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Part two of a three-part series, this paper describes chemical and toxicological results of a comprehensive shoreline ecology program that was designed to assess recovery in Prince William Sound following the Exxon Valdez oil spill of March 24, 1989. The program is an application of the “sediment quality triad” approach, combining chemical, toxicological, and biological measurements. Other parts of the program are described in Part 1: Study Design and Methods (Page et al., this volume) and Part 3: Biology (Gilfillan et al., this volume). The study was designed so that results could be extrapolated to the entire spill zone in the sound and projected forward in time. It combined one-time sampling of 64 randomly chosen study sites representing four major habitats and four oiling levels (including unoiled reference sites), with periodic sampling at 12 subjectively chosen “fixed” sites. Sediment samples—or when conditions required, filter-wipes from rock surfaces—were collected in each of three intertidal zones and from subtidal stations up to 30-m deep.
Oil removal was generally quite rapid: by 1991 the concentration of oil spilled from the Exxon Valdez had been dramatically reduced on the majority of shorelines by both natural processes and cleanup efforts. Moderate concentrations of petroleum residues remain only in limited, localized areas; however, most of these residues are highly asphaltic, not readily bioavailable, and not toxic to marine life.
Acute sediment toxicity from oil (as measured by standard toxicity tests) was virtually absent by 1990–'91, except at a small number of isolated locations. The petroleum residues had degraded below the threshold of acute toxic effects. Measurable polycyclic aromatic hydrocarbon (PAH) levels are, in general, well below those conservatively associated with adverse effects, and biological recovery has been considerably more rapid than the removal of the last chemical remnants. The remaining residues continue to degrade and are, in general, predicted to become indistinguishable from background hydrocarbon levels by 1993 or 1994. Localized residues of weathered oil will no doubt exist beyond 1994 at certain locations, but their environmental significance will be negligible compared with other stresses ongoing in the sound.
Samples of nearshore subtidal sediments showed surprisingly low concentrations of oil residue, as an increment to the natural petrogenic hydrocarbon background. Sediment toxicity tests showed that they were essentially non toxic. It appears that most of the oil leaving the shoreline was swept away and dissipated at sea. It is concluded that long-term ecological effects resulting from shoreline oiling or subtidal contamination are highly unlikely.
Exxon Valdez, polycyclic aromatic hydrocarbons, sediment chemistry, bioavailability, sediment toxicity, mussels, oil-spill recovery
Arthur D. Little, Inc., Cambridge, MA
Bowdoin College, Brunswick, ME
Bowdoin College, Brunswick, ME
ENSR Consulting and Engineering, Ft. Collins, CO
West Virginia University, Morgantown, WV