Energetic analysis of the relevance of WAF preparations to field mixing conditions

Abstract

Water accommodated fractions (WAFs) are standard laboratory preparations used in toxicity testing to simulate oil exposure in the water column. A traditional WAF is commonly created by mixing water with oil in an aspirator bottle using a stir bar without a vortex (i.e., low energy WAF or LEWAF). However, following the Deepwater Horizon accident, a new “High-Energy Water Accommodated Fraction” (HEWAF) was developed to represent the increased mixing energy from breaking waves. A HEWAF is generated using a commercial food blender to mix an oil and water solution, and the resulting chemical exposure from HEWAFs can be significantly different from the exposure resulting from LEWAF preparations. Consequently, assessment of the relevance of HEWAF toxicity test preparations to surface conditions in the field requires understanding how the energetics of the HEWAF preparation compare to the energetics in the ocean in general, and to the oil spill to which the test data are being applied. This study focused on the use of the HEWAF method used on the Deepwater Horizon oil spill. We measured the mixing intensity, or turbulent dissipation rate, generated in HEWAF and LEWAF preparations and compared the results to published turbulent dissipation rates for a variety of oceanic locations and conditions. The results indicate that HEWAFs subject the oil and water mixtures to a mixing intensity that is greater than 60,000 times more intense than the mixing intensity that has been recorded from breaking waves, and, on the basis of mixing intensity alone, even LEWAF preparations are over 7 times stronger than a breaking wave. Understanding the relevance of HEWAFs to environmental conditions is essential to ensure appropriate application of laboratory toxicity tests of oil exposure.