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Lingering Oil: Continued
All of the subsurface oil fingerprinted back to the source oil of the Exxon Valdez. Slightly weathered, the lightest fraction of aromatic hydrocarbons (single ring compounds like benzene and toluene) was missing, but most of the 2-4 ring polycyclic aromatic hydrocarbons (PAH) were intact and therefore toxic, and in the same proportions as Exxon Valdez oil collected in the first weeks of the spill.
In the weeks following the spill, oil often lay in some of the semi-enclosed bays for days to weeks, going up and down with the tides twice a day. With the daily stranding of the oil in the intertidal zone, some was pulled down into the sediments by the capillary action of the fine sediments beneath the coarse cobbles. The cleanup efforts and natural processes, particularly in the winter, cleaned the oil out of the top 2-3 inches, where oxygen and water can flow, but did little to affect the large patches of oil farther below the surface.
The lower half of the intertidal zone is the biologically-rich area where mussels, clams and other marine life are found in greatest abundance. This raised the question of bioavailability – were animals such as sea otters and harlequin ducks who feed in the intertidal, as well as the species that reside there, being chronically exposed to toxic PAH? In 1996-1998, the Nearshore Vertebrate Project investigated why the populations of several species on Northern Knight Island, which had been heavily oiled, were not recovering. Contrary to anticipated results, food availability was not a limiting factor. Instead, various vertebrate species showed elevated P450 levels compared to non-oiled areas; elevated levels of the enzyme P450 can be induced through exposure to oil. A series of studies in 2004, using passive samplers, also demonstrated that subsurface oil patches still leaked PAH and stimulated a P450 response in fish. Harlequins ducks have continued to show elevated levels through 2007. The elevated levels of P450 would have diminished following initial exposure to oil. Therefore, continuing elevated levels of P450 aren’t attributable to the initial impacts of the Spill, but indicate a continuing exposure to oil.