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Lingering Oil
Oil Remains: The Persistence, Toxicity, and Impact of Exxon Valdez Oil
Visitors today experience the spectacular scenery and wildlife of Prince William Sound and the North Gulf of Alaska. However, one of the most stunning revelations of Trustee Council-funded monitoring over the last ten years is that Exxon Valdez oil persists in the environment and in places, is nearly as toxic as it was the first few weeks after the spill.
This was not expected at the time of the spill or even ten years later. In 1999, beaches in the sound appeared clean on the surface. Some subsurface oil had been reported in a few places, but it was expected to decrease over time and most importantly, to have lost its toxicity due to weathering. A few species were not recovering at the expected rate in some areas, but continuing exposure to oil was not suspected as the primary cause.
In 2001, researchers at the Auke Bay Laboratories, NOAA Fisheries, conducted a survey of the mid-to-upper intertidal in areas of the sound that were heavily or moderately oiled in 1989. Researchers dug over 9,000 pits, at 91 sites, over a 95-day field season. Over half the sites were contaminated with Exxon Valdez oil. Oil was found at different levels of intensity from light sheening; to oil droplets; to heavy oil where the pit would literally fill with oil. They estimated that approximately 16,000 gallons (60,000 liters), of oil remained. The survey also showed a trend of an increasing number of oiled pits as they surveyed lower into the intertidal zone, indicating that there was more oil to be found lower down the beach. In 2003, additional surveys determined that while the majority of subsurface oil was in the mid-intertidal, a significant amount was also in the lower intertidal. The revised estimate of oil was now more than 21,000 gallons (80,000 liters). Additional surveys outside Prince William Sound have documented lingering oil also on the Kenai Peninsula and the Katmai coast, over 450 miles away.
The amount of Exxon Valdez oil remaining substantially exceeds the sum total of all previous oil pollution on beaches in Prince William Sound, including oil spilled during the 1964 earthquake. This Exxon Valdez oil is decreasing at a rate of 0-4% per year, with only a 5% chance that the rate is as high as 4%. At this rate, the remaining oil will take decades and possibly centuries to disappear entirely.
The amount of Exxon Valdez oil remaining substantially exceeds the sum total of all previous oil pollution on beaches in PWS, including oil spilled during the 1964 earthquake. This Exxon Valdez oil is decreasing at a rate of 0-4% per year, with only a 5% chance that the rate is as high as 4%. At this rate, the remaining oil will take decades and possibly centuries to disappear entirely.
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.