FISHERY BULLETIN: VOL. 74, NO. 1 



of the total DDT (102.7 metric tons). Sixteen sta- 

 tions representing 18.1 sq nautical miles (2.0% of 

 total area) contained 64.0% (193 metric tons) of 

 the total DDT. 



Subsamples taken from the tops and bottoms of 

 the blocks of sediment obtained with the Shipek 

 sampler indicated that most of the pesticide was 

 concentrated in the top strata of the samples ex- 

 cept for samples taken in the vicinity of the sewer 

 outfalls where deposition was very rapid. Cores 

 were taken from one sample taken near the sewer 

 outfalls and from a second taken at a greater dis- 

 tance from the outfalls to determine more about 

 vertical distribution of DDT in the sediments 

 (Table 3). 



At station 42-36 only p,p' DDE was measured 

 because DDT and DDD were not readily measur- 

 able in the deeper sediment sections. Half of the 

 DDE was found in the top 2 cm, 81% in the top 4 

 cm, and 95% in the top 6 cm. At station 42-20, 

 close to the sewer outfall where sewer sediment 

 deposition was heavy, there was very little 

 change in the chlorinated hydrocarbon concen- 

 trations at all five depths. 



Vance McClure (pers. commun.) has provided 

 me with a plot of the depth distribution of DDT, 

 DDE, DDD, and DDMU found in a box core sam- 

 ple taken about 1 nautical mile west-northwest of 

 the sewer outfall. Subsamples were taken from 

 the core at 3-cm intervals from to 12 cm and at 

 6-cm intervals from 12 to 36 cm. The pesticide 

 values remained high through 12 cm depth and 

 dropped off rapidly between 12 and 18 cm. DDMU 

 had a deeper distribution than the other three 

 components and increased to a maximum at 9 cm 

 and was still present at 36 cm. DDE was last 

 measured at 24 cm, and DDD and DDT at 18 cm. 

 Excluding DDMU, 72% of the pesticide was found 

 in the column corresponding to the top 10 cm and 

 28% below that depth. Including DDMU, 67% 

 was in the top 10 cm and 33% below. 



If the box core sample is typical of the stations 

 near the sewer undergoing rapid sedimentation, 

 about 30% of the pesticide was missed by sam- 

 pling only to a depth of 10 cm at these stations. 

 Because these stations near the sewer outfalls 

 contain most of the pesticide, the 217 metric tons 

 of pesticide estimated for the entire area in the 

 top 10 cm could be increased to roughly 300 met- 

 ric tons as a maximum estimate of total DDT in 

 the area. 



In the area of the Palos Verdes shelf only, 

 McDermott and Heesen (1974) estimated that 



Table 3. — Vertical distribution of DDT in the sediments as 

 determined from core samples taken at stations 42-20 and 

 42-36. 



Stn. 42-20 



Sfn. 42-36 



there were 218 tons of total DDT under 62 km^ of 

 bottom. They calculated that 85% of the total 

 DDT was in the top 12 cm of sediment. If the 

 pesticide is fairly equally distributed in the top 12 

 cm, about 14% would be in the 10- to 12-cm layer, 

 and the Shipek sampler would sample about 71% 

 of the total DDT. 



Sixteen contiguous stations on the Palos Ver- 

 des shelf sampled by us in 1971 represented an 

 area of 18.1 sq nautical miles (62.0 km^) and a 

 total DDT load of 139 metric tons. If this was only 

 71% of the total DDT in the area (the load of 

 the top 10 cm only), then the corrected esti- 

 mate including DDT below 10 cm would be 196 

 metric tons. 



McDermott et al. (1974) using a reduced sam- 

 pling area of 48 km^ determined that there were 

 156 tons of total DDT in their revised sampling 

 area. In this present study the area can be ad- 

 justed to 48 km^ by omitting the effect of ^Vi 

 peripheral stations. Estimated total DDT then 

 would be 132 metric tons. However, McDermott 

 et al. (1974, table 5) give estimates of total DDT 

 in the area in 2-cm increments down to a depth of 

 30 cm of sediments. This table indicates that only 

 about 59% of the total DDT is in the top 10 cm in 

 this area. This would increase my estimate of 

 total DDT to 224 metric tons for the 48 km^ area. 



The available data indicate that there is con- 

 siderable variation in the depth distribution of 

 total DDT in the sediments on the Palos Verdes 

 shelf However, the general conclusion that can 

 be drawn from the samples is that there are about 

 200 metric tons of total DDT in the bottom sedi- 

 ments in the 14 sq nautical mile area (48 km^) in 

 the vicinity of the sewer outfalls and another 100 

 metric tons in the remaining 897 sq nautical 

 miles of the 1971 survey area. 



On 27-28 June 1972, 11 mo after the first sam- 

 ples were taken, additional samples were ob- 

 tained from seven of the original stations. Four of 

 these stations were in deeper water, between 600 



34 



