4J_ 



it impossible to differentiate between the cap and the disposal mound based on grain size or 

 density. The 24 kHz survey of sediment density did not differentiate between the cap material 

 and other dredged material. 



The acoustic sediment density survey showed that, in general, the higher density 

 sediment (coarser grain size) was concentrated in water depths shallower than 54 m, and the 

 lower density sediment (silt and clay) was concentrated in the deepest areas. This overall 

 pattern indicates that potentially the finer-grained materials are settling in the deepest areas of 

 the site. The patchy data, however, prevented a conclusive identification of the project 

 dredged material, and the smoothing of the data tended to inhibit the identification of smaller 

 features related to dredged material disposal. The patchiness was due, in part, to the 

 variability due to rapidly changing slopes in the survey area (as in bathymetry), and the 

 heterogeneity of dredged material and, in particular, of the cap material that was being 

 mapped. Finally, the acoustic method of characterizing bottom sediment is still being refined; 

 the strength of the bottom reflection is a function of the acoustic impedance contrast between 

 the water column and the bottom sediments and is, in theory, directly related to sediment 

 density. More recent work has shown, however, that the strength of the return is also affected 

 by such sediment properties as porosity, surface "roughness" (particularly a problem with 

 heterogeneous dredged material), and grain size, among other factors (LeBlanc et al. 1992). 



A capping project is designed to isolate contaminants in the dredged material by 

 covering the dredged material with cleaner sediment which may have contaminant 

 concentrations comparable to, or somewhat greater than, reference. Reference data collected 

 at the PDS were compared to ambient sediment chemistry values (metals and PAHs) as 

 measured by the NO A A National Status and Trends Program in two areas near PDS: Casco 

 Bay (CSC) and Stover Point (MSSP). Metals and PAH data normalized to silt/clay were 

 collected from 1984 to 1986 for CSC and in 1988 for MSSP (Table 4-1). 



Mean metals values for the three PDS reference areas (Table 3-4) are within the ranges 

 for the NS&T stations. Based on the average concentrations of metals in the NS&T stations 

 and at the reference areas, it appears that the reference areas chosen for PDS are representative 

 of the ambient sediment in the Gulf of Maine. The total LMW and HMW PAHs at reference 

 areas EREF and SEREF (Table 3-5) are below the average total PAH values found at the 

 NS&T stations. At reference area SREF, the total HMW PAHs were 11.00 ppb compared to 

 9.90 ppb at MSSP. The total LMW PAHs at SREF, 2.39, was within the ranges found at the 

 NS&T stations. 



Given that the reference areas appear to be representative of the area, metals 

 concentrations from samples taken on the cap (Table 3-4) can be compared to the reference 

 areas. In general, all stations on the cap except for F7 and H5 had normalized metals values 

 within the ranges found at the reference areas, indicating that the cap has effectively isolated 



Monitoring Cruise at the Portland Disposal Site, July 1992 



