85 



most realistic possibility is that the 

 material recovered at CS-2 CTR was New 

 Haven cap material. The concentrations of 

 contaminants were within the ranges of 

 New Haven dredged material disposed 

 elsewhere (e.g., Cap Site 1, Mill- 

 Quinnipiac River). 



One of the original concerns about the 

 success of the capping project was that a 

 silt cap would make cap/mound distinction 

 difficult. However, cores from STNH-S 

 showed very clear chemical and visual 

 boundaries (Figure 4-4). Both metal and 

 organic data show a bimodal distribution 

 between the three samples taken from 

 areas in the core documented in the visual 

 core descriptions as being mound material 

 of sediments from Stamford, Black Rock, 

 and New Haven Harbors, and core 

 samples from Cap Site 2 (160-200 cm at 

 60NE and 160-180 at CTR) and cap 

 material (Figures 4-7). R 2 values are 

 0.841 at 60NE and 0.993 at CTR. This 

 suggests that silt caps are just as effective 

 at containing contaminants and may cause 

 even less disturbance than sand caps 

 deposited on silt. 



4.3 Dredging Effects on Sediment 

 Texture 



The uniform texture and contaminant 

 levels within the STNH-N cap may be, in 

 part, due to the use of a hopper dredge 

 (hydraulic) to collect and dispose of the 

 coarse material from outer New Haven 

 Harbor. STNH-N was deliberately 

 constructed with a coarse cap, and the 

 hopper dredge was used to produce an 

 even coverage of material. During the 



hydraulic dredging process, the sediment 

 texture is destroyed, and the pore waters 

 and sediments (with adsorbed 

 contaminants) are well mixed (Bohlen 

 1990). Sediments deposited using this 

 dredging method might meet the general 

 assumptions that cap material should have 

 relatively low and uniform contaminant 

 loading and texture (cap at STNH-N) and 

 be distinct from the mound material, 

 which would be expected to have relatively 

 high and variable contaminant loading and 

 texture. 



A clamshell dredge (mechanical) was 

 used for both the mound and cap at CS-2, 

 the cap at STNH-S, and the mound at 

 STNH-N. If there is any stratification or 

 variation in texture and contaminant 

 loading within the original deposit, 

 mounds and caps formed with this type of 

 dredging should not be expected to meet 

 the general assumption of uniform 

 contaminant loading and texture. If the 

 original deposit is relatively uniform in 

 texture or contaminant loading, the process 

 of clamshell dredging may preserve this 

 uniformity in mounds or caps (mound at 

 STNH-S). 



Applying this awareness of dredging 

 processes to the data leads to a new set of 

 hypotheses. With clamshell dredging, 

 some of the stratification, texture, and 

 contaminant distribution from the original 

 deposit may be preserved and observed in 

 long cores taken from disposal mounds and 

 caps. Based on results from a grab 

 sampling study, Morton and Karp (1980) 

 suggested that localized heterogeneity in 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



