Breaking down the science in the government's "oil budget" report on the Gulf

Valdez Fate_figure 3.bmp

Last week, the federal government released a report on the oil budget of the 2010 Gulf oil spill.  After reviewing it closely, there were a few things that stood out about the government’s analysis and the ‘roll-out’.

1. The statement that 75% of the oil has been ‘taken care of by mother nature’ - with the implication that it is no longer in the marine environment - is an overinterpretation of the data (and misleading). 

    After approximately 100 days, about one half of the oil had been removed from the marine environment by burning, direct recovery, and evaporation (note: that one bin combines evaporation and dissolution making the ‘removed’ oil slightly less than 50%).  However, approximately one half of the oil - the ‘dispersed’ and ‘residual’ fractions - may still be in the environment.  This is a lot of oil (over 100 million gallons -- equivalent to nine Exxon Valdez spills).  Given the lack of information about the rate of biodegredation, it is not clear what fraction of the oil remains in the ocean.  It is not justified to assume that the ‘dispersed’ fraction is ‘processed’ or gone.  Rates of biodegredation need to be measured in the environment at multiple sites and over time (the rates may be non-linear and change over time).     

    2. The oil budget in the report was a partial tally of the hydrocarbons released into the marine environment -- it did not include methane.  And, the oil budget itself was incomplete and lacking important information.

      One-half of the hydrocarbons released into the environment from the well (by weight) were in the form of methane gas.  These were not included in the budget, yet have important environmental implications.  Like oil, methane can be consumed by microbes that require oxygen; population explosions of these microbes can cause depletions in dissolved oxygen concentrations, which can stress or harm marine organisms.

      In addition, a number of environmental ‘compartments’ used to identify the different phases and fates of the oil, such as the oil ‘slick’, oil on the ‘coast’, oil on the ‘seafloor’, and ‘biodegraded’ oil,  were left un-estimated (see comparisions to oil-budgets from other spills).  Estimations for the volume of oil that passes through these various phases, or compartments, are important to developing a full understanding of impacts.  We hope that the government fills-in the rest of this story and continues to study the oil in transit, the various weathering patterns, and the fate of all of the hydrocarbons so that a fair and full assesmment of the impacts is conducted.

      Fate of Oil


      (> 1 year)1

      Exxon   Valdez        

      (100  days)2

      Deepwater Horizon   

      (≈100 days)3








      25% *





      Bio-chemically degraded




      On the coast








      % accounted for




      1. Jernelov A. and O. Linden.  1981.  Ixtoc. I: A case study of the world's largest oil spill, Ambio 10  299.

      2. taken from Fig. 3 in Wolfe, D. et al., 1994 The fate of the oil spilled from the Exxon Valdez.  Env. Sci. Technology (28) 13 561.

      3. Lubchenco J. et al., 2010 BP Deepwater Horizon Oil Budget:  What happened to the oil?

      * In the Deepwater oil budget report, this category was evaporated and dissolved. 

      3. The released oil budget represents a ‘snapshot’ of the oil, in a moment in time.  It does not directly address where the oil has been, where it is going, and how long it will remain in the system (or, importantly, the ecological impacts throughout).

        To fully understand the impacts of the oil one would, ideally, estimate the transit and fate of the oil, for the duration of the contamination.  Take, for example, the oil budget outlined by Wolfe and colleagues (1994) following the Exxon Valdez spill.  It shows that for the first ten days, the oil was mainly in the form of a slick (identified as “floating” in the figure below), encountering wildlife and impacting primarily bird and mammal populations.  Between day ‘10’ and day ‘100’, much of the oil had evaporated, dispersed, or was ‘beached’, thus entering a phase of coastal and open-water exposure.  By day 1000, most (≈ 85%) of the oil was removed from the environment.  However, 15% lingered in sediments in the coast and seafloor, continuing to contaminate the environment and organisms for the ensuing decades.  It’s important to note that a small proportion of the oil is recalcitrant and generally remains in the environment for decades. 


        Wolfe, D. et al., 1994 The fate of the oil spilled from the Exxon Valdez.  Env. Sci. Technology (28) 13 561.

        4. On the plus side, a relatively high percentage of the oil - 25% - was kept out of the Gulf or recovered in some fashion (through siphoning, skimming, and burning).

          This relatively high ‘recovery rate’ is largely because of the ability to siphon oil from the wellhead (17%) before it got into the water.  The small fraction of oil burned or recovered once it hit the water (5% and 3% respectively) is a sober reminder that once the oil is released into the environment, it is very difficult to clean up.

          5. The fraction of oil that was sucessfully chemically dispersed was arguably a modest percentage of the total (8%) given the unprecedented use of chemical dispersants. 

            A bound of uncertainty around this estimated value should be determined and that range of values should be factored into a thorough cost/benefit analysis of whether the extensive use of chemical dispersants was a good idea.  Important outstanding questions include, how chemical dispersants affected biodegredation rates, whether chemical dispersion increased the biological uptake of the oil, the potential bioaccumulation of the chemical dispersants, the health impacts of chemical dispersants to oil spill responders.

            6. The degree of precision associated with the estimated percentages for the fate of the oil was not addressed well in the report. 

              Typically oil that is recovered, burned, or skimmed is directly measureable, so estimates are likely to be accurate to within ± 10%.  The same is true for estimations of evaporation.  However, due to the combined challenges of measuring subsurface oil and natural spatial variability in the ocean, the estimation of the fraction of dispersed is much less precise and only accurate to within factors of two or more (see Dr. Jeffrey Short’s discussion of this).  The current estimate of 25% in this fraction should be regarded with significantly less certainty. 

              7. The oil budget is just that - an oil budget; it does not speak to environmental harm. 

                Oil budgets are an important exercise, but they are just a first step to determining the ecological harm caused by the oil contamination.  In order to truly understand the full scope of the impacts of the spill on the Gulf of Mexico, government and independent scientists need to research what harm the hydrocarbons and associated contaminants (e.g., metals) have done and are doing to organisms and the communities.  A long-term coordinated research program is needed with sufficient funds to back it up.