Pesticides Reported in Mississippi Floodwaters
October 25, 1993
Hydrologists at the U.S. Geological Survey (USGS) have found
surprisingly high concentrations of agricultural chemicals in
the Mississippi River and some of its tributaries, in the
aftermath of the river's flooding this summer. According to
USGS scientists, the results indicate that concentrations of
herbicides in the Mississippi are at, or near, the maximum
levels detected in a previous study that was conducted during
1991 and 1992. Scientists had expected concentrations to be
diluted by the record-high flows, but found instead that they
remained at the same levels.
Since flows were so much higher and concentrations of
herbicides were the same, the daily loads of these chemicals
in the rivers increased by almost 50% over previous
measurements. A USGS hydrologist stated, "On several days
during the peak flows, we estimated that the Mississippi
River at Thebes (Illinois) was carrying more than 12,000
pounds of atrazine per day."
A study of the Mississippi River and its tributaries by the
USGS during selected months in 1991-92 indicated that the
entire navigable reach of the river is contaminated with a
complex mixture of agrochemicals and their transformation
products. Scientists found strong indications that under
certain conditions there is a significant groundwater
contribution to the river. Therefore, the Mississippi River
serves as a vast drainage channel for pesticide-contaminated
surface and groundwater from the midwestern U.S.
The Mississippi River and its tributaries provide water to
about 70 cities, and accounting for 23% of the public
surface-water supplies for the U.S. Though untreated and
partially-treated industrial and domestic wastes are also
discharged into the river, USGS scientists state that
nonpoint-source pollution by agrochemicals from both surface
runoff and groundwater discharge may be the most significant
factor responsible for deteriorating water quality.
An estimated 200-300 million pounds of herbicides are applied
annually to control weeds in the Midwest. Atrazine, the most
widely used herbicide in the region, is of particular concern
because of its potential to migrate through the soil, and its
relatively long half-life. The U.S. Environmental Protection
Agency classifies atrazine as a possible human carcinogen
based on animal studies.
USGS found atrazine in every sample from the Mississippi
collected in April, May and June, 1991. This is especially
significant considering the large number of people that rely
on the Mississippi for drinking water. In an Iowa study in
the mid-1980s, atrazine was detected in 30 of 33 samples
collected from public water supplies. The results indicate
that current water treatment technology is ineffective in
substantially reducing or eliminating atrazine from drinking
water.
Sources: USGS, Department of the Interior, 119 National
Center, Reston, VA 22092, USA; phone (703) 648-4460. Journal
of Pesticide Reform, Winter 1991. Environmental Science and
Technology, Vol. 27 No. 8, 1993.
Contact: PANNA.
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The Pesticide Action Network Updates Service (PANUPS) is a
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Action Network North America Regional Center (PANNA). PANNA
is located at 116 New Montgomery Street, #810, San Francisco,
CA 94105. Tel: (415) 541-9140. Fax: (415) 541-9253. To
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