Frequently Asked Questions

How do dispersants work?

Dispersants work by enhancing the natural ability of petroleum degrading microorganisms to remove oil from the environment and they do this by breaking the oil into tiny droplets that rapidly dilute in the water column.This dilution allows aerobic biodegradation to proceed without exhausting nutrients or available oxygen.  In addition, the droplets of dispersed oil increase the surface area for microbial attack by several orders of magnitude compared to a surface slick, thereby speeding the natural biodegradation process.

How quickly does dilution happen?

Dilution in the open sea is so rapid that dispersed oil concentrations fall below acute toxicity thresholds of most water-column organisms in a matter of minutes to a few hours.

When are dispersants used?

Dispersants can be used in most marine offshore conditions encountered.Any decision to use dispersants involves a balanced judgment that dispersant use will reduce the overall impact of a particular spill, compared to not using dispersants. This requires a balancing of the advantages and disadvantages of dispersant use and a comparison with the consequences of other available response methods.Decision-makers must evaluate the trade-offs and challenges associated with the use of dispersants and make science-based decisions on the likely effects of dispersed oil on the resources and the Arctic habitats. A Net Environmental Benefit Analysis (NEBA) based approach should be employed to address the issues associated with oil remaining on the surface or dispersed into the water column.

Are chemical dispersants toxic?

Dispersants themselves are of low toxicity to marine life and are less toxic than the oil that is dispersed; concentrations start low and are rapidly diluted.There have been more than 30 years of research on dispersant efficiency and their potential impact on various ecosystems, revealing that the main toxicity effect of dispersed oil droplets comes from the oil itself and not from the dispersant.

Do we actually know about the environmental effects of dispersant use?

A significant body of scientific research and testing has been conducted over several decades, on the techniques and technologies available for responding to oil spills in icy conditions, including the Arctic. These hundreds of studies, allow us to make a decision on “environmentally safe” ways to use dispersants.

Is it true that dispersants just make the oil sink to the bottom and contaminate the sea floor?

Dispersants don’t add any weight to the oil, so dispersed oil droplets are still lighter than water and if they are small enough, they will remain suspended in the water column. So, dispersants DO NOT sink oil.Oil dispersed at the water surface tend to stay in the top 10 – 20 feet of the water column so it is highly unlikely to impact creatures living on the sea bottom.

Are dispersants effective in ice and freezing conditions?

There is a significant amount of research showing that dispersants are effective when applied at freezing and near-freezing temperatures and in cold waters.In fact, dispersants will, under certain conditions, be more effective in the presence of ice than in open water. There are several reasons for this.  One is that in a broken ice field the differential motion of the ice pieces actually increases the surface turbulence needed to cause dispersion.Another reason is that if oil can be dispersed, cold temperatures may actually increase the window of opportunity for dispersant use, by slowing the evaporation rate and extending the time period when the oil remains at a relatively low viscosity.In addition, in waters partially covered with ice, waves are greatly reduced, again slowing the rate of weathering of the oil through processes like emulsification that require surface mixing energy. Since weathered oil is more difficult to disperse, the presence of ice can increase the dispersant effectiveness.Under any ice condition, if energy is too low for rapid dispersion, a response vessel’s bow thrusters or propellers may be needed to provide the mixing energy needed for dispersion to occur.

Don’t dispersants just redistribute oil in the water column which increases the environmental impact?

It is true that dispersants do redistribute oil from the sea surface into the water column. If this is done in conditions that allow rapid dilution of dispersed oil to very low concentrations, the risk of ecological harm is small, compared to letting the oil impact the shoreline or other sensitive sites.  Further, the dispersed oil will biodegrade at rates that are likely more than an order of magnitude faster that surface slicks or oil that has stranded on shorelines.

Can dispersants be applied from aircraft as well as surface craft?

Dispersants can be applied via aircraft, including, when appropriate, helicopters and the industry is in the process of qualifying a 727-jet plane as a new high-speed dispersant delivery platform.Aircrafts can travel at speeds an order of magnitude faster than the fastest marine vessels, so they can very quickly be on scene responding to an oil spill.  Furthermore, once on scene, an aircraft can rapidly treat large areas as well as move between slicks located miles apart. Speed of initial response and the ability to treat large areas rapidly, are key to minimizing impacts from oil spills.

Can dispersants be used in high wind and wave conditions?

When high winds and waves prevent efficient mechanical recovery, dispersants become increasingly effective, this is because higher numbers of very tiny droplets are created under more energetic conditions.The safety aspects of poor weather related to flying safety and high concentrations of ice are the main reasons dispersants might not be used.  In extreme storm conditions, natural dispersion could be so effective that adding dispersants may accomplish little.

What is subsea dispersant injection?

Subsea dispersant injection is a method of applying dispersant directly at the source of a spill.  The technique is to simply spray dispersants into a plume of oil emanating from a point source, e.g., a well-control event.  In so doing, the jet of oil provides the energy to break up the oil into tiny dispersed oil droplets.  This will keep much of the oil in the deepwater where it will undergo the same dilution and biodegradation as dispersed oil at the surface.

Subsea dispersant injection has some important advantages over surface application of dispersants and other response options:

It is applied at the source of a spill before the oil has spread over a large area and potentially treats all the oil.

It can be applied day and night and in adverse weather conditions.

Dispersing the oil in deepwater can keep it from surfacing near the location of well response workers, which minimizes the potential for workers to be exposed to volatile organic vapors.

It can be applied in ice-covered waters with appropriate ice management.

It is more efficient allowing maybe five times less dispersant to be used.

Does the biodegration of oil slow down in the Arctic?

Studies have shown that natural biodegradation of oil under arctic conditions and in the deep ocean continue at a significant rate.

Are workers and the public at risk of any health issues with dispersants?

Although dispersants are formulated to be low in toxicity, response workers should use proper PPE and follow risk based management procedures, as in all cases when chemicals are handled. The general public is not likely to ever be exposed to dispersants, since dispersant application operations are generally required to be carried out in waters more than three miles from shore and only when people are out of the spray zone.

Is it not true that heavy fuel oils do not disperse?

The chemical makeup of heavy fuel oils (HFO) varies significantly depending on the source of crude.
Tests over many years have shown that some HFOs will disperse and some will not readily disperse. By using field tests prior to application it is possible to ascertain which heavy oil types can be dispersed.


References and further reading

  • Brown, H.M. and R.H. Goodman, “The use of Dispersants in Broken Ice,” in Proceedings of the 19th Arctic and Marine Oil Spill Program (AMOP) Technical Seminar, Environment Canada, Ottawa, Ontario, Canada, pp. 453-460, 1996.
  • Owens, C.K. and R.S. Belore, “Dispersant Effectiveness Testing in Cold Water and Brash Ice,” in the Proceedings of the 27th Arctic and Marine Oil Spill Program (AMOP) Technical Seminar, Environment Canada, Ottawa, Ontario, Canada, pp. 819-841, 2004.
  • Belore, R., “Wave Tank Tests to Determine the Effectiveness of Corexit 9500 on Hibernia Crude Oil under Cold Water Conditions,” in the Proceedings of the 25th Arctic and Marine Oil Spill Program (AMOP) Technical Seminar, Environment Canada, Ottawa, Ontario, Canada. pp. 735-740.
  • Joseph Mullin, Randy Belore, and Dr. Ken Trudel (2008) COLD WATER DISPERSANT EFFECTIVENESS EXPERIMENTS CONDUCTED AT OHMSETT WITH ALASKAN CRUDE OILS USING COREXIT 9500 AND 9527 DISPERSANTS. International Oil Spill Conference Proceedings: May 2008, Vol. 2008, No. 1, pp. 817-822.
  • Randy Belore (2003) Large Wave Tank Dispersant Effectiveness Testing in Cold Water. International Oil Spill Conference Proceedings: April 2003, Vol. 2003, No. 1, pp. 381-385.
  • S.L. Ross and MAR 2003. Cold-Water Dispersant Effectiveness Testing on Five Alaskan Oils at Ohmsett. Report to the U.S. Minerals Management Service, August 2003.
  • Margesin, R. and Schinner, F. (2001). Biodegradation and bioremediation of hydrocarbons in extreme environments. Applied Microbiology and Biotechnology, 56:650-663.
  • Prince, R.C. and Clark, J.R. (2004). Bioremediation of marine oil spills. In Studies in Surface Science and Catalysis, Chapter 18, 495 – 512, Elsevier B. V.
  • Kelly M McFarlin, K.M., Prince, R.C., Perkins, R.,Leigh, M.B., (2013). Biodegradation of dispersed oil in arctic seawater at -1°C.  PLoS ONE 01/2014.
  • Margesin, R. and Schinner, F. (2001). Biodegradation and bioremediation of hydrocarbons in extreme environments. Applied Microbiology and Biotechnology, 56:650-663.
  • Prince, R.C. and Clark, J.R. (2004). Bioremediation of marine oil spills. In Studies in Surface Science and Catalysis, Chapter 18, 495 – 512, Elsevier B. V.
  • Lee, K., Nedwed, T., Prince, R., Palandro, D., (2013).  Lab tests on the biodegradation of chemically dispersed oil should consider the rapid dilution that occurs at sea.  Marine Pollution Bulletin, Volume 73, Issue 1, 15 August 2013, Pages 314–318.