38 



4.0 DISCUSSION 



The July 1992 bathymetry at PDS showed a mound at the same location as the 

 January 1989 survey (Figure 3-1, 3-2). The depth difference comparison of these two 

 surveys (Figure 3-3) indicated an accumulation of 1 m about 25 m east of the 1992 buoy. 

 Below 43°34.100' N, another broad area of accumulation is also seen. Negative depth 

 differences between 1989 and 1992 (Figure 3-4) are clustered along steep slopes. These are 

 most likely caused by surveying lane offsets over steeply sloping hard rock ridges (Germano 

 et al. 1993). The extensive areas of dredged material accumulation are not on the steep 

 slopes and are therefore not affected by lane offsets. 



The pattern of sediment accumulation at PDS between January 1989 and June 1992 

 must be viewed in conjunction with the positions recorded in the barge logs for disposal 

 locations. From January 1989 to November 1990, 21 barge loads of material were recorded 

 as being released within 400' of the buoy (43°34.270' N, 70°01.968' W). During this time, 

 one barge load was recorded with a disposal position of 43°34.100' N, 70° 1.900' W. In 

 March and April 1991, 17 barge loads of material were again released within 200' of that 

 point. The release areas around these disposal points are shown in Figure 4-1. The disposal 

 position at 43°34.100' N was the location of the US Coast Guard deployed buoy prior to 

 1985. Since that time, navigational charts and the US Coast Guard light list continue to 

 denote a buoy at that location. Reported disposal points for the contaminated project 

 material released from October to December 1991, and the cap material released from 

 January to June 1992, are also indicated on Figure 4-1. Most of the contaminated project 

 material was released just north of the buoy location; one barge load on November 8, 1991 

 was released at 43°34.100' N, 70°01.900' W. Cap material was released at various points 

 over the project mound. However, three barge loads were released to the east of the project 

 area. These barge loads were released at a buoy location that was recorded by vessels in the 

 area on January 30, 1992 (Figure 4-2). 



The disposal locations for the material released in 1989/1990 and for the USCG 

 capping project correspond to the areas of accumulation between the 1989 and 1992 surveys 

 (Figure 4-1). There is no apparent accumulation at the release point recorded for the spring 

 of 1991, but there is accumulation approximately 100 m to the south. The July REMOTS® 

 survey detected dredged material around the designated disposal area as well as to the south. 

 The material detected along the southern edge of the deposit may have been a thin layer 

 deposited since 1989 (and therefore undetectable to bathymetric surveys), but it may also 

 have been material that was released prior to 1989 that is still visible in REMOTS® sediment 

 profile photographs. 



The silt/clay material from the USCG project that was unsuitable for unconfined open 

 water disposal was capped with cleaner silt/clay material as well as with sand from the 

 Northeast Petroleum project. The concurrent release of these two types of cap material made 



Monitoring Cruise at the Portland Disposal Site, July 1992 



