all sites are listed in Appendix A. The 

 figures and Appendix A highlight 141 

 sites that were sampled in at least four of 

 the five years between 1986 and 1990. 

 Data from those sites are used to iden- 

 tify temporal trends in this report . 



No single species of mollusk is com- 

 mon to all coasts. As a result, it has been 

 necessary to collect four different ones: 

 the mussel Mytilus echilis on the East 

 Coast from Maine to Cape May, NJ; the 

 oyster Crassostrea virginica from Dela- 

 ware Bay southward and throughout the 

 Gulf of Mexico; the mussels M. edulis 

 andM. californianus on the West Coast; 

 and the oyster Ostrea sandvicensis in 

 Hawaii. 



CHEMICALS MEASURED 



The NS&T program monitors concen- 

 trations of trace metals and organic com- 

 pounds. With the exception of chlori- 

 nated organic compounds such as DDT 

 and PCB, which exist entirely as the 

 result of human activities, a certain natu- 

 ral concentration of chemicals exists in 

 mollusks even in the absence of human 

 activity. Chemical concentrations ex- 

 ceeding natural levels should be consid- 

 ered "contamination," and the exact line 

 demarcating natural concentrations from 

 contamination is not easily drawn. It 

 depends on the species of mollusk itself 

 as well as on many local and regional 

 conditions. 



Data on concentrations of the 10 trace 

 metals and five groups of organic com- 



pounds listed in Table 1 are used in this 

 report to describe the distribution and 

 trends of chemical concentrations in 

 mollusks of the coastal and estuarine 

 United States. Concentrations of each 

 of these chemicals can serve as indica- 

 tors of human activity. While the metals 

 all have different uses, they can be cat- 

 egorized as chemicals that have been 

 increasingly discharged to the environ- 

 ment as a result of industrialization. 



The groups of organic compounds, how- 

 ever, cannot be categorized so generally. 

 Two of the groups, total DDT (tDDT) 

 and total chlordane (tCdane), are chlori- 

 nated pesticides. Use of DDT was banned 

 in the United States in 1972. Chlordane 

 use on U.S. crops ended in 1 983, and its 

 use for termite control effectively ended 

 in 1988(Shigenaka, 1990). Polychlori- 

 nated biphenyls (tPCB) are a mixture of 

 chlorinated compounds first used in the 

 1920s for a number of industrial pur- 

 poses. Their high heat capacities and 

 low dielectric constants were exploited 

 for use in electrical transformers and 

 capacitors. PCB use in the United States 

 began being phased out in 1971, and a 

 ban on new uses took effect in 1976. 

 Large changes in concentrations of tDDT 

 and tPCB were seen at some locations in 

 the 1970s following bans on further uses 

 of tDDT and tPCB (Mearnset al.. 1988), 

 but the compounds are still found in 

 tissues of organisms and marine sedi- 

 ments. PCB-containing devices are still 

 in use, chlordane remains in the ground 

 as a termiticide, and DDT remains in the 

 environment because of its resistance to 



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