Fingerprinting Hydrocarbons in the Biological Resources of the Exxon Valdez Spill Area
Authors: Bence AE , Burns WA 
Published: March 2009

A procedure has been developed that discriminates Exxon Valdez crude from other sources of hydrocarbons found in Prince William Sound and the Gulf of Alaska. The procedure uses polycyclic aromatic hydrocarbon (PAH) distributions, measured by gas chromatography/mass spectrometry (GC/MS), to fingerprint sample extracts. The relative abundances of alkylated phenanthrenes, dibenzothiophenes, and chrysenes are used to differentiate Exxon Valdez crude and its weathering products from other hydrocarbons. Saturate fraction distributions are used to confirm the PAH identification whenever possible.

The procedure has been applied to the more than 1 500 PAH analyses of tissues reported by the Oil Spill Health Task Force, formed after the spill to assess subsistence food safety, and nearly 4 700 PAH analyses of biological samples in PWSOIL, the government's damage-assessment chemistry database. These two datasets constitute the largest collection of hydrocarbon analyses of biological samples from the spill-impact zone.

Excluding shellfish, only a small fraction of the samples contain recognizable Exxon Valdez crude residues, and most of those are samples that were collected in 1989. Only rarely is Exxon Valdez crude identified in samples collected in 1990, and never in 1991 samples. The majority of samples containing Exxon Valdez crude residues are associated either with external surfaces (e.g., eggshells, skin, hair, carcasses) or with the gastrointestinal tract (e.g., stomach contents, intestines). Many of these samples contain diesel in various stages of weathering, as do sediment samples also archived in the database. For example, diesel is observed on approximately one-half of the bald eagle and common murre eggshells collected in both 1989 and 1990. Rarely is Exxon Valdez crude seen in internal tissues (e.g., liver) and never in body fluids (e.g., blood, milk), because of lack of exposure, lack of uptake, or metabolic effects. Low concentrations of low-molecular-weight PAH compounds, the source(s) of which cannot be identified, are present in some internal tissues and fluids at comparable levels in samples collected from both within the spill path and without. This suggests a source other than the spill. Additionally, more than 60% of the fish and wildlife samples in PWSOIL contain a distinct nonpetrogenic fingerprint, an artifact of laboratory analytical and reporting procedures. This fingerprint consists of 25 specific PAH compounds that were required to be reported, without corrections for baseline or detection limit, regardless of their concentration.

Mussels and clams from some heavily oiled shorelines contained high levels of Exxon Valdez crude in 1989. Subsequently, these levels dropped by approximately an order of magnitude annually, and many had reached or were approaching background by 1991. This is consistent with the sharply decreasing oiling levels detected during shoreline surveys in 1989 through 1992.

This empirical analysis shows that PAH fingerprinting is highly diagnostic for samples of external surfaces where selective uptake and metabolism are not issues. It also appears applicable to samples from the gastrointestinal tract. For external surfaces and gastrointestinal tract samples, it has a major advantage over other techniques in that it can quantitatively differentiate among multiple sources of hydrocarbons having generally similar compositions. Although the method is less diagnostic for internal tissues and fluids, it does offer a way to compare spill-path and non-spill-path sites, and it clearly identifies fingerprints that are artifacts of the analytical procedure.

Results show that the exposure of biota to spill oil in 1989 was restricted to the spill path where it was sporadically distributed. Very little evidence of Exxon Valdez crude was seen in 1990. Consequently, these data do not indicate widespread or persistent biologic exposure to Exxon Valdez crude.