Despite perceptions to the contrary, oil tanker accidents resulting in large marine oil spills are rare. When a large oil spill does occur in coastal waters, however, the immediate injury to marine ecosystems can in some cases be massive, especially if the oil washes ashore. Plants and animals living in the intertidal zone, as well as animals that use the sea surface, are the most vulnerable to spill injury, and large numbers can be killed in a short time.

Though oiled shorelines require cleanup, cleanup can cause harm to shoreline communities in addition to that caused by the oil itself. Thus, there is a strong need to answer the question, “how clean is clean?” How much oil should be removed to allow recovery to occur while minimizing additional harm from cleanup activities? Within a few days of the Exxon Valdez oil spill in March 1989 in Prince William Sound, Alaska, Exxon requested the help of senior Battelle scientists to answer these questions and to help determine the effects of the spill and the progress of ecosystem recovery following the spill.

The Exxon Valdez oil spill is the most contentious and extensively studied spill in history. Although the spill was smaller than many, it occurred in an area of high marine productivity and diversity. Exxon assumed responsibility for the cleanup and, in collaboration with the U.S. Coast Guard, mounted the largest oil spill cleanup in history.

The cleanup efforts supplemented by the scouring activity of severe winter storms removed most of the oil from the shore within one year of the spill. By 1992, the Coast Guard declared the Sound clean. Disagreement persists today as to the magnitude of the harm caused by the spill to marine ecosystems and the extent of ecosystem recovery.

There has been considerable disagreement between the trustees and Exxon-sponsored scientists concerning the impacts on commercial fishery species and shoreline communities. Studies performed by Battelle’s Dr. Jerry Neff showed that concentrations of petroleum hydrocarbons in the water column never reached high enough concentrations to harm pelagic fish. Additionally, Battelle’s Dr. Walter Pearson and colleagues showed that very little of the shoreline where herring deposited their eggs ever became oiled. Commercial catches of pink salmon and herring were at record highs for several years after the spill, indicating minimal harm to these populations.

Many scientists assumed that the Sound was essentially pristine before the spill and that any evidence of petroleum hydrocarbons in the environment or changes in biological communities could be attributed to the spill. Detective work by Battelle’s Dr. Paul Boehm and Dr. Gregory Douglas used advanced oil fingerprinting methods to show that there were and still are many other sources of petroleum hydrocarbons in the Sound. In fact, much of the hydrocarbons in offshore sediments were determined to be not from the spill, but instead from eroding shales and related natural oil seeps along the coast southeast of the Sound. Additional work showed that hydrocarbons in some western areas of the Sound were derived from earlier human activities.

Small amounts of oil from the Exxon Valdez can still be found in the Sound; most is present in the upper intertidal “splash zone” of rocky shores and has the appearance of hard asphalt. Some oil is buried deep in the sediments, removed from the environment. The weathered oil to which animals may be exposed is in a location and physical form that is not harmful to marine plants and animals living in the intertidal zone. Several shoreline studies performed by Dr. Paul Boehm and colleagues on ecological effects of the oil and recovery of oiled shorelines have shown that the formerly oiled shorelines recovered within two to four years after the spill.

Toxicity tests on shoreline sediments with a common shoreline crustacean, conducted as part of the well-established sediment quality “triad” approach (chemistry, toxicology, ecology) proved to be a good indicator of “how clean is clean?” These and several other Exxon-sponsored studies have shown that the toxicity of oiled sediments declined to clean levels within a few years and hydrocarbon-related toxicity was virtually absent in the shoreline sediments.

Although small amounts of weathered oil remain buried in sediments on a few boulder beaches, the remaining oil is in locations and physical forms that pose no health risk to shoreline biological communities, fish, birds, and mammals. Plant and animal populations that use the shore have recovered from the effects of the oil spill.

For more information on the Exxon Valdez project, please contact Dr. Jerry Neff at (781) 952-5229, neffjm@battelle.org, Dr. Greg Douglas at (781) 952-5393, douglasg@battelle.org, or Dr. Paul Boehm at (781) 952-5262, boehmp@battelle.org.