Science Advisory Report 2021/051

State of Knowledge on Chemical Dispersants for Canadian Marine Oil Spills

Summary

• Direct exposure to oil concentrated at the water surface or on shorelines and intertidal areas is typically very harmful to organisms and oil that reaches shorelines may persist for months to years. The use of dispersants is an important response option to mitigate the effects of an oil spill, including in cold climates and for treatment in ice-infested waters, particularly when there are limited viable removal options.
• Dispersion of oil into the water column is a natural process. Dispersants enhance the formation of smaller oil droplets, relative to natural processes, that remain in the water column and spread vertically and horizontally beneath the surface. This promotes the dissolution, dilution, and biodegradation of the oil over a larger volume of water.
• The dilution of smaller oil droplets following the effective use of dispersants reduces the potential for droplet collisions, thereby minimizing their coalescence and the reformation of surface slicks.
• Dispersants reduce the exposure to oil for organisms at the water surface and on shorelines and intertidal areas. Their use results in the temporary and localized increase in exposure to chemically dispersed oil for organisms (which may include species at risk) in the water column. Increased exposure to the benthic environment is possible.
• Typically, chemically dispersed oil has similar aquatic toxicity as oil alone (for commonly studied species); however, the duration and intensity of exposure to dispersed oil in the subsurface is mitigated by dilution, which quickly reduces oil concentration.
• Cold water species have similar sensitivities as temperate species to the acute toxicity of untreated and chemically dispersed oil.
• Lab-based toxicity tests provide critical information but are limited in representing the complexity of open water conditions. The effects of both untreated and chemically dispersed oil on marine biota are highly variable and are informed not only by toxicity but also by the interactions of physical, chemical, and biological factors. Modelling is useful for considering all parameters to predict potential effects to individuals, populations, and ecosystems.
• Monitoring requirements are site-, incident-, and context-specific. Operational monitoring is used to assess the effectiveness of dispersant application and measure against criteria to stop the application. Environmental monitoring is used to assess impacts and recovery from the spill event.

This Science Advisory Report is from the March 1 to 12, 2021 National Advisory Meeting on the State of Knowledge on Chemical Dispersants for Canadian Marine Oil Spills. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2021/051 (PDF, 676 KB)

Accessibility Notice

This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.