Organic Contamination of the Mississippi River from Municipal and Industrial Wastewater

Larry B. Barber, II, Jerry A. Leenheer, Wilfred E. Pereira,
Ted I. Noyes, Greg K. Brown, Charles F. Tabor, and
Jeff H. Writer

Fate of Contaminants in the River

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Figure 51. -- The most significant factors controlling the concentrations of organic contaminants in rivers are the physical processes of dispersion and dilution. Within this physical framework, the most significant chemical and biological processes controlling the fate of organic contaminants in the Mississippi River are (1) sorption to the sediment and removal by deposition, (2) desorption and diffusion of contaminants from bed sediments back into the water, (3) biological transformation to intermediate compounds, or biodegradation for complete removal, (4) volatilization to the atmosphere, (5) bioconcentration and magnification in the food chain, (6) photolysis, or the breakdown of contaminants under the influence of sunlight, and (7) hydrolysis, or the decomposition of contaminants by taking up the elements of water. Organic compounds of the type called "hydrophobic" (meaning that they prefer being sorbed onto sediment or organic particles to being dissolved in water) can be adsorbed onto sediments in concentrations that are a thousand to a million times greater than in the associated water. Once they are sorbed, the contaminants can be deposited and eventually become buried as sediments continue to accumulate. Buried contaminants can be remobilized, however, by resuspension of the sediments. Likewise, the sedimentary organic matter may decompose, reintroducing its sorbed contaminants to the river by desorption and diffusion of organic colloids. If the contaminants are sorbed onto the sediments in high concentrations, they can adversely affect bottom-dwelling organisms. The tendency of a contaminant to sorb onto the sediment is frequently indicative of its capacity to bioconcentrate and become magnified in the food chain.

Many organic contaminants are biodegraded in rivers. Readily degradable compounds may have a biodegradation half-life (time for one-half of the mass of a compound to be removed) in a river environment of less than 1 day, depending on factors such as temperature, population of bacteria, availability of oxygen, and availability of nutrients. Other organic contaminants are resistant to biodegradation and have half-lives on the order of years. Compounds that are rapidly biodegraded under aerobic conditions, such as those found in a flowing stream, may persist under the anaerobic conditions that exist in buried sediments.

Some organic contaminants are volatile, and the major pathway for their removal from water is transfer to the atmosphere. The rate of volatilization is a function of the vapor pressure and water solubility of the compound, water temperature, and amount of turbulence in the water. A typical half-life for volatile organic compounds is on the order of a few hours; thus, even high concentrations can be rapidly attenuated in river systems.

All these processes---sorption, biodegradation, and volatilization, as well as photolysis and hydrolysis---interact in complex ways in the natural environment. For example, biotransformations can increase the solubility of a hydrophobic organic contaminant, which results in less sorption and greater mobility in the river. The Mississippi River is dynamic, and the distribution of contaminants between air, water, sediment, and organisms is continually changing with variations in the chemical, hydrological, and climatic regimes. The transportation of sediment-bound contaminants, for example, is episodic; most of the transport occurs during periods of high flow and bed scouring. Instream biodegradation rates usually decrease at lower temperatures. Ice cover during cold weather decreases the importance of volatilization as a removal pathway. High concentrations of dissolved organic matter can affect sorption, biodegradation, and volatilization of organic contaminants.

Introduction

Among the topics that are discussed in this chapter and its figures are (1) a comparison of the present-day water quality of the river with historical trends, (2) the environmental processes controlling the occurrence and fate of organic contaminants, (3) the distributions and concentrations of organic contaminants in the water and sediments along the entire length of the river, and (4) the seasonal variability of some of the contamination patterns. Organic contaminants in the Mississippi River were assessed by collecting water and sediment samples between Minneapolis-St. Paul, Minnesota, and New Orleans, Louisiana, during 10 sampling cruises conducted in 1987--92, and analyzing the samples for the organic contaminants and indicator compounds listed in table 10.

Trends in Water Quality: A Historical Perspective

"In 1888, the Engineers were called to remove a bar forming near the St. Paul waterfront. Dredging discovered that this bar was formed entirely of garbage dumped into the river by St. Paul. This area of the river had been shoaling for several years; the Corps was called in only when the smell became so objectionable that private citizens obtained an injunction against the governments of Minneapolis and St. Paul. Minneapolis dumped 500 tons of garbage a day just below the Falls of St. Anthony, and St. Paul added even more than that." (Tweet, 1984, p. 125-126.)

"During the months of low flows recorded for June, July, and August (1926), the pool above the High Dam was in a septic condition with the ebullition of large quantities of gas at the surface. On many occasions the surface of the pool was covered with sewage sleek, the oily floating substances contained in sewage, and was highly discolored for a considerable distance below the sewer outlets. Odors were noticeable at times, but usually in the vicinity of the larger sewer outlets. Below the High Dam septic conditions also prevailed in the back water pools during the summer." (Wisconsin State Board of Health, 1927, p. 310).

Gross contamination problems like these were gradually eliminated during the mid-1900s, but less obvious problems caused the quality of the Mississippi River to decline as population centers continued to grow. After World War II, the synthetic-organic chemical industry rapidly expanded and thousands of new chemicals eventually made their way into natural waters. Beginning about 1950, investigations into the occurrence of organic chemicals indicated that the Mississippi River had been significantly degraded by organic contaminants. Since the passing of pollution-control laws during the early 1970s, many of the obvious and readily correctable sources of contamination from industrial and municipal wastewater have been eliminated or diminished. Improvements in sewage-treatment plants have improved the water quality even though the population has continued to increase.

Municipal-and Industrial-Derived Wastewater: A Perspective on Today

In many metropolitan areas, domestic and industrial wastes and stormwater runoff are drained into a combined sewer. Combined-sewer overflows can contribute high waste loads for short time periods, and pollution loads vary as a function of discrete storm events. Combined-sewer overflows vary in composition with the density of population and the type of industry.

Municipal sewage and industrial wastewater are typically treated prior to their discharge into surface waters. Most municipal wastewater is treated by activated-sludge or trickling-filter methods that rely on sorption and biodegradation to remove organic contaminants. Removal efficiency for strongly sorbing, biologically labile, and volatile organic compounds ranges from 25 percent for primary treatment, 80 to 98 percent for secondary treatment, and greater than 98 percent for tertiary treatment. However, poorly sorbing and biologically recalcitrant compounds are not completely removed even during tertiary treatment. Depending on the type of treatment, removal of micro-organisms varies from negligible to more than 95 percent.

After effluent is discharged into a river, dilution becomes a major factor regulating the concentrations of dissolved chemicals. The stream-dilution factor is a function of streamflow and effluent discharge (stream-dilution factor = streamflow divided by cumulative effluent discharge). Stream-dilution factors determined for individual cities along the Mississippi River during this study varied: about 50 for Minneapolis-St. Paul, 500 for St. Louis, and 1500 for New Orleans. Stream-dilution factors for many small cities along the lower river were greater than 100,000. However, cumulative stream-dilution factors, determined using the combined effluent volumes for all upriver municipal discharges, including tributaries, were relatively constant along the entire river and ranged from 25 to 50, indicating that 2 to 4 percent of the river volume is contributed by municipal wastewater discharges.

Stream-dilution factors vary as a function of season and long-term hydrological trends. For example, the Mississippi River discharges that were sampled during this study varied by a factor of two in the upper river and a factor of four in the lower river. Although stream-dilution factors can exceed 100,000, some contaminants such as fecal coliform bacteria can have concentrations up to millions of bacteria per liter in treated sewage effluents; thus, even small cities can contaminate the river with bacteria.

In addition to chemicals derived from domestic sources, municipal effluents also have an industrial component that varies depending on the type and number of industries present. For a given reach of the river, it may not be possible to distinguish contaminants introduced directly through onsite industrial discharges from contaminants introduced by industry through municipal wastewater. Likewise, urban runoff through storm sewers or from overland flow is a potential source of chemicals that may be difficult to distinguish from treated wastewater effluent. However, some synthetic-organic chemicals are unique and can be traced directly to specific industrial discharges.

Other significant sources of organic contamination along the Mississippi River are powerplants and pulp mills. A typical power- plant along the Upper Mississippi River uses about 6.6 cubic meters per second (m³/s) of cooling water (Water Quality Work Group of the Great River Environmental Action Team, 1980a), which amounts to approximately 3 percent of the average Mississippi River discharge at Minneapolis. Power- plants use a variety of chemicals in their cooling water, including chlo-rine as a disinfectant to prevent biofouling, and metal-complexing agents to prevent scale buildup. Pulp mills also can discharge large quantities of wastewater. The mean discharge for six pulp mills located along the Mississippi River and its tributaries is about 1 m³/s (Costner and Thornton, 1989), which is equal to about 10 percent of the municipal discharge for St. Louis. However, because of the high organic content of pulp-mill effluents, their organic loading to the river can be significant compared with municipal wastewaters. Pulp mills use a wide variety of organic chemicals, and the bleaching process produces significant amounts of chlorinated organic compounds. Another important source of organic contaminants is the application of agricultural chemicals such as fertilizers and pesticides. These chemical formulations are complex and consist of both active and inert ingredients. Many of the inert ingredients are solvents, surfactants, and builders that also are common in domestic and industrial products. Runoff from feedlots also contains high concentrations of organic matter and fecal coliform.

Surfactants in the River

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Figure 52. -- Anionic surfactants were studied in river waters during the 1950s to 1970s because they caused extensive foaming. Consequently, they have been measured for a long enough period (since 1958 in the Mississippi River) to provide an example of long-term trends in water quality. The foaming was the result of high concentrations of branched-chained alkylbenzenesulfonates (ABS) that were resistant to biological degradation during wastewater treatment and residence in rivers. Concentrations were frequently above the foaming threshold of about 0.5 milligram per liter (mg/L). Dilution was the primary attenuation mechanism for ABS. To remedy the foaming problem, the surfactant-manufacturing industry modified the chemical structure of the molecule to a more biodegradable linear side chain (LAS) which resulted in a significant decrease in concentrations during wastewater treatment and residence in rivers (Swisher, 1964). From 1959 to 1965, ABS concentrations in the Illinois River ranged from 0.4 to 9 mg/L; from 1965 to 1966, ABS+LAS concentrations ranged from about 0.2 to 0.3 mg/L (Sullivan and Evans, 1968; Sullivan and Swisher, 1969). After 1968, LAS concentrations decreased to less than 0.1 mg/L even though surfactant consumption increased by more than 30 percent. This example shows that it is possible to reverse certain pollution effects, in this case by the compulsory introduction of a more biodegradable material.

The box plots in the figure show the average annual concentrations of anionic surfactants (ABS plus LAS measured as methylene-blue-active substances) for all available sampling sites along the Mississippi River from 1963 to 1976 in the U.S. Environmental Protection Agency's STORET data base. Although the switchover from ABS to LAS occurred between 1964 and 1968, the concentrations of anionic surfactants in the Mississippi River continued to rise until 1971 or 1972, probably the result of increasing consumption rates and population density. Following 1972, concentrations decreased to the levels observed today, reflecting improvements in wastewater-treatment plants as the result of the Federal Water Pollution Control Act of 1972, which required secondary treatment of sewage effluents. The continued decrease of surfactant concentrations is shown in data collected during 1991--92, which are plotted on the right side of the graph.

Fecal Coliforms in the River

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Figure 53. -- Bacterial contamination of water is commonly assessed by measuring fecal coliform bacteria, which are present in untreated domestic sewage and animal wastes in extremely large concentrations (1,000 to 100,000 organisms per milliliter; American Public Health Association, 1992). The current maximum contaminant level for whole-body-contact recreation for fecal coliform bacteria is 200 organisms per 100 milliliter (mL). Coliform bacteria have been measured at many sites on the Mississippi River from the 1920s to the present. Fecal coliform concentrations as great as 100,000 organisms per 100 mL that were measured in the river in 1925--26 resulted from untreated sewage inputs near Minneapolis-St. Paul, Minnesota (Wisconsin State Board of Health, 1927). By the 1970s, improved wastewater treatment had greatly decreased fecal coliform concentrations in most of the river although high levels were still reported below Minneapolis-St. Paul and St. Louis, Missouri (Water Quality Work Group of the Great River Environmental Action Team, 1980a, 1980b). Fecal coliforms also are derived from animal waste and feedlot runoff.

The box plots in the figure show fecal coliform concentrations along the Mississippi River from 1982 to 1992 obtained from the U.S. Environmental Protection Agency's STORET data base and the U.S. Geological Survey's WATSTORE data base. Data for St. Louis from the Illinois River Watch program and data from the current study for 1991--92 also are plotted in the figure. Median fecal coliform concentrations exceeded the standard by a factor of 10 or more near Guttenberg, Iowa, Rock Island, Illinois (Quad Cities), St. Louis and Cape Girardeau, Missouri, and Memphis, Tennessee. The standard was exceeded to a lesser extent at several other locations. Standards also were exceeded in the Rock, Iowa, Des Moines, Missouri, and Kaskaskia Rivers (data not shown in the figure). Although earlier studies (Water Quality Work Group of the Great River Environmental Action team, 1980a) showed the standards being exceeded near Minneapolis-St. Paul, our measurements showed coliform counts in that area that were lower than the standard, indicating efficient removal during wastewater treatment. The fecal coliform contamination near St. Louis during 1991--92 was consistent with data reported by the Water Quality Work Group of the Great River Environmental Action Team (1980b), and probably results from lack of chlorination of treated sewage effluent in the metropolitan area.

Wastewater Contaminants Along the River

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Figure 54. -- Distributions and concentrations of dissolved organic contaminants along the length of the Mississippi River are best viewed in the context of other physical and chemical properties of the river water. During the three upriver cruises in summer 1991, fall 1991, and spring 1992 (see "Sampling the Length of the River"), samples were taken and measurements were made near the center of the river approximately every 10 miles between New Orleans and Minneapolis. On the left side of the figure are shown the distributions of three "background" variables---temperature, specific conductance, and dissolved organic carbon---to provide a context for viewing the contaminant data. On the right side of the figure are shown the distributions of three groups of dissolved organic contaminants---surfactants (MBAS and LAS) and adsorbable organic halogen (AOX). Complete tabulations of the data are given by Barber, Leenheer, and others (1995).

A

During the upriver cruise of summer 1991, water temperatures were fairly uniform along the length of the river and ranged from 22° to 27°C. By early fall 1991, the temperatures near Minneapolis had decreased about 10° while temperatures near New Orleans remained at summer levels; water temperature between Minneapolis and New Orleans showed a regular increase in the downstream direction. By early spring 1992, temperatures near Minneapolis were about 15°C lower than the previous summer and they increased downstream. During the winter (data not shown) water temperatures between Minneapolis and LaCrosse, Wisconsin (kilometer 2660), were near 0°C. A major influence of the various temperature regimes is their effect on rates of instream biodegradation, which decreases with decreasing temperature. The rate of volatilization also is affected by temperature, and the presence of ice cover during the winter can reduce the transfer of contaminants to the atmosphere.

B

Specific conductance is a measure of total inorganic dissolved solids, and the profiles measured during the three upriver cruises of 1991--92 generally resemble the long-term average profile of total dissolved solids in the Mississippi River shown in figure 12. Maximum values in the upper river were measured immediately below the confluence of the Minnesota River, which had the highest specific conductance of any tributary. Specific conductance decreased downstream as the result of dilution by low-conductivity tributaries such as the St. Croix, Chippewa, Black, and Wisconsin Rivers. The reach between the Illinois and Ohio Rivers (kilometers 1890 and 1530) had elevated specific conductance, reflecting the input of high-conductivity water from the Missouri River. Specific conductance decreased sharply as the Mississippi River received the more dilute waters of the Ohio River, and the lower river had relatively uniform values. There was little seasonal variability in specific conductance of the upper river. Seasonal variability in the reach between the Illinois and Ohio Rivers was controlled by the relative discharge from the Missouri River. Seasonal variability in the lower river was related to discharge, with the highest values recorded during low flow during the fall. Several spikes in the profiles were probably the results of point-source inputs and hydrological events that occurred upstream.

C

Dissolved organic carbon (DOC) is a measure of the total dissolved organic matter in the Mississippi River, most of which is natural. Wastewaters are only minor contributors of DOC. The most obvious features of the DOC profiles shown in the figure were the high concentrations in the upper river that decrease downstream, the sharp decreases in concentration below the Ohio River, and the relatively uniform concentrations (about 4 mg/L) in the lower river. There were distinct seasonal differences in DOC concentrations in the upper river; seasonal differences in the lower river were much less pronounced. DOC has important geochemical implications for the occurrence and fate of organic contaminants because it can (1) increase the solubility and facilitate the transport of organic contaminants, (2) alter rates of biodegradation, (3) form complexes with trace metals, and (4) react during water treatment to produce potentially toxic by-products (see next chapter).

D

Surfactants are major ingredients of soaps and detergents, and their presence is an indication of the effect of domestic wastewater on the water quality of the Mississippi River. Total anionic surfactants were determined as methylene-blue-active substances (MBAS), which is a composite measurement of linear alkylbenzenesulfonate (LAS), LAS biological metabolites and impurities, other synthetic-anionic surfactants, and naturally occurring compounds such as humic substances. The graph shows that during the upriver cruises of 1991--92, MBAS were present throughout the Mississippi River and its tributaries at concentrations ranging from about 20 µg/L to 100 µg/L, values that are below the drinking-water standard of 500 µg/L. The MBAS profiles showed some of the same general trends as specific conductance and DOC, including values in the upper river that were slightly larger than those in the lower river. The greater variability and scatter in the MBAS data indicate the effects of multiple point-source inputs from municipal wastewater discharges. MBAS concentrations had peaks in the vicinity of major cities and decreased rapidly downstream because of dilution, biodegradation, and sorption to the sediments.

E

Specific measurements for linear alkylbenzenesulfonate (LAS) were not strongly correlated with MBAS, and many of the samples with elevated MBAS had low levels of LAS. The annual consumption of LAS in the United States is about 300,000 metric tons per year (Schirber, 1989). LAS is readily biodegraded under aerobic conditions to form intermediate compounds and ultimately carbon dioxide, water, and sulfate (Swisher, 1987). Although LAS and its byproducts are nontoxic to humans, aquatic organisms can be sensitive to concentrations ranging from 10 to 1000 µg/L (Kimerle and Swisher, 1977; Kimerle, 1989). Measurable LAS concentrations in the Mississippi River were found near Minneapolis, St. Louis, and New Orleans. However, LAS concentrations drop below detectable levels within 80 km downstream from the major sources as the result of dilution and instream biodegradation. The St. Louis area has a stream-dilution factor (river discharge divided by municipal effluent discharge) of about 500, but has the highest LAS concentrations along the river. In contrast, the stream-dilution factor for Minneapolis is about 50 and concentrations are very low. The stream-dilution factor for New Orleans is about 1500, and concentrations are similar to those below Minneapolis-St. Paul.

F

Adsorbable organic halogen (AOX) is a measure of dissolved chlorinated organic matter, including both volatile and nonvolatile compounds, some of which may be toxic. These compounds are difficult to measure individually, so the AOX measurement serves as an overall index of their combined effect on water quality. AOX comes from a variety of sources, in particular, the chlorination of natural organic matter during disinfection of sewage effluents, biofouling control in powerplant cooling waters, pulp-wood bleaching, and other industrial discharges. There also are significant natural sources of AOX. The graph shows AOX profiles for the three upriver cruises of 1991--92. Although concentrations and distributions varied significantly along the river and among cruises, spatial trends relative to river reach were not as apparent as for other constituents. The greatest concentrations occurred during the spring and may have been the result of flushing natural and atmospherically transported AOX from the soils or increased anthropogenic sources during spring runoff events. The multiple spikes during the spring and fall probably represent sporadic inputs from point sources. The concentrations of AOX in the Mississippi River exceeded those that would be expected from discharge of treated municipal sewage effluent, indicating that other sources such as pulp mills and powerplants also contribute AOX. For example, a single pulp mill can contribute 840 kilograms (kg) per day of AOX (Costner and Thornton, 1989) which is 5--10 percent of the total Mississippi River AOX load near the confluence with the Ohio River. A typical powerplant can contribute a daily load of 600 kg per day (Water Quality Work Group of the Great River Environmental Action Team, 1980a). In contrast, the municipal-treated sewage discharge from St. Louis contributes an average of about 250 kg of AOX per day.

Wastewater Contaminants in Bed Sediments

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Figure 55. -- The sediments deposited and stored in the navigation pools of the Upper Mississippi River are contaminated with organic pollutants derived from municipal and other sources. Data in these three graphs are from samples of bed sediment collected during 1991--92 from the shallow areas of the navigation pools. Additional samples were collected from the shallow backwater areas of the unimpounded river at St. Louis and at Thebes, Illinois, 215 km downriver of St. Louis. Complete tabulations of the data are given in the reports by Writer (1992), Tabor (1993), Barber, Writer, and others (1995), and Writer and others (1995).

A

Linear alkylbenzenesulfonate (LAS) is found in soaps and detergents, and its presence in river- bottom sediments indicates contamination by domestic and municipal wastes. The concentrations of LAS on bed sediments (about 0.1 to 1 milligram per kilogram (mg/kg) of sediment) are one to three orders of magnitude greater than those detected in the overlying water (0.00005 to 0.012 milligrams per liter (mg/L) of water). The data in the graph show a pattern that can be attributed to inputs from cities such as Minneapolis-St. Paul, LaCrosse, and Rock Island. Biodegradation removes LAS from the sediment---at rates that apparently are variable enough to complicate patterns of high and low concentrations attributed to local sources.

B

Coprostanol is a fecal sterol that comes from human and livestock wastes. A properly operating wastewater-treatment plant will remove 95--99 percent of the coprostanol, resulting in concentrations ranging from 30 to 50 micrograms per liter (µg/L) in sewage effluents and from 1,400 to 7,900 mg/kg in sewage sludge (Walker and others, 1982). Between 85 and 95 percent of the coprostanol discharged in sewage effluents is associated with particulate matter that can be assimilated into bed sediments (Venkatesan and Kaplan, 1990). Data in the figure show that coprostanol concentrations ranged from 0.09 to about 0.8 mg/kg.

Concentrations in excess of 0.1 mg/kg indicate sewage contamination (Hatcher and McGillivary, 1979). Coprostanol is biodegraded more slowly than LAS, and its distribution in the upper river can be explained in terms of sources and sinks. Obvious sources of coprostanol are Minneapolis-St. Paul (Pool 2). Sites for the accumulation and storage of coprostanol-bearing sediments are the two largest pools in the upper river (Lake Pepin and Pool 19). During moderate to large flows, the sediments are likely to be swept out of the smaller shallower pools of the upper river and deposited in the larger deeper pools.

C

Polynuclear aromatic hydrocarbons (PNAs) are organic compounds that are common contaminants in sediments, several of which are sufficiently toxic to be listed as priority pollutants by the U.S. Environmental Protection Agency. The sources of PNAs are complex, but they typically come from combustion processes. Urban runoff, municipal-wastewater discharges, wood-treatment facilities, petroleum development and processing, coal-storage facilities, and transportation networks (both land and river) all contribute PNAs to the Mississippi River. PNAs detected on the bed sediments include the priority pollutants naphthalene, acenaphthene, acenaphthylene, fluorene, phenanthrene, fluoranthene, anthracene, chrysene, pyrene, benzo[b]fluoranthene, and benzo[k]fluoranthene. Concentrations of individual compounds ranged from less than 0.2 to about 10 mg/kg. Although only the elevated concentrations at Minneapolis-St. Paul (Pools 1 and 2) are clearly attributable to municipal sources, many of the PNAs detected in the other samples probably come from wastewater discharge and urban runoff.

Caffeine and EDTA in River Waters

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Figure 56. -- Caffeine and EDTA (ethylenediaminetetraacetic acid) are two of the more ubiquitous dissolved organic compounds in the Mississippi River. Neither compound is highly toxic to humans, but each is a specific indicator of contamination by wastewaters.

A

Caffeine is a stimulant chemical in beverages such as coffee and soft drinks and in a variety of food products. Its concentrations in municipal wastewaters range from 20--300 µg/L (Rogers and others, 1986). The graphs show the concentrations of caffeine measured in the Mississippi River and its tributaries during the three downriver sampling cruises of 1991--92 (a complete tabulation of data is given by Pereira and others, 1995). Caffeine is a fairly specific tracer of domestic wastewaters, and its profile illustrates the effect of population density on water quality. The concentration gradient between river kilometers 2950 and 2600 shows inputs from Minneapolis-St. Paul into the relatively small upper river and the progressive dilution downstream by the Minnesota and St. Croix Rivers. The elevated concentrations of caffeine in the Illinois River are from the large population in the Chicago metropolitan area, and the abrupt increase in the Mississippi River near river kilometer 1850 is the result of input from the Illinois River. The gradual downstream decreases in caffeine concentrations in the river indicate instream degradation through biotic and abiotic processes in addition to dilution by tributaries. Caffeine occurs at similar concentrations to EDTA in sewage effluents, but it occurs at much lower concentrations in river water because of degradation and because it is not supplied by the industrial sources that also introduce EDTA.

B

EDTA is used to form water-soluble complexes with insoluble metals in a wide variety of domestic and industrial applications, including stabilization of bleaching agents in laundry detergents, prevention of boiler scale formation in powerplants, addition as a preservative and clarifying agent in foods and beverages, use in fertilizers as a source of chelated metal micronutrients, and other applications in the metal-plating, photography, paper, and textile industries. The 1990 United States production of EDTA was 49,687 metric tons (U.S. International Trade Commission, 1990). EDTA is an indicator of total domestic and industrial wastewater inputs. EDTA has low toxicity to humans and aquatic organisms. The chief water-quality concerns are its ability to mobilize toxic metals from sediments and to stimulate algal growth by increasing the availability of nutrients such as iron and zinc. EDTA is not biologically degraded during wastewater treatment, but various EDTA salts and metal complexes have been observed to degrade in light (Alder and others, 1990). As a consequence of the lack of biodegradation, its high solubility in water, and large rates of production and usage, EDTA can be one of the most abundant organic contaminants in surface waters. The graphs show EDTA concentrations measured in the Mississippi River and four principal tributaries during the three downriver sampling cruises of 1991--92 (L.B. Barber, II, unpub. data, 1995). Concentrations were greater in the upper river than the lower river, and the peak concentrations occurred between kilometers 2250 and 1900 (between Dubuque and Keokuk, Iowa). The EDTA concentrations measured in the river were about five times greater than the concentrations that can be attributed to inputs of secondary treated municipal wastewaters. It is likely therefore that discharges from powerplants and other industrial sources contribute significantly to EDTA concentrations. The greatest tributary EDTA concentration was from the Illinois River, which probably reflects industrial and domestic wastewater inputs from the Chicago metropolitan area. In general, EDTA concentrations in the Mississippi River were lower than those reported for German rivers, which typically range from 10 to 60 µg/L (Frimmel and others, 1989).

Summary

The Mississippi River carries higher concentrations of organic contaminants in the vicinity of major metropolitan areas. Concentrations are typically greatest in the upper river where the stream- dilution factors are lowest. Major tributaries such as the Minnesota, Illinois, Missouri, and Ohio Rivers have significant effects on the organic chemistry of the Mississippi River. Seasonal differences are related to hydrologic, climatic, biological, and geochemical factors. Concentrations are greatest during periods of low flow (fall) and least during periods of high flow (spring). Likewise, concentrations of biologically labile and volatile organic compounds are greatestduring the winter when temperatures are lowest.

Although the data presented here provide only a brief glimpse of the water quality of the Mississippi River, comparisons with historical data show trends of improving water quality for several constituents. The improvements can be related to: (1) changes made by the chemical manufacturing industry to address the environmental fate of problematic chemicals, and (2) improved wastewater treatment by municipal and industrial dischargers. Converting primary treatment facilities to secondary treatment has resulted in improved water quality, although chemicals that are not completely removed present a challenge for treatment technology.

Much remains to be learned about the sources, fates, and effects of organic contaminants on human health and aquatic ecology. The data presented in the figures for this chapter and in the detailed reports of supporting data (Barber, Leenheer, and others, 1995; Barber, Writer, and others, 1995; Leenheer, Noyes, and Brown, 1995; Leenheer, Barber, and others, 1995; Pereira and others, 1995) represent a benchmark for future reference. They indicate critical areas where research is needed to more clearly define potential problems, and provide a baseline against which future changes in the water quality in the Mississippi River can be measured.

TTT and Flame Retardants in River Waters

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Figure 57. -- Some of the organic contaminants that are dissolved in the waters of the Mississippi River can be traced to unique sources. Two of these are TTT, an industrial by-product primarily contributed from the Kanawha River Valley of West Virginia, and flame retardant additives contributed almost exclusively from the Illinois River watershed. Because their sources are so restricted, these compounds can be used as tracers of the waters of their source tributaries as they mix with the other waters that make up the Mississippi River. Samples portrayed in the graphs were collected during the downriver cruises of 1987--92; complete tabulations of the data are given by Pereira and others (1995).

A

TTT (1,3,5-trimethyl-2,4,6-triazinetrione) is a by-product of the manufacture of methylisocyanate. TTT has been reported in residual materials from a tank after the leak of methylisocyanate in Bhopal, India (D'Silva and others, 1986), in secondary effluent from a municipal and industrial wastewater-treatment plant in Illinois (Ellis and others, 1982), and in water from the Ohio River where it continues to be detected at 1--2 µg/L (Don Mackell, Louisville Water Company, Louisville, Kentucky, written commun., 1989). During the sampling cruises of 1987--92, the Ohio River was the major contributor of TTT to the Mississippi River, and the source was eventually traced to a location on the Kanawha River where methylisocyanate is manufactured and transported. TTT is a very stable compound and was detected in all water samples collected from the Mississippi River between the Ohio River and the Gulf of Mexico, indicating little or no degradation during 2300 kilometers of river transport.

B

THAP (trihaloalkylphosphates) are used as flame-retardant additives in flexible and rigid polyurethane foams, and in textiles. THAP are relatively water-soluble compounds that have low bioconcentration factors and short half-lives in fish. THAP have been reported in municipal water supplies of cities adjoining the Great Lakes in Canada (Williams and others, 1982; Williams and Le Bel, 1981), in surface waters of Japan (Japan Environmental Agency, 1977--79), and in surface water, ground water, and drinking water in Italy (Galassi and Guzella, 1988).

The graphs show the concentrations of total THAP (sum of tris-2-chloroethylphosphate and two isomers of tris-2-chloropropylphosphate), that were detected in all water samples collected from the Mississippi River and its tributaries during the downriver sampling cruises of 1987--92. In addition to minor sources of THAP along the Mississippi River, there is a major source on the Illinois River. THAP are relatively stable and degrade only slightly during transport from St. Louis to the Gulf of Mexico, a distance of about 1850 kilometers.

Volatile Contaminants in the River

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Figure 58. -- Volatile organic compounds (VOC) appear to be short lived in the Mississippi River, as would seem to be self-evident from the fact that the compounds are called "volatile." However, considerable effort was spent in testing the waters of the Mississippi River for their presence because (1) there are degrees of volatility and (2) some of the compounds are highly toxic. Sources of volatile organic compounds are industrial, municipal, and transportational (urban runoff and river traffic).

The graphs show several of the volatile organic compounds that were detected in significant concentrations in the midriver samples collected during the three upriver cruises of 1991--92. Of the 64 target compounds, for which 440 samples were analyzed, 32 were detected and only 19 occurred in more than 1 percent of the samples. Concentrations of total VOC ranged from 0.2 to 3.1 µg/L and were typically an order of magnitude below maximum contaminant levels. There was a seasonal trend in volatile compound concentrations; the greatest frequency of detections and highest concentrations occurred during the fall and winter. During the winter (data not shown in the graphs) concentrations increase significantly in the reaches of the river that become ice covered. The areas with the most significant contamination by volatile organic compounds were the reach downstream from Minneapolis-St. Paul, the reach downstream from the confluence of the Ohio and Mississippi Rivers, and the reach between Baton Rouge and New Orleans.

SELECTED REFERENCES

Alder, A.C., Siegriest, H., Gujer, W., and Giger, W., 1990,

Behavior of NTA and EDTA in biological wastewater treatment: Water Research, v. 24, p. 733--742.

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