Contaminants in the Mississippi River
U.S. GEOLOGICAL SURVEY CIRCULAR 1133
Reston, Virginia, 1995
Edited by Robert H. Meade

Potentially Deleterious Effects of Chlorinating Mississippi River Water for Drinking Purposes

Ronald E. Rathbun

Introduction

An emerging issue in the treatment of drinking water is the role played by chlorine in the formation of toxic compounds. The idea that chlorine might be deleterious seems paradoxical because the addition of chlorine to public water supplies, which became a general practice during the early decades of this century, perhaps did more to increase human life spans than any other single event in history (Fairweather, 1990). Chlorine destroys the microbes that cause typhoid fever, cholera, and amoebic dysentery; as a result, these diseases have been virtually eradicated from the developed countries of the world. Within the last few decades, however, it has become evident that adding chlorine as a disinfectant to natural waters can result in the formation of chloroform and other carcinogenic or mutagenic compounds (Rook, 1974; Bellar and others, 1974). The importance of this issue may be summarized by the statement of R.J. Bull (1982) who concluded that "no other public health issue affects a larger proportion of the U.S. population than drinking water disinfection."

The Mississippi River is used by many of the cities and towns along its banks as a source of drinking water. As a standard practice, chlorine is added as a disinfectant to the waters that are extracted from the river before they are distributed to consumers through municipal water systems. As part of the assessment of the water quality of the Mississippi River, a set of laboratory experiments was done to assess the potential for forming toxic substances when river waters are chlorinated.

Background

The toxic by-product compounds formed during the chlorination of natural waters result from the reaction of free chlorine used for disinfection with the dissolved organic carbon (DOC) present in the water. The DOC is a complex mixture of organic compounds resulting from the decay of soil and plant organic matter (Thurman, 1985). Chemical and biological processes within aquatic systems also may contribute to the DOC. The DOC is present in varying concentrations and compositions in all natural waters. It is the DOC that serves as the precursor for the formation of the by-product compounds.

The by-products formed during the chlorination of natural waters can be divided into two classes of compounds. The first class consists of the trihalomethane (THM) compounds, which are chlorinated and brominated derivatives of methane. The four compounds commonly included in this class are chloroform (CHCl3), dichlorobromomethane (CHCl2Br), chlorodibromomethane (CHClBr2), and bromoform (CHBr3). Chloroform is a known animal carcinogen (Pieterse, 1988). The other THM compounds formed during the disinfection process are carcinogenic and mutagenic (Simmon and Tardiff, 1978), and positive correlations have been observed between the concentrations of these compounds in drinking water and bladder cancer (Cantor and others, 1978). Positive correlations also were observed for brain cancer in both males and females and for lymphoma and kidney cancer in males. Concerns about possible health effects of these compounds (U.S. Environmental Protection Agency, 1975, 1979; National Academy of Sciences, 1977) resulted in the U.S. Environmental Protection Agency's establishing a maximum contaminant level (MCL) in drinking water of 100 micrograms per liter (ug/L) total of the four THM compounds. Other countries have more stringent requirements. Germany and Switzerland have an MCL of 25 ug/L (Pieterse, 1988). In addition, Amsterdam has stopped the chlorination of drinking water because of health concerns about the ingestion of these compounds (Pieterse, 1988).

The second class consists of a complex mixture of relatively nonvolatile compounds that have higher molecular weights than the THM compounds. Because of the large number of different compounds present in this mixture at small concentrations, identification and quantification of the individual compounds is difficult. Consequently, the concentration of these compounds is determined as a bulk parameter called the nonpurgeable total organic halide (NPTOX) concentration. As of 1994, no maximum contaminant level has been established for the NPTOX concentration in drinking water.

Experimental Procedure

The experiments evaluated the factors that influence the formation of toxic by-products during the chlorination process. These factors are the concentration of free chlorine used in the chlorination process, the pH at which the water is chlorinated, the water temperature, and the concentration and composition of the DOC of the source water. The distribution of the compounds among the chlorinated and brominated compounds depends on the bromide concentration of the source water. The free chlorine concentration is determined by the particular water-treatment process being used, and the pH is determined by a combination of the pH of the source water and the water-treatment process being used. The DOC and bromide concentrations, the composition of the DOC, and the temperature are determined by the characteristics of the source water.

Potentials for the formation of THM and NPTOX compounds upon chlorination of Mississippi River water were determined in samples that were collected from 12 sites along the river between Minneapolis, Minnesota, and New Orleans, Louisiana; the samples were shipped to the laboratory in Denver, Colorado, where the experiments were conducted. Formation potentials were determined as a function of pH and initial free chlorine concentration for 7 days at a temperature of 25oC. Formation potentials also were determined as a function of reaction time over 7 days for a pH of 8.14 and an initial free chlorine concentration of 30 milligrams per liter. Complete details of the experimental procedure and the data resulting from this study have been presented in other reports (Rathbun, 1995; Rathbun and Bishop, 1993, 1994).

Experimental Results

Potentials for the formation of the THM and the NPTOX compounds when water from the Mississippi River is chlorinated are presented in figure 59 as a function of distance above Head of Passes, Louisiana. Also presented in figure 59 are the DOC concentrations. The formation potentials decrease with distance downstream, in each case paralleling the decrease of the DOC concentration with distance downstream. This behavior clearly demonstrates the importance of the DOC as the precursor in the by-product formation process.

The effect of pH on the by-product formation process is demonstrated by the fact that NPTOX formation potential is considerably larger than the THM formation potential for a pH of 6.73, whereas the THM formation potential is slightly larger than the NPTOX formation potential for a pH of 9.70. For the intermediate pH of 8.14, the NPTOX formation potential is larger than the THM formation potential, although the difference is considerably less than the difference for pH 6.73. These results indicate that pH conditions in the water-treatment process that minimize the formation of the THM compounds tend to maximize the formation of the NPTOX compounds.

Potentials for the formation of the THM and the NPTOX compounds as a function of reaction time are presented in figure 60 for water samples from the river at Minnea-polis, Memphis, and New Orleans. Concentrations of both types of compounds increased rapidly during the initial part of the experiment; more than one-half of the final concentrations was formed during the first day. Thereafter, the concentrations of both types of compounds increased gradually for the balance of the 7-day period.


Figure59

(Click on image for a larger version, 66K)

Figure 59. -- Results obtained in laboratory experiments show the potential for forming trihalomethane (THM) and nonpurgeable total organic halide (NPTOX) compounds during chlorination of waters collected at 12 sites along the Mississippi River during the summer of 1991. Potentials are graphed to show distance of the sampling sites upriver of Head of Passes, Louisiana, with Minneapolis to the left and New Orleans to the right. The graphs show the results of chlorination experiments conducted at three different values of pH: upper graph pH = 6.73 (slightly acid); middle graph pH = 8.14 (moderately alkaline); lower graph pH = 9.70 (strongly alkaline). The concentrations of dissolved organic carbon (DOC), measured at the beginnings of the experiments (shown by the triangles; same values in all three graphs), were greater in the upper-river waters than in the lower-river waters. Potentials of the formation of the toxic compounds generally parallel the DOC concentrations, but the relative importance of THM (squares) and NPTOX (circles) depends heavily on pH. At slightly acid and moderately alkaline pH (upper two graphs), NPTOX compounds form more readily than THM compounds. At strongly alkaline pH (lower graph), THM compounds form more readily than the NPTOX compounds.


Figure60

(Click on image for a larger version, 33K)

Figure 60. -- Results obtained in laboratory experiments show that the formation of trihalomethane (THM) and nonpurgeable total organic halide (NPTOX) compounds is initially very fast; the rate of formation decreases after about 1 day; several days are required to reach the maximum concentrations for both the THM and NPTOX compounds. The experiments portrayed in this figure were conducted on waters collected during the summer of 1991 from the Mississippi River at Minneapolis, Memphis, and New Orleans. All three experiments were conducted at a pH of 8.14, which is near the characteristic pH of most untreated waters of the Mississippi River.


Conclusion

Results of this study indicate that there is sufficient DOC in the Mississippi River that THM and NPTOX compounds will be formed when the water is disinfected with chlorine. The DOC concentration decreased with distance downstream between Minneapolis and New Orleans; the THM and NPTOX concentrations measured in the laboratory formation-potential experiments also decreased with distance downstream, paralleling the decrease of the DOC. The pH levels prevalent in the Mississippi River and pH values likely to be used in water disinfection are such that the NPTOX compounds are likely to form more abundantly than the THM compounds. However, if a more alkaline condition is used in the disinfection process, then the THM compounds are likely to form more abundantly than the NPTOX compounds. The formation potentials for the THM compounds, although less than the potentials for the formation of the NPTOX compounds, were always in excess of the present MCL of 100 ug/L. This conclusion should be tempered by the fact that experimental conditions of initial free-chlorine concentration and reaction time used in these laboratory experiments were such as to maximize the formation of the THM and NPTOX compounds. The large formation potentials observed, however, indicate that care must be exercised in disinfecting river water with chlorine, particularly in water from the Upper Mississippi River.

REFERENCES

Bellar, T.A., Lichtenberg, J.J., and Kroner, R.C., 1974,
The occurrence of organohalides in chlorinated drinking waters: Journal American Water Works Association, v. 66, no. 12, p. 703-706.
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Drinking water and health: Washington, D.C., Printing and Publishing Office, 939 p.
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Trihalomethane and nonpurgeable total organic-halide formation potentials for the Mississippi River and some of its tributaries, March-April 1992: U.S. Geological Survey Open-File Report 94-336, 55 p.
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Trihalomethane and nonpurgeable total organic halide formation potentials for the Mississippi River and some of its tributaries, June-August 1991: U.S. Geological Survey Open-File Report 93-158, 57 p.
___ 1994,
Trihalomethane and nonpurgeable total organic-halide formation potentials for the Mississippi River and some of its tributaries, September-October 1991: U.S. Geological Survey Open-File Report 94-36, 53 p.
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___ 1979,
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Contaminants in the Mississippi River
U.S. GEOLOGICAL SURVEY CIRCULAR 1133
Reston, Virginia, 1995
Edited by Robert H. Meade
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