EVOLUTION OF THE HYDROCARBONS PRESENT IN 

 THE SEDIMENTS OF THE ABER WRAC ' H ESTUARY 



by 

 Jean DUCREUX 



Institut Francais du Petrole 

 92506 Rueil-Malmaison - FRANCE 



Following the Amoco Cadiz accident, the Institut Francais du 

 Petrole analyzed various samples in an effort to learn the physico- 

 chemical characteristics of the oil pollutant released, and to observe 

 its evolution over time. These studies dealt principally with samples 

 of "chocolate mousse" and of beach-sand type surface sediments taken 

 at various depths. 



In March 1978 a study of the physico-chemical evolution of the 

 hydrocarbons trapped in the subtidal sediments of the Aber Wrac'h 

 estuary was undertaken with the collaboration of the Centre Oceanolo- 

 gique de Bretagne. 



INITIAL CHARACTERIZATION OF THE POLLUTANT 



The description of the Amoco Cadiz 's cargo was undertaken on the 

 basis of samples of foam ("chocolate mousse") taken at Portsall on 

 March 22 and 23, 1981. This method of identification is based on the 

 work of Pelet and Castex, and operates according to a breakdown by 

 chemical family, which has already been used by J. Roucache to make 

 a geochemical study of the organic matter extracted from sediments. 



The pollutant was identified as a mixture of two crude oils 

 — Iranian light and Arabian light — physico-chemical characteristics 

 of which are fairly similar ; they contain 45 to 47 % saturated hydro- 

 carbons, 31 to 34 % aromatic hydrocarbons, 16 to 17 % polar compounds, 

 and 4 to 5 % asphalt compounds (Fig. 1). The ratio of saturated hydro- 

 carbons to aromatic hydrocarbons is on the order of 1.3. 



The saturated fraction is constituted of normal paraffins, iso- 

 paraffins of isoprenoid compounds (pristane, phytane, etc..) and 

 cyclic and polycyclic alkanes or cycloparaf fins . With gas-phase 

 chromatography, n-alkanes in the sample taken at Portsall 23 March 

 follow a regular distribution curve ; her top corresponds to the 

 n-Cl5 and n-Cl6, after which it tapers off regularly to the n-C35 

 (Fig. 1). It is likely, incidentally, that compounds over n-C35 exist, 

 but they have not yet been detected. 



Ratios of pristanes/n-C17 and phytanes/n-C18 in crude oil are, 

 respectively, 0.37 and 0.51. To take into account evaporation pheno- 

 mena and compare with evolved samples, this oil sample was topped at 

 340°C. 



An unresolved complex mixture (UCM) appears under the n-alkanes, 

 constituted of isoparaffins and cycloparaf fins. Alkane (n+iso) 



111 



