Evaluation of PAH depletion of subsurface Exxon Valdez oil residues remaining in Prince William Sound in 2007-2008 and their likely bioremediation potential

This study examined the extent of oil weathering at the Exxon Valdez oil spill (EVOS) sites and estimated the bioremediation potential for shoreline segments by examining the depletion of total polycyclic aromatic hydrocarbons (PAHs) relative to an estimated applicability threshold of 70 per cent. The distribution of oil was examined by location and current ratios of nitrogen and non-polar oil in order to assess if biodegradation rates were nutrient-limited. The impact of sequestration on the effectiveness of bioremediation was also studied. Results of the study showed that the EVOS residues are patchy and infrequently found on sites that were heavily oiled in 1989. Only 0.4 per cent of the oil originally stranded in 1989 remained. The remaining EVOS residues are sequestered under boulder and cobble armour in areas with limited contact with flowing water. The study also showed that concentrations of nitrogen and dissolved oxygen in pore waters within strata adjacent to the sequestered oil can support biodegradation. Most remaining EVOS residues are highly weathered and biodegraded. It was concluded that nutrients added to the shorelines are unlikely to effectively contact the sequestered oil. 31 refs., 2 tabs., 14 figs