USGS Fact Sheet 163-98 April 1999
Download this fact sheet as a PDF file (137 KB)
Download a free copy of
Acrobat Reader
Discharge of Herbicides From the Mississippi River Basin to the Gulf of Mexico,
1991-97
Table of Contents
Figures
- Figure 1. Boundary
of Mississippi River Basin and location of sampling site.
- Figure 2. Stream discharge and total
herbicide concentration in the Mississippi River at Baton Rouge, Louisiana, 1991-97. The
total herbicide concentration represents the sum of the herbicides and metabolites listed
in table 1.
- Figure 3.
Graphs showing percent (A) detection and (B) concentrations of herbicides
and herbicide metabolites in samples from the Mississippi River at Baton Rouge, 1991-97.
The herbicides ametryn, prometryn, propazine, and terbutryn were detected in less than 2
percent of samples.
- Figure 4. Temporal variation in concentration of herbicides in
samples from the Mississippi River at Baton Rouge, 1991-97.
-
Figure 5. (A) Annual and (B) monthly loads of herbicides discharged from
the Mississippi River Basin to the Gulf of Mexico, 1991-97. The 1993 load of alachlor ESA
represents July-December only.
- Figure
6. Annual loads of selected herbicides discharged to the Gulf of Mexico, 1991-97.
- Figure
7. Relation of total annual load and use for selected herbicides discharged to the
Gulf of Mexico, 1991-97. Lines are annual load as a constant percentage of annual use.
Table
- Table 1. Herbicides and herbicide metabolites analyzed in samples from the
Mississippi River at Baton Rouge, Louisiana, 1991-97.
Current (1998) agricultural practices in the United States rely extensively on pesticides for
crop production. Of the more than 500 million kilograms (kg) of pesticides used annually in
the United States to control weeds, insects, nematodes, and other pests, about 20 percent are
herbicides applied to field crops in the Mississippi River Basin
(fig. 1). Because many herbicides
are relatively water soluble and mobile, they can be transported in surface runoff from
agricultural fields to streams. Although herbicides have many benefits, they may also produce
toxic side effects that may pose a potential hazard to human health and the environment. For
instance, the U.S. Environmental Protection Agency classifies the herbicide alachlor as a
human carcinogen, and several other herbicides including atrazine, cyan-azine, and metolachlor
are classified as possible human carcinogens. In addition, the effects of long-term, low-level
concentrations of herbicides or combinations of herbicides on aquatic eco-systems are largely
unknown.
Studies conducted by the U.S. Geological Survey (USGS) have documented the presence of
herbicides in the Mississippi River and its tributaries. These studies have found that
concentrations of herbicides are largest for several weeks to several months following their
application to farmlands. In some small watersheds in the Mississippi River Basin,
concentrations of herbicides in streams have been found to exceed 50 µ/L (micrograms
per liter) for short periods of time following spring storms. Discharge from small streams
transports herbicides into large rivers such as the Mississippi, Missouri, and Ohio. Although
herbicide concentrations in large rivers are generally not as high as in many small streams,
the cumulative mass of herbicides, which eventually discharge to the Gulf of Mexico, can be
large.
During 1991-97 use of five herbicides (acetochlor, alachlor, atrazine, cyanazine, and
metolachlor) on corn and soybeans accounted, on average, for about 70 percent of the annual
herbicide use on field crops in the Mississippi River Basin (U.S Department of Agriculture,
1992-97). However, some changes in the quantity and relative use of these herbicides occurred
over that time period. In 1997, total use of acetochlor, alachlor, atrazine, cyanazine, and
metolachlor was about 65,000 metric tons, a decrease of about 12,500 metric tons from the
amount used in 1991. Although use of atrazine and metolachlor remained relatively stable
during 1991-97, alachlor use decreased by about 19,000 metric tons or 85 percent, and
cyanazine use decreased by about 3,100 metric tons or 30 percent. The introduction and use of
the corn herbicide acetochlor in the United States in 1994 partially offset the decrease in
alachlor and cyanazine use, and by 1996 acetochlor was the third most extensively applied
herbicide on corn in the Midwestern United States.
To better understand the occurrence, temporal variability, and load of herbicides in the
Mississippi River, the USGS collected herbicide data from the Mississippi River at Baton
Rouge, Louisiana (fig. 1), during
1991-97. A total of 271 samples were collected and analyzed for 12 herbicides and 4
metabolites or breakdown products (table 1). These herbicides represent
the majority of herbicide usage in the Mississippi River Basin and are the ones most
frequently detected in streams. Samples were collected between 17 and 60 times per year.
Herbicides were removed from water samples by extraction with organic-phase (C-18) cartridges.
Samples were analyzed in the laboratory by either gas chromatography/mass spectrometry or
immunoassay. The analytical reporting limit for individual herbicides was 0.05 to 0.2
µ/L.
The total herbicide concentration (sum of the herbicides listed in table
1) in the Mississippi River at Baton Rouge during 1991-97 varied seasonally with the
largest concentrations occurring during May through August (fig.
2). This seasonal pattern has been noted in many streams in the Mississippi River Basin
and has been termed the "spring flush." Although individual herbicide concentrations in some
Mississippi River Basin streams have been reported to exceed 50 µ/L, total herbicide
concentrations in the Mississippi River at Baton Rouge did not exceed 10 µ/L during
1991-97 (fig. 2). Smaller, more drawn-out herbicide peaks in the
Mississippi River, compared with peaks in smaller streams, are attributable to the integrating
effect of the Mississippi River, which receives input from many smaller streams. These smaller
streams may drain areas of variable land use and crop groups and may deliver herbicides to the
Mississippi River at different times during the growing season. Peak herbicide concentrations
at Baton Rouge also may be attenuated by the presence of upstream reservoirs, which collect
and store the spring flush of herbicides, subsequently delivering smaller concentrations
downstream for longer periods of time (Goolsby and others, 1993).
Of the seven most frequently detected compounds in the Mississippi River at Baton Rouge, four
were breakdown products, or metabolites, of herbicides (fig. 3). Individual herbicides and metabolites detected in
more than 50 percent of the samples were alachlor ESA, atrazine, metolachlor, deethylatrazine,
and cyanazine. In general, concentrations of these five herbicides and metabolites also were
higher than concentrations of the other herbicides analyzed (fig. 3). No herbicide or metabolite was detected at a
concentration exceeding 5 µ/L.
Concentrations of atrazine, cyanazine, and metolachlor in the Mississippi River generally
peaked simultaneously in late May and early June, then decreased to preplant levels by the
end of August (fig. 4). Acetochlor was first detected in the Mississippi
River at Baton Rouge in 1995 (fig. 4), 1 year after its initial
introduction for use in the United States. Coincident with the increase in the number of
detections of acetochlor starting in 1995 was a decrease in concentrations and the number of
detections of alachlor. However, the alachlor metabolite alachlor ESA was detected in most of
the samples collected during the growing season during 1995-97. Alachlor ESA is less toxic
than the parent compound but is more persistent, has a greater mobility in soil and water, and
is more frequently detected in ground water throughout the Midwest.
Maximum Contaminant Levels (MCLs) and Health Advisories (HAs) for pesticide concentrations in
drinking water have been established for a number of herbicides by the U.S. Environmental
Protection Agency. Whereas MCLs are legally enforceable drinking-water regulations, HAs are
drinking-water criteria and nonenforceable. The concentration of atrazine exceeded the MCL of
3 µ/L in 11 of 271 samples (fewer than 5 percent). Cyanazine concentrations exceeded the
HA of 1 Îg/L in 15 of 245 samples (6 percent). Alachlor and simazine concentrations did not
exceed their MCLs of 2 and 4 µ/L, respectively. However, because MCLs and HAs are based
on annual average concentrations, one or more exceedances of the specified value does not
necessarily indicate noncompliance. None of the average annual concentrations of the
herbicides examined in this study exceeded MCLs or HAs.
The load, or mass per unit time, of herbicides delivered to the Gulf of Mexico was estimated
to examine trends in herbicide transport in the Mississippi River and to evaluate the amount
of herbicide leaving the basin as a percentage of the amount applied. Daily loads were
calculated by multiplying the daily herbicide concentration by the daily streamflow in the
Mississippi River at Baton Rouge. Herbicide concentrations on nonsampling days were estimated
by interpolating between concentrations measured on sampling days. Daily loads were summed to
estimate a total load over a specified period of time.
From 1991 through 1997, the annual load of herbicides from the Mississippi River Basin to the
Gulf of Mexico ranged from about 450 metric tons in 1992 to 1,920 metric tons in 1993
(fig.
5A). The load estimates indicate that alachlor ESA (analyzed beginning in July
1993) and cyanazine amide (analyzed beginning in 1995) compose a large part of the annual load
to the Gulf of Mexico. Nearly 80 percent of the annual herbicide load was discharged during
May through August (fig. 5B). The largest monthly load of herbicides was discharged to the Gulf
of Mexico in June when an average of about 300 metric tons, or 25 percent of the average
annual total, was discharged. In June 1995, nearly 650 metric tons of herbicides were
discharged to the Gulf of Mexico, the largest single monthly load of herbicides during 7 years
of sampling.
Of the herbicides analyzed, atrazine composed the largest part of the load discharged to the
Gulf of Mexico with a peak of about 640 metric tons in 1993 (fig. 6). This amount represented about 3
percent of the atrazine applied in the Mississippi River Basin in 1993. During 1991-97, the
average annual load of atrazine represented about 2 percent of its basinwide use. If the load
of atrazine metabolites (deethylatrazine and deisopropylatrazine) were included with the load
of atrazine, the average annual load would represent about 3 percent of the annual basin use
of atrazine. The loads of cyanazine and metolachlor in the Mississippi River were roughly
equivalent during 1991-97 (fig. 6). However, during most years the amount of cyanazine applied in the
Mississippi River Basin was about half that of metolachlor. Cyanazine, a triazine herbicide
with properties similar to those of atrazine, is relatively mobile in the environment and is
more persistent in streams than metolachlor, an acetanilide herbicide. As a result, the
average annual load of cyanazine during 1991-97 was about 1.5 percent of basinwide use, and
for metolachlor the average annual load was about 0.8 percent. The load of acetochlor in the
Mississippi River increased from less than 3 metric tons during its first year of use in 1994
to more than 30 metric tons per year during 1995-97 (fig. 6). The load of alachlor showed a corresponding
decrease from 49 metric tons in 1993 to less than 4 metric tons in 1997. The average annual
load of acetochlor and alachlor, both acetanilide herbicides, represented 0.3 and 0.15 percent
of the basinwide use, respectively.
The total load of acetochlor, alachlor, atrazine, cyanazine, and metolachlor as a percentage
of total basinwide use varied during 1991-97 (fig. 7). The largest total use of these five herbicides
occurred in 1992, when more than 81,000 metric tons were applied to crops in the Mississippi
River Basin. However, the total load of these herbicides in 1992 was only about 350 metric
tons, less than 0.5 percent of the use. Conversely, in 1993 the total use was about 73,000
metric tons, and the total load was about 1,200 metric tons, or about 1.6 percent of the use.
Obviously, factors other than total use are important in determining the concentration and
load of herbicides in the Mississippi River. Previous investigations have identified timing
and method of application, spatial and temporal variations in spring storms, and annual
variation in total streamflow as important factors that affect the temporal variability of
herbicides in streams.
We wish to acknowledge the personnel of the U.S. Geological Survey organic geochemistry
laboratory in Lawrence, Kansas, for their analysis of the herbicides samples used in this
publication. The analytical methods used have been described by Thurman and others (1990),
and Meyer and others (1993).
--Gregory M. Clark and Donald A. Goolsby
This Fact Sheet is based on information contained in the following publication and the
references contained therein:
Clark, G.M., Goolsby, D.A., and Battaglin, W.A., 1999, Seasonal and annual load of
herbicides from the Mississippi River Basin to the Gulf of Mexico: Environmental Science &
Technology, v. 33, no. 7, p. 981-986.
Goolsby, D.A., Battaglin, W.A., Fallon, J.D., Aga, D.S., Kolpin, D.W., and Thurman, E.M.,
1993, Persistence of herbicides in selected reservoirs in the Midwestern United
States--some preliminary results, in Goolsby, D.A., Boyer, L.L., and Mallard,
G.E., compilers, Selected papers on agricultural chemicals in water resources of the
midcontinental United States: U.S. Geological Survey Open-File Report 93-418, p. 51-74.
Meyer, M.T., Mills, M.S., and Thurman, E.M., 1993, Automated solid-phase extraction
of herbicides from water for gas chromatographic-mass spectrometric analysis: Journal of
Chromatography, V. 629, p. 55-59.
Thurman, E.M., Meyer, M.T., Pomes, M.L., Perry, C.A., and Schwab, A.P., 1990,
Enzyme-linked immunosorbent assay compared with gas chromatography/mass spectrometry for
the determination of triazine herbicides in water: Analytical Chemistry, v. 62, p.
2043-2048.
U.S. Department of Agriculture, 1992-97, Agricultural chemical usage--1992-98
field crops summary: Washington, D.C., U.S. Department of Agriculture, National
Agricultural Statistics Service, published annually.
Factors for converting metric units to inch/pound units measurements:
To convert from |
To |
Multiply by |
kilometer |
mile |
0.6214 |
kilogram |
pound |
2.205 |
ton (metric) |
ton (short, 2,000 pounds) |
1.102 |
microgram per liter |
part per billion |
1.0 |
For additional information and selected readings about the Miscontinent 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/ind-ex.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
|