18 



4.0 DISCUSSION 



Since 1985, the majority of dredged material has been disposed in the southwestern 

 quarter of MBDS near the MDA buoy (Figure 2-1; Germano et al. 1994). The distribution 

 of metals and organics was examined in order to determine the nature and extent of 

 contaminants in the western half of MBDS, and to identify areas that should be capped with 

 material suitable for open ocean disposal. 



Sediment samples have been collected at MBDS and at the MBDS reference areas 

 several times since 1985. Sample results from the 1989 survey were compared both with 

 these historical MBDS data, and with sediment chemistry data from coastal Massachusetts 

 collected by the National Oceanic and Atmospheric Administration (NOAA) through the 

 National Status and Trends (NS&T) Program. 



4.1 Spatial Distribution of Chemical Constituents 



Historical disposal activity north and west of the current disposal point may have 

 influenced the present distribution of sediment contaminants; the distribution of each metal 

 shows a similar pattern. The ranges of trace metal concentrations were limited. A contour 

 plot of Cu concentrations shows a band of higher concentrations in the center of western 

 MBDS (along Stations 12-3, 11-6, and 11-8; Figure 4-1). Station 12-3, located on the far 

 western edge of MBDS, had the highest measured concentrations of Cr (139 ppm), Cu 

 (78 ppm), Zn (221 ppm), and Hg (0.47 ppm). 



The distribution of DDE is relatively random, with two distinct highs at Stations 12-9 

 and 18-17 (> 10 ppb). The spatial distribution of PAHs is even more indicative of an 

 off site concentration to the west (Figure 4-2). Although PAH concentrations are questionable 

 at Stations 9-8 and 8-5 due to elevated detection limits, the contoured pattern suggests that 

 these samples were taken in an area of relatively lower PAH concentrations (Figure 4-2). 



4.2 Comparative Analysis of MBDS Sediment Chemistry Data 



4.2.1 Historical MBDS Data 



Reference area concentrations of trace metals were near or below the average of 

 MBDS stations sampled in the 1989 survey. Concentrations of trace metals at both reference 

 areas have remained relatively constant since reference area 18-17 was first sampled in 1985 

 (SAIC 1987; Table 4-1). Concentrations of trace metals detected at a few isolated stations at 

 MBDS, when compared with reference area results, indicated no significant accumulation of 

 trace metals, relative to the reference areas. 



Chemical Analyses of Sediment Sampling at the Massachusetts Bay Disposal Site, June 1989 



