DO BASINS AND BEAKERS NEGATIVELY BIAS DISPERSANT-EFFECTIVENESS TESTS?

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ABSTRACT Beaker and basin dispersant-effectiveness tests are used to help determine if application of dispersants in a real oil-spill incident is worthwhile. This paper provides evidence indicating that these tests negatively bias expected dispersion at sea because beakers and basins do not allow the spreading of oil slicks that occurs after application of dispersant in the unbounded open ocean. One reason is that closed system walls and/or oil-slick containment methods impose physical boundaries that restrict oil spreading. In addition to these physical constraints, surfactant films developed after applying dispersant form on the water surface surrounding an oil slick and act as chemical herders to keep slicks thick. This occurs in the field during a real incident and in beaker and basin dispersant-effectiveness tests. Surfactant films on the water surface, however, are fragile and can't persist in the open ocean but can persist throughout the short duration of standard dispersant-effectiveness tests. This paper provides a background discussion of how surfactant films contaminate the water surface on the perimeter of oil slicks to restrict spreading in both open and closed systems and evidence that these fragile films don't persist in the open ocean. The discussion is followed by a description of lab tests that showed even minimal water-surface contamination from the surfactants in a widely available dispersant significantly restricted spreading to keep slicks thick. Thick oil slicks, by their nature, will obviously require more turbulence to disperse than a thin film of the same oil. Thus, it is believed that the restricted slick spreading inherent in dispersant-effectiveness tests completed in basins and beakers results in significantly lower performance than would be expected at sea not only for heavier oils but for all oils in tests simulating low-energy conditions.