The natural petroleum hydrocarbon background in subtidal sediments of Prince William Sound, Alaska

A natural regional petroleum hydrocarbon background has been identified in the subtidal sediments of Prince William Sound. This hydrocarbon background, which is readily distinguished from Exxon Valdez spill oil by advanced hydrocarbon fingerprinting methods, is derived from petroleum seeps in the eastern Gulf of Alaska. The Alaska Coastal Current carries fine-grained sediments and associated hydrocarbons into the Sound where they are deposited. This activity has been ongoing for the past 160 years and probably for many thousands of years. Results of a stratified random study (SRS) of nearshore subtidal sediments conducted in 1990 and a fixed site study conducted in 1989, 1990, and 1991 show that this background is significant even in shallow water (3--30 m) and that it is a general phenomenon of the Sound. Polycyclic aromatic hydrocarbon (PAH) mixing model calculations show that, where present in nearshore subtidal sediments, residues of Exxon Valdez spill oil form a small increment on the natural background. The recognition of pre-existing natural and anthropogenic hydrocarbon backgrounds in a spill area is a fundamentally important component of natural resource damage assessment