Quantified Exposures from Potential Deepwater Releases for Comparative Risk Assessment of Oil Spill Response Options Including Dispersant Use

Abstract

ABSTRACT The goal of oil spill response is to mitigate the overall impacts of spilled oil on ecological and socioeconomic resources. Surface and subsea dispersant applications are effective tools that remain controversial after decades of research and discussion. The tradeoff that dispersants potentially increase effects on water column and benthic communities while reducing floating and nearshore/shoreline oil exposure is recognized, but inevitably are qualitatively considered when subjectivity and stakeholder interests prevail. To be objective and transparent, we developed a quantitative approach using oil spill modeling to evaluate response alternatives in a Comparative Risk Assessment (CRA) framework where the fractions of resources potentially exposed are compared, along with their recovery potential. The model quantifies exposure as water surface area, shoreline area and water volume exposed above thresholds of concern, multiplied by duration of exposure, in each environmental compartment. These exposure metrics (i.e., area-days or volume-days) are multiplied by relative densities across the environmental compartments to evaluate the fractions of the resources exposed in each modeled scenario. The fractions of resources exposed, along with their recovery potential, inform decisionmakers using a Spill Impact Mitigation Assessment (SIMA) approach with quantitative estimates of potential consequences, which they may consider along with stakeholder values. Previously, we evaluated a deepwater blowout in the Gulf of Mexico, assuming no intervention or various response options (mechanical recovery, in-situ burning, surface dispersant application, and subsea dispersant injection [SSDI]). The findings were that inclusion of SSDI reduced human and wildlife exposure to volatile organic compounds; 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 at depth. However, since organisms are less abundant at depth, overall exposure of valued ecosystem components was minimized by use of SSDI. Follow-up modeling shows the benefits of SSDI are due to reduction of the oil droplet sizes released to the water column. Droplet sizes are sensitive to oil and gas release rates, release depth, orifice size and dispersant-to-oil ratio. The exposure metrics resulting from a matrix of scenarios varying these inputs and response actions are expected to be generally representative of the fate and behavior of oil and gas blowouts in the offshore areas of the Gulf of Mexico, as well as other regions with similar oceanographic conditions.