The Mississippi River is important in several contexts. (1) It serves as drinking-water supply for 70 cities and towns. Thus, the contaminants in the waters of the Mississippi River have the potential for detrimental health effects on these consumers of river water. Some of these contaminants, of course, are removed in water-treatment plants, but some persist regardless of the treatment process. The analysis of river waters provides an indication of the potential for drinking-water contamination. (2) The birds, fish, and other organisms that live in or adjacent to the Mississippi River are affected by contaminants. Thus, there is a basis for interest from an ecological perspective as well as from the perspective of the health of humans who consume the birds and fish. (3) The Mississippi River is a significant source of water, sediment, and nutrients to the Gulf of Mexico. The chemical characteristics of these flows are important to the health and productivity of coastal and marine ecosystems. (4) The Mississippi River drains 41 percent of the conterminous United States, in which area reside 27 percent of the Nation's population. The contaminants moving through the river system are an excellent measure of what is being introduced into the Nation's waterways by human activities. In a sense, they are "report cards" on the Nation's efforts at water-pollution control. Or, to use the analogy of the great tropical ecologist Harald Sioli, "River water is the urine of the landscape," and the quality of the waters of the Mississippi is a monitor of the health of a substantial portion of the "body" of the Nation.
The study, the results of which are reported in this volume, included nearly the full length of the Mississippi River and sampled the waters within and along the boundaries of 11 different States. Ten research and sampling cruises were completed during 1987 through 1992. The study ended before the unprecedented flooding in the summer of 1993. However, the study has provided a baseline for comparison that has been used in the analysis of the movement of contaminants during the flood.
By sampling the river repeatedly, in high flows, in low flows, in the different seasons, one may learn something of the dynamics of contaminant movement through the river system. For example, one learns: what tributary contributes the largest amounts of certain contaminants; what times of year the contaminants enter the river; whether a contaminant, once it enters the river, continues to move downstream with the water, or is lost from the system as a result of chemical reaction, uptake by organisms, volatilization to the atmosphere, or storage on the riverbed or flood plain? Development of effective national and regional strategies for water-quality management demands knowledge of the sources, movement, and storage of contaminants and their reactions throughout the river system.
The purposes of this report are to provide an overview of the new knowledge developed in this study and to provide access to a body of comprehensive and consistent data that can be compared with data collected years and decades hence to help understand and evaluate the changes in the river and future progress (or lack of it) in water-quality management.