"Almost exactly 25 years apart, two catastrophes hit the Prince William Sound area, both coincidentally on Good Friday. In 1964, the Great Alaska Earthquake caused a spill of asphalt from storage tanks in the old town of Valdez.; in 1989, the supertanker Exxon Valdez spilled North Slope crude oil. Today, it seems easier for us to find asphalt residues from 1964 than oil residues from 1989 on the beach cobbles and bedrock surfaces around Prince William Sound."
Dr. Keith A. Kvenvolden, U.S. Geological Survey
In 1989, the supertanker Exxon Valdez spilled a large amount of crude oil into Prince William Sound.
About 258,000 barrels (11 million gallons) of crude oil from the North Slope of Alaska spilled into Prince William Sound when the supertanker grounded on Bligh Reef. The oil quickly spread southwest through the Sound into the Gulf of Alaska. Approximately 16 percent of the sandy, gravelly, and rocky shorelines of the Sound was coated with oily residues. Cleanup operations and natural processes have now removed much of the oil from affected shorelines. Although this spill caused significant environmental damage, especially to the wildlife, and cost millions of dollars to clean up, recent evidence suggests that an earlier spill event in the Sound may be more easily detected today than is the oil from 1989.
USGS scientists paid special attention to the carbon-13 isotope values and the chemical signatures of non-Exxon Valdez asphalt residues in an effort to determine how these residues were released to the environment. The tight range of heavy-carbon isotopic composition of these asphalt sample residues in Prince William Sound suggests not only that they came from Monterey-sourced oils but that they also came from the same batch of products.
The asphalt residues found in Prince William Sound are thought to come from a single event, such as the Great Alaska Earthquake which occurred on Friday, March 27, 1964. During this earthquake and the subsequent tsunami, an asphalt storage plant in Valdez was destroyed, spilling unknown amounts of asphalt into the Port Valdez fjord. A study described in a National Academy of Sciences publication shows that about 30 percent of the fjord was covered with asphalt after the earthquake. There is no record of any cleanup activity, which is understandable considering the enormous effort that was needed to clean up onshore earthquake damage and to rebuild towns.
Sampling of hydrocarbon residues has focused on easily-accessed surface exposures on beaches of Prince William Sound. Analytical results have only partially addressed the question of deposition of residues buried in the seafloor of the Sound; there is a paucity of information about the presence of Exxon Valdez oil in these sediments. Also, little is known about the movement and ultimate fate of residue-contaminated sediments resuspended by storm action. In addition, there may be other sources of oil contamination besides those already studied. For example, other workers believe that the source of some residues may be natural seeps in the Katalla region east of the Sound. USGS scientists intend to investigate this possibility in the future.
Dr. Paul Carlson
Email: kkvenvolden@usgs.gov or pcarlson@usgs.gov
This USGS Fact Sheet can be found at <URL:http://energy.usgs.gov/factsheets/Prince/friday.html>
Scientists at the U.S. Geological Survey (USGS) use standard geochemical techniques to distinguish oil residues.
During two cruises, conducted with help from the Minerals Management Service and the National Oceanic and Atmospheric Administration's National Marine Fisheries Service, USGS scientists collected samples from 27 beach sites around Prince William Sound. While tracking the geochemical fate of oil spilled from the Exxon Valdez, researchers found, in addition to weathered products from the spill, minor amounts of residues on beaches from a distinctly different source. By using carbon-isotope ratios of whole-oil residues as one method of identification, USGS scientists were able to distinguish Exxon Valdez oil and its residues from non-Exxon Valdez asphalt residues. The carbon-13 isotope values derived from Exxon Valdez oil are found to be significantly different than those of the asphalt residues.
USGS scientists conducted blind tests of residue samples to evaluate the idea that not all residues were from the same source.
The isotopic signature of the non-Exxon Valdez residues strongly suggests that its parent crude oil originated from the Monterey Formation of California, and that they are residues from oil products previously shipped by barge to Alaska. Carbon-13 isotope values of California oil are unique compared to other oils found around the world. Also, asphalt residues of California oils are chemically different from North Slope oil. A suite of biomarkers signifying the maturity of crude oils indicate that Monterey crude is somewhat immature, unlike North Slope crude. Also, specific biomarkers can be found in Monterey crude which are not present in North Slope crude. Thus, careful analysis of oil residues for particular hydrocarbon compounds can yield characteristics that allow USGS scientists to distinguish samples of oil residues from different sources.
USGS analyses of some samples of asphalt residues indicate that their release most likely resulted from a single event.
Researchers at the USGS believe that this single event was the 1964 Alaska earthquake.
Tests indicate that the non-Exxon Valdez residues may be related to paving and roofing tar.
USGS scientists obtained pavement samples from streets in Whittier, Alaska, and from the runways of nine Alaskan airports. Five of these pavement samples pre-date North Slope oil; their isotopic compositions are well within the range of the Monterey-sourced oils, but not so precisely matched. This suggests that the source of the paving material is California oil, though not from the same batch of California asphalt that spilled during the 1964 earthquake. Samples from five other airport runways which have been recently resurfaced show isotopic compositions similar to North Slope oil. Also, oil residues from a mine dump on a beach of Latouche Island, Alaska, have a carbon isotopic composition suggesting a California origin for that oil, but again, not from the same batch as that spilled in 1964.
USGS scientists hope to continue studying other possible sources and locations of hydrocarbon contamination.
For More Information:
Dr. Keith Kvenvolden
U.S. Geological Survey
345 Middlefield Road
MS 999
Menlo Park, CA 94025
U.S. Geological Survey
345 Middlefield Road
MS 999
Menlo Park, CA 94025
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Maintained by L. Friedrich Last updated 16-Oct-1997