76 



3.1.4 Sediment Coring 



Sediment cores were collected in 1997 and 1998 in a sampling pattern of concentric 

 zones defined by the monitoring plan (see Section 2, Figure 2-6). The objectives of this 

 sampling were to assess the physical and chemical composition of the sediments near the 

 surface of the mound and to verify the presence of at least 50 cm of cap material. 



The physical and geochemical measurements from the cores are reported in 

 comparison to the following: a core collected simultaneously at the reference area (WEST 

 REF); historic NLDS reference area values used as guidelines in permitting for NLDS 

 (Murray 1995); and samples collected from the dredging area (Maguire Group 1995). Data 

 from the dredging area were classified, based upon location within the Thames River, as 

 "UDM" and "CDM," assuming that the dredging sequence followed the project design 

 (Appendix E). For the Seawolf project, sediments were dredged from the Thames River 

 channel between the 1-95 Bridge and Navy Pier 33. The sediments from Piers 31 to 33 and 

 the central channel were classified as UDM, while sediments from the northern and southern 

 regions of the channel were classified as CDM based on chemical and biological testing. 

 Sediments were tested in 1990, 1992, and 1994 (Maguire Group 1995). There were some 

 discrepancies between the three sediment chemistry datasets used for comparison. The 

 samples collected from sediment cores, tested in 1990, represented sediments planned to be 

 dredged to a depth of -43 feet mean low water (MLW). The regulatory agencies reviewed 

 the data and determined that further testing was required. In 1992, surface layer (0 to 3 feet) 

 and deeper sediment (>3 feet) samples were collected using a clamshell bucket. Because of 

 the different sampling techniques employed, the samples from 1992 and 1990 could not be 

 statistically compared. 



The 1992 data did reveal that the near-surface sediments contained higher contaminant 

 levels than deeper sediments. In 1994, the required dredging depth was reduced to -41 feet 

 because the submarines were shown to be capable of transiting in a water depth of 39 feet. 

 The regulatory agencies conducted further testing for the revised dredging depth using the 

 same coring method as 1990. Although some differences in detection limits were observed, 

 the 1990 and 1994 sediment chemistry data were generally comparable. The grain size 

 analyses varied significantly in samples taken from nearby locations in 1990 and 1994. In 

 1990, the average fraction reported as "percent silt or finer" was 46.6% with reasonable 

 variation around the mean. All of the samples collected in 1994 were reported as 100% silt or 

 finer. No grain size data were reported in 1992. For comparison purposes, only grain size 

 data from 1990 were reported. 



Monitoring Cruise at the New London Disposal Site, Seawolf Mound 1995 - 1998 



