Data output from the Acoustic Core System® included amplitude and acoustic 

 impedance values. Acoustic impedances have been reliably assigned to different sediment 

 types and, therefore, can be used to detect changes between sediments with dissimilar 

 impedance characteristics (Hamilton 1970, 1971). Impedance values were converted to 

 density values and mapped to quantify changes in sediment type. For a more detailed 

 discussion of the analysis procedure, see Caulfield and Yim (1983) and Caulfield (1984). 

 The density values converted from the impedance values were compared to the density values 

 calculated for the surface sediment samples at the cap site to ground truth the data. 



2.4 Sediment Sampling and Analysis 



Sediment samples were collected from the center of the three PDS reference areas 

 (SREF, SEREF, and EREF) and from 13 stations located within the lateral limits of the cap 

 at PDS (H5, F5, D5, B5, H7, F7, El, F3, G3, E3, C3, D7, B7). The stations on the cap 

 correspond to the REMOTS® stations. The sediment samples were collected with a 0.1 m 2 

 teflon-lined Van Veen grab sampler. Three samples were collected for analysis at SEREF, 

 and two were collected from EREF and SREF due to difficulty in collecting sediments. One 

 grab sample was collected from each of the 13 stations on the cap (Figure 2-4). Each grab 

 at the reference stations was subsampled for metals, polycyclic aromatic hydrocarbons 

 (PAHs), and grain size and % total organic carbon. The grabs from the surface of the cap 

 sediment were subsampled for metals and grain size. Sediment to be analyzed for metals and 

 PAHs was placed in precleaned (acid- washed) glass jars. Sediment to be analyzed for grain 

 size and % total organic carbon (TOC) was placed in plastic bags. Samples were kept cold 

 (approximately 4° C) and delivered to the NED laboratory. 



2.4.1 Grain Size Analysis 



Physical analysis of sediments by the NED laboratory included visual classification, 

 and grain size analysis (sieve and hydrometer) using ASTM Method D-422 (ASTM 1990; 

 Table 2-2). Grain sizes were classified using the Wentworth (phi) scale: -2 to -1 phi for 

 gravel, between -1 and +4 phi inclusive for sand, between +4 and +8 phi inclusive for silt, 

 and greater than or equal to 9 phi for clay. Prior to initiating the grain size analysis, a 

 subsample (approximately 5-20 g) was taken for total solids analysis for determination of 

 moisture content. A sieve analysis was then performed in which the sample was separated 

 into size fractions greater than 62.5 iim (<4 phi - sand and gravel), and less than or equal to 

 62.5 iim (>4 phi - silt and clay). The gravel/sand fraction was subdivided further by 

 mechanically dry sieving it through a graded series of screens. The wet sieved and dry 

 sieved fractions less than 62.5 /xm were combined for each sample. The silt/clay fraction 

 was then subdivided using a pipet technique which utilizes the differential settling rates of 

 particles of different sizes. 



Monitoring Cruise at the Portland Disposal Site, July 1992 



