Contaminants in the Mississippi River
Reston, Virginia, 1995
Edited by Robert H. Meade

Nutrients in the Mississippi River

By Ronald C. Antweiler, Donald A. Goolsby, and Howard E. Taylor

Nutrients are chemical elements and compounds in the environment from which living things synthesize living matter: their body cells and tissues, their genetic material, their energy-bearing molecules, and their reproductive cells. In this discussion of water quality in the Mississippi River, four nutrient compounds are described: nitrate (NO3-), nitrite (NO2-), ammonium (NH4+), and orthophosphate (PO4-3). These are the most significant inorganic forms of two elements, nitrogen and phosphorus, that commonly limit the productivity of plants.

Nitrate and nitrite at high concentrations are known to have toxic effects on humans. The U.S. Environmental Protection Agency (USEPA), mandated by Public Laws 89-753 (the Clean Water Restoration Act of 1966) and 92-500 (the Federal Water Pollution Control Act Amendments of 1972), has established drinking-water standards for both of these to protect public health (Sceery, 1992; U.S. Environmental Protec-tion Agency, 1991a, 1991b; table 5). If the concentration of nitrate or nitrite is high in the raw water supply (for example, the Mississippi River), the cost of treatment increases. Ammonia also is known to be toxic to aquatic organisms, and the USEPA also has established criteria for this compound in surface waters (U.S. Environmental Protection Agency, 1985, 1992). Maximum contaminant levels have not been established for orthophosphate.


Plant productivity caused by nutrients is another factor of concern. In lakes, nutrient enrichment almost always increases algal production, a condition known as cultural eutrophication. One effect of eutrophication is that an abundance of algae may cause taste and odor problems in drinking-water supplies. A second effect of eutrophication is the increased uptake of dissolved oxygen by bacteria in response to higher concentrations of organic matter. If oxygen is taken up by decaying organic matter faster than it is imported from the atmosphere or produced by photosynthesis, it becomes depleted, and the aquatic species that require it are adversely affected. Furthermore, oxygen depletion causes basic changes in the chemical environment that allow materials (including many metals) that were formerly precipitated or tied to the sediments to become soluble and, therefore, mobile.

Nutrients Dissolved in River Waters


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Figure 30. -- Of four major nutrient compounds in the Mississippi River and its tributaries, only nitrate is found in concentrations approaching the U.S. Environmental Protection Agency (USEPA) maximum contaminant level (MCL). Orthophosphate usually is present in low concentrations, and concentrations of ammonium and nitrite usually are insignificant. The box plots show the medians and ranges of values determined in the Mississippi River main stem (Ml, M2) and in tributaries (Tl, T2). Values determined during 1991-92 are labeled Ml and T1 (see table 7 for a statistical summary of the data; see Antweiler and others, 1995, and Garbarino and others, 1995, for a detailed listing of the data), and those at monitoring stations of the U.S. Geological Survey's National Stream Quality Accounting Network (NASQAN) during 1979-91 are labeled M2 and T2.

In large turbid rivers such as the Mississippi, rapid stream velocities and reduced light penetration into the water may inhibit primary productivity. In the Mississippi River, eutrophication may be limited to slow-flowing reaches behind locks and dams and to backwaters and eddies. The major response of plants to the nutrients in the Mississippi River is delayed until the water reaches the estuarine regions along the coast of the Gulf of Mexico, where velocities decrease and sediment settles out of the water, allowing light to penetrate and algae to bloom. For example, the fish production of the Barataria Basin, one of these estuarine regions, has become endangered by eutrophication (Hanor, 1988; Madden and others, 1988; Craig and Day, 1976); thus, the nutrients supplied by the Mississippi River are of concern to the coastal areas of the Gulf of Mexico.

The sources of nutrients in surface waters can be broadly divided as natural and anthropogenic (see table 6). Natural sources are generally ubiquitous; however, their contribution is usually low because, over the course of time, natural systems have established balances between the production and consumption of nutrients. Anthropogenic sources arise from many activities. In the agricultural setting of the Mississippi River drainage, farmers increase the productivity and yield of their crops by use of chemical fertilizers. If more fertilizers are applied than are used by the crops, they can move into ground and surface waters and become a major source of nutrients in rivers. Other major sources of nutrients in surface waters are domestic and animal wastes. Although municipal wastewater is treated, only a fraction of the nutrients is removed. In addition to the nutrients derived from human sewage, municipal wastewater also contains nutrients from such things as lawn fertilizers and household cleaners and detergents. Other anthropogenic sources of nutrients are industrial, either from the manufacture of fertilizers or as by-products of other manufacturing processes.


During the last 80 years, there has been a marked increase in the concentration of nitrate in the Lower Mississippi River that has been attributed to the increasing use of fertilizers (Turner and Rabalais, 1991). Before 1940, nitrate concentrations ranged from 0.2-0.4 milligram of nitrogen per liter (mg N/L); since 1940, they have ranged from 1.0-1.2 mg N/L. In the last 10 to 15 years, however, nitrate concentrations do not appear to have changed.

Time-series data collected at Baton Rouge and elsewhere were combined and integrated to estimate that the Mississippi River delivered about 900,000 metric tons of nitrate and 35,000 metric tons of orthophosphate to the Gulf of Mexico during the year, April 1991-April 1992. If we assume that all nitrate in the Mississippi River before 1940 originated from natural sources, the increase since 1940 provides a rough estimate that at least 75 percent of the nitrate in the Mississippi River today is anthropogenic in origin. Most of that 75 percent apparently comes from nonpoint agricultural sources. It is estimated that more than 5.5 million metric tons of nitrogen fertilizer were applied to cropland in the Mississippi River Basin in 1991 (U.S. Department of Agriculture, 1992; J.J. Fletcher, West Virginia University, unpubl. data,1992). The 900,000 metric tons of nitrate discharged by the Mississippi River into the Gulf of Mexico was therefore equivalent to about 16 percent of the nitrogen fertilizer applied to cropland in one year. In contrast, most of the ammonium that enters the Mississippi River probably originates from either industrial or municipal waste.

Nitrate in River Waters


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Figure 31. -- During the last decade and a half, there has been no significant change in the concentrations of nitrate and nitrite in the Mississippi River. Shown here are the yearly mean concentrations of nitrate plus nitrite at monitoring stations of the U.S. Geological Survey's National Stream Quality Accounting Network (NASQAN) on the Mississippi River. The lower four lines represent the upper river at Royalton, Minnesota (upriver of the Twin Cities), and Alma, Wisconsin (Lock and Dam 4), and the lower river at Memphis, Tennessee, and St. Francisville, Louisiana. The uppermost line shows considerably more scatter than the other four, which probably is related to variable inputs of nitrate from the Illinois River; the Illinois enters the Mississippi at Grafton (a few kilometers upriver of Alton), and it usually contains higher concentrations of nitrate than the other major tributaries of the Mississippi (see fig. 32).

Nitrate Variations Through Time


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Figure 32. -- Nitrate concentrations in the Mississippi River and its tributaries vary seasonally, usually being higher in winter, spring, and early summer and lower in late summer and early autumn. The data shown in the graphs are from samples collected at weekly or biweekly intervals between March 1991 and September 1992 at six sites: three on the Mississippi River main stem and three in the lower reaches of principal tributaries. For a complete listing of the data, see Coupe and others (1995). The maximum contaminant level for nitrate in drinking water (10 milligrams of nitrogen per liter, a level that corresponds to the top of the graphs) was not exceeded in any of the samples collected at the six sites during the period shown here. The highest concentrations of nitrate were recorded in the Illinois River, which drains extensive agricultural areas and receives municipal wastes from Chicago and other cities.

Virtually all of the sharp increases of ammonium concentration in samples, such as those illustrated in figures 34 and 35, corresponded to metropolitan centers on the river. This association, coupled with data on fecal coliforms and surfactants (see the chapter, "Organic contamination of the Mississippi River from municipal and industrial wastewater"), suggests that most ammonium ions in the river originate from municipal waste.

The source of nitrite is more difficult to assess. Although there was probably a significant contribution from non-point sources, nitrite, like ammonium, was being removed by biotic utilization or conversion to nitrate, and hence this contribution was minimized.

To summarize: In terms of human health, nitrate is the only nutrient compound that represents a problem in the Mississippi River system; nitrate concentrations in many of the Mississippi River's tributaries in Iowa, Minnesota, and northern Illinois approach and occasionally exceed the USEPA drinking-water standard (table 5; see also Lucey and Goolsby, 1993). In addition to the public health question, nitrate represents an ecological problem as well. Because it is not removed quickly, nitrate is accumulating in the Gulf of Mexico; each year, the Mississippi delivers a large quantity (900,000 metric tons between April 1991 and April 1992) to the Gulf and therefore merits concern in terms of potential eutrophication. The largest sources of this nitrate are most likely fertilizers used in agriculture. Although the concentrations of nitrate in the Mississippi River have increased since the turn of the century, these levels have been virtually unchanged over the last 10 to 15 years. Hence, although nitrate concentrations may be high, they appear to have stabilized.

Ammonium Variations Through Time


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Figure 33. -- Ammonium concentrations generally are highest in winter (December through March) when river water temperatures are lowest. This pattern is not so much related to seasonal supplies of ammonium as it is to water temperatures. During the warmer months, ammonium is less persistent in oxygenated river water because it is more rapidly utilized by bacteria and algae or more rapidly oxidized to nitrate or nitrite. Data shown in the graphs are based on the same samples as the nitrate data shown in figure 32 (see Coupe and others, 1995, for a complete listing).


Nutrient Concentrations


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Figure 34. -- Spatial variations of nutrients along the length of the Mississippi River demonstrate the interplay between the input sources that increase their concentrations and the processes of dilution and decomposition that decrease their concentrations. Data shown here in A and B were collected during two upriver cruises (see "Sampling the Length of the River") wherein samples were taken approximately every 10 miles near the center of the river. Complete tabulations of the data are given by Antweiler and others (1995).

Data collected during the upriver cruise of June 23-July 2, 1991, showed (1) increases and decreases in nitrate and orthophosphate that were related mainly to inputs and dilutions by tributaries, and (2) downstream decreases of nitrite and ammonium that were related mostly to uptake by organisms or oxidation to nitrate. Concentrations of nitrate (NO3-) and orthophosphate (PO4-3) fluctuated in near parallel fashion, showing the following features, in downriver sequence: highest concentrations just below the mouth of the Minnesota River; rapid decreases in concentration between the mouth of the Minnesota River and kilometer 2500 (northern border of Iowa) owing mostly to dilution by tributary inflows that have lower concentrations; rapid increases in concentration as the river flowed past Iowa and northern Illinois, probably owing to more concentrated inputs from rivers like the Iowa and Des Moines; no appreciable changes in concentration at or below the mouth of the Missouri River; marked dilutions from the less concentrated waters of the Ohio River; few large changes in concentration in the lower Mississippi River, except for peaks between river kilometers 650 and 1100 that probably reflected pulses of runoff of stormwaters that fell a few weeks earlier in agricultural areas of Iowa and Illinois. Ammonium (NH4+) concentrations showed as three distinct spikes that appeared and disappeared rapidly: one below the Minnesota River and the Twin Cities, one below the Des Moines River and Keokuk, and one below St. Louis. Once ammonium has been introduced into the river, it decreases quickly as it is assimilated by bacteria and algae, converted to nitrate or nitrite, or removed by adsorption onto sediments. After reaching a peak concentration downriver of Lake Pepin (near river kilometer 2700), nitrite (NO2-) showed a general decrease downriver, punctuated most notably by an increase between kilometers 2400 and 2100 and another small increase near St. Louis. Nitrite, like ammonium, is removed by natural processes--its utilization by bacteria and algae and its oxidation to nitrate are the most likely possibilities--but not as rapidly as ammonium. Like ammonium, most of the nitrite probably originates from point sources.

Data collected during the upriver cruise of March 25-April 4, 1992, show substantial differences when compared with the data collected the previous summer (shown in A). These differences in concentrations reflected seasonal differences in the sources of nutrients and in the rates at which nutrients are utilized or transformed in the river. Nitrate (NO3-) concentrations again were highest in the Minneapolis-St. Paul area and decreased downriver to about kilometer 2400, below which point they remained fairly constant until they were diluted by the waters of the Ohio River. Orthophosphate (PO4-3) concentrations were lower than during the previous summer, especially in the upper river; the spikes or peaks of concentration between river kilometers 1100 and 650 may have resulted from storm events upriver that flushed in waters that were rich in orthophosphate. Ammonium (NH4+) concentrations again appeared as spikes, but these spikes were broader, reflecting the slower rates of removal or utilization during the early spring that are related to lower temperatures, slower growth cycles, or less incident radiation (sunlight). Persistent concentrations of nitrite (NO2-) along most of the length of the river suggest that mechanisms for its disappearance, such as uptake or oxidation, also were retarded during the early spring period of March-April 1992.

Nutrient Transport


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Figure 35. -- When nutrient data are graphed in terms of their fluxes or transports rather than their concentrations, they show that the transports of nitrate (NO3-) and orthophosphate (PO4-3) mostly increase downriver. This means that the processes that add nitrate and orthophosphate to the river are more prevalent than the processes that remove them. Transports of nitrite (NO2-) and ammonium (NH4+), on the other hand, generally decrease downriver because, in their cases, the removal mechanisms prevail. Shown here are the data from the upriver cruise of June 23-July 2, 1991: concentrations shown in figure 34A have been multiplied by water discharges to compute the downriver transports of dissolved nutrients in tons per day (1 metric ton = 1000 kg = 2205 pounds). Significant inputs from tributaries are shown by bars near the center of the graph; the individual nutrients are identified by bar colors which correspond to those of the curves. Concentrations of nitrate and orthophosphate showed parallel fluctuations during the sampling period. The initial loading to the Mississippi River from the Minnesota River represented the total transport of these two compounds for the next 600 km (to river kilometer 2250); this implies that over this distance, additions of nitrate and orthophosphate from tributaries and point sources were negligible, or that gains of these ions were balanced by their losses. From kilometer 2250 (near the confluence with the Iowa River) downriver to St. Louis (kilometer 1800), loads increased steadily from contributions from tributaries. The Iowa River at kilometer 2240, the Des Moines River at kilometer 2100, and the Missouri River at kilometer 1850 each added a large increment of nitrate and orthophosphate (see the bars in the middle of the graph). Because the Ohio River added relatively small amounts of nutrients, its effect can barely be distinguished on the graph. The large increases in the curves at about kilometer 1100 (south of Memphis, Tennessee) probably were caused by a storm event several weeks earlier in Iowa or Illinois, which flushed large quantities of these compounds into the river. The decreases in the curves at about kilometer 500 are caused by the Old River outflow, which removes substantial amounts of the water and dissolved nutrients from the Mississippi River. Ammonium (NH4+) loads were introduced into the river from point sources, most likely municipal wastes, and they decreased within relatively short distances downriver. Nitrite (NO2-) load behaved in a fashion somewhere between nitrate load and ammonium load: its points of introduction were generally not spikes, yet it clearly was being utilized or converted to other forms of nitrogen faster than it was being introduced--as reflected in the downriver decrease in nitrite load. Places where nitrite was introduced were not always related to entering tributaries; the sharp increase in nitrite load near river kilometer 2700 (Lake Pepin), for example, cannot be related to tributary inputs.

Nitrate Sources


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Figure 36. -- Disproportionately large quantities of the nitrate in the Mississippi River, on average, are derived from the upper-basin States of Minnesota, Iowa, and Illinois. Slightly more than one-half the total nitrate load in the system is derived from the Upper Mississippi River Basin, which supplies less than one-fourth of the total water. Conversely, the Ohio River supplies 41 percent of the total water, but only 21 percent of the total nitrate load. The Missouri River is the only major tributary whose nitrate load corresponds closely to its proportion of the water discharge.


Antweiler, R.C., Patton, C.J., and Taylor, H.E., 1995,
Nutrients, in Moody, J.A., ed., Chemical data for water samples collected during four upriver cruises on the Mississippi River between New Orleans, Louisiana, and Minneapolis, Minnesota, May 1990-April 1992: U.S. Geological Survey Open-File Report 94-523, p. 89-125.
Battaglin, W.A., and Goolsby, D.A., 1995,
Spatial data in geographic information system format on agricultural chemical use, land use, and cropping practices in the United States: U.S. Geological Survey Water-Resources Investigations Report 94-4176, 87 p.
Coupe, R.H., Goolsby, D.A., Iverson, J.L., Markovchick, D.J., and Zaugg, D.S., 1995,
Pesticide, nutrient, water discharge, and physical-property data for the Mississippi River and some of its tributaries, April 1991- September 1992: U.S. Geological Survey Open-File Report 93-657, 116 p.
Craig, N.J., and Day, J.W., Jr., 1976,
Barataria Basin eutrophication case history: Baton Rouge, Report to Louisiana State Planning Office, June 1976, 24 p.
Fletcher, J.J., 1992,
Unpublished nitrogen fertilizer use database: Morgantown, West Virginia University.
Garbarino, J.R., Antweiler, R.C., Brinton, T.I., Roth, D.A., and Taylor, H.E., 1995,
Concentration and transport data for selected dissolved inorganic constituents and dissolved organic carbon in water collected from the Mississippi River and some of its tributaries, July 1991-May 1992: U.S. Geological Survey Open-File Report 95-149, 149 p.
Goolsby, D.A., Coupe, R.C., and Markovchick, D.J., 1991,
Distribution of selected herbicides and nitrate in the Mississippi River and its major tributaries, April through June 1991: U.S. Geological Survey Water-Resources Investigations Report 91-4163, 35 p.
Goolsby, D.A., and Battaglin, W.A., 1993,
Occurrence, distribution and transport of agricultural chemicals in surface waters of the Midwestern United States, in Goolsby, D.A., Boyer, L.L., and Mallard, G.E., eds., Selected papers on agricultural chemicals in water resources of the Midcontinental United States: U.S. Geological Survey Open-File Report 93-418, p. 1-25.
Hanor, J.S., 1988,
Effects of discharge of municipal waste on water quality of the lower Mississippi River: Environmental Geology and Water Sciences, v. 12, p. 163-175.
Lucey, K.J., and Goolsby, D.A., 1993,
Effects of short-term climatic variations on nitrate concentrations in the Raccoon River, Iowa: Journal of Environmental Quality, v. 22, p. 38-46.
Madden, C.J., Day, J.W., and Randall, J.M., 1988,
Freshwater and marine coupling in estuaries of the Mississippi River deltaic plain: Limnology and Oceanography, v. 33, p. 982-1004.
Sceery, W.M., 1992,
Drinking water update: Environmental Testing and Analysis, v. 1, p. 26-30.
Turner, R.E., and Rabalais, N.N., 1991,
Changes in Mississippi River water quality this century-Implications for coastal food webs: Bioscience, v. 41, p. 140-147.
U.S. Department of Agriculture, 1992,
Agricultural resources--Inputs situation and outlook: U.S. Department of Agriculture, p. 13-20.
U.S. Environmental Protection Agency, 1985,
Criterion document: Washington, D.C., U.S. Environmental Protection Agency, 3 p.
___ 1991a,
Drinking water regulations and health advisories: Washington, D.C., U.S. Environmental Protection Agency, Office of Water, 12 p.
___ 1991b,
Summary of Phase II regulations--National Primary Drinking Water Regulation for 38 inorganic and synthetic organic chemicals--EPA Phase II Fact Sheet Series (Nitrate, no. 3 of 14, and Nitrite, no. 4 of 14): Office of Ground Water and Drinking Water, U.S. Environmental Protection Agency publication #570, 9-91-022, October.
___ 1992,
Revised tables for determining average freshwater ammonia concentrations: Washington, D.C., U.S. Environmental Protection Agency memorandum, dated July 30, 1992, 3 p.

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Contaminants in the Mississippi River
Reston, Virginia, 1995
Edited by Robert H. Meade

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