22 



contaminated material would be deposited 

 within CLIS at a point separate from, and 

 to the west of, the Stamford-New Haven 

 capped mounds. The new disposal site 

 was named Norwalk (NOR) after the sole 

 source area (Figure 2-1). The 

 contaminated deposit was to be capped by 

 cleaner (having low to moderate levels of 

 contaminants) material obtained from the 

 outermost sections of the navigation 

 channel. 



The estimated 1600 m 3 of sediment 

 containing nitrobenzene and naphthalene 

 would be placed as a conservative measure 

 in a subaqueous pit to be dredged within 

 the harbor and covered with 1-2 m of 

 clean sediment. Justifications for open- 

 water disposal were based on the 

 previously demonstrated ability to achieve 

 point placement and coverage in the 

 Stamford-New Haven project, and the 

 results of laboratory bioassays simulating 

 benthic conditions which showed that, 

 despite elevated contaminant 

 concentrations, exposure to the Norwalk 

 sediments resulted in negligible biological 

 impacts (ERCO 1979). 



2.3.1 NOR Disposal Operations 



Clamshell dredging of Norwalk Harbor 

 began on 11 April 1980. Harbor water 

 depths limiting dredge access necessitated 

 the removal of some low to moderately 

 contaminated sediments before the highly 

 contaminated material. The low to 

 moderately contaminated sediments were 

 transported to the selected disposal point 

 (Figure 2-1) via hopper barge. Dredged 



material deposition was concentrated in the 

 south of NOR (Morton 1981). 



Plans called for placement of highly 

 contaminated sediments prior to final 

 deposition of cleaner sediments to form the 

 finished cap. The initial dredging 

 continued until 30 May 1980 when 

 operations terminated to avoid the shellfish 

 spawning periods. On termination, 

 approximately 67,900 m 3 of sediment had 

 been dredged, of which 19,900 m 3 was 

 estimated to be highly contaminated 

 material (Feng 1982). 



The Norwalk dredging and open water 

 disposal resumed on 31 January 1981 and 

 continued through 3 June 1981. 

 Monitoring surveys during this period 

 indicated a rather haphazard distribution of 

 material and a less than optimum mound 

 and cap coherence (Morton 1981). Feng 

 (1982) and Brooks (1983) reported that 

 approximately 180,300 m 3 of additional, 

 relatively clean sediment from Norwalk 

 Harbor was placed at NOR in the period 

 between January and June 1981 (Table 

 2-4). 



A bathymetric survey conducted in the 

 middle of this period of disposal (27-29 

 April 1981) showed a new mound to the 

 north of the disposal buoy, containing 

 primarily sediments from the northern end 

 of Norwalk Harbor. The volume disposed 

 between 3 1 January and the period of the 

 survey (late April) was calculated to be 

 about 60,000 m 3 (Morton 1981), 

 approximately 1/3 of the total placed 

 through June (Table 2-4). 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



