On April 20, 2010, the Deepwater Horizon oil drilling rig was reportedly nearing completion of a deepwater oil well (owned by BP) when an uncontained release of hydrocarbons (oil and natural gas) caused explosions and fire, resulting in 11 crew member fatalities. The incident produced the largest oil spill that has occurred in U.S. waters, eclipsing the magnitude of the 1989 Exxon Valdez spill after four to five days. The spill spanned approximately 84 days, and following several attempts, responders gained control of the oil discharge on July 15, 2010. Based on the most recent estimate from the federal government's Flow Rate Technical Group, the deepwater well ultimately released more than 200 million gallons (or 4.9 million barrels) of crude oil (see "Well Flow Rate Estimates" below). The uncertainty range of this estimate (+/-10%) is equivalent to two Exxon Valdez -size spills in either direction. Following the well's containment, satellite data began to reveal decreasing daily quantities of oil on the ocean's surface. (See Appendix for a snapshot of satellite analyses from selected days during the spill.) According to the federal government's GeoPlatform database, August 20, 2010, was the first day since the spill's start in which no oil was observed on the ocean's surface. After several days without observed oil on the ocean surface, the satellite analyses were discontinued on August 25, 2010. Although decreasing amounts of oil were observed on the ocean surface following the well containment, oil spill response officials and researchers have found oil in other places. A pressing question raised by many stakeholders is where did the oil go? Assessing the oil's fate is the primary objective of this report. For the purposes of this report, the term fate refers to the oil itself, not its actual or potential impacts on the Gulf ecosystems or organisms. The impact of the 2010 Gulf spill is still being assessed and will likely continue to be assessed for years to come. Because evaluating the actual fate of the oil may take time and may prove difficult, perceptions of the oil's fate may influence congressional interest and action, with consequences for the affected stakeholders. These perceptions may be influenced by multiple factors, including oil spill assessments and their methods, the group that prepared the assessment (and perceived biases of the group), and the manner in which the assessment is presented. If policymakers have the perception that the oil has degraded with minimal impacts to the environment, attention to 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. The fate of the oil may be of interest to policymakers for other reasons, including oversight of oil spill response performance and assessment of the government's capability to monitor oil spills in the environment. Multiple factors play a role in determining the fate of oil released into a marine environment. These include the chemical composition of the oil, a complex collection of natural processes and pathways, the location of the spill or discharge, and human intervention. The chemical composition of oil plays a critical role in how it reacts in an environment. The 2010 Gulf spill involved crude oil, which is not a homogenous material, but contains a mixture of chemical compounds that interact differently when exposed to the environment. According to a 2003 National Research Council report, evaporation can remove about one-third of the volume of a medium crude oil slick within the first day of a release. However, other components of the crude oil may persist for much longer. For instance, the fraction of crude oil that is water soluble can persist for weeks to years. Other portions of spilled crude oil often form into tar balls, which are persistent and may be transported great distances from the release site. The crude oil released during the 2010 spill is relatively lighter than other crude oil, and thus may evaporate more readily. When oil enters a marine environment it encounters an array of natural processes that transport the oil from its point of origin and alter its physical and chemical makeup. These combined physical and chemical changes are often described as "weathering." The altered materials will be incorporated into the planet's various natural systems: water, soil, air, or living organisms. The time involved in these changes can range from a matter of minutes and hours for some crude oil components to years (possibly decades) for other components. Figure 2 illustrates the processes and pathways involved. The location of an oil release plays a significant role in determining the oil's fate. Various attributes of an affected marine environment will influence the degree to which natural processes (identified above) interact with the introduced oil. For example, warmer water temperatures will generally expedite some natural processes, such as evaporation, biodegradation, and dissolution. Severe weather events, which are common during the Gulf's hurricane season, could also influence an oil spill's fate. For example, a storm event may transport oil to sensitive locations or cause settled oil to become resuspended in the water column. Moreover, in the Deepwater Horizon incident, severe weather events can impede human response activities or allow oil to breach manmade barriers (e.g., booms). The 2010 Gulf spill presented a unique combination of location factors that likely played a part in the oil's fate. First, the origin of the release occurred at a water depth of approximately 5,000 feet. According to the 2003 National Research Council report, this type of scenario introduces a number of complications compared to oil released at the surface (e.g., from a tanker spill). Although some research has found that the majority of oil from a deepwater release will reach the surface in several hours, the resulting surface slick may be thinner (compared to a typical surface spill), because some of the oil will be naturally diffused and dispersed as it rises to the surface. Second, the oil well blowout was a multi-day event, discharging the oil into the Gulf waters at a rapid rate. Some have surmised that this high-speed discharge increased the rate at which the oil naturally dispersed. Third, some initial scientific studies have suggested that microbial organisms, which essentially eat certain oil components, may play a part in controlling the oil's ultimate fate (discussed below). In the 2010 Gulf spill, responders employed a combination of mechanical removal (e.g., booming and skimming), in situ burning, and surface and sub-surface chemical dispersant application to remove the oil from the open waters of the Gulf. These applications yield different consequences for the oil's fate. Mechanical removal essentially takes a portion of the surface oil out of the environment. In situ burning removes the oil from the marine environment, but may transfer some of the pollution risk to the air. Particulate matter (PM) from in situ burning is "considered by most health professionals to be the main combustion product to investigate and monitor." During the oil spill, EPA monitored for PM at the shoreline and found the levels to be consistent with Gulf Coast summertime levels. Use of chemical dispersants in response to the Gulf spill has received considerable attention. Dispersants are chemical agents that enhance the breakup of an oil slick into small oil droplets that mix with the water column. Dispersant application does not reduce the amount of oil entering the environment; instead, dispersants alter the physical properties of oil, changing its transport, fate, and potential effects. In general, a decision to use dispersants poses trade-offs for oil spill responders. The objective of dispersant use is to minimize the amount of surface oil that reaches shoreline habitats, where it threatens a wide range of animals and plants. The downside is that dispersants increase water column organisms' exposure to oil. As stated in a 2005 National Research Council study, "[d]ispersant application thus represents a conscious decision to increase the hydrocarbon load (resulting from a spill) on one component of the ecosystem (e.g., water column) while reducing the load on another (e.g., coastal wetland)." Federal responders authorized the use of approximately 1.8 million gallons of dispersants in the Gulf. Approximately 42% of this dispersant volume was applied at the point where oil was escaping the wellhead, an unprecedented sub-surface application. Excepting a relatively minor application on July 19, dispersant use halted when the well was sealed on July 15. While dispersants have proven effective in breaking up the oil, impacts from the chemically dispersed oil (and the chemical dispersants) are unknown. On August 4, 2010, the National Incident Command (the spill response leadership team of the federal government) made public an estimate of the oil spill budget for the Deepwater Horizon incident. The release of the estimate received considerable media attention and was accompanied by a press conference, attended by key Administration personnel: Thad Allen (National Incident Commander), Jane Lubchenco (Administrator of the National Oceanic and Atmospheric Administration, NOAA), Carol Browner (Director of the White House Office of Energy and Climate Change Policy), and Robert Gibbs (White House Press Secretary). A number of questions were raised about the August 4 estimates, in part because of limited detail about how the estimates were developed (see "Criticism of the August 2010 Oil Budget Estimates" below). On November 23, 2010, the federal government released a peer-reviewed "Technical Document" that further explained how the estimates were derived; in some cases, the November document modified the August estimates. The Technical Document explained that the oil budget estimates were and will continue to be used for operational and response purposes, not to track the final fate of the spilled oil. Although the November Technical Document stresses this point in its text, the August oil budget estimates appear to have been used exactly for this latter purpose. The August and November oil budget estimates describe the oil as being in one of seven categories, which can be placed into three groups: human intervention, natural processes, and other.