Bioremediation of marine oil spills: when and when not--the Exxon Valdez experience

Abstract

In this article we consider what we have learned from the Exxon Valdez oil spill (EVOS) in terms of when bioremediation should be considered and what it can accomplish. We present data on the state of oiling of Prince William Sound shorelines 18 years after the spill, including the concentration and composition of subsurface oil residues (SSOR) sampled by systematic shoreline surveys conducted between 2002 and 2007. Over this period, 346 sediment samples were analysed by GC-MS and extents of hydrocarbon depletion were quantified. In 2007 alone, 744 sediment samples were collected and extracted, and 222 were analysed. Most sediment samples from sites that were heavily oiled by the spill and physically cleaned and bioremediated between 1989 and 1991 show no remaining SSOR. Where SSOR does remain, it is for the most part highly weathered, with 82% of 2007 samples indicating depletion of total polycyclic aromatic hydrocarbon (Total PAH) of >70% relative to EVOS oil. This SSOR is sequestered in patchy deposits under boulder/cobble armour, generally in the mid-to-upper intertidal zone. The relatively high nutrient concentrations measured at these sites, the patchy distribution and the weathering state of the SSOR suggest that it is in a form and location where bioremediation likely would be ineffective at increasing the rate of hydrocarbon removal.

Figure 1

Nitrogen applications during bioremediation in PWS 1989–1990. Diameter of circles scaled to represent pounds of nitrogen.

Figure 2

Changes in composition of oil on sediments from PWS in 1990 laboratory microcosm tests of bioremediation as measured by HPLC chromatography (different from more detailed GC‐MS reported for field samples). Sats is the saturates or alkanes. Aromatics (2–4+ rings) include the 41 target PAHs, plus other multi‐ring species. The polars (or NSO fraction) include asphaltenes, resins and oxygenated by‐products of biodegradation, and are considered largely inert to biodegradation. (Wt % is the percentage composition based on mass.)

Figure 3

Depletion of PAH groups in subsurface oil residue within fertilized section of KN135B bioremediation field test, PWS, 1990.

Figure 4

Depletion indices measured at Prestige oil spill bioremediation test (Moreira site) during a 1‐year sampling period (Gallego et al., 2006). Degradation rates slowed once light alkanes were depleted.

Figure 5

Depletion of total resolvable alkanes in all oiled sediment samples analysed from 2002 to 2007 as function of Total PAH depletion. Resolvables include C9‐ to C40‐normal alkanes plus pristane and phytane. Almost all are totally depleted.

Figure 6

Bioremediation indices for all PWS samples analysed between 2002 and 2007.

Figure 7

Bioremediation indices for samples collected in 2007 as function of pit elevation above mean low tide (MLLW). Samples least degraded reside in upper intertidal zone (A and B), while few were found that might respond to bioremediation in lower intertidal (C and D).

Figure 8

Depletion of C1‐ to C4‐alkylated phenanthrenes versus Total PAH depletion in samples from 2002 to 2007. Pattern is typical of biodegadation.

Figure 9

Ratio of (nitrogen concentration/non‐polar hydrocarbons) in 2006–2008 pore water samples normalized to same ratio measured in fertilized portion of KN135B during 1990 bioremediation test. Ratios reflect relatively high natural background nutrient concentrations currently at most sites.

Figure 10

Photo of site SM006B showing the % Total PAH depletion at pits sampled in 2007. All pits along a tidal elevation transect are 10 m apart. Green dots reflect no oiling above reference background. Only two small patches of oil remain that might respond to bioremediation.