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Advanced hydrocarbon fingerprinting methods and improved analytical methods make possible the quantitative discrimination of the multiple sources of hydrocarbons in the benthic sediments of Prince William Sound (PWS) and the Gulf of Alaska. These methods measure an extensive range of polycyclic aromatic hydrocarbons (PAH) at detection levels that are as much as two orders of magnitude lower than those obtained by standard Environmental Protection Agency methods. Nineteen hundred thirty six (1 936) subtidal sediment samples collected in the sound and the eastern Gulf of Alaska in 1989, 1990, and 1991 were analyzed.
Fingerprint analyses of gas chromatography-mass spectrometry data reveal a natural background of petrogenic and biogenic PAH. The petrogenic background is derived largely from oil seeps in the eastern Gulf of Alaska. Age-dated (210pb) sediment cores indicate that significant input of seep hydrocarbons into Prince William Sound has been going on for at least 160 years and probably for thousands of years. Superimposed on this natural background are locally elevated concentrations of anthropogenic hydrocarbons from sources such as diesel fuel and pyrogenic PAH that are found primarily adjacent to active or historical sites of human use. Exxon Valdez crude, its weathering products, and diesel fuel refined from Alaska North Slope crude are readily distinguished from the natural seep petroleum background and from each other because of theirdistinctive PAH distributions.
Mixing models were developed to calculate the PAH contributions from each source to each sediment sample. These calculations show that most of the seafloor in PWS contains no detectable hydrocarbons from the Exxon Valdez spill, although elevated concentrations of PAH from seep sources are widespread. In those areas where they were detected, spill hydrocarbons were generally a small increment to the natural petroleum hydrocarbon background. Low levels of Exxon Valdez crude residue were present in 1989 and again in 1990 in nearshore subtidal sediments off some shorelines that had been heavily oiled. By 1991 these crude residues were heavily degraded and even more sporadically distributed.
Polycyclic aromatic hydrocarbons (PAH), oil seeps, oil fingerprinting, Exxon Valdez, subtidal sediments, petrogenic hydrocarbons, pyrogenic hydrocarbons
Bowdoin College, Brunswick, ME
Exxon Production Research Company, Houston, TX