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USGS Fact Sheet 076-98
July 1998
Herbicides in Ground Water of the Midwest:
A Regional Study of Shallow Aquifers,
1991-94
The intensive herbicide use associated with the "Corn Belt" marks the Midwestern United
States as a region where herbicide contamination of ground water could be a problem. To
better understand the regional occurrence of herbicides in shallow aquifers of the Midwest, a
sampling network of 303 wells across 12 States was developed. The results documented
relatively widespread, low-level concentrations of herbicides in the shallow aquifers
sampled. The most frequently detected compounds, however, were the transformation products of
these herbicides. A relation was determined between herbicide occurrence and the general age
of the ground water sampled. Water that recharged ground water within the past 40 years was
much more likely to contain herbicides than water recharged earlier.
Table of Contents
Parts of 12 Midwestern States (Illinois, Indiana, Iowa, Kansas,Michigan, Minnesota, Missouri,
Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin) comprise a region commonly
referred to as the "Corn Belt." The Corn Belt is the largest and most intensive
crop-producing region of the United States, accounting for about 65 percent of the total
harvested cropland and about 60 percent of the herbicide use in the Nation. This intensive
herbicide use marks the Corn Belt as a region where the potential for herbicide contamination
of ground water could be significant. Although herbicides have many benefits, they may also
produce a wide range of toxic side effects that could pose a potential hazard to human health
and the environment.
Previous State and national surveys conducted in the Midwest have produced a wide range in
results regarding the detection of herbicides. For example, the reported frequency of
detection of atrazine ranged from less than 1 to 47 percent for the 14 State or national
studies that analyzed for atrazine (Burkart and Kolpin, 1993). Differences between these
studies (such as analytical reporting limits, target population, well-selection criteria,
time of sample collection, and objective of study) make interpretations of data collected in
prior studies difficult. These differences make it hard to distinguish between the natural
(due to real differences) and the artificial (due to differences between studies)
distribution of herbicides in Midwestern ground water.
To better understand the regional occurrence of herbicides in shallow aquifers of the
Midwest, the U.S. Geological Survey designed a monitoring network of 303 wells completed in
unconsolidated and bedrock aquifers (figs. 1 and 2) located
throughout the Corn Belt (Kolpin and Burkart, 1991). Unconsolidated aquifers in the Midwest
are commonly consist of sand and gravel deposited by glacial meltwater or recent streams.
Whereas, bedrock aquifers in the Midwest generally consist of sandstone, limestone, or
dolomite. From 1991 to 1994, more than 800 ground-water samples were collected from these
wells and analyzed for selected herbicides and herbicide degradation products (degradates)
(Kolpin and others, 1993, 1996c). The consistency of this data set allows for a unique
investigation of herbicide occurrence in shallow aquifers across the Midwest.
Detections of herbicides were relatively widespread in shallow aquifers across the Midwest
(fig.
1), with one or more compounds being detected at greater than 0.05 µg/L (microgram
per liter, which is roughly equivalent to "part per billion") in 40.3 percent of the 303
wells sampled. The concentrations encountered, however, were generally low, with the median
total herbicide concentration being approximately 0.5 µg/L. Only one sample had a
herbicide concentration (alachlor = 4.3 µg/L) that exceeded a Maximum Contaminant Level
(MCL) or Health Advisory Level (HAL) for drinking water (U.S. Environ-mental Protection
Agency, 1995). However, these drinking-water criteria may not answer all questions related to
health and environmental risks associated with the presence of herbicides in ground water.
First, only 7 of the 13 compounds detected at greater than 0.05 µ/L have MCLs or HALs
established. Second, these criteria only consider the effects of individual pesticides and do
not account for possible additive or synergistic toxicity from the presence of more than one
compound. The co-occurrence of multiple herbicide compounds in ground-water samples was
common during this study (fig. 3). Two or more compounds
were present in 60 percent of samples where pesticides were detected. Third, these criteria
only consider acute toxic effects and do not consider potential chronic effects such as
reproductive, developmental, and neural-behavioral toxicity.
Although numerous studies have been conducted investigating the occurrence of herbicides in
ground water, few have considered the degradates of these herbicides in their investigations.
Degradates (also referred to as metabolites or transformation products) are formed as
herbicides break down to different compounds in the environment. Degradates were commonly
found in shallow aquifers across the Midwest, being four of the five most frequently detected
compounds for this study (fig. 4). The frequencies shown have been
adjusted to a common detection threshold of 0.05 µg/L to take into account variations
in reporting limits among the compounds examined. The frequency of detection for a given
herbicide increased substantially when its degradates are considered
(fig. 5).
Also, a substantial part of the measured concentration for a given herbicide was in the form
of its degradates (fig. 6). Consequently, both the
overall occurrence (measurement of "how often") and concentration (measurement of "how much")
of the herbicides are underestimated in shallow aquifers if data on herbicide degradates are
not considered. Information on degradates is essential to fully understanding the fate and
occurrence of the parent herbicides and to determine the complete consequences of a
herbicide's use on human health and the environment. Although some degradates appear to be
less toxic than their parent compounds (Heydens and others, 1996; Stamper and Tuovinen,
1998), others have been shown to have similar acute (Kaufman and Kearney, 1970; Reddy and
others, 1997) and chronic (Babic-Gojmerac and others, 1989; Lang and others, 1997) toxicity
as their corresponding parent compounds.
Atrazine was the most frequently detected parent compound in this study
(fig.
4). This is likely the result of a comparatively slow rate of atrazine degradation under
environmental conditions (Agertved and others, 1992; Widmer and Spalding, 1995) and its long
history of extensive use across the Midwest in both agricultural and nonagricultural
settings. Indeed, atrazine has been the most frequently detected parent compound in many
studies (Kross and others 1990; Holden and others, 1992; Kolpin and others, 1998).
Surprisingly, prometon was the second most frequently detected parent compound
(fig.
4). Prometon is used primarily for nonagricultural purposes, such as domestic and
commercial applications to driveways, fence lines, lawns, gardens, and as an asphalt additive
(Healy, 1996; Pasquarell and Boyer, 1996). A direct association to nonagricultural land and
prometon occurrence was found for this study (Burkart and Kolpin, 1993). Thus, agricultural
activities are not the only sources of herbicide contamination of ground water,
nonagricultural activities (such as urban and suburban use) also contribute to such
contamination. The limited information available for prometon suggests that its use is far
less than most of the other herbicides examined. What prometon lacks in use may be
compensated for by its persistence in the environment, having the longest half-life of the
herbicides examined (Wauchope and others, 1992).
A relation was determined between herbicide occurrence and the general age of the ground
water sampled. General age was obtained by measuring the tritium (3H) concentration in ground
water. Tritium is a radioactive isotope of hydrogen (H) that was greatly increased in the
atmosphere with the advent of atmospheric testing of nuclear weapons beginning in 1953. Thus,
the amount of tritium in a sample can be used as a general tracer to determine whether ground
water was recharged before or after 1953. As expected, water that recharged ground water
within the past 40 years is more likely to contain herbicides than water recharged earlier
(fig. 7). Because the first significant use of herbicides to control
weeds in crops also roughly coincides with the start of atmospheric testing of nuclear
weapons, ground water determined to be older than 1953 would predate the use of herbicides.
The general age of the water does not cause herbicide contamination but simply identifies an
aquifer's susceptibility to contamination by indicating the presence of post-1953 recharge
water.
Pre-1953 water was much more likely to occur in near-surface bedrock aquifers (47.1 percent,
16 of 34 randomly selected samples) than in near-surface unconsolidated aquifers (7.8
percent, 5 of 64 randomly selected samples). This provides insight as to the reason for the
frequency of herbicide detection (using a 0.05-µg/L detection threshold), being
substantially less in the bedrock aquifers (21.9 percent, 23 of 105 wells) than in the
unconsolidated aquifers (50.05 percent, 99 of 198 wells) sampled.
The results of this study have been published in a variety of publications. Additional
reading on this study can be obtained from the following reports: Kolpin and others (1994,
1995, 1996a, 1996b), Kolpin and Thurman (1995), and Kolpin (1997).
--D.W. Kolpin, J.K. Stamer, and D.A. Goolsby
- Agertved, J., Rugge, K., and Baker, J.F., 1992, Transformation of the herbicides
MCPP and atrazine under natural aquifer conditions: Ground Water, v. 30, p. 500-506.
- Babic-Gojmerac, T., Kneiwald, Z., and Kneiwald, J., 1989, Testosterone metabolism
in neuroendocrine organs in male rats under atrazine and deethylatrazine influence:
Journal of Steroid Biochemistry, v. 33, p. 141-146.
- Battaglin, W.A., and Goolsby, D.A., 1995, Spatial data in geographic information
system format on agricultural chemical use, land use, and cropping practices in the
United States: U.S. Geological Survey Water-Resources Investigations Report 94-4176,
87 p.
- Burkart, M.R., and Kolpin, D.W., 1993, Hydrologic and land-use factors associated
with herbicides and nitrate in near-surface aquifers: Journal of Environmental Quality,
v. 22, p. 646-656.
- Healy, D.F., 1996, Water-quality assessment of the Rio Grande Valley, Colorado,
New Mexico, and Texas--Occurrence and distribution of selected pesticides and nutrients
at selected surface-water sites in the Mesilla Valley, 1994-95: U.S. Geological Survey
Water-Resources Investigations Report 96-4069, 85 p.
- Heydens, W.F., Siglin, J.C., Holson, J.F., and Stegeman, S.D., 1996, Subchronic,
developmental, and genetic toxicology studies with the ethane sulfonate metabolite of
alachlor: Fundamental and Applied Toxicology, v. 33, p. 173-181.
- Holden, L.R., Graham, J.A., Whitmore, R.W., Alexander, W.J., Pratt, R.W., Liddle,
S.K., and Piper, L.L., 1992, Results of the National Alachlor Well Water Survey:
Environmental Science and Technology, v. 26, no. 5, p. 935-943.
- Kaufman, D.D., and Kearney, P.C., 1970, Microbial degradation of s-triazine
herbicides: Residue Review, v. 32, p. 235-265.
- Kolpin, D.W., 1997, Agricultural chemicals in groundwater of the midwestern United
States--Relations to land use: Journal of Environmental Quality, v. 26, no. 4, p.
1025-1037.
- Kolpin, D.W., Barbash, J.E., and Gilliom, R.J., 1998, Occurrence of pesticides in
shallow groundwater of the United States--Initial results from the National
Water-Quality Assessment Program: Environmental Science and Technology, v. 32, no. 5,
p. 558-566.
- Kolpin, D.W., and Burkart, M.R., 1991, Work plan for regional reconnaissance for
selected herbicides and nitrate in ground water of the Mid-continental United States,
1991: U.S. Geological Survey Open-File Report 91-59, 18 p.
- Kolpin, D.W., Burkart, M.R., and Thurman, E.M., 1993, Hydrogeologic,
water-quality, and land-use data for the reconnaissance of herbicides and nitrate in
near-surface aquifers of the Mid-continental United States: U.S. Geological Survey
Open-File Report 93-114, 61 p.
- _____1994, Herbicides and nitrate in near-surface aquifers of the Midcontinental
United States, 1991: U.S. Geological Survey Water-Supply Paper 2413, 34 p.
- Kolpin, D.W., Goolsby, D.A., and Thurman, E.M., 1995, Pesticides in near-surface
aquifers--An assessment of highly sensitive analytical techniques and tritium: Journal
of Environmental Quality, v. 24, no. 6, p. 1125-1132.
- Kolpin, D.W., Nations, B.K, Goolsby, D.A., and Thurman, E.M., 1996b, Acetochlor in
the hydrologic system in the Midwestern United States: Environmental Science and
Technology, v. 30, no. 5, p. 1459-1464.
- Kolpin, D.W., and Thurman, E.M., 1995, Postflood occurrence of selected
agricultural chemicals and volatile organic compounds in near-surface unconsolidated
aquifers in the upper Mississippi River Basin, 1993: U.S. Geological Survey Circular
1120-G, 20 p.
- Kolpin, D.W., Thurman, E.M., and Goolsby, D.A., 1996a, Occurrence of selected
pesticides and their metabolites in near-surface aquifers of the Midwestern United
States: Environmental Science and Technology, v. 30, no. 1, p. 335-340.
- Kolpin, D.W., Zichelle, K.E., and Thurman, E.M., 1996c, Water-quality data for
nutrients, pesticides, and volatile organic compounds in near-surface aquifers of the
Midcontinental United States, 1992-1994: U.S. Geological Survey Open-File Report
96-435, 47 p.
- Kross, B.C., Hallberg, G.R., and others, 1990, The Iowa State-wide rural
well-water survey--Water-quality data, initial analysis: Iowa Geological Survey
Technical Information Series Report 19, 142 p.
- Lang, D.H., Rettie, A.E., and Bocker, R.H., 1997, Identification of enzymes
involved in the metabolism of atrazine, terbuthylazine, ametryne, and terbutryne in
human liver microsomes: Chemistry and Residue Toxicology, v. 10, p. 1037-1044.
- Pasquarell, G.C., and Boyer, D.G., 1996, Herbicides in karst groundwater in
southeast West Virginia: Journal of Environmental Quality, v. 25, p. 755-765.
- Reddy, K.N., Locke, M.A., and Zablotowicz, R.M., 1997, Soil type and tillage
effects on sorption of cyanazine and degradation products: Weed Science, v. 45, p.
727-732.
- Stamper, D.M., and Tuovinen, O.H., 1998, Biodegradation of the acetanilide
herbicides alachlor, metolachlor, and propachlor: Critical Reviews in Microbiology, v.
24, no. 1, p. 1-22.
- U.S. Environmental Protection Agency, 1995, Drinking water regulations and health
advisories: Washington, D.C., Office of Water.
- Wauchope, R.D., Buttler, T.M., Hornsby, A.G., Augustijn-Beckers, P.W.M., and Burt,
J.P., 1992, The SCS/ARS/CES pesticide properties database for environmental
decision-making: Review of Environmental Contamination and Toxicology, v. 123, p.
1-156.
- Widmer, S.K., and Spalding, R.F., 1995, A natural gradient transport study of
selected herbicides: Journal of Environmental Quality, v. 24, p. 445-453.
For additional information and selected reading about the Midcontinent Herbicide Project,
write to:
U.S. Geological Survey
Mail Stop 415, Box 25046
Building 53, Wing F-200
Denver Federal Center
Lakewood, CO 80225
Additional information on the Midcontinent Herbicide Project and other USGS programs can be
found by accessing "http://wwwrcolka.cr.usgs.gov/midconherb/index.html" on the World Wide Web.
For more information please contact:
District Chief
U.S. Geological Survey
4821 Quail Crest Place
Lawrence, Kansas 66049-3839
(785) 842-9909
email: waucott@usgs.gov
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