Nearly 771 thousand gallons of chemical dispersants were injected into the oil and gas emitted by BP's Deepwater Horizon oil spill. Based on prior research, it was anticipated that the dispersants would biodegrade with modest rapidity. Instead, researchers found that the dispersants, which were studied at depths of 1,000+ meters, moved with the oil plumes until they were "lost to dilution" in the Gulf's depths. Dispersants were detected 300 kilometers from the wellhead even two months after their application.
The report on the study can be found at http://pubs.acs.org/doi/abs/10.1021/es103838p.
There is some debate among researchers on the role of dispersants in oil degradation. Some view it as helping; others, that it inhibits degradation. Expect more papers on that topic in the near future.