A Review of Literature Related to Oil Spill Dispersants

This report is a review of the literature on oil spill dispersants published from 1997 to August, 2008. The report identifies and focusses on recent advances in dispersant effectiveness, toxicity, and biodegradation. Other topics such as application, use, behaviour and fate are also covered. The prime motivation for using dispersants is to reduce the impact of oil on shorelines, thus the application must be successful and effectiveness high. As some oil would come ashore, discussion remains on what effectiveness is required to significantly reduce the shoreline impact. A major issue is the actual effectiveness during spills so that these values can be used in estimates for the future. The second motivation for using dispersants is to reduce the impact on birds and mammals on the water surface. The benefits of using dispersants to reduce impacts on wildlife still remain unknown. The third motivation for using dispersants is to promote the biodegradation of oil in the water column. The effect of dispersants on biodegradation is still a matter of dispute. Some papers state that dispersants inhibit biodegradation others indicate that dispersants have little effect on biodegradation. The most recent papers, however, confirm that inhibition is a matter of the surfactant in the dispersant itself and the factors of environmental conditions. It is clear, on the basis of current literature that the surfactants in some of the current dispersant formulations can inhibit biodegradation. No enhancement of biodegradation was clearly shown in any recent studies. Effectiveness remains a major issue with oil spill dispersants. It is important to recognize that many factors influence dispersant effectiveness, including oil composition, sea energy, state of oil weathering, the type of dispersant used and the amount applied, temperature, and salinity of the water. The most important of these is the composition of the oil, followed closely by sea energy. It is equally important to note that the only thing that is important is effectiveness on real spills at sea. More emphasis might be put on monitoring to provide reliable information for assessment and modeling. The results of the review indicate that dispersant effectiveness continues to be a major issue and is unresolved for Alaska North Slope (ANS) crude oil. Results of laboratory testing yield values ranging from 5 to 35%. Field tests show effectiveness values that are fractions of higher energy lab tests even of the moderate-energy tests. Tank tests show very high results, but testing is still to be conducted according to recommended procedures. The results of dispersant toxicity testing are similar to that found in previous years, namely that dispersants vary in their toxicity to various species, however, dispersant toxicity is less than the toxicity of dispersed oil. Of the recent toxicity studies of dispersed oil, most researchers found that chemically-dispersed oil was more toxic than physically-dispersed oil. About half of these found that the cause for this was the increased PAHs, typically about 5 to 10 times, in the water column. Others noted the increased amount of total oil in the water column. Some noted the damage to fish gills caused by the increased amount of droplets. Few researchers noted that the toxicity of chemically-dispersed oil was roughly equivalent to physically-dispersed oil. The interaction of droplets, particularly chemically-dispersed droplets appears to be an important facet of oil fate. It appears that high concentrations of sediment will have significant effect on dispersed oil droplets and the formation of stable OMAs (Oil-Mineral-Aggregates). OMAs appear to be stable over time and sink slowly and sediment on the bottom. Oil spill