22 



reasonable surrogate and matrix spike recoveries, which also indicates that these data are 

 valid. Therefore, the decrease in PCB concentrations in MBDS sediment is apparently real. 



Commercial production of PCBs ceased in 1977 (Kennish 1992). These compounds 

 persist in estuarine and marine sediments and have a half-life in environmental samples from 

 8 to 15 years (Kennish 1992). They may be nearly undetectable in the water column, but 

 tend to accumulate in biota (Pequegnat et al. 1990). The observed decreases in PCB 

 concentrations may be related to a lower environmental input than in the past. Reduced 

 inputs through better management have now allowed reductions to occur over the past 5-10 

 years or more. Subsequent burial or dilution with newer deposited (natural) sediments 

 containing lower levels of PCBs seems the most likely explanation. 



4.2.2 Regional Data Comparison 



Samples from coastal and estuarine sediment data from 300 sites in the United States 

 have been collected and analyzed during NOAA's NS&T since 1984. Several sites along the 

 coast of Massachusetts and in Boston Harbor were sampled from 1984 to 1989 (NOAA 

 1991). Stations closest to MBDS, including Cape Ann, Duxbury Bay, Salem Harbor, and 

 Quincy Bay, were compared to the MBDS results. None of the NS&T stations were located 

 as far offshore as MBDS. However, since much of the material sampled at MBDS was 

 presumably material dredged from Massachusetts harbors, the comparison is appropriate to 

 place the chemistry data in a relative context. 



In comparing the MBDS results with NS&T data, several qualifiers must be made. 

 The analytical methods vary somewhat between the two; NS&T methods were developed for 

 very low detection limits. For example, although many PAHs were lower than the MDL in 

 MBDS samples, detection limits were higher than those of the NS&T data, so the two 

 datasets are not directly comparable. Individual concentrations of detected PAHs (rather than 

 total HMW or LMW PAHs) were compared with NS&T data, because of the uncertainty of 

 summing values which are below detection. PCBs are analyzed by NS&T methods as 

 individual isomers, and are not comparable with the total PCB measurement obtained by the 

 NED laboratory. 



Metals. MBDS metals data were compiled, and several ranges of values were 

 compared both with regional Massachusetts and national NS&T data. The maximum and 

 minimum metals values, and the statistical range of values (mean ± one standard deviation) 

 were reported for MBDS data, and compared with four regional sites and the values reported 

 by NS&T for national mean and tiigh" values (greater than one standard deviation above the 

 national mean; NOAA 1991). The four regional areas used for comparison are: Cape 

 Ann-Gap Head (CAGH), Salem Harbor (SAL), Quincy Bay (QUI), and Duxbury Bay-Clarks 

 Island (DBCI; Figure 1-1). Data over several years were averaged from each of these sites. 



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



