134 



PCB concentrations in the sand cap were found to be as much as 60% of 

 the concentration detected below the sand-mud interface and all PCB 

 concentrations measured in the sand cap were greater than the mean PCB value 

 calculated for the original capping material. Consequently, the authors concluded 

 that the PCBs in the sand cap were derived from the contaminated material. In 

 other words, PCBs had moved from the dredged material into the cap material 

 within three and one-half years. The authors final conclusion was that PCBs 

 were being forced out of the muddy sediments during mound compaction and 

 residing in solution in the pore water of the cap. 



iv. Edges of sand caps are vulnerable to bioturbation 



Benthic organisms that live in the top layers of sediments become exposed 

 to contaminants as they feed and burrow through the sediments. This activity is 

 called bioturbation and is the major route of exposure for contaminants to marine 

 organisms that capping of dredged material is supposed to prevent. The 1987 

 study of the Mud Dump Site did find bioturbation on the sand cap itself. But 

 bioturbational mixing of the thin sand layer with underlying muds on the flanks 

 of the EMD. resulting in a "fairly healthy benthic community. .established in the 

 peripheral areas" was documented" 3 . The authors predicted that as bioturbational 

 mixing continues, the sand cap would continue to shrink. 



c. There is substantial evidence of material moving outside the Mud Dump 

 Site boundaries 



An ACOE sponsored field study was conducted in 1986 to "continue 

 research into the effectiveness of capping 24 ." It found significant accumulation of 

 sediment to the south and east of the present disposal point in the northeast 

 quadrant, which the authors attributed to a "lack of tight controls on disposal 

 operations and/or hydrographic energy regime." More work was considered 

 necessary to discover which reason was the cause of the movement off site. The 

 authors predicted that high energy storms could develop waves of sufficient height 

 to resuspend bottom sediment at least at the shallower depths of disposal mounds. 

 Obviously, once suspended, dredged material can be transported by prevailing 

 currents. 



:3 See footnote 17. pg. 24. 

 u See footnote 17 



