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Oil Spill Dispersants: Efficacy and Effects
ratio data for aliphatic:aromatic concentrations showed no evidence of significantly enhanced dissolution of lower- and intermediate-molecular-weight aromatics as a result of the dispersant treatment.
Payne et al. (1991d) demonstrated that dissolved aromatic compounds from oil introduced into refreezing leads can be advected as conservative components in the brine generated during frazil ice formation to the benthic bottom boundary layer in field experiments completed in the Chukchi Sea. If dispersants were applied to oil released in open water during freezing conditions or to oil contained in open leads/broken sea ice during a refreezing event, it is conceivable that the enhanced dissolution process predicted by French-McCay and Payne (2001) could lead to transport of dissolved aromatic components to the benthos before significant evaporative weathering could otherwise occur. Clearly, any such enhanced transport would be spill or location specific, as it assumes only minor horizontal transfer.
The effects of surfactants and commercial oil dispersant mixtures on the rate and extent of biodegradation of crude oil, petroleum products, and individual hydrocarbons have been intensively investigated for over thirty years with mixed results. In some studies, biodegradation is stimulated, others find evidence of inhibition, and others observe no effects attributable to the presence of surfactants or commercial dispersants. Experimental systems have used a wide variety of substrates (e.g., crude oil, individual hydrocarbons), surfactants (e.g., commercial dispersant mixtures, pure surfactants), and microbial communities (e.g., natural seawater microbiota, microbial communities enriched by growth on crude oil, pure cultures). None of these factors appear to systematically affect the outcome. Instead, the effects of surfactants or commercial dispersant mixtures on the biodegradation rates of crude oil and defined hydrocarbons appear to depend on the chemical characteristics of the surfactants, the hydrocarbons, and the composition of the microbial community. Other factors, such as nutrient concentrations, oil-water ratios, and mixing energy, can also be expected to affect the observed biodegradation rate of dispersed oil.
One source of confusion in the literature on dispersant effects on oil biodegradation is that conclusions are often based on indirect evidence. For example, Corexit 9527 (DOR = 1:10) was shown to increase the rate of oxygen uptake in suspensions of South Louisiana and Kuwait crude oils relative to suspensions of physically dispersed oil (Traxler and Bhattacharya, 1978), and several dispersants increased the rate or extent of microbial growth on Arabian (Mulkins-Phillips and Stewart, 1974) or For-