87 



contaminant loading might be diagnostic of 

 dredged material. 



Since both the cap and the mound are 

 composed of dredged material, any 

 certainty that the interface between the cap 

 and the mound will be distinct and easily 

 recognizable visually or chemically is 

 reduced. There may be several interfaces 

 between successive barge loads. If the top 

 of the disposal pile happens to contain the 

 low end of the range of contaminants (i.e., 

 deeper or coarser dredged material), and 

 the first barge load of cap material 

 happens to have the high range for this 

 material (i.e., surface or finer dredged 

 material), the interface may appear to be 

 blurred or mixed. Therefore, it may be 

 impossible to distinguish inherent 

 variability from variability due to mixing 

 during the disposal process. 



For example, at CS-2 the transition 

 area within core 80NE could be interpreted 

 as a result of limited mixing of materials 

 between the cap and the mound during 

 deposition. However, it seems more likely 

 that the top of CS-2 around this core 

 originally consisted of a mixture of sand 

 and shell which was mistaken for cap 

 material. The potential for incorporation 

 of coarse, relatively clean material into the 

 mound material during the dredging 

 operation is relatively high, given the 

 nature of clamshell dredging operations. 

 These results suggest that variation in 

 contaminant levels within horizons and 

 correlation of contaminants with sediment 

 texture are not necessarily diagnostic 

 criteria. Without further evidence to 

 distinguish the cause, both conditions 



could result from mixing of cap and 

 mound materials during disposal, from 

 preservation of variability introduced 

 during dredging, or from a combination of 

 the two. 



The criterion that is most likely to 

 influence management decisions is the 

 presence or absence of gradients of 

 contaminants within the cap. Given the 

 processes that govern the deposition of the 

 dredged material, gradients would not be 

 expected unless the contaminants were able 

 to migrate from higher concentrations in 

 the mound material up through the cap 

 toward the lower concentrations at the 

 sediment- water interface. If this 

 hypothesis is the most crucial, future 

 sampling of cores taken from capped 

 mounds should target testing for the 

 presence of chemical gradients. The 

 absence of a correlation between sediment 

 texture and contaminant levels would also 

 be an indicator that contaminant 

 mobilization may have taken place. This 

 effort, while not practical for individual 

 disposal projects, could be implemented as 

 part of an organized research effort. If 

 there is no evidence of gradients, or there 

 is strong correlation between contaminant 

 concentration and sediment texture, there 

 may be no cause for further investigation. 

 If evidence of gradients does exist, pore 

 water sampling could confirm or exclude 

 the hypothesis of contaminant mobilization 

 and potential availability to the benthic 

 ecosystem. 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



