Prince William's Oily Mess: A Tale of Recovery

How Much Oil Remains?

What was the ultimate fate of the 10.8 million gallons of oil released from the Exxon Valdez?

The ultimate fate of the 10.8 billion gallons of oil that leaked from the Exxon Valdez. NOAA scientists estimate that 2% of the oil remains on the beaches.

Nobody knows for sure, but based on the areas that were studied in the aftermath of the spill, scientists made estimates of the ultimate fate of the oil. A 1992 National Oceanic and Atmospheric Administration (NOAA) study provided some insight, estimating that the great majority of the oil photolysed in the atmosphere, dispersed into the water column or degraded naturally (biodegraded by microorganisms or photolysed in the water). Cleanup crews recovered about 14 percent of the oil, and approximately 13 percent sank to the sea floor. About 2 percent (some 216,000 gallons) remained on the beaches.

Whodunit? Fingerprinting Oil

Considering that nearly 11 million gallons escaped from the tanker, and that large quantities eventually fouled shorelines in the sound and elsewhere, very little remains. The graph below shows actual measurements made by NOAA scientists at their study sites in Prince William Sound from 1989 to 1997.

 

The observed maximum percent of the surface covered by oil at eight NOAA study sites in Prince William Sound, 1989-1997. Values are mean percent cover of oil. Are the data what you expected? What data point on the graph seems out of place?

Residual oil below the surface of a beach. In some places in Prince William Sound the remaining oil is only evident below the surface. (Photo credit: OR&R, NOAA)

At the sites being studied by scientists, surface oil had all but disappeared by 1992, three years after the spill. The apparent increase in surface oiling in 1991 (two years after the spill) was likely to have been caused by heavy equipment digging up buried oil (called "berm relocation"), which was used as a remedial technique that year.

However, oily traces of the spill can still be found on some beaches. The remaining oil generally lies below the surface of the beaches in places that are very sheltered from the actions of wind and waves (which help to break down and remove stranded oil), and on beaches where oil initially penetrated very deeply and was not removed. At these beaches, there are signs of weathered oil on the surface and deposits of fresher oil buried beneath. Sometimes this oil makes its way to the surface and can be seen as a sheen on the water as the tide comes in. Interestingly, despite the fresh appearance of oil at these sites, chemical analysis and biological observations indicate that the oil is actually of such low toxicity that many intertidal organisms can tolerate its presence, even though it can accumulate in their tissues.

Residual oil on a cobble beach on Smith Island, Prince William Sound, Alaska in 1997. Here, large volumes of oil have penetrated so deeply into this beach that substantial quantities continue to leach out. Oil sheens were observed at this site in undisturbed tide pool water. (Photo credit: OR&R, NOAA)

One of the scientists' goals is to determine whether this residual oil is causing environmental harm to organisms living there, since one of the most difficult questions to answer during any oil spill is, "How clean is clean?" That is, when does cleanup begin to cause more harm than simply leaving the oil in place to degrade naturally?

In addition, reports both in the news and in scientific journals have stated that not all of the oil found in Prince William Sound can be traced back to the Exxon Valdez. This is not surprising. Many potential alternate sources of hydrocarbons exist in the marine environment, even in a region that is relatively unpolluted. As examples:

Some of the hydrocarbons are natural, coming from undersea oil seeps or forest fires.

Others are definitely of human origin, such as the rupturing of oil storage tanks during the Alaskan earthquake of 1964, the pumping of ship ballast tanks, and fuel leakage from commercial ships and recreational boats traveling through the area.

Scientists dig for subsurface, residual oil on a beach in Prince William Sound. (Photo credit: OR&R, NOAA)

Chemists who  "fingerprinted" hydrocarbon residues in both beach sediments and in animal tissues found that not all of the oil came from the Exxon Valdez. More recently, the highest concentrations of oil in mussel tissues have come from small boat harbors and diesel fuel. However, scientists hypothesize that most of the oil contamination found in Prince William Sound does trace back to the Exxon Valdez.