Executive Summary

By Robert H. Meade and Jerry A. Leenheer

This volume contains a selection of some of the more interesting results of a 5-year study conducted by the U.S. Geological Survey of contaminants in the Mississippi River and some of its major tributaries. During the first 3 years of the study, 1987-90, the rivers were sampled on seven different occasions between St. Louis, Missouri, and New Orleans, Louisiana. During the last 2 years of the study, 1991-92, the scope of the program was increased to include three further samplings of the full length of the river between Minneapolis-St. Paul, Minnesota, and New Orleans. The sampling and analytical efforts were focused on three phases of contaminants in the rivers: (1) contaminants transported in the dissolved phase, (2) contaminants transported in the adsorbed phase-that is, in association with the suspended silts and colloids-and (3) contaminants stored in the bottom sediments in the navigation pools of the Upper Mississippi River.

Contaminants were assessed for the period of sampling, 1987-92. Given that samples were collected no more frequently than twice a year, the 5-year period was not sufficiently long to establish any time trends-whether, that is, the contaminant levels were increasing or decreasing. Included in the analysis were only a few data that had been collected before our study. Yet to be assessed are the subsequent effects on river quality of the great flood of 1993. Contaminants are assessed mostly in a spatial rather than a temporal context. This report, in other words, is a snapshot rather than a chronicle.

Some of the findings represent new information, whereas many of the results confirm previously known aspects of the distribution of contaminants in the river, as described, for example, in the volumes by Costner and Thornton (1989), Scarpino (1985, especially pages 151-186), and Wiener and others (1984). This summary includes three figures that demonstrate the three phases of contaminant occurrence listed in the first paragraph. Further details of each of the contaminants portrayed here, as well as of other contaminants, are given in the chapters that follow.

SUGGESTIONS FOR READING THIS REPORT

The chapters on the individual contaminant groups (heavy metals, pesticides, nutrients, polychlorinated biphenyls, and other organic contaminants) are organized to be read easily at different levels. For readers who want an overview in the shortest amount of time, the figures and their captions contain much of the relevant information. For readers having more time or interest, the main text of each chapter provides further information and analysis. For readers who want to inspect entire data sets and evaluate analytical methods, each chapter contains bibliographic references to the U.S. Geological Survey Open-File Reports and Water-Resources Investigations Reports that contain the basic data and complete descriptions of the procedures that were used to obtain them.

Dissolved Contaminants

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Figure 1. -- The waters of the Mississippi River carry dissolved contaminants and bacteria that originate from a variety of municipal, agricultural, and industrial sources. The map shows the amounts of water discharged by the Mississippi River and its main tributaries during an average year. About 2 percent of the average discharge of the Mississippi River comes from municipal and industrial point sources. The distribution of contaminants along the Mississippi River depends on the nature and location of their sources, the degree of wastewater treatment, the stability of the contaminants, and their dilution by receiving waters. The graphs below the map show the concentrations of contaminants dissolved in the Mississippi River between Minneapolis-St. Paul and the Gulf of Mexico. The data in the graphs are generalized from the results of chemical analyses of representative samples of water collected at 10-15 sites along the Mississippi River: on as many as 10 separate occasions during 1987-92 downriver of St. Louis, and on 3 separate occasions during 1991-92 upriver of St. Louis.

As the Mississippi River flows southward from its headwaters in the northern Midwest, its discharge is more than doubled by the waters it receives from the Illinois and Missouri Rivers. This combined discharge is more than doubled again as it is joined by the waters of the Ohio River. About 500 kilometers upriver of its principal mouth, the Mississippi River bifurcates, and one-fourth of its discharge is diverted down the Atchafalaya River to the Gulf of Mexico.

A

Fecal coliform bacteria derived from human and animal wastes survive only briefly in river water, but their average concentrations exceed the maximum contaminant level (MCL) established by the U.S. Environmental Protection Agency of 2,000 per liter for recreational use in much of the Mississippi River because of incomplete wastewater treatment.

B

Linear alkylbenzenesulfonate (LAS) is a biodegradable detergent, primarily derived from domestic sewage. Its presence in high concentrations in the Mississippi River in the St. Louis metropolitan area corresponds with the elevated counts of coliform bacteria, and probably reflects the incomplete treatment of wastewaters discharged into the river.

C

Caffeine is a stimulant chemical in coffee and soft drinks. Because it is consumed only by humans, it is an indicator of domestic sewage and illustrates the extent to which sewage is diluted by the river. Concentrations of caffeine in municipal wastewaters usually range between 20 and 300 micrograms per liter (mg/L). The much lower concentrations of 0.02-0.04 mg/L of caffeine shown in the graph indicate that municipal wastewaters may be diluted as much as a thousandfold after they are well mixed into the Mississippi River.

D and E

Agricultural contaminants enter the rivers from mostly nonpoint sources, usually as runoff from croplands during spring and summer.

D

Nitrate in the Mississippi River comes mostly from fertilizers. Its concentration in the river fluctuates seasonally, depending on when fertilizers are applied to farmlands and the timing of rainfall and runoff. Nitrate concentrations are generally smaller in the Mississippi River below the confluence of the Ohio River; the major portion of nitrate in the Mississippi River is derived from the tributaries that drain intensively farmed regions in Illinois, Iowa, and Minnesota.

E

Atrazine is a pre-emergent herbicide used mainly on cornfields. It is nearly ubiquitous in the Mississippi River. Atrazine concentrations usually are greatest near St. Louis, Missouri, because of inputs from the Missouri, Illinois, and other rivers that drain the farming regions of the Corn Belt. Concentrations usually are smaller in the Lower Mississippi because of dilution by water from the Ohio River. Atrazine concentrations vary seasonally, and occasionally exceed the maximum contaminant level for treated drinking water of 3 micrograms per liter for a few days during the spring runoff in the Mississippi River between St. Louis and the Ohio River confluence.

F

Ethylenediaminetetraacetic acid (EDTA) is the dissolved organic chemical contaminant present at the greatest concentration in the Mississippi River. Generally considered nontoxic, this chemical is a general indicator of industrial contamination, and it is found in the Mississippi River at about one-fourth of the concentration found in some European rivers.

G and H

Two examples of contaminants from industrial point sources are tris-2-chloroethylphosphate (TCLEP) and 1,3,5,-trimethyl-2,4,6-triazinetrione (TTT).

G

TCLEP is a flame retardant that is added to polyurethane foams and textiles, and, in the Mississippi River system, it is derived almost exclusively from the Illinois River Basin. Its exclusive source and its persistence in solution make TCLEP a useful tracer and indicator of waters from the Illinois River as they mix with waters from other tributaries down the Mississippi River.

H

TTT is a by-product of the manufacture of methylisocyanate. Its overwhelmingly singular source in the Mississippi River system is in the basin of the Kanawha River of West Virginia, a tributary of the Ohio River. Proportions of TTT dissolved in the water can be used to follow the mixing of the Kanawha River with the Ohio, and the Ohio River with the Mississippi River.

Contaminants in Suspended Sediment

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Figure 2. -- The suspended sediments that are transported by the Mississippi River and its tributaries adsorb and carry contaminants. Organic contaminants such as polychlorinated biphenyls (PCBs) and inorganic contaminants such as lead are many times more likely to adhere to sediment particles than to remain in the dissolved phase. The map shows the amounts of suspended sediment discharged by the Mississippi River and its main tributaries during an average year near 1990. The graphs arranged below the map show the concentrations of constituents adsorbed on the sediments in suspension in the main-stem Mississippi River between Minneapolis-St. Paul and the Gulf of Mexico. The data in the graphs are generalized from the results of chemical analyses of representative samples of suspended sediment collected at 10-15 sites along the Mississippi River: on as many as 10 separate occasions during 1987-92 downriver of St. Louis, and on 3 separate occasions during 1991-92 upriver of St. Louis.

Suspended-sediment discharges in the Upper Mississippi River are fairly small when compared to those of the major tributaries. The sediment discharge of the Upper Mississippi River is increased five- to tenfold by the sediment discharge of the Missouri River. The average sediment load in the main stem is increased by another significant increment by the contribution from the Ohio River.

A

Organic carbon (expressed here as weight percent of dried suspended silt) is proportionately greater in the uppermost Mississippi River, and its proportion decreases downriver. Particulate organic carbon in the Mississippi River is mostly natural, but it affects the ways in which contaminants, especially organic contaminants, are adsorbed by suspended sediment. The Missouri and Illinois Rivers transport suspended sediment in which organic carbon is somewhat less concentrated; where these two tributaries enter the Mississippi (near river kilometer 1850), the organic carbon percentages are decreased by dilution. Organic carbon percentages in the suspended sediment of the Ohio River, on the other hand, are typically greater than those in the Missouri and Illinois Rivers, and the organic carbon in suspended sediment is increased slightly where the Ohio River joins the Mississippi (river kilometer 1535).

B

Polychlorinated biphenyls (PCBs), which are organic contaminants that were formerly used widely in industrial applications, are typically most concentrated on the suspended sediments in the Upper Mississippi River near Minneapolis-St. Paul. The difference between PCB concentrations on the suspended sediments near Minneapolis and those near St. Louis is due mostly to the greater amounts of suspended sediment in the river at the latter city, rather than an indication that Minneapolis-St. Paul contributed 5-10 times more PCBs to the river than did St. Louis. The high concentrations in the upper river decrease rapidly downriver, and they are increased significantly only as the suspended sediment from the Ohio River, which usually contains more PCBs than that in the middle reaches of the Mississippi River, enters and mixes.

C

Hexachlorobenzene, another organic contaminant of industrial origin, is predominantly derived from two main sources in the Mississippi River Basin: (1) the Ohio River, which enters the Mississippi at river kilometer 1535, and (2) the industrial corridor along the lowermost 400 kilometers of the Mississippi River.

D

Lead and other heavy metals are associated with the suspended sediments along the length of the Mississippi River. Spatial variations in their concentrations are less pronounced than those of PCBs and hexachlorobenzene. However, they do tend to be most concentrated on the suspended sediments in the river just downstream from Minneapolis-St. Paul (as in the case of PCBs, because of the relative scarcity there of suspended sediment), and they show slight increases related to more concentrated inputs from the Ohio River.

Contaminants in Bed Sediments

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Figure 3. -- The Upper Mississippi River is partly impounded by control structures which form pools that trap sediments and their adsorbed contaminants. Twenty-nine locks and dams, built mostly for navigation, control the depth and, to a lesser extent, the flow of the river between Minneapolis and St. Louis. Twenty-six of these control structures form major pools that trap and retain sediment and contaminants in varying degrees. The two largest pools are Lake Pepin, a natural pool formed by the partial damming of the Mississippi River by the sand delta of the tributary Chippewa River, and Pool 19, the impoundment behind the hydroelectric dam at Keokuk, Iowa, the highest major dam on the upper river.

The bottom sediments of 25 of these pools (1-26, except for Pool 17) were sampled during 1991-92. In each pool, 15-20 samples of the surficial sediment stored in the backwater areas of the lower reaches of the pool were collected and combined into a composite sample, which was analyzed for a number of characteristics and contaminants, some of which are portrayed in the figure. On the horizontal axes of the graphs: UP = upper Lake Pepin; LP = lower Lake Pepin.

A

Percent clay, a measure of the fineness of the sediment particles stored on the bottoms of the pools, is shown in the graph as percent finer than 0.004 millimeter. Fineness is an important attribute in the retention of contaminants because the finer sediments have greater total surface areas (compared to coarser sediments) with which contaminants may react or onto which contaminants may be adsorbed. The most concentrated accumulation of the finest sediments in the pools of the Upper Mississippi River is in Lake Pepin. Other large impoundments, Pools 19 and 26, have trapped sediments that are finer than those in smaller pools. Clay and other fine sediment particles have more time and opportunities to settle and be deposited in the larger deeper pools than in the smaller shallower ones.

B

Particulate organic carbon usually is found most abundantly in association with the finest sediments. In the pools of the Upper Mississippi River, organic carbon shows a strong correlation (although not an identical distribution) with percent clay. Independent of its associations with fine sediment, organic carbon is a strong determinant in the adsorption of contaminants-especially organic contaminants-by sediments.

The distributions of the contaminants adsorbed by the bottom sediments in the pools of the Upper Mississippi River reflect two main factors: (1) the affinities of the contaminants for fine particles and organic carbon, and (2) local sources of contaminants.

C

Polychlorinated biphenyls (PCBs) are manmade organic compounds, formerly used extensively in industrial applications. Although banned for many years, PCBs still persist in the bottom sediments in the pools of the Upper Mississippi River. They are found in the greatest concentrations in the pools farthest upriver, and especially in the fine-grained carbon-rich sediments on the bottom of Lake Pepin. Lower concentrations in the pools farther downriver suggest that the principal sources of PCBs in the Upper Mississippi River were centered in and near the Minneapolis-St. Paul metropolitan area, and that Lake Pepin has trapped and retained most of the PCBs, retarding their further transport downriver.

D

Lead comes from natural and manmade sources, but its sources in the Upper Mississippi River are mostly industrial and municipal. Sediments having concentrations greater than 40 micrograms of lead per gram of sediment are considered "moderately polluted." The highs and lows of lead concentration in the pools of the Upper Mississippi River mimic almost exactly the highs and lows of the percent clay with one notable exception. Pool 12 includes the mouth of the Galena River, which drains an area that was mined extensively for lead some years ago; the higher concentrations in Pool 12 may reflect the input of lead-bearing sediment. The high concentrations of lead in the sediments of Lake Pepin and Pool 19, however, probably reflect mostly the general affinity of lead for fine sediment.

E

Coprostanol is a fecal sterol found in the excrement of humans and animals. It is not destroyed completely by normal sewage treatment, and so its presence in the sediments of the Upper Mississippi River is an indicator of human or animal wastes. The high concentrations of coprostanol in Lake Pepin and Pool 19 at least partly reflect the affinity of coprostanol for fine sediment. The high concentrations of coprostanol in the sediments of Pool 2 reflect the large discharges of treated human wastes in the Minneapolis-St. Paul area. Less easy to explain is the high concentration of coprostanol in Pool 14, which is upriver of the Quad Cities (Moline-Rock Island, Illinois, and Davenport-Bettendorf, Iowa), and therefore cannot be due to a large municipal source.

Costner, Pat, and Thornton, Joe, 1989,

We all live downstream-The Mississippi River and the national toxics crisis: Washington, D.C., Greenpeace USA, 120 p., 1 app. (61 p.)

Scarpino, P.V., 1985,

Great river-An environmental history of the Upper Mississippi, 1890-1950: Columbia, Missouri University Press, 219 p.

Wiener, J.G., Anderson, R.V., and McConville, D.R., eds., 1984,

Contaminants in the Upper Mississippi River-Proceedings of the 15th Annual Meeting of the Mississippi River Research Consortium: Boston, Butterworth Publishers, 368 p.