Weathering of Tanker Prestige and Erika Fuel Oils N°6 in Marine Environment

The shipwreckage of the oil tanker Prestige and Erika occurred respectively on November the 19 2002 off the coast of Galice in Spain and on December the 12 1999 in the Golf of Gascogne. Both led to large oil spills that contaminated hundreds of kilometres coast: more than 10 000 tons of fuel oil n°6 were released in the Atlantic Ocean in the south of Finistère as a consequence of the Erika and probably four times more from the Prestige east of Galicia. As for the product transported by the tanker Erika, the fuel n°6 of the Prestige was little sensitive to the action of dispersing agents. The water-in-oil emulsion formed during its contact with water led to large sheets that floated and drifted under the sea surface in the direction of the coast. Samples reaching the coast after different residence times in the ocean were analysed and the results were compared to those of the reference product charged. The results showed that the fuel composition were similar in both products. For both fuels, laboratory experiments have been performed in closed systems at 20°C in order to evaluate the maximu m transfer of hydrocarbons to seawater with slow agitation of the water, ensuring a constant homogenisation of the aqueous phase. For both fuels, the hydrocarbon solubilisation at equilibrium after 2 days were very similar. The concentration of BTEX solubilised in seawater was between 210 and 260 μg/L for Erika fuel and 185 μg/L for the Prestige. The concentration of total hydrocarbons was about 560 to 750 μg/L for Erika fuel and 570 μg/L for the Prestige. At the same time, the concentration of solubilised polyaromatic hydrocarbons (according the list of the US-EPA) was from 140 to 190 μg/L for Erika fuel and 133 μg/L for the Prestige. Biodegradability tests of these fuel oil n°6 have been performed using a hydrocarbon degrading microflora whose intrinsic capacity to degrade hydrocarbons has been checked in previous works. In both cases, it appeared that 9% of the maltenes fraction could be degraded under optimised laboratory conditions, linear alkanes below nC40 were nearly totally biodegraded, 98% for Erika and 92 % for Prestige. Concerning the biodegradation of the saturated and aromatic fractions, the biodegradation behaviour is comparable. Even though some differences exists that could be attributed to the respective composition of each petroleum cut. The biodegradation ratio of the total fraction of saturated hydrocarbons was about 21% for Erika and 61% for the Prestige. The polyaromatic hydrocarbons (PAH) with less than four rings were completely biodegraded for Erika while 85% were biodegraded for the Prestige. With 70% and 66 % respectively, the fluoranthene biodegradation are similar. The pyrene concentration, degraded at 25% for Erika, remain constant for the Prestige after 80 days. For higher molecular weight PAH, no significant disappearance could be noticed.