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EIWA Abstracts
THE
OCCURRENCE OF CHLOROACETANILIDE AND TRIAZINE HERBICIDE DEGRADATES
IN STREAMS IN EASTERN IOWA
Stephen J. Kalkhoff, Supervisory Hydrologist, U.S. Geological
Survey, Iowa City, Iowa
Douglas J. Schnoebelen, Hydrologist, U.S. Geological Survey, Iowa
City, Iowa
E. Michael Thurman, Research Hydrologist, U.S. Geological Survey,
Lawrence, Kansas
Proceedings from Agriculture and the Environment: State and Federal
Initiatives conference at Iowa State University, March 5-7, 2001
The occurrence of chloroacetanilide (acetochlor, alachlor, and
metolachlor) and triazine (atrazine, cyanazine, and simazine)
herbicides and degradates were investigated from 1996 to 1998
as part of the U.S. Geological Survey's National Water-Quality
Assessment Program. More than 300 water samples were collected
from 12 stream sites in the eastern Iowa. Monthly samples were
collected for one year beginning in March 1996 and during the
growing season (April to September) in 1998. Samples were collected
weekly or biweekly at three sites in 1997 to determine temporal
variability. Samples were analyzed for the chloroacetanilide herbicides,
acetochlor, alachlor, and metolachlor, and their sulfonic (ESA)
and oxanilic acid (OA) degradates. Samples were also analyzed
for the triazine herbicides, atrazine, cyanazine, and three of
their common degradates.
Chloroacetanilide and triazine herbicide degradates are substantial
components of the pesticide load in eastern Iowa streams. Degradates
are detected more frequently and at greater concentrations than
the parent compounds. When adjusted to a common detection threshold
of 0.20 micrograms per liter, the four most frequently detected
compounds were metolachlor ESA (detected in 99.7 percent of the
samples), alachlor ESA (99.1 percent), metolachlor OA (94.3 percent),
and acetochlor ESA (71.8 percent). Metolachlor, the most commonly
occurring parent compound, was detected in 54.1 percent of the
samples. On average, degradates accounted for most (93 percent)
of the total pesticide mass measured in each sample. Metolachlor
compounds account for 63 percent; alachlor for 19 percent; acetochlor
for 9.5 percent; atrazine for 6.9 percent and cyanazine for 1.2
percent of the degradates. The monthly median total metabolite
concentration ranged from 4.6 micrograms per liter in February
to 10.0 micrograms per liter in June. In contrast, the monthly
median total pesticide concentration ranged from 0.16 micrograms
per liter in October to 3.8 micrograms per liter in June. The
data suggest that differing chemical properties, application rates,
and precipitation may affect metabolite concentrations in streams.
The largest quantities of pesticides and pesticide degradates
are transported from eastern Iowa basins during the late spring
and early summer soon after application. Monthly degradates loads
in the Iowa River at Wapello were higher than the loads of the
parent compounds throughout the year but degradates loads were
particularly dominant during the spring and early summer. In the
fall and winter months, however, the pesticide degradates accounted
for nearly all of the pesticide compounds transported to the Mississippi
River. Parent compounds accounted for from 3 percent (December)
to 27 percent (May) of all pesticide compounds transported by
the Iowa River. During the relatively dry late fall and winter
months, much of the water and dissolved pesticide compounds originated
from ground-water discharge to streams. The presence of relatively
high concentrations of alachlor and metolachlor degradates in
the fall and winter, several months after pesticide application,
indicates that these compounds are relatively stable.
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