Often, there are no witnesses, but hydrocarbon chemist Dave Shaw at the University of Alaska Fairbanks says that doesn't mean the polluter won't get caught. He says oil, just like a person's DNA or fingerprints, has a unique signature.
Petroleum is a mixture of many different chemical substances.
Those things arise deep in the earth in processes in which oil is formed and they vary for every oil. Shaw uses a sophisticated technique called gas chromatography to separate and identify the thousands of complex chemical compounds found in oil. The result is called a trace, and it looks very much like a grocery store barcode. When you've got these traces that have all the different peaks, all the individual compounds from the oil, they serve very much like a fingerprint, and it's possible to compare two different oils and see whether they are the same or different. The Coast Guard's National Oil Identification Lab in Groton, Connecticut, uses gas chromatography traces to identify oil polluters.