MacGREGOR: DDT OFF SOUTHERN CALIFORNIA 



Table l. — Correlation coefficients for 11 parameters relating to DDT and its metabolites in bottom sediments off southern California. 

 Values above 1.000 correlation diagonal are for 103 stations. Values below diagonal are for 76 stations leaving out those 27 stations 

 having total DDT readings greater than 100 mg/m^. For 100 observations, a correlation coefficient of 0.254 indicates a probability 

 of 0.01. 



Los Angeles County sewer outfalls (r = —0.871). 

 Values ranged from 6,600 mg of total DDT/m^ of 

 bottom near the sewer outfalls to about 1 mg/m^ 

 at more distant stations. 



The distribution of DDT was modified some- 

 what by currents which tended to deposit the 

 DDT along the coast and to the northwest more 

 than to the east (Figure 1). The apparent relation 

 between total DDT and depth results from the 

 fact that the sewers discharge into relatively 

 shallow coastal waters and the sludge tends to 

 remain there. The deeper waters are merely 

 farther from the sewer outfalls and the areas 

 along the coast favored by the currents. 



McDermott et. al. (1974) took sediment sam- 

 ples from the Palos Verdes shallow-water shelf 

 area in the vicinity of the sewer outfalls only. 

 Their tables A-1 and A-4 give total DDT in parts 

 per million dry weight from gravity core samples 

 taken in 1972. I have contoured their data 

 (Figure 2B) for the top 10 cm of sediment to 

 compare with the 1971 data (Figure 2A) which 

 has been converted to parts per million dry 

 weight. Their 1973 data in their table A-5 repre- 

 sents parts per million dry weight of total DDT in 

 the top 5 cm of Shipek samples taken in the same 

 area (Figure 2C). In each of the 3 yr the patch of 

 sediment representing more than 100 ppm. total 

 DDT tends to retain its integrity fairly well as an 

 oblong area stretching to the northwest of the 

 sewer outfalls. The contours representing 10 to 

 100 ppm. seem to be expanding somewhat to the 

 northwest and in 1973 to the southeast also. 



At the Los Angeles County sewage disposal 

 plant, most of the solids are removed by centri- 

 fuging, but the supernatant is pumped into the 

 ocean along with the water from the settling 



tanks. This reduces the amount of particulate 

 matter being discharged into the ocean. Never- 

 theless, quantities of relatively DDT-free partic- 

 ulate matter have been deposited on the Palos 

 Verdes shelf since dumping of DDT into the sew- 

 er system was stopped. In time this could cause 

 a change in the DDT profile of the sediments. 



On the other hand, most of the shallow inshore 

 areas along this section of coast tend to have 

 sandy bottoms, and the silt bottoms in the vicin- 

 ity of the sewer outfalls would appear to be 

 unstable artifacts. Storms, tides, and currents 

 could remove or deposit layers of bottom silts in 

 this shallow-water area and further change the 

 DDT profiles. 



Based on the paired samples taken in 1971, the 

 variation within a sampling area for one sample 

 would be roughly plus 100% minus 50% at a one 

 standard deviation level. For an average for two 

 samples it would be plus 70% minus 40%. This 

 could account for the differences in the distribu- 

 ion of total DDT for the 3 yr. However, the 

 similarities are much more striking than the 

 differences. 



High sample weight and high water content 

 both indicate samples containing more silt, while 

 lower weights and lower water content indicate 

 samples containing more sand. Both of these 

 measurements are related to depth, with the bot- 

 tom in deep basins tending to be fine silt while 

 shallow areas tend to be sandy. This tendency is 

 masked in shallow-water areas where there are 

 sewer outfalls which deposit large quantities of 

 fine material in the shallower waters. This is in- 

 dicated by the improvement of the correlation 

 coefficients from 0.643 to 0.761 for sample weight 

 and from 0.771 to 0.921 for percent water with 



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