distribution of contaminated material is quite heterogenous. The 

 concentrations of lead ranged from 14 6 to < 12 ppm and the range 

 of concentrations measured for petroleum hydrocarbons was 13 00 

 to <50 ppm. Total organic carbon values were between 0.94 and 

 0.01 %. The concentrations of several of the contaminants were 

 below the analytical detection limits; these included cadmium, 

 nickel and DDTs. 



The levels of contaminants in the dredged material 

 deposited at this site are shown in Table 1-1; these levels are 

 generally similar to those measured at the disposal site except 

 for mercury and PCBs, which had much higher levels in the 

 original dredged material. 



4.0 DISCUSSION 



4 . 1 Bathymetry 



Approximately 197,000 m 3 of dredged material has been 

 deposited at the "DGD" buoy since this disposal location was 

 established in November 1985. It is not certain whether the lack 

 of a distinct, bell-shaped disposal mound, typical of shallower 

 sites, is due to the depth of water and the behavior of the 

 dredged material during convective descent in these depths or to 

 the distribution of the actual locations of individual scow loads 

 during disposal (Figure 4-1) . Previous studies have indicated 

 FADS to be a containment site with no significant bottom currents 

 capable of sediment resuspension and transport. Because of the 

 depth of the site, the acoustic bathymetric records can reliably 

 detect large-scale changes in depth over the entire area surveyed 

 on the order of 50 cm or more. This value reflects the 

 accumulation of errors introduced by the positioning system, 

 tidal corrections, and the calibration of the fathometer (speed 

 of sound through the water column) as well as the vertical motion 

 of the research vessel during the survey. During disposal, the 

 falling sediment is expected to spread because the greater depth 

 offers more time for entrainment of water into the sediment 

 (Figure 4-2) . As more water is mixed into the sediment during 

 convective descent, a larger percentage of the original dredged 

 material volume forms sediment/water mixtures with densities 

 approaching that of the surrounding seawater. The falling 

 velocity of these mixtures is greatly reduced so that the 

 increased descent time to the bottom results in a greater 

 spreading of the material as compared with a shallower location. 



Results of the REMOTS® survey in this area revealed the 

 presence of dredged material within a circle with a radius of 

 approximately 500-600 meters. Spreading 197,000 m 3 of dredged 

 material over that area in an even layer would result in an 

 estimated thickness between 17 - 25 cm. This thin layer of 

 dredged material would not be detectable acoustically. 



