Introduction 

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, cyanazine, 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 ecosystems 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 μg/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 μg/L. 

Herbicide Occurrence in the 

Mississippi River at Baton Rouge 

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 μg/L, total herbicide concentrations in the Mississippi River at 

Baton Rouge did not exceed 10 μg/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 

Discharge of Herbicides From the Mississippi River 

Basin to the Gulf of Mexico, 1991–97 

U.S. Department of the Interior 

U.S. Geological Survey 

USGS Fact Sheet FS–163– 98 

April 1999 

0 300 600 KILOMETERS 

0 300 600 MILES 

Boundary of 

Mississippi 

River Basin 

Gulf of Mexico 

Baton Rouge 

CANADA 

River 

Figure 1. 

sampling site. 

Boundary of Mississippi River Basin and location of 
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 μg/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 

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 drinkingwater regulations, HAs are drinkingwater criteria and nonenforceable. The 

concentration of atrazine exceeded the 

MCL of 3 μg/L in 11 of 271 samples 

(fewer than 5 percent). Cyanazine 

concentrations exceeded the HA of 

1 μg/L in 15 of 245 samples 

(6 percent). Alachlor and simazine 

concentrations did not exceed their 

MCLs of 2 and 4 μg/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. 

Herbicide Load to the Gulf of Mexico 

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 

1991 1992 1993 1994 1995 1996 1997 0 

2 

4 

6 

8 

10 

Total herbicide concentration, 

in micrograms per liter 

Stream discharge, in cubic 

meters per second 

0 

10,000 

20,000 

30,000 

40,000 

50,000 

60,000 

Total herbicide 

concentration 

Stream 

discharge 

Figure 2. 

the Mississippi River at Baton Rouge, Louisiana, 1991–97. 

total herbicide concentration represents the sum of the herbicides 

and metabolites listed in table 1. 

Table 1. Herbicides and herbicide metabolites analyzed in samples from the 

Mississippi River at Baton Rouge, Louisiana, 1991–97 

[m g/L, micrograms per liter] 

Herbicide or 

metabolite 

Type of 

herbicide 

Number of 

samples 

Years 

sampled 

Analytical 

reporting limit 

(m g/L) 

Acetochlor acetanilide 120 1994–97 0.05 

Alachlor do. 271 1991–97 .05 

Alachlor ESA metabolite 160 1993–97 .10 

Ametryn triazine 198 1991–97 .05 

Atrazine do. 271 1991–97 .05 

Cyanazine do. 245 1991–97 0.05–0.20 

Cyanazine amide metabolite 88 1994–97 .05 

Deethylatrazine do. 245 1991–97 .05 

Deisopropylatrazine do. 245 1991–97 .05 

Metolachlor acetanilide 271 1991–97 .05 

Metribuzin triazine 243 1991–97 .05 

Prometon do. 199 1991–97 .05 

Prometryn do. 199 1991–97 .05 

Propazine do. 199 1991–97 .05 

Simazine do. 271 1991–97 .05 

Terbutryn do. 115 1993–97 .05 

detections of 

Stream discharge and total herbicide concentration in 

The 
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. 

Acknowledgments 

We wish to acknowledge the personnel 

0 

0 

0.05 

0.10 

1.0 

10 

20 

10 

30 

40 

50 

60 

70 

80 

90 

100 

Percent of detection Concentration, in micrograms per liter 

A. 

B. 

Parent herbicide 

Metabolites 

Number of samples 

Maximum value 

75th percentile 

Median 

25th percentile 

Minimum value 

(160) 

(271) 

(271) 

(271) 

(245) 

(245) 

(245) 

(88) (271) 

(271) (120) 

(243) (199) 

Alachlor ESA

Atrazine

Metolachlor

Deethylatrazine

Cyanazine

Deisopropylatrazine

Cyanazine amide

Simazine

Alachlor

Acetochlor

Metribuzin

Prometon 

Reporting level 

1991 1992 1993 1994 1995 1996 1997 0 

0.3 

0.6 

0.9 

1.2 

1.5 

0 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

3.5 

4.0 

4.5 

Herbicide concentration, in micrograms per liter 

Atrazine 

Cyanazine 

Alachlor 

Alachlor ESA 

Acetochlor 

Metolachlor 

A. 

B. 

Figure 3. A) Percent detection and (B) concentrations of herbicides and herbicide metabolites in samples from the Mississippi River at Baton Rouge, 1991–97. 

prometryn, propazine, and terbutryn were detected in less than 

2 percent of samples. 

Figure 4. emporal variation in concentrations of herbicides in 

samples from the Mississippi River at Baton Rouge, 1991–97. 

(

The herbicides ametryn, 

T
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. 

Additional References 

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 midcontin-ental 

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—1992–97 field crops 

summary: Washington, D.C., U.S. 

Department of Agriculture, National 

Agricultural Statistics Service, 

published annually. 

For additional information and 

selected readings about the 

Miscontinent Herbicide Project, 

write to: 

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. 

U.S. Geological Survey 

Mail Stop 415, Box 25046 

Building 53, Wing F–200 

Denver Federal Center 

Lakewood, CO 80225 

To convert from o 

kilometer 

kilogram 

ton (metric) 

microgram per 

liter 

mile 

pound 

ton (short, 

2,000 pounds) 

part per 

billion 

0.6214 

2.205 

1.102 

1.0 

Factors for converting metric units to

inch/pound units measurements: 

1991 

J F M A M J J A S O N D 

1992 1993 1994 1995 1996 1997 

350 

0 

500 

1,000 

1,500 

2,000 

300 

250 

200 

150 

100 

50 

0 

Mean monthly load for 

1991–97, in metric tons 

Annual load, in metric tons

Alachlor 

ESA Cyanazine amide 

Herbicides and 

metabolites analyzed 

during 1991–97 

other than cyanazine 

amide and alachlor ESA 

All herbicides analyzed 

during 1991–97 

A. 

B. 

Atrazine 

Cyanazine 

Alachlor 

Acetochlor 

Metolachlor 

1990 1991 1992 1993 1994 1995 1996 1997 1998 

0 

10 

100 

1,000 

Herbicide load, in 

metric tons per year 

1995 

1994 

1993 

1996 

2.0 percent 

1.0 percent 

0.5 percent 

1991 

1992 

1997 

60,000 70,000 80,000 90,000 

2,000 

1,000 

1,500 

300 

500 

Total load of acetochlor, alachlor, 

atrazine, cyanazine, and metolachlor, 

in metric tons per year 

Total use of acetochlor, alachlor, atrazine, 

cyanazine, and metolachlor in the Mississippi 

River Basin, in metric tons per year 

Figure 5. A) Annual and (B) monthly loads of herbicides 

discharged from the Mississippi River Basin to the Gulf of 

Mexico, 1991–97. 

July–December only. 

Figure 6. 

charged to the Gulf of Mexico, 1991–97. 

Figure 7. 

selected herbicides discharged to the Gulf of Mexico, 

1991–97. ­

centage of annual use. 

T Multiply by 

(

The 1993 load of alachlor ESA represents 

Annual loads of selected herbicides disRelation of total annual load and use for 

Lines are annual load as a constant per