How much oil is spilling in to the Gulf of Mexico? And, yes, it does matter.

Soon after the explosion of the Deepwater Horizon rig, the coast guard and BP estimated that 42,000 gallons (1000 barrels) were leaking from the well each day into the Gulf of Mexico.  Within a week this estimate was revised by the government to 210,000 gallons (5,000 barrels) a day.

This round estimate of 210,000 gallons has stuck – accepted with certainty by the federal government - despite multiple independent estimates that suggest the spill is substantially larger.

Oil Slick Size

Initial estimates were based on oil slick size, following the Bonn Convention method.  However, independent estimates – also based on the size of the oil slick – questioned the widely reported value of 210,000 gallons per day. 

In a statement to the press, Dr. Robert Howarth of Cornell University pointed out that the government’s figure rested at the lower bound of potential spillage rates using oil-slick size as an estimator.  His own estimate ranged from 170,000 gallons to 630,000 gallons (4047 to 15,000 barrels) per day.

Using satellite imagery of the oil slick and thickness estimates from visual descriptions, John Amos of Sky Truth and Dr. Ian MacDonald of Florida State University estimated a minimum average flow rate of 1.1 million gallons (26,500 barrels) per day – five times the original estimate.  Differences between their calculations and the governments can not be identified because the government’s computation has not been made public.

In truth, it appears that oil-slick size is not the best estimator given the depth of the spill, the heavy use of dispersants during the response, and the evidence that a substantial amount of oil is remaining below the surface (quite possibly the result of physical dispersion of the hot liquid during entry into the cold, high pressured deep waters), resulting in a likelihood of underestimating the spillage rates.

Other Available Methods

Experts point out that a range of alternative, and more suitable, methodologies exist.  They include direct physical measures of velocity with flow meters (this would, admittedly, be challenging), acoustic methods, and visual analysis of particle flow from the leaking pipe (particle image velocimetry).

Immediately following the release of footage of the leak, a number of updated measures were provided by independent scientists.  According to an NPR story, Steven Wereley, an associate professor of mechanical engineering at Purdue University, conducted visual analysis of the leaking pipe estimating between 2 and 3.5 million gallons (56,000 barrels to 84,000 barrels) per day.  According to NPR other scientists employing similar methods also generated estimates in the millions of gallons (or 10,000s of barrels) per day.

 Why the size of the spill matters

 BP and the federal government repeatedly argue that measuring the volume of oil entering the Gulf is not a priority, that a more precise measure of flow rate is not important.  Their rationale is that it would not influence the response in any way.

We disagree for a number of reasons:

  1. Scale. The flow rate estimates differ by a factor of ten.  Differences on this scale are not quibbles; they are big, fundamental differences. 
  2. Response. The discrepancy is sufficiently large enough to influence response strategies.  For example, to promote the efficacy of dispersants, they are applied at a specific ratio to the volume of oil.  This is not possible if the volume is unknown, by this large of a degree.  In addition, the ability to successfully cap the well, engineer a dome, or pump the oil to the surface depends on a good estimate of the oil flow rate (both in terms of volume of oil and the force with which it is exiting the pipe).
  3. Law. Under the Oil Pollution Act of 1990, a natural resource damage assessment (NRDA) must be conducted.  This entails assessing the input of oil, its fate (i.e., where it goes, what it coats and contaminates), and the damage it caused.  The ability to fully conduct this accounting – or ‘mass balance’ - requires knowing the initial volume of oil.
  4. Financial Penalty. Following discharge of oil into a water body, the federal Clean Water Act allows for a civil penalty of up to $1,000 per barrel of oil spilled. This penalty can not be calculated to its fullest extent without knowing the total volume of oil.
  5. Future emergency plans. Knowing the magnitude of this spill is necessary to inform future emergency response plans.  Substantial underestimates of the volume of oil leaking from Deepwater Horizon will leave us unprepared in the future.

There are multiple reasons why BP may not want the true amount of oil to be known.  Just take for example, the $1000 per barrel of oil spill civil penalty under the Clean Water Act.  Using the “official” number of 5,000 barrels per day, their current tally is $140 million (and counting).  Using some of the higher estimates provided by visual analysis of the leaking pipe, BP’s current tally is in the billions (and counting).

The bigger mystery is why the federal government is sitting on the sidelines.  Why has the federal government been reluctant - and so slow - to undertake its own assessment of the size of the spill, particularly given the available expertise and alternative methods? 

About the Authors

Lisa Suatoni

Senior Scientist, Oceans program

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