Effect of long term natural weathering on oil composition: study of the 41-years-old Amoco Cadiz and 20-years-old Erika oil spills

Abstract

On 16 March 1978, the oil tanker the Amoco Cadiz, transporting 223,000 tons of crude oil and 4,000 tons of bunker fuel oil, suffered a failure of her steering mechanism and ran aground on Portsall Rocks, on the Breton coast. The entire cargo spilled out as the breakers split the vessel in two, progressively polluting 360 km of French shoreline from Brest to Saint Brieuc. This was the largest oil spill caused by a tanker grounding ever recorded in the world. The consequences of this accident were significant, and it caused the French Government to revise its oil response plan (the Polmar Plan), to acquire equipment stocks (Polmar stockpiles), to impose traffic lanes in the Channel and to create Cedre. On 12 December 1999, the tanker Erika broke up and sank off the coast of Brittany (France) leading to the spill of 20,000 tons of a heavy fuel oil. 400 km of the French Atlantic coastline were polluted. Because of the characteristics of the oil (a very heavy fuel oil with a high content of light cracking oil) and the severe weather conditions (a centennial storm with spring tides) when the oil came on shore, the Erika spill was one of the most severe accidental releases of oil along the French coastlines. All types of habitat were concerned, and pollution reached the supratidal zone affecting terrestrial vegetation and lichens. In 2019, respectively 41 years and 20 years after these major oil spills affecting the French shoreline, a sampling round was conducted at two sites recorded to present some residual traces of oil. Samples of weathered oil were collected, extracted with methylene chloride and then purified through an alumina-silica microcolumn. SARA fractionation and GC-MS analyses were performed in order to assess respectively the total degradation of the weathered oil (amount of saturates, aromatics and polar fraction) and the specific degradation of nalkanes from n-C9 to n-C40, biomarkers (such as terpanes, hopanes and steranes) and PAHs (parents and alkylated derivatives).