TABLE 3. Average hydrocarbon concentrations (ppm) in the sediments in 

 the Bays of Morlaix and Lannion from July 1978 to February 

 1979. 



Oil Pollution in Core Sediments 



In August 1978, 24 several-meter-long vibracores were taken to as- 

 certain the vertical distribution of oil incorporated into bottom 

 sediments. Complete data concerning sedimentary characteristics and oil 

 chemical analysis are presented by Marchand and d'Ozouville (1981). A 

 comparison of grain size analyses of surface (Hamon) grab and vibracore 

 samples revealed some differences. The vibracoring technique disturbed 

 the first 10 cm of surface sediment layer as expressed by a shortage of 

 fine particles. In another respect, we noticed that hydrocarbon levels 

 in surface layer sediments were low, from 1 to 50 ppm (m = 22 + 19 

 ppm), except for two stations in the Bay of Morlaix (AF 132: 100 ppm; 

 AF 139: 174 ppm) . These values were lower than those reported for 

 surface sediments collected by the Hamon grab in July and November 1978 

 (Table 4) . These observations are an indirect confirmation that oil is 

 inclined to be adsorbed on fine sedimentary particles. 



Hydrocarbon analyses of deep layers in the sediment cores did not 

 reveal any significant vertical penetration of oil. Organic extract 

 (EXT) and/or hydrocarbon (HC) concentrations very often remained 

 homogeneous in the whole of the cores. The most important levels were 

 found in the first 20-30 centimeters, corresponding to the layer sampled 

 by the Hamon grab. A diving survey undertaken during the same time by 

 d'Ozouville et al. (1979) gave the same conclusion: the depth of oil 

 penetration was usually given to less than 7 cm, possibly related to the 

 depth of biological reworking. Table 5 presents some results showing 

 the absence of a deep diffusion of oil into bottom sediments. Reference 

 data relating hydrocarbon content to sediment type for 17 cores are 

 presented in Table 6. 



150 



