A Tale of the Two Worst Oil Spills in U.S. History

A review of: Atlas, R. M., and Hazen, T. C. (2011) Oil Biodegradation and Bioremediation: A Tale of the Two Worst Spills in U.S. History. Environmental Science and Technology 45: 6709-6715

This paper looks at biodegradation and bioremediation by focusing on two major marine oil spills, the Exxon Valdez spill in the Gulf of Alaska and the BP Deepwater Horizon oil leak in the Gulf of Mexico. The paper describes each of the spills in depth and looks at how bioremediation was used in each case. Hydrocarbons found in crude oil are natural products that are derived from aquatic algae 180 to 85 million years ago and therefore, frequently escape into the environment through underground reservoirs etc. Because of this and the natural origin of these hydrocarbons, many microorganisms have been able to evolve adaptations that enable them to utilise the hydrocarbons as sources of carbon and energy for microorganism growth. There are hundreds of such species.

The oil tanker, Exxon Valdez, ran aground on the Gulf of Mexico in 1989, spilling 11 million gallons of crude oil into the surrounding water. Because of the practical difficulty of removing the oil by washing and collecting in this case, bioremediation was selected as the primary method of oil pollution treatment. Field tests were conducted in which fertilisers were added to the oil spill in the hope of increasing the rate of oil biodegradation. Rates of biodegradation showed the decrease of hydrocarbons of up to 1.2% per day, but this slowed as the more readily accessible components were depleted. As a result of these tests, this method was adopted and two fertilisers were chosen and applied to the whole spill. Sediment sample collected in the same year as the spill found that 25-30% of the oil had been degraded within the first few weeks of the spill as well as a significant increase in oil-degrading bacterial population which made up 40% of the population in that area. By 1992, most of the oil had been degraded and oil-degrading bacterial populations had dropped back down to normal levels.

In the Gulf of Mexico, oil and gas escaped from the BP Deepwater Horizon exploratory well in 2010. It took 84 days to stop the leak and in that time, an estimated 205.8 million gallons of oil and gas had been released. In the treatment of this leak, it was estimated that 3% was skimmed, 8% chemically dispersed, 17% captured, 5% burnt, 16% naturally dispersed, 25% evaporated or dissolved and 26% still remaining. Dispersion was one of the main strategies adopted in the treatment of this spill as it was required to ensure safety for operations to stop the leak to happen. This involved the injection of a disperser at depth in order to avoid a slick forming on the surface of the water and reaching the shoreline. Oil dispersed both as depth, forming a ‘cloud’ and on the surface as oil droplets which were dispersed by the current.

Research on the oil ‘cloud’ found that there was an increased microbial concentration within the cloud, significantly higher than outside the cloud, suggesting microbial activity. 16 subfamilies of γ-proteobacteria were found to be the predominant species found in the cloud and further investigation revealed that the increased bacterial concentration was due to an increase in oil-degrading bacteria.

The paper is interesting to look at the practical aspects of bioremediation and to see how each case requires different treatment in terms of controlling the oil spill. It also highlights the usefulness of bioremediation as a natural method of controlling pollution, but, as in other research, it finds that there are limitations. In this case, bioremediation cannot control 100% of the oil pollution.