[an error occurred while processing this directive]
EIWA Abstracts
OCCURRENCE
AND DISTRIBUTION OF PESTICIDES IN STREAMS OF THE EASTERN IOWA
BASINS, 1996-98
Douglas J. Schnoebelen, Stephen J. Kalkhoff, and Kent D. Becher
Hydrologists
U.S. Geological Survey
Iowa City, Iowa
Proceedings from Agriculture and the Environment: State and Federal
Initiatives conference at Iowa State University, March 5-7, 2001
The U.S. Geological Survey began collection of water samples in
streams of the Eastern Iowa Basins in 1996 for the analysis of
pesticides and pesticide degradates as part of the National Water
Quality Assessment Program (NAWQA). This study provides some of
the first large scale monitoring data on pesticides and pesticide
degradates in Eastern Iowa. Three hundred and forty-four samples
were collected from 1996-98 to document the occurrence, distribution,
and transport of pesticide compounds. Pesticide analysis included
80 pesticide compounds and 10 pesticide degradates. The Eastern
Iowa Basins study encompasses about 50,500 square kilometers (19,500
square miles) and is drained by four major rivers--the Wapsipinicon,
Cedar, Iowa, and Skunk. Agriculture accounts for approximately
93 percent of the land use in the study area.
The most commonly detected pesticides were those most heavily
used on crops. The triazine (atrazine and cyanazine) and chloroacetanilide
(alachlor, acetochlor, and metolachlor) pesticides are some of
the most heavily used (by weight) historically and during the
period of data collection 1996-98. Atrazine and metolachlor were
detected in all samples. Acetochlor, alachlor, and cyanazine were
detected in more than 70 percent of all surface-water samples.
Few non-agricultural herbicides were detected. One exception,
prometon was detected in more than 80 percent of the samples at
very low concentrations (less than 0.1 micrograms per liter).
Pesticide degradates were some of the most frequently detected
pesticide compounds in the study. Four pesticide degradates--metolachlor
ethane sulfonic acid (metolachlor ESA), alachlor ethane sulfonic
acid (alachlor ESA), metolachlor oxanilic acid (metolachlor OA),
and acetochlor ethane sulfonic acid (acetochlor ESA) were detected
in more than 75 percent of the samples.
A few insecticides that may pose potential risk to aquatic invertebrates
were detected in streams from May through September, the months
when most application normally occurs. Carbofuran was the most
commonly detected insecticide (16 percent of all samples). Although
detected in less than 20 percent of all samples, carbofuran was
detected in 68 percent of the samples in June. When present, carbofuran
concentrations were generally less than 0.80 micrograms per liter.
Chloropyrifos was detected in about seven percent of the samples.
As with other insecticides, chlorpyrifos was detected most frequently
in June (30 percent). The highest concentration was 0.06 micrograms
per liter. Diazinon, a common urban insecticide found in other
NAWQA studies throughout the Nation, was detected in only 2 percent
of the samples in the Eastern Iowa Basins study.
Pesticides were found to occur in mixtures with several compounds
rather than individually. Four or more parent pesticide compounds
were detected in 91 percent of the water samples and seven or
more parent compounds were detected in 46 percent of the water
samples. Four or more pesticide degradates were detected in 93
percent of the water samples and seven or more pesticide degradates
were detected in 46 percent of the water samples.
Commonly applied parent pesticide compounds (acetochlor, alachlor,
atrazine, cyanazine, and metolachlor) were generally detected
at low concentrations with median concentrations ranging from
0.01 to 0.22 micrograms per liter. The median concentrations for
the pesticide degradates were larger than their parent compounds.
Median concentrations for the pesticide degradates ranged from
0.07 to 3.7 micrograms per liter. Acetochlor, alachlor, atrazine,
cyanazine and metolachlor pesticides compounds were present at
least an order of magnitude or higher in the late spring and summer
than at other times of the year. The maximum measured concentrations
for acetochlor, atrazine, cyanazine and metolachor were approximately
11 to 48 micrograms per liter (the maximum for alachlor was 0.56
micrograms per liter). In contrast, maximum measured concentrations
for the total pesticide degradates were lower than their parent
compounds and ranged from approximately 0.7 to 12 micrograms per
liter. The maximum measured concentration of a single pesticide
compound was for atrazine at 48 micrograms per liter.
Seasonal patterns of atrazine, acetochlor, alachlor, cyanazine,
and metolachlor generally show peak concentrations following application
in May and June and decreasing during remainder of the growing
season. In addition, a small secondary peak in atrazine, acetochlor,
alachlor, cyanazine, and metolachlor concentrations occurred at
all sites in late winter. This secondary peak may be attributed
to early "winter thaw" that can release pesticide residue
from soil, making pesticides available to be transported to surface
water by snowmelt and early spring rains.
Pesticide degradates account for a significant portion of the
total pesticide load at all sites. Eighty-one percent of the total
pesticide load in samples from Iowa River near Rowan, Wolf Creek
near Dysart, and the Iowa River at Wapello were as pesticide degradates.
The pesticide degradates for the triazine compounds tended to
follow the load pattern of the parent pesticide compounds closely
throughout the year. In general, the degradate loads calculated
for the triazine compounds were smaller than loads calculated
for their parent pesticides. The loads for the chloroacetanilide
degradate compounds were larger than those for the parent pesticides.
The loads for alachlor were found to be small or nonexistent.
Alachlor has been heavily used in the past, but since 1995 has
been largely replaced by acetochlor or other herbicides. The loads
for all degradates were higher than the parent compounds during
the winter months. Overland flow may be diminished during the
winter months, but shallow sub-soil drainage and ground-water
flow may be a source of many pesticide degradates during the late
fall and winter.
Occurrence of pesticide compounds varied by landform region. The
triazine herbicides, atrazine and cyanazine and their degradates
were present in significantly greater concentrations in the Southern
Iowa Drift Plain (predominantly loess soils) than either the Des
Moines Lobe or the Iowan Surface (predominantly till soils). Less
atrazine and cyanazine are applied to till soils because of pH
and organic carbon content. Alachlor, metolachlor, and acetochlor
have often been used to offset triazine pesticide reductions in
area with till soils.
[EIWA Homepage][EIWA
Publications]
U.S. Department of the Interior
| U.S. Geological Survey