Degradation of dispersed crude oil

As we have found out in lectures dispersants can play an important role in aiding biodegradation of large oil spills. Many studies have demonstrated how effective the use of such dispersants but few studies have actually assessed the biodegradation of oil once it has been dispersed in environmentally relevant conditions. Once dispersants have been added oil droplets are very small and become diluted within the water column. This study looked at the degradation of oil at these low concentrations, close to those observed following a real spill when dispersants have been used in order to discover degradation is affected by the presence of a dispersant. 

This study is carried out in laboratory with sea water collected in April and January 2010 from the shore of New Jersey and stored in Carboys. No bacteria or nutrients were added so only indigenous bacteria and nutrients were present throughout the study in an attempt to mimic natural conditions for biodegradation. Oil was added to the samples, one lot of oil samples was lightly weathered in the laboratory by evaporation at room temperature until 20% of its weigh was lost (to mimic 24 hours exposure to sea water) and the other was fresh oil. To both types of the dispersant Corexit 9500 was added. A second set of experiments were carried out where no dispersant was added the sea water samples. The experiment was carried out in a cold room at 8 °C.

At designated times water was extracted from each Carboy 3 times and analysed using gas chromatography and mass spectrometry. Oil biodegradation was monitored with respect to 17α(H),21β(H)-hopane as a conserved internal marker within the oil. Oil was found to be degraded quickly and extensively (more than 80% after 60 days). The half times of the biodegradation were found to be similar with and without dispersant at 13.8 and 11 days respectively. The level of oil present was 2.5 ppm by volume which is clearly low and represents levels expected when dispersants have successfully been added. The authors note that this study cannot be used to assess the effectiveness of dispersants but the study shows that under natural conditions biodegradation is not affected by the presence of dispersants but is rapid and extensive.

I found this study interesting as it used natural nutrient levels and only indigenous bacteria. I think for future work it would be good to look at which bacteria are actually present in the sample and in what numbers to gain a more clear understanding of the whole process. It would also be good to compare the results from this study to with water taken from other areas as there would presumably be different naturally occurring bacteria and nutrient levels which would be likely to change the degradation rates.

Prince. R, McFarlin. K, Butler. J,  Febboe. E, Wang. F, Nedwed, T. 2013 The primary biodegradation of dispersed crude oil in the sea. Chemosphere. 90; 521-526.