Herbicides and Herbicide Degradation Products in Upper Midwest Agricultural
Streams during August Base-Flow Conditions
Stephen J. Kalkhoff,* Kathy E. Lee, Stephen D. Porter, Paul J. Terrio, and E. Michael Thurman
ABSTRACT of the Midwest generally decrease through summer due
Herbicide concentrations in streams of the U.S. Midwest have been to degradation and flushing from the soils. However,
shown to decrease through the growing season due to a variety of factors that affect the occurrence of herbicide com-
chemical and physical factors. The occurrence of herbicide degrada- pounds in small- to medium-sized streams during late
tion products at the end of the growing season is not well known. summer base-flow conditions in the upper Midwest have
This study was conducted to document the occurrence of commonly not been thoroughly documented.
used herbicides and their degradation products in Illinois, Iowa, and Atrazine degrades through a combination of physical,
Minnesota streams during base-flow conditions in August 1997. Atra- chemical, and biological processes. The three major
zine, the most frequently detected herbicide (94%), was present at degradation products of atrazine are deethylatrazine
relatively low concentrations (median 0.17 g L 1). Metolachlor was
(DEA), deisoproplyatrazine (DIA), and hydroxy-atra-
detected in 59% and cyanazine in 37% of the samples. Seven of
nine compounds detected in more than 50% of the samples were
zine (HA). Deisoproplyatrazine and DEA form primar-
degradation products. The total concentration of the degradation ily through biological processes in the soil (Kaufman
products (median of 4.4 g L 1) was significantly greater than the and Kearney, 1970). Adams and Thurman (1991) sug-
total concentration of parent compounds (median of 0.26 g L 1). gest that the presence of increased DEA concentrations
Atrazine compounds were present less frequently and in significantly is primarily due to a large number of microbes, high
smaller concentrations in streams draining watersheds with soils devel- organic carbon content, and relatively warm soil tem-
oped on less permeable tills than in watersheds with soils developed peratures. Hydroxy-atrazine forms primarily through
on more permeable loess. The detection and concentration of triazine chemical hydrolysis (Armstrong et al., 1967) and binds
compounds was negatively correlated with antecedent rainfall (April– to soils with high organic matter and low pH. Mesocosm
July). In contrast, acetanalide compounds were positively correlated
studies (Lee et al., 1995) indicated that atrazine in the
with antecedant rainfall in late spring and early summer that may
transport the acetanalide degradates into ground water and subse-
aquatic environment degraded more rapidly in the pres-
quently into nearby streams. The distribution of atrazine degradation ence of emergent vegetation (cattail [Typha spp.]) than
products suggests regional differences in atrazine degradation pro- in open water.
cesses. Substantially less research has been conducted on
the degradation of chloroacetanilide herbicides such as
acetochlor, alachlor, and metolachlor. Although the ap-
plication of chloroacetanilide herbicides has, at times,
T he Corn Belt region of the U.S. Midwest is one
of the most intensive and productive agricultural
regions in the world. Nearly 80% of the USA’s corn
exceeded that of the triazine herbicides in parts of the
Midwest (Meyerfeld et al., 1996), the occurrence of the
(Zea mays L.) and soybean [Glycine max (L.) Merr.] is chloroacetanilide herbicides has been substantially
grown in the region and more than 100 000 Mg of pesti- lower than triazine herbicides in surface water (Burkart
cides are applied to cropland in the Midwest annually and Kolpin, 1993; Thurman et al., 1992). Chloroacetani-
(Battaglin and Goolsby, 1999). Intensive use of agricul- lide herbicides degrade more rapidly than atrazine
tural chemicals may result in nonpoint-source contami- (Buhler et al., 1993; Miller et al., 1997), thus reducing
nation of surface and ground water throughout the Mid- their potential transport to ground water or streams.
west. Results of studies conducted during the past Complete breakdown for all compounds, however, has
decade indicate that large amounts of herbicides are not been established (Stamper et al., 1997; Miller et
washed from cropland and transported into tributary al., 1997). Relatively stable and persistent intermediate
streams of major rivers of the Midwest during periods herbicide degradation products occur for these chloro-
of rainfall in late spring and early summer (Thurman acetanilide herbicides. Research has identified an
et al., 1991, 1992; Coupe et al., 1995; Battaglin and Gool- alachlor degradation product, alachlor ethanesulfonic
sby, 1999). Following spring runoff of herbicides, con- acid (alachlor ESA), found in both ground water and
centrations of parent compounds in streams and rivers surface water (Kolpin et al., 1996, 1997; Thurman et al.,
1996). Field and Thurman (1996) suggest that alachlor
ESA may be the result of a glutathione conjugation
S.J. Kalkhoff, U.S. Geological Survey, 400 S. Clinton St., Rm. 269, process occurring in plants, algae, and terrestrial micro-
Iowa City, IA 52244. K.E. Lee, U.S. Geological Survey, 2280 Woodale organisms. Additionally, it is hypothesized that mobile
Dr., Mounds View, MN 55112. S.D. Porter, U.S. Geological Survey,
Denver Federal Center, Box 25046, MS 406, Lakewood, CO 80225-
sulfonated and nonsulfonated degradation products of
0046. P.J. Terrio, U.S. Geological Survey, 221 N. Broadway Ave., other chloroacetanilide herbicides may result from this
Suite 101, Urbana, IL 61801. E.M. Thurman, U.S. Geological Survey, glutathione conjugation pathway (Aga et al., 1994; Field
4821 Quail Crest Pl., Lawrence, KS 66049. Received 12 Feb. 2002.
*Corresponding author (firstname.lastname@example.org).
Abbreviations: DEA, deethylatrazine; DIA, deisoproplyatrazine;
Published in J. Environ. Qual. 32:1025–1035 (2003). ESA, ethanesulfonic acid; HA, hydroxy-atrazine; OA, oxanilic acid.
1026 J. ENVIRON. QUAL., VOL. 32, MAY–JUNE 2003
and Thurman, 1996; Thurman et al., 1996). Outdoor need to understand how geological and climatological
mesocosm studies (Graham et al., 1999) have shown factors affect the transport of pesticides to streams and
that alachlor and metolachlor were transformed to etha- how they affect breakdown of these compounds.
nasulfonic (ESA) and oxanilic acid (OA) degradation The purpose of this paper is to document the occur-
products. Alachlor OA is the more prominent break- rence of commonly used triazine and chloroacetanilide
down product of alachlor and OA and ESA degradates herbicides and their degradates in streams draining ag-
of metolachlor are formed in more equal proportions ricultural watersheds in southern Minnesota, eastern
in aquatic systems (Graham et al., 1999). Recent studies Iowa, and central Illinois during base-flow conditions
(Kalkhoff et al., 1998; Phillips et al., 1999) have shown in August 1997. The occurrence of the pesticides and
that the relatively stable and soluble chloroacetanilide pesticide degradates are related to differences in soils
ESA and OA degradation products are commonly de- in the watersheds and to differences in amount of rain-
tected in streams. fall during the growing season. The results presented
Soils are an important factor in transformation and in this paper are one part of the U.S. Geological Sur-
transport of triazine and chloroacetanilide herbicides in vey’s National Water Quality Assessment Program’s
the environment. Physical and chemical characteristics (NAWQA) Midwest Regional Synoptic Study (Soren-
of soils influence the breakdown of pesticides and their son et al., 1999).
movement to ground water and nearby streams. Al- In Minnesota, the study area selected was the Minne-
though deethylatrazine accounts for only a small part sota River basin of the Upper Mississippi River basin
of atrazine degradates, it is a significant compound in NAWQA study unit (Stark, 1994). The study area in
surface water because of its selective removal from soil Iowa incorporated the entire Eastern Iowa basins
(Thurman et al., 1996). Hydroxy-atrazine is bound more NAWQA study unit (Kalkhoff, 1994). These basins in-
tightly to the soil (Lerch et al., 1998) and may be trans- cluded the Wapsipinicon, Cedar, Iowa, and Skunk Riv-
ported to streams by soil erosion due to rainfall runoff. ers. The study area in Illinois included the Lower Illinois
Fine-grained soils favor the transport of metolachlor River basin NAWQA study unit (Warner, 1998).
ESA over metolachlor and metolachlor OA (Phillips et
MATERIALS AND METHODS
Soil permeability may influence the delivery of water
and pesticides to streams and affects runoff and base- Candidate sites (Fig. 1) were selected on streams at the
flow conditions. Dissolved pesticides may reach streams mouth of watersheds that ranged from 150 to 2700 km2 and
through runoff or through ground water discharge into are located in each of three NAWQA study units (Upper
the stream depending on soil texture and slope. Pesti- Mississippi River basin, Eastern Iowa basins, and Lower Illi-
nois River basin). In Minnesota, 24 sites were selected in
cides may reach the stream through adsorption to sedi-
watersheds located in the Minnesota River basin. Twenty-five
ment and subsequent transport to the stream. Areas subwatersheds were located in the Wapsipinicon, Cedar, Iowa,
with well-drained soils may have a greater potential and Skunk Rivers in the Eastern Iowa basins. Twenty-one
for herbicide infiltration of more soluble compounds sites were selected in the Lower Illinois River basin. Water-
(Larson et al., 1997; Burkart et al., 1999) into ground sheds with more than 75% agricultural land use were targeted
water and subsequent transport to streams through for selection (Sorenson et al., 1999).
ground water discharge. In contrast, areas with poorly Soils in the study area generally are moderately to poorly
drained soils have reduced infiltration to ground water permeable and were formed from glacially derived material.
and subsequently provide more runoff to streams. Predominant soil parent materials, based on the USDA (1981)
Poorly drained soils may be more vulnerable to erosion, Major Land Resource Areas (MLRA), are shown in Fig. 1.
however, providing more particulate forms of contami- The most recent glaciation of Wisconsinan age deposited
calcareous clay-rich tills from central and southern Minnesota
nants and sediments to enter streams. Areas with poorly
into central Iowa. These deposits are commonly referred to
drained soils in the Midwest that result from high water as the Des Moines Lobe. Another glacial lobe moved into
tables due to low relief are typically tile-drained to in- northeastern Illinois. Soil properties can be extremely variable
crease drainage. Because tile drains facilitate drainage in a watershed, but these areas contain a large proportion of
from agricultural fields to the stream, they may short- poorly drained soils that were generally developed on till
circuit the mitigating effects of subsurface flow through deposits. The Minnesota River basin and the northern part
riparian soils and buffer strips and serve as point sources of the Eastern Iowa basins are in three MLRAs: Rolling Till
of contaminants (Moorman et al., 1999). Prairie (MLRA 102A), Central Iowa and Minnesota Till Prai-
Although soils may be an important factor in the ries (MLRA 103), and the Eastern Iowa and Minnesota Till
transformation and transport of triazine and chloroacet- Prairie (MLRA 104). The eastern part of the Lower Illinois
anilide herbicides, few studies have documented the River basin is in the Northern Illinois and Indiana Heavy Till
relation between soil characteristics in the watershed Plain (MLRA 110).
Older Pre-Illinoian glacial deposits have been eroded and
and the presence of pesticide degradates in nearby
leached for a longer period and are generally covered by a
streams. Transport of herbicides to streams is of concern thicker layer of wind-blown loess in southern Iowa and west-
because these compounds may potentially affect aquatic ern Illinois (Fehrenbacher et al., 1968; Anderson, 1983). Mod-
communities (Fairchild et al., 1998, Hartgers et al., 1998) erately permeable soils were developed on these loess depos-
and drinking water supplies. Therefore, there is a need its. The southern part of the study area in Iowa and a large
to better understand the persistence and transformation part of the Illinois study area is in the Illinois and Iowa Deep
of heavily used herbicides the Midwest. There is also a Loess and Drift (MLRA 108). Areas adjacent to the Lower
KALKHOFF ET AL.: HERBICIDES IN MIDWESTERN AGRICULTURAL STREAMS 1027
Fig. 1. Sampling sites and predominant soil parent materials in the study area.
Illinois River and its major tributaries are in the Central Missis- Stream discharge at the time of sample collection in August
sippi Valley Wooded Slopes (MLRA 115). reflected the precipitation that fell during late spring and sum-
Differences between till and loess soil permeability may mer. Discharge in southern Minnesota streams ranged from
be mitigated to some extent by the presence of tile drains. near to substantially greater than the long-term average (Mit-
Generally, where tile drains are present, water that has leached ton et al., 1998) during the sampling period. In contrast,
through the soil moves faster to nearby streams than in areas streamflow in Iowa and Illinois ranged from less than 10 to
without subsurface drainage. more than 50% of the long-term August monthly mean during
Rainfall was greatest in the northern part of the study area the sampling period (May et al., 1998; Wicker et al., 1998).
(Minnesota River basin) and decreased southeast through the Abundant rainfall and fertile soils are conducive to row-
Eastern Iowa basins and Lower Illinois River basin during crop agriculture in the study area. Corn and soybean are the
the three months before sampling (Sorenson et al., 1999). main crops (median of 75% of the watersheds is planted in
Rainfall generally increased in the Minnesota River basin and, corn and soybean; Sorenson et al., 1999) with corn comprising
in contrast, decreased in the Lower Illinois River basin from slightly greater than 50% and soybean slightly less than 50%
May through July (Mitton et al., 1998). July rainfall was signifi- of the cropped area. Although the amount of each watershed
cantly less in the Eastern Iowa basins and the Lower Illinois planted in corn was not significantly different, the subbasins
River basin than in the Minnesota River basin, with total sampled in the Minnesota River basin had a small but statisti-
amounts ranging from less than 130 mm to more than 400 mm cally significantly greater amount of soybean acres than in the
during the period of May through July (Sorenson et al., 1999). sampled Eastern Iowa and Lower Illinois River subbasins.
This rainfall pattern resulted in watersheds with predomi- Herbicides used to control competing vegetation in corn
nantly till parent material having significantly greater rain and soybean vary by state and may vary by soil parent material.
than watersheds with predominantly loess parent material Atrazine and cyanazine were used on substantially less crop
(Table 1). area and at a substantially lower rate in Minnesota than in
Table 1. Hydrologic conditions at sites on streams draining selected agricultural watersheds in the U.S. Midwest, August 1997.
Rainfall during the 90 d before sampling†
parent soil material Sites Drainage area† Streamflow† May June July
2 3 1
km m s mm
Till 43 448 0.93 100 130 120
Loess 27 587 0.28 89 84 50
† Median values.
1028 J. ENVIRON. QUAL., VOL. 32, MAY–JUNE 2003
Table 2. Use of selected triazine and chloroacetanilide herbicides on corn and soybean in Iowa, Illinois, and Minnesota, 1997.†
Iowa Illinois Minnesota
Percent crop Percent crop Percent crop
Herbicide Crop Application rate area applied Application rate area applied Application rate area applied
1 1 1
kg a.i. ha % kg a.i. ha % kg a.i. ha %
Acetochlor corn 2.13 28 2.32 29 1.28 29
Alachlor corn 2.57 2 2.57 1 2.23 1
Alachlor soybean – – – – 3.06 4
Atrazine corn 1.10 72 1.31 79 0.68 35
Cyanazine corn 2.71 16 2.52 15 1.28 6
Metribuzin soybean 0.27 1 0.21 13 0.39 2
Metolachlor corn 2.48 43 2.40 31 3.07 22
Metolachlor soybean 3.13 2 2.81 4 2.29 5
Simazine corn – – 1.56 2 – –
† USDA (1998).
Iowa and Illinois in 1997 (Table 2). Metolachlor is used on Laboratory Analysis
more crop area in Iowa than in Illinois and Minnesota. Data
shown in Table 2 are state-wide averages and probably are The U.S. Geological Survey’s Organic Geochemistry Re-
the best available data, but do not necessarily represent the search Laboratory (OGRL) in Lawrence, Kansas, analyzed
usage rates in each watershed. Several reports suggest that water samples. The parent compounds (ametryn, atrazine,
alachlor, acetochlor, cyanzine, metolachlor, metribuzin, pro-
herbicides are not uniformly applied within each state. Fallon
meton, prometryn, propachlor, propazine, simazine, and ter-
et al. (1997) illustrated that atrazine use (on a county level)
butryn) and the triazine degradation compounds (cyanazine-
increased from northwest to southeast in the Minnesota River
amide, deethylatrazine, and deisoproplyatrazine) (Table 2)
basin. Stoltenberg and Pope (1990) reported that atrazine was
were analyzed with gas chromatography–mass spectrometry
applied to only 25% of the corn in north-central Iowa in
following extraction on C18 cartridges (Meyer et al., 1993;
contrast to 69% of the corn in southeastern Iowa during the
Thurman et al., 1990; Zimmerman and Thurman, 1999). The
1980s. Application rates also were less in north-central Iowa in
analytical reporting limit for this method was 0.05 g L 1 for
areas with lower-permeability till soils that contained greater
all compounds. Six chloroacetanilide herbicide degradation
organic carbon content because of farmers’ concern that “car-
products (acetochlor ESA, acetochlor OA, alachlor ESA,
ryover” of atrazine residues in soils may damage soybean.
alachlor OA, metolachlor ESA, and metolachlor OA) and the
Although data are not available, the concern for atrazine car- atrazine degradate (hydroxy-atrazine) (Table 3) were ana-
ryover also may result in reduced application rates by farmers lyzed by high-performance liquid chromatography (HPLC)
in areas of Minnesota and Illinois with till soils. Atrazine use following solid-phase extraction on C18 cartridges (Ferrer et
data reported in Minnesota (Table 2) appear to reflect this al., 1998; Zimmerman et al., 2000). Approximately 5% of the
pattern. Also, in response to the detection of atrazine in samples were verified with HPLC–MS using the method of
ground water, atrazine management areas were established Ferrer et al. (1998). This combination of methods was able to
in northeastern Iowa in areas where bedrock aquifers are close clearly distinguish between alachlor ESA and acetochlor ESA
to landsurface (Iowa Department of Agriculture and Land in the samples. The analytical reporting limit for this method
Stewardship, 1999). No more than 1.7 kg of atrazine per hect- was 0.2 g L 1.
are may be applied per year in these management areas. Pesti- Although the analytical reporting limits for the parent com-
cide use data suggest that farmers alternatively applied pounds and three atrazine degradation products were four
alachlor, metolachlor, and most recently acetochlor to control times lower (0.05 versus 0.20 g L 1) than the acetanalide
competing vegetation in pesticide-management areas. degradation products under investigation, the uncensored data
were used when comparing the frequencies of detection among
Sample Collection compounds. Loss of a substantial amount of data would occur
if a common detection threshold of 0.20 g L 1 were used to
Sampling for analysis of pesticides and pesticide degradates compute the frequency of detection for a compound. The
took place concurrently in the three National Water-Quality result may be an underestimation of the frequency of detection
Assessment Program study units during August 1997. Sam- of the acetanalide degradation products in relation to the parent
pling methods are described in detail by Sorenson et al. (1999) compounds. Although not known, lower analytical detection
and are briefly summarized here. Water was collected with a limits probably would have increased the frequency of detection
depth-integrated sampler at 5 to 10 verticals equally spaced for the degradate compounds. Also, because frequency of detec-
across the stream or river (Edwards and Glysson, 1988; Ward tions of the degradation products generally were greater than
and Harr, 1990; Shelton, 1994) to integrate vertical and hori- the parent compounds (see Results), the patterns of detection
zontal water quality variability. Samples were then filtered would not have been changed with lower frequency of detec-
through baked glass-fiber filters with a nominal 0.7- m pore tions but probably would have been strengthened.
diameter to remove suspended particulate matter (Shelton,
1994). Following field processing, all samples were immedi-
ately chilled (placed in a cooler with wet ice) for shipment to Data Analysis
the analytical laboratories. Sampling and filtering equipment Data from all 70 sites were used in summarizing the occur-
were decontaminated between samples by washing in a deter- rence of the herbicide compounds. Because of the nonnormal
gent solution, rinsing with deionized water, and finally rinsing distribution of the data, comparison between groups of data
with methanol. After air-drying, the equipment was stored in was made with the nonparametric Kruskal–Wallis test and
plastic bags or wrapped in aluminum foil in preparation for correlations between constituents were made with the Spear-
transport to the next sampling site (Shelton, 1994). man’s Rho test (Helsel and Hirsch, 1992).
KALKHOFF ET AL.: HERBICIDES IN MIDWESTERN AGRICULTURAL STREAMS 1029
Table 3. Herbicides and herbicide degradation products analyzed.
Common name Chemical name Use or origin
Acetochlor 2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide herbicide
Acetochlor ethanesulfonic 2-[(2-ethyl-6-methylphenyl)(ethoxymethyl)amino]-2-oxoethane sulfonic acid herbicide degradate (acetochlor)
acid (acetochlor ESA)
Acetochlor oxanilic acid 2-[(2-ethyl-6-methylphenyl)(ethoxymethyl)amino]-2-oxoacetic acid herbicide degradate (acetochlor)
Alachlor 2-chloro-2 -6 -diethyl-N-(methoxymethyl)-acetanilide herbicide
Alachlor ethanesulfonic acid 2-[(2,6-diethylphenyl)(methoxymethyl)amino]-2-oxoethane sulfonic acid herbicide degradate (alachlor)
Alachlor oxanilic acid 2-[(2,6-diethylphenyl)(methoxymethyl)amino]-2-oxoacetic acid herbicide degradate (alachlor)
Ametryn 2-(ethylamino)-4-isopropylamino-6-methyl-thio-s-triazine herbicide
Atrazine 2-chloro-4-ethylamino-6-isopropylamino-s-triazine herbicide
Deethylatrazine (DEA) 2-amino-4-chloro-6-(isopropylamino)-s-triazine herbicide degradate (atrazine,
Deisopropylatrazine (DIA) 2-amino-4-chloro-6-(ethylamino)-s-triazine herbicide degradate (atrazine,
Hydroxy-atrazine (HA) 2-hydroxy-4-(ethylamino)-6-(isopropylamino)-s-triazine herbicide degradate (atrazine)
Cyanazine 2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methyl propionitrile herbicide
Cyanazine-amide 2-chloro-4-(1-carbamoyl-1-methyl-ethylamino)-6-ethylamino-s-triazine herbicide degradate (cyanazine)
Metolachlor 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methyl herbicide
Metolachlor ethanesulfonic 2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2- oxoethanesul- herbicide degradate
acid (metolachlor ESA) fonic acid (metolachlor)
Metolachlor oxanilic acid 2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2- oxoacetic acid herbicide degradate
(metolachlor OA) (metolachlor)
Metribuzin 4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H )-one herbicide
Prometon 2,4-bis(isopropylamino)-6-methyoxy-s-triazine herbicide
Prometryn 2,4-bis(isopropylamino)-6-(methylthio)-s-triazine herbicide
Propachlor 2-chloro-N-isopropylacetanilide herbicide
Propazine 2-chloro-4,6-bis(isopropylamino)-s-triazine herbicide
Simazine 2-chloro-4,6-bis(ethylamino)-s-triazine herbicide
Terbutryn 2-tert-butylamino-4-ethylamino-6-methylthio-s-triazine herbicide
RESULTS roads, rights-of-way, and industrial sites that may be
susceptible to erosion. Although its total use is probably
Because of their chemical and physical properties,
substantially less than other herbicides in the Midwest,
commonly used triazine and chloroacetanilide herbi-
prometon is much more persistent (aerobic soil half-life
cides were expected to degrade by biological and chemi-
is 932 d) and is commonly applied at substantially greater
cal processes through the growing season. Degradation
rates (Capel et al., 1999). Simazine is used for agricul-
should be evident by the presence (or absence) of herbi-
tural purposes, but it also commonly occurs in samples
cide compounds in rivers and streams that drain agricul-
with prometon (Gilliom et al., 1999), suggesting an ur-
tural areas of the Midwest. Degradation should also
ban or nonagricultural source.
be reflected in the amount of the parent compound in
Seven of nine compounds detected in greater than
relation to degradation compounds. The concentration
50% of the samples were herbicide degradation prod-
of degradate compounds in streams may be influenced ucts (Table 4). Metolachlor degradation products were
by conditions in the watershed, such as soil properties two of the four most commonly detected degradation
and rainfall, that affect breakdown and transport. Dilu- products. Metolachlor ESA was detected in 96% of the
tion by ground water free of pesticides also will influence samples and was detected more often than any other
degradate concentrations. compound. Metolachlor OA was detected in 80% of
During the study, both triazine and chloracetanalide the samples. Surprisingly, alachlor ESA, a degradate of
residues (parent plus degradation products) were de- a herbicide that has low use and that was not detected
tected at the 70 sites in the upper Midwest in August in the streams, was the second-most commonly detected
1997 (Table 4). Among all sites, 6 parent compounds compound. The frequent presence of alachlor ESA in
and 10 degradation products were detected. Ametryn, these streams may be the legacy of the heavy historical
alachlor, metribuzin, prometryn, propachlor, propazine, use of alachlor (Meyerfeld et al., 1996). The four triazine
and terbutryn were not detected at the 0.05 g L 1 level degradation products were detected in 41 to 81% of the
and, with the exception of alachlor, are not discussed samples (Table 4).
in the remainder of this report. The frequency of detec- Concentrations of parent compounds were generally
tion of the parent compounds was similar to use: low, ranging from less than 0.05 to 1.5 g L 1, and did not
atrazine metolachlor cyanazine. Atrazine, meto- exceed established drinking water standards (USEPA,
lachlor, and cyanazine were the most frequently de- 2000). Several pesticides analyzed do not have estab-
tected herbicides and were present in 94, 59, and 37% lished standards. With two exceptions, atrazine and pro-
of the samples, respectively. Although not listed among meton, the parent compounds were present at concen-
the most heavily used herbicides (USDA, 1998), pro- trations less than 1.0 g L 1. Median concentrations
meton and simazine were detected in 16 and 7% of the were less than the analytical reporting limit for all parent
samples, respectively. Prometon is generally used to compounds except atrazine (0.17 g L 1) and meto-
control vegetation in noncrop areas that include paved lochlor (0.06 g L 1).
1030 J. ENVIRON. QUAL., VOL. 32, MAY–JUNE 2003
Table 4. Statistical summary of herbicide compounds in 70 agricultural streams of the U.S. Midwest, August 1997.
Compound of detection Minimum 25th percentile Median 75th percentile Maximum
Acetochlor 6 0.05† 0.05 0.05 0.05 0.21
Alachlor 0 0.05 0.05 0.05 0.05 0.05
Atrazine 94 0.05 0.10 0.17 0.28 1.5
Cyanazine 37 0.05 0.05 0.05 0.09 0.64
Metolachlor 59 0.05 0.05 0.06 0.11 0.42
Prometon 16 0.05 0.05 0.05 0.05 1.4
Simazine 7 0.05 0.05 0.05 0.05 0.20
Acetochlor ethanesulfonic acid (ESA) 67 0.20 0.20 0.33 0.50 1.6
Acetochlor oxanilic acid (OA) 40 0.02 0.20 0.20 0.34 1.4
Alachlor ESA 84 0.20 0.26 0.58 1.2 3.5
Alachlor OA 23 0.20 0.20 0.20 0.2 0.54
Cyanazine-amide 41 0.05 0.05 0.05 0.10 1.2
Deethylatrazine 81 0.05 0.05 0.09 0.12 0.39
Deisopropylatrazine 74 0.05 0.05 0.07 0.12 0.36
Hydroxy-atrazine 60 0.20 0.20 0.28 0.69 8.8
Metolachlor ESA 96 0.20 1.0 1.7 3.0 6.7
Metolachlor OA 80 0.20 0.24 0.34 0.66 1.3
† The symbol indicates values less than analytical reporting limit indicated.
Degradate concentrations ranged from less than 0.2 summed concentrations of the degradation products in
to 8.8 g L 1. The ethanesulfonic acid degradation prod- each sample (4.4 g L 1) was almost 17 times greater
ucts were generally present in greater concentration than the median value for the summed concentrations
than the triazine degradation products. In addition to of the parent pesticides (0.26 g L 1). During the study,
being detected most frequently, metolachlor ESA was multiple herbicide compounds were commonly detected
present in the greatest concentration (median of 1.7 g in each stream. Multiple parent herbicide compounds
L 1). With the exception of alachlor ESA (0.58 g L 1), were detected in 70% of the samples and multiple degra-
median concentrations of the remaining compounds date compounds were detected in all samples. The me-
were less than 0.35 g L 1. dian number of parent pesticide compounds detected in
Metolachlor and its degradation products were the the samples was two and the median number of pesticide
prevalent pesticide compounds (by mass) in streams in degradation products detected in the samples was seven.
the Midwest in August 1997. On average, greater than The composition of the chloroacetanilide residue was
50% of the total pesticide residue (by mass) consisted substantially different than the composition of the tri-
of metolachlor compounds (parent and degradation azine residues in August 1997. Chloroacetanilide com-
products). Alachlor and atrazine compounds each con- pounds in the streams consisted almost exclusively of
tributed about 14%, acetochlor about 9%, and cyana- degradation products. The total acetochlor residue (par-
zine compounds less than 1% of the total pesticide mass. ent plus degradation products) contained less than 1%
Although chemical properties such as solubility and parent compound. Alachlor residue consisted entirely
degradation rates are factors, the greater metolachlor of the ESA and OA degradation products. Metolachlor
compound mass can also be partially attributed to the residue consisted of about 2% parent and about 98%
high rate of metolachlor usage (Table 2). In addition to degradation products. In contrast, the triazine com-
being frequently detected, alachlor degradation prod- pounds are present as a mixture (25–50%) of parent
ucts were present in surprisingly high concentrations and degradation products. Both atrazine and cyanazine
based on the amount of parent compound applied in degrade biologically to DIA. However, it is assumed
1997. The disproportionately high concentration in rela- for this study that the DIA originates from atrazine due
tion to use suggests that alachlor degradation products to the difficulty in partitioning the source and to the
are persistent and may originate from applications in fact that atrazine use was substantially greater than cya-
previous years. Kolpin et al. (1998) have shown that nazine. Because of this assumption, atrazine degra-
alachlor ESA is one of the most commonly detected dation products may be slightly overestimated and cy-
pesticide degradation products in shallow ground water. anazine degradation products slightly underestimated.
Cyanazine residue consisted of 50% parent and 50%
DISCUSSION cyanazine-amide. Reddy et al. (1997) found that cyana-
zine-amide was more likely to remain in the aqueous
Relation between Parent Compounds and phase and thus have greater transport potential by water
Degradation Products than cyanazine. Atrazine residue consisted of about
In August 1997, after pesticide application, degrada- 25% parent, 25% DIA plus DEA, and about 50% HA.
tion products were the most frequently detected herbi- Although HA was commonly present in streams
cide compounds and were present in significantly throughout the Midwest, it was more prevalent in the
greater concentrations than the parent compounds in southern part of the study area. Generally, concentra-
streams of the Midwest. The median value for the tions of HA would be expected to be higher in streams
KALKHOFF ET AL.: HERBICIDES IN MIDWESTERN AGRICULTURAL STREAMS 1031
Fig. 2. Relation between hydroxy-atrazine to atrazine ratio (HAR)
and deethylatrazine plus deisoproplyatrazine to atrazine ratio
(DDAR) and the latitude.
draining basins with higher atrazine application rates,
but increased concentrations also may result from dif-
fering degradation pathways. The ratios of degradation
compound to parent compound were calculated to com-
pensate for the differing application rates (Fig. 2). If
atrazine degraded to HA, deethylatrazine, and deiso-
proprylatrazine uniformly, the degradate to the parent
compound ratios would be uniform across the study
area. These ratios were not uniform. The hydroxy-atra-
zine to atrazine ratio (HAR) increased and the deeth-
ylatrazine plus deisoproprylatrazine to atrazine ratio
(DDAR) decreased from north to south (Fig. 2). An
increasing HAR with a concurrent decreasing DDAR
from southern Minnesota to central Illinois suggests atra-
zine degradation is not consistent across the Midwest.
Relation between Herbicide Compounds
and Soil Parent Material
Pesticide compounds found in streams of the Midwest
during late summer 3 to 4 mo after application were
partially dependent on the predominant soil parent ma- Fig. 3. Frequency of detection and median concentration of selected
terial in the watershed. The two dominant classes of herbicides and herbicide metabolites in streams draining till and
windblown loess-derived soils of the upper U.S. Midwest, Au-
herbicides (triazines and acetanalides) considered in this gust 1997.
study were detected at different rates and at different
concentrations, dependent on whether till- or loess-type primary degradation products were present more fre-
soils predominated in the watershed. Chemical proper- quently in streams draining watersheds with predomi-
ties of these herbicides may influence their use and nantly loess-type soils than in watersheds with predomi-
subsequent detection in basins with different soil types. nantly till soils (Fig. 3). Although the frequency of
Atrazine and cyanazine, the two most heavily used detection for atrazine was not significantly different,
triazine herbicides in the Midwest, and several of their atrazine degradation products were detected more fre-
1032 J. ENVIRON. QUAL., VOL. 32, MAY–JUNE 2003
Herbicidal properties, such as degradation rate, may
determine application rates, which in turn partially ac-
count for the presence of herbicides in streams during
late-summer base-flow conditions. Triazine pesticide
use rates are apparently less in areas with predominantly
till soils (Stoltenberg and Pope, 1990) than in areas of
predominantly loess soils. This is due to the relatively
high pH and organic carbon content of till soils that
hinder the degradation of triazine herbicides, resulting
in the “carryover” of atrazine in the soil to the following
growing season that may harm soybeans in a corn–
soybean crop rotation. Because of lower use on till soils,
triazine degradation products would be expected to be
lower. Cyanazine sorption has been correlated with fine
soil texture and greater organic carbon content (Reddy
et al., 1997). Alachlor, metolachlor, and acetochlor pos-
sibly were used to offset triazine pesticide reductions
in areas with till soils. Acetanalide herbicides also are
adsorbed to organic matter (Miller et al., 1997), but
because of their generally shorter half-life (Barbash et
al., 1999), they are not present in substantial quantities
in soils the following growing season.
Relation between Herbicide Compounds
Triazine and chloroacetanilide herbicide transport to
streams of the Midwest in late summer was influenced
by the timing and amount of rainfall during the growing
season. Concentrations of metolachlor compounds in
Fig. 4. Concentrations of atrazine and metolachlor compounds in re- streams (consisting primarily of degradation products)
lation to rainfall during selected periods in the watershed during were greatest in streams that received greater amounts
the growing season.
of rainfall early in the growing season (Fig. 4). In con-
trast, concentrations of atrazine compounds (consisting
quently in streams draining loess soils than in streams of a mixture of parent and degradation products) were
draining till soils. least in streams draining watersheds that received the
Field and laboratory studies suggest that soil composi- most rainfall during the late summer (Fig. 4). The sum
tion and temperature may influence which compound of the concentrations of atrazine compounds commonly
is produced through atrazine degradation. Early labora- exceeded 2 g L 1 in streams draining watersheds that
tory research (Hance and Segal, 1980) found that greater received less than 100 mm of rain in June and July, and
concentrations of HA were formed in acidic rather than rarely exceeded 1 g L 1 in streams draining watersheds
neutral soils. Although HA leached from soils in small that received more than 200 mm of rain in June and July.
amounts in relation to atrazine, Mersie and Seybold Although atrazine and metolachlor adsorb to soil ma-
(1996) found that almost twice as much HA desorbed terials in similar amounts (Mersie and Seybold, 1996),
from a low clay content, lower pH, and lower organic metolachlor has been found to be less susceptible to
carbon content soil similar to loess soils. Hydroxy-atra- runoff transport than atrazine (Jaynes et al., 1999). Ini-
zine adsorbs readily to soil and may be transported to tially, chloroacetanilide pesticides adsorb to soil and
the streambed during summer runoff. Desorption by may not be readily available to transport to streams
ground water seeping into streams through contami- and leach to ground water. Chloroacetanilide pesticides
nated streambed sediment has been postulated as a probably then degrade in the soils within weeks of appli-
source of HA in streams during base-flow conditions cation. As the pesticides degrade to the more soluble
(Lerch et al., 1998). ESA and OA degradation products, summer rains may
In contrast, two ESA degradation products, alachlor transport degradation products to streams and ground
ESA and acetochlor ESA, were detected more fre- water. Kolpin et al. (1998) have documented the trans-
quently in samples from streams draining till soils than port of metolachlor and alachlor degradation products
in streams draining loess soils (Fig. 3). Several chloroac- to shallow ground water. As a result, degradation prod-
etanilide degradation products (alachlor ESA, meto- ucts will be present in ground water and potentially
lachlor ESA, metolachlor OA, and acetochlor ESA) available for transport to streams during late summer.
were present in significantly (P 0.05) greater concen- Since the triazine pesticides are generally more solu-
trations in samples from streams draining watersheds ble than the chloroacetanilide pesticides, excess rainfall
with till soils than watersheds with loess soils. may have flushed much of the atrazine from the soil
KALKHOFF ET AL.: HERBICIDES IN MIDWESTERN AGRICULTURAL STREAMS 1033
early in the growing season. Atrazine concentrations that drain corn- and soybean-producing areas. By late
have been found to be greatest in small (Kalkhoff and summer of 1997, several months after application, the
Schaap, 1996) and large (Clark et al., 1999) streams and majority of the herbicide residues in streams of the
rivers of the Midwest during May and June when rainfall Midwest are degradation compounds. Degradation
runoff is greatest. However, atrazine is persistent (half- products of the commonly used triazine (atrazine and
life of more than 140 d; Barbash et al., 1999) and any cyanazine) and chloroacetanilide (acetochlor, alachlor,
remaining after the spring flush, for the most part, has metolachlor) herbicides comprise the majority of pesti-
not degraded and is available to be transported from cide compounds found in the streams during base-flow
the soil. Atrazine may be flushed both into nearby conditions in late summer. Although degradation prod-
streams and transported to ground water because of its ucts constitute a major part of the residue, the two
relatively high solubility. classes of herbicides have substantially different compo-
Although water originating from rainfall may require sitions. The chloroacetanilide residue was comprised
several decades to move through local sand and gravel almost exclusively of degradate compounds, but the tri-
aquifers to streams (Puckett and Cowdery, 2002), part azine residue consisted of a mixture of parent and degra-
of the ground water contributing to flow in the streams date compounds.
studied during this investigation was in aquifers for only As would be expected, differences in use patterns
a short period of time. The age of ground water entering contributed to differences in the types of pesticides pres-
these 70 streams is difficult to accurately quantify with- ent in late summer in streams of the Midwest. Atrazine
out detailed hydrologic studies, but may be a mix of and to some extent cyanazine compounds are more
old water contributed from deeper regional flow and prevalent in watersheds with a majority of moderately
younger water from shallower localized flow systems. permeable loess soils than in watersheds with mostly
Two lines of evidence suggest that at least part of the poorly permeable till soils. The predominant atrazine
water in the streams of the Midwest in August is rela- degradates change from the biologically derived DIA
tively young ground water. The presence of acetochlor and DEA in the north to the chemically and biologically
or acetochlor degradates in 75% of the streams indicates derived hydroxy-atrazine in the southern part of the
that at least part of the water was three years old or Midwest study area.
younger. Acetochlor was registered for use beginning The timing of rainfall during the growing season is
in the 1994 growing season, three years before this inves- an important factor in transport of pesticide residue to
tigation began. The correlation of metolachlor and met- streams during late summer. Increased rainfall early
olachlor degradates with late spring and early summer in the growing season (June) contributed to increased
rains (Fig. 4) also suggests that ground water originating concentrations of metolachlor degradation products in
as rainfall during the current growing season was part streams during August, whereas increased rainfall dur-
of the streamflow in the streams of the Midwest. This ing mid-summer (July) flushed most atrazine com-
correlation would not be expected if water transporting pounds from the watersheds, resulting in lower concen-
pesticides applied during the 1997 growing season were trations in August.
not present in the streams. Results of this study indicate that even though con-
The young ground water that comprises part of the centrations of commonly used triazine and chloroacet-
streamflow may originate from water infiltrating soil anilide herbicides are low in streams of the Midwest
relatively close to the stream (Puckett and Cowdery, during late summer, some of their degradation products
2002) that then moves through the shallow water table are present in substantial concentrations. Initial studies
to the streams. Crops are commonly grown on the flat (Heydens et al., 1996) suggested that ESA degradation
flood plain and pesticides are applied in close proximity products are less toxic than the parent compounds.
to many of the streams of the Midwest. In addition, However, studies documenting toxicological effects of
young ground water may also originate from consider- the OA degradation products and effects of chronic
able distance from the stream, being transported to the exposure on aquatic organisms are sparse.
stream in tile lines. Phillips et al. (1999) have shown Scarcity of detailed herbicide use information (county
that pesticides and pesticide degradates are rapidly or watershed level) complicates the interpretation of
transported from the soil to the water table after rain water quality results from similar geologic areas in the
events. Since tile lines are generally located in or near Midwest. Most of the degradation products studied were
the top of the water table, recently recharged water the initial breakdown products and the presence and
containing any dissolved pesticide compounds will be concentration of degradation products further down the
routed to a nearby stream. breakdown pathway in streams is unknown. Additional
studies to document and quantify these secondary and
tertiary degradation products are needed to fully under-
CONCLUSIONS stand the fate and movement of pesticides through the
Though the Corn Belt region of the U.S. Midwest is hydrologic system.
commonly thought of as a relatively homogeneous area
(Omernick, 2000), relatively small differences (in rela- ACKNOWLEDGMENTS
tion to the rest of the United States) in soils and climate The authors would like to thank the U.S. Geological Sur-
have a substantial influence on the amount and types vey’s National Water-Quality Assessment Program National
of herbicide compounds that are transported to streams Leadership Team for providing additional funds for the Mid-
1034 J. ENVIRON. QUAL., VOL. 32, MAY–JUNE 2003
west Regional Synoptic Study, and Steve Sorensen and study ter by HPLC/MS using negative ion spray. Anal. Chem. 69:4547–
unit biologists Mitch Harris and Linda Roberts, who were 4553.
able to persevere in the face of many setbacks and obstacles Field, J.A., and E.M. Thurman. 1996. Glutathione conjugation and
to make the synoptic study a reality. Appreciation is expressed contaminant transformation. Environ. Sci. Technol. 30:1413–1417.
Gilliom, R.J., J.E. Barbash, D.W. Kolpin, and S.J. Larson. 1999. Test-
to the numerous USGS field personnel in Illinois, Iowa, and
ing water quality for pesticide pollution. Environ. Sci. Technol.
Minnesota who spent many hours in the streams collecting 33:164A–169A.
samples and gathering information, without which this report Graham, W.H., D.W. Graham, F. Denoyelles, V.H. Smith, C.K. Lar-
would have not been possible. T. Steinheimer of the USDA- ive, and E.M. Thurman. 1999. Metolachlor and alachlor breakdown
ARS Soil Tilth Laboratory and S. Larson and D. Dupre of the product formation patterns in aquatic field mesocosms. Environ.
U.S. Geological Survey reviewed earlier drafts of the paper. Sci. Technol. 33:4471–4476.
Suggestions by three anonymous reviewers greatly improved Hance, R.J., and G.M. Segal. 1980. A comparison of the decomposition
the report. of atrazine and diuron in soils of different fertilizer content and
pH. Agrochimica 24:266–273.
Hartgers, E.M., G.H. Aalderink, P.J. Van den Brink, R. Gylstra,
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