44 



5.0 CONCLUSIONS 



The July 1992 monitoring survey at PDS succeeded in further delineating the dredged 

 material within the site boundaries. Two apparent disposal mounds were found by 

 comparing the 1989 and the 1992 bathymetric surveys, one representing the capped mound 

 around the 1989/1992 buoy location and one to the south nearer the 1984 buoy location, an 

 area that received material after 1989. Dredged material distribution, as detected by the 

 REMOTS® sediment-profile survey, includes these two areas of accumulation and extends 

 over a broad area to the south as well. The distribution of dredged material in the 

 REMOTS® photographs may reflect historical (pre- 1989) dredged material as well as material 

 released between 1989 and 1992 that was not thick enough to be detected acoustically. 



The 24 kHz survey was not effective in distinguishing the cap material. The 

 heterogeneity of the cap, a mix of coarse and fine-grained sediment, resulted in patchy data 

 over a large area. The high degree of smoothing necessary during data processing decreased 

 the ability to identify specific features in this data set. The "density" as measured by the 

 system is only related to the strength of the acoustic signal. Recent evidence that other 

 factors may influence signal strength (i.e, surface roughness, porosity) make the attempt to 

 distinguish the cap material by this method alone even more difficult. 



The effectiveness of the cap in isolating contaminants was determined by examining 

 the sediment chemistry values and the benthic biology. Normalized sediment chemistry 

 results from samples on the cap, with the exception of stations F7 and H5, show metals and 

 PAH values within the ranges found in the ambient sediment at the reference areas. Further, 

 these values were generally similar to or lower than values measured in cap material prior to 

 dredging. The location of F7, near the site of the 1984 buoy where a small volume (625 m 3 ) 

 of contaminated material was disposed, suggests that additional cap material should be placed 

 there as a precautionary measure. Stage UJ organisms were prevalent on the cap, indicating 

 that the benthic environment on the cap is healthy and that the cap material has most likely 

 isolated contaminated material from the sediment/ water interface. 



Monitoring Cruise at the Portland Disposal Site, July 1992 



