SEDIMENT TOXICITY MEASUREMENTS IN OIL SPILL INJURY ASSESSMENT: A STUDY OF SHORELINES AFFECTED BY THE EXXON VALDEZ OIL SPILL IN PRINCE WILLIAM SOUND, ALASKA

Beginning in 1989, scientists supported by ExxonMobil conducted studies to assess the fate and effects of the Exxon Valdez oil spill on shorelines in Prince William Sound (PWS), Alaska. These included the shoreline ecology program (SEP) which consisted of: (1) a stratified random sampling program of 64 sites to generally assess spill effects in 1990, (2) a non-random sampling of heavily oiled ("worst-case") sites in 1990, 1991, 1998 and 1999 to assess oil persistence and worst-case effects, and (3) a 1993 field study of oiled mussel beds. The 1990-1993 field programs included concurrent sediment sampling for hydrocarbon chemistry and sediment toxicity. This sediment quality assessment found that spill residues on the oiled shorelines rapidly lost toxicity through weathering. The relative amounts of naphthalenes and chrysenes in the polycyclic aromatic hydrocarbons (PAH) in the sediments were found to be good indicators of weathering and toxicity. Using a standard sediment amphipod bioassay on the held samples, increased mortality above background was found at exposures levels above c 2,600 ng/g total PAH (TPAH) for oil that had weathered a year or more. For samples with TPAH>2,300 ng/g, samples with high mortalities (>80%) were dominated by relatively high fractions of naphthalenes (median = 0.26) and samples in the low mortality category (<30%) was dominated by relatively high fractions of chrysenes (median = 0.24). The amphipod mortality data fit significantly to a logistic model. Estimated LC10 and LC50 values were approximately 4,100 and 10,750 ng/g TPAH, respectively. The observed LC50 concentration of 10,750 ng/g TPAH (including alkyl homologues) in this field study relates well to 50th percentile probability of effect sediment quality guideline (SQG) values reported in the literature. Sediment infauna species richness and Shannon diversity decreased with increasing TPAH above the 2,600 ng/g threshold, demonstrating a correspondence between sediment bioassay results and biological community effects in the field. The 1999 median sediment TPAH concentration at the 1990-1999 worst-case sites was 117 ng/g, well below the 2,600 ng/g toxicity threshold value and indicative of a lack of potential for long term toxic effects. Comparison of SQG values with sediment hydrocarbon and toxicity data in a long-term oil spill recovery study provides a means to answer the question: "How clean is clean?". For the covering abstract see ITRD E122846.