Deepwater Horizon Oil Spill:
The Fate of the Oil
Jonathan L. Ramseur
Specialist in Environmental Policy
December 16, 2010
Congressional Research Service
7-5700
www.crs.gov
R41531
CRS Report for Congress
P
repared for Members and Committees of Congress
Deepwater Horizon Oil Spill: The Fate of the Oil
Summary
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?
On August 4, 2010, the federal government released an estimate of the oil spill budget for the
Deepwater Horizon incident. On November 23, 2010, the federal government released a peer-
reviewed “Technical Document” that further explained how the estimates were derived, and in
some cases, modified the initial estimates. The oil budget estimates divide the released oil into
seven categories, accounting for the following percentages of the total oil released. These
categories generally fall into three groups:
• Human intervention: direct recovery from the well (17%); in situ burning (5%);
skimmed (3%); chemically dispersed (16%).
• Natural Processes: naturally dispersed (13%); evaporated or dissolved (24%).
• Other (22%): refers to the oil remaining after subtracting the above estimates
from the total estimated release; possible fates include remaining in the water
column, settling to the sea floor, mixing with sediment, ingested by microbes, or
collected during shore cleanup activities.
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. Researchers are
continuing to study various components of the Gulf, specifically damages to natural resources.
Some of these efforts may provide clues to the oil’s fate.
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. The perception of the spill’s fate may be influenced by multiple factors, including
oil spill assessments, the group that prepared the assessment, 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 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.
Congressional Research Service
Deepwater Horizon Oil Spill: The Fate of the Oil
Contents
Introduction ................................................................................................................................ 1
Factors that Impact an Oil Spill’s Fate ......................................................................................... 3
Composition of the Oil.......................................................................................................... 3
Natural Processes .................................................................................................................. 4
Location of Spill ................................................................................................................... 5
Human Intervention .............................................................................................................. 5
The Federal Government’s Oil Budget Estimates......................................................................... 6
Oil Budget Categories ........................................................................................................... 7
Human Intervention ........................................................................................................ 8
Natural Processes .......................................................................................................... 10
Other ............................................................................................................................ 10
Inherent Uncertainty ........................................................................................................... 11
Where is the Oil that Remains in the Gulf? ................................................................................ 12
Oiled Shorelines.................................................................................................................. 12
Water Column..................................................................................................................... 14
Biodegradation.................................................................................................................... 15
Sea Floor ............................................................................................................................ 16
Conclusions .............................................................................................................................. 17
Figures
Figure 1. Flow Rate Estimates during Response .......................................................................... 2
Figure 2. Fate of Oil in a Marine Environment ............................................................................ 4
Figure 3. Federal Government’s Oil Budget................................................................................. 8
Figure 4. Surface and Subsurface Chemical Dispersant Application during the 2010 Gulf
Oil Spill ................................................................................................................................... 9
Figure 5. Federal Government Oil Budget Estimates and Ranges of Uncertainty ....................... 12
Figure 6. Oiled Shoreline by State ............................................................................................. 13
Figure 7. Estimates of Percentage Contribution of Oil into North American
Coastal Waters, by Major Source Categories .......................................................................... 15
Figure A-1. NOAA Satellite Analysis of Gulf Oil Spill .............................................................. 18
Figure A-2. NOAA Satellite Analysis of Gulf Oil Spill .............................................................. 19
Figure A-3. NOAA Satellite Analysis of Gulf Oil Spill .............................................................. 20
Appendixes
Appendix. Selected Satellite Images of Deepwater Horizon Oil Spill......................................... 18
Congressional Research Service
Deepwater Horizon Oil Spill: The Fate of the Oil
Contacts
Author Contact Information ...................................................................................................... 20
Congressional Research Service
Deepwater Horizon Oil Spill: The Fate of the Oil
Introduction
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,1 the deepwater
well ultimately released more than 200 million gallons (or 4.9 million barrels) of crude oil (see
“Well Flow Rate Estimates” below).2 The uncertainty range of this estimate (+/-10%) is
equivalent to two Exxon Valdez-size spills in either direction.3
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.4 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.5
1 U.S. Department of the Interior, Press Release, “U.S. Scientific Teams Refine Estimates of Oil Flow from BP’s Well
Prior to Capping,” August 2, 2010, at http://app.restorethegulf.gov/release/2010/08/02/us-scientific-teams-refine-
estimates-oil-flow-bps-well-prior-capping.
2 As will be discussed, an estimated 17% of this oil did not enter the Gulf environment, but was directly recovered from
the wellhead by BP.
3 Several non-governmental groups have subsequently produced estimates that are comparable to the federal
government’s calculation. See Timothy J. Crone et al., “Magnitude of the 2010 Gulf of Mexico Oil Leak,” Science
Express, September 23, 2010. A Wood’s Hole Oceanographic Institution team also produced a similar estimate,
described in National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, The Amount and Fate
of Oil, Staff Working Paper, released October 6, 2010, at http://www.oilspillcommission.gov.
4 See the GeoPlatform interactive mapping and database tool at http://www.geoplatform.gov/gulfresponse/index.html.
To track the observed oil data, click “View Interactive Map.” Use the “layers” tab and select “BP Deepwater Horizon
Oil Spill” and then “Satellite, Radar, and Aerial Images of the Spill.”
5 For further reading, see e.g., CRS Report R41407, Deepwater Horizon Oil Spill: Highlighted Actions and Issues, by
Curry L. Hagerty and Jonathan L. Ramseur; CRS Report R41396, The 2010 Oil Spill: Natural Resource Damage
Assessment Under the Oil Pollution Act, by Kristina Alexander; CRS Report R41485, Reorganization of the Minerals
Management Service in the Aftermath of the Deepwater Horizon Oil Spill, by Henry B. Hogue.
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Deepwater Horizon Oil Spill: The Fate of the Oil
Well Flow Rate Estimates
Throughout the oil spill response, the estimated flow rate received considerable attention and generated considerable
controversy. Figure 1 illustrates the Unified Command’s flow rate estimate over time, culminating in an August 2
estimate of 62,000 barrels per day.6 For comparison purposes, Figure 1 identifies other estimates that were offered
during the response and the general time frame in which they were made. These additional estimates come from
independent scientists and an environmental advocacy group, as well as a worst-case scenario BP provided
responders. A staff paper from the National Commission on the BP Deepwater Horizon Oil Spill and Offshore
Drilling documents this chain of events.7
The flow rate estimates and associated issues8 are beyond the scope of this report. For the purposes of this report,
the ultimate flow rate estimate is an important piece of information, because it determines the magnitude of the spill
volume. The uncertainty in the flow rate estimate affects the uncertainty in the magnitude of various oil fates. In
addition, the underestimated flow rate may have played a role in the degree to which the oil was monitored.9
Figure 1. Flow Rate Estimates during Response
Comparison of Selected Estimates
80,000
70,000
60,000
y
a 50,000
/D
ls 40,000
rre
a 30,000
B
20,000
10,000
0
/10
/10
/10
/10
/10
/10
/10
/10
/10
04/20
05/03
05/16
05/29
06/11
06/24
07/07
07/20
08/02
Unified Command
MacDonald
SkyTruth
BP "worst case"
Wereley
Crane
Chiang
Source: Prepared by CRS; estimates and timeline from National Commission on the BP Deepwater Horizon Oil
Spill and Offshore Drilling, The Amount and Fate of Oil, Staff Working Paper, released October 6, 2010, at
http://www.oilspillcommission.gov.
Notes: Details regarding the sources of the above estimates are provided in the Commission publication (cited
above). In some cases, the estimates were presented as a range. For example, the Chiang estimate was between
20,000 and 100,000 barrels/day. In such cases, the middle point of the range was used in the figure.
6 U.S. Department of the Interior, Press Release, “U.S. Scientific Teams Refine Estimates of Oil Flow from BP’s Well
Prior to Capping,” August 2, 2010, at http://app.restorethegulf.gov/release/2010/08/02/us-scientific-teams-refine-
estimates-oil-flow-bps-well-prior-capping.
7 The Amount and Fate of Oil, Staff Working Paper, released October 6, 2010, at http://www.oilspillcommission.gov.
8 Issues may include (1) the relationship between flow rates and potential Clean Water Act penalties (see CRS Report
R41370, Federal Civil and Criminal Penalties Possibly Applicable to Parties Responsible for the Gulf of Mexico Oil
Spill, by Robert Meltz); and (2) possible impacts of underestimated rates on response efforts (see National Commission
on the BP Deepwater Horizon Oil Spill and Offshore Drilling, Stopping the Spill: The Five-Month Effort to Kill the
Macondo Well, Staff Working Paper, released November 22, 2010).
9 For example, if the early flow rates had indicated a greater volume of oil was not reaching the ocean surface, might
responders have initiated a more comprehensive subsea monitoring program?
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Deepwater Horizon Oil Spill: The Fate of the Oil
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.
Factors that Impact an Oil Spill’s Fate
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.
Composition of the Oil
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,10 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.11 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.
10 National Research Council, Oil in the Sea III: Inputs, Fates, and Effects, (hereinafter NRC Report, 2003) National
Academies Press, 2003.
11 This fraction includes a wide range of polycyclic aromatic compounds (PAH), which appear to pose the greatest
toxicity to the environment. NRC Report, 2003, p. 114.
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Deepwater Horizon Oil Spill: The Fate of the Oil
Natural Processes
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.12 The
time involved in these changes can range from a matter of minutes and hours for some crude oil
components to years (possibly decades)13 for other components. Figure 2 illustrates the processes
and pathways involved.
Figure 2. Fate of Oil in a Marine Environment
Processes and Pathways
Source: Reproduced from National Research Council, Oil in the Sea III: Inputs, Fates, and Effects, National
Academies Press, 2003, Figure 4-1. This document provides a comprehensive discussion of these
processes/pathways.
12 For a more detailed discussion, see NRC Report, 2003, Chapter 4 (“Behavior and Fate of Oil.”)
13 See e.g., Daniel Esler et al., “Cytochrome P4501A Biomarker Indication of Oil Exposure in Harlequin Ducks up to
20 Years after the Exxon Valdez Oil Spill,” Environmental Toxicology and Chemistry, Vol. 29, No. 5, published online
January 2010, pp. 1138–1145.
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Deepwater Horizon Oil Spill: The Fate of the Oil
Location of Spill
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.14
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.15
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).
Human Intervention
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.”16 During the
oil spill, EPA monitored for PM at the shoreline and found the levels to be consistent with Gulf
Coast summertime levels.17
14 For further discussion, see NRC Report, 2003.
15 NRC Report, 2003, p. 108, citing a 2001 study: Johansen et al., “DeepSpill—Field Study of a simulated oil and gas
blowout in deep water,” Proceedings Intern. Marine Environmental Modeling Seminar. New Orleans, LA, 2001.
16 Nir Barnea (National Oceanic and Atmospheric Administration), “Health and Safety Aspects of In-situ Burning of
Oil,” at http://response.restoration.noaa.gov/book_shelf/655_health.pdf.
17 See EPA, “Air Data from the Gulf Coastline,” at http://www.epa.gov/bpspill/air.html#aqi.
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Deepwater Horizon Oil Spill: The Fate of the Oil
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.18 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).”19
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 15.20 While
dispersants have proven effective in breaking up the oil, impacts from the chemically dispersed
oil (and the chemical dispersants) are unknown.21
The Federal Government’s Oil Budget Estimates
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.22 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).
18 For a more comprehensive discussion, see National Research Council, Oil Spill Dispersants: Efficacy and Effects
(National Academies Press, 2005).
19 Ibid.
20 EPA, The Federal Government Response: Dispersant Use in the BP Oil Spill, at http://www.restorethegulf.gov/sites/
default/files/imported_pdfs/external/content/document/2931/835583/1/
Fact%20Sheet%20Dispersants%20july%2029%202010-1.pdf.
21 EPA conducted some laboratory tests on the dispersants used in the Gulf spill. Of the eight dispersants evaluated (on
the two test species), EPA found that the dispersants alone were less toxic than the dispersant-oil mixture. For more
information, see EPA’s Toxicity Testing of Dispersants, at http://www.epa.gov/bpspill/dispersants-testing.html.
22 The release includes two documents, both made public August 4, 2010: “Deepwater Horizon MC252 Gulf Incident
Oil Budget: Government Estimates through August 01 (Day 104)” and “BP Deepwater Horizon Oil Budget: What
Happened To the Oil?”
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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.23 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.
Criticism of the August 2010 Oil Budget Estimates
After the government released its initial oil budget estimate, some scientists questioned specific aspects of the
analysis; others criticized the way in which the estimates were presented by high-level administration officials. This
chain of events is documented in a staff working paper prepared by the National Commission on the BP Deepwater
Horizon Oil Spill and Offshore Drilling.24
One of the primary critiques of the oil budget was that it offered precise figures (to the barrel) for various estimates.
These fixed amounts give the impression that the estimates carry more certainty than is the case. Similarly, the budget
documents do not provide uncertainty ranges for various estimate categories, some of which may have considerable
error bars (as shown in the November release). As a possible demonstration of the uncertainty, a group of scientists
with the Georgia Sea Grant program estimated substantial y lower amounts for the evaporation category.25
Another critique of the August oil budget was the way in which certain oil fate categories were initially described
during interviews and press conferences, arguably giving the impression that large percentages of the oil budget were
of little concern. For example, the categories of naturally dispersed, chemically dispersed, and evaporated/dissolved
were initially grouped together and described as “gone” in some verbal communications.26 However, the National
Commission working paper charts a progression of statements from Administration officials, which over time,
included more caveats and clarifications. Some of these caveats and clarifications were present in the August oil
budget document from the start. For example, the August oil budget stated that “until it is biodegraded, natural y or
chemically dispersed oil, even in dilute amounts, can be toxic to vulnerable species.”27
Oil Budget Categories
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. The estimates below are from the November 2010 oil budget document, which are based on
calculations made July 14, 2010 (Figure 3).
23 The Federal Interagency Solutions Group, Oil Budget Calculator Science and Engineering Team, Oil Budget
Calculator: Deepwater Horizon-Technical Documentation, November 2010.
24 National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, The Amount and Fate of Oil,
Staff Working Paper, released October 6, 2010, at http://www.oilspillcommission.gov. President Obama established the
National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling on May 22, 2010. The commission
is tasked with providing recommendations on how to prevent—and mitigate the impact of—any future spills that result
from offshore drilling. The commission’s final report is due to the President by January 12, 2010.
25 Chuck Hopkinson, Outcome/Guidance from Georgia Sea Grant Program: Current Status of the BP Oil Spill, August
17, 2010.
26 Recounted in the National Commission Working Paper.
27 National Commission, Working Paper, pp. 20-23.
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Deepwater Horizon Oil Spill: The Fate of the Oil
Figure 3. Federal Government’s Oil Budget
Based on the “Expected” Estimates Calculated July 14, 2010
Directly
Recovered
Other
17%
22%
Burned
5%
Skimmed
3%
Chemically
Evaporated or
Dispersed
Dissolved
16%
24%
Naturally
Dispersed
13%
Source: Reproduced by CRS using estimates provided the Federal Interagency Solutions Group, Oil Budget
Calculator Science and Engineering Team, Oil Budget Calculator: Deepwater Horizon-Technical Documentation,
November 2010.
Human Intervention
• Direct recovery from wellhead: This refers to the direct capture of oil from the
discharging well and its pipeline transport to surface level vessels. Of the seven
categories, this quantity—820,000 barrels (~34 million gallons)—is the only
category based on direct measurement and is thus the most precise figure,
accounting for 17% of the total estimated release.
• Burned: The Oil Budget estimates that responders burned 260,000 barrels (~11
million gallons) of oil in situ—5% of the total estimated release. The Oil Budget
states that the authors used American Society for Testing and Materials (ASTM)
burn rate standards in their calculations, with different rates for non-emulsified
and emulsified oil.28
• Skimmed: Although the estimate of skimmed oil is based on an actual
measurement—barrels of oil/water mixtures collected—the estimate is a “rough
calculation” due to the uncertainty of net oil in the oily water. Measurements of
oil-water ratios were not available, so the oil budget authors based skimmer
performance on sea conditions, nature of the oil, and past experience.29 The oil
28 An EPA document suggests the burned estimates are more uncertain than presented in the oil budget documents.
According to EPA, responders conducted 411 controlled burn events, combusting an estimated 222,000 to 313,000
barrels of oil (or 9.3 million to 13.1 million gallons), at http://www.epa.gov/research/dioxin.
29 The Federal Interagency Solutions Group, Oil Budget Calculator Science and Engineering Team, Oil Budget
Calculator: Deepwater Horizon-Technical Documentation, November 2010, p. 33.
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Deepwater Horizon Oil Spill: The Fate of the Oil
budget estimate is 160,000 barrels (~6.7 million gallons)—3% of the total
estimated release. Although oil spill recovery rates average approximately 15%,30
“historically, recovery from major spills has amounted to only a few percent , if
there was any recovery at all.”31
• Chemically dispersed: The budget estimates that 770,000 barrels (~32 million
gallons) of oil were dispersed through the application of 1.8 million gallons of
surface and sub-surface chemical dispersants (Figure 4)—16% of the total
estimated release. This estimate represents a near doubling (88%) compared to
the August document, by far the most the most significant modification. The
authors note that the original estimate was conservative, and now represents the
low end of the estimate range. The range of uncertainty for chemically dispersed
oil dwarfs the uncertainties for other categories (Figure 5). The Technical
Document states: “Estimating effectiveness of these dispersant operations was
the most difficult challenge in constructing the Budget Calculator, given the
present state of oil response technology.”32 In particular, experts could not reach
an agreement on the underlying assumption for subsurface application.
Figure 4. Surface and Subsurface Chemical Dispersant Application
during the 2010 Gulf Oil Spill
Cumulative Application
D
a
2,000,000
90,000 ily D
)
s 1,800,000
Combined Daily
80,000 i
n
sp
Application Rates
1,600,000
e
allo
70,000 san
Cumulative Surface Application
G
( 1,400,000
t A
n
60,000
1,200,000
p
50,000 plicat
licatio 1,000,000
p
p
40,000 io
800,000
n
30,000 R
600,000
a
t
lative A
e
u
400,000
20,000 (Gallo
m
u
200,000
10,000
C
Cumulative Subsurface Application
n
0
0
s
)
10
10
10
10
10
0
0
/10
10
10
10
/1
/1
30/
7/10
21/
28/
11/
18/
25/
4/23/ 4/
5/
5/14/ 5/
5/
6/4 6/
6/
6/
7/2
7/9
Source: Prepared by CRS with data from daily “Current Operations” updates, available at
http://www.restorethegulf.gov/.
30 National Research Council, Spills of Non-Floating Oils, National Academy Press, 1999.
31 Office of Technology Assessment, Coping with an Oiled Sea, 1990.
32 The Federal Interagency Solutions Group, Oil Budget Calculator Science and Engineering Team, Oil Budget
Calculator: Deepwater Horizon-Technical Documentation, November 2010, p. 25.
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Deepwater Horizon Oil Spill: The Fate of the Oil
Natural Processes
• Naturally dispersed: The budget estimates that 630,000 barrels (~26 million
gallons) of the oil were naturally dispersed—13% of the total estimated release
(Figure 3).33 The authors define dispersed oil as droplets smaller than 100
micron (approximately the diameter of a human hair). The Technical Document’s
discussion of the natural dispersion calculation suggests this estimate is based on
several assumptions. Although the Technical Document data used to generate the
uncertainty ranges in Figure 5 showed no uncertainty range for the “naturally
dispersed” category, the text of the Technical document discusses an uncertainty
factor for both natural subsurface and natural surface dispersion.34 The reason for
this apparent omission is not specified.
• Evaporated or dissolved: The budget estimates that 1.2 million barrels (~50
million gallons) of oil evaporated or dissolved—24% of the total estimated
release (Figure 3). The authors state that they achieved this estimate by applying
an evaporation rate specific to the type of oil released to the amount of oil
estimated to have reached the surface (and not removed through in situ burning).
Although the evaporation and dissolution processes are lumped together, the
evaporation process likely dominates this grouping. According to a 2003
National Research Council report, medium crude oil can lose up to 40% of its
volume within a few days. By contrast, “dissolution accounts for only a small
portion of oil loss.”35 However, the Oil Budget Technical Document indicates
that dissolution likely played a larger role in the Gulf spill than typical surface
spills: “because of the nature and depth of the release, dissolution of oil into the
water column was a significant weathering factor.” 36
Other
The “other” category is essentially the portion that remains after subtracting the above category
measurements/estimates from the total amount of oil estimated to have released. Such a
calculation yields 1.1 million barrels (~46 million gallons)—22% of the total estimated release
(Figure 3). This category includes “oil on or just below the surface as light sheen and weathered
tar balls, washed ashore or collected from shore, or buried in sand and sediments and may
resurface through time.”
33 This is a 3% decrease from the August estimate. Although not explained, the decrease is likely due to the increase in
chemically dispersed oil. See pp. 5-6 of the Technical Document.
34 See p. 23 and p. A1.9.
35 NRC Report, 2003, pp. 90-93.
36 The Federal Interagency Solutions Group, Oil Budget Calculator Science and Engineering Team, Oil Budget
Calculator: Deepwater Horizon-Technical Documentation, November 2010, p. 16.
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Inherent Uncertainty
Most of the estimated categories of the oil’s fate contain some degree of uncertainty, because the
estimates are not based on direct measurement or observation.37 Instead, the oil budget analysts
employed formulas and assumptions based on “theoretical considerations and expert knowledge
from past incidents.”38
Although the uncertainty behind the estimates was not stressed in the August document, the
November Technical Document underscores the uncertainties in multiple instances. The
document points out the estimates represent a snapshot in time and are likely to change with new
information, stating “the final mass balance calculation of the Deepwater Horizon Spill lies
somewhere in the future, to be produced only after all the appropriate and necessary studies will
have been completed.”39
The November Technical Document includes uncertainty ranges for some of the various
categories. These are illustrated in Figure 5. The columns illustrate the “expected” case estimates.
The uncertainty bars represent the range between the best and worst case scenarios.
Some may question the usefulness of releasing the estimates to the public, considering (1) the
level of uncertainty (particularly the magnitude—tens of millions of gallons) and (2) the stated
purpose of the oil budget being solely for oil spill responders. A potential concern of making the
estimates public is that the estimates may be misapplied, leading to false perceptions of the oil’s
fate. Others may counter that the government should share data/information when it is available.
This debate is beyond the scope of this report, but it may be an issue policymakers should
consider for future oil spill responses.
37 “There are very few variables in the overall set of theoretical equations which are actually populated by real
measurements, observations, or even observation-based estimates …. it cannot be stressed enough that it is not
appropriate to use the results in post-spill evaluations of impact or response actions.” Peer review comments in
Appendix 10 of Technical Document, p. A10.24.
38 Technical Document, p. iv.
39 Ibid., p. 2.
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Figure 5. Federal Government Oil Budget Estimates and Ranges of Uncertainty
Based on July 14, 2010 Estimates
70
Natural Processes
Other
Human Intervention
d
60
e
s
d
re
per
ed
lv
ve
is
o
50
D
s
co
ly
d
is
s
al
rse
D
n
ic
e
o
or
ll
ctly Re
d
a
40
e
hem
te
C
Disp
ra
Dir
lly
o
f G
ra
p
a
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v
s
E
n
30
Na
illio
M
d
20
e
d
e
rn
m
Bu
im
Sk
10
-
Source: Prepared by CRS using estimates provided the Federal Interagency Solutions Group, Oil Budget
Calculator Science and Engineering Team, Oil Budget Calculator: Deepwater Horizon-Technical Documentation,
November 2010, p. 39.
Notes: The columns illustrate the “expected” case estimates. The uncertainty bars represent the range between
the best and worst case scenarios. Although the data (from Figure 12) used to generate the uncertainty ranges
above showed no such range for the “natural y dispersed” category, the text of the Technical document
discusses an uncertainty factor for both natural subsurface and natural surface dispersion.
Where is the Oil that Remains in the Gulf?
Even assuming that approximately half of the oil has been removed from the Gulf marine
environment through direct recovery, burning, skimming, or evaporation,40 a substantial portion
of oil—over 100 million gallons—remained in the Gulf as of the Oil Budget estimate. Where is
the remaining oil and what is its physical and chemical state? Some data and ongoing studies
offer some clues that could provide a partial and preliminary answer.
Oiled Shorelines
Although many of the response activities were intended to prevent oil from reaching shorelines,
oil eventually contaminated hundreds of miles of Gulf state shorelines. This oil falls into the
“other” category of the government’s oil budget. Figure 6 illustrates the oiled shoreline miles by
state over time. These data reflect “daily snapshots,” not cumulative miles of oiled shoreline. The
40 Evaporated oil may remain in the atmosphere for some period of time. The 2003 NRC states that “relatively little
work has been conducted on the basic physics and chemistry of oil spill evaporation” (p. 90).
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oiled shoreline data above combines two categories: “moderate to heavy” and “light” oiled
shoreline miles. The Unified Command incident reports began to make this distinction on August
23, 2010. Two dramatic increases in the data occurred on June 21 and 29—increases of 193% and
94%, respectively. These increases were not explained in the incident reports. As of October 27,
2010, approximately 17% of the oiled shoreline was classified as “moderate to heavy.” In a
November 10, 2010 public conference call, the federal on-scene coordinator, Admiral Paul
Zukunft, stated that 27 shoreline miles are currently considered “heavily oiled.” None of the daily
incident reports defines or quantifies the terms “light,” “moderate to heavy,” or “heavily oiled.”
Figure 6. Oiled Shoreline by State
800
700
600
Florida
e
lin 500
e
Alabam a
hor 400
S
of
Mississippi
s 300
ile
M
200
100
Louisiana
0
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
2010
4/
1/
8/
5/
3/
13/
20/
27/
7/
11/
18/
25/
8/
8/
15/
22/
29/
9/
12/
19/
26/
10/
17/
24/
6/
6/
6/
7/
7/
7/
8/
8/
8/
9/
9/
9/
10/
10/
10/
10/
Source: Prepared by CRS with data from Unified Command daily and weekly incident reports.
Notes: According to the incident reports, the data reflect “daily snapshots.” The oiled shoreline data above
combines two data categories: “moderate to heavy” and “light” oiled shoreline miles. The Unified Command
incident reports began to make this distinction on August 23, 2010. The dramatic increase in oiled shoreline
occurred in the data on June 29, when the Louisiana shoreline figure increased from 69 to 259 miles in one day.
This increase was not explained in the incident report. As of October 27, 2010, approximately 17% of the oiled
shoreline is classified as “moderate to heavy.” In a November 10, 2010 public conference call, the federal on-
scene coordinator, Admiral Paul Zukunft, stated that 27 shoreline miles are currently considered “heavily oiled.”
Although the hundreds of miles of oiled shoreline might appear to account for a considerable
amount of oil, CRS is not aware of an authoritative source that quantifies this oil (in terms of
volume). Such a quantification is important in determining the extent to which the beached oil
contributes to the “other” category. The November Technical Document states: “no estimates
were made of the percentage of oil in [oiled shoreline debris] so it is not possible to give a
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reliable estimate of this amount. Based upon past spills, the oil content of collected debris mass is
only a few percent.”41
Water Column
In the immediate weeks following the Deepwater Horizon incident, independent researchers
began to disclose findings of potential underwater plumes of weathered oil near the spill source.
Some of the initial evidence generated some controversy and was met with skepticism.42
However, a different research team subsequently reported evidence of an undersea plume, which
NOAA confirmed was linked to the Deepwater Horizon incident.43
Peer-reviewed papers followed, documenting further evidence of subsea plumes. For example, in
an August report in Science, a team from Woods Hole Oceanographic Institution described a
plume44 southwest of the well site that was over 35 kilometers long at a depth of approximately
3,300 feet.45 In an interview the researchers point out that the earlier efforts—the research that
was greeted with skepticism—may have identified the same plume.46
The Administration established a Joint Analysis Group (JAG), including officials from NOAA
and EPA, to facilitate cooperation and coordination regarding subsea monitoring in the Gulf. The
JAG has issued several reports on collected data in the Gulf.47 The most recent JAG report
(August 16, 2010) reviewed data collected through August 9, 2010.48 This report documented
diminished dissolved oxygen (DO) levels, which serve as a proxy measure for subsurface
dispersed oil (i.e., subsea plumes). The decreased DO levels where found in areas where plumes
have been observed by other scientists.
41 The Federal Interagency Solutions Group, Oil Budget Calculator Science and Engineering Team, Oil Budget
Calculator: Deepwater Horizon-Technical Documentation, November 2010.
42 See e.g., NOAA Administrator’s Press Release discussing the findings from the Pelican research vessel, which
documented irregular dissolved oxygen levels. “Media reports related to the research work conducted aboard the R/V
Pelican included information that was misleading, premature and, in some cases, inaccurate.” May 17, 2010 press
release at http://www.restorethegulf.gov/release/2010/05/17/statement-noaa-administrator-jane-lubchenco-ongoing-
efforts-monitor-subsea-impact.
43 Robert Haddad and Steven Murawski (NOAA), Analysis of Hydrocarbons in Samples Provided from the Cruise of
the R/V WEATHERBIRD II, May 23-26, 2010, released June 8, 2010, at http://www.noaanews.noaa.gov/stories2010/
PDFs/noaa_weatherbird_analysis.pdf
44 Characteristics of the plume material have been described in different ways. According to the Woods Hole team
(Interview with Richard Camilli and Christopher Reddy): “visually, these water samples look like clean springwater
and have no smell.” However, a different research team extracted samples from a suspected plume in the same vicinity
that did have an oil sheen and an oily odor (Samantha Joye, “Trust Your Senses,” Gulf Oil Blog, University of Georgia,
May 30, 2010, at http://gulfblog.uga.edu.)
45 Richard Camilli et al., “Tracking Hydrocarbon Plume Transport and Biodegradation at Deepwater Horizon,” Science
Express, August 19, 2010.
46 Interview with Richard Camilli and Christopher Reddy (Woods Hole Oceanographic Institution), Science Magazine
podcast, August 20, 2010.
47 See Joint Analysis Group website, at http://ecowatch.ncddc.noaa.gov/JAG/index.html.
48 Joint Analysis Group (JAG), Review of Preliminary Data to Examine Oxygen Levels In the Vicinity of MC252#1
May 8 to August 9, 2010, at http://ecowatch.ncddc.noaa.gov/JAG/index.html.
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Biodegradation
Several scientific studies have looked into the role that microbial organisms are playing in
biodegrading the oil remaining in the Gulf waters.49 The Gulf’s natural oil seeps, which have
existed for eons, support a community of oil-eating bacteria, which have possibly increased in
response to the Gulf spill.
Natural Seeps
As illustrated in Figure 7, the majority of oil in U.S. waters comes from natural seeps—geologic openings on the
ocean floor. Wel -known natural seeps are found in the Gulf of Mexico and off the coast of southern California,
regions with extensive oil exploration and production. Although the seeps release large volumes of oil each year, the
surrounding ecosystem can adapt, and even thrive, because the rate of release is relatively slow.50 A 2003 study
estimated that the natural seep volume in North American waters ranges from 24 million to 71 million gallons each
year.51
Figure 7. Estimates of Percentage Contribution of Oil into North American
Coastal Waters, by Major Source Categories
Based on Average Annual Releases, 1990-1999
Oil
Consumption
33%
Natural Seeps
62%
Oil
Transportation
4%
Oil Extraction
1%
Source: Prepared by CRS with data from the National Research Council (NRC) of the National Academies of
Science, 2003, Oil in the Sea III: Inputs, Fates, and Effects, p. 69.
Notes: Extraction includes platform spills, produced waters, and atmospheric deposition. Transportation
includes spills from tanker/barges, pipelines, coastal facilities, and atmospheric deposition. Oil consumption
includes river and urban runoff; oil spills from cargo ships; operational discharges from commercial vessels and
recreational craft; and atmospheric deposition. For further details of these inputs, see NRC Report, 2003.
49 See National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, Testimony during
September 27-28 Meeting (Panel II); Terry C. Hazen et al., “Deep-Sea Oil Plume Enriches Indigenous Oil-Degrading
Bacteria,” Science Express, August 24, 2010; Richard Camilli et al., “Tracking Hydrocarbon Plume Transport and
Biodegradation at Deepwater Horizon,” Science Express, August 19, 2010; Joint Analysis Group, Review of
Preliminary Data to Examine Oxygen Levels in the Vicinity of MC252#1 (May 8 to August 9, 2010), August 16, 2010.
50 NRC report, 2003, p. 2.
51 The “best estimate” (included in Figure 7) is 47 million gallons (ibid., p. 69).
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However, some scientists have questioned the magnitude of the microbe contribution.52 In the
paper from the Woods Hole research team (cited above), the authors surmised that the plume had
“persisted for months without substantial biodegradation.” A subsequent paper published
evidence that suggested otherwise. A team largely composed of scientists from the Lawrence
Berkley National Laboratory documented results indicating:
that a variety of hydrocarbon-degrading populations exist in the deep-sea plume [the same
plume studied by the Woods Hole team] and that the microbial communities appear to be
undergoing rapid dynamic adaption in response to oil contamination. These results also
imply that there exists a potential for intrinsic bioremediation of oil contaminants in the
deep-sea, and that oil-degrading communities could play a significant role in controlling the
ultimate fates of hydrocarbons in the Gulf.53
Sea Floor
Although oil spilled or released at a subsea location generally rises to the surface, some fraction
of the oil could instead sink, after mixing with suspended solids in the water column. 54 Studies
regarding the fate of the 1979 Ixtoc55 oil spill indicate that a substantial portion—25%, according
to one estimate—of the oil settled to the bottom of the Gulf.
Some sampling of sea floor sediments suggests that some of the oil from the 2010 Gulf spill may
be resting on the ocean floor. For example, in early September, a university research vessel
extracted sediment samples near the well site that had a layer of oil on the top sediment layer.
Although the researchers could not definitively link the oil to the spill, according to their initial
observations, the oil layer did not resemble oily deposits from natural seeps.56 However, the
November Technical Document appears to discount the September findings, stating that “ongoing
sampling programs have not confirmed the claim of extensive oil being present on the bottom of
the Gulf from Deepwater Horizon.”57
52 See Ian R. MacDonald, (Florida State University), National Commission on the BP Deepwater Horizon Oil Spill and
Offshore Drilling, Testimony during September 27-28 Meeting (Panel II).
53 Terry C. Hazen et al., “Deep-Sea Oil Plume Enriches Indigenous Oil-Degrading Bacteria,” Science Express, August
24, 2010.
54 Ian R. MacDonald (Professor of Oceanography, Florida State University), Testimony before the National
Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, September 27, 2010.
55 On June 3, 1979, the Ixtoc I exploratory well in the Bay of Campeche, Mexico, in 167 feet of water, blew out. A
resulting explosion and fire destroyed the platform, which sank to the seafloor and damaged the drilling pipe and well
casing. This incident led to the largest oil spill in North American waters (at that time): an estimated 140 million
gallons (more than 3 million barrels). The release was ultimately contained—290 days later—by pumping mud into
geologically linked wells, a strategy similar to the relief well approach employed in the 2010 Gulf oil spill. See Arne
Jamelov and Olof Linden. “Ixtoc I: A Case Study of the World’s Largest Oil Spill.” Ambio, v. 10, no. 6, 1981, p. 299-
306.
56 See Samantha Joye, Gulf Oil Blog, University of Georgia, Department of Marine Sciences, September 5, 2010, at
http://gulfblog.uga.edu/2010/09/focusing-in-on-oil/.
57 Oil Budget Technical Document, p. 9.
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The federal government established a “Sub-Sea and Sub-Surface Monitoring Strategy” in
August.58 This strategy involved sampling in nearshore, offshore, and deepwater environments.
Scientists completed the sampling plans in late October. As of the date of this report, the
Operational Science Advisory Team (OSAT) is analyzing the results.59
Conclusions
A complete and definitive answer to the question of the remaining oil is unknown at this juncture.
The most recent authoritative assessment—the November Oil Budget document—contained
estimates based on calculations made in July 2010. At the time of these calculations, a substantial
portion of the oil had been effectively removed from the Gulf environment through human
interaction. However, a greater portion remained, in some form, in the Gulf. Months later, it is
unknown what happened to the oil that remained.
It is debatable whether the fate of the remaining oil will ever be established conclusively.
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 addressed through an incremental process.60 Researchers are
continuing to study various components of the Gulf, specifically damages to natural resources.
Some of these efforts may provide clues to the oil’s fate.
58 The plan is available here: http://www.restorethegulf.gov/sites/default/files/documents/pdf/
13_NOV_2010_SMU_Strategic_Plan.pdf. For further discussion of the response framework see CRS Report RL33705,
Oil Spills in U.S. Coastal Waters: Background and Governance, by Jonathan L. Ramseur.
59 Unified Command website (“Restore the Gulf”), “Subsurface Monitoring Unit strategy document available,” Gulf
Science Monitor Newsletter, November 18, 2010, at http://www.restorethegulf.gov/sites/default/files/documents/pdf/
GulfScienceMonitor_Issue7-November182010.pdf.
60 National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, The Amount and Fate of Oil,
Staff Working Paper, released October 6, 2010, p. 24.
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Appendix. Selected Satellite Images of Deepwater
Horizon Oil Spill
Figure A-1. NOAA Satellite Analysis of Gulf Oil Spill
June 15, 2010
Source: National Oceanic and Atmospheric Administration, GeoPlatform.gov, at
http://www.geoplatform.gov/gulfresponse.
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Figure A-2. NOAA Satellite Analysis of Gulf Oil Spill
July 15, 2010
Source: National Oceanic and Atmospheric Administration, GeoPlatform.gov, at
http://www.geoplatform.gov/gulfresponse.
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Figure A-3. NOAA Satellite Analysis of Gulf Oil Spill
August 15, 2010
Source: National Oceanic and Atmospheric Administration, GeoPlatform.gov, at
http://www.geoplatform.gov/gulfresponse.
Author Contact Information
Jonathan L. Ramseur
Specialist in Environmental Policy
jramseur@crs.loc.gov, 7-7919
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