Bioremediation of the Exxon Valdez oil in Prince William Sound beaches

Abstract

Oil from the Exxon Valdez laden with polycyclic aromatic hydrocarbons (PAH) has persisted on some beaches in Prince William Sound, Alaska, > 20 years after these beaches became contaminated. The degradation rate of the total PAH (TPAH) is estimated at 1% per year. Low oxygen concentrations were found to be the major factor causing oil persistence, and bioremediation through the injection of hydrogen peroxide and nutrients deep into four beaches in PWS were conducted in the summers of 2011 and 2012. It was found that due to the treatment, the TPAH biodegradation rate was between 13% and 70% during summer 2011 and summer 2012. The results also showed high efficiency in the delivery of oxygen and nutrient to the contaminated areas of the beach. However, the approach has an environmental cost associated with it, and stakeholders would need to conduct a rigorous net environmental benefit analysis (NEBA) for pursuing the bioremediation of submerged contaminated sediments, especially in higher latitudes.

Introduction

The 1989 Exxon Valdez oil spill (EVOS) polluted around 800 km of intertidal shorelines within Prince William Sound (PWS), Alaska (Neff et al., 1995, Neff and Stubblefield, 1995). Studies conducted by scientists from the National Oceanic and Atmospheric Administration (NOAA) estimated that between 60 and 100 tons of subsurface oil persists in many initially-polluted beaches in Prince William Sound (PWS) (Short et al., 2004, Short et al., 2006). The persistence of oil was also noted by other studies (Page et al., 2008, Hayes and Michel, 1999, Michel and Hayes, 1999, Taylor and Reimer, 2008, Li and Boufadel, 2010). The lingering oil contains relatively high concentrations of polycyclic aromatic hydrocarbons, PAH, (Short et al., 2004), which are known to be toxic to intertidal organisms (Carls et al., 2001), as sea otters and harlequin ducks maybe exposed to subsurface lingering oil while foraging on the beaches of northern Knight Island (Short et al., 2006).

Earlier findings based on six beaches in Prince William Sound contaminated with moderate to heavy oil residue (MOR to HOR) from the EVOS, revealed that generally the beaches consist of an upper high-permeability layer underlain by a lower layer whose permeability is two to three orders of magnitude lower than that of the upper layer (Li and Boufadel, 2010, Xia et al., 2010, Xia and Boufadel, 2011, Bobo et al., 2012). On these beaches, the lingering Exxon Valdez oil was located a few inches (0.10 m) below the interface of the two layers. Oil-contaminated sediments were anoxic with a dissolved oxygen (DO) concentration around 1.0 mg/L), and had a high ratio of ammonia to nitrate as nitrogen-N (Boufadel et al., 2010, Sharifi et al., 2011), whereas similar oil-free sediments were oxic (DO > 3 mg/L) and have a moderate ammonia to nitrate ratio (as N), suggesting that oil biodegradation is oxygen-limited in oil-contaminated sediments. In addition, the concentrations of pore water nutrients in contaminated sediments were < 0.5 mg N/L and < 0.04 mg P/L (Boufadel et al., 2010, Sharifi et al., 2011). These values are 5 folds smaller than the nutrient concentrations that support maximal rates of oil biodegradation within sediments; they are ≥ 2 mg-N/L (Venosa et al., 1996a, Boufadel et al., 1999, Du et al., 1999), and ≥ 0.2 mg-P/L in phosphate, with an approximate optimal ratio of N/P around 10 (Atlas and Bartha, 1972, Venosa et al., 1996a, Wrenn and Venosa, 1996, Smith et al., 1998).

A laboratory study by Venosa et al. (2010) found that around 80% of the PAHs in Exxon Valdez oil obtained from three beaches in PWS biodegraded within six months. The oil they used was already 70% weathered (Atlas and Bragg, 2007). However, the Venosa et al. (2010) study required disturbing the oiled sediments to place them in microcosm, and it was argued by Atlas and Bragg (2009a); Atlas and Bragg (2009b) that oiled areas are sheltered hydraulically from input of dissolved oxygen and nutrient. For this reason, it was important to evaluate whether biodegradation of the lingering oil could be enhanced in situ through the injection of amendments into the subsurface of polluted beaches in Prince William Sound.

In any beach subjected to tide, the net (or time-averaged) movement of pore water is seaward throughout the beach with the exception of the region near the high tide line where the flow is usually landward and downward (Boufadel et al., 2006a, Li et al., 2007, Li and Boufadel, 2010). Therefore, water solutions applied onto the beach surface would tend to move seaward after they percolate into the beach (Boufadel et al., 2006b). Due to the two-layer structure of the beaches in PWS, where the upper layer's permeability is 100 to 1000 times that of the lower layer (Li and Boufadel, 2010), solutions applied onto the surface tend to dilute with pore water and wash out to sea much more rapidly than they can be transported into the contaminated layer (Li and Boufadel, 2011). Xia et al. (2010) found based on numerical simulations of solution migration in a PWS beach that the nutrient concentration in the oil-contaminated sediments would be only 1% of the concentration applied onto the beach surface.

Tracer studies using a conservative tracer released directly into the lower layer of three beaches in PWS resulted in much less dilution (Boufadel and Bobo, 2011, Boufadel et al., 2011) due to the small porosity of the lower layer. Therefore, subsurface delivery of nutrients was expected to be superior to surface application, and was selected for a pilot bioremediation study.

This paper reports the findings of an in situ bioremediation investigation at four beaches in Prince William Sound (Fig. 1): EL056C (Northwest Bay on Eleanor Island; 60.5506/− 147.5795), LA015E (Latouche Island; 60.0596/− 147.8171), PWS3A44 (Mears Point, Perry Island; 60.6567/− 147.9319), and SM006B (Smith Island; 60.5278/− 147.3851). Hydrogen peroxide and nitrate and phosphate solutions were injected into the beaches to enhance microbial growth and subsequently oil biodegradation. The focus was on the biodegradation of the polycyclic aromatic hydrocarbons (PAH) due to their toxicity and their persistence in the oiled beaches. Surrogate measurements, including the concentration of dissolved oxygen and nutrient in pore water, and the concentration of microorganisms are also reported.