Dynamics of spilled oil natural transformation on the sea coasts in different geographical conditions.
Résumé
The paper is devoted to the comparative analysis of results of long-term field observations carried out on geographically different sea coasts contaminated by spilled fuel oil. Such investigations are of particular interest as they make it possible to assess the rates of oil pollution natural transformation as well as their correlation with principal geographical factors such as climatic and hydrological conditions, coast exposure and geomorphology, sediment types, intensity of biogeochemical cycles.
For these purposes, the authors have chosen three zones where the accidental oil spills occurred in the last years: on the north-western coast of France (tanker “Erika” accident in December 1999; accident at an oil refinery in the Loire River Estuary in March 2008), on the north-western coast of Spain (tanker “Prestige” accident in November 2002) and in the Strait of Kerch (tanker “Volgoneft-139” accident in November 2007). The duration of long-term observations ranged from 5 to 13 years.
The present investigation included visual assessment and sampling of oil slicks and sediment cores. The analysis of samples were carried out with the use of thin layer and column chromatography, optical and gravimetric methods making possible to determine separately the sum of saturated, mono- and di-aromatic hydrocarbons (HC), the sum of polycyclic aromatic hydrocarbons (PAH) and the sum of asphaltic components (AC) as well as the presence of hydrocarbons of contemporary biological origin.
The obtained results show that even 10 to 13 years after the oil spills their weathered traces are still present in the coastal zone, especially in its upper part. In the course of time an exponential diminution in the oil pollution level and the ratio between hydrocarbons and asphaltic components (HC/AC) was revealed. In the studied cases, the half-periods of fuel oil natural transformation varied from 1 to 5 years. The highest activity of the self-cleaning process was observed for small fuel oil traces on the rocky substrate exposed to the influence of fresh air, solar radiation, storm waves and surf action. On the Strait of Kerch coasts influenced by shallow, slightly salted and highly bio-productive waters of the Sea of Azov the spilled fuel oil tends to disappear more rapidly than on French and Spanish coasts washed by the Atlantic Ocean.