61 



concentrations may also be slightly higher 

 than reference values. Finally, surface 

 concentrations are expected to equilibrate 

 as ambient sediments are deposited over 

 time and mixed with the cap surface until 

 background levels are once again 

 established (4). 



Chemistry data from STNH-N and 

 STNH-S show that capping of disposal 

 mounds has effectively produced a "layer 

 cake" effect such that lower concentrations 

 of contaminants are measured in surface 

 sediments overlying contaminated dredged 

 material. Time-series data along E-W and 

 N-S transects are available for each 

 depositional phase of the STNH capped 

 mounds. These data provide a 

 representative picture of the change in 

 surface chemistry during the period of 

 dredged material deposition, and serve to 

 test the geochemical model. Sediment 

 copper (Cu) levels at STNH-N will be 

 used to demonstrate the variation in 

 chemical concentrations. In general, 

 metals and oil and grease vary in a similar 

 fashion; specific differences in particular 

 analytes are discussed in the section that 

 follows. 



In May 1979, after formation of the 

 mound of Stamford material, Cu 

 concentrations of surface sediments were 

 three to six times higher than in March 

 before disposal (Figure 3-12). By June, 

 after placement of the sand cap from New 

 Haven harbor, surface Cu concentrations 

 near the center of STNH-N decreased to 

 below reference levels. These results are 

 not surprising considering the lithology of 

 the capping material at STNH-N; sand 



tends to have lower metal concentrations 

 than clay-rich silt because many clay 

 minerals contain metals in the natural 

 environment. At the margin of the capped 

 mound, Cu concentrations were in the 

 range of background levels. By early 

 August, the Cu concentrations near the 

 center of STNH-N began to increase, as 

 predicted by the geochemical model. 

 Eventually, surficial mixing of cap 

 sediments with ambient sediments and 

 deposition of local-source silt onto the 

 capped mound should result in the 

 reestablishment of background contaminant 

 concentrations (Figure 3-12). 



The depositional, and therefore 

 chemical, history at STNH-S is more 

 complex. As with STNH-N, the 

 contaminant load of the surface sediments 

 increased with mound deposition, then 

 decreased with cap deposition. The silt 

 cap effectively reduced the contaminant 

 concentrations in surface sediments to near 

 or slightly above reference levels. 



A series of contour plots of Cu during 

 each depositional phase at STNH-S 

 demonstrate the evolution of that capped 

 mound. The presence of relatively higher 

 Cu levels 200 m south of the center 

 following cap deposition agrees with the 

 detection of thin cap cover noted in 

 bathymetric profile analyses (Figure 3-13; 

 Morton and Karp 1980). Results of the 

 August 1979 survey indicated an increase 

 of contaminant levels to the west of 

 STNH-S, suggesting mixing of Stamford 

 and New Haven sediments, or an errant 

 barge disposal (Figure 3-13). This 

 increase was flagged, and plans for the 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



