The long-term persistence of relatively un-weathered, bioavailable oil was not expected at the time of the Spill or even ten years later. In 1999, beaches in the Spill area appeared clean on the surface, although occasional tar balls or hardened oil residues could be encountered, which remains true today. Some subsurface oil had been reported in a few places (Hayes and Michel, 1999), but it was expected to decrease over time and most importantly, to have lost its toxicity due to weathering. This was not always the case, and a series of surveys funded by the Council has continued to locate, further identify, and increase understanding of the persistence and toxicity of the lingering oil (Short et al., 2004, 2006, 2007; Irvine et al. 2006, 2014; Nixon et al. 2013; Nixon and Michel 2015; Li and Boufadel, 2010; Xia et al. 2010; Xia and Boufadel, 2011).
To predict potential areas where subsurface oil lingers, and to quantify the amount of lingering oil, scientists constructed geospatial models using data collected between 2001 and 2008 from pits dug at 314 sites where residual oil from the Exxon Valdez was found. As a result of this work, it was estimated that approximately 23,000 gallons of oil remained in Spill-area shorelines.
Maps of the oiled areas showing the degree of oiling (medium, high) are included in the Final Report, which was published in 2010 (Michel et al., 2010). In 2015, a series of models using field data for subsurface oil in excavated pits and predictor variables was used to further refine the probability of encountering subsurface oil (Nixon and Michel, 2015).
At the time of the Spill, most scientists believed that in a relatively short period of time, natural weathering processes would decompose the spilled oil or cause it to become non-toxic, stable asphalt. Contrary to expectations, even two decades later, subsurface oil encountered in pit surveys of PWS beaches fingerprinted back to the source oil of the Exxon Valdez. Slightly weathered, the lightest fraction of aromatic hydrocarbons (single ring compounds like benzene and toluene) was missing, but most of the 2-4 ring polycyclic aromatic hydrocarbons (PAH) were intact and therefore toxic, and in the same proportions as Exxon Valdez oil collected in the first weeks of the Spill (Short et al., 2007).
Samples of 19-year-old oil from the subsurface of PWS, still toxic, were evaluated in a laboratory to investigate the factors limiting biodegradation. Removed from the subsurface environment, the oil was biodegradable within 6 months, with the addition of nutrients and oxygen (Venosa et al., 2010). Subsequent sampling of the lingering oil in place, in the field pits, revealed that although the available nutrient supply was somewhat limited, the main limiting factor to biodegradation was a lack of available oxygen (Boufadel et al., 2010).