(ASTM, 1996); however, standard guides should be developed for ensuring that dispersant stockpiles meet minimum efficacy standards.

D.4 Are the Environmental Conditions Conducive to the Successful Application of Dispersant and Its Effectiveness?

Water temperature, wind velocity, wave height, and other environmental factors play key roles in determining whether dispersant can be applied safely and effectively. Just as these environmental factors define a safe and effective operational window for mechanical response techniques, they also define an operational window for dispersant application. Generally, these operational windows are often dissimilar and sensitive to different environmental parameters. For example, booming and skimming (standard mechanical response techniques) work well in calm conditions and weak currents, whereas dispersants require some minimum wave energy to disperse the surface slick and entrain individual oil droplets. There are guidelines for minimum/maximum conditions for wind speed, sea state, and temperature, and conditions often change during the actual application (Fingas and Ka’aihue, 2004a). Thus, decisionmakers should continuously monitor the character of the surface slick and on-site conditions and frequently reassess the decision to apply, or continue to apply, dispersant.

Existing capabilities to characterize and predict evolving environmental conditions beyond sea state and weather are limited. Unlike surface slicks that are affected primarily by surface winds, the nature and trajectory of subsurface dispersed oil plumes are more sensitive to currents. Even wave height, a critical component for predicting dispersant effectiveness, may be difficult to predict more than a few hours in advance.

D.5 Will the Effective Use of Dispersants Reduce the Impacts of the Spill to Shoreline and Water Surface Resources without Significantly Increasing Impacts to Water-Column and Benthic Resources?

As discussed throughout Chapters 3, 4, and 5, there are still many uncertainties about the fate of dispersed oil droplets and the many different factors and processes that control that fate in different biophysical settings. Understanding the relative risk posed to various portions of the ecosystem at a spill, however, requires an adequate understanding of the physical and toxicological effects that dispersed oil may have on many different components of that ecosystem. In open, offshore waters, physical mixing processes tend to rapidly dilute a plume of dispersed oil droplets, reducing the potential for significant impacts on organisms in the water column or associated with the seafloor. The effective use of dispers-



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