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Oil Remaining in the Environment
How much oil from the Exxon Valdez spill remains on the shorelines of Prince William Sound? 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 (as reflected in the graph below).
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Observed maximum surface oiling at eight NOAA study sites in Prince William Sound, 1989-1997. Values in mean percent cover. Apparent increase in 1991 is attributable to unearthing of buried oil by heavy equipment, "berm relocation," used as a remedial technique that year.
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However, oily vestiges of the spill can still be found on some beaches. Some places are still contaminated enough to raise the concerns of residents in Prince William Sound and the decision-makers who oversee spill restoration activities. In the National Oceanic and Atmospheric Administration (NOAA) monitoring program, we see a wide range of oiling conditions at our study sites:
- No oil can be seen or measured on some beaches, either on the surface or below.
- At other beaches, we can find 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 sheen on the water as the tide flushes back in.
Difficult Questions
One of the goals of the NOAA program is to determine whether the residual oil at our study sites and elsewhere is causing environmental harm to resident organisms, 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?
There have been reports both in the news and in scientific journals that not all of the oil found in Prince William Sound can be traced back to the Exxon Valdez. This is not surprising. There are many potential alternate sources of hydrocarbons in the marine environment, even in a region we consider to be relatively unpolluted:
- Some of these are natural, such as undersea oil seeps and forest fires.
- Others are definitely of human origin, such as the rupturing of oil storage tanks during the Alaskan earthquake, the pumping of ship bilges, and fuel leakage from commercial and recreational vessels traveling through the area.
In our monitoring program, chemists "fingerprint" hydrocarbon residues in both beach sediments and in animal tissues, and we find that not everything we measure looks like oil spilled from the Exxon Valdez. In fact, some of the highest concentrations of oil in mussel tissues recently have come from small boat harbors and diesel fuel. But there is little doubt that most of the oil contamination we find at our study sites does trace back to the Exxon Valdez.
More Information about Oil in the Sound
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- How Toxic Is Oil? There are many kinds of oil, and each is a complex mixture of chemicals. So how can oil toxicity be assessed?
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- What Is Weathering? Here's an explanation of how oil weathers and how the degree of weathering of oil samples can be determined.
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Other Pages in this Series
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