Many Mississippi River water quality issues of today resemble the issues of the early 1970s, when the Clean Water Act was being drafted, but their relative importance has shifted in the past 35 years. Water pollution control measures (e.g., the National Pollutant Discharge Elimination System, discussed further in Chapter 3) have reduced point source pollutant inputs from industrial and municipal discharges. This has, in turn, reduced many serious water quality problems such as oxygen depletion caused by organic wastes, thermal pollution, oil slicks, phosphate detergent wastes, and sediments from larger construction sites. In addition, removal of lead from gasoline and the banning of some industrial chemicals such as polychlorinated biphenyls (PCBs) and pesticides such as chlordane, aldrin, dieldrin, and DDT (dichlorodiphenyltrichlorethane) have greatly reduced the amount of toxic substances in the Mississippi River. Pretreatment programs in larger cities have reduced discharges of heavy metals and other toxic materials from municipal wastewater treatment plants (see Chapter 4, Box 4-1 for further discussion of water quality improvements under Clean Water Act-related projects).
Despite these advances, the Mississippi River today is affected by water quality problems and challenges that include nutrients, sediments, toxics, and fecal bacteria. Toxic substances—metals and organic chemicals—are primarily legacy contamination issues, although there are continuing inputs, especially of pesticides. These substances have chronic ecosystem and human health impacts and are difficult to address, because river bottom sediments are the primary reservoir and source of these materials in many reaches of the river. High counts of fecal bacteria, once a public health problem at raw sewage discharges all along the Mississippi River, were substantially reduced with the implementation of secondary sewage treatment in many areas. Today, some parts of the river—mainly near large municipalities—still experience fecal bacteria counts that exceed water quality standards.
Fecal bacteria and new inputs of toxic substances can be controlled through existing mechanisms in the Clean Water Act. By contrast, water quality problems related to nonpoint source inputs—especially (1) nutrients, primarily nitrogen and phosphorus from agricultural runoff and other agriculture activities, and (2) sediments, from upland or farmland erosion and river bed and bank erosion—are not as readily addressed by existing mechanisms. Accordingly, this report focuses primarily on Mississippi River water quality problems as they relate to nutrients and sediments.
TABLE 2-1 Relative Proportions of the Mississippi River Watershed Within Its Larger Subbasins
Watershed Land Area (%) Discharge (%)
Upper 15 19
Missouri 42 13
Ohio 16 38
Arkansas 13 10
Lower Mississippi 7 13
Red 7 7
SOURCE: Reprinted, with permission, from Turner and Rabalais (2004). © 2004 by Springer Netherlands.