Identification of Exxon Valdez oil in sediments and tissues from Prince William Sound and the northwestern Gulf of Alaska based on a PAH weathering model

We used a first-order loss-rate kinetic model of polynuclear aromatic hydrocarbon (PAH) weathering to evaluate 7767 environmental samples collected for the Exxon Valdez oil spill (EVOS) for the presence of spilled oil. The model was developed from experiments with gravel coated with crude oil and washed for 6 months. The modeled PAH included the 14 most persistent compounds of 31 analyzed by GC/MS. Parameters include loss-rate constants related to the energy required for PAH to escape from petroleum and a quantitative index of weathering. The model accounts for 91% of the temporal variability of modeled PAH concentrations. We compared the discrepancies between measured and model-predicted PAH concentrations of EVOS samples with a probability distribution of these discrepancies derived from the experimental weathering results, Only 1541 field samples contained sufficient PAH for valid application of the model; three-fourths fit the model at alpha greater than or equal to 0.01 type I error, 9% fit an alternate model characterized by the absence of weathering, 17% fit neither model, and a few fit both models. The 1164 total samples that fit the weathering model account for 86% of the summed PAH concentrations detected in all 7767 samples. We conclude that first-order loss-rate kinetics account for the dominant PAH weathering processes in the EVOS and that the rate of weathering is determined mainly by the ratio of surface area to volume of petroleum in the environment.