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Oil Spill Dispersants: Efficacy and Effects
ants, therefore, reduces the threat posed by a surface slick to organisms on the surface or, eventually, nearer to shore by altering the fate of that oil. As a consequence, a more limited and less robust set of information is needed to support the decision to use dispersants in such offshore conditions.
Use of dispersant in treating nearshore spills, however, raises many questions that are difficult to answer with the current understanding of the dispersed oil fate and effects. As pointed out in the previous discussion of environmental risk assessment, decisions regarding the use of dispersants in the nearshore settings often involve trade-offs and, therefore, call for more diverse and robust information (e.g., toxicological and population-level information about a particular species). As a consequence, questions about the fate and possible effect of dispersed oil or refined products make up a significant portion of the discussion in Chapters 4 and 5. Environmental monitoring of the operations usually focuses on preventing the direct application of dispersants onto wildlife or sensitive habitats. Additional monitoring is used in post-dispersant evaluations and model validation.
The models most commonly used to support real-time decision-making were designed to predict the trajectory of a surface slick, not a three-dimensional dispersed plume. Such models, which are in active use in the North Sea (Reed et al., 1999) and under development in the United States, are particularly sensitive to the quality of information about the subsurface current structure. In addition, current information is insufficient to evaluate dissolved components (e.g., toxic compounds) or concentrations of dispersed droplets for their impacts on nearshore environments. Ironically, as the effectiveness of dispersant increases, so does the potential threat to organisms exposed to the dispersed plume, due to the increased concentration of dissolved compounds and dispersed droplets in the water column. In open deep water, it may be reasonable to assume rapid dilution of the plume would take place. It is a generally held view, however, that such dilution should not be expected in shallower waters; hence a general avoidance of the use of dispersants in shallower waters exists. In addition, the current catalog of maps indicating the location and type of species or habitat that may be at risk from surface slicks is more adequate for areas along the shoreline. Information about the relative abundance of species in the water column or on the seafloor is inherently more difficult to obtain and tends to vary over shorter time scales. Greater capabilities to predict the trajectory of subsurface plumes of dispersed oil and the distribution of water-column and benthic species are needed, especially in shallower water where the impact of a dispersed oil plume may be more significant.