11 



3.0 RESULTS 



3.1 Bathymetry 



A comparison of the August 1990 and November 1988 precision bathymetric surveys 

 showed that a distinct mound was formed at the "MDA" buoy between these two surveys. In 

 August 1990, the minimum depth at the disposal point was approximately 88.50 m (Figure 3- 

 1), compared to a depth of 89.25 m in November 1988 (Figure 3-2). A depth difference 

 contour chart (Figure 3-3) indicated that the deposit had a maximum thickness of 0.8 m and 

 was centered slightly east of the buoy. The average diameter of the deposit was 420 meters. 

 Depth differences on the order of 20 cm (i.e., approaching the limits of detection in this 

 comparison of the 1990 and 1988 surveys) occurred within 400 m of the disposal mound 

 center. Depths within the surveyed area ranged from 87.25 m in the southwest to 92.25 m 

 in the northwest. 



A depth matrix comparison of the 1988 and 1990 bathymetric surveys resulted in a 

 volume calculation of 78,075 m 3 (95% confidence limits; 55,500 m 3 to 100,650 m 3 ) of 

 material deposited since the November 1988 survey. Total volume estimates, including an 

 estimate based on dredged material detected with REMOTS®, are discussed in the next 

 section. 



3.2 REMOTS® Sediment-Profile Photography 



The major modal grain size over the surveyed area ranged from fine sand (3-2 phi) to 

 silt-clay (> 4 phi; Figure 3-4). For most of the disposal site stations and the three reference 

 areas the major mode was > 4 phi (Figure 3-5). Coarser sediments, consisting of patches of 

 fine (4-2 phi) to medium (2-1 phi) sands intermixed with some silt-clay, were located within 

 200 m north, 200 m south, 300 m east, and at the center of the disposal site (Figure 3-6). 



Small-scale surface boundary roughness values at the disposal site stations were 

 significantly greater than those for the reference areas (p<0.05, Mann- Whitney U-test). 

 Frequency distributions for small-scale surface boundary roughness indicated a major mode 

 at 0.6-1.0 cm (class 2) for disposal site stations and at 0.0-0.6 cm (class 1) for the reference 

 areas (Figure 3-7). Values for the disposal site stations reflected the physical disturbance 

 related to disposal operations. 



Dredged material layers presumed to be recently deposited (i.e., since the January 

 1989 survey) were evident in the REMOTS® photographs from stations surrounding the 

 disposal buoy (Figure 3-8). The presence of "relic" dredged material at most of these same 

 stations made the precise boundaries of this deposit difficult to determine. This "relic" 

 material was presumed to be the result of disposal operations which have been conducted at 



Monitoring Cruise at the Massachusetts Bay Disposal Site, August 1990 



