23 



A bathymetric survey was conducted in 

 August 1981 following the completion of 

 disposal operations. The final reported 

 disposal volumes for the 1981 disposal 

 season were approximately 70,000 m 3 of 

 highly contaminated material capped with 

 280,000 m 3 of low to moderately 

 contaminated material from the outer 

 Norwalk Harbor, resulting in a 4:1 

 capping ratio (Table 2-4; Morton et al. 

 1984b). There is some potential error in 

 the estimates of volumes of low, medium, 

 and highly contaminated material dredged 

 from Norwalk Harbor because these 

 classifications are an arbitrary scale used 

 to describe natural sediment variations 

 (Section 3.3). The estimates of "cap" and 

 "mound" ratios at NOR are problematic 

 because the records of each barge load 

 sediment contaminant class and its ultimate 

 disposal point are no longer available. In 

 addition, this cap:mound ratio is 

 misleading because again, as at STNH-S, 

 the cap and mound materials were 

 interspersed, especially since a significant 

 portion of the "cap" material was 

 deposited first. 



The bathymetry from a January 1982 

 survey showed a decrease of 

 approximately 1 m across the capped 

 surface, probably due to consolidation 

 (Section 3.1.3; Morton et al. 1984b). 

 Subsequent bathymetric surveys have 

 shown a stable dual mound configuration 

 (Figure 2-5). 



2.3.2 NOR Operations: Conclusions 



Subsequent monitoring has shown no 

 negative impact of the disposal operations 

 at NOR. Several factors were notable: 



• Operational procedures should 

 emphasize coherent mound disposal 

 followed by cap disposal. 



• If relatively uncontaminated 

 sediments need to be removed first, 

 they should not be included as part 

 of the mound. 



• Accurate sediment classification of 

 source material for mound and cap 

 should be established; the 

 gradational character of Norwalk 

 "cap" and "mound" sediments made 

 later distinction between these two 

 problematic. 



2.4 Mill and Quinnipiac Rivers 



During the spring of 1982, NED 

 initiated a third capping project to 

 accommodate sediments to be removed as 

 part of federal maintenance dredging of 

 areas in the Mill and Quinnipiac Rivers 

 adjoining the northern limits of New 

 Haven Harbor (Figure 2-6). Preliminary 

 surveys showed sediments within the Mill 

 River to contain concentrations of oil and 

 grease sufficient to place them in the 

 NERBC highly contaminated category 

 (Tables 2-1 and 2-2). The material was 

 characterized by high concentrations of 

 fibrous residue or wood pulp, which 

 limited sediment cohesion. This unique 

 sediment texture, combined with the 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



