26 



lower values (4, 3, and 4) were all located at SREF. Stations at SEREF had very high OSI 

 values with four stations having a +11 OSI value. 



3.3 Sediment Density 



Density of the upper sediment surface was calculated from the 24 kHz survey 

 conducted concurrent with the bathymetric survey. The software calculated density values 

 from the acoustic return every 20 seconds along the survey track. Density values ranged 

 from 1.2 (clay) to 1.7 gcc 1 (sand) (Figure 3-10). The highest sediment densities, greater 

 than 1.6 gcc 1 , were all located in water depths shallower than 54 m. The apex of the 

 disposal mound had a density of 1.5 to 1.6gcc'. The less dense material ( — 1.4 gcc 1 ) was 

 concentrated in the deeper areas, although the results were somewhat patchy. The patchiness 

 of the data over a large area required a high degree of smoothing during the processing of 

 the data, which tended to decrease the ability to identify specific features. The variability in 

 the acoustic reflection also was a function of the heterogeneity of dredged material; the 

 acoustic reflection of the 24 kHz is affected by differences in porosity, surface "roughness", 

 and grain size, among other factors. 



3.4 Sediment Grain Size and Chemistry 



3.4.1 Grain Size and Total Organic Carbon 



The major mode of the grain size in samples from PDS and reference stations ranged 

 from gravel to clay (Table 3-1). Sediments at the reference stations were medium brown 

 silty sand with gravel at EREF and SREF, and medium to dark brown silty clay at SEREF. 

 At EREF and SREF, the sediment was mostly gravel (18 and 22%) and fine sand (38 and 

 33%). At SEREF, 51% of the samples were silt, and 39% were clay. At the disposal site, 

 the sediments ranged from light grey clay at F3 to poorly graded gravel at F7. Most of the 

 disposal site stations contained more than 50% silt/clay (B5, B7, C3, D5, D7, El, F3, F5, 

 G3, H5, and H7). Stations with coarser grained sediments included E3 (65% fine sand) and 

 F7 (60% gravel). In addition to grain size, the sediment descriptions note shell fragments at 

 F7 and H7, and some grass at B5. 



Total organic carbon was measured at the three reference areas as part of the baseline 

 chemistry data. Average TOC values were lowest at EREF (0.60%) and highest at SEREF 

 (1.3%) (Table 3-2). 



Monitoring Cruise at the Portland Disposal Site, July 1992 



