By Douglas J. Schnoebelen, Stephen J. Kalkhoff, Kent D. Becher, and E.M. Thurman
U.S. Geological Survey, Iowa City, IA
The full report is available in pdf. Links to the pdf.
samples were collected in streams of the Eastern Iowa Basins study unit from
1996 to 1998 as part of the U.S. Geological Survey’s National Water-Quality
Assessment (NAWQA) Program. More than 350 samples were collected to document the
occurrence, distribution, and transport of pesticides and pesticide degradates.
The Eastern Iowa Basins study unit encompasses about 50,500 square kilometers
(19,500 square miles) in eastern Iowa and southern Minnesota and is drained by
four major rivers—the Wapsi-pinicon, Cedar, Iowa, and Skunk—which flow into
the Mississippi River at the eastern border of Iowa.
most commonly detected pesticides— acetochlor, alachlor, atrazine, cyanazine,
and metolachlor—were those most heavily used on crops during the study.
Atrazine and metolachlor were detected in 100 percent, and acetochlor, alachlor
and cyanazine were detected in more than 70 percent of all surface-water
samples. Four pesticide degradates—metolachlor ethane sulfonic acid, alachlor
ethane sulfonic acid, metolachlor oxanilic acid, and acetochlor ethane sulfonic
acid were detected in more than 75 percent of the samples. Only one
nonagricul-tural herbicide, prometon, was detected in more than 80 percent of
the samples. Carbofuran, the most commonly detected insecticide, was found in 16
percent of all samples.
of pesticide compounds commonly occurred in the samples. Five or more parent
pesticide compounds were detected in 50 percent of the samples. Four or more
pesticide degradates were detected in 68 percent and seven or more pesticide
degradates were detected in 17 percent of the samples. Acetochlor, alachlor,
atrazine, cyanazine, and metolachlor were generally present at low
concentrations; median concentrations ranged from 0.01 to 0.22 microgram per
liter. However, median concentrations for the pesticide degra-dates, 0.07 to 3.7
micrograms per liter, were larger than their parent compounds. Acetochlor,
alachlor, atrazine, cyanazine, and metolachlor pesticide compounds were detected
at an order of magnitude or higher in the late spring and summer than at other
times of the year. Pesticide concentrations generally peak following application
in May and June and decrease during the growing season. A small secondary peak
of atrazine, acetochlor, alachlor, cyanazine, and metolachlor concentrations
occurred in late winter at all sites. The seasonal patterns for the triazine
(atrazine and cyanazine) degradates were similar to the parent compounds
(increasing in the spring), but the triazine degra-dates often had higher median
concentrations than their parent compounds in the fall and winter. The
chloroacetanilide (acetochlor, alachlor, and metolachlor) degradates did not
follow a strong seasonal pattern like their parent compounds. In general, the
chloroacetanilide degradates had constant and higher median concentrations when
compared to their parent compounds throughout the year. The median
concentrations for the chloroacetanilide pesticide degradates were often an
order of magnitude higher than their parent compounds.
of pesticides varied by land-form region. Atrazine and cyanazine and their
degradates were present in significantly greater concentrations in streams of
the Southern Iowa Drift Plain than streams of either the Des Moines Lobe or the
Purpose and Scope
Description of the Eastern Iowa Basins
Pesticide Use and Properties
Study Design and Methods of Study
Sampling Site Selection.
Pesticides and Pesticide Degradates in Streams
Occurrence and Distribution
Relevance of Pesticides in Streams
Summary and Conclusions
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