FIGURE 3.35. 



Offshore surface sediment and l'Aber Benoit sampling 

 locations (Marchand, Courtot) . 



Hydrocarbon concentrations and source classifications for the 

 entire data set are shown in Table 8. Individual aromatic hydrocarbon 

 determinations by GC/MS appear for several time series in Tables 9 

 through 13 and for two of the L'Aber Benoit samples in Table 14. 



An instructive way of viewing the time series information is 

 presented in Figures 3.36 and 3.37. At both the Terenez/ Morlaix and 

 lie Grande time series, concentrations increased between April and July 

 1978. In the case of the Terenez samples, the increase is due to 

 offshore transport of weathered oil as evidenced by 1) an increase in 

 absolute concentrations, 2) a decrease in the ALK/ISO ratio, and 3) an 

 increase in phenanthrenes (total P, C L , C 2 , C 3 , C 4 P) and dibenzothio- 

 phenes, without an accompanying increase in the pyrogenic PAH compounds 

 (m/e 202). However, the lie Grande benthic samples show an increase in 

 total hydrocarbons along with increases in the aromatics including the 

 pyrogenic PAH. This latter finding indicates that both petroleum 

 hydrocarbons and combustion-related PAH material are being transported 

 to and deposited in the offshore sediments near lie Grande by a similar 

 mechanism, most likely in association with suspended matter from 

 riverine plumes. Figure 3.38, a plot of phenanthrene and its alkyl 

 homologues at the Morlaix Station (Station B) , reveals that while the 

 source of the phenanthrenes is petroleum in July 1978, as evidenced by 

 the greater abundance of alkylated compounds versus the parent (unsub- 

 stituted) compounds, the input in February of 1979 is largely pyrogenic 

 (i.e. greater amounts of parent phenanthrene). This illustrates both 

 the usefulness of detailed GC/MS-derived data and their subsequent 

 presentation in alkyl homologue distribution plots. 



63 



