Evolution des Hydrocarbures et de l'Activité Bactérienne dans des Sédiments Marins Contaminés par un Déversement de Pétrole Brut et Traité

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Abstract
The fate of an experimental oil pollution of intertidal sediments in a sheltered beach of North Brittany (France) has been investigated over a 16-month period. Chemical treatments were applied to two of the three contaminated plots by premixing oil respectively with dispersant and biodegrading agents. The physico-chemical and bacteriological characteristics of the polluted areas were followed with the purpose of identifying the limitating parameters for oil microbial degradation and the effect of treatments. The concentration of hydrocarbons in the oiled sediments did not change significantly during the experimental period. Spectrofluorimetric and chromatographic data showed that the main evolution of oil concerns the degradation of n-alkanes and the removal of light aromatics. Biodegradation of hydrocarbons occurred at a measurable rate only during the warm seasons (average temperature 18± 2[ddot]C) causing after sixteen months the disappearance of more than 80% of the n-alkanes fraction independently of the pollution sediment level and the chemical treatment of the experimental plots. However the biodegradation of n-alkanes proceeded during the first months, at different rates, inversely depending on oil content in the collected samples. The main limitating factor is dissolved oxygen according to the fact that spilled oil was located at 3–5cm depth in a poorly oxygenated zone characterized by low redox potential. Nutrients were not a limitating factor probably due to domestic and agricultural inputs in this area. A marked bacterial growth was observed two weeks after the oil spill with a relative increase in hydrocarbon degrading bacteria with respect to total heterotrophs. Degradation rates, based on C14 n-hexadecane experiments, seem to follow the same way than specific bacterial counts (plate technique). Specific bacteria are always high at the end of our 16 months' field experimentation. In the laboratory as well as in the field experiments, the same behaviour of untreated and chemically treated oil was observed in partially anaerobic sediment.