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
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.
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
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
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).
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
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.
(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.
(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
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.
- 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.
- Bull, R.J., 1982,
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disinfectant by-products: Environmental Science and Technology, v. 16, no.
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- Cantor, K.P., Hoover, R., Mason, T.J., and McCabe, L.J., 1978,
of cancer mortality with halomethanes in drinking water: Journal National
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drinking waterĐ A perspective: South African Journal of Science, v. 84,
March, p. 166-70.
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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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- ___ 1994,
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potentials for the Mississippi River and some of its tributaries,
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halogenated compounds found in chlorinated drinking water, in Water
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Michigan, Ann Arbor Science, p. 417-431.
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Martinus Nijhoff/Dr. W. Junk Publishers, 497 p.
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suspected carcinogens in drinking water: Report to Congress: Report PB-250
961, National Technical Information Service, 52 p.
- ___ 1979,
- National interim primary drinking water regulations; control of
trihalomethanes in drinking water; final rule: Federal Register, v. 44,
no. 231, p. 28641-28642.
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Contaminants in the Mississippi River
U.S. GEOLOGICAL SURVEY CIRCULAR 1133
Reston, Virginia, 1995
Edited by Robert H. Meade
h2o Webserver Team
Last Modified: 1230 01 Oct 96 ghc