copper is decreasing at six, cadmium at 

 five (Fig. 6a) , silver at four, and zinc at 

 three. Silver is decreasing at all four 

 sites in Delaware Bay (Fig. 6b). Arsenic 

 is decreasing (Fig. 6c) and zinc increas- 

 ing at both sites in Terrebonne Bay. Just 

 as finding clusters of sites with "high" 

 concentrations (Appendix A) argues for 

 those concentrations to be representa- 

 tive of an area, similar trends among 

 sites in an area argues for the trend being 

 real and area-wide. 



NATURAL AND HUMAN 

 INFLUENCES 



If chemical concentrations in mollusks 

 at any particular site increase because of 

 human activities such as industry, agri- 

 culture, mining, or the wastes of daily 

 living, the mollusks can be said to be 

 chemically contaminated. If the chemi- 

 cal supply is purely natural, such as 

 cadmium from deep ocean water bath- 

 ing mollusks in Northern California, 

 then concentrations do not represent con- 

 tamination. For trace elements, there is 

 no absolute way, based on concentra- 

 tion alone, to separate natural from hu- 

 man factors. An approximation of the 

 extent of human influence at a site is the 

 number of people in its proximity. Us- 

 ing 1990 census data (B. Davis, TIGER 

 System Staff, U.S. Census Bureau), 

 Spearman correlations were calculated 

 between numbers of people residing 

 within 20 km (12.4 miles) of each Mus- 

 sel Watch site and chemical concentra- 

 tions in mollusks collected in 1990. As 

 expected, because their existence is en- 



tirely or, to a large extent, due to human 

 activity, the highest correlations were 

 between population and concentrations 

 of chlorinated organic compounds and 

 PAHs. Statistically significant correla- 

 tions were found between population 

 and concentration for lead and zinc in 

 both oysters and mussels, and for cop- 

 per, mercury, and silver in mussels. There 

 is no correlation between population and 

 copper, mercury, or silver concentra- 

 tions in oysters, and no or negative cor- 

 relations between population and con- 

 centration of arsenic, cadmium, chro- 

 mium, nickel, and selenium in either 

 mussels or oysters. 



A lack of correlation on a national scale, 

 however, does not prove that concentra- 

 tions are not affected by human activity 

 at any individual site. There can well be 

 human influences in rural areas. For 

 example, the fact that mercury concen- 

 trations are high in oysters at two rural 

 sites in Matagorda Bay, TX (Appendix 

 A) is probably related to a reservoir of 

 mercury contamination remaining from 

 a major discharge of that element from a 

 chlor-alkali plant in the 1970s (Holmes, 

 1 977). Furthermore, there are rural sites 

 with high concentrations of chlorinated 

 organic compounds whose only source 

 is human activity. Thus, it is not pos- 

 sible, simply on the basis of proximity to 

 population centers, to separate human 

 from natural effects on chemical con- 

 centrations in mollusks. This inability 

 complicates trend detection since it is 

 only the human-influenced component 

 of chemical concentrations in mollusks 



23 



