Effects of Oil Spill Dispersants on Marine Organisms Maya Hutchins Environmental Science Program, Appalachian State University, Boone, NC [email protected]

Editor’s Preface The ENV 3100 Issues in Environmental Science course is the “Writing in the Discipline” course for the Environmental Science program. In partial satisfaction of the course requirements, students must write a literature review on a topic of their choice that is both timely and relevant to the environmental science community. The most outstanding literature review is published in the JSRESA the following summer. Starting in April of 2010, the British Petroleum drilling rig Deepwater Horizon began leaking an estimated 8,400 cubic meters per day of oil into the Gulf of Mexico until September of 2010 when the relief well was completed and the oil discharge ceased. Oil spill dispersants were used to help mitigate the effects of the oil in the gulf waters. In particular, the use of Corexit 9527 and Corexit 9500A in the Gulf of Mexico caught the author’s attention because this was the first time that the Corexit dispersants were used underwater (at high pressures and low temperatures) and because of the toxicity of both dispersants, alone and in combination with crude oil. Herein, Maya Hutchins reviews the available literature in the Fall of 2011, focussing on an article by Schor (2010) [1], toward a better understanding of the impacts resulting from the use of the Corexit disperants on the Gulf of Mexico ecosystem in the long term.

1.0 Introduction Due to the recent Deep Water Horizon (DWH) oil spill in the Gulf Coast, many questions are being raised about the toxicity and long-term effects of oil dispersants used to clean the DWH oil spill [1]. The Deep Water Horizon oil spill released an estimated 4.9 million barrels of Louisiana sweet crude (LSC) oil into the Gulf of Mexico between April 20 and July 15, 2010. Of the quantities of oil removed from surface waters, 17% were from the direct capture of oil from the well head, 3% from skimming of oil from the surface, 5% from booming and burning, and 8% from the application of oil dispersants [2]. The purpose of oil dispersants is to facilitate the reduction and dilution of an oil slick [3]. The efficiency of the dispersant on the breakdown of the oil slick is dependent on salinity, temperature, pressure, and oil properties such as viscosity, density, and chemical composition [4,5]. In response to the DWH spill, over 1.1 million gallons of a commercial dispersant, Corexit 9500A and 9527, were used. Initially, a small amount of Corexit 9527 was applied to the spill [6]. Due to the toxicity of Corexit 9527, 4,059,845

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L of Corexit 9500A, a reformulation of Corexit 9527, was sprayed onto the surface of the oil spill and 2,914,767 L was injected 1,544 m below sea level directly at the well head [2]. Another process effective in the removal of an oil slick from surface waters is biodegradation where marine bacteria breakdown hydrocarbons [6,7]. The review herein seeks to examine the data, analysis, and discussion presented by Schor (2010) in Oil Spill Dispersants Shifting Ecosystem Impacts in Gulf, Scientist Warn [1].

2.0 Effects on Biodegradation The major active components in oil dispersants are surfactants. Surfactants are surface-active agents which possess both hydrophilic and hydrophobic properties, causing dispersants to be both water and oil compatible [2,8]. When an oil slick is highly disturbed, usually by wave and current action, the surfactants forms a film at the water-oil boundary. As the oil is agitated into smaller and smaller droplets, the surfactant prevents the oil from recombining into an oil slick [8]. When the oil slick breaks into small droplets (< 100 µm), the surface-to-volume ratio of the oil

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increases, making them more vulnerable to hydrocarbon degrading bacteria (HDB), which can only degrade oil at the water-oil interface [7]. A study by Xia et al. (2009) [4] examined the effects of salinity, temperature, and oil dispersant additives on biodegradation processes. The effectiveness of the HDB was measured as the difference in total petroleum hydrocarbons (TPH) from day 0 to day x. The experiment showed that biodegradation by HDB is most effective at high salinities (33ppt - see Figrue 1) and high temperatures (300 C - see Figure 2), and had a positive response to oil dispersant additives [4]. As seen in Figure 3, the relationship between efficiency of biodegradation and oil dispersant ratios is not linear. The HDB was most efficient at degrading hydrocarbons when oil dispersants were at a 2:10 ratio, followed by 3:10, 1:10, and 4:10 (see Figure 3). The efficiency of HDB to degrade spilled oil was more effective with any dispersant:oil ratio compared to spilled oil with no dispersant added [4]. A more recent study, however, indicates that while the use of oil dispersants may enhance the availability of hydrocarbons to hydrocarbon degrading bacteria, oil dispersants may also kill HDB [7]. Hamdan & Fulmer (2011) [7] collected bacteria samples from beached oil samples in May 2010, stating that these samples are ‘ideal candidates’ because the microbial communities have been acclimatized to conditions of high contamination by hydrocarbons, a process which occurs in real oil spills. The bacteria in these samples were identified using DNA analysis and separated into six different microbial communities. Four different concentrations of Corexit 9500A were tested in the absence of oil on three hydrocarbon degrading microbial communities at concentration ratios of 0, 1:10, 1:25, and 1:50, as suggested by the EPA. The percent of live bacteria cells was then measured using a Baclight. The control, at a concentration of 0, resulted in 100% bacteria viability, while concentrations of 1:10 and 1:25 had significantly greater negative effects on HDB survivability, where percentages of live bacteria dropped to