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Status of Restoration

Lingering Oil: Continued

Michel et al. (2010) estimated the total area and mass of oil by oiling categories for the same area that Short et al. (2004) used in their calculations. A comparison shows that there is very good agreement between the two approaches. The Exxon Valdez spill released 10,800,000 gallons of North Slope crude oil, which converts to 39,560 metric tons. Thus, the Michel et al. (2010) estimate of any subsurface oil in Prince William Sound as of the period of 2001-2007 represents 0.25% of the total spill, the estimate for areas with lightly oiled residues represents 0.09% of the total spill, the estimate for areas with medium-oiled residues represents 0.08% of the total spill, and the estimate for areas of highly-oiled residues represents 0.07% of the total spill volume.

Carls et al. (2016) re-sampled six segments in Prince William Sound in 2015, using the same methods as Short et al. (2004) in 2001. They dug 400 pits in these six segments and found that there has been essentially no change in the percent of pits that were oiled and thus no significant changes in the estimated total area with subsurface oiling at each site. The authors also reported no significant differences in average subsurface oil mass per unit area for areas reported by Short et al. (2004) as having light, medium or heavily-oiled residues.

Though this investigation was more limited in location and scope, these results imply that there is little ongoing change in the amount of subsurface oil in Prince William Sound at present.

Aerial photograph of Smith Island showing the general locations of upper, middle and lower intertidal zones.
Aerial photograph of Smith Island showing the general locations of upper, middle and lower intertidal zones. This is a view typical of one of the oiled beaches, armored with cobbles, with oil at depth. Oil encountered in pits dug into the subsurface in PWS was most often found, 75% of the time, in the middle intertidal zone (ITZ), 18% of the time in the upper intertidal zone, and 7% of the time in the lower intertidal zone. (Michel et al., 2016).

Why Is The Oil Still There?

After review of all the beach segments studied in 2001-2008, it was clear that the presence and absence of lingering subsurface oil were being influenced by a number of factors. The initial degree of oiling is a very important influence on the presence of lingering oil, because the heavier the oiling, the deeper the oil would penetrate into the subsurface, which occurred only where the shoreline was permeable. Low exposure to wave action slowed the natural reworking of the oiled sediments. The importance of "armoring," which is where the finer gravels on the beach surface are transported away by waves, leaving a layer of coarse gravel on the surface that is very stable, was a new discovery following the spill (Hayes et al. 2010). Armored gravel beaches occur in Prince William Sound because of the wide range in the sizes of the gravel and in the variations in wave energy. In addition, the ruggedness of the shoreline creates intertidal and nearshore bedrock outcrops that act as natural breakwaters, creating micro-sheltered habitats and bending the waves in ways that pile up sediment behind them. Accumulations of boulder-sized rubble along sheltered bays provide semi-permeable sediments with very limited wave energy. Li and Boufadel (2010) found that, in beaches with low freshwater seepage from the land, there were two layers in the beach: an upper layer that was permeable and a lower layer that was 100 times less permeable. The dissolved oxygen in the lower layer was low, which slowed microbial degradation of oil that penetrated into this lower layer.

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