Polychlorinated biphenyls are organic contaminants that were formerly used widely in industrial applications. Along the Mississippi River, they are typically most highly concentrated in suspended sediments near Minneapolis and St. Louis. Industrial activities in the Minneapolis-St. Paul region led to PCB concentrations there that were five to ten times higher than in other parts of the river. Concentrations of PCBs were greatest in sediments between Minneapolis-St. Paul and Lake Pepin. Increased concentrations near St. Louis reflect the input of suspended sediments from the Ohio River, which usually contain more PCBs than do the waters in the middle reaches of the river. Hexachlorobenzene, another organic contaminant of industrial origin adsorbed to suspended sediments, is derived predominantly from the Ohio River and the industrial corridor along the lowermost 400 kilometers (248 miles) of the Mississippi River. There are hundreds of different kinds of PCBs, and numerous medical studies show that they have a variety of human health effects. In addition to the direct implications of PCBs for human health, bioaccumulation of PCBs in fish tissue is another key concern in the Mississippi River (see Box 2-1).
Toxic Substances and Fish Contamination
A key concern for commercial and recreational fishermen on the Mississippi River is the existence of toxic substances in the river’s fish populations. States along the Mississippi River issue various versions of fish consumption advisories, which are usually based on concentrations in fish tissue. Fish tend to accumulate long-lived, slightly soluble chemicals such as PCBs, pesticides, and herbicides in their fatty tissue. Concentrations of toxic substances in fish tissue can be much higher than in the water. Most of the 10 states along the mainstem Mississippi River list some reach as being of impaired water quality, and most of these impairments are based on fish tissues that contain unacceptable concentrations of toxic substances. For example, Illinois, Minnesota, Missouri, Tennessee, and Wisconsin list the entire river for PCBs; Tennessee lists dioxin and chlordane; and Minnesota and Wisconsin list mercury, all on the basis of high concentrations in fish.
PCBs are legacy contaminants that are stored in bed sediments in the navigation pools of the upper Mississippi River. The concentrations in the upper (10 centimeters) sediments are high below Minneapolis-St. Paul, reach their highest values in Lake Pepin, and are significantly lower in the pools downriver of Lake Pepin (Rostad et al., 1994). After they were banned in 1977, concentrations of PCBs in the upper layers of bed sediments decreased dramatically, especially in pools 2-9 (UMRCC, 2002). Evidence of the contaminant legacy, however, is seen in deeper buried sediments, where concentrations are much higher (Rostad et al., 1994). Chlordane concentrations also decreased, especially in pools 10-26 (UMRCC, 2002).