"The April 20, 2010, explosion of the Deepwater Horizon offshore drilling rig led to the largest oil spill in U.S. waters. Federal government officials estimated that the deepwater well ultimately released (over 84 days) over 200 million gallons (or 4.9 million barrels) of crude oil. Although decreasing amounts of oil were observed on the ocean surface following the well's containment on July 15, 2010, oil spill response officials and researchers have found oil in other places. A pressing question that has been raised by many stakeholders is where did the oil go? [...] Direct observation and measurement of the fate of the vast majority of the estimated 200 million gallons of oil presents a considerable challenge. In some cases, the estimates used to calculate these percentages contain considerable uncertainty. Even assuming that approximately half of the oil has been removed from the Gulf ecosystem through direct recovery, burning, skimming, or evaporation, the fate of the remaining ('other') oil is unknown. A complete and definitive answer to the question of the remaining oil is unknown at this juncture. It is debatable whether the fate of the remaining oil will ever be established conclusively, because multiple challenges hinder this objective: the complexity of the Gulf system; resources required to collect data; and varied interpretations over the results and observations. Moreover, as time progresses, determining the fate of the oil will likely become more difficult. Regardless, the question of oil fate will likely be answered through an incremental process. [...] If policymakers have the perception that the oil has degraded with minimal impacts to the environment, attention to the oil spill's consequences and associated impacts may wane. On the other hand, a perception that a substantial volume of oil remains and poses a threat to the environment could result in continuing pressure on Gulf industries and livelihoods."
CRS Report for Congress, R41531