TABLE 8. Organic carbon content of the Aber Wrac'h sediments, 

 number of observations. 



(n) = 



In this discussion, three areas of Aber Wrac'h are described in 

 terms of oil degradation:: (1) the mouth of the estuary (fine-grained 

 sands, station 3), (2) the downstream part (stations 1, 2, 4, and 5), 

 and (3) the upstream part (stations 6, 7, 8, and 9). The evolution of 

 oil pollution in sediments from 1978 to 1981 for each station is given 

 in Figure 8; the change in average oil concentrations for each of the 

 three defined areas is given in the Table 9 and Figure 9. In March 

 1978, 15 days after the AMOCO CADIZ wreck, concentrations ranged from 

 773 ppm to 12,000 ppm. At the mouth of the Aber which is well exposed 

 to high marine energy, hydrocarbon content dropped from 773 ppm in March 

 1978 to 25 ppm in March 1981, illustrating a fairly rapid 

 decontamination of these fine-grained sands. 



Since the sediments of the Aber Wrac'h are relatively homogeneous, 

 the decontamination process is mainly related to the energy level of the 

 zone. In the downstream part of the Aber, the sediments were more 

 polluted in March 1978 (average about 5,000 ppm) than the sediments 

 collected in the upstream part (average about 1,500 ppm). For the first 

 39 months after the spill, a natural but slow decontamination was 

 observed with some temporary increases in January 1980 (about 1,800 ppm) 

 and March 1981 (about 780 ppm). In June 1981, the average residual oil 

 content was about 600 ppm. In the upper part of the Aber, the sediments 

 were initially less polluted, but since this is a low-energy area, a 

 decrease in hydrocarbon content was not observed until January 1981. As 

 had been observed in the downstream portions of the Aber, significant 

 increases in hydrocarbon levels were observed during January 1980 in 

 upstream areas. Since March 1981, oil levels have decreased; average 

 residual oil concentrations were about 670 ppm. 



Three years after the wreck, the petroleum pollution of the Aber 

 Wrac'h sediments seems to be relatively uniform. Figure 10 gives the 

 observed decontamination rate of the sediments. At first approximation, 

 station 3 (located at the mouth of the Aber) is well decontaminated (3% 

 of the initial residual oil level observed in March 1978). However, 

 sediments within Aber Wrac'h remain quite polluted with about 20 percent 

 of the residual oil content still remaining in March 1978. 



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