Almost half the sites are near urban areas within 20 km of urban population centers of more than 100,000. Mussels are sampled at 274 sites around the nation. Half of those sites are within 20 km of urban areas. These levels are not based on toxic effects of the chemicals on organisms. Concentrations defining the high end of the overall distribution of concentrations measured in mollusks by the Mussel Watch Project.
Many chemicals have high concentrations at sampling sites near cities.
For example, there is a strong statistical correlation between human population density and chemical concentrations in oysters and mussels for chlordane, DDT, PCBs, PAHs, butyl tins, and lead. For trace metals other than lead, no association of high concentrations with urban areas is evident. High concentrations of chemicals in mollusks may not diminish the health, longevity, or reproductive capacity of the mollusks or any other organisms. If the organic contaminants or lead are causing biological effects, however, the most likely areas to find these effects would be near the urban areas.