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Comparative Risk Assessments of Dispersant and Other Oil Spill Response Options Quantifies Oil Exposure Tradeoffs Weighed by Relative Abundance and Recovery Potential of Affected Biota

SessionCase Studies - Dispersants

DescriptionThe usefulness of subsea dispersant injection (SSDI) for reducing environmental impacts of a deepwater oil and gas release continues to be debated. Oil spill modeling has been used to evaluate tradeoffs of SSDI and other response measures when responding to a spill. For example, oil exposure assessments of the Deepwater Horizon (DWH) oil spill were modeled as it occurred and with various response alternatives included or not in the simulations. A quantitative Comparative Risk Assessment (CRA) methodology was used to weigh the relative consequences of differing response application scenarios. Inclusion of SSDI reduced human and wildlife exposure to volatile organic compounds (VOCs); dispersed oil into a large water volume at depth; enhanced biodegradation; and reduced surface water, nearshore and shoreline exposure to ﬂoating oil and entrained/dissolved oil in the upper water column. Tradeoﬀs included increased exposures of benthic and water column biota in deep offshore waters. The CRA method provided a quantitative comparison of the relative exposures of Valued Ecological Components (VECs) in the Gulf of Mexico ecosystem. Exposure areas and volumes of VECs are weighed by exposure duration, relative abundance across environmental compartments (ECs) occupied, and recovery potential. Since shoreline habitats and wildlife (marine mammals, sea turtles and birds) are vulnerable to surfaced oil and relatively slow to recover, and organisms were much less abundant at depth, overall exposure of VECs was minimized by use of SSDI.
Application of the CRA methodology to other deep water oil spill scenarios in the Gulf of Mexico and elsewhere resulted in similar conclusions. The reductions in exposures of wildlife, surface water biota, and nearshore habitats are most sensitive to the oil droplet size distributions of untreated and dispersant-treated oil. Droplet size distributions vary considerably with oil and gas release rates and the concentration of dispersant in the SSDI application. The degree to which deep water VECs are affected are also subject to spill location, current patterns at depth, and presence of vulnerable deep water habitats downstream of a spill. In the CRA framework, the fractions of resources exposed, along with their recovery potential, inform decisionmakers using a Spill Impact Mitigation Assessment (SIMA) approach with objective and transparent quantitative estimates of potential consequences of spills assuming various response alternatives, which they may consider along with stakeholder values.

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