controlled (in situ) burning, biostimulation, and monitored natural attenuation.
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As with other response options, there are potential limitations and trade-offs associated with subsea dispersant injection. Like surface application, subsea dispersant injection requires special approvals, is subject to regulatory requirements, and requires specialized equipment and expertise.
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Each response method has a complex suite of advantages and disadvantages, including and not limited to encounter rate, effectiveness, and ecosystem and human health effects that should be considered when developing and executing oil spill response plans. These complex trade-offs are best addressed using NEBA tools such as CERA, SIMA, and CRA.
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