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As part of a comprehensive water-quality assessment program performed in Prince William Sound and the western Gulf of Alaska following the Exxon Valdez oil spill of March 24, 1989, water samples were collected from 417 locations, most of them in areas through which the oil drifted, to assess the distribution and concentrations of petroleum hydrocarbons in the water column. Over 5 000 water samples were analyzed for individual and total petroleum alkanes and for aromatic hydrocarbons by very sensitive gas Chromatographic techniques. A total of 2 461 of these samples were analyzed for polycyclic aromatic hydrocarbons (PAHs). Concurrent with some of these samples, an additional 123 water samples were collected in April 1989 (a week to a month after the spill) at 32 offshore locations and in June 1989 at 7 nearshore sites in Prince William Sound to determine the toxicity of the water to representative species of marine organisms. The toxicity of Prince William Sound water was assessed with standard Environmental Protection Agency (EPA) and American Society for Testing and Materials (ASTM) marine toxicity tests with representative species of three taxonomic groups: (1) Skeletonema costatum (a marine diatom), (2) Mysidopsis bahia (a crustacean), and (3) larval/juvenile Cyprinodon variegatus (a fish, the sheepshead minnow).
Highest concentrations of total PAHs, the best indicator of the potential toxicity of the spilled oil to water-column organisms, were observed in the water column of Prince William Sound in the first two months after the spill. Measured PAH levels were below the state of Alaska's water quality standard of 10 ppb total aromatic hydrocarbons. This finding is consistent with National Oceanic and Atmospheric Administration (NOAA) water samples collected during the same time period. PAH concentrations in the water column of the sound declined rapidly with time after the spill to essentially background concentrations of 0.01 to 0.1 ppb by mid to late summer of 1989. No water samples from spill path areas of the western Gulf of Alaska contained more than 1.0 ppb total PAHs. These concentrations are well below concentrations reported in the scientific literature to be toxic to marine plants and animals. Eight of 329 samples (2.4%) of the water surface contained between 10 and 29.3 ppb total PAHs; these samples probably contained oil sheen material.
Toxicity tests with full strength Prince William Sound water showed no acute or chronic toxicity to three representative species of marine organisms. There was no dose-response relationship for any species or seawater sample, and there was no relationship between the concentration of total PAHs in the undiluted seawater samples and percentage of survival or relative growth rates in the test organisms.
These studies show that the traces of petroleum hydrocarbons present in the water column of Prince William Sound and the western Gulf of Alaska shortly after the spill were well below concentrations capable of producing harmful effects in populations of sensitive species and life stages of marine organisms.
Prince William Sound, Exxon Valdez, oil spill, water column, crude oil, toxicity, marine organisms, polycyclic aromatic hydrocarbons, PAH
Senior Research Leader, Battelle Ocean SciencesArthur D. Little, Inc., DuxburyCambridge, MAMA
Senior Aquatic Toxicologist, ENSR Consulting and Engineering, Fort Collins, CO