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3.2.1 Bathymetry 



We compared the September 1997 bathy metric survey results with the July 1996 

 survey and calculated the depth difference (Figure 3-2). The bathymetric chart shows the 

 development of the I mound just southeast of the buoy location. The mound reached a 

 maximum height of 3.73 m above the 1996 baseline chart, with an approximate width of 150 

 m. The 0.25-m apron covers the northwestern portion of the G mound. Previous 

 REMOTS® surveys had indicated poor environmental conditions, characterized by slow 

 recolonization rates and decreased RPD depths due to the oxidation of high labile organic 

 concentrations, in an isolated region south of the D mound and west of the G mound. 

 Although the mound was not placed directly over this area, apron mound material potentially 

 has covered some of the problem region area at 1996 REMOTS® station D200S. 



3.2.2 REMOTS® Sediment-Profile Photography 



REMOTS® sediment-profile photography was used to document benthic 

 recolonization as well as delineate the placement and any environmental impacts of the 

 recently disposed dredged material. The September 1997 survey occupied a 13-station 

 cross grid with stations located 50 m apart over the I mound (Figure 2-2). The March 

 1998 survey sampled a 5-station grid, including the center and four 50-m stations for 

 comparison. Complete REMOTS® results for the new disposal mound are available in 

 Appendices B and C. 



3.2.2.1 Sediment Grain Size and Stratigraphy 



In September 1997, we detected dredged material at all I mound stations and in 

 most replicates. Replicates at 150E indicated a thin layer of dredged material, 6 cm to 

 7 cm, over ambient sediments. Dredged material thickness was greater than penetration at 

 most of the stations. The average mean camera penetration was 13.40 cm, which was 

 greater than measured at the reference areas (Section 3.4). 



A thin layer of brown, oxidized silt over black sulfidic mud characterized the 

 sediment in the photographs. The lighter surface layer indicated the depth of RDF and 

 oxygenation. Redox rebounds, which are stratigraphic differences in color or shade below 

 the surface layer on the sediment-profile image, were evident at Stations lOOS, lOOW, and 

 150E, all of which were outside of the 0.25 m depth mound apron identified in the 

 bathymetric survey. The presence of a redox rebound interval in a new sediment deposit 

 suggests a gradual decline in pore water oxygen content, which could be attributable to a 

 decrease in regional bottom water dissolved oxygen concentrations. Redox rebounds tend to 

 be driven by seasonal effects. For the central five stations only, redox rebounds increased 



Monitoring, Cruise at the WLIS Disposal Site, September 1997 and March 1998 



