59 



of the DAM OS Program were primarily 

 heavy metals and hydrocarbons such as oil 

 and grease. In the following discussion 

 the comparisons stem from these 

 measurements, although it is important to 

 recognize that, as research continues, more 

 information is available on the toxicity of 

 various compounds. In recent years, data 

 have been collected on additional 

 contaminants such as PAHs, PCBs, and 

 pesticides, which pose distinct ecological 

 and potential human health risks. 



Most of the sediment samples were 

 obtained using a 0.1-m 2 Smith-Mclntyre 

 mechanical grab sampler. On maximum 

 penetration, expected for most of the fine- 

 grained dredged material, this sampler will 

 extract a sediment section extending to 

 approximately 25 cm below the sediment- 

 water interface. Typically, subsamples of 

 this section are obtained using individual 

 sections of plexiglass core liner 

 approximately 6.5 cm in diameter and 

 10 cm in length (e.g., SAIC 1990a). In 

 the early years of the program, similar 

 cylindrical plastic tubes were used by 

 divers to directly sample the surface of the 

 capped disposal mound for subsequent 

 chemical analysis (Morton et al. 1984b). 



Sediment chemical analyses have been 

 conducted by the NED laboratories. The 

 quality of laboratory data was assessed 

 primarily by its reproducibility. In 

 general, each station was sampled three 

 times, but the variation between sets of 

 data is not consistent. Considering the 

 variable composition of dredged material, 

 the majority of scatter between three 

 replicate points may reflect the true nature 



of the sediment. For example, if three 

 samples were taken at a station which has 

 only a thin cover of a particular type of 

 sediment, the three sample results could 

 show a mix of the types of sediment 

 present. In the ensuing discussion, data 

 which are presented are station averages. 

 This averaging serves to "smooth" the 

 relative concentrations at each station. 



The CLIS reference station has been 

 sampled repeatedly since the beginning of 

 the DAMOS Program. A measure of data 

 precision was obtained by comparing 

 measurements of samples taken at different 

 times (Section 3.3.3). In general, the 

 reference values have been consistent. 



3.3.2 Surface Sediment Geochemical 

 Model 



A model was developed to describe the 

 changing chemistry of surface sediment 

 collected at a station during different 

 phases of capped mound development 

 (Figure 3-11). Before disposal, measured 

 concentrations of contaminants should be 

 within the range of reference station values 

 (1 on Figure 3-11). After mound sediment 

 deposition, the concentration levels should 

 increase to within the range of those values 

 measured in the source harbors (2). With 

 the deposition of the cap, the contaminant 

 concentrations of the surface sediments 

 should decrease (3). The amount of this 

 decrease is again dependent upon the 

 chemistry of the source area. Sand caps 

 will tend to have lower contaminant 

 concentrations than silt caps because most 

 contaminant species are associated with the 

 fine-grained fraction. Cap contaminant 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



