Consensus Report

Oil Spill Dispersants: Efficacy and Effects (2005)


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Although significant steps have been taken over the last 15 years to reduce the size and frequency of oil spills, the sheer volume of petroleum consumed in the United States and the complex production and distribution network required to meet the demand make spills of oil and other petroleum products inevitable. Oil dispersants (chemical agents such as surfactants, solvents, and other compounds) are used to reduce the effect of oil spills by changing the chemical and physical properties of the oil. By enhancing the amount of oil that physically mixes into the water, dispersants can reduce the potential that a surface slick will contaminate shoreline habitats. This report reviews the adequacy of existing information and ongoing research regarding the effectiveness of dispersants as an oil spill response technique, as well as the effect of dispersed oil on marine and coastal ecosystems, and includes recommended steps to be taken to better support policymakers faced with making hard choices regarding the use of dispersants as part of spill contingency planning efforts or during actual spills.

Key Messages

  • Better information is needed to determine the window of opportunity and percent effectiveness of dispersant application for different oil types and environmental conditions.
  • Data from field studies (both with and without dispersants) are needed to validate models and provide real-world data to improve knowledge of oil fate and effects.
  • Oil trajectory and fate models used by the technical support staff advising on-scene decisionmakers for dispersed oil behavior are not adequate in terms of: (1) their representation of the natural physical process involved, (2) verification of the codes, and (3) validation of the output from these models in an experimental setting or during an actual spill.
  • One of the most significant weaknesses in correlating laboratory-scale and meso-scale experiments with conditions in the open ocean results from a lack of understanding of the turbulence regime in all three systems.
  • Research funds in the United States to support oil spill response options in general are extremely limited and declining.
  • Serious consideration should be given to determining the value and potential role of field testing. The body of work done to date has provided important, but still limited understanding of many aspects of the efficacy of dispersants in the field and the behavior and toxicity of dispersed oil.
  • The factors controlling rates of the biological and physical processes that determine the ultimate fate of dispersed oil are poorly understood. Of particular concern is the fate of dispersed oil in areas with high suspended solids and areas of low flushing rates.
  • The mechanisms of both acute and sublethal toxicity from exposure to dispersed oil are not sufficiently understood.
  • There are many important, unanswered questions about how dispersed oil might be consumed by plankton and deposited on the seafloor with fecal matter or otherwise passed through the food chain.
  • There is insufficient information to determine how chemically dispersed oil interacts with suspended sediments, both short- and long-term, compared to naturally dispersed oil.
  • There is insufficient understanding of the actual concentrations and temporal or spatial distributions and behavior of chemically dispersed oil from field settings (from either controlled experiments or actual spills).
  • To date, there have been no wave-tank or laboratory studies that can be used reliably to predict the performance of dispersants on water-in-oil emulsions (i.e., mousse) generated from the weathering of oil on the water surface.
  • Two general types of modeling efforts and products should be recognized: (1) output intended to support decisionmaking during preplanning efforts, and (2) output intended to support emergency response to provide rough-cut outputs in hours.
  • Their ability to predict the concentrations of dispersed oil and dissolved petroleum hydrocarbons of concern in the water column with sufficient accuracy to aid in real-time spill decisionmaking has yet to be fully determined.