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What Is Weathering?
Oil is a mixture of many different chemicals. Not only does each of these have its own toxicity, but each also behaves differently in the environment.
Some components are much more volatile than others, and so they tend to evaporate more rapidly when oil is spilled:
- Some are more easily broken down by microbes on the beaches.
- Sunlight can also degrade oil components.
The sum of these physical and biological processes results in what we call "weathering" of the oil, reflected in the changes in chemical composition of oil residues over time. The more closely the chemical composition of a residue resembles that of the unspilled oil, the "fresher" it is.
In Prince William Sound, our chemists tell us that the remaining Exxon Valdez oil ranges from very weathered to relatively fresh.
Here are graphs showing the chemical composition of the fresh Exxon Valdez crude oil and two different oil residues found at our study sites. One residue sample, which bears little resemblance to the original oil, shows a highly weathered material. The other, still quite different from the cargo oil, nonetheless retains more of the compounds found in the original product and so is considered as less weathered.
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Differences in composition of oil from the Exxon Valdez tanker, and surface and subsurface residues found in 1994. The subsurface oil, buried and thus protected from many weathering processes, resembles the original oil much more than surface residue.
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What does this mean? In general, we have found that the more exposed to the elements the oil is, the more rapidly it weathers. Accordingly, the least weathered oil we can still find is the subsurface, or buried residues.
Fingerprinting Oil
The chemical composition of oil found in the environment also yields important clues about its origin. The process of determining where a hydrocarbon residue originated is what we call "fingerprinting." Source fingerprinting is a complex procedure that is part art and part science, relying both on the experience of the analytical chemist and on the results of ratios between certain discriminating hydrocarbon components in a mix. Similar to the literal uses of fingerprinting, experienced chemists can analyze the evidence left at a "crime scene" (spill site) to make a reasonable determination of "whodunit"--that is, if a hydrocarbon residue is in fact oil, where it might have originated, and, hopefully, who spilled it. The U.S. Coast Guard uses such forensic methods to determine a responsible party when an oil spill with no known source washes up on a shoreline.
Most of the hydrocarbon residue we find at our Prince William Sound study sites originated with the Exxon Valdez spill. Other potential sources we have identified include diesel fuel, non-specific combustion sources that could include anything from wood stoves to vessel exhaust, and spilled oil not linked to the Exxon Valdez.
Other Pages in this Series
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