Oral Presentation Society of Environmental Toxicology and Chemistry Australasia 2021

Assessment of dispersant impacts when deriving toxicity threshold values for oil spills (#84)

Jill Woodworth 1
  1. SLR Consulting, Perth, WESTERN AUSTRALIA, Australia

Dispersants are a response tool for application in an oil spill situation and, if used correctly, can assist in the protection of shorelines and sensitive organisms from oil spills by dispersing oil and distributing it throughout the water column. While dispersants reduce surface oil load, they also increase the amount of dispersed oil in the water column in the immediate vicinity where they are applied. This action of dispersants has led to the erroneous impression that toxicity of the oil/dispersant mixture is higher than the oil alone.


Several dispersants are registered for use in Australia and this presentation will focus on the toxicity of three. Two of these are similar in composition in that they both contain petroleum hydrocarbons and dioctyl sulfosuccinate (DOSS) albeit at different concentrations. Review of ecotoxicity data for these dispersants indicates the petroleum hydrocarbon in the dispersant is responsible for dispersant’s ecotoxicity , rather than DOSS or other constituents. 95% species protection levels for dispersants were calculated as being greater than 3,000 µg/L dispersant using a large international data set.


It has been suggested in the literature that interactive effects between a dispersant and an oil may increase the toxicity of the dispersed oil to exposed organisms. The differences in toxicity observed between the water accommodated fraction (WAF) of oil without dispersant and the oil/dispersant mixture (termed CEWAF) are a result of increased oil concentrations in the CEWAF because of dispersant addition. Therefore, the addition of dispersants does not increase the toxicity of oils per se, rather they permit more petroleum hydrocarbon to enter the aqueous phase. It is this increased concentration which results in higher ecotoxicity. Therefore, for compliance and risk management, a single threshold concentration for the relevant oil can be used for application with any dispersant.