The Effect of Pore Water Chemistry on the Biodegradation of the Exxon Valdez Oil Spill

Abstract

Knowledge of the oxygen and nutrient concentrations in an oil-contaminated Prince William Sound (PWS) beach is important for understanding of the oil persistence over two decades after the Exxon Valdez spill. It was traditionally believed that there was enough oxygen in the contaminated shorelines to sustain aerobic microbial metabolism of oil and that nutrients were the major factors limiting oil biodegradation. In the present study, we analyzed the oxygen and nutrients levels in both clean and oily areas on a PWS beach that was heavily contaminated by the Exxon Valdez oil spill. We found that the level of nitrogen and phosphorous were 0.454 mg-N L −1 and 0.033 mg-P L −1 , respectively, which is not sufficient to fully support microbial growth, confirming that nutrient concentration was a major factor limiting oil transformation by biological ways. We also observed that the oxygen level varied from higher than 3 mg L −1 in the clean wells to about 1 mg L −1 (near anoxic level) in the oily wells. In addition, the lowest nitrate levels were observed at the oily wells. Altogether, these results suggest that the low level of efficient electron acceptors (oxygen and nitrate) detected at the oily spot is responsible for slow and potentially inefficient biodegradation of the oil.

Role of freshwater in the persistence of theExxon Valdezoil spill in a wave-exposed beach

“…High water retention time in the lower layer caused depletion of water dissolved oxygen which was confirmed in the study of Sharifi et al (2011).…”

“…Based on the particle tracking results, the average pore water velocity in the upper layer and the lower layer was 8 m/hour and 5.7× 10 -3 m/hour, respectively. Sharifi et al (2011) studied the effect of dissolved oxygen and nutrient concentration of the pore water on the biodegradation of the Exxon Valdez oil spill in two transects in the same beach studied here (beach SM006C) during their field study in summer of 2009. Their right transect coincided with the same transect which is investigated here.…”

Beach geomorphic factors for the persistence of subsurface oil from the Exxon Valdez spill in Alaska

“…In addition, Li and Boufadel (2010) reported also low concentration of oxygen at oiled pits. Boufadel et al (2010) and Sharifi et al (2010) further confirmed that the low level of efficient electron acceptors (oxygen and nitrate) detected at the oily spots is responsible for slow and potentially inefficient biodegradation of the oil. Thus, the natural biodegradation rate of oil is expected to be slow due to the small concentrations of nutrients and dissolved oxygen.…”

“…They also found that oil is present in the transect with the smaller fresh groundwater flow (flowing seaward). Boufadel et al (2010) and Sharifi et al (2010) confirmed the presence of low concentration of oxygen and nutrients which thus could be the limiting factor for oil biodegradation. Taylor and Reimer (2008) reported that sites with the most extensive visibly recorded subsurface oil residues were on Smith Island, Knight Island, and Eleanor Island.…”

“…The maximum concentration of nutrients in PWS is less than 0.4 mg N/L (Eslinger et al 2001;Bragg et al 1994;Boufadel et al 2010;Sharifi et al 2010), an order of magnitude lower than the concentration needed for maximum microbial growth, which ranges from 2 (sometimes 5) to 10 mg N/L (Lewis et al 1995;Venosa et al 1996;Boufadel et al 1999;Wrenn et al 2006). In addition, Li and Boufadel (2010) reported also low concentration of oxygen at oiled pits.…”