The chemical composition of oil found
in the environment yields
important clues about where it came from. The process
of determining where an oil residue originated is what
scientists call "fingerprinting” or “source 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 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 residue is, in fact, oil, where it might have originated, and, possibly, who spilled it. For example, the U.S. Coast Guard uses such forensic methods to determine whom the "responsible party" is when an oil spill with no known source washes up on a shoreline.
Oil is a mixture of many different chemicals. Not only does each chemical have its own toxicity, but each also behaves differently in the environment.
The sum of these physical and biological
processes results in what scientists call "weathering" of the oil, which is 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, chemists tell us that the remaining Exxon Valdez oil ranges from very weathered to relatively fresh. What does this mean? In general, scientists have found that the more exposed to the elements the oil is, the more rapidly it weathers. Accordingly, the least weathered oil can still be found under the
surface, or as buried residues of the original oil.