do not affect the analyses of trends based 

 on changes at individual sites. 



NATIONWIDE DISTRIBUTION OF 

 CHEMICAL CONCENTRATIONS 



In 1990, the Mussel Watch Project 

 sampled 214 sites — the most sites 

 sampled in any single year since the 

 project began. Data from 1990 are used 

 to describe the distribution of chemical 

 concentrations throughout the Nation. 



As the mercury and chlordane concen- 

 trations in Figure 3 illustrate, chemicals 

 in mollusks at most sites are at the low 

 end of the overall concentration range, 

 and as concentrations increase they are 

 found at fewer and fewer sites. This type 

 of distribution was also found for chemi- 

 cal concentrations in sediments at NS&T 

 sites (O'Connor, 1990), and is common 

 among environmental data sets. As Fig- 

 ure 3 also exemplifies, when the loga- 

 rithms of the concentrations are plotted, 

 the distributions become bell-shaped and 

 can be said to be "log-normal." An 

 advantage of this distribution is that it 

 allows a statistically objective defini- 

 tion of a "high" concentration as one 

 where the logarithmic value is more 

 than the mean plus one standard devia- 

 tion of the logarithms for all concentra- 

 tions. As demonstrated in Figure 3, the 

 mean and "high" concentrations for 

 mercury and chlordane are 0.094 and 

 0.24 ug/g (dry) and 14 and 31 ng/g, 

 respectively. For those and the other 

 chemicals being evaluated in this report, 

 the mean and "high" concentrations for 



1990 are listed in Table 2. 



This definition can be used to identify 

 which Mussel Watch sites have mol- 

 lusks with "high" concentrations of each 

 chemical. Appendix A lists, in clock- 

 wise geographic sequence from Maine 

 to Hawaii, all sites sampled from 1986 

 through 1990. It also indicates which 

 chemicals, if any, had concentrations in 

 the high range. Copper, zinc, silver, lead 

 and chromium have been excluded be- 

 cause, as just discussed, concentration 

 comparisons for those elements can only 

 be done among sites with a common 

 species. 



On a national scale, high levels of or- 

 ganic contamination are clearly indicted 

 in the urbanized areas of Boston, New 

 York, Mobile, San Diego, San Fran- 

 cisco, and Los Angeles. There is also a 

 tendency for concentrations to increase 

 in these and other areas as sites are 

 located closer to population centers. In 

 Galveston Bay, for example, the higher 

 concentrations are found near the ship 

 channel to Houston rather than toward 

 the Gulf of Mexico. 



High concentrations associated with sites 

 away from population centers are not all 

 readily explained. Total butyltin (tBT) 

 is high near marinas because it was used 

 on boats, but high concentrations of 

 other chemicals may be unrelated to 

 human activity. High concentrations of 

 cadmium in mussels collected along the 

 coast of Northern California have been 

 attributed to naturally high levels in 



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