DISCOBIOL: Assessment of the Impact of Dispersant Use for Oil Spill Response in Coastal or Estuarine Areas

ABSTRACT

Dispersants are known to be an appropriate solution for offshore spill response when sea conditions provide enough energy to disperse and then dilute oil into surface waters. In shallow coastal areas, the use of dispersant is restricted due to the potential that the dispersed oil might come into contact with sensitive resources before dilution can take place. However, after assessing the advantages and potential risks of dispersing oil in coastal areas, it may emerge after careful consideration that and in some cases the use of dispersants could provide a net environmental benefit.

The DISCOBIOL research program aimed to provide practical recommendations on dispersant use in coastal and estuarine areas by acquiring relevant (in terms of likely dispersed oil concentrations) and robust experimental information on the impact of mechanically and chemically dispersed oil on living resources. The main conclusion from these experiments was that there is no significant difference between the impacts from oil with and without dispersant use in terms of acute toxicity. However there are some observable sub-lethal effects from exposure to dispersed oil which do not persist more than a few weeks. In a natural environment, on a medium or long timescale, biota which have been exposed to oil (with and without dispersant) do exhibit some symptoms which could affect their survival rate in the field even though they do not lead to acute toxicity effects. However the DISCOBIOL project demonstrated that effects of dispersed oil were less severe than previously recorded for near shore environments.

In terms of applying these results to decision making at an oil spill, it highlights the need in coastal areas prior to the use of dispersant to complete a “Net Environmental Benefit Analysis” (NEBA) to determine whether the use of dispersant is expected to minimize the overall damage resulting from the pollution. As it is difficult to cover the number of possible spill scenarios at the contingency planning stage, instead of completing a NEBA, many countries define geographical limits where dispersion can be undertaken, based on the water depth and the distance to the shore as well as the presence of sensitive resources. The DISCOBIOL study confirmed the appropriateness of these pre-defined limits for France's coastal waters but demonstrated that they could be less restrictive since the exposure to dispersed oil could be at least five times higher than was previously considered the safe limit.