3.0 RESULTS 



3 . 1 REMOTS® Sediment-Profile Photography 



3.1.1 Presence of Dredged Material 



Dredged material was observed at five of the six transects sampled (Figure 3-1; 

 Appendix). Dredged material in the REMOTS® photographs appeared in two forms: 1) 

 dark silty material typically found in dredged material throughout New England and 2) 

 Boston Blue Clay (Figure 3-2, A and B). Boston Blue Clay, formed from silt and clay 

 particles in glacial melt water, is a common constituent of material dredged from Boston 

 Harbor (Camp, Dresser, and McKee, Inc. 1991). In most cases dredged material appeared 

 as a horizon below a sandy mud layer of reworked sediments several centimeters thick and 

 extended below camera penetration (Figure 3-3). However, in two instances (Figure 3-2, 

 A and B) the sandy layer was very small or absent, and dredged material was near the 

 sediment- water interface. 



Spatial distribution of dredged material was patchy. Dredged material was often 

 seen in only one replicate per station even though the replicates were only a few meters 

 apart. The two transects containing the greatest number of stations with dredged material 

 (six stations) were C3 and C6 (Figure 3-1). Transects CI, C2, and C5 contained two to 

 three stations with dredged material while none was observed at transect C4. 



3.1.2 Grain Size Distribution 



Sediment grain size distribution was uniform throughout the area sampled. 

 Sediments were predominantly silts and clays with a major mode size of > 4 phi 

 (Appendix). With the exception of two stations, C4-2 and 4, grain sizes ranged from very 

 fine sands (3 phi) to silts and clays (>4 phi). Fine-grained sediments were overlain by a 

 surface layer of very fine sands in most of the photographs. Stations C4-2 and 3 contained 

 coarse-grained surface material as large as -1 phi. 



3.1.3 Apparent Redox Potential Discontinuity (RPD) Depth 



The apparent redox potential discontinuity describes the region in which sediments 

 change from a surface oxidizing environment to an underlying reducing environment. The 

 depth of the RPD is determined by the net rate of transport of dissolved oxygen (DO) 

 across the sediment- water interface, and the rate of consumption of DO in the sediment 

 column. Mean apparent RPD depths (Figure 3-4), averaged for each transect by station, 

 were between 1.5 cm and 2.0 cm below the sediment- water interface (Appendix). The 



Monitoring Cruise at the Historic Boston Lightship Disposal Site, August 1994 



