These "highs" serve as a basis for categorizing sites but, as will be discussed, "high" concentrations are 

 not always indicative of environmental contamination. Because organic compounds showed no con- 

 centration differences between mussels and oysters or between two species of mussels, there is some 

 confidence in comparing concentrations of such compounds from all sites, including sites in Hawaii, 

 Puerto Rico, the Great Lakes, and the Florida Keys, even though species from those sites could not be 

 compared with others in terms of their ability to accumulate chemicals. On the other hand, we know 

 that all species are not the same with regard to accumulation of all trace elements. 



For some chemicals there is a tendency for concentrations to decrease with time, so comparisons of 

 concentrations among sites are limited to concentrations measured since 1990. The results are listed in 

 Table 3 and plotted in Figures 2 through 13. If a site was sampled in 3 or 4 of the years from 1990 

 through 1993 and a "high" concentration was found in only one year, that site is not shown on the 

 corresponding figure for that chemical. Such occurrences usually mean that the concentrations were 

 close to but usually below the "high" value. If the site was sampled only once or twice over those years, 

 then even a single exceedance qualified it for inclusion on a figure. In effect, sites are plotted in Figures 

 2 to 15 if the concentration of the given chemical exceeded the "high" value in at least half the years 

 since 1990 in which the site was sampled. 



Despite all those qualifications, two general features about the spatial distributions of "high" concen- 

 trations are worth noting. First, that "high" concentrations can often be attributed to human activities 

 because they are found where human populations are high. Second, there are many instances where 

 trace elements are at "high" concentrations for purely natural reasons and are not evidence of contami- 

 nation. 



Chlorinated hydrocarbons are synthesized chemicals with no natural sources. It is evident in Table 3 

 and Figures 2(EPCB), 3(ZDDT), 4(XChlordane), and 5(ZDieldrin) that their concentrations tend to be 

 high in urban areas. The connection of high concentrations with population centers also holds for 

 polycyclic aromatic hydrocarbons (Fig. 6(XPAH)), mercury (Fig. 7) and zinc (Fig. 8). Butyltin (Fig. 9) 

 concentrations are high at sites near marinas, which are usually near populated areas. 



For some of the other elements, "high" concentrations are found near Boston, New York, San Diego, 

 Los Angeles, Honolulu, and other urban centers, indicating contamination. For nickel, selenium, cop- 

 per, zinc, cadmium, and arsenic, however, at least some of the "high" concentrations are natural. Find- 

 ing such concentrations for nickel (Fig. 10), selenium (Fig. 11), and copper (Fig. 1 2) at every site in the 

 Great Lakes most likely does not indicate environmental enrichment with those elements but, rather, a 

 strong affinity for those elements by the species D. polymorpha and the fact that trace elements are 

 more bioavailable in fresh than in marine waters (Cross and Sunda, 1985). The nickel concentrations 

 on the West Coast may be high simply because rocks in that area are enriched relative to those else- 

 where in the U.S. (USGS, 1981). High concentrations of cadmium (Fig. 13) on the West Coast have 

 been attributed to upwelling of deep ocean water (Goldberg et al., 1983) because such water naturally 

 contains higher concentrations of cadmium than surface ocean water. Elevated arsenic (Fig. 14) levels 

 in the southeast have been attributed to the natural occurrence of economically valuable phosphate 

 deposits in that part of the country (O'Connor, 1992). The high selenium in San Francisco Bay may 

 reflect soil enrichment with that element in California's Central Valley. Drainage from agricultural 

 soils into the Kestersen National Wildlife Refuge has caused selenium poisoning among fish-eating 

 birds (Presser et al., 1990). However, selenium and cadmium are both also high at sites along the Gulf 

 Coast west of the Mississippi River and we have no data on natural enrichments either in that area or in 

 the Mississippi River. 



