The fate of oil and oil-dispersant mixtures in freshwater ponds.

The fate, distribution and composition of oil and oil-dispersant mixtures were studied in a series of five, lined, inground ponds containing sandy gravel sediment and mesotrophic water. Normal Wells crude oil and Corexit 9527 were added at nominal concentrations of 100 and 20 ppm, respectively, to two of the ponds, and the crude oil alone was added at 100 ppm to a third pond. The water surface, water column, the sediment, pond liner and attached biota were systemically sampled for a year. While only about 2% of the oil remained in the water column of the pond with no dispersant addition, in the pond with the dispersant, about 10% of the oil persisted in the water for several weeks. Most of the oil initially dispersed in the water returned to the water surface, then eventually sank to the sediment. Thinner surface films showed a higher dispersant content than the thicker slicks , and the thinner films had higher infrared carbonyl absorption. Final distribution calculations revealed that about 45% of the oil had degraded in the oil-dispersant-treated ponds during the one year study, while only 23% could not be accounted for in the oil pond. Changes in the oil composition during the experiment were similar in all ponds, with no evidence to suggest that the dispersant affected oil composition in any special manner.