64 



placement of additional cap material at 

 STNH-S during the spring of 1980 

 included coverage of the south and west 

 areas of the cap (Morton and Karp 1980). 



The next bathymetric survey (in 

 association with sediment sampling) at 

 STNH-S occurred in November 1979, 

 primarily to measure the effect of the 

 6,000 m 3 of Stamford material which had 

 been deposited in the preceding month. 

 The bathymetric survey indicated an 

 apparent large loss of material (see Section 

 3.1.3). To confirm this unexpected 

 result, and because of the passage of 

 Hurricane David in early September, an 

 additional survey at both STNH-N and 

 STNH-S was conducted in December 

 1979. 



The chemical results of both the 

 November and December surveys showed 

 a shift of Stamford-range contaminant 

 levels from the south and west of STNH-S 

 to the center (Figure 3-14). The increase 

 at the top was most likely due to the 

 September-October deposition of Stamford 

 material. It is unlikely that this increase 

 could be due to stripping off of cap 

 material, or mixing of Stamford and New 

 Haven, during Hurricane David. Cores 

 taken within 60 m of the center of STNH- 

 S in 1990 showed a minimum of 1.5 m of 

 New Haven material above the mound/cap 

 interface (Section 4.0). 



The reasons for the decrease of 

 contaminant levels at the stations west and 

 south of the center between August 1979 

 and November 1979 are unclear. One 

 possibility is that surficial New Haven cap 



material from the center was transported to 

 the flanks as a result of Hurricane David. 

 Geotechnical modeling has suggested that 

 much of the reduction in STNH-S 

 contaminant concentrations could have 

 been due to consolidation. This conclusion 

 does not discount the possibility of minor 

 surficial (cap) reworking and/or slumping. 



Following the deposition of 110,000 m 3 

 of New Haven capping material in the 

 spring of 1980 at STNH-S, contaminant 

 concentrations returned to reference levels 

 everywhere on the capped mound. 

 Subsequent sampling through 1986 has 

 shown no substantial increase in surface 

 contaminants; concentrations have 

 remained near background at STNH-S and 

 continue to approach background at 

 STNH-N. These data support the model 

 described above which predicts that, 

 barring physical disturbance or pore water 

 migration and precipitation, surface 

 sediment contaminant values should remain 

 equivalent to those measured at the 

 reference station (Figure 3-11). 



3.3.3 The Chemistry of Dredged 

 Material 



The attempt to classify sediment by the 

 "amount" of contamination is limited by 

 our knowledge of the effects of 

 bioaccumulation and associated mortality 

 rates, as well as pathways to human 

 consumption, for any given element or 

 compound. In comparing the tables of cap 

 and mound characteristics (Tables 2-1 and 

 2-3), it is apparent that there are no 

 "indicator" species of contamination for 

 the early CLIS projects. For example, 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



