Trace metal Region Cadmium Sites in the Great Lakes Copper Sites in the Great Lakes Nickel Sites in the Great Lakes Selenium Sites in the Great Lakes Arsenic Buzzards Bay, MA Arsenic All sites from Cape Hatteras, NC to Matanzas River, FL Cadmium Sites in MS and LA close to the Mississippi River Nickel Chesapeake and Delaware Bays Selenium Many sites in LA and TX Selenium Prince William Sound, AK High concentrations of four trace metals in zebra mussels of the Great Lakes are probably due to those metals being more bioavailable in fresh water than in seawater or to that species' being more capable of accumulating the metals than marine mollusks. Because a fresh-water species and a marine species have no common site, it is impossible to directly test these ideas. There is no tendency for the concentrations among the sites at the Great Lakes to be high only near population centers, however, so it is likely that they are high for natural reasons. Contamination at non-urban sites can be related to natural causes. Nickel concentration is high in the Chesapeake and Delaware Bays due to nearby ore deposits enriched with this trace element.
Independent evidence indicates that some of the other regionally high concentrations are natural.
The area around the Chesapeake and Delaware Bays was the world's major source of chromium in the 19th century, and nickel, found today in high concentrations in both bays, is enriched in the same minerals that provide chromium. Similarly, the facts that arsenic is chemically similar to phosphorus and that the Southeast is rich in phosphate deposits may be responsible for the high arsenic concentrations in that region. As discussed in the following Case Study, although the source is unknown, the high cadmium concentrations in Louisiana and Texas may be caused by human activity.