Study of the Fates and Effects of the Exxon Valdez Oil Spill on Benthic Sediments in Two Bays in Prince William Sound, Alaska. 1. Study Design, Chemistry, and Source Fingerprinting

Abstract

This study was conducted to assess the subtidal effects of the Exxon Valdez oil spill in a large embayment in Prince William Sound, AK. A stratified random-sampling design was used to compare stations in an oiled bay, the Bay of Isles, with stations in Drier Bay, a bay that received little impact from the spill. The study included sediment chemistry, benthic ecology, and bioaccumulation elements. Only the results on chemistry of the oil in the bottom sediments are reported here. Analyses of sediment samples revealed four types of polycyclic aromatic hydrocarbons (PAH) in the two bays: (1) Alaska North Slope (ANS) crude oil attributable to the spill, (2) natural oil seep-derived background, (3) pyrogenic, and (4) diagenetic. The Bay of Isles subtidal sediments contained significantly higher levels of weathered ANS-PAH attributable to the spill than did Drier Bay. However, the levels of ANS-PAH in the Bay of Isles were generally lower than those of the petrogenic background PAH naturally present in the subtidal sediments of the sound. This natural petrogenic background PAH component increased in concentration with increasing depth zone (and increasing sediment clay content) for each bay. Drier Bay, a location of past cannery and mining activity, had significantly greater concentrations of pyrogenic PAH than did the Bay of Isles. All sediment PAH concentrations were well below the effects range-low (ER-L) sediment toxicity threshold value of 4000 ng/g for total PAH. The highest sediment total PAH concentration (1500 ng/g) occurred in Drier Bay, and the highest sediment ANS (spill)-PAH concentration (201 ng/g) occurred in the Bay of Isles. There is no evidence for large-scale offshore transport of Exxon Valdez crude to the subtidal sediments.