77 



4.1 Results of the Coring 

 Investigation 



Both CS-2 and STNH-N had sand 

 caps; based on previous studies, the visual 

 interface between cap and mound was 

 expected to be more obvious than in cores 

 from STNH-S, which had a silt cap. At 

 STNH-N, the coarse-grained cap (sand and 

 shells) was fairly uniform in texture (some 

 bands of shell hash) and had low levels of 

 contaminants (Figure 4-2). There was a 

 sharp visual transition from the cap to the 

 mound sediments in all of the cores except 

 40W. The mound sediments were 

 relatively uniform in texture (black organic 

 silt) with high levels of contaminants. 



In comparison to STNH-N, the cap 

 material at CS-2 was variable in sediment 

 texture (sand, shell, and silt) and 

 contaminant loading (Figure 4-3). Based 

 on chemical results, it is apparent that 

 mound material was recovered only at 

 80NE and CTR, even though it was 

 described as being present at 40E. The 

 visual transition from the cap to the mound 

 was not as obvious as at STNH-N. The 

 zone of transition at 80NE appeared to 

 extend over 30 cm, and the transition in 

 contaminant levels occurred within the 

 bottom of a sand layer which had been 

 defined visually as cap material. The 

 mound material was also variable in 

 texture (shell and silt) and contaminant 

 loading. 



At STNH-S, the cap material was 

 highly variable in sediment texture (Figure 

 4-4). The visual appearance was one of 

 very distinct bands of high organic (black) 



and low organic (grey) silt and clay. The 

 contaminant loading was moderate and 

 variable. Despite this variability, the 

 visual and chemical transition to mound 

 material was distinct. Again, it is 

 apparent from the visual descriptions and 

 the chemical results that mound material 

 was recovered only in two cores: 60NE 

 and CTR. STNH-S received a large 

 amount of cohesive cap material which 

 formed a relatively thick layer on top of 

 the mound. Despite success in taking long 

 cores, most of the material recovered was 

 cap material. The mound material 

 contained high levels of contaminants and 

 a uniform texture of dry, black organic 

 silt. In this case (in contrast to STNH-N 

 and CS-2), the variability of the cap 

 material made it distinctive and 

 recognizable, and the uniformity of the 

 mound material made it easier to 

 distinguish. 



4.2 Geochemistry of CLIS Cores 



The hypothesis that the contaminants 

 are chemically isolated is dependent, in 

 our model, on the lack of chemical 

 gradients in the sediment samples. This 

 hypothesis is constrained largely by 

 sampling. Samples were taken every 

 20 cm in order to avoid bias introduced by 

 field interpretation of a boundary. In 

 some cases, the sample boundary did 

 happen to coincide with a visual boundary. 

 However, if a sample was taken in a 

 transition zone, it is impossible to 

 distinguish whether an intermediate level 

 of contamination resulted from an actual 

 gradient in the sediment (which may 

 indicate remobilization of contaminants) or 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



