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sediment-profile camera data also were collected in February 1996. The precision 

 bathymetric and side-scan sonar surveys were performed over an expanded 1950 m x 

 1000 m survey area in the southern region of the disposal site. The additional survey area 

 was added to aid in establishing a deployment location for a physical oceanographic 

 instrument array (McDowell and Pace 1997). Sediment-profile and planview photographs 

 were collected within the project area to track the newly deposited material and provide 

 photographic baseline data of the project area. 



4.3.1 Bathymetry 



The results of the 1950 m x 1000 m bathymetric survey revealed an irregular 

 topography to the west of the Royal River Project Area (Figure 4-5). A significant 

 difference in depth (approximately 30 m) is noticeable between the northwest and southeast 

 corners of the survey area. At a contour interval of 1 m, the steep slopes and rough 

 topography throughout the 1.95 km^ area were clearly visible, suggesting the presence of 

 exposed bedrock over much of the PDS seafloor (Figures 4-6). The February 1996 

 bathymetric data were re-gridded to the smaller 800 m x 800 m project area, improving 

 resolution for comparisons with the August 1995 dataset. Not unexpectedly, depth 

 difference calculations between the two surveys were not able to resolve accurately the 

 small volume (2,800 m^) of dredged material from the Harraseeket River. Further surveys 

 using photographic technologies did reveal the presence of fresh dredged material (Section 

 4.3.2). 



4.3.2 REMOTS® Sediment-Profile Photography 



SAIC collected three replicate photographs from a total of 24 stations within the 

 REMOTS® sampling grid during the February 1996 field effort (Figure 4-7). Evidence of 

 dredged material deposition was seen primarily in the REMOTS® photographs collected 

 northwest of the buoy location. The character of the dredged material varied from "fresh" 

 looking low optical reflectance fine-grained clay attributed to Harraseekt River material 

 (Figure 4-8), to relic dredged material with a higher reflectance, attributed to older, 

 historical dredged material. 



The recently deposited Harraseeket River material, consisting of a thin layer of dark 

 silt and clay, was seen within 100 m northwest of the PDA buoy position (Figure 4-7). In 

 several replicates, a relic redox potential discontinuity (RPD) layer (the depth of 

 oxygenation in the upper sediment strata) was visible below a newly formed oxygenated 

 layer at the sediment/water interface. The presence of a relic RPD is indicative of recent 

 sediment deposition (Figure 4-8). 



Dredged material thicknesses greater than camera penetration depth were detected to 

 the northwest, more than 100 m away from the center of the survey grid. This suggested 

 The Portland Disposal Site Capping Demonstration Project, 1995-1997 



