3.0 RESULTS 



3.1 Bathymetry 



Within the MBDS survey area, depths sloped northeastward from 87 m in the 

 southwest corner to more than 90 m in the northern portion of the survey grid (Figure 3-1). 

 The April 1992 bathymetric survey detected two mounds located west of the buoy location. 

 The minimum water depth at the highest mound located within 100 m west of the buoy was 

 85.75 m. The formation of the mound was more distinct than in the August 1990 survey 

 which showed the maximum height of the mound to be located 100 m east of the buoy and a 

 minimum water depth of 88.50 m (Figure 3-2). 



A comparison of the depth matrices from 1990 and 1992 indicated that the dredged 

 material deposited during that time had a maximum thickness of 2 m (Figure 3-3). The 

 shape of the deposit was elliptical, with most of the material accumulated within a 200-400 m 

 area. A depth difference between the 1990 and 1992 bathymetric surveys gave a volume 

 calculation of 151,764 m^ (95% confidence limits; 121,801 m^ to 181,727 m^) for material 

 deposited since the August 1990 survey. 



3.2 REMOTS® Sediment-Profile Photography 



REMOTS® sediment-profile photography detected dredged material out to 600 m east, 

 400 m south, 800 m west, and 400 m north of the center of the mound (Figure 3-5). The 

 most recent material released at MBDS was Boston Blue Clay from the Central Artery/ Third 

 Harbor Tunnel Project. The distinctive signature of the Blue Clay, its light greenish-gray 

 color and homogeneous, fme-grained appearance, was seen in those stations closest to the 

 disposal site center (Plates 1 & 2) (Figure 3-5). Relic dredged material, i.e., dredged 

 material deposited prior to the Boston Blue Clay (Plates 3 & 4) was found further from the 

 center of the site, particularly to the west. The REMOTS® survey indicated that all of the 

 dredged material was contained well within the disposal site boundaries with the exception of 

 dredged material found at station B07 (Plates 5 & 6). All reference stations contained 

 ambient silt-clay sediments. 



Both Stage I and Stage III taxa were present at MBDS. Stage III taxa, indicative of a 

 mature, healthy benthic community, were evident at all reference site stations and at '35 out 

 of 43 stations at the disposal site (Figure 3-6). Stations immediately north and west of the 

 mound center had evidence of Stage I organisms (small, pioneering polychaetes). At the 

 center of the mound, penetration by the camera prism was low due to the high shear strength 

 of Boston Blue Clay, and only one out of the three replicate photographs was analyzed. The 

 successional stage was indeterminate because of the low penetration. Successional stage data 

 were not obtained at three stations located at 18-17 REF (lOON, 200N, and 2(X)W) because 

 of overpenetration by the camera prism in the soft sediments. 



Monitoring Cruise at the Massachusetts Bay Disposal Site, March 31 - April 4, 1992 



