The Science of the Spill
Oil Spill Questions and Answers

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How do the November 23, 2010 oil budget numbers differ from the August 4 estimates?

The information below is extracted from a press release issued by the Deepwater Horizon Incident Joint Information Center on November 23, 2010.

Today’s report provides the technical basis underlying the Calculator’s oil fate estimates used to help respond to the spill. This report, following additional assessment and peer-review, is largely consistent with early results released by the federal government. The most significant change is a doubling of the expected amount of oil classified as “chemically dispersed” — revised from 8% to an estimated 16% with a possible range of between 10% and 29%. Additional data and studies have over the course of the past few months led the oil budget team to relax certain initial conservative assumptions with regard to the effectiveness of dispersant operations. The early estimate of the percentage of "other" (or, "residual") oil was 26%; the current version of the Calculator estimates it as 23%, and qualifies this estimate with the belief that, with high confidence, the true percentage should be between 11% and 30%.

Oil Budget (Released Aug. 4)
Category........................% of Total
Direct Recovery.................17%
Chemically Dispersed.......8%
Naturally Dispersed.........16%
Evaporated or Dissolved...25%

Oil Budget Technical Report
Category......................+_% of Total......Change
Direct Recovery................17%.............None
Chemically Dispersed......16%..........+8%
Naturally Dispersed.........13%...........-3%
Evaporated or Dissolved....23%.........-2%

What happened to the oil? Why don't the reports agree?

The are two analyses of where the oil from the MC252 well has gone:

Summary Findings

NIC - "In summary, it is estimated that burning, skimming and direct recovery from the wellhead removed one quarter (25%) of the oil released from the wellhead. One quarter (25%) of the total oil naturally evaporated or dissolved, and just less than one quarter (24%) was dispersed (either naturally or as a result of operations) as microscopic droplets into Gulf waters. The residual amount — just over one quarter (26%) — is either on or just below the surface as light sheen and weathered tar balls, has washed ashore or been collected from the shore, or is buried in sand and sediments."

GSG - "However, after accounting for oil that has been skimmed and burned (10% collectively), evaporated (8-12%) and degraded (4-8%), we estimate that the oil remaining at or below the surface is between 70 and 79% or between 2.9 and 3.2 million barrels.

We note that this does not account for oil that we know has washed into coastal wetlands. This is a particularly difficult form to quantify, since much of it has settled in tidal creek and bay bottoms or has been buried in salt marsh and creek bottom sediments."

Why don't the two reports agree?

There are three primary reasons for the differences. Additional confusion arises from the common, but flawed, interpretation that the NIC reports states that only 25% of the oil remains. As GSG states, "The news media’s tendency to interpret
“dispersed” and “dissolved” as “gone”is wrong."

Different amounts of oil considered. The two reports consider different total amounts of oil. NIC uses a figure of 4.9 million barrels - the estimate for the total amount of oil released from the well, including the 0.8 million barrels which was recovered to ships. GSG works with a figure of 4.1 million barrels - the amount of oil that was spilled into the Gulf of Mexico. In assessing the effects of the oil, the GSG approach is more appropriate. NIC's use of the larger figure reduces the percentages of each figure describing where the oil went. For example, had NIC based their numbers on the 4.1 million barrel spilled oil figure their residual oil figure would be 31% rather than 25%.

Dispersed oil is not "gone." The NIC reports estimates that 24% of the 4.9 million barrels was dispersed, either naturally or as a result of chemical dispersants. Based on the 4.1 million barrel figure the dispersed oil fraction would be 29%. This dispersed oil is not included in their residual oil figure, even though they state that it is still a hazard in the marine environment - "Until it is biodegraded, naturally or chemically dispersed oil, even in dilute amounts, can be toxic to vulnerable species." Unfortunately, many have interpreted the "residual oil" figure as the net amount of oil remaining from the spill. Such an interpretation is incorrect and misleading.

Considering that the dispersed oil and residual oil remain using the 4.1 million barrel basis, the NIC figures indicate that the remaining oil is 60% of the total spilled. This figure more accurately represents the oil remaining in the marine environment and compares more closely with the GSG range of 70 to 79%.

GSG considered dispersed oil to be part of the residual oil remaining in the environment. However, the GSG report estimates that 4 to 8% of the spilled oil has been degraded and no longer remains as a threat.

Estimates of evaporated and dissolved oil differ. Most of the remaining difference between the two reports is due to differences in the estimates of evaporated and dissolved oil. NIC estimates evaporation and dissolution as 30% of the 4.1 million barrel spilled oil total. GSG estimates evaporation alone at 8 to 12% of the spilled oil. The GSG report points out that oil can evaporate only when it is at the surface of the water and that the distribution of the oil in the 5000-foot deep water column near the wellhead is not well understood. GSG uses a range of evaporation based on a low and high figure for the fraction of the oil that reached the surface.

The NIC estimates lump evaporation and dissolution together. Since dissolved oil remains in the water, it would have been more appropriate to address it separately instead of considering it to be gone from the marine environment like the evaporated oil. The GSG report took this more appropriate approach.

So what's the "right" answer?

Based on the findings of the two reports it is reasonable to estimate that about 2/3 (60% to 79%) of the oil spilled from the MC252 well remains in the marine environment. The amount of residual oil is then 2.4 to 2.9 million barrels. This is approximately 9 to 11 times the size of the Exxon Valdez spill. Note that the lower range does not include the unknown quantity of dissolved oil.

When the Deepwater Horizon rig sank, approximately 700,000 gallons of Diesel fuel were released. It is not known what fraction of that fuel burned immediately or what happened to the remainder.

Note: Due to round-off errors, insignificant differences may appear in summed figures.

Are whale sharks at risk because of the Deepwater Horizon spill?"

Response provided by Dr. Eric Hoffmayer of the Gulf Coast Research Laboratory, September 3, 2010.

The University of Southern Mississippi’s Gulf Coast Research Laboratory Whale Shark Research Program has been collecting information on whale shark sightings in the northern Gulf of Mexico since 2003. Over 350 reports have been collected to date and more than one third of those sightings occurred in the region impacted by the Deepwater Horizon oil spill (see image below).  Given the amount of time whale sharks spend at/near the surface of the water and the fact that they aggregate in large numbers to feed, there is considerable potential for harm or death to individuals from direct exposure to and contamination from the spill (via oiling or clogging of their gills), as well as from depletion of prey, or consumption of oil-contaminated prey. In addition to oil observed at the surface of the water, dispersants currently being used to ‘breakup’ the oil will significantly increase the oil's presence throughout the water column, resulting in exposure to sharks below the surface.  It is unknown if these sharks can sense the oil or the dispersant, however it seems unlikely as aerial surveys observed whale sharks feeding at the surface just 4 miles away from the spill site on June 22, 2010. Unfortunately, because sharks are negatively buoyant, if they succumb to the oil they will sink to the bottom and there will be no record of how many sharks have died or the exact cause of death.

In order to study the impact of the oil on whale sharks in the region, GCRL scientists have been and will continue to make offshore trips to deploy satellite tags on whale sharks. Satellite tagging of whale sharks provides a reliable method to assess the behavioral aspects of their response to the oil spill within an area of the northern GOM identified as whale shark essential fish habitat.  Satellite position only tags (SPOT) provide real-time location data when the sharks are at the surface, and PSAT tags provide archived data on movements and diving behavior. PSAT tags also provide useful data in estimating short-term mortality rates of whale sharks that might swim into affected areas.  The combined use of SPOT and PSAT tags will allow us to estimate the short-term mortality rate of whale sharks that move into the area affected by the oil spill.  Once we have a better understanding of how the whale sharks respond to the oil spill, we will be able to estimate impacts to the population on a regional basis.  

How much oil has been released by the Deepwater Horizon well?

The National Incident Command’s Flow Rate Technical Group (FRTG) was established to make an official assessment of the oil flow rate and total volume. The FRTG was led by United States Geological Survey (USGS) Director Marcia McNutt, and a team of Department of Energy (DOE) scientists and engineers led by Energy Secretary Steven Chu. The FRTG estimates that the flow rate varied from 53,000 to 62,000 barrels per day during the 87-day spill and that a total of 4.9 million barrels of oil had been released from the BP Deepwater Horizon well as of July 15 when the well was capped. It was estimated that approximately 800,000 barrels of oil was captured, so that the total volume of oil released into the Gulf was approximately 4.1 million barrels.

The FRTG estimates have an uncertainty of plus or minus approximately 10 percent.

Reference: NIC press release - U.S. Scientific Teams Refine Estimates of Oil Flow from BPs Well Prior to Capping, August 2, 2010


What is a dispersant? How does it work?

NOAA, the National Oceanic and Atmospheric Administration, has an excellent explanation of dispersants and how they're used. Use the menu in the blue/gray bar to move from page to page.

Dispersants: A Guided Tour

What will happen if a hurricane strikes during an oil spill?

Reference: "Hurricanes and the Oil Spill." National Oceanic and Atmospheric Administration, May 27, 2010. pdf, 1.4 mb

Key points:

  • Experience to date with oil spills and hurricanes is primarily with spills caused by the hurricane, not with spills that existed before the hurricane arrived.
  • An oil slick should have little effect on the development of the hurricane or on wind and waves.
  • Significant oil spills resulted from Hurricane Katrina and Hurricane Rita. The oil released was widely dispersed by the storms.
  • Wind and wave action during a hurricane will break down masses of oil and disperse the oil, accelerating the natural weathering and decomposition of the oil.
  • A hurrican may drive oil inshore, possibly causing contamination of hurricane debris with oil. The risk would be more significant with larger amounts of surface oil close to the shore.
  • There is no danger of oil falling as rain.
 I am still leary of eating shrimp and fish from the gulf. Is anybody getting samples off the floor of the gulf since that’s where the shrimp and other things eat that are eaten by the bigger fish and so on. Also are any tests being run to see if there are any species going up fresh water rivers such as the bull shark that we know can tolerate fresh water. thank you for your time.
We discussed your question with several scientists at the Gulf Coast Research Laboratory.   Seafood for human consumption has been tested throughout the four affected states and the adjacent federal waters of the Gulf.  No toxic levels of any contaminants have been found and the seafood has been deemed safe.  The resources listed below provide some specifics information on testing and results.
Bull sharks can and do run up rivers as do Gulf sturgeon, shad, and mullet.  While these species of fish have not been specifically tested, it is not thought that they would have higher contaminant levels than the species that have been tested.  We see no concern that fish could carry a significant amount of contaminants into the coastal rivers.


National Science Foundation University of Southern MississippiMississippi State UniversityMississippi Public Broadcasting Gulf Coast Research Laboratory Centers for Ocean Sciences Education Excellence, Central Gulf of Mexico (COSEE CGOM)
Copyright © 2010-2012. All rights reserved. This material is based upon work supported by the National Science Foundation under Grant 1048433. Any opinions, findings, and conclusions or recommendations expressed here are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.